Origin of postcollisional magmatic rocks in the Dabie orogen: Implications for crust-mantle interaction and crustal architecture

Abstract

Zircon Hf and whole-rock Sr-Nd isotopic compositions were determined for postcollisional mafic-ultramafic and felsic intrusive rocks in the Dabie orogen. The results provide not only insights into the character of their source rocks and the nature of crust-mantle interaction, but also constraints on the crustal architecture of continental collision orogen. SHRIMP zircon U-Pb dating gave concordant ages of 121±6 to 131±2Ma for the bimodal intrusives. Zircon Hf isotope analyses gave negative εHf(t) values of -26.3±0.6 to -7.0±0.5 for the mafic-ultramafic rocks, with two-stage Hf model ages of 1.62 to 2.83Ga. Zircons from felsic granitoids also gave negative εHf(t) values of -31.6±0.5 to -16.9±0.9 with two-stage Hf model ages of 2.25 to 3.16Ga. Both the mafic-ultramafic and felsic rocks have high initial 87Sr/86Sr raios of 0.7065 to 0.7084 and very low εNd(t) values of -21.7 to -11.7 for whole-rock. The crust-like geochemical signatures in the mafic-ultramafic rocks suggest their derivation from partial melting of an orogenic lithospheric mantle source that is enriched in radiogenic isotopes as well as incompatible trace elements such as LILE and LREE. It would be generated by reaction of the overlying subcontinental lithospheric mantle (SCLM) wedge peridotite with the felsic melt derived from the subducted continental crust during the continental collision. Therefore, these postcollisional mafic-ultramafic rocks record recycling of the subducted continental crust and consequent crust-mantle interaction in the continental subduction zone. The granitoids have Hf model ages as old as Paleoarchean, which cannot be derived from partial melting of surrounding orthogneisses alone, but requires involvement of more ancient Archean crust in their source region. Thus, their source represents a mixture of crustal rocks from Paleoproterozoic and Paleoarchean basement. In combination of the present and previous studies, the Dabie orogen is suggested to have a three-layer crustal structure prior to the postcollisional magmatism: Central Dabie in the upper with dominantly young Hf model ages of late Mesoproterozoic to Neoproterozoic, North Dabie in the middle with dominantly middle Paleoproterozoic Hf model ages, and the source region of the postcollisional granitoids in the lower with Paleoproterozoic to Paleoarchean Hf model ages. © 2011 Elsevier B.V.