Late Triassic Batang Group arc volcanic rocks in the northeastern margin of Qiangtang terrane, northern Tibet: partial melting of juvenile crust and implications for Paleo-Tethys ocean subduction

Abstract

The Batang Group (BTG) volcanic rocks in the Zhiduo area, with NW-trending outcrops along the northeastern margin of the Qiangtang terrane (northern Tibet), are mainly composed of volcaniclastic rocks, dacite and rhyolite. Major and trace element, Sr and Nd isotope, zircon U–Pb and Hf isotope data are presented for the BTG dacites. Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating constrains the timing of volcanic eruption as Late Triassic (221 ± 1 Ma). Major and trace element geochemistry shows that the BTG volcanic rocks are classified as calc-alkaline series. All samples are enriched in large-ion lithophile elements and light rare earth elements with negative–slightly positive Eu anomalies (Eu/Eu* = 0.47–1.15), and depleted in high field strength elements and heavy rare earth elements. In addition, these rocks possess less radiogenic Sr [(87Sr/86Sr) i = 0.7047–0.7078], much radiogenic Nd (eNd(t) = −4.2 to −1.3) and Hf (eHf(t) = 4.0–6.6) isotopes, suggesting that they probably originated from partial melting of a crustal source containing a mantle-derived juvenile component. The inferred magma was assimilated by crustal materials during ascending and experienced significant fractional crystallization. By combining previously published and the new data, we propose that the BTG volcanic rocks were genetically related to southwestward subduction of the Ganzi–Litang ocean (a branch of Paleo-Tethys) in the northeastern margin of the Qiangtang terrane. Given the coeval arc-affinity magmatic rocks in the region, we envisage that the Ganzi–Litang ocean may extend from the Zhongdian arc through the Yidun terrane to the Zhiduo area, probably even further northwest to the Tuotuohe area.