Zircon U-Pb ages and O-Nd isotopic composition of basement rocks in the North Qinling Terrain, central China: evidence for provenance and evolution

摘要

The Qinling Group was previously interpreted as the oldest Precambrian basement unit of the North Qinling Terrain, recording its formation and early crustal evolution. The Qinling Group consists predominantly of gneisses, amphibolites, and marbles, which underwent multi-phase deformation and metamorphism. In order to better constrain the provenance and tectonic setting of this group and the evolution of the North Qinling orogenic belt, in situ U-Pb dating and oxygen isotopic analysis of zircons in combination with whole-rock geochemistry and Sr-Nd isotope analysis was performed on the two dominant rock types, amphibolite and felsic gneiss. Felsic gneisses exhibit enrichment of LREEs and LILEs (Rb, Ba, Th, K, Pb), negative Eu anomalies and depletion of HFSEs (Nb, Ta, P, Ti). The rocks have slightly elevated δ~(18)O values (6.5-9.3 %o) and initial ε_Nd values of -4.6 corresponding to two-stage Nd model age of 1.99 Ga. Amphibolites are also enriched in LILEs and LREEs and depleted in Nb and Ta and have homogeneous δ~(18)O values (5.0-6.0 %o), but higher initial ε_Nd values (2.8-3.3) and younger two-stage Nd model ages (1.29-1.24 Ga) compared to the gneisses. The zircon age record indicates that the gneisses and amphibolites were formed in a ～960 Ma volcanic arc environment rather than in a rift setting as previously suggested. A major metamorphic event took place during the Early Paleozoic. Based on the age spectrum of detrital zircons, the Qinling Group is interpreted as an autonomous geological unit, which was mainly derived mostly from 1,000 to 900 Ma old granitoid rocks. The North Qinling Terrain can be regarded as a remnant of the Grenville orogenic belt with an early Neoproterozoic evolution different from that of the North and South China blocks.