On The Compositional Diversity Of The Early Mesozoic Granitic Magmatism In The West Qinling Orogenic Belt In Central China

Figuring out the compositional diversity of granitic rocks is the key to understand the generation and evolution of continental crust.The composition of granitic magma is determined by consititutes of source rocks and conditions of partial melting,and can be modified by processes such as mixing,assimilation and fractional crystallization during the transfer.The surge of large-scale granitic magma is usually controlled by specific tectonic setting.Extensive Triassic granitic magmatism develops in Western Qinling orogen and provides a compelling case to understand the diversity of granitoids.In this dissertation,the Baijiazhuang(BJZ)and Zhongchuan(ZC)plutons have been chosen as concrete objects of study.Detailed field investigation combined with studies about mineralogy,petrology,and geochemistry have been carried out to give insights into the roles of different magmatic processes involved with compositional diversity of granitic magmatism from source to emplacement level.Furthermore,integrating with previous works,the petrogenesis of Triassic granitoids in Western Qinling has been discussed.The main progress and advances are as follows:(1)The BJZ pluton is characterized by peraluminous affinity with primary muscovite.The obvious positive correlations among Ba vs.Zr for whole rock and molar P vs.molar(REE+Y)in zircons,combined with low oxygen fugacity conditions and occurrence of abundant inherited zircons,indicate an S-type granitic affinity for the BJZ pluton.Low CaO/Na2O ratios suggest that it sourced from partial melting of metapelite.In order to contrast the petrogenesis of S-type granites in Western Qinling,the previously reported S-type granites have been collected and two groups of S-type granites in terms of geochemical features are recognized,namely,high-Sr low REE(Group A)and low-Sr high REE(Group B).Incongruent melting including muscovite fluid-present and-absent melting can perfectly explain their geochemical characteristics of major and trace elements,respectively,such as high Eu/Eu*values,Sr,CaO contents and low Nb,Ta contents and low zircon and monazite sarutated temperatures in Group A relative to Group B.For the BJZ pluton,the negative correlations among Rb vs.(87Sr/86Sr)i and P2O5 vs.?Nd(t)indicate that incongruent melting process cannot explain largely variable Sr-Nd isotopic ratios.Negative correlation between 1/Sr vs.(87Sr/86Sr)i,combined with no indicative of mantle materials in field,further preclude the effect of magma mixing on the isotopic variation.Thus,hetergeneous source is invoked to be responsible for the variation of Sr-Nd isotopic composition.As a result,we conclude that the BJZ pluton is originated from the heterogeneous sources through incongruent melting.The large inhomogeneous Sr-Nd isotopic characteristics presented in emplacement level imply incremental assembly of magmas.(2)The ZC pluton presents concentric zoned texture and comprises four kinds of lithology from periphery to center based on detailed field investigation,e.g.,porphyritic coarse-grained monzogranite/granodiorite(Xujiaba unit),medium-grained monzogranite bearing quartz and k-feldspar phenocrysts(Zhangjiazhuang unit),medium-grained monzogranite and fine-grained monzogranite(Guandigou unit).The Zhuangjiaba unit intrudes the Xujiaba unit and no clear contact relationship is found between other boundaries of unit/lithology.Mafic microgranular enclaves(MMEs)are mainly distributed in Xujiaba unit and rare in Zhangjiaba unit.LA-ICPMS zircon U-Pb datings yield indistinguishable ages at 215 Ma within age errors for samples from different units,which suggests the timescale of magma cooling to solidus is less than 5 millions of years.Major-major element variations on Harker diagram show no progressive fractionation between different lithological units,combined with different mineral assemblages and textures among different lithological units,suggesting that multiple batches of magmas assembled into the ZC pluton.This is further supported by similar Hf contents in zircons from different lithological units.Similarily,the sample GN1935 collected from Xujiaba unit adjacent to Zhangjiaba unit cannot be explained by magma differentiation,and rather should reflect another injection of magma.Therefore,construction process of the ZC pluton is involved in at least six batches of magmatic injection.Magma mixing process has an impact on the composition of the ZC pluton,especially on Xujiaba unit,which can be demonstrated by compositional fluctuation in plagioclase zonation from Xujiaba and Zhangjiazhuang units and resorption texture of plagioclase from MMEs.Similar petrographical and compositional features of zircons from Xujiaba unit and its mafic enclaves indicate that the zircons in MMEs is probably captured from Xujiaba unit during magma mixing,which can be supported by similar zircon Hf isotopic composition.For the Guandigou unit without occurrence of MMEs,the zircon trace elements and negative correlations between SiO2 and P2O5 indicates the I-type granitic affinity of the ZC pluton.(3)The incremental growth of the ZC pluton reflects the importance of construction process for the petrogenesis of a pluton.Similar to the case of the ZC pluton,these granitoids in western Qinling share the same Sr-Nd-Hf isotopic compositions with their mafic enclaves,which implys extensive magma mixing in Triassic.However,coeval mafic rocks are seldom found in Western Qinling.We consider that a deep hot zone can develop due to continuous underplating of mafic magma,which leads to mafic magma rarely passing through and intruding into shallow crust.(4)The Nd-Hf model ages for the Triaasic granitoids in Western Qinling are clustered at 1.3-1.6 Ga,at which there is no rock recorded in Western Qinling and Yangtze Block.Given that the Neoproterozoic zircon grains(?750 Ma)developed in Triassic granitoids in Western Qinling,the Neoproterozic mafic rocks dominated in northwestern Yangtze Block can serve as the source.The occurrence of age peaks of 870-960 Ma and?430 zircon grains in Triassic granitoids imply that early Neoproterozoic and Paleozoic rocks from North Qinling terrane also can be as candidates of source rocks of Triassic granitoids in Qinling orogen.In addition,sparse Paleoproterozoic zircon grains were observed.Therefore,we conclude that the source components of Triassic granitoids in Western Qinling are probably constituted by Neoproterozoic mafic rocks in northwestern Yangtze Block,Neoproterozoic and Paleozoic rocks in North Qinling terrane,and Paleoproterozoic rocks.The Triaasic granitoids from Western Qinling belong to high-K calcic alkaline rock,however,the Neoproterozic mafic rocks from northwestern Yangtze Block have high content in Na but low content in K,which cannot explain the high K characteristics of Triaasic rocks in Western Qinling.The high K characteristics of Triaasic rocks,on one hand,can be attributed to the addition of Neoproterozoic and Paleozoic rocks in North Qinling terrane in source.On the other hand,the mafic enclaves originated from metasomatic mantle commonly exhibit high K characteristics,hence,the input of potassium-enriched mantle component during magma mixing can also contribute to high-K characteristic of Triassic granitoids in Western Qinling.