PETROGENESIS AND DYNAMIC SETTING OF SILURIAN HIGH-MG DIOR-ITES IN THE DULAN AREA,NORTHERN QAIDAM:CONSTRAINTS OF ZIR-CON U-Pb CHRONOLOGY,GEOCHEMISTRY AND Hf ISOTOPE
High magnesium diorite serves as a crucial indicator for investigating magmatic dynamics and tectonic evolution.This study focuses on newly discovered high magnesium diorite from the Dulan area on the eastern part of the northern margin of Qaidam.The research utilizes petrography,chronology,whole rock geochemistry,and Hf isotope geochemistry of zircon to explore its genetic mechanisms and dynamic background.LA-ICP-MS zircon U-Pb chronology reveals that the Dulan high magnesium diorite crystal-lized at approximately 430±3 Ma(MSWD=0.85,N=15),placing its formation in the late Silurian Age.Geochemical analysis indicates that the Dulan high magnesium diorite exhibits low SiO2(52.8％-59.8％),medium total alkali(Na2O+K2O)=4.52％-6.74％,and high Al2O3(16.0％-17.85％),FeOT(6.23％-8.10％)contents,characteristic of calc-alkaline quasi-aluminous rocks.The rocks are characterized by a high MgO content(3.21％to 5.09％),Mg# values ranging from 48 to 53,and low Sr/Y ratios(5.08 to 13.6),indicating they belong to the high magnesium diorite category.These high mag-nesium diorites are enriched in large ion lithophile elements such as Rb,Ba,Th,U,and K,while show-ing depletion in high field strength elements like Nb,Ta,and Ti.They exhibit a weak negative Eu anoma-ly(δEu=0.74 to 0.97).Zircons from these rocks display high Zr/Hf ratios(33.0 to 45.7)and Nb/Ta ra-tios(13.5 to 16.9),with depleted Lu-Hf isotopic compositions and relatively young model ages(eHf(t)=,2.51 to 5.01,TDM1=859 to 996 Ma).Based on a comprehensive analysis of metamorphic peaks,sedimen-tation,and magmatic activity records in the northern margin of Qaidam,the Dulan high magnesium diorite is interpreted to have formed from magma that was generated during the fragmentation of the plate slab af-ter collision events.The magma likely originated from partial melting of the neocrust,with mixing of crust-mantle magma facilitated by the invasion of mantle-derived magma.
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