Journal of Asian Earth Sciences | 2021
Melting of the Neoproterozoic Yangtze crustal remnants beneath the North Qinling Terrane induced by the Paleo-Pacific plate subduction: Evidence from the Early Cretaceous Laojunshan granitoids
Abstract
Abstract Zircon U-Pb ages and Hf-isotopes, and whole-rock geochemical and Sr-Nd-Pb isotopic compositions of the Laojunshan granitoids in the North Qinling Terrane (NQT) are presented to discuss their petrogenesis. The Laojunshan granitoids were emplaced at ca. 120\xa0Ma and contain a minor population of Neoproterozoic inherited zircons. They are monzogranite to porphyritic biotite granites, high-K calc-alkaline to shoshonitic and classified as metaluminous I-type granites. They exhibit enriched LREE, flat HREE patterns and low Sr/Y ratios of 4.53–9.05. They are also characterized by enrichment in Th, U, Pb but depletion in Eu, Sr, Nb, P and Ti, suggesting fractional crystallization of plagioclase, apatite and Fe-Ti oxides. The granitoids have slightly radiogenic (87Sr/86Sr)i (0.7046\xa0~\xa00.7056), less negative eNd(t) (-3.7\xa0~\xa0-2.5) and positive zircon eHf(t) (+0.1~+8.8), corresponding to Nd and Hf model ages of 1055–1195\xa0Ma and 616–1173\xa0Ma, respectively. Their Pb-isotope compositions overlap with those of the Late Mesozoic magmatic rocks of the Yangtze Block. These features suggest that the Laojunshan granitoids were mainly generated by partial melting of juvenile Neoproterozoic rocks in the subducted Yangtze crust. The differences between the Laojunshan and the ca. 158–128\xa0Ma granitoids in the southern margin of the central North China Craton (S-CNCC) indicate that the Laojunshan rocks were formed under relatively low-pressure conditions. We suggest that the tectonic regime in the NQT and S-CNCC during late Early Cretaceous (126–112\xa0Ma) was dominated by extension attributed to lithospheric thinning that associated with the westward subduction of the Paleo-Pacific plate.