大洋俯冲和陆-陆碰撞过程中陆壳的改造与成熟OA

The oceanic subduction zones and collision orogenic belts are critical regions for the evolution and matura-tion of continental crust.However,the specific mechanisms driving crustal reworking and maturation remain subjects of considerable controversy.In this study,we employ an integrated approach that combines petrography,geochemistry,zircon U-Pb geochronology,and zircon Lu-Hf isotopic analyses to analyze the felsic gneisses and their internal leu-cosomes,basic gneisses,and diorites within the Guaijiaoliang-Shuangkoushan arc-related rock unit,in order to in-vestigate the key mechanisms driving continental crust evolution in this region.Whole-rock geochemical analysis re-veals that the felsic gneisses,basic gneisses,and diorites are enriched in large ion lithophile elements and light rare earth elements,while being depleted in high field strength elements,characteristic of typical arc magmatic geo-chemical signatures.The basic and felsic gneisses exhibit a continuous evolution in major element composition,suggesting that the protolith of felsic gneisses originated from the crystallization differentiation of basic magmas.The leucosomes within the felsic gneisses exhibit high SiO2,Al2O3,and Na2O contents but low K2O,CaO,Rb/Sr,and Th/U ratios,characteristic of the geochemical features of trondhjemite,consistent with fluid-present melting.Zir-con U-Pb geochronology reveals that the protolith of basic gneiss and diorites formed at 470.9±6.5 Ma and 441.6±3.0 Ma,respectively.Hf isotopic analysis reveals that the zircon εHf(t)values of the basic gneisses range from+6.74 to+11.26,whereas the εHf(t)values of the diorites vary from+5.13 to+8.06.The leucosomes in the fel-sic gneisses crystallized at 441.6±3.5 Ma,and their zircon εHf(t)values range from+7.24 to+12.27,which is consistent with those of the basic gneisses.Comprehensive analysis indicates that the Guaijiaoliang-Shuangkoushan arc magmatic-metamorphic unit records two phases of arc magmatic activity and one phase of metamorphic-anatexis.During the～470 Ma oceanic crust subduction phase,subducted fluids facilitated partial melting of the mantle wedge,leading to the formation of the first phase of arc magmatism,i.e.,the protoliths of the basic gneisses and felsic gneisses.During the transition from oceanic subduction to continental collision at～440 Ma,the upwelling of the asthenosphere triggered partial melting of a hydrated mantle,resulting in the formation of diorites.The emplace-ment and crystallization of the diorites released heat and fluids,inducing hydrous partial melting of the felsic gneiss protolith.Both phases of arc magmatic activity and the subsequent anatexis events promoted the transformation of the continental crust from basic to acidic within the North Qaidam subduction-collision mélange belt.This suggests that arc magmatism and anatexis processes during the transition from oceanic subduction to continental collision were key factors driving the reworking and maturation of the continental crust.