Petrogenesis of the Microgranular Enclaves and Their Host Granites from the Xitian Intrusion in South China: Implications for Geodynamic Setting and Mineralization

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Article Petrogenesis of the Microgranular Enclaves and Their Host Granites from the Xitian Intrusion in South China: Implications for Geodynamic Setting and Mineralization

Miao He *, Qing Liu *, Quanlin Hou, Jinfeng Sun and Quanren Yan CAS Key Laboratory of Computational Geodynamics, College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China; [email protected] (Q.H.); [email protected] (J.S.); [email protected] (Q.Y.) * Correspondence: [email protected] (M.H.); [email protected] (Q.L.)



Received: 21 September 2020; Accepted: 23 November 2020; Published: 26 November 2020


Abstract: The South China Block had experienced a significant tectonic transition during the Mesozoic in response to the subduction of the Paleo- and the Pacific Ocean. Large-scale granitic intrusions with massive mineralization are widespread in South China, and their tectonic settings are not defined. The Xitian intrusion is ideal for probing the geodynamic setting and mineralization in South China because they comprise an abundance of microgranular enclaves (MEs) and diverse types of granite associated with mineralization. Age determined by zircon U-Pb dating suggests that the MEs and their host granites are coeval within error, of ca. 152 Ma. The MEs have a similar initial Hf-O isotopic composition as host granites, and the rapid cooling mineral textures indicate that they are autoliths. Geochemical data show that the host granites are high-K, calc-alkaline, and transitional from metaluminous to peraluminous, slightly enriched in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), with obvious negative Eu anomalies, belonging to I-type. The Nb/Ta and Zr/Hf ratios indicate the volatile penetrates the magmatic-forming process, and the fluid with abundant volatile could extract metal element effectively from the mantle.

Keywords: microgranular enclaves; zircon geochronology; Hf-O isotopes; South China

1. Introduction The South China Block in the southwestern part of the Circum-Pacific tectonic domain consists of the Cathaysia Block in the southeast and the Yangtze Block in the northwest. It is characterized by the extensive occurrence of granites, associated with numerous world-famous polymetallic deposits [1–3]. The origin of large-scale intrusions was commonly associated with mantle magmatism [4,5]. MEs (including mafic and intermediate microgranular) are common in volcanic and plutonic rocks in South China [6,7]. They were interpreted as globules of the mantle- and crust-mantle derived magma [8–10]. These enclaves can tend to equalize their composition with host rocks during hybridization [11]. The mixing of the mantle and crustal magmas at different levels can mark the transition from crustal to mixed granites [12]. Nevertheless, the origin of MEs has been interpreted to restite fragments that have been transported from the source region of the magma [13,14] or as the result of the accumulation of early crystals trapped within its residual liquid at emplacement pressures [15–18]. Evidently, the origin of enclaves in granitic intrusions has a key role in indicating magma differentiation mechanism [19]. The South China Block had experienced multiple tectonothermal events during the Mesozoic. As a result, widespread Mesozoic granites were formed accompanied by a series of large W-Sn

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The South China Block had experienced multiple tectonothermal events during the Mesozoic. As a result, widespread Mesozoic granites were formed accompanied by a series of large W-Sn deposits [3,20,21]. However, whether the mantle was involved in metallogenic magmatism is still debated [2,22,23]. In this paper, the MEs provide geological evidence of a genetic link with their host rocks. A detailed investigation of the petrography, zircon U-Pb geochronology, geochemistry, zircon Hf-O isotopes was carried out on the MEs and the host granites from the Xitian intrusion. This research aims to identify the petrogenesis of granitoids, the origin of mineralization, and to reveal theMinerals geodynamics2020, 10, 1059 setting and mineralization of magmatism. 2 of 20

2. Geological Setting deposits [3,20,21]. However, whether the mantle was involved in metallogenic magmatism is still debatedThe [Xitian2,22,23 intrusion]. In this is paper, located the in MEs the providecentral part geological of the Nanling evidence Range, of a genetic accompanied link with by their tin- polymetallichost rocks. A deposits detailed investigation[24]. Nanling of Range the petrography, is one of zirconthe world’s U-Pb geochronology,most important geochemistry, magmatic- hydrothermalzircon Hf-O isotopes W-Sn provinces was carried (Figure out on 1) the [25–27]. MEs and It thecontains host graniteswidely fromdistributed the Xitian granites intrusion. and Thisnumerous research W-Sn aims todeposits, identify theincluding petrogenesis Shizuyuan, of granitoids, Furong, the Taoxikeng, origin of mineralization, Da’ao, Xihuashan, and to reveal and Guposhanthe geodynamics (Figure setting 1) [1–3,22]. and mineralization of magmatism. The Xitian deposit also belongs to the Qin-Hang belt, an economic ore belt in South China [3,23]. It2. is Geological a Neoproterozoic Setting suture belt result from the collisions of the Yangtze Block and the Cathaysia Block, along the Jiangshan-Shaoxing fault [24,28]. Numerous Middle to Late Jurassic (171–150 Ma) The Xitian intrusion is located in the central part of the Nanling Range, accompanied by calc-alkaline granitoids rocks and related Cu-Au-Mo-Pb-Zn-W-Sn deposits are distributed along the tin-polymetallic deposits [24]. NanlingRangeisoneoftheworld’s most important magmatic-hydrothermal Qin-Hang belt in response to the Jurassic Paleo-Pacific Plate subduction [29,30]. Because of the W-Sn provinces (Figure1)[ 25–27]. It contains widely distributed granites and numerous W-Sn deposits, superimposed influence of multistage tectonmagmatic events, the source of magma associated with including Shizuyuan, Furong, Taoxikeng, Da’ao, Xihuashan, and Guposhan (Figure1)[1–3,22]. polymetallic deposits remains highly controversial.

Figure 1. GeologicalGeological sketch sketch map map of of the South China Bl Blockock showing the distribution of Mesozoic granite-volcanic rocks. The locations of granites and volcanics refer to [[28].28].

The Xitian deposit also belongs to the Qin-Hang belt, an economic ore belt in South China [3,23]. It is a Neoproterozoic suture belt result from the collisions of the Yangtze Block and the Cathaysia Block, along the Jiangshan-Shaoxing fault [24,28]. Numerous Middle to Late Jurassic (171–150 Ma) calc-alkaline granitoids rocks and related Cu-Au-Mo-Pb-Zn-W-Sn deposits are distributed along the Qin-Hang belt in response to the Jurassic Paleo-Pacific Plate subduction [29,30]. Because of the superimposed influence of multistage tectonmagmatic events, the source of magma associated with polymetallic deposits remains highly controversial. Minerals 2020, 10, 1059 3 of 20

Minerals 2020, 10, x FOR PEER REVIEW 3 of 20 In this region, the stratigraphical sequences consist of the Palaeozoic to Early Proterozoic In this region, the stratigraphical sequences consist of the Palaeozoic to Early Proterozoic clastic clastic rocks, Mesozoic carbonate, and clastic rocks, partly unconformable overlain by Neogene clay rocks, Mesozoic carbonate, and clastic rocks, partly unconformable overlain by Neogene clay rocks rocks (Figure2). The intensive folding and deformed basement are mainly composed of weakly (Figure 2). The intensive folding and deformed basement are mainly composed of weakly metamorphosed clastic sedimentary rocks in Sinian to Silurian. Overlying upper Jurassic to Cretaceous metamorphosed clastic sedimentary rocks in Sinian to Silurian. Overlying upper Jurassic to sedimentary rocks are mostly sandstones deposited in rift basin [31]. Cretaceous sedimentary rocks are mostly sandstones deposited in rift basin [31].

Figure 2. Geological sketch map of the Xitian ore fieldfield (modified(modified after [[32]).32]).

The Xitian intrusion is dumbbell-shaped and mainly occurred in two episodes: the Late Triassic (ca.(ca. 230230 Ma) Ma) and and the the Late Late Jurassic Jurassic (ca. 150(ca. Ma) 150 [ 24Ma),32 –[24,32–37].37]. The Triassic The Triassic plutons occurredplutons occurred as the batholiths as the intrudedbatholiths into intruded Ordovician into Ordovician rocks and Devonian rocks and carbonate Devonian rocks. carbonate The Late rocks. Jurassic The Late granites Jurassic are scattered granites andare scattered intruded and into intruded the older into Triassic the older plutons. Triassic They plutons. are mainly They distributed are mainly in distributed the interior in of the the interior pluton andof the along pluton the and eastern along orebody the eastern (Figure orebody2). The (Figur two tectonice 2). The events two tectonic accompanying events large-scaleaccompanying magmatic large- activityscale magmatic result in activity the E-W result trending in the and E-W NE trending trending and structures NE trending [36]. The structures Xitian ore [36]. field The developed Xitian ore a seriesfield developed of symmetrical a series high-angle of symmetrical normal high-angle faults, as thenormal bounding faults, faultsas the ofbounding Chahan faults rift basin of Chahan [37,38]. Therift basin metallogenic [37,38]. magmatismThe metallogenic mainly magmatism explodes in main 175–150ly explodes Ma [22, 27in]. 175–150 The ore Ma veins [22,27]. mainly The occurred ore veins in themainly contact occurred zone between in the contact the granite zone andbetween Devonian the gr carbonateanite and rocks.Devonian It is obviouscarbonate that rocks. the faults It is obvious control thethat ore the veins faults in control the area the [39 ore]. Tungsten veins in is the chiefly area hosted[39]. Tungsten in wolframite is chiefly and scheelite,hosted in and wolframite tin is mainly and inscheelite, cassiterite. and Thetin is ore mainly veins in of thecassiterite. W-Sn deposit The ore are veins mainly of the distributed W-Sn deposit as veinlet-disseminated are mainly distributed [22,38 as]. veinlet-disseminated [22,38].

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3. Petrography The XitianMinerals 2020 intrusion, 10, x FOR PEER mainly REVIEW consists of biotite granite and biotite monzogranite4 (Figureof 20 3a–d). The biotite3.granite Petrography consists of quartz (35–40 vol.%), K-feldspar (30–36 vol.%), plagioclase (20–25 vol.%), biotite (5–7 vol.%),The Xitian with intrusion granitic mainly texture consists (Figure of biotite3b). granite The and biotite biotite monzogranite monzogranite (Figure is characterized 3a–d). by 10–15 vol.%The megacrysts biotite granite composed consists of quartz mainly (35–40 of K-feldspar vol.%), K-feldspar and matrix (30–36 vol.%), containing plagioclase 30–35 (20–25 vol.% quartz, 30–35 vol.%vol.%), K-feldspar, biotite (5–7 25–30 vol.%), vol.% with granitic plagioclase, texture (Figure 5–6 vol.% 3b). The biotite biotite (Figure monzogranite3d). The is characterized dark microgranular enclaves wereby 10–15 discovered vol.% megacrysts in both composed types mainly of the of late K-feldspar Jurassic and granite. matrix containing They are 30–35 rounded-spheroid vol.% quartz, and 30–35 vol.% K-feldspar, 25–30 vol.% plagioclase, 5–6 vol.% biotite (Figure 3d). The dark scattered spot-like,microgranular with enclaves a plastic were shape discovered (Figure in both3e). type Biotites of the is late gathered Jurassic togranite. cluster They (Figure are rounded-3f), while large feldspar crystallizedspheroid and atscattered the contact spot-like, zone with of a plastic MEs and shape granites (Figure 3e). (Figure Biotite3 isg). gathered The mineral to cluster compositions (Figure are quite similar3f), while to those large of feldspar the host crystallized granites at the (Figure contact3h). zone of MEs and granites (Figure 3g). The mineral compositions are quite similar to those of the host granites (Figure 3h).

Figure 3. Field photographs and microscope images of rocks and microgranular enclaves (MEs) of the Figure 3. FieldXitian intrusion. photographs (a,b) Biotite and microscopemonzogranite; images (c,d) Bitotie ofrocks granite; and (e) Field microgranular photograph showing enclaves MEs (MEs) of the Xitian intrusion.in the granite (a,b )of Biotite the Xitian monzogranite; intrusion; (f) Biotite (c,d )agglomerates; Bitotie granite; (g) The (e feldspar) Field crystal photograph passes through showing MEs in the graniteMEs; of the (h) MEs. Xitian Abbreviations: intrusion; (Qtzf) Biotite (quartz); agglomerates; Kfs (K-feldspar); ( gPl) (plagiocla The feldsparse); Bt (biotite). crystal passes through MEs; (h) MEs. Abbreviations: Qtz (quartz); Kfs (K-feldspar); Pl (plagioclase); Bt (biotite).

4. Sampling and Analytical Methods After the petrographic study, eleven fresh and representative rocks were crushed and powdered (<200 mesh) for geochemical study. Four samples (2708, 2614, 2711, and 2713) were classified as biotite Minerals 2020, 10, 1059 5 of 20 granite, while four samples (2609, 2706, 2702, and 2704) represent biotite monzogranite. Both types were collected from the margin to the central parts of the pluton. For in situ geochronology analysis, two samples of the host granites (2706 and 2708) and two samples of the microgranular enclaves (XTS-1 and XTS-2b) were collected. Sampling locations were shown in Figure2.

4.1. Zircon U-Pb Dating by SIMS and LA-ICP-MS Zircons for U-Pb analysis were separated by using conventional magnetic and density techniques. The sample and standards were put together into epoxy, which was then polished to section the crystals in half. In order to reveal their internal structures and target sites for U-Pb dating, all zircons were documented with transmitted and reflected light micrographs as well as cathodoluminescence (CL) images. U, Th, and Pb of zircons (2706 and 2708) were measured using the Cameca IMS-1280 secondary ion mass spectrometry (SIMS) at the Institute of Geology and Geophysics, Chinese Academy of Sciences in Beijing (IGGCAS). The mount was vacuum-coated with high-purity gold before analysis. U-Th-Pb isotopic ratios and absolute abundances were determined relative to the standard zircons Plešovice [40], interspersed with those of the unknown grains. The O2− primary ion beam with an intensity of ca. 10 nA was accelerated at 13 kV. A single electron multiplier was used on − ion-counting mode to measure the secondary ion beam intensities by peak jumping sequence [41]. Measured compositions were calibrated of common Pb using non-radiogenic 204Pb. An average of present-day crustal composition [42] was used for the common Pb correction. Data reduction was carried out using the Isoplot program [43]. Zircons used for laser inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating of samples (XTS-1 and XTS-2b) were conducted using an Agilent 7500a ICP-MS equipped with a 193 nm GeoLas laser at IGGCAS. The analysis parameters and detailed procedures are described by [44]. The reference material (NIST 610) and internal calibrant (29Si) were used to calibrate the U, Th, and Pb concentrations. The 207Pb/206Pb and 206Pb/238U ratios were calculated using the GLITTER program. Data were corrected using the standard zircon (91500) as an external calibrant, with a standardization error of ~ 1% (circa 16 Ma). Because the 204Pb isotope cannot be precisely measured, ± common Pb contents were evaluated using the method described by [45]. The age was calculated and plotted using ISOPLOT by omitting highly discordant and compositionally anomalous data [43]. Age uncertainties are reported for precision and including a 1% standardization error. Unless otherwise specified, all errors are reported at 2σ confidence [46].

4.2. Whole-Rock Major and Trace Elements For major-element analyses, samples with fluorescent agents were mixed and fused into glass disks. Whole-rock major element analysis was done by X-ray fluorescence (XRF-1500) at the IGGCAS, with an analytical precision better than 1%. Standard GSR-3 was used to monitor the analytical accuracy. Trace element concentrations were determined by ICP-MS on a solution basis, using the techniques described by [47]. An internal standard Rh solution monitored counting signal drift. According to duplicate analyses on these standards, the precision is generally better than 5% for most trace elements.

4.3. In Situ Zircon Hf and O Isotopes Analysis In situ zircon oxygen isotopes were also conducted using the same Cameca ion microprobe at IGGCAS. Isotopes 16O and 18O were collected simultaneously using Faraday cups. The instrumental mass fraction factor (IMF) was corrected using standard zircon Penglai. Measured 16O/18O ratios were normalized using Vienna Standard Mean Water composition (VSMOW, 16O/18O = 0.0020052). The detailed analytical procedures were similar to those described by [44]. After the oxygen isotopic analyses, the sample mountings’ conductive coatings were removed, but the grains were not repolished, leaving the analyzed spots visible to guide the Hf analyses. Zircon Hf isotope analyses were conducted by MC-ICP-MS using a Neptune system. The detailed analytical Minerals 2020, 10, x FOR PEER REVIEW 6 of 20

MineralsAfter2020 the, 10, 1059oxygen isotopic analyses, the sample mountings’ conductive coatings were removed,6 of 20 but the grains were not repolished, leaving the analyzed spots visible to guide the Hf analyses. Zircon Hf isotope analyses were conducted by MC-ICP-MS using a Neptune system. The detailed analytical procedure and correction for interferences were followed as that described by [ 48]. During During analysis, analysis, the 176Hf/177Hf ratio of the standard zircon GJ-1 was 0.2820217 1.3 (2σ, n = 21), similar to the the 176Hf/177Hf ratio of the standard zircon GJ-1 was 0.2820217 ±± 1.3 (2σ, n = 21), similar to the 176 177 σ commonly accepted 176Hf/Hf/177HfHf ratio ratio of of 0.281999 0.281999 ± 88 (2 (2σ, ,nn == 10)10) measured measured using using the the solution solution method. method. ± 176 177 The zircon HfHf isotopicisotopic compositionscompositions ofof thethe granitesgranites (the(the initialinitial 176HfHf//177HfHf ratio ratio for for each spot) were calculated usingusing thethe U-Pb U-Pb crystallization crystallization date date of theof the grain grain or the or weighted the weighted mean mean age of age the hostof the granite. host The age-corrected 176Hf/177Hf ratio was then expressed as ε (t), calculated using the parameters of granite. The age-corrected 176Hf/177Hf ratio was then expressedHf as εHf(t), calculated using the 176 177 176 177 parametersHf/ Hf =of 0.282785176Hf/177Hf and = 0.282785Lu/ andHf = 1760.0336Lu/177Hf [49 =]. 0.0336 [49]. 5. Analytical Results 5. Analytical Results 5.1. Zircon U-Pb Ages 5.1. Zircon U-Pb Ages Zircon grains from the biotite granite and biotite monzogranite are colorless and transparent, behavingZircon homogeneous grains from the and biotite compositionally granite and zoning biotite inmonzogranite CL images (Figureare colorless4). They and are transparent, euhedral, prisms,behaving and homogeneous pyramids in and shape, composit rangingionally from 80zoning to 250 inµ mCL in images length. (Figure Concentric 4). They oscillatory are euhedral, zonings prisms,are generally and pyramids developed in inshape, zircon ranging grains, from whose 80 to cores 250 exhibitμm in length. segments Concentric with sector oscillatory zoning. zonings Only a arefew generally zircon grains developed contain in ovoid zircon cores. grains, Others whose show co typicalres exhibit oscillatory segments zoning with without sector zoning. inherited Only cores. a fewZircons zircon in Xitiangrainsgranites contain haveovoid variable cores. Others Th (124–3650 show typical ppm) oscillatory and U (209–3596 zoning ppm) without contents, inherited with cores. Th/U Zirconsratios of in 0.1–1.7. Xitian Allgranites these have characteristics variable Th suggest (124–3650 that theseppm) zirconsand U (209–3596 are magmatic ppm) in contents, origin [50 with–52]. TheTh/U analytical ratios of 0.1–1.7. results ofAll zircon these U-Pbcharacteristics dating are suggest summarized that these in Supplementary zircons are magmatic Table S1. in Despiteorigin [50– the 52].captured The analytical and inherited results crystals, of zircon zircon U-Pb grains dating (2706) are summarized from the biotite in Supplementary monzogranite Table yield aS1. weighted Despite themean captured206Pb/238 andU age inherited of 150.5 crystals,1.1 Ma(MSWD zircon =grains0.95, Figure(2706)5 ).from For thethe Xitianbiotite intrusion, monzogranite samples yield of the a 206 238 ± weightedbiotite granite mean (2708) Pb/ giveU aage weighted of 150.5 mean ± 1.1206 MaPb (MSWD/238U age = of0.95, 151.3 Figure1.8 5). Ma For (MSWD the Xitian= 1.6, intrusion, Figure5), 206 238 ± whichsamples is of the the best biotite estimate granite of the (2708) magma give crystallization a weighted mean time. Pb/ U age of 151.3 ± 1.8 Ma (MSWD = 1.6, Figure 5), which is the best estimate of the magma crystallization time.

Figure 4. Cathodoluminescence (CL) images of representative zircons from (a) biotite monzogranite, Figure 4. Cathodoluminescence (CL) images of representative zircons from (a) biotite monzogranite, (b) biotite granite, and (c,d) microgranlar enclaves. The circles show ing the locations of in situ U-Pb, (b) biotite granite, and (c,d) microgranlar enclaves. The circles show ing the locations of in situ U-Pb, Hf, and O isotopes analyses. Hf, and O isotopes analyses.

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Zircon grains from microgranular enclaves are stubby to prismatic, whose lengths range from 100 to 200 μm with length/width ratios of 1.1–1.5. They have variable Th/U ratios (Th/U = 0.1–1.2). In general, these zircons are brown or colorless and have a few snatchy cracks in their crystals, suggesting a magmatic origin [50–52]. The analytical results of zircon U-Pb dating are summarized in Supplementary Table S2. Fifteen spots analyses of sample (XTS-1) plot close to the Concordia curve (Figure 5) and yield a weighted mean age of 153.5 ± 1.8 Ma (MSWD = 1.3). In addition, the remaining 11 spots analyses of XTS-2b yield a weighted mean 206Pb/238U ages of 154.2 ± 1.5 Ma (MSWD = 6.1), which is interpreted as the best estimate of the crystallization age (Figure 5). The ages within Mineralsanalytical2020 error, 10, 1059 suggest that the host granites and microgranular from the Xitian intrusion are coeval7 of 20 within the error.

Figure 5. Zircon U-Pb ages of the host granites (a,b) and MEs (c,d) from the Xitian intrusion. Figure 5. Zircon U-Pb ages of the host granites (a,b) and MEs (c,d) from the Xitian intrusion.

5.2. Whole-RockZircon grains Major from and microgranular Trace Element enclaves are stubby to prismatic, whose lengths range from 100 toBased 200 µ onm the with whole-rock length/width major ratios element of 1.1–1.5. result (Su Theypplementary have variable Table Th S3),/U ratiosthe biotite (Th/ Ugranites= 0.1–1.2). and Inbiotite general, monzogranite these zircons from are the brown Xitian orintrusion colorless are and weakly have peraluminous a few snatchy with cracks A/CNK in their (molar crystals, ratios suggestingAl2O3/[CaO a + magmatic Na2O + K origin2O]) index [50–52 values]. The analyticalaround 1.1, results belonging of zircon to U-Pbthe high-K dating calc-alkaline are summarized series in Supplementary(Figure 6). They Table exhibit S2. high Fifteen SiO spots2 (70.6–77.9 analyses wt.%), of sample slightly (XTS-1) high plotK2O close(3.9–5.6 to thewt.%), Concordia and constant curve (Figure5) and yield a weighted mean age of 153.5 1.8 Ma (MSWD = 1.3). In addition, the remaining Na2O (3.0 to 4.0 wt.%). They contain Al2O3 varying± from 12.2–14.2 wt.% and low CaO contents (0.6– 11 spots analyses of XTS-2b yield a weighted mean 206Pb/238U ages of 154.2 1.5 Ma (MSWD = 6.1), 1.6 wt.%). MEs are intermediate to felsic in composition (Figure 6a) and exhibit± variable SiO2 from which59.0 wt.% is interpreted to 72.0 wt.%. as the Their best A/CNK estimate values of the crystallizationrange from 0.95–1.98 age (Figure (Figure5). The 6a). ages These within MEs analytical have low errorand variable suggest MgO that the(0.09–0.17 host granites wt.%) andand microgranularrelatively low TFe from2O the3 values Xitian varying intrusion from are 0.23–0.52 coeval within wt.%. theThe error. host granites have significantly higher K2O contents than MEs (Supplementary Table S3). Consequently, these granites and monzogranites are similar to the shoshonitic series (Figure 6b). In 5.2. Whole-Rock Major and Trace Element the Harker diagrams, host granites (biotite monzogranite and biotite granite) and microgranular enclavesBased define on the broadly whole-rock linear major trends. element Their result CaO, (Supplementary Al2O3, TiO2, and Table P2 S3),O5 contents the biotite decrease granites with and biotiteincreasing monzogranite SiO2 contents from (Figure the Xitian 7). intrusion are weakly peraluminous with A/CNK (molar ratios Al2O3/[CaO + Na2O + K2O]) index values around 1.1, belonging to the high-K calc-alkaline series (Figure 6). They exhibit high SiO 2 (70.6–77.9 wt.%), slightly high K2O (3.9–5.6 wt.%), and constant Na2O (3.0 to 4.0 wt.%). They contain Al2O3 varying from 12.2–14.2 wt.% and low CaO contents (0.6–1.6 wt.%). MEs are intermediate to felsic in composition (Figure6a) and exhibit variable SiO 2 from 59.0 wt.% to 72.0 wt.%. Their A/CNK values range from 0.95–1.98 (Figure6a). These MEs have low and variable MgO (0.09–0.17 wt.%) and relatively low TFe2O3 values varying from 0.23–0.52 wt.%. The host granites have significantly higher K2O contents than MEs (Supplementary Table S3). Consequently, these granites and monzogranites are similar to the shoshonitic series (Figure6b). In the Harker diagrams, host granites (biotite monzogranite and biotite granite) and microgranular enclaves define Minerals 2020, 10, 1059 8 of 20

broadly linear trends. Their CaO, Al2O3, TiO2, and P2O5 contents decrease with increasing SiO2 contents (Figure7). Minerals 2020, 10, x FOR PEER REVIEW 8 of 20 Minerals 2020, 10, x FOR PEER REVIEW 8 of 20

2 2 FigureFigureFigure 6. 6. 6.( a ((a)Aa)) A/CNKA/CNK/CNK vs. vs.vs. A A/NK/NKdiagram diagram [ [53];53[53];]; ( (b(bb))) SiOSiO SiO22 vs. vs. K KK22OO diagram diagram (solid (solid lines lineslines from fromfrom [54].) [[54].)54]).

Figure 7. Harker diagrams showing a variation of selected major oxides (e.g., TiO2 (a), CaO (b), Al2O3 FigureFigure 7.7. HarkerHarker diagrams diagrams showing showing a variation a variation of selected of selected major major oxides oxides (e.g., (e.g., TiO2 TiO(a), 2CaO(a), ( CaOb), Al (2bO3), (c), P2O5 (d)) versus SiO2 in the Xitian intrusion. Al(c2),O3 P2O (c5), (d P))2O versus5 (d)) versusSiO2 in SiOthe2 Xitianin the intrusion. Xitian intrusion. Primitive mantle-normalized trace element patterns for granites and MEs show the enrichments PrimitivePrimitive mantle-normalizedmantle-normalized tracetrace elementelement patternspatterns forfor granitesgranites andand MEsMEs showshow thethe enrichmentsenrichments in Rb, Th, and U and depletions in Ba, Sr, Eu, and Nb (Figure 8a). Therefore, they both have high inin Rb, Rb, Th, Th, and and U U and and depletions depletions in Ba,in Ba, Sr, Eu,Sr, Eu, and and Nb (FigureNb (Figure8a). Therefore, 8a). Therefore, they both they have both high have Rb high/Sr Rb/Sr (4.1–39.6) and Rb/Ba (1.2–29.6). Chondrite-normalized REE patterns (Figure 8b) for the biotite (4.1–39.6)Rb/Sr (4.1–39.6) and Rb /andBa (1.2–29.6). Rb/Ba (1.2–29.6). Chondrite-normalized Chondrite-normalized REE patterns REE patterns (Figure8 b)(Figure for the 8b) biotite for the granites biotite granites and biotite monzogranite show a different enrichment of heavy rare earth elements (HREEs). andgranites biotite and monzogranite biotite monzogranite show a di ffshowerent a enrichment different enrichment of heavy rare of heavy earth elementsrare earth (HREEs). elements Both (HREEs). host Both host granites and MEs are rich in LREE with Eu negative anomaly (σEu = 0.007–0.146). The MEs granitesBoth host and granites MEs are and rich MEs in LREEare rich with in LREE Eu negative with Eu anomaly negative (σ anomalyEu = 0.007–0.146). (σEu = 0.007–0.146). The MEs and The some MEs granitesand some show granites a smaller show Eu a anomalysmaller Eu and anomaly a minor and amount a minor of HREE. amount The of detailedHREE. The trace detailed element trace data andelement some data granites from theshow Xitian a smaller intrusion Eu are anomaly listed in and Suppl a minorementary amount Table of S4. HREE. The detailed trace fromelement the Xitiandata from intrusion the Xitian are listed intrusion in Supplementary are listed in Suppl Tableementary S4. Table S4.

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8 Figure 8.. (a) Primitive-normalized(a) Primitive-normalized trace element trace elementspider diagram spider of diagram the Xitian of intrusion; the Xitian (b) Chondrite- intrusion; (normalizedb) Chondrite-normalized REE patterns for REE the patterns Xitian intrusion. for the Xitian Normalized intrusion. values Normalized are from values [55]. are from [55].

5.3. Zircon Hf and O Isotopes Five samples, samples, including including two two biotite biotite monzogranite monzogranite (2609 (2609 and and 2706), 2706), one one biotite biotite granite granite (2708), (2708), and andtheir their microgranular microgranular enclave enclave (XTS-1 (XTS-1 and and XTS-2b), XTS-2b), were were chosen chosen for for zircon zircon Hf Hf and and O O isotopic isotopic analysis using LA-ICP-MS and SIMS, respectively. Another fivefive samples, including two biotite monzogranite (2702 and 2704) and three biotite granites (2614, 2711, and 2713), were selected for zircon Hf isotopic analysis. The results are shownshown in Supplementary Table S5.S5. Hf isotope analyses were conducted on zircon grains that were previously analyzed for U-Pb dating. Zircons from the MEs have similar Hf isotopicisotopic compositions compared with the host biotite granites (Figure 99).). Most of the the analyzed analyzed zircon zircon grains grains show show low low 176176Lu/Lu177/Hf177 Hfratios ratios (<0.002); (<0.002); thus, thus, the theanalytical analytical 176Hf/176177HfHf/177 ratiosHf ratios can be can used be usedto represent to represent the isot theopic isotopic compositions compositions of magmas of magmas at the at time the timeof formation of formation [48]. The [48]. dated The dated at about at about 150 Ma 150 magmatic Ma magmatic zircons zircons from fromMEs samples MEs samples (XTS-1 (XTS-1 and XTS- and 176 176177 177 XTS-2b)2b) have haveHf/ Hf/ ratiosHf ratiosranging ranging from 0.282448 from 0.282448 to 0.282600, to 0.282600, corresponding corresponding to εHf(t) to εvaluesHf(t) values from − from8.16 to8.16 −2.93 to (average2.93 (average of −5.61). of Zircons5.61). Zircons from the from granites the granites (samples (samples 2609, 2706, 2609, 2702, 2706, 2704, 2702, 2708, 2704, 2614, 2708, 2711, 2614, − − − 2711,and 2713) and 2713)have havevariable variable Hf isotopic Hf isotopic compositions compositions with with εHfε(t) (t)values values ranging ranging from from −9.089.08 to to 0.32 Hf − (average ofof −6.19). The granites and MEs have two-stage HfHf modelmodel ageage (T(TDM2)) values values of of 1.18–1.78 Ga − DM2 and 1.39–1.72 Ga, respectively.respectively. Three grains have thethe εεHfHf(t)(t) values around zero, indicatingindicating mantlemantle involvement in their genesis. Zircons from the ME MEss have similar O isotopic compositions compared with the host granites (Figure9 9).). ZirconsZircons fromfrom thethe MEsMEs (samples(samples XTS-1XTS-1 andand XTS-2b)XTS-2b) havehave OO isotopicisotopic compositions with δ1818O values ranging from 8.4%8.4‰ to 9.3%9.3‰.. Eighteen Eighteen spot spot analyses were performed on the biotitebiotite monzogranitemonzogranite (2609),(2609), yieldingyielding δδ1818O valuesvalues fromfrom 8.6%8.6‰ toto 9.4%9.4‰.. Fifteen-spot analyses were performedperformed onon thethe granitesgranites (2706),(2706), yieldingyielding δδ1818O values fromfrom 8.4%8.4‰ to 9.5%9.5‰.. Zircon grains from biotite granites (2708) (2708) have have similar similar δδ1818OO values values from from 8.1‰ 8.1% to 9.2‰.to 9.2% The. Theindistinguishable indistinguishable Lu-Hf Lu-Hf and andO isotope O isotope compositions compositions between between biotite biotite granites granites and and biotite biotite monzogranites monzogranites further further confirm confirm their genetic relation. Moreover, both zircon grains of MEs and host granites havehave similar Lu-Hf isotope compositions, indicating their parental magmasmagmas areare fromfrom thethe samesame source.source.

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Figure 9. The histogram of εHf(t) (a,c,d) and δ1818O values (b,e,f) from the Xitian intrusion and MEs. Figure 9. The histogram of εHf(t) (a,c,d) and δ O values (b,e,f) from the Xitian intrusion and MEs.

6.6. DiscussionDiscussion

6.1.6.1. OriginOrigin ofof thethe MEsMEs IgneousIgneous microgranularmicrogranular enclavesenclaves areare widespreadwidespread inin graniticgranitic rocksrocks andand point point to to magma magma mixing mixing processesprocesses [ 56[56].]. However, However, enclaves enclaves are are common common in graniticin granitic rocks, rocks, and and their their presence presence should should not be not used be toused infer to any infer particular any particular tectonic tectonic setting [setting57] specifically. [57] specifically. Nevertheless, Nevertheless, there are there considerable are considerable models proposedmodels proposed to decipher to thedecipher origins the of MEs,origins for of instance, MEs, for whether instance, they whether are restites they of are source restites rocks of [ 13source,14], autolithsrocks [13,14], [58,59 autoliths], or they [58,59], were formed or they by were the mantle-crustformed by the magma mantle-crust mixing magma [12,60]. mixing [12,60]. InIn general, general, restite restite is is entrained entrained by by a a partial partial melt melt and and is is considered considered to to represent represent the the magmatically magmatically equilibratedequilibrated source source material material [13 [13].]. The The chemical chemical composition composition of restiteof restite is usually is usually complementary complementary to the to meltthe melt component component in the in host the host magma. magma. The xenolithThe xenolith is formed is formed from from an older an older melting melting event event and oftenand often has magmatichas magmatic reaction reaction textures textures [60]. The[60]. MEs The andMEs their and hosttheir graniteshost granites show show similar similar zircon zircon U-Pb agesU-Pb of ages ca. 150of ca. Ma 150 (Figure Ma (Figure5), indicating 5), indicating that they that are they formed are formed by a contemporaneous by a contemporaneous magmatic magmatic event ratherevent rather than residuesthan residues of partial of partial melting melting or xenoliths or xenoliths of the of country the country rocks. rocks. TheThe magmamagma mixing model model is is the the most most common common and and prevailing prevailing mechanism mechanism to interpret to interpret the origin the originof MEs of in MEs granites, in granites, especially especially in South in SouthChina China [6,7,61]. [6, 7Generally,,61]. Generally, there thereare significant are significant contrasts contrasts of the chemical and isotopic compositions between MEs and the host granites derived from the mixing magma [62]. In our study, MEs and the host granites from the Xitian intrusion have a similar isotopic

Minerals 2020, 10, 1059 11 of 20 of the chemical and isotopic compositions between MEs and the host granites derived from the mixing magma [62]. In our study, MEs and the host granites from the Xitian intrusion have a similar isotopic character. Previous studies suggested that similar chemical affinities are considered to be the consequence of the complete equilibrate of dissimilar materials representing variable degrees of diffusive exchange between the felsic and more mafic magmas during slow cooling [8,63]. However, some results show that this isotopic equilibration can only be partially achieved. In some cases, highly mobile elements (especially alkali metal) equilibrate quickly than other elements [64]. So it is physically unlikely for isotopic ratios to be homogenized, whereas major and trace elements are not [65]. The Hf isotopic and O compositions of magmatic zircon grains from the MEs in the Xitian intrusion exhibit isotopic features similar to those of their host granites, requiring a common source or complete equilibrate of dissimilar materials. In this situation, only minerals and textural evidence can be used to determine the processes whereby the MEs from the Xitian intrusion are formed. The MEs are commonly darker than their host rocks in color and have a large volume of biotite. The textural relationship of porphyritic feldspar and MEs suggest the disequilibrium during melting (Figure3g). Meanwhile, the features of the subhedral to allotriomorphic biotite and needle-like apatite, as observed in MEs from the Xitian intrusion, indicate a rapid cooling process. The similarity of enclaves with different colors in the Xitian intrusion suggests that they are probably formed from their host granites by kinetically induced, accelerated crystallization of ferromagnesian minerals, particularly biotite, related to rapid cooling [16]. All characters imply the MEs and their host granites were from the same parental magma rather than late isotope equilibrium, which excludes the magma mixing processes. Numerical models show that the granite magmas underwent a short period of chaotic convection and were generated in the upper cooling zone by melt-to-solid phase transformation [66]. These autolithic enclaves can be preserved in the chamber’s upper and hotter interior and dragged downwards as descending convecting fingers. Such enclaves contain low-SiO2 and high-temperature minerals phases [67,68], which are coincident with the MEs in Xitian (Supplementary Table S4).

6.2. Petrogenesis of the Xitian Intrusion The Xitian host granites are devoid of a characteristic of Al-rich minerals (e.g., cordierite and muscovite) and have a slightly negative correlation between P2O5 and SiO2 (Figure7), excluding S-type [69,70]. The weakly peraluminous feature of the Xitian host granites also precludes they are differentiated from a sedimentary source because this process would produce strongly peraluminous melts [71]. The host granites of the Xitian intrusion are high-K calc-alkaline rocks with much lower TFe2O3/MgO ratios (2.88–11.64) than the A-type granites (mean TFe2O3/MgO = 13.4). The temperature calculated by the geothermometer of Waston suggests that the cooling temperature of the Xitian granite (Mean = 781 ◦C, Supplementary Table S4) is lower than the temperature of typical A-type granite [72–74]. In contrast, all the above features of the Xitian intrusion suggest that the Xitian granite belongs to I-type, derived from dehydration melting of igneous rock or their metamorphic equivalents [75]. Both the host granites and the MEs display negative anomalies of Eu, Ba, Sr, Zr, P and Ti (Figure8a). The decreasing trends of Al2O3, TiO2, and CaO with increasing SiO2 for the Xitian host granites and the MEs are related to ilmenite, plagioclase, and biotite presence (Figure7). It is worth noting that these chemical variations of host granites are in accordance with the crystal fractionation sequence of ferromagnesian minerals, plagioclase, Ti-Fe oxides, and apatite. Garnet in the residue could result in substantial depletion of HREE in produced melts because of its extremely high partition coefficients for the HREE [76]. So the flat HREE patterns and higher HREE concentration may not be responsible for the entertainment of garnet in the residue. However, the hydrothermal process could justify the abundance of HREE [77]. Because of the physical and chemical properties, the content of rare earth elements in nature presents four groups of upper convex or lower concave curves [78]. According to the formula for quantitative analysis of the intensity of the four grouping effects proposed by [79], when t > 0, rare earth elements’ distribution form presents the Minerals 2020, 10, 1059 12 of 20

four grouping effects. It can be concluded that the Xitian granite has an obvious four grouping effects (Figure 10). A lot of studies suggest that the four-group effect of rare earth elements in granite is the Mineralsfinal result 2020, 10 of, x rock-fluid FOR PEER interactionREVIEW produced by highly evolved magma experiencing the separation12 of 20 ofMinerals melt-fluid 2020, 10 and, x FOR fluid-gas PEER REVIEW [77,80 ]. It can be found that the late Jurassic Xitian pluton underwent12 of 20 separationmagmatic crystallizationof melt-fluid and and fluid-gas hydrothermal [77,80]. magmatism It can be process.found that the late Jurassic Xitian pluton underwentseparation magmaticof melt-fluid crystallization and fluid-gas and [77,80]. hydrothermal It can bemagmatism found that process. the late Jurassic Xitian pluton underwent magmatic crystallization and hydrothermal magmatism process.

FigureFigure 10. 10. TheThe four four group group effects effects of of rare rare earth earth elements elements in in the the Xitian Xitian intrusion. intrusion. The The data data of of Qianlishan Qianlishan Figure 10. The four group effects of rare earth elements in the Xitian intrusion. The data of Qianlishan plutonpluton are are from from [77]. [77]. pluton are from [77]. InIn the the process process of of magmatic magmatic evolution, evolution, granite granite can can lead lead to a to significant a significant decrease decrease in Cr, in Ni, Cr, Sr, Ni, Ba Sr, a,Ba, as aswellIn well the as process asa significant a significant of magmatic increase increase evolution, in inthe the contentcontent granite of ofcanLi, Li, Rb, lead Rb, and to and aCs significant Cs[81]. [81 Sr]. is Sr decrease mainly is mainly concentrated in concentratedCr, Ni, Sr, inBa magma’sina, as magma’s well early as earlya significantstage, stage, while while increase Rb Rbis enrichedin is enrichedthe content in the in of there Li,sidual residual Rb, andmelts. melts. Cs Therefore, [81]. Therefore, Sr is mainlyRb/Sr Rb /ratio Srconcentrated ratio is close is close to in constanttomagma’s constant in early the in theearly stage, early stage while stage of magma Rb of magmais enriched evolution, evolution, in withthe re withansidual average an averagemelts. value Therefore, valueslightly slightly less Rb/Sr than less ratio 0.5 than [82].is 0.5close With [82 to]. theWithconstant enhancement the enhancementin the early of differentiation, stage of di offf erentiation,magma Rb/Sr evolution, Rbratio/Sr rapi ratiowithdly rapidlyan increases average increases valueto more toslightly morethan less than10, sothan 10, Rb/Sr so0.5 Rb [82].ratio/Sr With ratiocan becanthe used beenhancement used to represent to represent of thedifferentiation, thedegree degree of differentiation. of Rb/Sr differentiation. ratio rapi It dlycan It can increases be befound found to that more that both both than Nb/Ta Nb 10,/Ta soand and Rb/Sr Zr/Hf Zr/ Hfratio ratios ratios can showshowbe used a a significant significant to represent decrease decrease the degree with with theof the differentiation. in increasecrease of of evolution evolution It can bedegree found (Figure that both 11).11). Nb/Ta and Zr/Hf ratios show a significant decrease with the increase of evolution degree (Figure 11).

Figure 11. Variations of Rb/Sr versus Nb/Ta (a) and Zr/Hf (b) from the Xitian intrusion. Figure 11. Variations of RbRb/Sr/Sr versus NbNb/Ta/Ta ((aa)) andand ZrZr/Hf/Hf ((bb)) fromfrom thethe XitianXitian intrusion.intrusion. The variable range of Nb/Ta and Zr/Hf ratios are traditionally considered as the result from the separationTheThe variablevariable and crystallization range of NbNb/Ta /Taof accessoryandand ZrZr/Hf/Hf minera ratiosratios arearels containing traditionallytraditionally these consideredconsidered elements asas [83,84]. thethe resultresult Rutile fromfrom and thethe ilmeniteseparationseparation are andandessential crystallizationcrystallization carrier minerals ofof accessoryaccessory of Nb and mineralsminera Ta. Sincels containingcontaining the DNb/Ta thesethese in rutile elementselem andents ilmenite [[83,84].83,84]. is RutileRutile less than andand 1,ilmeniteilmenite the separation areare essentialessential and crystallization carriercarrier mineralsminerals of of ofrutile Nb Nb and and Ta. Ta.ilmenite Since Since thewill the D DdecreaseNbNb/Ta/Ta in rutile the Nb and content ilmenite and isis produce lessless thanthan an1,1, thetheelevated separationseparation Nb/Ta andand ratio crystallizationcrystallization in the residual ofof rutilerutile magma andand [85] ilmeniteilmenite. However, willwill decreasedecrease the Nb thethecontent NbNb contentcontent in the andlateand produceJurassicproduce graniteanan elevatedelevated in the NbNb/TaXitian/Ta ratiointrusionratio inin thethe increases residualresidual significantl magmamagma [[85]85y with].. However,However, the increase thethe NbofNb the contentcontent degree inin of thethe differentiation latelate JurassicJurassic (Figuregranite 11). in the It suggests Xitian intrusion that the increasesseparation significantl and crystallizationy with the of increase Ti-rich of minerals the degree is not of thedifferentiation reason for the(Figure Nb/Ta 11). fractionation. It suggests that A previousthe separation study and has crystallization shown that the of Ti-richaddition minerals of fluid is containing not the reason F will for reducethe Nb/Ta the viscosityfractionation. of magma A previous and the study temperature has shown of the that solid the phase addition line, of resulting fluid containing in the increase F will ofreduce the solubility the viscosity of Nb ofover magma Ta in silicateand the melts, temperature Ta being of less the soluble solid phase [86]. Thus,line, resulting the Nb and in Tathe contents increase willof the increase solubility while of Nb the over Nb/Ta Ta inratio silicate decrease melts, (F Taigure being 12). less Some soluble researchers [86]. Thus, summarized the Nb and Tathe contents Nb/Ta ratiowill increaseof granite while related the Nb/Ta to tungsten ratio decrease and tin (F iguremineralization 12). Some andresearchers divided summarized the petrogenesis the Nb/Ta of ratio of granite related to tungsten and tin mineralization and divided the petrogenesis of

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Mineralsperaluminous2020, 10, 1059 granite into normal crystallization differentiation and magma-hydrothermal13 of 20 interaction by the index of Nb/Ta = 5 [87]. The majority of Nb/Ta ratio in the late Jurassic granite of the Xitian intrusion are below five, indicating the magma forming process was related to magmatic- granite in the Xitian intrusion increases significantly with the increase of the degree of differentiation hydrothermal processes (Supplementary Table S4). (Figure 11). It suggests that the separation and crystallization of Ti-rich minerals is not the reason for Zircon is the main carrier mineral of Zr and Hf in granite. When magma reaches Zr saturation, the Nb/Ta fractionation. A previous study has shown that the addition of fluid containing F will reduce it will crystallize zircon. Furthermore, because of the high Zr/Hf ratio (generally higher than 40) in the viscosity of magma and the temperature of the solid phase line, resulting in the increase of the zircon, the Zr/Hf ratio in the residual magma will significantly decrease after the separation and solubility of Nb over Ta in silicate melts, Ta being less soluble [86]. Thus, the Nb and Ta contents will crystallization. However, the Zr/Hf ratio (Figure 11) and the Zr content in the Xitian intrusion increase while the Nb/Ta ratio decrease (Figure 12). Some researchers summarized the Nb/Ta ratio decreased significantly with the increase of the degree of differentiation (Figure 12). Also, the Hf of granite related to tungsten and tin mineralization and divided the petrogenesis of peraluminous content of the Xitian late Jurassic granite did not decrease during the fractional process (Figure 12), granite into normal crystallization differentiation and magma-hydrothermal interaction by the index which indicates that the variation of Zr/Hf ratio in the Xitian intrusion was not only due to the zircons of Nb/Ta = 5 [87]. The majority of Nb/Ta ratio in the late Jurassic granite of the Xitian intrusion are crystallization. A previous study also showed that the Zr/Hf ratio, which decreases significantly with below five, indicating the magma forming process was related to magmatic-hydrothermal processes magmatic differentiation, is related to the magmatic system rich in F. It will increase the solubility of (Supplementary Table S4). Zr and Hf, thus making Zr/Hf significantly differentiated [84].

FigureFigure 12.12. VariationsVariations ofof RbRb/Sr/Sr versusversus thethe concentrationconcentration ofof NbNb ((aa),), TaTa ((bb),), ZrZr ((cc),), andand HfHf ((dd)) fromfrom thethe XitianXitian intrusion.intrusion.

6.3. ImplicationsZircon is the for main Geodynam carrieric mineralSetting and ofZr Mineralization and Hf in granite. When magma reaches Zr saturation, it willAs crystallize stated above, zircon. fractional Furthermore, crystallization because ofimplies the high effective Zr/Hf separation ratio (generally of crystal higher and than liquid 40) infractions, zircon, thewhich Zr/Hf modifies ratio in the residualchemical magma equilibr willium significantly and enables decrease magmatic after differentiation. the separation andThe crystallization.separation of the However, early-crystallized the Zr/Hf ratio minerals (Figure fr 11om) and melt the would Zr content cause in the Xitianresidual intrusion magma decreased to move significantlyupwards through with thebuoyancy increase and of thebeing degree enriched of di ffinerentiation Si, Al, Na, (FigureK, and volatiles12). Also, [88,89]. the Hf Calculations content of the of Xitianmelt density late Jurassic changes granite during did notfractionation decrease duringshow th theat fractionalcompositional process effects (Figure on 12density), which are indicates usually thatgreater the variationthan associated of Zr/Hf thermal ratio in effects the Xitian [90]. intrusionThe magma was chamber not only in due the to middle the zircons crustcrystallization. is under large Astatic previous pressure, study where also the showed volatiles that can the be Zr highly/Hf ratio, dissolved which in decreases the residual significantly magma [88]. with The magmatic volatiles differentiation, is related to the magmatic system rich in F. It will increase the solubility of Zr and Hf, (F, Cl, CO2, etc.,) can decrease the solidus temperature of the silicate melt, prolonging the process of thusfractional making crystallization Zr/Hf significantly [91]. Recent differentiated studies [show84]. that the injection of mafic magma into an early formed crystal-rich chamber would add heat and decrease magma viscosity, which can significantly

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6.3. Implications for Geodynamic Setting and Mineralization As stated above, fractional crystallization implies effective separation of crystal and liquid fractions, which modifies the chemical equilibrium and enables magmatic differentiation. The separation of the early-crystallizedMinerals 2020, 10, x FOR minerals PEER REVIEW from melt would cause the residual magma to move upwards through14 of 20 buoyancy and being enriched in Si, Al, Na, K, and volatiles [88,89]. Calculations of melt density promote the fractional crystallization process [92,93]. Mantle-derived mafic magmas are not changes during fractionation show that compositional effects on density are usually greater than necessary for direct material contributions to such a magma mixing but may provide a critical heat associated thermal effects [90]. The magma chamber in the middle crust is under large static pressure, source for crustal melting [62]. where the volatiles can be highly dissolved in the residual magma [88]. The volatiles (F, Cl, CO , Some researchers interpreted the Jurassic magmatism in South China as products due to the2 etc.,) can decrease the solidus temperature of the silicate melt, prolonging the process of fractional upwelling of the asthenosphere mantle, which is in response to the full-scale slab foundering and crystallization [91]. Recent studies show that the injection of mafic magma into an early formed slab rollback [94]. According to the Pacific plate subduction model proposed by [94], the basalt layer crystal-rich chamber would add heat and decrease magma viscosity, which can significantly promote in the lower middle crust of the Cathaysia block was probably closely related to the mantle upwelling the fractional crystallization process [92,93]. Mantle-derived mafic magmas are not necessary for direct in the asthenosphere caused by Pacific subduction during the Mesozoic [30]. After studied basalts of material contributions to such a magma mixing but may provide a critical heat source for crustal different ages in the South China plate, some scholars found that plate fragmentation occurred at melting [62]. 110~160 Ma, which caused disturbance of the asthenosphere and extensive magmatism in the Some researchers interpreted the Jurassic magmatism in South China as products due to the Yanshan Period of South China, and large-scale polymetallic mineralization occurred in this period upwelling of the asthenosphere mantle, which is in response to the full-scale slab foundering and slab [95]. In our study, the existence of εHf(t) value above zero also suggests the involvement of the mantle rollback [94]. According to the Pacific plate subduction model proposed by [94], the basalt layer in component (Figure 13). Many researchers believe that fluids from the mantle are supercritical fluids, the lower middle crust of the Cathaysia block was probably closely related to the mantle upwelling which are easy to form in subduction zones due to their characteristics of high temperature and high in the asthenosphere caused by Pacific subduction during the Mesozoic [30]. After studied basalts pressure, and are important media for element migration and mantle metasomatism in subduction of different ages in the South China plate, some scholars found that plate fragmentation occurred zones [96,97]. With the upwelling of the asthenosphere mantle, the mantle-derived fluid could extract at 110~160 Ma, which caused disturbance of the asthenosphere and extensive magmatism in the the metallogenic element efficiently [98,99]. Cavity expansion generates extreme reductions in Yanshan Period of South China, and large-scale polymetallic mineralization occurred in this period [95]. pressure that cause the fluid to flow and expand the vapor. Such flash vaporization of the fluid results In our study, the existence of ε (t) value above zero also suggests the involvement of the mantle in the co-deposition of silica andHf metal elements [100]. component (Figure 13). Many researchers believe that fluids from the mantle are supercritical fluids, As shown in Figure 13, all the MEs samples plot below the CHUR reference line. Moreover, the which are easy to form in subduction zones due to their characteristics of high temperature and high host granites have similar εHf(t) values with the MEs and the zircon two-stage Hf model ages are pressure, and are important media for element migration and mantle metasomatism in subduction similar to the Cathysia Block [101]. These data provide additional support to the origin of the MEs zones [96,97]. With the upwelling of the asthenosphere mantle, the mantle-derived fluid could extract and granites in the Xitian intrusion from a single isotopically homogeneous, lower crustal source. the metallogenic element efficiently [98,99]. Cavity expansion generates extreme reductions in pressure Accordingly, given all these observations, we propose that the partial melting of the lower crust with that cause the fluid to flow and expand the vapor. Such flash vaporization of the fluid results in the mantle fluid resulted in the origin of the Xitian intrusion. It resulted from a change in the geodynamic co-deposition of silica and metal elements [100]. regime, including break off/roll back of the subducted slab and subsequent regional extension.

Figure 13. (a) U-Pb ages vs. εHf(t) diagram of zircon of the host granites and (b) MEs from the Xitian Figure 13. (a) U-Pb ages vs. εHf(t) diagram of zircon of the host granites and (b) MEs from the Xitianintrusion. intrusion.

7. ConclusionsAs shown in Figure 13, all the MEs samples plot below the CHUR reference line. Moreover, the host granites have similar ε (t) values with the MEs and the zircon two-stage Hf model ages A comprehensive study concerningHf the Late Jurassic Xitian intrusion and associated MEs are similar to the Cathysia Block [101]. These data provide additional support to the origin of the provides evidence for the implication of geodynamic and mineralization in South China. MEs and granites in the Xitian intrusion from a single isotopically homogeneous, lower crustal source. (1) MEs and their host granites are coeval and were emplaced at ca. 152Ma. (2) MEs were autoliths that experienced a period of chaotic convection and rapid cooling. Biotite granite and biotite monzogranite were generated from igneous sources, and they show evidence of volatiles addition. (3) The mantle provides heat for crustal melting in our region. Separation and accumulation of evolved melts were facilitated by magma replenishment. It may result from the Pacific subduction.

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Accordingly, given all these observations, we propose that the partial melting of the lower crust with mantle fluid resulted in the origin of the Xitian intrusion. It resulted from a change in the geodynamic regime, including break off/roll back of the subducted slab and subsequent regional extension.

7. Conclusions A comprehensive study concerning the Late Jurassic Xitian intrusion and associated MEs provides evidence for the implication of geodynamic and mineralization in South China. (1) MEs and their host granites are coeval and were emplaced at ca. 152 Ma. (2) MEs were autoliths that experienced a period of chaotic convection and rapid cooling. Biotite granite and biotite monzogranite were generated from igneous sources, and they show evidence of volatiles addition. (3) The mantle provides heat for crustal melting in our region. Separation and accumulation of evolved melts were facilitated by magma replenishment. It may result from the Pacific subduction.

Supplementary Materials: The following are available online at http://www.mdpi.com/2075-163X/10/12/1059/s1. Author Contributions: Conceptualization, M.H. and Q.L.; Data curation, Q.L.; Formal analysis, M.H.; Funding acquisition, Q.H.; Investigation, M.H., Q.L., Q.H. and J.S.; Methodology, J.S.; Writing—original draft, M.H. writing—review and editing, Q.L. and Q.Y.; All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Key R&D Program of China (2016YFC0600401), Public Welfare Project of the Ministry of land and Resources of China (201211024-04). Acknowledgments: We thank Song G.X. and Zhang J.H. of the University of Chinese Academy of Sciences for assistance in major elements, trace elements, and zircon Hf-O isotope analyses. We thank Niurui at Henan Rock and Mineral Testing Center and Wu S.C., Zhu H.F. at 416 Brigade, Bureau of Geology and Mineral Exploration and Development of Hunan Province for their help with fieldwork. Conflicts of Interest: The authors declare no conflict of interest.

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