Chaolei Yan

Liangshu Shu
email: lsshu@nju.edu.cn

Yan Chen

Michel Faure

Zuohai Feng

Mingguo Zhai

The construction

Keywords: South China, Jiangnan Orogenic Belt, Neoproterozoic S-type granite, Anisotropy of magnetic susceptibility, Bouguer gravity modeling

whether they are published or not. The documents may come

Introduction

Magmatic rocks are important constituents of the continental crust and good markers for the crust evolution [START_REF] Castro | The off-crust origin of granite batholiths[END_REF][START_REF] De Saint-Blanquat | Internal structure and emplacement of the Papoose Flat pluton: An integrated structural, petrographic, and magnetic susceptibility study[END_REF][START_REF] Paterson | Magmatic tubes, pipes, troughs, diapirs, and plumes: Late-stage convective instabilities resulting in compositional diversity and permeable networks in crystal-rich magmas of the Tuolumne batholith, Sierra Nevada, California[END_REF], and magmatic activity plays a significant role in the crust-mantle interaction and recycling of crustal material [START_REF] Faure | Crustal thinning recorded by the shape of the Namurian-Westphalian leucogranite in the Variscan belt of the northwest Massif Central, France[END_REF][START_REF] Roman | The fate of mafic and ultramafic intrusions in the continental crust[END_REF]. Therefore, it is essential to study the magmatism to have a better understanding of the magmatic process and the crustal evolution.

Several aspects, such as: i) magma generation, ii) differentiation, iii) transport and ascent, and iv) emplacement, must be distinguished in the magmatic evolution [START_REF] Glazner | Is stoping a volumetrically significant pluton emplacement process?[END_REF]. The magma emplacement is an important stage of the process as it is the only one that can be directly investigated through field and laboratory studies. During the emplacement, some features observed both in the pluton and country rocks embody the interaction between the magma fluids and lithosphere as well as the mechanism of space accretion during the magma emplacement [START_REF] Paterson | Is stoping a volumetrically significant pluton emplacement process?: Discussion[END_REF].

Previous studies showed that the magma intrusion are not only dominated by the physical properties (i.e., magma composition, temperature, pressure and the rheology of the crust etc.), but also controlled by syn-magmatic regional tectonic setting [START_REF] Caricchi | Non-Newtonian rheology of crystal-bearing magmas and implications for magma ascent dynamics[END_REF][START_REF] Castro | On granitoid emplacement and related structures. A review[END_REF][START_REF] Castro | Fractionation and incipient self-granulitization during deep-crust emplacement of Lower Ordovician Valle Fértil batholith at the Gondwana active margin of South America[END_REF][START_REF] De Saint-Blanquat | Internal structure and emplacement of the Papoose Flat pluton: An integrated structural, petrographic, and magnetic susceptibility study[END_REF][START_REF] Hutton | Granite emplacement mechanisms and tectonic controls: inferences from deformation studies[END_REF][START_REF] Moyen | Late Archaean granites: a typology based on the Dharwar Craton (India)[END_REF][START_REF] Pitcher | The nature, ascent and emplacement of granitic magmas[END_REF][START_REF] Wei | An early extensional event of the South China Block during the Late Mesozoic recorded by the emplacement of the Late Jurassic syntectonic Hengshan composite granitic massif (Hunan, SE China)[END_REF]. Consequently, deciphering the process of magma emplacement, i.e., (1) the 3D structure of the pluton [START_REF] Cruden | Deformation around a rising diapir modeled by creeping flow past a sphere[END_REF][START_REF] Mathieu | Dykes, cups, saucers and sills: Analogue experiments on magma intrusion into brittle rocks[END_REF][START_REF] O'driscoll | The Great Eucrite intrusion of Ardnamurchan, Scotland: Reevaluating the ring-dike concept[END_REF][START_REF] Stevenson | Laccolithic, as opposed to cauldron subsidence, emplacement of the Eastern Mourne pluton, N. Ireland: evidence from anisotropy of magnetic susceptibility[END_REF]; (2) granite internal fabrics [START_REF] Liu | Incremental Emplacement of the Late Jurassic Midcrustal, Lopolith-Like Qitianling Pluton, South China, Revealed by AMS and Bouguer Gravity Data[END_REF][START_REF] Tarling | Magnetic anisotropy of rocks[END_REF][START_REF] Wei | A multidisciplinary study on the emplacement mechanism of the Qingyang-Jiuhua massif in southeast China and its tectonic bearings. Part I: Structural geology, AMS and paleomagnetism[END_REF];

(3) the magma flow pattern [START_REF] Clemens | Granitic magma transport by fracture propagation[END_REF][START_REF] Ernst | Evidence from magnetic fabric for the flow pattern of magma in the Mackenzie giant radiating dyke swarm[END_REF][START_REF] Moreira | Magma flow pattern in dykes of the Azores revealed by anisotropy of magnetic susceptibility[END_REF][START_REF] Paterson | Magmatic tubes, pipes, troughs, diapirs, and plumes: Late-stage convective instabilities resulting in compositional diversity and permeable networks in crystal-rich magmas of the Tuolumne batholith, Sierra Nevada, California[END_REF]; (4) the thermal conditions and deformation of the granite and country rocks [START_REF] Byerly | Internal fabrics of the Idaho batholith[END_REF][START_REF] De Saint-Blanquat | Internal structure and emplacement of the Papoose Flat pluton: An integrated structural, petrographic, and magnetic susceptibility study[END_REF][START_REF] Paterson | A review of criteria for the identification of magmatic and tectonic foliations in granitoids[END_REF][START_REF] Žák | Four magmatic fabrics in the Tuolumne batholith, central Sierra Nevada, California (USA): implications for interpreting fabric patterns in plutons and evolution of magma chambers in the upper crust[END_REF], can help to understand the evolutions of the granitic pluton and the continental crust. S-type granite is considered as produced by partial melting and crystallization of Al-rich rocks, such as orthogneiss and pelitic sedimentary rocks forming the lower to middle continental crust. It is commonly accepted that peraluminous magma is a syn-to post-orogenic product formed by the melting of the thickened orogenic root [START_REF] Chappell | Two contrasting granite types: 25 years later[END_REF]. Moreover, S-type granite may emplace in a post-orogeny setting (e.g., most of the post-orogenic plutons in the Early Paleozoic orogen of SE China; [START_REF] Song | Late Early Paleozoic and Early Mesozoic intracontinental orogeny in the South China Craton: Geochronological and geochemical evidence[END_REF]Wang et al., 2012b;[START_REF] Yan | Early Paleozoic intracontinental orogeny in the Yunkai domain, South China Block: New insights from field observations, zircon U-Pb geochronological and geochemical investigations[END_REF], and also in an active continental margin (Cenozoic western Pacific; [START_REF] Collins | Geodynamic significance of S-type granites in circum-Pacific orogens[END_REF].

In the Jiangnan Orogenic Belt (JOB), which marks the collision belt between the Yangtze and Cathaysia blocks, the peraluminous granitic plutons sporadically crop out from east to west, such as the Shexian, Xucun, Shiershan, Jiuling, Yuanbaoshan, Sanfang, Bendong and Fanjingshan plutons (Fig. 1). A large quantity of work on the geochronology and geochemistry suggest that these plutons are post-orogenic ones (e.g., [START_REF] Shu | An analysis of principal features of tectonic evolution in South China Block[END_REF][START_REF] Shu | Tectonic evolution of the eastern Jiangnan region, South China: New findings and implications on the assembly of the Rodinia supercontinent[END_REF][START_REF] Wang | Geochronological, geochemical and Nd-Hf-Os isotopic finger printing of an early Neoproterozoic arc-back-arc system in South China and its accretionary assembly along the margin of Rodinia[END_REF]Wang et al., , 2014b;;Yao et al., 2014a, Zhang andWang, 2016;[START_REF] Zhang | Early Neoproterozoic (~850 Ma) back-arc basin in the Central Jiangnan Orogen (Eastern South China): geochronological and petrogenetic constraints from meta-basalts[END_REF]. So far, rare studies have focused on the physical construction process of the S-type granite in the JOB, however it is essential for understanding the process of the Jiangnan Orogeny. The geological observation, anisotropy of magnetic susceptibility (AMS) and gravity are efficient ways to decipher the construction process and deep geometric shape of the plutons [START_REF] Liu | Incremental Emplacement of the Late Jurassic Midcrustal, Lopolith-Like Qitianling Pluton, South China, Revealed by AMS and Bouguer Gravity Data[END_REF]. In order to obtain a better knowledge of the Neoproterozoic Jiangnan Orogeny, studies on the emplacement of the Neoproterozoic S-type granitic plutons along the JOB as well as their detailed consideration of the pluton construction, syn-magmatic and syn (post)-tectonic settings are necessary. Consequently, we choose the Sanfang-Yuanbaoshan plutons as the study targets to decipher the evolution of the Jiangnan Orogeny.

Geological setting

The Jiangnan Orogenic Belt (JOB)

The JOB is a key tectonic element of the South China Block, representing the Neoproterozoic NE-SW to E-W trending belt developed as a consequence of the amalgamation between the Yangtze and Cathaysia blocks. The oceanic subduction of the Paleo-South China Ocean beneath the Yangtze Block since ca. 1000 Ma [START_REF] Shu | Early Paleozoic depositional environment and intracontinental orogeny in the Cathaysia Block (South China): implications from stratigraphic, structural, geochemical and geochronologic evidence[END_REF][START_REF] Shu | Tectonic evolution of the eastern Jiangnan region, South China: New findings and implications on the assembly of the Rodinia supercontinent[END_REF][START_REF] Wang | Detrital zircon geochronology of Precambrian basement sequences in the Jiangnan orogen: dating the assembly of the Yangtze and Cathaysia Blocks[END_REF]Yao et al., 2014b[START_REF] Yao | Jiangnan Orogen, South China: A ~970-820 Ma Rodinia margin accretionary belt[END_REF] was responsible for the formation of the Jiangnan magmatic arc, and followed by the collision (or continental subduction) of the Cathaysia Block after its final closure [START_REF] Shu | An analysis of principal features of tectonic evolution in South China Block[END_REF]Wang et al., 2012a). Groups of ultramafic, mafic magmatic, volcanic rocks, limestones, cherts and turbidites are interpreted as ophiolites and dated from ca. 1000 Ma to ca. 840 Ma, distributing along the suture zone [START_REF] Bgmrah | Regional Geology of Anhui Province[END_REF][START_REF] Bgmrjx | Regional Geology of Jiangxi Province[END_REF][START_REF] Bgmrzj | Regional Geology of Jiangxi Province[END_REF][START_REF] Shu | Early Paleozoic and Early Mesozoic intraplate tectonic and magmatic events in the Cathaysia Block, South China[END_REF][START_REF] Shu | Tectonic evolution of the eastern Jiangnan region, South China: New findings and implications on the assembly of the Rodinia supercontinent[END_REF][START_REF] Sun | Neoproterozoic tectonic evolution of the Jiuling terrane in the central Jiangnan orogenic belt (South China): Constraints from magmatic suites[END_REF][START_REF] Xia | Neoproterozoic amalgamation between Yangtze and Cathaysia blocks: The magmatism in various tectonic settings and continent-arc-continent collision[END_REF].

The Precambrian strata in the JOB are composed of the Sibao and Danzhou groups and their equivalents. The Danzhou group unconformably covers the Sibao group. The unconformity between these two groups indicates that an orogenic event took place before the sedimentation of the Danzhou group and its equivalents. This collision resulted in essentially folding and weak metamorphism (mainly under greenschist facies conditions) in the Sibao group and its equivalents [START_REF] Bgmrgx | Regional Geology of Guangxi Autonomous Region[END_REF][START_REF] Bgmrhn | Regional Geology of Hunan Province[END_REF][START_REF] Charvet | The Neoproterozoic-early Paleozoic tectonic evolution of the South China Block: an overview[END_REF][START_REF] Shu | Pre-Devonian tectonic evolution of South China: from Cathaysia Block to Caledonian period folded Orogenic Belt[END_REF][START_REF] Shu | An analysis of principal features of tectonic evolution in South China Block[END_REF]. The tight NE-SW to N-S striking folding and faulting are widely developed in the Sibao group and its equivalents in the whole JOB resulted by the northwestward subduction of the Paleo-South China oceanic crust and the subsequent collision of the Yangtze and Cathaysia blocks [START_REF] Shu | An analysis of principal features of tectonic evolution in South China Block[END_REF], whereas the structure of the Danzhou group was characterised by broad and gentle folds with a weak metamorphism, or even no metamorphism [START_REF] Bgmrgx | Regional Geology of Guangxi Autonomous Region[END_REF][START_REF] Bgmrhn | Regional Geology of Hunan Province[END_REF]Yao et al., 2014b).

In the JOB, the Sibao group was intruded by peraluminous granitic plutons dated at ca. 840-815 Ma with a peak of 830 Ma (e.g., Bendong, Fanjingshan, Sanfang, Shexian, Shiershan, Jiuling, Xucun and Yuanbaoshan plutons; Fig. 1; [START_REF] Guo | The pre-Devonian tectonic patterns and evolution of south China[END_REF][START_REF] Shu | Structural analysis of the Nanchang-Wanzai sinistral ductile shear zone (Jiangnan region, South China)[END_REF][START_REF] Shu | Study on the high pressure metamorphic blueschist and its Late Proterozoic age in the Eastern Jiangnan belt[END_REF][START_REF] Shu | Tectonic evolution of the eastern Jiangnan region, South China: New findings and implications on the assembly of the Rodinia supercontinent[END_REF][START_REF] Xin | Neoproterozoic post-collisional extension of the central Jiangnan Orogen: Geochemical, geochronological, and Lu-Hf isotopic constraints from the ca. 820-800 Ma magmatic rocks[END_REF]Yao et al., 2014b). After the formation of the original South China continent, the Nanhua intracontinental rifting developed at ca. 810 Ma-760 Ma by probably upwelling mantle magma just ending the Jiangnan Orogeny [START_REF] Li | Neoproterozoic granitoids in South China: crustal melting above a mantle plume at ca. 825 Ma?[END_REF]. The NE-SW striking Nanhua rifting was coeval with the generation of bimodal igneous rocks in the Jiangnan and Cathaysia regions, including the western Yunnan, northern Guangxi, northern Hunan, northern Jiangxi, northern Zhejiang and Northern Fujian basins [START_REF] Li | Assembly, configuration, and break-up history of Rodinia: A synthesis[END_REF][START_REF] Shu | SHRIMP zircon U-Pb age, litho and biostratigraphic analyses of the Huaiyu Domain in South China-Evidence for a Neoproterozoic orogen, not Late Paleozoic-Early Mesozoic collision[END_REF][START_REF] Shu | Geochronological and geochemical features of the Cathaysia block (South China): New evidence for the Neoproterozoic breakup of Rodinia[END_REF][START_REF] Wang | History of Neoproterozoic rift basins in South China: implications for Rodinia break-up[END_REF].

Sanfang-Yuanbaoshan granitic plutons and country rocks

The Sanfang and Yuanbaoshan granitic plutons are located in the western segment of the JOB (Fig. 1), and show sub a N-S elongated elliptic shape with an area of ca. 1,000 km 2 and ca. 300 km 2 , respectively, and an aspect ratio about 2:1 (Fig. 2).

These two granitic plutons both intruded into the Sibao group which is mainly composed of bathyal to abyssal terrigenous arenaceous-argillaceous metasediments with bedded flysch, and chaotic turbidity deposits with olistoliths [START_REF] Bgmrgx | Regional Geology of Guangxi Autonomous Region[END_REF]Yao et al., 2014a). Previous studies reveal that the crystallization age of these plutons ranged from 823±5 Ma to 833±6 Ma with high Aluminium Saturation Index ranging in 1.15-1.40 (Yao et al., 2014b). Their geochemical features indicate that the magma of Sanfang and Yuanbaoshan plutons were derived from supracrustal source rocks [START_REF] Wang | LA-ICP-MS U-Pb zircon geochronology of the Neoproterozoic igneous rocks from Northern Guangxi, South China: implications for petrogenesis and tectonic evolution[END_REF]. The widespread occurrence of tourmaline in these plutons [START_REF] Li | U-Pb zircon ages of granites from the southern margin of the Yangtze block: timing of Neoproterozoic Jinning: orogeny in SE China and implications for Rodinia assembly[END_REF] reveals that the granites are highly evolved and incorporated a significant amount of fluids.

As for the deformation, [START_REF] Li | U-Pb zircon ages of granites from the southern margin of the Yangtze block: timing of Neoproterozoic Jinning: orogeny in SE China and implications for Rodinia assembly[END_REF] proposed that all the granitic intrusions were deformed in different degrees showing foliation with gneissic texture. Faults are developed in both plutons as well as in their country rocks (Fig. 2), mainly striking in the NE-SW and N-S directions. Some kilometer to meter-scale mafic and ultramafic blocks enclosed in the mudstone-sandstone of the Sibao group crop out around the plutons. The well folded but low greenschist-facies metamorphosed Sibao group is commonly recognized by the accretionary complex coeval with the subduction of the Paleo-South China oceanic crust (Wang et al., 2014a;[START_REF] Zhou | Geochronology of Neoproterozoic mafic rocks and sandstones from northeastern Guizhou, South China: Coeval arc magmatism and sedimentation[END_REF], and recently work reveals that the upper Sibao group was deposited during the Yangtze-Cathaysia continental collision [START_REF] Yan | Time constraints on the closure of the Paleo-South China Ocean and the Neoproterozoic assembly of the Yangtze and Cathaysia blocks: Insight from new detrital zircon analyses[END_REF].

The Danzhou group unconformably covers the Sibao group, with distinct differences in the deformation style. It mainly consists of sandstone and mudstone featured by gentle folds with a low grade metamorphism, or even no metamorphism [START_REF] Bgmrgx | Regional Geology of Guangxi Autonomous Region[END_REF].

Field observation and microscopic investigation

In the field, we made observations on these two plutons and their adjacent country rocks as much as possible in order to obtain maximal information on macroscopic features of rocks with a large surface covering, and figured out their geological nature.

Granitic plutons

The Sanfang and Yuanbaoshan plutons stand at a varying elevation ranging from ca. 200 to ca. 2000 meters above sea level (a.s.l.). These two porphyritic monzogranitic plutons are mainly made up of quartz, K-feldspar, plagioclase, muscovite and biotite. Tourmaline and plagioclase nodules can be observed in some outcrops where the monzogranite shows fine or coarse grain facies (Fig. 2).

It is worthy to note that these two plutons are generally undeformed in the relatively lower part though some deformation can be occasionally observed in contact zone, whereas deformed in the upper part (generally above 700 meters a.s.l.). In our visited localities, all the massive and undeformed granites are at the relatively low elevation (e.g., A51: 566m, A55: 317m, B14: 234m, B19: 329m). These undeformed granites display magmatic textures with euheral crystals of quartz, feldspars and micas (Fig. 3A and3B). The quartz grains show a slightly undulose extinction (Fig.

4A, 4B and 4C).

However, the most deformed granites crop out in the relatively higher part (e.g., A43: 993m, A46: 983m, B08: 802m, B17: 837m).A consistent low angle (~25°) foliation is developed within the deformed granites, dipping to the W and SW. Augen structure, gneissic structure and shear band are common in this deformed part. The mineral and stretching lineations are developed by elongated quartz grains, biotite and muscovite aggregates, and pressure shadows around the Kfeldspar megacrysts trending in the E-W direction, with the top-to-the-W kinematics (Fig. 3C).

Moreover, in the highly deformed granites, the quartz and muscovite are totally recrystallized and show a preferred orientation. Myrmekites develop within the K-feldspars (Fig. 4D), indicating that the granites have experienced dynamic metamorphism as the myrmekite is an exolution of quartz and plagioclase due to fluid circulation during deformation [START_REF] Castle | An exsolution silica-pump model for the origin of myrmekite[END_REF][START_REF] Faure | Crustal thinning recorded by the shape of the Namurian-Westphalian leucogranite in the Variscan belt of the northwest Massif Central, France[END_REF]. And mica fishes from these two granitic plutons both show the top-to-the-W or -SW kinematics (Fig. 4E and4F).

Country rocks

The Sibao group is well exposed in the Sanfang-Yuanbaoshan area. Previous studies indicate that the Sibao group was tightly folded with general N-S and NNE-SSW striking axes [START_REF] Bgmrgx | Regional Geology of Guangxi Autonomous Region[END_REF]. However, our investigation reveals that the structural occurrence of the Sibao group is also accommodated to the shape of the pluton with generally outward dipping. The original sedimentary bedding (S0) and foliation (S1) with moderate to steep angle mostly strike parallel with the border of the Sanfang and Yuanbaoshan plutons (Fig. 2). Moreover, we can find that the dip angle of the S0-1 gradually decreases as the distance away from the contact border (Fig. 3E and3F). In our visited area, two hornfel outcrops with width of 20-30cm are found in the western and eastern contact borders between the Sanfang pluton and Sibao group (Fig. 3D). The top-to-thedownward and -outward deformations are founded in the contact zones in the northern margin of the Sanfang pluton and eastern margin of the Yuanbaoshan one (Fig. 3K).

Previous studies also revealed a greenschist facies metamorphism for the Sibao group [START_REF] Bgmrgx | Regional Geology of Guangxi Autonomous Region[END_REF]. Based on our macro-/microscopic observations, we found the degree of deformation and metamorphism of the Sibao group varies greatly in different zones.

Similar to the phenomenon of elevation vs metamorphism and deformation within the granitic plutons, in general, the degree of the metamorphism and deformation of the Sibao group seems gradually increases from the lower part to the higher part, the field investigation revealed that: (i)

The Sibao group at the elevation below 200 meters mainly consists of sandstone and mudstone and doesn't display metamorphism neither deformation (Fig. 3E and3F); (ii) At the elevation of ca. 200-700 meters, the degrees of the metamorphism and deformation are higher than those of below the 200 meters. In this unit, the rocks mainly consist of slate and phyllite. The foliation (S1) transposed from the original sedimentary bedding (S0) is rather scattily distributed, the lineations are better developed with a sub E-W direction, meanwhile the textures reveal top-to-the-W kinematics (Figs. 3G,4G and 4H); (iii) For the Sibao group at the altitude higher than 700 meters, the degrees of deformation and metamorphism are much more developed with respect to the former lower parts. The original beddings (S0) are well folded and even overturned as recumbent fold (S1), with the axis gently plunging to the north (Fig. 3H). However, the later foliations S2 consistently dip to the W with angles of 15-45°, with consistently oriented sub E-W lineations (Fig. 3I).

Moreover, mylonite and ultramylonite of the Sibao group were observed at the altitude of 1000-1600 meters, with horizontal foliation and E-W directed lineation (Fig. 3J).

Magma crystallization age and Hafnium isotopic analysis

The magma U-Pb crystallization age and geochemical features of the Sanfang and Yuanbaoshan plutons have been investigated since the past twenty years (e.g., [START_REF] Li | U-Pb zircon ages of granites from the southern margin of the Yangtze block: timing of Neoproterozoic Jinning: orogeny in SE China and implications for Rodinia assembly[END_REF]Yao et al., 2014b). With the recent development of analysis techniques, we have collected two dating samples, A37 (1403) and B13 (2024) (Figs. 2 and7), to obtain constraints on the age of these plutons.

Zircon U-Pb age

Heavy liquid and magnetic techniques are used for separating the zircon grains, which were afterward mounted in epoxy resin, polished and coated with gold. The zircon U-Pb isotopes were analyzed at the Testing Center of Shandong Bureau of China Metallurgical Geology Bureau (TCSBCMGB), using a ThermoX2 ICP-MS connected to a GeoLas Pro 193 nm laser ablation system. The detailed measurements and data calculation can be referred in [START_REF] Liu | Continental and oceanic crust recycling-induced melt-peridotite interactions in the trans-North China orogen: U-Pb dating, Hf isotopes and trace elements in zircons from mantle xenoliths[END_REF].

Cathodoluminescence (CL) images of zircons were acquired by using a JEOL JXA8230 electron probe microanalyzer at TCSBCMGB. Typical CL images of zircons are showed in Figure 5A and 5B, revealing that the grains are characterized by oscillatory zoning. Moreover, the high Th/U values (0.35 in average) indicate the zircons are magmatic originated [START_REF] Hoskin | The composition of zircon and igneous and metamorphic petrogenesis[END_REF].

All of the U-Pb ages are listed in Table 1 and graphically displayed in Figure 5. Thirty seven ages are obtained for each pluton (Sanfang and Yuanbaoshan), yielding the range of 814-1150 Ma and 827-2720 Ma, respectively (Fig. 5D and5G). However, most of the data provide ages between 820 to 850 Ma (Fig. 5E and5H), with weighted mean 206 Pb/ 238 U ages of 830±2 Ma and 830±5 Ma, respectively (Fig. 5F and5I). Some ages older than 2.0 Ga may indicate the existence of Paleoproterozoic rocks below the plutons. These grains may be xenocrystals involved in the granitic magma.

Zircon Hf isotopic analysis

The zircon Hf isotopic model age tells a minimum age for the material from which the zircons sourced. In order to have an idea about the evolution of the zircons, we carried out the analysis on the Zircon Hf isotopes by Neptune MC-ICP-MS at Nanjing University. For calculating the model ages of the zircons, we used the standard zircon 91500 as external sample with a 176 Hf/ 177 Hf value of 0.282300 ± 8 (2σ). During analysis, the 176 Lu/ 177 Hf of average crust is 0.015. Lu decay constant of 1.865x10 -11 was applied for the calculation of the initial 176 Hf/ 177 Hf values [START_REF] Scherer | Calibration of the Lutetium-Hafnium clock[END_REF].

The isotopic ratios of the 176 Hf/ 177 Hf and 176 Lu/ 177 Hf for the chondrite and depleted mantle are 0.282772 and 0.0332, 0.28325 and 0.0384, respectively [START_REF] Blichert-Toft | The Lu-Hf isotope geochemistry of chondritesand the evolution of the mantle-crust system[END_REF].

About half of the U-Pb dated zircons from the Sanfang granite were selected for in-situ Hf isotopic analysis that performed where used for U-Pb dating. All of the Hf isotopic analytical results and εHf(t) versus U-Pb age diagram are presented in Table 2 andFigure 5C, respectively.

The sample of A37 (1403) shows negative εHf(t) values, ranging from -1.89 to -11.15, with an average of -4.63. Correspondingly, the two-stage model ages chiefly plot on 1828-2143 Ma, while separately project at 2418 Ma (Fig. 5C). The results suggest that such granite sample was derived from some Paleoproterozoic basement rocks. Moreover, all the εHf(t) values are under the Chondritic uniform reservoir (CHUR) line, which indicates that little or no mantle component has been involved into the source rock during the partial melting and magma ascent.

Rock magnetic investigation and results

Field sampling

A total of 352 cores from 55 sites were sampled for the AMS study, 35 and 20 for the Sanfang and Yuanbaoshan plutons, respectively, along all possible roads with an interval of ~2 km (Fig. 7). Each site of fresh outcrop covers several tens of square meters and at least six cores are drilled with an interval of 3 to 4 meters between each other from each site.

Magnetic mineralogical analysis

In order to define the magnetic susceptibility carriers in our samples, we have carried out following methods: ( 1 A total of nine samples were chosen for magnetic mineralogical analysis, and the representative results are presented in Figure 6. The hysteresis curves (Fig. 6A) present similar Sshaped hysteresis loops with significant difference between the corrected and uncorrected curves, suggesting that the main magnetic susceptibility carriers could be both ferromagnetic and paramagnetic minerals. Rapid saturation of isothermal magnetic remanence at about 200 mT is featured by Sample A46 (Fig. 6B), indicating the existence of ferromagnetic minerals with low magnetic coercivity. However, for Sample A48, the gradual saturation until to 1.0 Tesla (Fig. 6B) highlights that the existence of ferromagnetic minerals with high magnetic coercivity. The rates of saturation of Samples A65 and A67 are in-between A46 and A48 (Fig. 6B), revealing both the weak and high coercive ferromagnetic minerals in the granites. The thermomagnetic measurements on Samples A10 and A23 yield a quick decrease of the magnetic susceptibility at about 580℃ (A10 in Fig. 6C) and continuous drop until to 680°C (A23 in Fig. 6C), which suggest the existence of (titano) magnetite and hematite. Moreover, these two samples show a continuous rise of magnetic susceptibility in the cooling stage (owing to oxidation reactions) revealing that a significant mineral transformation occurred during the heating, e.g. iron sulfide to iron oxide (Fig. 6C). The SEM investigation reveals that the existence of titanomagnetite with the mass fraction of the TiO2 ranged from 45.51% to 50.65%, and they are commonly intergrown or partly enclosed along the cleavage and boundary of biotite (Fig. 6D). The Day-plot of hysteresis parameters is presented in Figure 6E, showing that (titano) magnetite of the measured samples drop in the multidomain zone (e.g., [START_REF] Dunlop | Theory and application of the Day plot (Mrs/Ms versus Hcr/Hc) 2. Application to data for rocks, sediments, and soils[END_REF]. It indicates that the major (elongated) axis of the magnetic susceptibility ellipsoid can be correlated to the major morphological axis of minerals (e.g., [START_REF] Borradaile | Tectonic applications of magnetic susceptibility and its anisotropy[END_REF].

Consequently, the carriers of magnetic susceptibility are composed by ferromagnetic minerals, for instance, multidomain (titano) magnetite and hematite, and of paramagnetic minerals, such as biotite, muscovite and feldspar. According to [START_REF] Rochette | Rock magnetism and the interpretation of anisotropy of magnetic susceptibility[END_REF], the magnetic fabrics of these minerals are comparable to the petrographic ones. Therefore, the AMS measurements will be an effective way to obtain the information on the petrofabrics of granite, so that to decipher the granitic magma emplacement [START_REF] Martín-Hernández | Magnetic fabric: methods and applications-an introduction[END_REF].

AMS parameters

The measurements of AMS were performed with a KLY5 Kapprabridge at ISTO. A total of 352 (228 and 124 from the Sanfang and Yuanbaoshan, respectively) specimens with 2.5 cm in diameter and 2.2 cm in length from 55 sites were prepared. Detailed information can be found in Table 3. The mean bulk magnetic susceptibility (Km) of each sampling site from the Sanfang and Yuanbaoshan plutons varies from 27.5 to 133.0x10 -6 SI and 32.7 to 197.0x10 -6 SI, respectively (Fig. 8A, 8B and 8D). The site-mean value of the anisotropy degree of the magnetic susceptibility (PJ) ranges in [1.017, 1.156] and [1.009, 1.172] for the Sanfang and Yuanbaoshan plutons, respectively (Fig. 8B, 8C and8E). The site-mean of shape parameter (T) ranges in [-0.384, 0.912] and [-0.384, 0.817] for the Sanfang and Yuanbaoshan, respectively (Fig. 8C and8F). No obvious correlation of PJ can be observed with Km (Fig. 8B) and elevation (Fig. 8C), however, the PJ and T are positively related (Fig. 8F). Moreover, PJ and T values both increase with the rising of elevation (Fig. 8D and8E), which is illustrated in the cross section of Figure 8H.

AMS results

The AMS results for all sampling sites are given in Figure 7. Differences are obvious and significant between the undeformed and deformed granites according to the features of their magnetic foliation and lineation, degree of anisotropy and shape parameter (Figs. 7 and8). In our visiting area, the undeformed granites with magmatic fabrics can be roughly partitioned as NW and SW borders, S part, and SE border of the Sanfang pluton, and SE part and NE border of the Yuanbaoshan pluton, meanwhile in order to read easily, these zones are also shown by Arabic numbers 1, 2, 4 and 6, respectively in Figure 8. In the NW and SW borders of the Sanfang pluton (Zone 1), the magnetic foliations consistently dip to the E with the inclination ranged from 33° to 79°, but the magnetic lineations span from N to S with a relative concentration on the SE dipping.

Almost all the specimens show PJ value lower than 1.1 and magmatic fabrics with even shape parameters between oblate (T>0) and prolate (T<0). In the S part of the Sanfang pluton (Zone 2), the magnetic foliations mainly dip to the W with inclinations varying from 31° to 63°, and magnetic lineations span from ~230° to 330°, with a relative concentration in nearly E-W directed.

94% of the specimens have PJ values lower than 1.1 and the ratio between the oblate and prolate shapes is about 2:1. The SE border of the Sanfang pluton (Zone 4) displays the gentle magnetic foliations with dispersed magnetic lineations. All the PJ values are lower than 1.1 and the ratio of shape parameter between the oblate and prolate is about 2:1. Moreover, in the SE part and NE border of the Yuanbaoshan pluton (Zone 6), the strikes of the magnetic foliations mainly dip SW with relative steep inclinations from 35° to 76°, and the magnetic lineations are essentially SSE.

Most of the PJ values are lower than 1.1, and the ratio between the oblate and prolate shapes is about 1:1.

However, the deformed granites are recognized in the Central to NE Sanfang pluton and in the Central to NW Yuanbaoshan pluton (Zones 3 and 5, respectively; Fig. 8). In the Central to NE part of the Sanfang pluton (Zone 3), the PJ values, with the range from 1.041 to 1.187, are higher than other zones in the Sanfang pluton. 37% of the specimens with the PJ value higher than 1.1, suggesting that the magnetic fabric could have been modified in the post-solidus state [START_REF] Tarling | Magnetic anisotropy of rocks[END_REF]. The magnetic foliations gently dip to the SW, with coherent NEE-SWW directed magnetic lineations. More than 95% of the shape parameters of specimens are greater than zero, i.e., oblate shape. Similarly, the Central to NW part of the Yuanbaoshan pluton (Zone 5) highlights PJ values ranging from 1.017 to 1.183. 57% of sites in this zone provide PJ values higher than 1.1, and almost all of these sites present positive values for shape parameter, i.e., oblate shape. They show moderate magnetic foliations dipping ranging from 4° to 27°, with consistent NW-SE directed magnetic lineations.

Consequently, the AMS results of these two plutons can be divided into two aspects, namely, the magmatic fabric and tectonic one, corresponding to the undeformed and deformed granites, respectively (Figs. 7 and8). These two parts seem to be qualitatively distinguished at the approximate altitude of 600-800m. To simplify the description, we utilize the average altitude line of 700m as the separatrix between the undeformed and deformed granites. Moreover, it is worth noting that a few samples (about less than 5 per cents) show the incoherent results with respect to above description. The samples B01, B03 and B04 are in the relative low elevation (~500m), but their petro/magnetic fabrics seem like a tectonic origin, with obvious sigmoid feldspar and high PJ values ranges in [1.064, 1.079] (Fig. 7), therefore, these localities have been classified to Zone 5. Meanwhile, the samples A48, A60 and A61 are in the relative higher part, but displaying massive granite and lower PJ values ranges in [1.026, 1.042] compared to the ductilely deformed granites (Figs. 7 and8G), hence, these localities are categorized in Zone 1. Furthermore, the two massive granite samples A33 and B11 display different magnetic features with respect to the adjacent ones, showing vertical magnetic foliations and gentle lineations which are all parallel to the boundary between the granitic plutons and country rocks (Fig. 9).

In order to have an integrated view of features of the magnetic foliations and lineations, a synthetic distribution map of magnetic foliations and lineations are presented in Figure 9.

Generally, the magnetic foliations and lineations can be separated into two parts as previously documented, respectively. For the magmatic fabrics, the predominant strike of the foliation of the Sanfang and Yuanbaoshan plutons is sub N-S directed, and the secondary group is sub E-W directed. Two major groups of lineations are identified in the Sanfang pluton, namely the NEE-SWW and NNW-SSE directed ones. And the distinct magmatic lineations are sub NNW-SSE directed in the Yuanbaoshan pluton. However, for the tectonic fabrics, a set of consistent gentle SW dipping magnetic foliation and gently SWW directed magnetic lineation is recognized in the Sanfang pluton. Whereas dispersed magnetic foliations with gentle inclination are identified in the deformed granite in the Yuanbaoshan within a group of gently NW directed magnetic lineations (Fig. 9).

Gravity modeling

The regional gravity anomaly embodies the density heterogeneity of the underground, hence, the proper decipherment of the gravity anomaly data would help us to know the construction of the geological bodies in the deep [START_REF] Martelet | Fast mapping of magnetic basement depth, structure and nature using aeromagnetic and gravity data: combined methods and their application in the Paris Basin[END_REF], especially, the intrusive geological bodies usually present obvious density contrast with the wall rocks. Consequently, the gravity modeling method has been applied to deduce the Sanfang and Yuanbaoshan pluton shapes within the Sibao strata. The Bouguer gravity anomaly map for the Sanfang-Yuanbaoshan area is acquired at the China Geological Survey. The Digital Elevation Model (DEM) for the Sanfang-Yuanbaoshan area is obtained from the CGIAR-CSI database.

We collected both the granites and country rocks for the density measurements, which have been carried out at the University of Nanjing by using the Matsuhaku Electronic Densimeter GH-300. For each sample, we measured twice and took the average value at a precision of 0.001g/cm³. Detailed density data for the samples from the granites, Sibao group, Danzhou group and Sinian strata can be referred in Table 4. As the post Sinian strata is poorly exposed, we take it as same as the density of the Sinian strata. Besides, the density of the mafic-ultramafic block is referred to the geological survey of the Guangxi province [START_REF] Bgmrgx | Regional Geology of Guangxi Autonomous Region[END_REF]. The density value of 2.670g/cm³ for the background in the study area is referred in CRUST 1.0.

In the Bouguer gravity anomaly data, both the short and long wavelength signals are involved, which reflect the surface and deep signatures, respectively. Thus the long wavelength signals should be removed for the purpose of getting appropriate gravity anomaly information just induced by the sub-surface geological units. The Butterworth Low-Pass Filter is applied for removing the long wavelength signals with cut-off wavelengths of 10, 20, 30… to 130, 140 and 150 kilometers.

Through the comparison of the residual gravity anomaly and geological maps, we found that the residual gravity anomaly with the cut-off wavelength of 130 km is the most suitable one, which matches best with ground surface features (Fig. 10).

The selected residual Bouguer gravity anomaly map shows two obvious negative anomaly centers in the central and southern parts of the Sanfang and Yuanbaoshan plutons, respectively (Fig. 10). The negative anomaly regions for these two plutons are both N-S directed, which concur the geometry of these two plutons. According to the residual Bouguer gravity anomaly data and regional tectonic features, five profiles are chosen for gravity modeling, two of them (WE1 and WE2) are perpendicular to the regional tectonic lines, and three (NS1, NS2 and NS3) are parallel with the regional structures.

Five interpreted profiles are presented in Figure 10. Their deep structures are modelled with the residual Bouguer gravity anomaly data, however, the surface contact relationship between each unit is depicted according to the field observations and geological maps [START_REF] Bgmrgx | Regional Geology of Guangxi Autonomous Region[END_REF].

According to the Bouguer gravity modeling along these profiles, we can characterize these two plutons as following: (1) Negative gravity anomalies are N-S long axis orientated within the Sibao group; (2) The roots of the granitic plutons are presented in a N-S orientated dyke shape; (3) In the E-W direction, two plutons are in a tongue shape; (4) For the Sanfang pluton, the deepest root is located in its southern part, and the depth is progressively decreased northwards with the shallowest depth of ca. 0.5-1 km in its northern part; (5) For the Yuanbaoshan one, the thickest part is situated in the central pluton; (6) The Sanfang and Yuanbaoshan plutons seem to be connected in the deep.

Discussion

Origin of the magma and its crystallization age

Numerous researches have studied the petrogeneses and tectonic settings of the Neoproterozoic granites in the JOB, the Neoproterozoic S-type granitic plutons in the JOB have been dated with U-Pb crystallization ages ranged from 840 to 815 Ma, peaking at ca. 830 Ma (e.g., [START_REF] Wang | LA-ICP-MS U-Pb zircon geochronology of the Neoproterozoic igneous rocks from Northern Guangxi, South China: implications for petrogenesis and tectonic evolution[END_REF][START_REF] Wu | Reworking of juvenile crust: Element and isotope evidence from Neoproterozoic granodiorite in South China[END_REF][START_REF] Xue | Geochronology and geochemisty of the Neoproterozoic granitoid association from eastern segment of the Jiangnan orogen, China: Constraints on the timing and process of amalgamation between the Yangtze and Cathaysia blocks [in Chinese with English abstract[END_REF][START_REF] Zhong | SHRIMP U-Pb zircon geochronology of the Jiuling granitic complex batholith in Jiangxi Province [in Chinese with English abstract[END_REF]. Besides, these plutons are characterised by the high Aluminium Saturation Index ranging from 1.15 to 1.40 (Yao et al., 2014b and references therein) and low initial ɛNd values from -4.8 to -7.6 with high enrichment in Rb, K and Th elements [START_REF] Li | U-Pb zircon ages of granites from the southern margin of the Yangtze block: timing of Neoproterozoic Jinning: orogeny in SE China and implications for Rodinia assembly[END_REF]. Thus the geochemical features indicate that the granites were sourced from supracrustal rocks [START_REF] Wang | LA-ICP-MS U-Pb zircon geochronology of the Neoproterozoic igneous rocks from Northern Guangxi, South China: implications for petrogenesis and tectonic evolution[END_REF].

In this study, several methods are carried out for deciphering the origin of the magma.

Petrologically, the granite samples are K-feldspar megacryst rich (Fig. 3C). Structurally, the cathodoluminescence (CL) images (Fig. 5A and5B) show zircons with clear oscillatory zoning, revealing the typical feature of zircons from acidic magma. Isotopically, moreover, all of the εHf(t) values are under the CHUR line (Fig. 5C), suggesting the melting of magma with little or even no mantle components. Consequently, we suggest the granites of the Sanfang and Yuanbaoshan plutons were generated by the partial melting of crustal materials, furthermore, the relatively homogeneous lithological features of these granitic plutons may suggest that the magma was derived from single component within the chamber.

Moreover, the new zircon results from the Sanfang and Yuanbaoshan granites provide the mean ages of these two plutons at 830±2 Ma and 830±5 Ma, respectively (Fig. 5F and5I), which further indicates the magma crystallized at ca. 830 Ma. Few zircon grains give ages older than 2.0 Ga (Fig. 5G), indicating that a Paleoproterozoic basement probably exists below the upper crust in this area, and these old zircons were captured during the magma ascent. Furthermore, the Hf isotopic analyses show a group of two-stage model ages at ca. 1800-2400 Ma, which suggest the possible existence of Paleoproterozoic rocks beneath the JOB.

Acquisition of magnetic fabrics

According to macro/microscopic observations and AMS study, the surveyed areas in the Sanfang and Yuanbaoshan plutons can be divided into two fabric domains, namely, the sub-solidus (or magmatic) and post-solidus (or solid-state) ones (Figs. 8 and9).

The magmatic domain concerns the zones with the elevation below ~700 meters a.s.l., in this domain, most of the granites are macroscopically quasi isotropic (Zones 1, 2, 4 and 6 in Fig. 8G).

Meanwhile, relatively coherent, magnetic fabrics with N-S to NNE-SSW striking lineations and eastward and westward dipping foliations have been observed for the Yuanbaoshan pluton, however, relatively dispersed lineations with general E-W and N-S dipping foliations for the Sanfang pluton. The microscopic investigations show that quartz grains are euhedral with slightly undulose extinction (Fig. 4A and4B), however, the biotite grains in the granite do not show any post-solidus deformation (Fig. 4B). These evidence suggest the granites have not suffered significant post-solidus deformation. Furthermore, 96% of the PJ values in this domain lower than 1.1, which indicates that the magnetic fabrics were probably acquired during the magma crystalization [START_REF] Borradaile | Tectonic applications of magnetic susceptibility and its anisotropy[END_REF]. Therefore, we propose that the magnetic fabrics in this domain are primary ones, i.e. without post-solidus overprint due to post-emplacement geological events. The shape parameters indicate both oblate and prolate shapes with equivalent proportion, which indicates that the magnetic foliation and lineation were almost equally developed, thus we may propose that the original magma probably flowed without significant influence of a regional tectonics [START_REF] Tarling | Magnetic anisotropy of rocks[END_REF].

On the contrary, in the post-solidus domain (Zones 3 and 5 in Fig. 8G) at altitudes above ~700m a.s.l., the granites are foliated with augen gneissic structures and shear bands. Microscopic studies reveal that the quartz grains are mostly recrystallized as neograins with serrated boundaries, showing the evidence of high temperature and high strain deformation. A top-to-the-W kinematics is macroscopically documented by the sigmoidal augen structure and shear bands, and microscopically by mica fish and pressure shadows. In addition, the PJ values of this zone are generally higher than 1.1, i.e., higher than those in the magmatic domain, suggesting that the magnetic fabrics were acquired after the magma cristalization. Moreover, it is worthy to note that almost all the shape parameters for the specimens in this domain are greater than zero (Fig. 8G), indicating the planar magnetic fabric is more developed than the linear magnetic one probably due to the deformation posterior to the crystallisation. Thus, the evidence above reveals that the magnetic fabrics of this domain are secondary, i.e., the synmagmatic ones are overprinted.

However, the formation time of these post-solidus fabrics is not yet constrained, they might have been developed during the later stage of the magma emplacement or recrystallized during regional tectonic events that post-dated the plutons emplacement [START_REF] Tarling | Magnetic anisotropy of rocks[END_REF]. As this is out of the scope of this study, the possible hypotheses for this ductile deformation will not be discussed in detail in this text.

Mechanism of the magma emplacement and space creation

In this section, we concentrated on the lower part (primary magmatic fabrics) of the plutons and their wall rocks to decipher the magma emplacement process of the Sanfang and Yunabaoshan plutons, using our macro/micro observations, geochronological results, fabric measurements, gravity modelling as well as the previous data.

Sanfang pluton

The Sanfang pluton is elliptic shaped in the N-S long axis. Previous geological survey reveals that both of the granitic plutons and their country rocks are cut by the sub N-S directed faults, which were probably generated in the Neoproterozoic period and reactivated in the Paleozoic or in the Mesozoic (Fig. 2).

Our field investigation indicates that the bedding of the Sibao group close to the Sanfang pluton conform the geometry of the pluton, the original sedimentary bedding (S0) and foliation (S1) are nearly parallel with the border of the granite, suggesting a dome shaped pluton (Fig. 2).

Moreover, the dip angle of the S0-1 gradually decreases as the strata away from the contact border (Fig. 3E and3F). At the exposure level, the Sanfang pluton seems having rarely heated the country rocks with no evidence for obvious thermal aureole, and hornfels have been only observed in two localities in the contact boundary (Fig. 3D), with length 3~5m by width 20~30cm. This means that the amount of heat was not so large and thus we can infer that the first magma injection was probably in a small volume. It probably reveals that the magma injections took place though multi episodic injections but with a small magma volume at each injection, which could be referred to magma pulse amalgamation (Fig. 11; de [START_REF] De Saint-Blanquat | Internal structure and emplacement of the Papoose Flat pluton: An integrated structural, petrographic, and magnetic susceptibility study[END_REF][START_REF] Horsman | Emplacement-related fabric and multiple sheets in the Maiden Creek sill, Henry Mountains, Utah, USA[END_REF].

In the NW and SW borders of the Sanfang pluton (Zone 1), the magnetic foliations dip Ewards with moderate dipping angles (31-60°), whereas the magnetic lineations in this zone show two direction groups, i.e., sub E-W plunging with steep angles (61-90°) in the southern part, and N-S plunging with gentle angles (0-30°) in the northern part (Fig. 9). Moreover, in the southern part of the Sanfang pluton (Zone 2), the foliations dipping W-wards with steep angles and mainly W-dipping lineations may imply that the magma dominantly flowed quasi vertically and accreted in the E-W direction. Furthermore, the magnetic foliations dip SE/SW-wards with moderate angles, and the magnetic lineations generally show a sub E-W preferred orientation in the SE border of the Sanfang pluton (Zone 4, Figs. 8 and9).

From the integrated results, it is obvious that the magmatic foliations of the Sanfang pluton can be roughly distinguished as two sets, namely, the sub N-S striking (major) and sub E-W striking (minor). Meanwhile, the orientations of the magmatic lineations seem more scattered (Fig. 9).

The residual Bouguer gravity anomaly data show that the Sanfang pluton is also N-S elongated at depth, and reveal that the root of the Sanfang pluton is located in its southern part.

And the interpreted profile NS1 suggests that the thickness of the Sanfang pluton decreases northwards (Fig. 10). From the unravelled profiles WE1 and WE2, moreover, the dyke shaped channels seem to be characterized in the pluton.

Yuanbaoshan pluton

The geometry of the Yuanbaoshan pluton is N-S elongated with an aspect ratio of 2:1. The field observation shows that the pluton geometric shape also well affects the occurrence of the country rocks of the Sibao group, i.e., the bedding of the sedimentary rocks follows the shapes of the pluton (Fig. 2). This appearance seems becoming weaker leaving away the pluton. The sigmoid quartz vein within the Sibao group locates to the east of the Yuanbaoshan pluton might be interrelated with the ductile shearing during the magma emplacement since such top-to-the-E kinematic is rarely developed in the study area except in the contact area between the eastern Yuanbaoshan pluton and its country rock (Fig. 3K). In the SE part and NE borders (Zone 6) of this pluton, the strikes of the magmatic foliations are principally NNW-SSE directed with steep and even vertical inclinations. And the magmatic lineations are mostly sub N-S directed and few NW-SE directed with gentle dipping angles (Fig. 9), suggesting that the magma might vertically ascend and mainly horizontally flow in the sub N-S direction in these parts. Moreover, two samples of B02 and B11, close to the boundary between the Yuanbaoshan pluton and Sibao strata, display foliation with moderate to vertical inclination, and gentle lineations which parallel to the boundary of country rocks. It may suggest that the magma probably flowed along the boundary of the country rocks (Fig. 11).

Moreover, the residual Bouguer gravity anomaly yield that the root of the pluton is located in the central to southern part (Fig. 10). Similar to the case of Sanfang pluton, the inferred profiles NS2 and NS3 suggest that the thickness of pluton is slightly decreasing from south to north, and the profiles WE1 and WE2 suggest that the dyke shaped channels seem alike also within the Yuanbaoshan pluton.

Construction processes of the two plutons

Following observations and results may help to understand the pluton construction process:

1. the strata surrounding the pluton strike parallel to its border with progressive decrease of the bedding inclination; 2. magnetic fabric patterns are characterised by principal sub N-S striking foliation and partial sub E-W ones with relatively dispersed lineations for the Sanfang pluton and sub N-S directed ones for the Yuanbaoshan pluton; 3. gravity modelling reveals a thick 'root' located in the central to south part of the plutons with possible N-S directed dyke-shaped channels.

By integrating the above phenomena, we propose that the magma probably made its upwelling into the Sibao group in the central to southern part of the plutons through dyke-shaped channels, probably resulted by the pre-existing abundant and large-scale folds/faults within the Sibao group, and laterally pushed the country rocks outwards. Due to probably the shape of preexisting structures, i.e. N-S oriented folds/faults, the magma dominantly flowed north-southwards to construct the tongue-and/or sill-shaped plutons. Meanwhile, the granitic bodies were progressively accreted in sub E-W direction to form the principal sub N-S oriented foliation.

Therefore, the N-S flow of magma and E-W accretion of pluton seem likely forming two dome shaped plutons (Fig. 11). Nevertheless, the depth of emplacement is unclear and worthy for future investigation.

Tectonic implications and perspectives

S-type granitic plutons are interesting geological markers to understand the tectonic process, numerous such plutons are distributed along the JOB. However, their tectonic setting is still hotly debated in the past decade. Some scholars suggested that the peraluminous plutons were generated and made their intrusion during the amalgamation of the Yangtze and Cathaysia blocks, i.e., the syn-orogenic stage, while considering the coeval mafic rocks dated at ca. 850-830 Ma [START_REF] Ding | Neoproterozoic ( ~ 850 Ma) subduction in the Jiangnan orogen: evidence from the SHRIMP U -Pb dating of the SSZtype ophiolite in southern Anhui Province[END_REF][START_REF] Zhou | Geochemistry of meso-and Neoproterozoic mafic-ultramafic rocks from northern Guangxi, China: arc or plume magmatism?[END_REF]. Nevertheless, some others hold the view that the granitic plutons were formed at the post-orogenic stage [START_REF] Wang | Geochronological, geochemical and Nd-Hf-Os isotopic finger printing of an early Neoproterozoic arc-back-arc system in South China and its accretionary assembly along the margin of Rodinia[END_REF]2014b;[START_REF] Yan | Time constraints on the closure of the Paleo-South China Ocean and the Neoproterozoic assembly of the Yangtze and Cathaysia blocks: Insight from new detrital zircon analyses[END_REF][START_REF] Zhang | Early Neoproterozoic (~850 Ma) back-arc basin in the Central Jiangnan Orogen (Eastern South China): geochronological and petrogenetic constraints from meta-basalts[END_REF][START_REF] Zhang | Early Neoproterozoic (~840 Ma) arc magmatism: geochronological and geochemical constraints on the meta-basites in the Central Jiangnan orogenic belt[END_REF]. These suggestions are, however, proposed only on the basis of the geochronological logic, without direct geological and physical evidence.

In this study, the field observations show some pluton emplacement related deformation in the country rocks, but no any evidence for a syn-magmatism regional scaled tectonic deformation.

The primary magmatic fabrics in both plutons seem having been acquired in a regional tectonic free environment. Thus we propose that the crustal partial melting magma in the western Jiangnan area might be produced by the thickened crust and emplace into the Sibao group during a period of tectonic quiescence, in other words, the S-type granitic magma emplacement probably signifies the ending stage of the Jiangnan Orogeny.

However, the timing and mechanism of the top-to-the-W post-solidus deformation distinguished by the macro-/micro observations, and AMS study is still poorly studied. Two possible hypotheses are supposed, one is that the deformation occurred during the magma intrusion at ca. 830 Ma, whereas the other one is that it maybe caused by the Paleozoic or Mesozoic tectonic events. However, the latter one is puzzled because of the regional deformation/metamorphism is not yet recognized in the Jiangnan region (Shu, 2012 and references therein). In order to solve these scientific issues, detailed field work on the regional deformation as well as the geochronology study, e.g., Ar-Ar dating, should be carried out in the further research.

Conclusions

Through this multidisciplinary study, the emplacement process and tectonic setting of the Neoproterozoic S-type Sanfang-Yuanbaoshan granitic plutons in the JOB can be concluded as following:

(1) The magma of the plutons sourced from the partial melting of crustal material, and crystallized at ca. 830 Ma;

(2) Magnetic/petrographic fabrics can be divided into two groups, namely, a primary subsolidus, or magmatic, fabric developed during the pluton emplacement; and a secondary postsolidus, or solid-state, one related to a still uncertain tectonic process;

(3) The crustal partial melting magma probably intruded into the pre-existing mechanically weak (fold/fault) system in the Sibao group through dyke-shape channels, and afterwards dominantly flowed north-southwards to construct the tongue-and/or sill-shaped plutons, meanwhile, the granitic bodies were progressively accreted in sub E-W direction to form two dome shaped plutons;

(4) The intrusion of the S-type granitic magma probably occurred in a regional tectonic free environment, the emplacement probably marked the latest stage of the Jiangnan Orogeny. For the cross-section, the contact relationship between each geological unit is depicted according to the field observations, however, the deep structure of the profile is inferred from the gravity modeling. Note that the vertical scale is exaggerated in order to show more details.

Figure captions

  ) magnetic hysteresis loop characterization by using a vibrating magnetometer in the Laboratoire de Paleomagnétisme of the Institue de Physique du Globe de Paris (IPGP); (2) isothermal remanence magnetization characterization with an IM30 pulsed magnetizer and a JR5 spinner magnetometer at the Insititut des Sciences de la Terre d'Orléans (ISTO); (3) thermomagnetic characterization using a KLY5 kappabridge coupled with a CS3 furnace at ISTO; and (4) scanning electron microscope (SEM) with the ZEISS MERLIN Compact for the minerals at ISTO.

Figure 1 .

 1 Figure 1. Simplified tectonic map of the South China Block.

Figure 2 .

 2 Figure 2. Geological sketch map of the Sanfang-Yuanbaoshan area with a NW-SE cross-section.

Figure 3 .

 3 Figure 3. Representative field photos in the Sanfang-Yuanbaoshan area, with the geographic

Figure 4 .

 4 Figure 4. Microphotographs with the representative features of the granites and country rocks in

Figure 5 .

 5 Figure 5. The Cathodoluminescence, U-Pb ages and epsilon Hf(t) results of the granite samples of

Figure 6 .

 6 Figure 6. Magnetic mineralogic analyse of the granites in the Sanfang pluton. (A) Hysteresis loop

Figure 7 .

 7 Figure 7. Sketch map with the sampling sites and AMS results for each site.

Figure 8 .

 8 Figure 8. Magnetic parameters of the granites of the Sanfang-Yuanbaoshan plutons. (A)

Figure 9 .

 9 Figure 9. A synthetic distribution map of the magnetic foliations and lineations in the Sanfang and

Figure 10 .

 10 Figure 10. The residual Bouguer gravity anomaly map of the Sanfang-Yuanbaoshan area with five

Figure 11

 11 Figure 11. 3-D model for the emplacement mechanism of the Sanfang and Yuanbaoshan plutons

Figures

  Figures

Figure 1

 1 Figure 1

Figure

  Figure 2

  Figure 2

  Figure 3 934

Table 1 . LA-ICP-MS zircon U-Pb dating results for the granite samples from the Sanfang and Yuanbaoshan plutons

 1

	Analysi s			CORRECTED RATIOS					CORRECTED AGES (Ma)
		207Pb/206P b		207Pb/235 U		206Pb/238 U		207Pb/206P b		207Pb/235 U	206Pb/238 U		concordanc e	Th/U ratio s
			1σ		1σ		1σ		1σ	σ		1 σ
							Sample A37 (1403)
	1403-1	0.064170	0.00337 0	1.248450	0.07149 0	0.134730	0.00351 0	747	11 4	823	815	2 0	99%	0.94
	1403-2	0.066760	0.00355 0	1.296120	0.09280 0	0.134580	0.00609 0	830	11 4	844	814	3 5	96%	0.24
	1403-3	0.067440	0.00349 0	1.306550	0.07720 0	0.134610	0.00385 0	851	11 0	849	814	2 2	95%	0.27
	1403-4	0.075440	0.00315 0	1.773600	0.07894 0	0.164030	0.00335 0	1080	86	1036	979	1 9	94%	0.15
	1403-5	0.068760	0.00293 0	1.337000	0.05990 0	0.136220	0.00367 0	892	90	862	823	2 1	95%	0.37
	1403-6	0.079070	0.00305 0	2.212030	0.09938 0	0.195370	0.00488 0	1174	78	1185	1150	2 6	97%	0.56
	1403-7	0.065360	0.00255 0	1.264680	0.05093 0	0.136980	0.00301 0	786	84	830	828	1 7	99%	0.43
	1403-9	0.078520	0.00310 0	2.067010	0.08720 0	0.186330	0.00433 0	1160	80	1138	1101	2 4	96%	0.40
	1403-10	0.067610	0.00318 0	1.294370	0.06193 0	0.135810	0.00329 0	856	10 0	843	821	1 9	97%	0.43
	1403-11	0.066860	0.00324 0	1.283580	0.06133 0	0.137200	0.00355 0	833	10 3	838	829	2 0	98%	0.05
	1403-12	0.067490	0.00379 0	1.281600	0.07151 0	0.136250	0.00398 0	853	12 0	838	823	2 3	98%	0.23
	1403-13	0.069100	0.00291 0	1.331060	0.05784 0	0.138100	0.00443 0	902	89	859	834	2 5	96%	0.12
	1403-14	0.067310	0.00287 0	1.289490	0.05486 0	0.136940	0.00410 0	847	91	841	827	2 3	98%	0.10
	1403-15	0.071780	0.00315 0	1.362270	0.06041 0	0.137590	0.00394 0	980	92	873	831	2 2	95%	0.14
	1403-16	0.067260	0.00376 0	1.288130	0.06947 0	0.137630	0.00411 0	846	11 9	840	831	2 3	98%	0.13
	1403-17	0.067700	0.00432 0	1.298600	0.07225 0	0.138830	0.00552 0	859	13 6	845	838	3 1	99%	0.26
	1403-18	0.066130	0.00269 0	1.280030	0.06139 0	0.136630	0.00441 0	811	87	837	826	2 5	98%	0.09
	1403-19	0.068080	0.00310 0	1.317510	0.06638 0	0.137140	0.00387 0	871	97	853	828	2 2	97%	0.07
	1403-20	0.067920	0.00320 0	1.305320	0.06168 0	0.137340	0.00473 0	866	10 0	848	830	2 7	97%	0.35
	1403-21	0.075510	0.00217 0	1.759020	0.06791 0	0.166670	0.00344 0	1082	59	1030	994	1 9	96%	0.29
	1403-22	0.066880	0.00210 0	1.280900	0.03741 0	0.138870	0.00489 0	834	67	837	838	2 8	99%	0.38
	1403-23	0.064280	0.00123 0	1.228120	0.02578 0	0.138060	0.00182 0	751	41	813	834	1 0	97%	0.16
	1403-24	0.066260	0.00124 0	1.264570	0.02497 0	0.137730	0.00143 0	814	40	830	832	8	99%	0.16
	1403-25	0.064920	0.00113 0	1.234900	0.02308 0	0.137450	0.00157 0	772	37	817	830	9	98%	0.28
	1403-26	0.066510	0.00157 0	1.263570	0.03267 0	0.136920	0.00216 0	823	50	830	827	1 2	99%	0.40
	1403-27	0.065110	0.00115 0	1.235710	0.02398 0	0.137220	0.00177 0	778	38	817	829	1 0	98%	0.27
	1403-28	0.068180	0.00124 0	1.297320	0.02711 0	0.137290	0.00183 0	874	38	845	829	1 0	98%	0.17
	1403-29	0.066420	0.00139 0	1.260180	0.02649 0	0.137220	0.00174 0	820	45	828	829	1 0	99%	0.35
	1403-30	0.066360	0.00140 0	1.263390	0.03091 0	0.137100	0.00212 0	818	45	829	828	1 2	99%	0.13
	1403-32	0.081160	0.00173 0	2.164630	0.06578 0	0.190400	0.00386 0	1225	43	1170	1124	2 1	95%	0.09
	1403-33	0.065520	0.00112 0	1.251830	0.02261 0	0.137720	0.00141 0	791	37	824	832	8	99%	0.20
	1403-34	0.067610	0.00164 0	1.291380	0.03148 0	0.138000	0.00200 0	857	52	842	833	1 1	98%	0.63
	1403-35	0.066110	0.00187 0	1.252750	0.03496 0	0.136620	0.00229 0	810	61	825	826	1 3	99%	0.16
	1403-36	0.082480	0.00111 0	1.993340	0.03110 0	0.173880	0.00179 0	1257	27	1113	1033	1 0	92%	0.21
	1403-38	0.066920	0.00118 0	1.276980	0.02320 0	0.137420	0.00139 0	835	38	836	830	8	99%	0.20
	1403-39	0.067410	0.00133 0	1.284140	0.02711 0	0.137860	0.00190 0	850	42	839	833	1 1	99%	0.24
	1403-40	0.066030	0.00123 0	1.266980	0.02943 0	0.137720	0.00193 0	807	40	831	832	1 1	99%	0.21

Table 2 . LA-MC-ICP-MS zircon Lu-Hf isotopic data for the granite sample from the Sanfang pluton

 2

	Sample	176Yb/177Hf	2σ	176Lu/177Hf	2σ	176Hf/177Hf	2σ	εHf(t)	εHf(0)	Age(Ma)	TDM1(Ma)	TDM2(Ma)	f(Lu/Hf)

Table 3 . Sampling sites and site-mean AMS measurement results for the Sanfang-Yuanbaoshan plutons

 3 Note: Alt.: Altitude, n: number of specimen at sampling site, Km: bulk magnetic susceptibility PJ: degree of susceptibility anisotropy, T: shape parameter of the AMS ellipsoid, K1: Magnetic lineation, K3: The pole of the magnetic foliation, Inc.: Inclination, Dec.: Declination, α95 (max/min): the long and short axes of the confidence ellipsoid at 95% level

									Site mean AMS results
					Km
	Site	Geographic coordinates	Alt.(m)	n	(10 -6 SI)	PJ	T	K1	α95(max/min)	K3	α95(max/min)
								Dec (°)/Inc(°)	(°)	Dec (°)/Inc(°)	(°)
						Sanfang
	A02	N25°36.269′ E108°46.973′	547	8	40.2	1.017	0.178	155.1/17.4	61.2/35.7	259.7/38.6	47.4/39.1
	A04	N25°35.191′ E108°46.424′	680	6	27.5	1.074	0.007	159.2/10.1	28.2/15.6	263.1/53.5	17.6/12.7
	A15	N25°22.597′ E108°49.759′	860	6	81.6	1.104	0.776	263.0/43.0	42/4.7	58.9/44.3	11.8/4.3
	A20	N25°24.531′ E108°46.455′	1330	6	33.0	1.130	0.377	255.2/18.9	34.5/16.1	48.8/69.1	26.1/12.8
	A26	N25°17.328′ E108°44.369′	402	6	76.9	1.083	0.000	223.2/61.7	8.7/5.4	56.9/27.6	16.5/6.2
	A31	N25°13.954′ E108°40.646′	770	6	90.5	1.041	-0.155	83.9/42.4	29.7/15.0	292.0/44.0	36.5/12.5
	A33	N25°13′14.7" E108°51′33.4"	293	8	47.9	1.117	0.054	304.4/0.9	62.3/32.7	34.8/24.3	54.2/25.3
	A34	N25°17′08.1" E108°53′10.3"	263	6	52.5	1.022	-0.384	78.6/0.9	6.6/4.8	346.3/68.4	37.4/3.9
	A35	N25°17′24.0" E108°52′26.8"	715	6	55.2	1.063	0.417	251.6/19.7	11.0/4.7	46.5/68.4	11.6/7.6
	A36	N25°17′44.6" E108°51′38.9"	705	6	88.1	1.081	0.392	262.1/21.2	19.1/7.0	40.7/62.7	19/7.9
	A37	N25°18′45.8" E108°50′34.5"	724	6	67.8	1.099	0.148	256.9/14.5	7.9/4.9	54.3/74.3	11.0/3.1
	A38	N25°26′07.7" E108°48′50.7"	1153	7	42.6	1.140	0.737	263.9/36.6	14.7/4.9	86.4/53.4	10.1/4.7
	A40	N25°22′52.5" E108°54′03.7"	772	7	48.9	1.071	0.649	289.5/13.2	23.2/5.3	58.1/69.4	6.9/4.1
	A41	N25°22′24.9" E108°53′38.4"	674	6	46.9	1.059	0.569	264.2/20.8	17.3/2.6	44.9/63.9	7.3/2.6
	A42	N25°21′49.1" E108°52′57.9"	636	6	57.5	1.071	0.722	216.8/26.1	26.6/7.7	33.1/63.9	10.4/7.7
	A43	N25°20′43.1" E108°50′39.9"	993	7	99.5	1.083	0.562	265.4/23.0	16.4/4.5	76.2/66.7	6.7/4.4
	A44	N25°27′23.2" E108°50′42.3"	1085	7	100.0	1.116	0.687	253.3/14.8	15.7/9.1	91.6/74.4	10.3/5.5
	A45	N25°28′15.0" E108°51′27.9"	1001	6	109.0	1.156	0.912	224.1/1.4	37.8/3.7	30.7/88.6	7.4/0.6
	A46	N25°23′25.3" E108°49′03.4"	983	8	88.8	1.097	0.671	253.1/20.8	13.2/6.4	61.4/68.8	7.9/4.2
	A47	N25°25′51.6" E108°44′30.3"	1196	7	93.3	1.071	0.776	210.7/53.0	27.3/5.8	95.3/17.9	8.0/4.1
	A48	N25°28′29.2" E108°43′21.2"	788	8	46.6	1.042	-0.111	163.0/31.9	10.8/2.9	302.6/50.7	17.9/2.5
	A49	N25°29′45.9" E108°43′24.5"	704	8	55.5	1.055	-0.221	173.5/19.8	4.6/2.1	276.5/32.1	12.2/4.0
	A51	N25°32′04.4" E108°43′51.3"	566	6	50.3	1.067	-0.282	150.8/14.4	4.1/2.3	264.5/57.3	12.0/3.2
	A52	N25°31′10.6" E108°43′39.7"	612	6	44.0	1.030	-0.065	3.5/10.2	9.4/4.4	261.2/49.8	18.0/5.1
	A53	N25°21′09.7" E108°53′37.2"	677	6	41.9	1.066	0.725	166.0/32.7	19.9/9.0	355.4/56.9	10.1/3.7
	A54	N25°15′25.5" E108°51′46.0"	296	6	74.4	1.022	0.193	306.7/6.9	67.3/6.6	45.8/52.6	13.8/7.0
	A55	N25°15′52.1" E108°45′18.9"	317	6	124.0	1.062	0.413	245.2/36.5	22.3/6.8	87.8/51.3	8.0/6.9
	A56	N25°15′03.6" E108°45′05.5"	353	6	66.8	1.037	0.161	304.1/26.4	35.2/9.9	89.6/58.9	14.8/11.8
	A57	N25°12′51.7" E108°45′04.4"	630	5	57.3	1.042	0.071	292.5/42.4	16.1/7.1	91.9/45.7	16.2/7.1
	A58	N25°13′36.6" E108°44′15.8"	426	6	91.6	1.043	-0.073	264.2/48.0	17.4/6.9	112.2/38.5	25.3/8.5
	A59	N25°17′32.8" E108°43′37.6"	441	6	96.2	1.028	0.104	243.1/38.8	21.8/6.7	115.4/37.3	25.5/10.2
	A60	N25°19′20.4" E108°41′58.6"	870	6	41.9	1.026	0.268	49.1/44.1	68.8/10.6	289.8/26.8	16.1/8.5
	A61	N25°20′23.1" E108°42′24.2"	940	7	133.0	1.038	0.460	172.1/55.1	18.1/5.2	278.2/11.0	18.0/3.7
	A63	N25°16′04.1" E108°41′01.9"	533	7	112.0	1.038	0.128	122.9/53.3	18.6/7.2	298.1/36.6	12.0/5.2
	A65	N25°14′17.1" E108°40′46.5"	722	8	107.0	1.030	-0.178	161.9/42.6	67.4/11.4	310.7/43.0	17.9/11.2
						Yuanbaoshan
	B01	N25°24′11.4" E109°06′22.4"	494	7	108.0	1.079	0.390	274.4/11.5	16.1/4.4	36.0/68.8	17.3/4.7
	B02	N25°24′00.0" E109°06′18.9"	489	5	32.7	1.026	0.361	101.2/6.3	34.0/13.8	5.8/40.7	21.1/10.3
	B03	N25°24′30.1" E109°06′51.7"	514	6	135.0	1.064	0.464	112.8/2.0	13.5/5.9	19.0/62.8	7.6/5.1

Table 4 . The density of the Sanfang-Yuanbaoshan granites and country rocks

 4

	Strata/Rock	Lithology	Sampling number	Density of each sample (g/cm 3 )	Average of measured density (g/cm3)	Standard derivation
			1404	2.630
			A35	2.534
			A37	2.586
			A38	2.601
			A39	2.663
			A40	2.549
			A42	2.559
			A45	2.670
			A46	2.671
			A48	2.592
	γ2 Sanfang	Granite	A49	2.615	2.606	0.0425
			A51	2.591
			A52	2.622
			A55	2.673
			A57	2.576
			A58	2.648
			A59	2.534
			A62	2.582
			A63	2.631
			A65	2.602
			A66	2.587
			B07	2.597
			B09	2.594
	γ2 Yuanbaoshan	Granite	B02 B05	2.621 2.623	2.606	0.0129
			B19	2.591
			B20	2.612
		Slate	1406	2.741
		Slate	1415	2.673
		Slate	1419	2.664
		Slate	1422	2.757
		Slate	1425	2.740
		Sandstone	1427	2.648
	Sibao Group	Sandstone	1429	2.684	2.678	0.0577
		Slate	1439	2.706
		Slate	1447	2.679
		Slate	1451	2.622
		Sandstone	1453	2.763
		Sandstone	1458	2.568
		Slate	1459	2.763

Acknowledgments

We wish to express our thanks to Drs. Liu Hongsheng and Wei Wei for their help on the analyses of AMS and Bouguer gravity modeling. This study is supported by the National Key R & D Program of China [No. 2016YFC0600202], National Natural Science Foundation of China [Nos. 41772204], Major Program of National Natural Science Foundation of China [Grant Nos. 41890834] and Natural Science Foundation of Guangxi [Grant Nos. 2019GXNSFDA245009].

We also gratefully acknowledge financial support by China Scholarship Council. Datasets related to this article can be found at https://doi.org/10.6084/m9.figshare.11858388.