Neoproterozoic crustal growth and reworking in South Qinling, central China: Evidence from zircon U-Pb age and Hf isotopic composition of paragneiss in the Foping dome

Basement rocks within orogenic belts are useful for reconstructing crustal evolution. In the present study, we report the U-Pb ages and Hf isotopic compositions of detrital zircon and the geochemical of whole rock for Precambrian paragneiss from the Foping complex in South Qinling. The paragneiss samples had TiO2/Al2O3 ratios of 1.84-5.70, Th/Sc ratios of 0.67-1.20, and Zr/Sc ratios of 11.0-18.1, suggesting felsic rocks as the major source. The immobile trace elements indicate that the source rocks were mostly derived from a continental arc environment. The U-Pb ages of detrital zircon are notably concentrated around two major peaks at -850 Ma and -700 Ma and two subordinate peaks at -2500 Ma and -2000 Ma with & epsilon;Hf(t) values varied widely from -26.0 to +12.4. The predominantly Neoproterozoic ages of the detrital zircon grains show a similar age distribution to those of Neoproterozoic igneous rocks in South Qinling and the Yangtze Block. Combining the results with the existing data for Neoproterozoic rocks in the study area, we propose that South Qinling was generally characterized by growth of the crust in the early Neoproterozoic (ca. 1000-900 Ma) that could have resulted from an intra-oceanic subduction along the northern margin of the Yangtze Block. Reworking of the ancient crust strengthened thereafter, which could be attributed to tectonic shift from an island-arc setting to a continental arc environment at ca. 900-860 Ma. Extensive formation of igneous rocks in the continental arc provided the main source for the Precambrian paragneisses in the Foping dome.

Automated summary

The U-Pb ages and Hf isotopic compositions of detrital zircon, as well as the geochemical characteristics of whole rock, from Precambrian paragneiss in the Foping complex were studied in this research. The results suggest that the paragneiss samples were mainly derived from felsic rocks in a continental arc environment. The U-Pb ages of the detrital zircon show concentrations around two major peaks at -850 Ma and -700 Ma, as well as two minor peaks at -2500 Ma and -2000 Ma, with variable εHf(t) values ranging from -26.0 to +12.4. The Neoproterozoic ages of the detrital zircon grains in the Foping dome are similar to those of Neoproterozoic igneous rocks in South Qinling and the Yangtze Block. This study suggests a crustal growth in the early Neoproterozoic (ca. 1000-900 Ma) in South Qinling, possibly caused by intra-oceanic subduction along the northern margin of the Yangtze Block, followed by reworking of the ancient crust attributed to a tectonic shift from an island-arc setting to a continental arc environment at ca. 900-860 Ma. The extensive formation of igneous rocks in the continental arc provided the main source for the Precambrian paragneisses in the Foping dome.