Geochemical Constraints On The Partial Melting Of Deeply Subducted Continental Crust And Subsequent Magmatic Evolution Process

The partial melting and magmatic evolution of the deeply subducted continental crust in the continental collision orogen will significantly affect the tectonic evolution of the orogenic belt,the chemical differentiation of continental crust,as well as crust-mantle interactions,which is of great research significance.At present,the genetic link between magma formation and tectonic evolution in orogenic belts is still unclear,especially on how the widespread granite in orogenic belts can be related to the evolution of orogenic belts.In addition,it is still of hot debate on whether the high-silicon granitic melts can undergo significant mineral crystallization differentiation during magma evolution.Syn-exhumation magmatism in the collision orogen is an important process of crustal differentiation and crust-mantle interaction at the converging plate margins.It is necessary to elucidate which factors control the geochemical composition of such magmatic products.The Dabie-Sulu orogenic belt was formed during the continental collision between the South China Block and the North China Block during the Triassic.As a typical continental collision orogen in the world,there are abundant ultra-high pressure(UHP)rocks,granitic rocks and leucosomes that have undergone partial melting in different degrees,which provides a typical place for studying the partial melting of UHP rocks and its geochemical effects.Based on these considerations,this thesis selected the granites in the Lanshan and Mibaishan areas of the Sulu orogen as the main research object,and systematically carried out mineralogy,petrology,chronology,whole-rock and mineral geochemical studies.The results provide important constraints for the melting process and timing of UHP rocks,magma evolution and granite genesis in continental collision orogens.The combined analysis of whole-rock major and trace elements,Nd-Fe isotopes,mineral O isotopes,zircon U-Pb ages and trace elements of the syn-exhumation granites in the Lanshan area of the Sulu UHP metamorphic belt has discovered several Triassic granite plutons,with a total exposed area of about 10 km2.Field observations show that these syn-exhumation granites are generally distributed in blocks with almost no deformation.They are high-K calc-alkaline and exhibit enrichment in LREE and LILE but depletion in HREE and HFSE in trace element distribution patterns.Zircons in these granites contain relict magmatic cores characterized by steep HREE patterns with strong negative Eu anomalies.These cores show middle Neoproterozoic U-Pb ages of 724-779 Ma,consistent with the protolith age of UHP metaigneous rocks in the Dabie-Sulu orogenic belt.Zircon rims exhibit oscillatory zoning,and are characterized by Triassic U-Pb ages of 210-216 Ma,low LREE contents and steep HREE patterns with significant negative Eu anomalies,suggesting their growth from granitic magmas during exhumation of the deeply subducted continental crust.The granites have low δ18O values for quartz,zircon and whole-rock and Neoproterozoic U-Pb ages of zircon cores,indicated a large-scale partial melting of the deeply subducted continental crust of the South China Block at the end of the continental collision.One coesite inclusion was found in the anatectic zircon rim,pointing to the deep origin of anatectic melts at a subarc depth of>80 km.Whole-rock SiO2 contents are correlated with major and trace elements,which are ascribed to crystal fractionation of mainly biotite and plagioclase during magma evolution.In particular,these granites exhibit highly variable δ56Fe values from 0.05-0.30‰,which are correlated with Fe3+/ΣFe,SiO2,Al2O3,Nb/Ta and Eu/Eu*.This suggests that the crystal fractionation of biotite controls the Fe isotope variation in the granites.Therefore,the syn-exhumation granites experienced significant geochemical differentiation mainly through the fractional crystallization during the magma evolution.A compilation of syn-exhumation granites in the typical collisional orogens of the world shows that such granites were mainly formed through dehydration melting of hydrous minerals during decompressional exhumation.They fall into the magnesian group with relatively low REE and HFSE(like Ti,Nb and Zr)contents,distinct from the geochemical compositions of A-type granites.Therefore,syn-exhumation granites and post-collisional A-type granites were formed at different tectonic environments,which record different styles of partial melting processes.A comprehensive analysis of whole-rock major and trace elements,microbeam titanite geochemistry(major and trace elements,U-Pb ages and Nd isotopes)of the Mibaishan synexhumation granite in the Sulu UHP metamorphic belt revealed that these high-silicon granites were formed by partial melting of the deeply subducted continental crust and subsequent magma crystallization.Significant geochemical differentiation has been revealed by wholerock and mineral geochemistry.Titanite in the granite samples can be divided into anatectic and magmatic origins according to the Fe/Al ratios and trace elements like REE.Both types of titanite give similar U-Pb ages of 209 ± 23 Ma to 233 ± 26 Ma,corresponding to the exhumation stage of the Sulu UHP metamorphic belt.These titanites show consistent εNd(t)values in a range of-14.8 to-7.2,which are generally consistent considering the analysis error.This observation indicates that these titanites formed in a nearly closed system without significant mixing of different sources of magma.The highly variable Nb/Ta ratios(3.8-1063)show good correlations with FeOt,Al2O3,Ta,Sr,LREE and Lu/Gd.This is ascribed to crystal fractionation of titanite,plagioclase and allanite/epidote during magma evolution.Therefore,accessory minerals like titanite play a key role in constraining granite differentiation.Titanite geochemistry can record complex geological processes from partial melting at the source in depth to fractional crystallization at the shallow crust level for the syn-exhumation granites.The comprehensive analysis of microbeam trace element and Sr-Nd-Hf isotopes of accessory minerals including apatite,epidote,allanite and zircon reveals that granites record Sr-Hf isotope disequilibrium partial melting processes.The εNd(t)ranges of accessory minerals apatite and allanite are very limited.Thus,it is reasonable to infer that such apatite and allanite are formed in a system with a nearly closed Nd isotopic composition,and there is no significant source heterogeneity for such granites.The Sm-Nd isochron ages obtained from apatite and allanite are consistent with the timing of partial melting during the exhumation stage of the deeply subducted South China continental block.However,apatite and epidote show a wide range of Sr isotopic composition.The maximum(87Sr/86Sr)i value ranges in each individual sample are 0.741596-0.767946 and 0.755106-0.768578,respectively,which indicate that the granitic magma underwent Sr isotope disequilibrium melting,and the proportion of Srcontrolling minerals such as feldspar and mica may be different in the partial melting process.In addition,the εHf(t)values of zircon are consistent with those of the whole rock samples,and the εHf(t)variation of syn-magmatic zircons in the granite samples can be as high as 1.4-10.5,indicating the combined role of the dissolution of residual zircon and the contribution of other Hf-rich minerals with high radiogenic Hf isotope composition.Therefore,the microbeam analysis of Sr-Nd-Hf isotopic composition of accessory minerals can be used as an effective geochemical tracer to provide key information for the partial melting and magmatic evolution of the deep subducted continental crust in collision orogens.