Zircon U–Pb geochronology, Sr-Nd-Hf isotopic composition and geological significance of the Late-Triassic Baijiazhuang and Lvjing granitic plutons in West Qinling Orogen.

摘要

The Qinling Orogen was a consequence of continental collision of the South China Craton with the North China Craton in the Triassic, and caused widespread granitoid magmatism. However, the petrogenesis of these granitoids remains controversial. In this paper, we choose the Baijiazhuang (BJZ) and Lvjing (LJ) plutons in the West Qinling Orogen for a combined study of the zircon U–Pb geochronology, whole-rock major and trace element compositions and Sr-Nd-Hf isotopic characteristics. We obtained zircon crystallization ages of ~ 216 Ma and ~ 212 Ma for the BJZ and LJ plutons, respectively. The granitoid samples from both plutons have high K2O metaluminous to peraluminous compositions. They are enriched in large ion lithophile elements (LILEs), light rare earth elements (LREEs) and depleted in high field-strength elements (HFSEs) with significant negative Eu anomalies. The BJZ samples have initial Sr isotopic ratios of 0.7032 to 0.7078, εNd (t) of − 10.99 to − 8.54 and εHf (t) of − 10.22 to − 6.41. The LJ granitoids have initial Sr isotopic ratios of 0.7070 to 0.7080, εNd (t) of − 5.37 to − 4.58 and εHf (t) of − 3.64 to − 1.78. The enriched isotopic characteristics of the two plutons are consistent with their source being dominated by ancient continental crust. However, two BJZ samples show depleted Sr isotope compositions, which may infer possible involvement of mantle materials. Mantle-derived melt, which formed from partial melting of mantle wedge peridotite facilitated by dehydration of the subducted/subducting Mianlue ocean crust, provide the required heat for the crustal melting while also contributing to the compositions of these granitoids. That is, the two granitic plutons are magmatic responses to the closure of the Mianlue ocean basin and the continental collision between the Yangtze and South Qinling crustal terranes.