Geochronology and geochemistry of granites from the Hengjian area, Qinling Orogenic Belt: Implications for the Late Palaeozoic tectonic evolution of the North Qinling Terrane, China

The North Qinling Orogen underwent subduction and collision processes during the Palaeozoic, but the tectonic framework remains unclear. To investigate the tectonic evolution of the North Qinling Orogen, we selected the Devonian granites from the northeastern margin of the North Qinling Terrane. Integrated petrographic, geochemical, and geochronological studies, as well as Sr-Nd-Hf isotopic analyses, were carried out on the newly identified Devonian granites from the Hengjian area in the northeastern North Qinling Terrane, Qinling Orogenic Belt. Zircon laser ablation inductively coupled plasma mass spectrometry U-Pb dating reveals that these granites were crystallized at 396-387 Ma. The granite samples are metaluminous to weakly peraluminous (A/CNK = Al2O3/(CaO+Na2O+K2O) = 0.86 to 1.12), with high SiO2 (60.6-76.5 wt%) and high total alkaline (Na2O + K2O = 8.45-11.0 wt%) contents. The granites are enriched in Th, rare-earth element, Zr, and Hf and depleted in Sr, Nb, Ta, P, Eu, and Ti. They are characterized by high Zr + Nb + Ce + Y contents (511-990 ppm) and 10,000*Ga/Al ratios (2.71-3.32), as well as high zircon-saturation temperatures (T-Zr = 819-885 degrees C, average = 845 degrees C), which indicate that they are typical A-type granites. The granites have low Y/Nb (0.39-0.67) and Yb/Ta (0.56-0.83) ratios, resembling those of the A(1)-type granite, which indicates that they are formed in anorogenic settings. They also have low (Sr-87/Sr-86)(i) ratios (0.7055-0.7071) and negative epsilon(Nd)(t) values (-10.1 to -8.40) and variable zircon epsilon(Hf)(t) values (-8.46 to +14.7), with the two-stage model ages of 2.0-1.8 Ga and 1.9-0.4 Ga, respectively. These geological features suggest that the granites of the Hengjian area were most likely generated by partial melting of mafic lower crust induced by asthenospheric ascent. The A-type granites were generated in a continental rift environment during 396-387 Ma, corresponding to the closure of the Shangdan Ocean and the opening of the Palaeo-Tethyan Ocean during the Early Devonian.

Automated summary

The tectonic evolution of the North Qinling Orogen is complex, and the formation of Devonian A-type granites in the northeastern North Qinling Terrane indicates they were generated in a continental rift environment, related to the opening of the Palaeo-Tethyan Ocean and the closure of the Shangdan Ocean.