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Early cretaceous collapse of the zircon granitoids and their implications for the Dabie orogen, eastern China SHRIMP U-Pb dating and geochemistry [查看] HaijinXu、ChangqianMa、KaiYe

Two stages of early Cretaceous post-orogenic granitoids are recognized in the Dabie orogen,eastern China,which recorded processes of extensional collapse of the orogen.The early stage granitoids(~132Ma)are foliated hornblende quartz monzonites and porphyritic monzogranites.They are of high-K calc-alkaline series and mataluminous to weakly peraluminous,with high K2O and Low MgO contents(Mg# values:32.0-46.0),they contain high Sr,low Y and heavy rare earth elements(HREE),and have high Sr/Y and (La/Yb)N ratios,without clear negative Eu,Sr and Ti noomalies.The early stage deformed granitoids have adakitic geochemical compositions and are equilibrated with residues rich in garnet and poor in anorthite-rich plagioclase,and thus indicate the existence of an over-chickened(〉50km)crustal root beneath the orogen at ~132Ma.The later stage granitoids(~128Ma)are undeformed fine-grained monzogranites,fine-grained K-feldspar granites and coarse-grained K-feldspar granite-porphyry.They belong to a peraluminous and high-K calc-alkaline to shoshonite series,the display a flat HREE pattern and have strong negative Eu,Sr and Ti anomalies,with Low Sr/Y and (La/Yb)Nratios.The late stage grantoids are equilibrated with residues rich in anortbite-rich plagioclase,hornblende.ilmeite/titanite and poor in garnet,indicating that the crust of the Dabie orogen became thinner(〈35km)at ~128Ma.SHRIMP zircon U-Pb ages and changing compositional trends for these two stages of granitoids indicate that the over-thickened crust formed by the Triassic continental subduction/collision under the Dabie orogen remained until the early Cretaceous,and collapsed quickly in a few million year during the early Cretaceous.