David Grémillet
email: david.gremillet@cefe.cnrs.fr

Jérôme Fort

Françoise Amélineau

Elena Zakharova

Tangi Le Bot

Enric Sala

Maria V Gavrilo

Arctic warming: non-linear impacts of sea-ice and glacier melt on seabird foraging

Keywords: Arctic marine fauna, Arctic climate change, Behavioural plasticity, Cryosphere, Historical data, Little auk

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Introduction

Global warming is twice as marked in the Arctic than at lower latitudes, and this has a rapid and profound impact on the cryosphere. For instance, the Intergovernmental Panel on Climate Change Fifth Assessment Report (IPCC, 2013) predicts a sea-ice free Arctic in summer from the 2030s, or towards the end of 21 st century, depending on scenarios. The Arctic being 70% marine, a reduction in sea-ice extent and thickness radically modifies its seascape, with major impacts on atmospheric and ocean circulation, biogeochemical cycles and ecosystem functioning (AMAP, 2012). Endemic arctic species, such as the emblematic polar bear (Ursus maritimus), are predicted to go extinct, and to be replaced by species from lower latitudes (review in [START_REF] Gilg | Climate change and the ecology and evolution of Arctic vertebrates[END_REF]. Yet arctic warming also leads to pronounced glacier and permafrost melt (IPCC, 2013), which strongly modify terrestrial and coastal landscapes, and associated ecological dynamics.

Determining and predicting the overall impacts of cryosphere changes is therefore a key research objective (AMAP, 2012), particularly at the interface between terrestrial and marine ecosystems [START_REF] Post | Ecological Consequences of Sea-Ice Decline[END_REF]. Beyond polar bears, a limited number of sea-ice associated arctic species function as sentinel species, whose ecological responses to the consequences of climate change deserve special attention. This is the case of the little auk (Alle alle), for a number of reasons: it is the most abundant seabird in the Atlantic Arctic, with over 40 million individuals, and therefore plays a major ecological role within marine ecosystems [START_REF] Karnovsky | Estimation of carbon flux to dovekies (Alle alle) in the North Water[END_REF][START_REF] Karnovsky | Foraging behavior of little auks in a heterogeneous environment[END_REF]. It is a specialized zooplankton consumer, and participates in a short food chain with strong responses to sea-ice presence/absence and to sea-surface temperature [START_REF] Grémillet | Little auks buffer the impact of current Arctic climate change[END_REF][START_REF] Jakubas | Foraging by little auks in the distant marginal sea ice zone during the chick-rearing period[END_REF]. It is also the smallest high arctic, diving seabird, and operates at the upper limit of its energetic capabilities, something that strongly enhances its sensitivity to environmental change (Harding et al., 2009a). Current knowledge shows that ocean warming and a vanishing cryosphere have a negative impact on little auks, because they preferentially feed on large, lipid-rich zooplankton (copepods) associated with cold water in the marginal ice zone [START_REF] Karnovsky | Foraging distribution of little auks (Alle alle) across the Greenland Sea: implications of present and future Arctic climate change[END_REF]. It is therefore predicted that little auk colonies situated in warmer areas will be more impacted than those from areas that are less exposed to global warming. Yet ocean warming progresses rapidly in the North Atlantic, and it seems appropriate to test the impact of cryosphere changes on little auks at their northernmost breeding site, which is situated in Franz-Josef Land (Russian State Federal Refuge), at 80°N. This extremely remote area is often regarded as an 'arctic Eden', still shielded from the consequences of global warming, with 85% of its land mass covered by glaciers and extensive, year-round, sea-ice cover surrounding islands of the archipelago. It is home for over a million seabirds, including an estimated 500,000 little auks [START_REF] Anker-Nilssen | The status of marine birds breeding in the Barents Sea region[END_REF], as well as large proportions of the world populations of further endemic arctic species (e.g. polar bear, walrus Odobenus rosmarus rosmarus, ivory gull Pagophila eburnea, bowhead whale Balaena mysticetus). Previous investigations showed that little auks mainly respond to marine environmental change through marked plasticity in their foraging behaviour and diet [START_REF] Grémillet | Little auks buffer the impact of current Arctic climate change[END_REF]. We therefore investigated the foraging ecology of little auks from Franz-Josef Land using miniaturised temperature-depth recorders (TDR) and dietary analyses. Adult body measurements and chick growth rates were also recorded. These fitness indicators were compared with data collected at the same study site twenty years before, and with little auk foraging behaviour from three other breeding sites across the Atlantic Arctic.

Seabird data were interpreted in the context of Franz-Josef Land sea-ice and glacier dynamics, which we analyzed over the past 30 years using satellite remote-sensing. Specifically, we tested the hypothesis that little auks from Franz-Josef Land benefit from conditions of the pre-industrial age, with a foraging effort predicted to be lower than at other, more southern breeding sites in the Arctic.

Materials and methods

The study site was situated in Tikhaya Bay, Hooker Island, Franz-Josef Land (80°20.293'N ; 52°49.062'E), at a little auk colony facing coastal waters and in the vicinity of the 7.2 km 2 Sedov glacier (Fig. 1), during National Geographic's Pristine Seas 2013 Franz-Josef Land expedition.

Little auks were studied in July-August 2013 during the chick-rearing phase, within the framework of a scientific programme approved by the board of the National Park Russian Arctic and the Ministry of Education and Science of the Russian Federation (Permit N°71 issued on 03/06/2013). This phase was chosen because birds are accessible, have their highest foraging effort for self-maintenance and chick-feeding, and are therefore assumed to be most susceptible to environmental change. Four different sets of birds/nests were studied synoptically within the same colony. First, 15 birds were caught in the vicinity of their nest and equipped with TDRs (G5; CEFAS Technologies, Lowestoft, UK; 31 mm length, 8 mm diameter; 2.7 g, corresponding to 1.7% of lightest handled little auk body mass, or DST micro-TD; StarOddi, Iceland; 25.4 mm length, 8.3 mm diameter; 3.3 g, corresponding to 2% of the lightest handled little auk body mass), which were attached to ventral feathers using waterproof Tesa® tape. TDRs were left on the birds for 3-6 days, upon which birds were recaught and tags downloaded. TDRs recorded hydrostatic pressure and temperature every 4 sec for the entire period (G5), or for 24h starting on the day following deployment (DST microTD). Recorded temperature signals allow discrimination of periods spent at the nest, in flight, and in the water, and were therefore used to calculate the number of foraging trips per 24h, foraging trip duration and flight time per trip and per 24h (for full details on analyses see Harding et al., 2009b). Pressure recordings were analyzed with MultiTrace (Jensen Software Systems, Laboe, Germany), to determine (per 24h) the number of dives, average and maximum dive depth, average and maximum dive duration, and the percent time spent underwater. Foraging parameters of little auks from Franz-Josef Land were compared with those of birds from East Greenland and from two locations on Svalbard, which were studied using the same methodology (data from [START_REF] Grémillet | Little auks buffer the impact of current Arctic climate change[END_REF]. This biotelemetry study was complemented by direct observations of feeding little auks, assessed on three occasions from a boat in August 2013. During these 2h sessions birds were observed from <100m on a calm sea using 12x42 binoculars, and positions recorded using a hand-held GPS. These at-sea observations were compared with similar surveys conducted in the summer of 2001. Second, breeding little auks carry undamaged zooplankton prey back to the nest in a sublingual pouch. Twenty birds carrying such food loads were also caught, and complete food loads stored in 70% ethanol. Prey identification was performed to the lowest-possible taxonomic level under a binocular microscope. Third, we marked a set of 18 little auk nests and determined chick growth (body mass to 1g, wing and tarsus length to 1mm) every second day throughout the chick-rearing phase (1-26 August 2013). Finally, all birds caught for TDR deployments and diet sampling were weighed to the nearest 2g and measured (wing and tarsus) to the nearest 1mm. This sample was complemented with adult breeding birds caught synoptically within the same area for other purposes.

Diet, adult morphometrics and chick growth rates were compared with similar data collected by [START_REF] Stempniewicz | The little auk Alle alle polaris of Franz Josef Land: A comparison with Svalbard Alle alle populations[END_REF] and [START_REF] Weslawski | Summer diet of seabirds from the Frans Josef Land archipelago, Russian Arctic[END_REF] within the same area in 1991-1993. We only had access to average body mass and body size measurements for 1991-1993, and therefore could not calculate/compare individual body condition indexes for the two periods.

Mean monthly sea-ice concentrations (percentage of ocean surface covered by sea-ice) were determined for July and August, across 1979-2013, within 100 km of the study colony (i.e. the maximum foraging range of breeding little auks; Anker-Nilssen, 2000; [START_REF] Jakubas | Foraging closer to the colony leads to faster growth in little auks[END_REF] using daily EUMETSAT OSI SAF ice concentration satellite products for 1979-2009. This consisted of reprocessed SMMR and SSM/I instruments data with a spatial resolution of 12.5 km [START_REF] Eastwood | Global Sea Ice Concentration Reprocessing -Product User Manual[END_REF]. AMSR-E instrument measurements with 6.25 km spatial resolution [START_REF] Spreen | Sea ice remote sensing using AMSR-E 89-GHz channels[END_REF] were used as a complement for the 2002-2013 time period, also allowing cross-checking of the two data sets across 2002-2009.

Dynamics of the Sedov glacier were studied using high resolution (60 -15 m) Landsat images.

For the 1985-2013 period, 13 images were used, most of them acquired during July-August, the period of highest ablation, except for 1998 (April) and 2006 (September). Images were digitised, and we determined the glacier terminus position for each year. Yearly glacier progression/retreat was then calculated using 2001 as an arbitrary reference.

Results

Summer sea-ice cover (Fig. 2A) showed strong variability during 1979-2013. Until 2003 the mean monthly sea ice concentration within 100 km of the little auk colony varied between 40 and 70%, and between 15-55% in July and August, respectively. Since 2003-2004 the archipelago has been almost ice-free in August, since sea-ice tended to melt earlier each year. Only small ice-filled areas occurred at the north and north-east periphery of the archipelago, partly advected by winds and currents.

Coastal glaciers also retreated markedly in the study area (Fig. 2B). Since the middle of 1990s the Sedov glacier retreated progressively, and from the year of its maximal surface area in 1994 to 2012 it has been shrinking by 0.64 km 2 . This corresponds to a 9% reduction of its 1994 surface (7.2 km 2 ; 0.50% decrease per year). In order to verify that these observations are robust, we compared this dynamic with that of the neighbouring Yuria glacier (Fig. 1). For this glacier the total 1985-2012 retreat of the terminus position was even higher, with a 18% decrease of its initial 12.5 km 2 area (0.66% per year), and a major decline between 2002-2012.

All 15 TDRs deployed were recovered. One data file was corrupted and we therefore present data for 14 birds (S1). Little auks foraged extensively, performing an average of 6 feeding trips and 361 dives per day. Dives were usually short (36 sec) and shallow (6.2m), but birds occasionally dived >1 min to >20m depth. Diving effort was comparable to that of little auks diving in three other regions of the Arctic (Fig. 3A&B, see also S2). However, and most surprisingly, flight time per foraging trip was at least one order of magnitude lower in little auks from Franz-Josef Land than in birds from three other arctic regions (Fig. 3C&D). Such short flight times (usually <4 min) between breeding colony and feeding area indicated that birds did not feed offshore, but extremely close to the colony. We confirmed this through direct at-sea observations of little auks: birds were seen between foraging dives, positioned along the front separating glacier melt from coastal waters (Fig. 1 andS3). Birds were only present at this front, on the sea side where water was much clearer than in glacial melt water. Conversely, at-sea surveys conducted in the summer of 2001 showed that little auks foraged in the marginal ice zone, northwest of the Franz-Josef archipelago (S4).

Little auk diet was largely dominated by copepods, which were present in all samples and represented 95.7% of all prey items (Tab. 1). These copepods were mainly Calanus glacialis and C. hyperboreus, two cold-water associated species. This diet differed markedly from data collected in 1991-1993 at the same study site [START_REF] Weslawski | Summer diet of seabirds from the Frans Josef Land archipelago, Russian Arctic[END_REF]. Those showed that at that time little auks not only fed on copepods, but also to 13% on Apherusa glacialis, a sea-ice associated amphipod which was present in the food of 49% of all little auks sampled in 1991-1993. This particular prey was absent from all but one of our 2013 samples (i.e. a proportion of 0.5% of all prey and 5% occurrence, see also S5). Growth curves of little auk chicks measured in August 1992 [START_REF] Stempniewicz | The little auk Alle alle polaris of Franz Josef Land: A comparison with Svalbard Alle alle populations[END_REF] and in August 2013 (our study) were identical (Fig. 4).

Adult body measurements (n=57 in 1992 and n=108 in 2013) showed that birds were 4% lighter in 2013 (194.1 ± 11.9g) than in 1992 (202.3 ± 12.5g; t=4.07, p<0.001), but that other measurements were not significantly different between the two periods (Wing length: 133.3± 3.7mm in 1992and 133.0 ± 4.0mm in 2013. Tarsus length: 23.1 ± 0.8mm in 1992and 23.5 ± 1.0mm in 2013).

Discussion

Our results allow two important conclusions: (1) the consequences of global warming are strongly modifying coastal marine ecosystems even in Franz-Josef Land, an extremely remote, high-Arctic locality. (2) Disappearance of summer sea-ice from the archipelago and pronounced coastal glacier melt have antagonistic effects on little auk foraging behaviour, leading to a non-linear response to climate change in this endemic, ecologically highly important arctic species (Fig. 5). Our work is strongly multidisciplinary, and features the first comprehensive investigation of little auk foraging ecology in relation to sea-ice and coastal glacier melt.

Studying the overall impacts of cryosphere changes on arctic biodiversity has been identified as a major scientific challenge by the Arctic Council and its working groups (e.g. AMAP, 2012), as well as by international research initiatives such as the Belmont Forum. Specifically, there is currently a lack of studies that integrate terrestrial and marine processes and investigate the impacts of arctic climate change in coastal areas [START_REF] Post | Ecological Consequences of Sea-Ice Decline[END_REF]. Further, the IPCC (IPCC, 2013) and the Belmont Forum have stressed the importance of identifying non-linear responses of organisms and populations to global change. Our research fits tightly to these objectives, and builds upon existing, yet limited, work on sea-ice associated marine predators in the context of climate change [START_REF] Gaston | Climate change, ice conditions and reproduction in an Arctic nesting marine bird: Brunnich's guillemot (Uria lomvia L.)[END_REF][START_REF] Eamer | Life linked to ice. A guide to sea-ice associated biodiversity in this time of rapid change[END_REF]. In a global perspective, little auks appear as the arctic ecological equivalent to Adélie penguins (Pygoscelis adeliae). For this species, extensive Antarctic research has demonstrated the importance of the marginal ice zone for efficient foraging on krill (Euphausia sp.) and fish, whereby birds require specific, intermediate sea-ice concentrations for an optimal provisioning efficiency [START_REF] Emmerson | Sea ice cover and its influence on Adelie Penguin reproductive performance[END_REF][START_REF] Ballard | Coexistence of mesopredators in an intact polar ocean ecosystem: The basis for defining a Ross Sea marine protected area[END_REF]. Interestingly, recent investigations also demonstrated nonlinear responses of Adélie penguins to climate change, whereby their plastic phenotypic response to variable sea-ice concentration was perturbed by extreme climatic events [START_REF] Lescroël | Antarctic Climate Change: Extreme Events Disrupt Plastic Phenotypic Response in Adelie Penguins[END_REF]. Further, our results are also in line with recent studies and syntheses from other arctic regions, which stressed the importance of glacier outflow into coastal ecosystems, and the ecological consequences of a climate-induced increase of such outflow. In Alaska, for instance, researchers demonstrated that enhanced glacier discharge generate trophic cascades in arctic fjord ecosystems [START_REF] Arimitsu | Oceanographic gradients and seabird prey community dynamics in glacial fjords[END_REF], whereby melt water fronts attract pelagic fish suitable for seabird foraging [START_REF] Renner | Structure of marine predator and prey communities along environmental gradients in a glaciated fjord[END_REF]. For the Svalbard region, Lydersen and colleagues [START_REF] Lydersen | The importance of tidewater glaciers for marine mammals and seabirds in Svalbard, Norway[END_REF] identified similar dynamics as those demonstrated by our study, with seaice rapidly disappearing as a resting and foraging habitat for a series of seabirds and marine mammals, and glacier melt water enhancing primary and secondary productivity in coastal ecosystems, to the benefit of some members of the marine predator guild. Their important conclusion was that this effect might only be temporary, as some of the coastal glaciers might retreat from the coastline if arctic warming persists. The little auk population of Franz-Josef Land is therefore predicted to benefit from profitable foraging conditions close to shore as long as coastal glaciers are melting. According to glacier melt recorded over the last decades (0.5-0.6% of total surface per year, see results), and assuming a linear process, a complete melt of the Sedov glacier would be completed in approximately 180 years. However, the glacier might have retreated sufficiently far inland to stop influencing coastal ecological dynamics decades before. Karnovsky and colleagues (2010) predicted that 40% of all major little auk colonies would disappear from the Atlantic Arctic by the end of the 21 st century. Their prediction, which was based on zooplankton thermal niche modelling under the influence of increasing sea-surface temperature, now has to be revisited in the context of arctic coastal glacier melt and little auk behavioural plasticity. Little auks were so far considered as an archetype of endemic, specialized arctic predator bound to disappear from a warming Arctic. Actually they show surprising flexibility, allowing them to maintain fitness levels [START_REF] Grémillet | Little auks buffer the impact of current Arctic climate change[END_REF]this study). Work conducted at little auk breeding sites across the Arctic clearly demonstrates the importance of cold water and associated large, lipid -rich copepods for their profitable foraging [START_REF] Frandsen | Composition of chick meals from one of the main little auk (Alle alle) breeding colonies in Northwest Greenland[END_REF][START_REF] Kwasniewski | The impact of different hydrographic conditions and zooplankton communities on provisioning Little Auks along the West coast of Spitsbergen[END_REF][START_REF] Karnovsky | Inter-colony comparison of diving behavior of an Arctic top predator: implications for warming in the Greenland Sea[END_REF], as well as their close association with the marginal ice zone [START_REF] Jakubas | Foraging closer to the colony leads to faster growth in little auks[END_REF]. However, our study demonstrates that glacier melt water can also generate optimal foraging conditions for this species, as a refuge coastal habitat in the absence of sea-ice. Higher variability in diving effort for little auks from Franz-Josef Land compared to other localities (Fig. 3A&B) nonetheless suggests lower predictability in food availability at glacier melt-water fronts. Further, it is unclear to us why little auks make such profitable use of this new foraging habitat in Franz-Josef Land, and not at other arctic localities.

This may be due to the geography of the archipelago, with a very high number of small islands covered by glaciers. Franz-Josef Land therefore has a many more glacier fronts exposed to coastal waters than Svalbard, for instance. This presumption calls for further studies assessing the importance of coastal glacier frontal zones to foraging little auks at the meta-population level.

There is currently a huge demand for predicting the fate of arctic biodiversity exposed to ongoing climate change. At the species level, this is achieved by building habitat models linking distribution ranges with a suite of abiotic and biotic environmental variables (Elith & Leathwick, 2009). This environmental envelope is then extrapolated to generate predicted future distributions (e.g. [START_REF] Péron | Projected poleward shift of king penguins' (Aptenodytes patagonicus) foraging range at the Crozet Islands, southern Indian Ocean[END_REF][START_REF] Beaugrand | Marine biological shifts and climate[END_REF]. In this context, and as clearly demonstrated by our study, it is essential to identify and implement key environmental drivers. Ignoring some of these factors, and neglecting non-linear, plastic behavioural responses of the species involved (Fig. 5) may lead to erroneous predictions [START_REF] Barnagaud | Relating Habitat and Climatic Niches in Birds[END_REF][START_REF] Sorte | Tracking of climatic niche boundaries under recent climate change[END_REF][START_REF] Schurr | How to understand species' niches and range dynamics: a demographic research agenda for biogeography[END_REF]. Island, Franz-Joseph Land, in 2013(n = 20, our study) and in 1991-1993(n = 47, (Weslawski et al., 1994)). Note the importance of Calanus copepods (bold) and quasi disappearance of ice-associated Apherusa glacialis and Gammarus wilkitzkii (bold) in 2013. 15,00 0,00 0,00 Thysanoessa spp 0,52 6,00 0,00 0,00 Mysis oculata 0,12 11,00 0,00 0,00 Fishs 0,12 9,00 0,36 40,00 Ostracoda spp 0,12 2,00 0,00 0,00 Acanthostepheia spp 0,02 2,00 0,00 0,00 Chaetognats 0,00 0,00 0,01 5,00 Undetermined 0,00 0,00 0,77 40,00

Table 1. Comparison of little auk diet on Hooker

Figure 1 .

 1 Figure 1. (a) Study area on Hooker Island, Franz-Josef Land (from a 2006 Landsat image),

Figure 2 .

 2 Figure 2. (a) Interannual variability of August and July mean sea ice concentration within 100

Figure 4 :

 4 Figure 4: Little auk chick growth curves from Franz-Josef Land measured in 1992 (black circles;

Figure 5 :

 5 Figure 5: Hypothetical non-linear impacts of sea-ice disappearance and coastal glacier melt onto

Acknowlegdements

This study was conducted in the specially protected area of the Franz-Josef Land State Federal Refuge, within the framework of the joint international expedition of National Park Russian Arctic and a National Geographic 'Pristine Seas Expedition FJL 2013'. The expedition was funded by National Geographic, Blancpain, and Davidoff Cool Water. DG and JF are funded by CNRS and the European Union (Marie Curie CIG; Project 631203 to JF), and by the French Polar Institute (ADACLIM program N°388). We are grateful to the Russian Geographical Society for its support, and to the Russian Arctic National Park for research permits. We warmly thanks the international 'Pristine Seas' team and all persons present on board Polaris during the expedition. We are also immensely grateful to all Russian colleagues present at Tikhaya Bay station in August 2013 for their hospitality, in particular to our bodyguard Vladimir Kretov for endless patience during fieldwork. We thank Craig R. White for advice on allometry, Delphine Bonnet and Rumsais Blatrix for their assistance throughout the zooplankton analyses.

Land are from this study, others from [START_REF] Grémillet | Little auks buffer the impact of current Arctic climate change[END_REF]. Picture credits: Manfred Enstipp & David Grémillet. arbitrary. Note that sea-ice disappearance is initially predicted to have a negative impact on seabirds (1), until glacier discharge is sufficiently strong to create a new coastal foraging habitat (2; current situation in Franz-Josef Land). Reduction of glacier discharge in the absence of seaice is then predicted to further diminish seabird performance (3), but foraging plasticity might allow birds to switch to another feeding mode and stabilize/improve their performance (dotted line). Picture credits: NASA & David Grémillet.


Fang-Fang Huang

Ru-Cheng Wang

Lei Xie

Jin-Chu Zhu

Saskia Erdmann

Xu-Dong Che

Rong-Qing Zhang

Differentiated rare-element mineralization in an ongonite-topazite composite dike at the Xianghualing tin district, Southern China: An electron-microprobe study on the evolution from niobium-tantalum-oxides to cassiterite

Keywords:  Ongonite,  Topazite,  Niobium-tantalum-oxides,  Cassiterite,  Nb-Ta-Sn mineralization,  Fluorine,  Fractional crystallization

Our study characterizes in detail the mineralogical, textural and compositional features of a highly evolved, composite ongonite-topazite dike and its magmatic differentiation history. We present compositional data collected by established techniques, i.e. by electron microprobe and wet-chemical analysis, which provide a detailed framework for future studies that employ state-of-the-art analytical techniques. The studied dike (referred to as the No. 431 dike) crops out within the Xianghualing area in the Nanling Range of southern China, in close spatial association with Jurassic Sn-Nb-Ta granite plutons. The rock samples in the No. 431 dike were collected from a structurally lower drill hole and a trench at higher level. The ongonite is encountered throughout the dike, but the topazite is only revealed along the margin of the upper, near-surface dike. The results of whole-rock major and trace element analyses show that the rocks of the No. 431 dike are strongly peraluminous with an average ACNK value of ~ 1.5 for ongonite and > 3.9 for topazite. They are enriched in F, 1.7 wt.% and 5.4 wt.% on average for ongonite and topazite, respectively. The rocks have low Zr/Hf and Nb/Ta ratios, and high levels of ore-forming elements including Nb, Ta, Sn, and W. Silicate and oxide mineral assemblages, textures, and compositions are also distinct for the two rock types studied. In the lower ongonite of the dike, there are abundant phenocrysts of K-feldspar, quartz, and albite, and microphenocrysts of topaz and zinnwaldite in a matrix dominated by quartz, K-feldspar, and albite. Characteristic oxides are columbite-tantalite, tapiolite, and microlite, but cassiterite is absent. The upper ongonite of the dike has a silicate assemblage similar to the lower ongonite; columbite-(Mn), uranomicrolite, and limited amounts of cassiterite are the dominant accessory minerals. The topazite is characterized by large amounts of topaz and zinnwaldite intergrown with quartz, while K-feldspar, albite, and quartz phenocrysts have rounded shapes and are relatively rare. Cassiterite is the most abundant ore mineral, while Nb-Ta oxide minerals are less abundant. We interpret the wholerock compositional trends, mineral textures, assemblages, and compositions to reflect the differentiation of an evolved, initially homogeneous magma that separated into aluminosilicate and hydrosaline melts, corresponding to crystallization of ongonite and topazite, respectively. The crystallization of Nb-Ta-and Sn-bearing ore minerals was strongly controlled by the separation of the two melt phases. We hypothesize that dike propagation/widening subsequent to the initial dike emplacement may have driven the separation of the aluminosilicate and hydrosaline melt phases that crystallized to ongonite in the core and topazite along the margins of the structurally higher part of the dike.

Introduction

Ongonite and topazite, described frequently in the 1970s (Eadington and Nashar, 1978, Kovalenko et al., 1971and Kovalenko et al., 1975), are two unusual types of felsic rock that are rich in rare elements. They are characterized by volcanic to shallow-intrusive textures, and abundant volatile-rich minerals (mainly topaz). Ongonite is defined as the phenocrystic subvolcanic analogue of granite that is rich in the rare elements Li and F (Kovalenko and Kovalenko, 1976). Topazite has been used in reference to felsic dikes that consist mainly of quartz and topaz, for which mineralogical, textural, and field relationships suggest a magmatic origin (Eadington and Nashar, 1978). Mineralization associated with these two particular types of rock has been described from many localities worldwide. Examples include W deposits associated with F-rich rhyolites (ongonites) from Ongon Khairkhan, Mongolia (Štemprok, 1991), the type locality of topaz rhyolite with W, Nb, Ta, and Sn mineralization (Burt, 1992), similar to topaz-albite granite, e.g., Limu in Southern China [START_REF] Zhu | Huang Topaz-albite granites and rare-metal mineralization in the Limu district, Guangxi Province, southeast China Mineral[END_REF], the French Massif Central (Cuney et al., 1992), Southern New Brunswick, Canada (Taylor, 1992), and the Eastern Desert, Egypt (Helba et al., 1997). The mineralization associated with topazite in Southern China is complex but dominated by Sn, such as deposits at Shicheng, Xunwu, Huichang, Yanbei and Taishun (Liu et al., 1996).

Although both ongonite and topazite are F-rich peraluminous rocks, their coexistence is not common. Kortemeier and Burt (1988) reported the first example of ongonite and topazite dikes occurring together in the Flying W Ranch area, Arizona, and they considered the rocks to be dominantly magmatic with fluorine-controlled transitions for both the ongonite and topazite, according to field, textural, mineralogical, and geochemical criteria. However, the ongonite and topazite in that area are not strongly mineralized.

The No. 431 dike in the Xianghualing tin district of southern China contains both ongonite and topazite in spatial association (Chen, 1984, Du and Huang, 1984[START_REF] Zhu | Liu Topazite-ongonite relationship and its bearing on vertical zonation in rare-metal granites: evidence from Xianghualing District, Hunan Province[END_REF][START_REF] Zhu | Ongonite and topazite in dike No. 431 of Xianghualing district and their spatial zonation and genetic relationship[END_REF], and the typical mineralization involves Nb, Ta, and Sn. The coexistence of both ongonite and topazite in such a small-scale dike provides a unique opportunity to compare the rare-element mineralization in the two rock types. Our objective was to determine if the two rock types represent co-magmatic melts, and if so by which mechanisms they formed? Most interestingly, how can we explain the mineralization of these two different types of rock? We report comprehensive major and trace element whole-rock data, as well as detailed petrographic observations and mineral compositional data determined using electron microprobe analysis (EMPA). We evaluate the genetic relationship between ongonite and topazite exposed in a narrow dike, and discuss differentiation trends, particularly the coupled fractionation of F with Nb-Ta-Sn and the role of liquid immiscibility in their distribution between ongonite and topazite. We explore the potential role of oxides as an efficient container of rare metals and as a monitor of changes in fluid composition in mineralized highly-evolved silica systems. We believe that a detailed study of this kind provides an excellent framework for future investigations using advanced and new, evolving analytical techniques, particularly those targeting melt inclusions of the various mineral populations.

Geological setting and sampling of the Xianghualing No. 431 dike

The Xianghualing metallogenic district is located in the northern part of Linwu county, South Hunan province, which is in the center of the Nanling Range in South China. In terms of its tectonic setting, the Xianghualing district is situated where the middle part of the E-Wtrending Nanling tectonic belt converges with the N-S-trending Leiyang-Linwu tectonic belt [START_REF] Yuan | Dai 40 Ar-39 Ar isotopic dating of the Xianghualing Sn-polymetallic orefield in Southern Hunan, China and its geological implications[END_REF]. The Xianghualing district itself consists of a tectono-magmatic anticlinal dome that is composed mainly of Paleozoic strata with minor Mesozoic and Cenozoic strata. The main part of the lower Paleozoic consists of Cambrian arenaceous, argillaceous, and siliceous rocks together with minor carbonates. The upper Paleozoic strata lie unconformably on the lower Paleozoic, and are dominated by Middle Devonian conglomerates, sandstones, and shales of the Tiaomajian Formation (D 2 t); Middle Devonian limestones and dolomites of the Qiziqiao Formation (D 2 q); Upper Devonian dolomitic limestones and sandstones of the Shetianqiao Formation (D 3 s); and Carboniferous carbonates and clastic rocks [START_REF] Yuan | A precise U-Pb age on cassiterite from the Xianghualing tin-polymetallic deposit (Hunan, South China) Mineral[END_REF].

Jurassic red beds and Cretaceous continental clastic rocks are found scattered in small basins in the northern and eastern parts of the district. Three early Yanshanian granite stocks have been identified in the area, and from south to north they are the Jianfengling granite (K-Ar age of 167 Ma; Mo et al., 1980), the Tongtianmiao granite, and the Laiziling granite (154-155 Ma, zircon U-Pb dating; [START_REF] Zhu | Fractionation, evolution, petrogenesis and mineralization of Laiziling Granite Pluton, Southern Hunan Province[END_REF]. The mineralization associated with these granites includes W, Sn, Nb, Ta, Be, F, Pb, and Zn. This is also the type locality of hsianghualite, the first new mineral named by a Chinese mineralogist in China (Huang et al., 1958).

The No. 431 dike is located ~ 70 m northeast of the Laiziling granite (E112° 34′ 06″; N25° 27′ 26″) (Fig. 1). It is ~ 1770 m long and 1.8 to 18.0 m wide. The dike dips to the south at 42°-78°. The dike intrudes Devonian carbonates along a sharp contact, and the dike margins are characterized by flow structures and xenoliths of Devonian limestone which have chilled margins [START_REF] Zhu | Liu Topazite-ongonite relationship and its bearing on vertical zonation in rare-metal granites: evidence from Xianghualing District, Hunan Province[END_REF]. There is no direct evidence that the dike connects with the Laiziling granite body. However, vertical drilling has proved that the intrusive contact of the granite is inclined beneath the dike at an angle of 40°, and it thus seems possible that granite and the No. 431 dike are connected at depth. Modified after [START_REF] Zhu | Fractionation, evolution, petrogenesis and mineralization of Laiziling Granite Pluton, Southern Hunan Province[END_REF] Ground (or near-surface) trenches and subsurface drillings were carried out in the early 1970s in order to investigate the dike. Earlier work had described the dike as heterogeneous, and it was classified as a topaz-bearing felsophyre and aplite (Du and Huang, 1984), but the later work of [START_REF] Zhu | Liu Topazite-ongonite relationship and its bearing on vertical zonation in rare-metal granites: evidence from Xianghualing District, Hunan Province[END_REF] showed that the dike contains at least two major rock types, ongonite and topazite.

The samples we describe in this paper were collected from the No. 431 dike at two different locations. They capture the apparent petrographic variations both vertically and horizontally (Fig. 2). On the unexposed western side of the dike, the No. 840 drill revealed ongonite between depths of -87.4 m and -96.3 m. The samples acquired from the eastern side of the dike are exposed in the No. 5 exploratory trench. They contain spatially associated ongonite, occurring at the core of the upper dike, and topazite, occupying the marginal zones of the upper dike (Huang et al., 1988). Thus, ongonite appears throughout the dike, revealed both as lower ongonite in the subsurface drill hole and as upper ongonite in the near-surface trench, whereas topazite is only found in the upper part of the dike. Details of the whole-rock major and trace-element compositions of the No. 431 dike are given in Table 1. The data show that ongonites have similar compositions characterized by relatively high contents of SiO 2 (on average, 68.59 wt.%) and Al 2 O 3 (on average, 18.47 wt.%), where SiO 2 contents decrease and Al 2 O 3 contents increase from lower to upper parts of the dike. The rocks have a strongly peraluminous character with an average ACNK value of 1.5 [Al 2 O 3 /(Na 2 O + K 2 O + CaO) in molars], and high F contents (on average, 1.7 wt.%). These characteristics are typical of ongonites elsewhere (Kovalenko andKovalenko, 1976 andŠtemprok, 1991). We further note that the marginal zone of lower ongonite (against the country rocks) is enriched in F and Li relative to its core zone (i.e., samples 10-11-20 versus samples 12-14; Table 1). Moreover, two slight differences in the major element contents between the lower and upper ongonites should not be ignored. Compared with ongonite in the near-surface, lower ongonite has higher contents of FeO t and CaO (up to 3.03 and 1.00 wt.%, respectively), possibly corresponding to the higher mica and fluorite abundances. Relative to the ongonites, the topazite has a lower SiO 2 content, ranging from 56.57 to 63.08 wt.%, and higher Al 2 O 3 content (up to 27.77 wt.%, average of 25.14 wt.%) and significantly higher F contents (up to 6.22 wt.%, average of 5.44 wt.%). The whole-rock SiO 2 content and FeO t /MnO decrease from lower ongonite to upper ongonite and topazite in the No. 431 dike, while Al 2 O 3 inversely increases (Table 1). Compared to the ongonite rocks, the Na 2 O content of the topazite is extremely low (< 0.08 wt.% versus 3.80-6.88 wt.% for ongonites) while K 2 O contents are relatively high, but variable (3.56-5.57 wt.% versus 1.05-4.59 wt.% for ongonites). The samples from the No. 431 dike are enriched in Li, Rb, Cs, Nb, Ta, Be, Sn, W, and Hf, and depleted in Sr, Ba, P, and Ti, with significantly low Zr/Hf (on average 6.04 for ongonites and 4.23 for topazite) and Nb/Ta ratios (on average 1.55 for ongonites and 1.06 for topazite) (Table 1). The contents of the ore-forming elements Nb, Ta, Sn, and W are high, up to 248, 175, 62.6, and 68.5 ppm for the ongonites, and 223, 207, 802, and 771 ppm for the topazite, respectively. The contents of rare-earth elements (REE) in the ongonite and topazite samples are markedly low, and the normalized REE pattern is similar to the M tetrad effect of [START_REF] Takahashi | W-and M-type tetrad effects in REE patterns for water-rock systems in the Tono uranium deposit[END_REF] (Fig. 5). There are slight negative Eu anomalies; the δEu value for the ongonites and topazite average 0.22 and 0.47, and the (La/Yb) N ratio varies from 1.00 to 2.89 and 1.80 to 5.50, respectively.

Mineral chemistry

Feldspars

Feldspars, including K-feldspar and albite, are common in the ongonites but absent in the topazite. In the ongonites, K-feldspar exists mainly as subhedral phenocrysts up to 1 mm long, and locally it encloses albite. All K-feldspar has low Ab contents of 1.8-3.2 mol%. Albite occurs mostly as euhedral to subhedral tabular phenocrysts, but may also be included within K-feldspar or quartz to form a snowball texture which is usually observed in granites rich in rare-elements (Huang et al., 2002 andPollard, 1989). Compositionally, the feldspars in the ongonites are nearly pure end members (K-feldspar has 1.79-3.18 mol% Ab, and albite has 97.00-99.1 mol% Ab) (Table 2). Note: -: Below detection limits; structural formulae of topaz were calculated on the basis of (Al + Si = 3).

Mica

In the ongonites, mica occurs as phenocrysts about 0.1 to 1.5 mm in size, and is also present in the groundmass. But it is not as abundant as in the topazite, and it is only locally found as one of main rock-forming minerals. According to electron-microprobe analyses, the micas in the ongonites and topazite mostly classify as zinnwaldite, together with minor lepidolite and rare lithian muscovite (following the classification of Foster, 1960a andFoster, 1960b). The FeO content of zinnwaldite in the lower ongonite may be up to 16.88 wt.%, while that in the upper ongonite from the trench is 6.62 wt.%, close to the composition of lepidolite. In contrast, the MnO content is highest in upper ongonite (up to 2.78 wt.% MnO), but very low in lower ongonite (only up to 0.95 wt.% MnO). Zinnwaldite from the topazite has FeO and MnO contents of about 10 wt.% and 1.9 wt.%, respectively, and the composition is similar to the micas in the coexisting ongonite. The F content of the zinnwaldite is on average 5.71 wt.% in lower ongonite from the drill hole, while in upper ongonite and topazite from the trench the values are higher, with averages of 8.89 wt.% and 8.17 wt.%, respectively (Table 3). Tindle and Webb (1990) and Monier and Robert (1986), respectively.

Topaz

Topaz is only observed as an accessory mineral in the ongonites, but it is one of the main rock-forming minerals in topazite, where it can reach approximately 30 vol.%. Two kinds of topaz crystal are found in the ongonites of the dike: short prisms in lower ongonite and fine acicular crystals in upper ongonite. They usually coexist with other rock-forming minerals in the groundmass. In the topazite, topaz occurs dominantly as small grains in the groundmass, rarely as phenocrysts, but it also forms aggregates with cassiterite (see below). Radial arrays of needle-like topaz characterize the groundmass, of which some are included in quartz phenocrysts. In addition, EMPA results indicate that the topaz is fluorine-dominated with F contents of 18.88 to 21.67 wt.% (Table 2).

Niobium-tantalum oxides

Nb-Ta oxide minerals in the No. 431 dike include the columbite-group minerals which are always present, and the rarely observed tapiolite and microlite. The columbite-group minerals are present in all the ongonites and topazite to different degrees. Only a small amount of tapiolite is observed in lower ongonite, while microlite occurs in the ongonites as well as the topazite. Most of the Nb-Ta oxides are present in association with rock-forming minerals such as quartz, albite, and mica. Approximately 400 electron-microprobe analyses were obtained from about 80 Nb-Ta oxide grains in the studied ongonites and topazite, and representative results are presented in Table 4.

Niobium-tantalum oxides in ongonite of the lower dike

Columbite-group minerals appear in lower ongonite of the dike mostly as fine grains less than 300 μm in size, which are commonly interstitial to the rock-forming minerals. Sometimes the columbite contains inclusions of quartz and topaz, indicating its simultaneous crystallization with those rock-forming minerals (Fig. 6). Progressive zoning is occasionally observed in some crystals, characterized by increasing brightness in BSE images (Fig. 6a). In particular, the grains commonly display < 20 μm wide, very bright rims (Fig. 6a,b). Electronmicroprobe results indicate that the columbite-group minerals can be divided into two groups on the basis of their compositions. The dominant part in some zoned crystals displays variations in Mn/(Fe + Mn) (shortened hereafter as Mn # ) from 0.46 to 0.70, and in Ta/(Nb + Ta) (shortened to Ta # ) from 0.04 to 0.38. Overall, most of them fall into the field of columbite-(Mn) (Table 4; Fig. 6e). The brightest outermost parts of the grains are tantalite-(Fe) or Ta-rich columbite-(Fe). They have moderate Ta # of 0.30-0.58 and a variable Mn # of 0.17-0.62. Microgranular aggregates in the marginal zone of lower ongonite make up the second-most important occurrence of Nb-Ta oxide minerals. Back-scattered electron images demonstrate that tantalite-(Fe) and tapiolite are the main minerals in the centers of the aggregates, and these are surrounded by many irregular microlite grains (Fig. 6c,d). Uraninite and fluorite may also be found in this complex association. The results of the analyses of tantalite-(Fe) show a restricted Ta # ranging from 0.52 to 0.58 and a Mn # ranging from 0.20 to 0.42. They plot precisely near the bottom boundary of the miscibility gap between tantalite and tapiolite (Černý et al., 1992). Tapiolite, a kind of highly Fe-dominated Ta-enriched oxide, has extremely high values of Ta # and low values of Mn # , ranging from 0.76 to 0.91 and 0.03 to 0.04, respectively. Its composition plots close to the top boundary of the miscibility gap (Fig. 6e). The third main mineral in the aggregates is microlite, containing 8.69 wt.% Na 2 O, 5.54 wt.% CaO, and 6.23 wt.% UO 2 on average (Table 5). In addition, the fluorine content of the microlite in subsurface ongonite is up to 4.23 wt.%, the highest of all the microlites from the different rocks of this dike. In summary, the Nb-Ta oxides display an overall Ta dominance toward the marginal zones of lower ongonite of the dike, especially for the structurally highest level, characterized first by the appearance of Ta-dominated minerals such as tapiolite and microlite, and second by the overall increasing Ta # value in the columbite, as revealed in Fig. 7.

Niobium-tantalum oxides in ongonite of the upper dike

In comparison with the lower ongonite of the dike, the columbite-group minerals in upper ongonite are compositionally simpler. Intergranular subhedral tabular crystals up to 100 μm long (Fig. 8a) that contain low Ta # values of 0.09 to 0.11 and Mn # values of 0.55 to 0.60 classify as columbite-(Mn) (Table 4). Moreover, abundant subhedral to anhedral fine-grained crystals occur interstitially in near-surface ongonite, and they have moderate Ta # values of 0.21 to 0.41 and high Mn # values of 0.86 to 0.96. This type of columbite also classifies as columbite-(Mn). The columbite-(Mn) is characteristically enriched in tungsten with a maximum of 18.86 wt.% WO 3 and an average of 4.54 wt.%. The third type of columbite grains is part of a mineral association that includes microlite (Fig. 8b) and/or rutile + zircon, and these grains are commonly 10-50 μm in size. They have distinctively high values of Ta # and Mn # , ranging from 0.32 to 0.37 and 0.91 to 0.94, respectively. All the columbite group minerals found in upper ongonite are columbite-(Mn), and the latter two kinds of Nb-Ta oxide minerals share close positions in the columbite-tantalite quadrilateral diagram (Fig. 8c).

The associated microlite is characterized by high UO 2 contents up to 15.36 wt.% but low Na 2 O and CaO levels. It could therefore be classified as uranomicrolite according to the classification suggested by Hogarth (1977) and Černý (1989). Tantalian rutiles are only found in upper ongonite as euhedral-subhedral crystals in aggregates of Nb-Ta oxide minerals, and they contain up to 21.92 wt.% Ta 2 O 5 and trace amounts of Fe, Mn, Sn, and W.

Niobium-tantalum oxides in topazite

In the topazite, columbite-group minerals are fewer and smaller in size (10-15 × 10-30 μm), and they commonly occur as subhedral crystals associated with microlite or hubnerite (Fig. 9a,b). Chemical analyses show that the columbite-group minerals also correspond to columbite-(Mn) in the classification diagram (Fig. 9c). Overall the Ta # and Mn # values of columbite-(Mn) in topazite overlap those in the associated upper ongonite. Subhedral to anhedral microlite is present along with the columbite-(Mn), and it is rich in uranium with up to 17.46 wt.% UO 2 , which means it can be classified as an uranomicrolite. The association of columbite and microlite is consistent with the assemblage observed in the spatially associated ongonite.

Cassiterite

Cassiterite is almost absent in lower ongonite and scarce in upper ongonite. It occurs as anhedral crystals or fine veinlets along zinnwaldite cleavages. In contrast, in the topazite, cassiterite is abundant as an accessory mineral. According to the textural evidence and mineral compositions, two main types of cassiterite may be distinguished in the topazite: disseminated and aggregated cassiterites, and they are considered below (Table 6; Fig. 10). able 6.

Representative electron-microprobe results of cassiterites in the Xianghualing No. 431 dike.

Rock type

Upper ongonite Topazite

Minera l

Cassiterite

Discussion

In this part, we first discuss the magmatic fractionation of the composite ongonite-topazite dike using mineral assemblages and their textures as well as whole-rock compositional data.

We then evaluate mineral-scale textural and compositional evidence for the fractionation of the volatile and incompatible elements F and Nb-Ta-Sn. Lastly, we discuss mechanisms that may have driven the spatial segregation of magmas that crystallized to ongonite and topazite.

Co-magmatic relationships between ongonite and topazite in the Xianghualing No. 431 dike

Although both ongonite and topazite have equivalents among granitic rocks that are rich in rare elements, their coexistence was only described in 1988 from the Flying W Ranch of Arizona (Kortemeier and Burt, 1988). In addition, Johnston and Chappell (1992) also described co-magmatic, topaz-bearing aplite (very similar to ongonite) and topazite. They suggested their co-magmatic origin on the basis of petrographic investigations, in spite of their presence in different dikes. The Xianghualing No. 431 dike exposes coexisting ongonite and topazite in a very small scale dike (less than 18 m wide). The ongonite and topazite of the No. 431 dike have highly evolved compositions with ~ 1.1-6.2 wt.% F, ~ 750-6250 ppm Li, and an ACNK index of > 1.1 to ~ 6.9, where the degree of differentiation increases from lower ongonite over upper ongonite to associated topazite. Additionally, both Nb/Ta and Zr/Hf ratios decrease in the same order. Compositionally, the ongonite and topazite rocks are very similar to F-rich, peraluminous, highly evolved, and volatile-rich eruptive rocks such as the Macusani glass in SE Peru (Pichavant et al., 1987) and topaz rhyolites [START_REF] Breiter | Breiter Nearly contemporaneous evolution of the A-and S-type fractionated granites in the Krušné hory/Erzgebirge Mts[END_REF], Gioncada et al., 2014[START_REF] Raimbault | Burnol The Richemont rhyolite dike, Massif Central, France; a subvolcanic equivalent of rare-metal granites Can[END_REF][START_REF] Xie | Zhang A tin-mineralized topaz rhyolite dike with coeval topaz granite enclaves at Qiguling in the Qitianling tin district[END_REF]. Other topazites (Songling, Hailuoling, Tongkengzhang, Fenghuangdong and Yangbin) besides the Xianghualing No. 431 dike in southern China also share similar petrological and geochemical characteristics (Liu et al., 1996). Most of them contain quartz and topaz as the rock-forming minerals and occasionally have fine-grained zinnwaldite. They are generally relatively rich in SiO 2 , strongly peraluminous and have a high content of F (up to 6.55 wt.%) (Liu et al., 1996).

Trace elements of topazites are rich in HFSE (Nb, Ta, Sn, W, Zr and Hf) with high values of Rb/Sr and low values of Zr/Hf and Nb/Ta.

How the evolved magma emplaced in the No. 431 dike crystallized and how the ongonite types and topazite are related to each other are discussed in the following sections, using the compositional variation and the distribution of mineral assemblages, their textures, and composition. The main features we seek to explain are: (i) the strong increase in F, Li, Sn, and W and the decrease in Na 2 O from ongonite to topazite in the upper dike; (ii) the highly porphyritic texture of ongonites throughout the dike; (iii) the euhedral-subhedral phenocrysts in core zone of the lower ongonite and upper ongonite, and the subrounded phenocrysts in marginal zone of the lower ongonite and the topazite; and (iv) the highly contrasting abundance and mineral textures of topaz, Nb-Ta oxide minerals, and cassiterite in ongonite and topazite (Fig. 4).

Studies of natural examples and experimental results

have demonstrated that liquid immiscibility may play an important role in the evolution volatile-rich magmatic system and particularly in the evolution of Li-F-rich granitic magmas (e.g., Kamenetsky and Kamenetsky, 2010[START_REF] Peretyazhko | Sapozhnikov Immiscibility of calcium fluoride and aluminosilicate melts in ongonite from the Ary-Bulak intrusion, Eastern Transbaikal region[END_REF], Roedder, 1992, Roedder and Coombs, 1967, Shchekina et al., 2013, Veksler, 2004[START_REF] Veksler | Partitioning of elements between silicate melt and immiscible fluoride, chloride, carbonate, phosphate and sulfate melts, with implications to the origin of natrocarbonatite Geochim[END_REF]. High concentrations of alkali and earth alkali metals favor liquid immiscibility and high concentrations of non-silicate anions (e.g., F -, CO 3 2-) permit their efficient separation. Low-pressure (72 -100 MPa), hightemperature (650-1100 °C) experiments performed by [START_REF] Veksler | Partitioning of elements between silicate melt and immiscible fluoride, chloride, carbonate, phosphate and sulfate melts, with implications to the origin of natrocarbonatite Geochim[END_REF] have revealed that in immiscible silicate-fluoride systems, the fluoride melt strongly partitions F, Li 2 O, MgO, and CaO and moderately partitions Al 2 O 3 , Na 2 O, and K 2 O. Trace elements that most strongly partition into the fluoride melt are W, Th, and the REEs. Additionally, in carbonatesilicate systems, the carbonate liquid strongly partitions Na 2 O and CaO and moderately partitions K 2 O, Mo, and W.

The strong and abrupt increase in the concentrations of F and Li from ongonite to topazite and the increase in MgO from the upper ongonite to coexisting topazite in the No. 431 dike is thus consistent with unmixing of silicate and fluorite melts. The very low Na 2 O content and the variable K 2 O content of the topazite samples in the margin of the dike are not only suggested by the experimental silicate-fluoride partition coefficients, but could also reflect the unmixing of carbonate species, as the dike wall rocks are dominated by carbonate strata. Trace-element partitioning could have also been governed by carbonate and fluorine species, as they seem to most strongly partition the ore minerals Nb, Ta, Sn, and W that are strongly enriched in the topazite of the No. 431 dike.

The presence of the phenocrysts of K-feldspar, quartz, albite, zinnwaldite, minor topaz and columbite-(Mn)in the structurally lower part (ongonite in the subsurface, lower part of the dike; Fig. 11a,b) and also in the near-surface part of the dikesuggests that early crystallization enriched the melt phase in elements including the network-modifiers Li and F and ore elements Sn and W (Time 1, ongonite in the near-surface, upper part of the dike, Fig. 11c,d). The large size of the phenocrysts in ongonite indicates that crystal nucleation was slow relative to crystal growth (e.g., Hersum andMarsh, 2007 and[START_REF] Pietro | Pietro Decryption of igneous rock textures: crystal size distribution[END_REF]. Such crystallization is consistent with the presence of large amounts of volatiles and other networkmodifiers and/or extended crystallization time (Fenn, 1977 andSwanson, 1977). In contrast, the strongly porphyritic texture of ongonites with abundant fine-grained quartz and albite along with K-feldspar, zinnwaldite, topaz, and ore minerals is consistent with subsequent rapid crystallization (e.g., Fig. 3b). Such rapid crystallization is typically invoked to be triggered by supercooling as a function of decreasing temperature and/or the exsolution of volatile components from an originally homogeneous melt phase (e.g., [START_REF] Breiter | Gabašová Textural and chemical evolution of a fractionated granitic system: the Podlesí stock[END_REF], Cobbing et al., 1986, London and Morgan, 2012and Štemprok et al., 2008). Crystallization as a result of abrupt cooling alone is inconsistent with the occurrence of the most fractionated rocks, the topazite, along the margins of the No. 431 dike. Consistent with the compositional variation between ongonite and topazite rocks, we suggest that the fine-grained assemblage started to form when the granitic magma separated into aluminosilicate and hydrosaline melt phases that later crystallized ongonite and topazite lithologies, respectively (Time 2, Fig. 11eh).

Once the two melt phases had unmixed (Time 3), the low-viscosity hydrosaline melts appear to have segregated from the more viscous aluminosilicate melt toward the margin of the dike (Fig. 11i-l). Possible driving forces for this segregation are discussed in the following section. Rare, subrounded silicate phenocrysts both in the rim zone of lower ongonite and in topazite were likely entrained from the assemblage that crystallized from the originally homogeneous silicate melt phase. They were then partially resorbed while hosted by the hydrosaline melt phase (e.g., Fig. 11f,h). At this stage, abundant quartz, topaz, zinnwaldite and ore minerals crystallized. Notably, most of the Sn that was originally present in the magma was carried in the hydrosaline melt to the margin of the dike, where cassiterite precipitated.

Comparison of ore mineral assemblages in the ongonites and topazite

The common accessory minerals in the No. 431 dike are columbite-tantalite, tapiolite, microlite, uraninite, rutile, hubnerite-ferberite, and zircon. Details of the distribution of the main ore minerals in the different rock types are provided in Table 7. As shown by the mineralogical descriptions, the ongonites and topazite in the No. 431 dike display distinct Nb-Ta-Sn oxide mineral associations. In the ongonite of the lower dike, raremetal elements are dominated by columbite-tantalite, together with tapiolite and microlite, but are free of cassiterite. Commonly, the subhedral Ta-poor columbite-(Mn) crystals are intergrown with rock-forming minerals, and they may indicate early magmatic crystallization.

The progressive zoning in columbite-(Mn), comprising an increase in the Ta content from core to rim, can be attributed to the gradual differentiation of the magma. The experimental work of Linnen and Keppler (1997) and Linnen, 1998 andLinnen, 2004 suggests that the solubility of the Ta-rich end-member is higher than that of the Nb-rich member of columbite in peraluminous granites or pegmatite melts, similar to the relationship between the Fe-rich members relative to the Mn-rich members. Therefore, the coexisting tapiolite and tantalite-(Fe) represent the products of late-stage magmatic or possibly deuteric hydrothermal crystallization. Tapiolite is commonly found in pegmatites and granites that have undergone a highly evolved magmatic process [START_REF] Beurlen | Nb-Ta-(Ti-Sn) oxide mineral chemistry as tracer of rare-element granitic pegmatite fractionation in the Borborema Province, Northeastern Brazil Mineral[END_REF], Černý et al., 1989a, Černý et al., 1989b, Chudik et al., 2008, Novák et al., 2003[START_REF] Rao | Complex internal textures in oxide minerals from the Nanping No. 31 dike of granitic pegmatite, Fujian province, southeastern China Can[END_REF][START_REF] Wang | Monchoux The association of columbite, tantalite and tapiolite in the Suzhou granite, China Can[END_REF]and Zhang et al., 2004), and the fact that the tapiolite-tantalite pair plot at the appropriate boundary of the miscibility gap can probably be attributed to exsolution from an unstable homogeneous precursor (Černý et al., 1989a and Černý et al., 1989b). Fluorite is usually present in the mineral assemblage of tapiolite, tantalite, and microlite, suggesting crystallization from an Frich liquid. Overall, the variations in the composition of columbite-tantalite and tapiolite reflect a highly differentiated and relatively Fe-richer magma system.

In the ongonite of the upper dike, columbite-(Mn) also occurs, and it is associated with a small amount of cassiterite. In the topazite that coexists with ongonite in the upper dike, Nb-Ta oxide minerals become less abundant, but their composition is similar to those in the coexisting ongonite. Interestingly, there is an abundance of Nb-Ta oxide minerals in the ongonite, but a large amount of cassiterite only appears in the topazite. In particular, the cassiterite occurs as two distinct types. First, subhedral grains of cassiterite are associated with zircon and hubnerite (Fig. 10a), which may have crystallized from a melt phase; second, abundant, highly anhedral cassiterites with very fine-grained inclusion of topaz and varlamoffite in cavities may have crystallized at a later stage from an epigenetic hydrothermal fluid. et al., 1992 and[START_REF] Wang | Monchoux Mineraux dissemines comme indicateurs du caractere pegmatitique du granite de Beauvoir, Massif d'Echassieres, allier[END_REF], in the Yichun granite of Southern China (Belkasmi et al., 2000 andHuang et al., 2002), and in the Czeck Podlesí granite [START_REF] Breiter | Uher Nb-Ta-Ti-W-Sn-oxide minerals as indicators of a peraluminous P-and F-rich granitic system evolution: Podlesí[END_REF]. It is particularly noted that Nb-Ta-Sn ores may be deposited at various stages of differentiation as coexisting columbite-tantalite and cassiterite are commonly disseminated in various facies of these granites. However, cassiterite may also be deposited in hydrothermal quartz veins, whereas columbite-tantalite is never found in such veins. In this study of the Xianghualing No. 431 dike, it was noted that the dike has two distinct types of rare-mental mineralization: Nb-Ta mineralization in the ongonites and Sn mineralization in the topazite. This apparently contrasting feature of Nb-Ta versus Sn mineralization differs from what is found in granites that are similarly enriched in rare-elements, and the difference may possibly be linked specifically to the highly F-enriched, high-level or near-surface emplacement of the Xianghualing Nb-Ta-Sn-enriched melt that permitted the efficient segregation of aluminosilicate and hydrosaline melts. As mentioned above, the ongonites of No. 431 dike are thought to be the product of extreme fractional crystallization of an F-rich granitic magma and unmixing of a silicate and fluoride-rich melt phases. The ore elements were likely present in the melt as complexes of TaF 8 3-, NbF 7 2 -, and SnF 6 2 -. The solubility of Nb, Ta, and Sn in the high-F fluids was maintained for a long time. At 600 °C, 120-490 ppm Nb is required for columbite saturation in a melt with 2 wt.% F (Linnen, 1998 and[START_REF] Bhalla | Solubility of cassiterite in evolved granitic melts: effect of T, fO 2 , and additional volatiles[END_REF]Keppler, 1997). Simultaneously, the Nb-and Ta-complexes are sensitive to the Li-content in the melt. [START_REF] Breiter | Uher Nb-Ta-Ti-W-Sn-oxide minerals as indicators of a peraluminous P-and F-rich granitic system evolution: Podlesí[END_REF] confirmed that columbite often crystallized immediately after Li-mica (zinnwaldite), while the Li-contents decreased rapidly. As a result of both factors, the complexes of Nb and Ta were depolymerized easily, and the Nb-Ta minerals began to precipitate early from the ongonite melt. However, [START_REF] Bhalla | Solubility of cassiterite in evolved granitic melts: effect of T, fO 2 , and additional volatiles[END_REF] observed an increase in cassiterite solubility with increasing F in peraluminous melts. Obviously, the crystallization of abundant topaz in the ongonites and topazite would have rapidly caused a decrease in the activity of F in the melt. As a result, the cassiterite could crystallize during the post-magmatic and hydrothermal stages, and even varlamoffite could form in the cavities. Therefore, the increase in the F content of the melt that produced the No. 431 dike would inevitably have led to tin mineralization in the strongly F-enriched topazitic melts.

The observed trends of increasing Fe in columbite group minerals in ongonite of the lower dike as compared to increasing Mn in columbite group minerals in ongonite and topazite of the upper dike (Fig. 12; Table 4) reflect a decrease of whole-rock FeO t /MnO value from lower to upper part of the dike (Table 1). The compositional trends of the columbite minerals also concur with Mn-poor zinnwaldite in lower ongonite and Mn-bearing zinnwaldite in upper ongonite and topazite (Table 3). We therefore interpret the two different compositional trends shown in Fig. 12 to reflect the overall increase of differentiation from lower to upper parts in the dike. Aluminosilicate-dominated and hydrosaline melt phases as those inferred to have crystallized ongonite and topazite of the No. 431 dike differ in density and viscosity. Hydrosaline melts have low densities and low viscosities compared to aluminosilicate-dominated melts [START_REF] Badanina | Trumbull Magmatic evolution of Li-F, rare-metal granites: a case study of melt inclusions in the Khangilay complex, Eastern Transbaikalia (Russia)[END_REF][START_REF] Peretyazhko | Peretyazhko Genesis of mineralized cavities (miaroles) in granitic pegmatites and granites[END_REF], Veksler, 2004[START_REF] Veksler | An experimental study of B-, P-and F-rich synthetic granite pegmatite at 0.1 and 0.2 GPa Contrib[END_REF], and the density contrast of the two melt phases could thus have caused segregation of the hydrosaline melt into structurally higher positions. However, the zoning of both lower ongonite from core to margin (as revealed in drill core) as well as upper ongonite to topazite (as exposed in the exploratory trench) suggests that the hydrosaline melt separated toward boththe upper and the lowerdike margins, which cannot be explained by simple densitydriven segregation. Temperature gradients between dike interior and country rocks could have also affected the segregation of the two melt phases. However, if temperature was the main driving force for segregation we would expect that the least evolved ongonite rocks would have crystallized along the colder dike margins and the more evolved ongonite rocks and topazite would have crystallized in the dike interior. Another possible explanationand our preferred interpretationis that continued dike propagation/widening caused the partial (in subsurface ongonite) to significant (from nearsurface ongonite to topazite) segregation of hydrosaline from aluminosilicate melt. Elastic propagation rates for fractures exceed viscous flow rates of silicate magmas (e.g., Clarke et al., 1998 andRubin, 1993), and dike propagation/widening will therefore create a pressure gradient and may shortly create a vacuum. The least viscous material of the dikethe hydrosaline meltwill have been instantly drawn into the low-pressure zone, and thereby largely separated from the aluminosilicate melt. The greater width of the dike as exposed in the exploratory trench relative to the drill core could reflect that the dike opened further at this structurally higher position, which caused a more advanced segregation of the two melt phases into ongonite and topazite in the upper dike than at the structurally lower position with zoned subsurface ongonite.

We believe that there might be records in the rocks such as melt inclusions of two immiscible melts, which may be present in minerals from both types of rocks and can be good for further studies. A detailed investigation of melt inclusions from the different mineral types and crystal populations will be ideally suited to testand to refineour proposed model. However, the detailed compositional and textural study presented here is an important guide for the sensible collection and interpretation of such data.

Conclusions

The Xianghualing No. 431 dike is composed of ongonite and topazite which are predominantly mineralized by Nb-and Ta-bearing and Sn-bearing ore minerals, respectively. Ongonite crystallized from an aluminosilicate magma that reached saturation in F and other volatile components. Upon exsolution of a hydrosaline melt phase (or phases), the aluminosilicate melt rapidly crystallized to a quartz-, K-feldspar-, and albite-dominated assemblage with Nb-Ta ore minerals of the columbite-tantalite, tapiolite and microlite as characteristic accessories. Topazite formed from the hydrosaline melt that segregated toward the margin of the small-scale dike, crystallizing a topaz-, zinnwaldite-, and quartz-dominated assemblage. Exsolution of the two melt phases and their efficient segregation into core and rim zones of the dike may have been driven by dike propagation/widening subsequent to the initial dike emplacement. Melt inclusions should provide a detailed record of the proposed evolution and further provide compositional details on the fractionating liquids.

Fig. 1 .

 1 Fig. 1. : Simplified geological map of the No. 431 dike, Hunan Province, South China. (a) Location of Hunan Province in China; (b) the No. 431 dike in close spatial association to the ~ 154 -155 Ma Laiziling granite; and (c) the No. 431 dike sharply intrudes into carbonate-dominated strata.

Fig. 3 .

 3 Fig. 3. : Photographs, photomicrographs, and BSE (back-scattered electron) images of ongonite and topazite. (a) Fine-grained upper ongonite with quartz phenocryst (gray); (b) quartz phenocryst in ongonite containing tabular albite crystals, under crossed polarized light; (c) short prismatic or columnar topaz in a groundmass of lower ongonite (BSE); (d) needle-like topaz in a groundmass of upper ongonite (BSE); (e) flow structure in fine-

Fig. 5 .

 5 Fig. 5. : Chondrite-normalized REE patterns for the No. 431 dike (chondrite REE values from Anders and Grevesse, 1989).

Fig. 6 .

 6 Fig. 6.

Fig. 7 .

 7 Fig. 7. : Chemical variations of the columbite-group minerals and tapiolite from lower ongonite, upper ongonite and topazite.

Fig. 8 .

 8 Fig. 8. : BSE images and compositional classification of the columbite-group minerals from the ongonite of the upper dike. (a) Euhedral tabular columbite-(Mn) is interstitial to the rock-forming minerals, (b) columbite-(Mn) intergrowth with U-rich microlite, and (c) compositions of the columbite-group minerals from upper ongonite, plotted on the Mn/(Fe + Mn) versus Ta/(Nb + Ta) quadrilateral diagram. Minerals abbreviations: U-Mic: uranomicrolite.

Fig. 9 .

 9 Fig. 9. : BSE images and compositional classification of the columbite-group minerals from topazite of the upper dike. (a) Fine-grained columbite-(Mn) intergrown with U-rich microlite, interstitial to the rock-forming minerals; (b) subhedral columbite-(Mn) intergrown with hubnerite; and (c) compositions of the columbite-group minerals from topazite, plotted on the Mn/(Fe + Mn) versus Ta/(Nb + Ta) quadrilateral diagram.

Fig. 11

 11 Fig. 11. : The proposed magmatic evolution of the No. 431 dike now exposed at subsurface and nearsurface locations. Crystallization of an initially homogeneous, highly differentiated magma was

6. 3 .

 3 Differentiated Nb-Ta-Sn mineralization in the ongonites and topazite of the No. 431 dike Granitic rocks rich in the rare elements are generally strongly enriched in F, and they are characterized by the crystallization of Nb-Ta-Sn ores, which may be directly linked to magmatic processes. Typical examples are found in the Beauvoir granite of the French Massif Central (Cuney

Fig. 12

 12 Fig. 12. : Fractionation trends of the columbite-group minerals and tapiolite from the No. 431 dike. Arrows indicate direction of compositional fractionation.

Table 1 .

 1 Major-and trace-element compositions of the Xianghualing No. 431 dike.

	Rock type		Lower ongonite			Upper ongonite		Topazite
	Sample		ZK840-		XXL-		XHL-		XXL-
	no.	10* 11* 12* 14* 20* 33	34 310 73	08	10	11	12
	SiO 2 (wt.%)	65.83 70.21 69.67 70.76 71.74 67.23 67.51 67.14 67.20 57.77 59.60 63.08 56.57
	TiO 2						0.01 0.01 0.01 0.01 0.03 0.03 0.02 0.03
	Al 2 O 3	19.63 16.08 17.11 16.93 17.23 19.91 19.49 19.78 20.05 27.77 23.94 23.08 25.78
	Fe 2 O 3	0.44 0.40 0.29 0.10 0.42 0.17 0.02 0.02 0.02 0.44 1.07 0.36 1.68
	FeO	2.63 2.66 1.73 0.79 0.55 0.14 0.26 0.15 0.22 0.92 2.14 1.15 2.08
	MnO	0.06 0.06 0.04 0.03 0.02 0.03 0.08 0.09 0.08 0.83 0.47 0.36 0.49
	MgO	0.16 0.30 0.14 0.12 0.62 0.01 0.01 0.02 0.01 0.11 0.08 0.04 0.05
	CaO	0.49 0.57 0.71 0.70 1.00 0.03 0.02 0.02 0.05 0.04 0.03 0.01 0.03
	Na 2 O	4.00 3.80 6.88 5.70 4.52 4.54 4.54 4.80 4.13 0.08 0.07 0.06 0.07
	K 2 O	4.32 3.80 2.04 3.30 1.05 4.59 4.23 4.26 4.26 3.56 5.57 5.18 3.62
	P 2 O 5	0.00 0.00 0.01 0.00 0.00 0.01 0.03 0.02 0.02 0.01 0.01 0.02 0.01
	Li 2 O	0.21		0.18 0.13 0.05 0.31 0.45 0.48 0.54 1.47 1.67 1.38 1.49
	H 2 O	0.85 1.11 1.06 1.16 1.82
	LOI						2.13 2.74 2.49 3.06 5.99 4.73 4.61 7.34
	F	3.30 2.32 1.20 1.16 2.06 1.11 1.19 1.42 1.34 4.99 5.18 5.36 6.22
	Total	100.5 3	100.3 4	100.5 6	100.3 9	100.2 1	99.10 99.37 99.26 99.65 99.01 99.41 99.35 99.25
	A/CNK	1.62 1.41 1.15 1.19 1.66 1.59 1.61 1.58 1.74 6.86 3.87 4.03 6.29
	FeO t	3.03 3.02 1.99 0.88 0.93 0.29 0.28 0.17 0.24 1.32 3.10 1.47 3.59
	FeO t /MnO 50.43 50.33 49.78 29.33 46.40 9.77 3.48 1.87 2.98 1.59 6.60 4.09 7.33
	Li (ppm)		1230		886		765 958 1130 1450 6061 6110 5608 6252
	Be		8.5		7.7		179 30	13	11	249 34	34	30
	Sc		6.5		7.3		8.5 8.7 8.6 8.6 7.1 6.8 5.4 4.9
	V		4.7		2.4		1.3 1.2 0.85 1.5 2.5 2.0 1.4 4.2
	Cr		9.7		7.9		4.2 11	4.2 11	1.6 2.4 3.5 7.4
	Cs		52		39		28	49	60	51	67	74	61	93
	Rb		1418		1073		3880 4310 4670 4780 4918 5087 4440 4640
	Ba		63		51		14	14	19	17	11	12	7.3 5.4
	Th		6.6		6.3		20	11	15	10.0 39	27	30	23
	U		25		15		9.5 4.9 7.4 6.9 18	6.9 20	11
	Nb		149		248		97	99	133 105 223 105 193 163
	Ta		57		175		114 65	97	69	207 100 175 161
	Pb		42		30		140 106 52	40	180 139 193 58
	Sr		59		78		11	14	15	8.2 16	13	32	12
	Zr		71		18		46	30	32	35	77	4.2 86	70
	Hf		16		3.2		7.7 3.9 5.7 5.1 22	0.69 24

Rock type Lower ongonite Upper ongonite Topazite Sample no. ZK840- XXL- XHL- XXL- 10* 11* 12* 14* 20* 33 34 310 73 08

									10	11	12
	Y	1.7	1.4	0.34 0.35 0.30 0.44 0.54 0.18 0.33 0.57
	Ga	59	66	57	55	53	59	25	41	24	33
	Sn	35	13	18	38	63	29	142 438 247 802
	W	4.6	5.5	39	25	69	24	114 60	758 771
	La	1.5	4.5	1.9 2.5 2.5 1.4 2.1 1.0 1.9 1.5
	Ce	5.5	17	5.2 6.9 6.2 4.0 5.2 3.8 5.8 3.9
	Pr	0.73	1.7	0.52 0.70 0.65 0.40 0.70 0.44 0.67 0.53
	Nd	1.8	3.4	0.97 1.3 1.2 0.85 1.6 0.86 1.1 1.2
	Sm	0.55	0.88	0.26 0.34 0.29 0.31 0.34 0.16 0.19 0.23
	Eu	0.03	0.02	0.02 0.02 0.02 0.02 0.05 0.02 0.01 0.02
	Gd	0.32	0.47	0.12 0.14 0.12 0.16 0.13 0.06 0.09 0.15
	Tb	0.09	0.08	0.04 0.04 0.03 0.05 0.04 0.02 0.02 0.02
	Dy	0.60	0.56	0.27 0.21 0.21 0.34 0.22 0.06 0.10 0.15
	Ho	0.11	0.11	0.05 0.04 0.04 0.06 0.05 0.02 0.02 0.03
	Er	0.36	0.39	0.21 0.15 0.16 0.25 0.12 0.03 0.10 0.09
	Tm	0.08	0.09	0.07 0.05 0.05 0.08 0.05 0.02 0.04 0.04
	Yb	0.95	1.2	0.95 0.59 0.69 0.98 0.58 0.13 0.55 0.59
	Lu	0.14	0.17	0.15 0.09 0.11 0.16 0.09 0.02 0.07 0.06
	Nb/Ta	2.62	1.42	0.85 1.52 1.37 1.53 1.08 1.04 1.10 1.01
	Zr/Hf	4.41	5.73	5.97 7.70 5.64 6.76 3.54 6.12 3.63 3.63
	Rb/Sr	24.11	13.84	352.7 3	314.6 0	313.4 2	585.0 7	314.4 2	406.4 2	137.1 7	389.9 6
	Total REE	12.76	30.06	10.72 13.01 12.24 9.07 11.26 6.67 10.68 8.61
	LREE/HR EE	3.80	8.90	4.77 8.96 7.66 3.36 7.92 17.38 9.71 6.52
	δEu	0.20	0.08	0.30 0.24 0.26 0.23 0.70 0.55 0.24 0.37

Note: Major element of the samples with * is cited from

[START_REF] Zhu | Ongonite and topazite in dike No. 431 of Xianghualing district and their spatial zonation and genetic relationship[END_REF]

; FeO t = FeO + Fe 2 O 3 * 0.9.

Table 2 .

 2 Representative electron-microprobe results of feldspars and topaz in the Xianghualing No. 431 dike.

	Rock			Ongonite			Topazite
	type
		K-feldspar	Albite	Topaz (matrix)	Topaz
	Miner
	al	Phenocry	Phenocry	Phenocry	Matri	Column	Acicula	Matri	Phenocry
		st	st	st	x	ar	r	x	st
	SiO 2 (wt.%)	62.80	63.55	69.08	68.79 31.47	31.38 32.30 32.40
	TiO 2 -	-	-	-	-	-	-	0.01
	Al 2 O 3 18.40	18.38	20.51	20.17 57.76	56.30 55.92 55.50

Table 3 .

 3 Representative electron-microprobe results of micas in the Xianghualing No. 431 dike. : -: Below detection limits. Structural formula calculated based on O = 23. *: LiO 2 and H 2 O calculation of Fe-rich mica (Lower ongonite) and Fe-poor mica (upper ongonite and topazite) are after

		Lower ongonite Upper ongonite	Topazite
	Rock type
		1	2	3	4	5	6	7	8	9
	SiO 2 (wt.%) 42.51 42.76 42.06 52.62 52.83 52.55 52.08 53.35 49.35
	TiO 2	0.08 0.12 0.15 0.04 0.01 0.07 -	0.03 -
	Al 2 O 3	21.97 21.81 22.16 22.51 20.59 19.79 19.99 19.54 20.61
	FeO	13.32 14.16 15.66 5.34 5.70 4.34 7.71 7.05 9.19
	MnO	0.68 0.62 0.83 1.10 0.88 2.00 1.69 1.37 1.74
	MgO	0.12 0.05 0.09 -	0.07 0.14 0.09 0.10 0.05
	CaO	0.01 0.01 0.04 -	-	-	-	0.02 0.01
	Na 2 O	0.32 0.40 0.25 0.27 0.11 0.26 0.15 0.17 0.09
	K 2 O	10.00 10.09 9.28 10.82 9.22 11.03 10.41 9.67 10.28
	Rb 2 O	0.91 0.85 0.81 -	-	1.42 -	-	-
	Cs 2 O	0.04 0.02 0.04 -	-	0.04 0.02 -	0.01
	F	6.35 7.85 6.14 8.58 8.36 8.45 8.04 8.28 7.92
	F O	2.67 3.31 2.58 3.61 3.52 3.56 3.39 3.49 3.33
	Li 2 O*	2.65 2.72 2.52 2.60 2.53 2.56 2.43 2.50 2.39
	H 2 O*	1.03 0.35 1.15 0.37 0.36 0.32 0.53 0.43 0.49
	Total	97.30 98.48 98.58 100.63 97.13 99.40 99.75 99.02 98.80
	Si	6.311 6.302 6.211 7.107 7.320 7.288 7.197 7.341 6.967
	Al IV	1.689 1.698 1.789 0.893 0.680 0.712 0.803 0.659 1.033
	Al VI	2.155 2.092 2.068 2.692 2.684 2.524 2.454 2.511 2.396
	Ti	0.009 0.013 0.016 0.004 0.001 0.007 0.000 0.003 0.000
	Fe	1.654 1.745 1.934 0.604 0.660 0.503 0.891 0.812 1.085
	Mn	0.085 0.077 0.103 0.126 0.103 0.235 0.198 0.159 0.209
	Mg	0.026 0.010 0.020 0.000 0.015 0.029 0.019 0.021 0.009
	Li*	1.581 1.612 1.497 1.410 1.408 1.426 1.350 1.385 1.356
	Sum oct	5.509 5.549 5.638 4.835 4.871 4.724 4.911 4.891 5.056
	Ca	0.002 0.001 0.006 0.000 0.000 0.000 0.000 0.002 0.002
	Na	0.093 0.114 0.071 0.070 0.031 0.070 0.040 0.045 0.025
	K	1.893 1.897 1.747 1.864 1.629 1.951 1.836 1.696 1.852
	Rb	0.087 0.080 0.077 0.000 0.000 0.126 0.000 0.000 0.000
	Cs	0.002 0.001 0.002 0.000 0.000 0.003 0.001 0.000 0.001
	Sum int	2.078 2.094 1.903 1.934 1.660 2.150 1.877 1.744 1.879
	OH*	1.018 0.340 1.135 0.335 0.337 0.292 0.485 0.396 0.466
	F	2.982 3.660 2.865 3.665 3.663 3.708 3.515 3.604 3.534
	Al total	3.844 3.790 3.857 3.585 3.364 3.236 3.256 3.170 3.429
	Fe/(Fe + Mg) 0.985 0.994 0.990 1.000 0.978 0.946 0.980 0.975 0.991

Note

Table 4 .

 4 Representative electron-microprobe results of columbite-group minerals and tapiolite in the Xianghualing No. 431 dike. Below detection limits. 1, 2 are the representative results of the columbite-(Mn) core; 3, 4 are the representative results of the columbite-(Fe) or tantalite-(Fe) rim; 5, 6 are the representative results of the tantalite-(Fe) associated with tapiolite.

	Rock type						Lower ongonite						Upper ongonite			Topazite
					Columbite-group minerals				Tapiolite		Columbite-group minerals	Columbite-group minerals
	Mineral
		1	2	Range	3	4	Range	5	6	Range	7	8	Range	9	10	11	12	Range	13	14	15	Range
	WO 3 (wt.%) 0.36 0.48	0.00-2.33	1.58 1.73	1.18-19.83	10.21 3.62	0.70-5.68	0.91 0.80	0.00-1.24	1.44 0.09 8.82 4.05	0.00-14.34	4.67 2.02 13.07	1.13-17.46
	Nb 2 O 5	62.54 67.64	38.40-74.55	28.59 35.02	20.73-45.93	23.34 25.09	22.83-28.05	5.54 8.42	4.82-12.62	65.41 41.93 32.55 47.82	28.20-65.49	37.96 40.38 33.89	33.89-44.83
	Ta 2 O 5	16.70 10.51	5.38-39.89	51.04 45.89	32.48-54.14	48.53 51.94	48.53-55.32	79.79 73.65	67.93-79.79	10.19 38.11 35.46 25.32	9.97-38.51	35.68 34.08 31.03	24.22-37.47
	TiO 2	1.74 0.48	0.23-3.34	1.09 0.80	0.32-2.89	1.04 0.48	0.48-2.04	0.74 2.31	0.40-2.32	1.19 1.56 3.85 3.20	0.51-4.14	3.99 3.13 3.68	1.54-5.09
	SnO 2	0.07 0.01	0.00-0.15	0.01 0.01	0.00-0.96	0.16 0.19	0.07-0.20	0.13 0.17	0.05-0.23	0.01 0.29 0.67 0.08	0.00-1.24	0.87 0.48 1.32	0.09-11.72
	Sc 2 O 3	0.67 0.16	0.12-2.27	0.59 0.09	0.01-0.80	-	0.29	0.00-0.29	-	-	0.00-0.20	0.27 1.24 1.14 0.82	0.22-1.26	0.68 1.26 1.07	0.30-2.56
	MnO	11.13 13.13	8.09-14.55	5.74 4.77	3.01-10.67	3.72 3.99	3.28-7.99	0.51 0.55	0.41-0.59	12.00 15.44 16.53 18.02	10.95-18.85	15.97 16.76 14.45	13.23-17.48
	FeO	5.80 8.37	6.43-10.87	12.11 12.67	6.57-15.58	13.36 14.83	11.16-14.83	14.02 14.18	13.83-16.48	9.62 1.38 0.96 1.39	0.67-9.62	0.49 1.33 1.93	0.40-6.51
	Total	99.01 100.78		100.74 100.97		100.36 100.43		101.63 100.08		100.12 100.03 99.97 100.68		100.30 99.43 100.44
	W	0.006 0.007		0.029 0.030		0.191 0.067		0.019 0.016		0.022 0.001 0.152 0.065		0.079 0.034 0.222
	Nb	1.694 1.781		0.905 1.078		0.760 0.815		0.202 0.300		1.734 1.239 0.977 1.340		1.116 1.185 1.006
	Ta	0.272 0.166		0.973 0.850		0.950 1.015		1.746 1.578		0.163 0.677 0.640 0.427		0.631 0.602 0.554
	Ti	0.078 0.021		0.057 0.041		0.056 0.026		0.044 0.137		0.053 0.077 0.192 0.149		0.195 0.153 0.182
	Sn	0.002 0.000		0.000 0.000		0.005 0.005		0.004 0.005		0.000 0.008 0.018 0.002		0.023 0.012 0.034
	Sc	0.035 0.008		0.036 0.006		0.000 0.018		0.000 0.000		0.014 0.071 0.066 0.044		0.038 0.071 0.061
	Mn	0.565 0.648		0.341 0.275		0.227 0.242		0.035 0.037		0.596 0.855 0.930 0.946		0.879 0.922 0.804
	Fe	0.291 0.408		0.710 0.722		0.804 0.891		0.943 0.934		0.472 0.075 0.053 0.072		0.027 0.072 0.106
	Mn/(Fe + Mn) 0.66 0.61 0.46-	0.32 0.28 0.17-	0.22 0.21 0.20-	0.04 0.04 0.03-	0.56 0.92 0.95 0.93 0.55-	0.97 0.93 0.88 0.68-

Table 5 .

 5 Representative electron-microprobe results of microlite in Xianghualing No. 431 dike.

		Lower ongonite Upper ongonite		Topazite
	Rock type
		1	2	3	4	5	6	7	8	9
	Nb 2 O 5 (wt.%) 10.41 16.21 3.93 5.64 7.63 7.25 7.38 7.62 13.06
	Ta 2 O 5	62.25 53.14 73.30 66.81 63.23 57.80 60.48 59.34 56.06
	TiO 2	0.73 0.61 0.27 1.26 1.33 1.18 1.07 1.62 3.45
	Na 2 O	7.70 9.22 8.50 0.20 0.28 0.11 7.41 7.62 13.06
	CaO	6.23 5.28 5.62 3.28 2.18 1.54 3.50 3.96 3.22
	MnO	-	-	0.01 0.20 0.57 0.38 0.18 0.13 0.46
	FeO	0.03 0.09 0.05 0.25 0.38 0.81 0.23 0.41 0.09
	UO 2	6.97 6.91 4.26 12.71 13.61 13.22 12.48 12.50 17.46
	PbO	0.39 0.48 0.26 0.33 0.66 6.56 0.39 0.38 0.56
	WO 3	-	1.43 0.30 0.75 1.24 1.62 1.09 0.65 0.73
	SnO 2	0.26 0.18 0.15 0.32 0.25 0.34 0.33 0.28 0.34
	Sc 2 O 3	1.09 0.96 1.09 1.25 0.91 1.03 1.01 1.06 0.50
	F	3.15 3.35 3.96 2.70 1.81 1.01 2.24 2.40 1.50
	F O	1.33 1.41 1.67 1.14 0.76 0.42 0.94 1.01 0.63

Note: -: Below detection limits. Structural formulae based on Nb + Ta + W + Ti = 2.

Table 7 .

 7 Summary of distribution of ore minerals in different rock types in the Xianghualing No. 431 dike.

	Sample

no. Columbite-group minerals Tapiolite Microlite Cassiterite

		Sample no. Columbite-group minerals Tapiolite Microlite Cassiterite
		ZK840-20 + +	-	-	-
		XXL-33	+ +	-	+ +	+
		XXL-34	+ +	-	-	+
	Upper ongonite
		XHL-310	+	-	-	+
		XHL-73	+	-	-	-
		XXL-08	+	-	+	+ +
		XXL-10	+ +	-	+	+ + +
	Topazite
		XXL-11	+	-	-	+ + +
		XXL-12	+	-	-	+ + +
		ZK840-11 + + +	+ +	+ +	-
		ZK840-12 + + +	+ +	+ +	-
	Lower ongonite	ZK840-14 + +	-	+	-
		ZK840-15 + +	-	-	-
		ZK840-16 + + +	-	-	-

Note: +++: most frequent; ++: more abundant; +: relatively abundant; -: rare or absent.

Acknowledgments

This study was supported financially by the Natural Science Foundation of China (Grant Nos. 41230315 and 41172061), the National Key Basic Research Program of China (973 Program) (No. 2012CB416704), and the Key Research Project of the China Geological Survey (Grant No. 12120113067300). We thank Dr. W.L. Zhang and Mr. B. Wu for their kind assistance with the laboratory work, Mr. X.G. Luo and Mr. T. Ding for their help in the field work. We greatly appreciated the comments by the guest editor Dr. M.F. Zhou, and two reviewers, Dr. K. Breiter and Dr. T.P. Zhao, which helped to considerably improve the manuscript.

(1) Disseminated cassiterite occurs as independent euhedral to subhedral granular crystals (10-20 μm across), either in the interstices between topaz and quartz, or in a mineral association with zircon, columbite-(Mn), hubnerite, and/or microlite (Fig. 10a). The composition of this type cassiterite is similar to that of magmatic cassiterite in pegmatites rich in rare metals (Černý, 1989 andNeiva, 1996), and it is characterized by significant amounts of Ta (up to 7.77 wt.% Ta 2 O 5 ), Nb (up to 4.01 wt.% Nb 2 O 5 ), and W (up to 5.13 wt.% WO 3 ), together with minor Mn (up to 1.34 wt.% MnO) and Fe (up to 3.89 wt.% FeO). In the Nb + Ta vs. Fe + Mn diagram, disseminated cassiterite is mostly dispersed along the 2:1 line, corresponding to the general substitution of columbite solid-solution in cassiterite: 3Sn 4 + ⇔ 2 (Nb, Ta) 5 + + (Fe, Mn) 2 + , similar to that described by Černý and Ercit (1989) and Tindle and Breaks (1998).

(2) Aggregated cassiterite is typically present as micron-sized crystals in aggregates with needle-like topaz (Fig. 10b-d). This type of cassiterite has low levels of trace elements (less than 1 wt.%) (Fig. 10e). Moreover, minor amount of galena is locally found included in cassiterite aggregates, suggesting their crystallization was part of a later hydrothermal process.