Petrogenesis Of Late Paleozoic To Early Mesozoic Granites In Southeastern Guangxi And Southwestern Guangdong Provinces And Their Geodynamic Implications

The geodynamic mechanism of the late Paleozoic to early Mesozoic magmatism in South China is always a hot topic of international interest.The late Paleozoic granites are rarely reported,while the early Mesozoic granitic magmatism is much more extensive and intense.In spite of decades of research,the geodynamic mechanism of the related granites has long been debated.The focus of the debate is whether the granitic magmatism is controlled by the Paleo-Tethys or Paleo-Pacific tectonic domains.This study first identified two late Permian S-type granites(Napeng and Shicun)and an earliest early Mesozoic A-type granite(Xinxing)in the Yunkai area and adjacent areas.In this study,on the basis of the field and petrographic works,we present systematically mineral chemistry,LA-ICP-MS zircon U-Pb dating,in-situ zircon trace elements and Hf isotopes,whole-rock geochemical and Sr-Nd isotopic research for these three plutons.We use these data to clarify their emplacement ages,confirm their genetic types,discuss their source characteristics,magmatic evolution and tectonic settings,and further define the geodynamic mechanism of the late Paleozoic to early Mesozoic magmatism in South China.Our study shows that:1.The Napeng pluton consists of cordierite-and/or garnet-bearing biotite granites and the Shicun pluton is composed of two-mica granites and muscovite granites.Detailed LA-ICP-MS zircon U-Pb dating shows that both plutons were emplaced in the late Permian(ca.259 Ma)and contain abundant inherited zircons with a spread of ages including Neoarchean-Paleoproterozoic(2682-2434 Ma),Neoproterozoic-Cambrian(980-503 Ma),Ordovician-Silurian(452-435 Ma),Devonian(412-391 Ma),Carboniferous(342-309 Ma)and early Permian(294-270 Ma).The cordierite-and/or garnet-bearing biotite granites show variable SiO₂ contents(66.7-76.9 wt.%)and are strongly peraluminous(ASI=1.31-2.40).They have higher maficity(Fe₂O₃^T+MgO>4.0wt.%)than experimental melts of metasedimentary rocks,and their ASI,TiO₂ and Yb all increase with increasing maficity.These granites have high zircon saturation temperatures(T_Zr,830-897oC)and high initial ⁸⁷Sr/⁸⁶Sr(0.7162-0.7261),negative?_Nd(t)(-10.8 to-13.5)and?_Hf(t)(in-situ zircon)(-9.0).The two-mica granites have SiO₂contents of 71.3-73.5 wt.%with ASI=1.04-1.21 and T_Zr=770-795oC.The muscovite granites have higher SiO₂(74.1-76.5 wt.%)and ASI(1.2-1.5)and lower T_Zr(736-759oC)than the two-mica granites.Both the two-mica and muscovite granites have low maficity(Fe₂O₃^T+MgO<4.0 wt.%)similar to experimental melts of metasedimentary rocks,and have high initial ⁸⁷Sr/⁸⁶Sr(0.7246-0.7280),negative?_Nd(t)(-9.4 to-11.8)and?_Hf(t)(in-situ zircon)(-6.1).Detailed petrographical and geochemical characteristics of the studied granites indicate that they belong to S-type granites and were formed in the continent-continent collison setting.The origin of the Napeng granites is as follows:biotite incongruent melting of subducted oceanic sediments of the eastern Paleo-Tethys Ocean occurred within the garnet stability field(>12 kbar)at temperarutes of 830-897?,generating strongly peraluminous granitic melt coexising with a garnent-dominated solid peritectic mineral assemblage.Such a granitic melt with the entrainment of different proportions of peritectic garnent and ilmenite ascended from the source and intruded high levels in the crust,during which some peritectic garnet broke down to cordierite at shallow depth on the ascent path.The origin of the Shicun granites can be sumarized as follows:the Shicun two-mica granites were formed by muscovite-dehydration melting of subducted oceanic sediments of the eastern Paleo-Tethys Ocean within the depth of normal lower crust in the continental collision zone at temperatures of 770-795?.Subsequent fractionation of such a partial melt by removal of plagioclase and biotite generated the magma of the Shicun muscovite granites.2.The Xinxing granitic pluton is a large batholith cropping out on the east side of the Yunkai area with its long axis parallel to the regional NE-trending faults.Our new data indicate the pluton was emplaced during the interval 234-232 Ma rather than the late Mesozoic as previously considered.The southern pluton consists of syenogranite,monzogranite and alkali-felspar granite,which are composed of K-feldspar,oligoclase,quartz and ferri-biotite.The northern pluton consists of monzogranite and granodiorite,which are composed of andesine,K-feldspar,quartz and magnesio-biotite.The southern granites have high SiO₂ contents(72.5–79.2 wt%)and high FeO_T/(FeO_T+MgO)ratios with low Mg#[atomic Mg/(Mg+Fe _T)].They are enriched in rare earth elements and depleted in Sr and Ba with Zr+Y+Ce+Nb>350 ppm and 10000×Ga/Al>2.6.Their initial magma temperatures are unusually high(>886°C).All those features strongly suggest an affinity with A-type granites.The northern granites have lower SiO₂ contents(63.4–72.5 wt%)and lower FeO_T/(FeO_T+MgO)ratios as well as higher Mg#than the southern granites.They are also enriched in rare earth elements and depleted in Sr and Ba,with high Ga/Al ratios and high Zr+Y+Ce+Nb contents.Geochemical data and major element modeling confirm that the southern A-type granites were generated by the moderate degree(?49%)shallow(ca.15–20 km depth)dehydration melting of the early Paleozoic granitoids triggered by intraplating of basaltic magmas.Subsequent moderate degree(?57%)fractionation of such crustal melts produced the more felsic granites in the southern pluton,whereas subsequent mixing of such crustal melts with about 8 to 48%intraplated mafic magmas generated the less felsic granites in the northern pluton.Interaction of mafic magmas with A-type crustal melts causes the northern granites to be transitional towards I-type granites.We further suggest their origin to have been related to the transtension of the regional NE–trending strike-slip faults.3.The origin of the Napeng and Shicun granites might mark the onset of the continental collison between the Indochina and South China Blocks at the beginning of the late Permian(?259 Ma).The origin of the Xinxing A-type granites indicates that a near N-S compressive stress exsited in the South China Block during the period of 259-231 Ma,which was caused by the continuing collision between the Indochina and South China Blocks.We thus further suggest that the late Paleozoic to early Mesozoic magmatism is mainly controlled by the eastern Paleo-Tethys tectonic domain with no impact from the Paleo-Pacific tectonic domain.