Early Cretaceous continental arc evolution in South Pamir:Response to sediment relamination
Early Cretaceous continental arc magmatism was developed in South Pamir,with the transition of magmatic products from initial S-type granites to later I-type granites recording the arc evolution.This study presents new zircon U-Pb ages and Hf-O isotopes,whole-rock elements and Sr-Nd isotopes for the Murgob two-mica granites and biotite granites to constrain their petrogenesis and geodynamic processes.Zircon U-Pb dating indicates that the two-mica granites formed at 115.2 Ma and the biotite granites formed at 104.8 Ma.The geochemical data show that the two-mica granites have high SiO2,Al2O3,and low MgO contents.They belong to the high-K calc-alkaline series with peraluminous features,and belong to S-type granites.They are enriched in LREEs and LILEs,depleted in HREEs and HFSEs,and have enriched Sr-Nd isotopic compositions((87Sr/86Sr)i=0.710543-0.712025,εNd(t)=-9.4),variable zircon εHf(t)(-10.0 to 1.8)and δ18O values(6.2‰-9.3‰).The geochemical characteristics of the samples are consistent with muscovite dehydration melting of the metagreywacke and metapelite.Their source was 70%metasedimentary rocks,20%lower crustal basement,and 10%mantle-derived magma.The incorporation of garnet-rich deep crustal components to their sedimentary source results in the Nd-Hf isotopic decoupling.The biotite granites have high SiO2,high K2O,and low MgO contents.They are enriched in LREEs and LILEs and depleted in HFSEs,with negative Eu anomalies(Eu/Eu*=0.43-0.76).They have enriched whole-rock Sr-Nd isotopic compositions((87Sr/86Sr)i=0.710781-0.711090,εNd(t)=-10.8 to-10.7)and enriched zircon Hf isotopic compositions(εHf(t)=-11.9 to-4.1)and variable zircon δ18O values(6.9‰-9.7‰).They were formed by high-degree partial melting(15%-50%)of a mixed source comprising 65%lower crust,15%metasedimentary rocks,and 20%asthenospheric mantle.The Karakul-Mazar Triassic sedimentary rocks were relaminated in the Central-South Pamir lower crust during the closure of the Paleo-Tethys Ocean.The dehydration reaction of the Early Cretaceous low-angle subduction of the Neo-Tethys oceanic slab triggered partial melting of the Karakul-Mazar sedimentary rocks,with the addition of deep crustal components to form the Murgab two-mica granites.Subsequently,as the Neo-Tethys oceanic slab underwent slab foundering,the upwelling of the asthenospheric mantle triggered magmatic flare-up,leading to partial melting of the lower crust with the subordinate contribution of relaminated sediments,forming the Murgob high δ18O biotite granites.The predominant and subordinate status between the sediments and the lower crust in the magmatic source controlled the evolution of the South Pamir Early Cretaceous continental arc,and the relamination and consumption of sediments caused the transition from the initial S-type granite to late I-type granite.
South Pamircontinental arcrelaminationtwo-mica granitebiotite granite
万方数据
维普
