Sources and petrogenesis of Late Triassic Zhiduo volcanics in the northeast Tibet: Implications for tectonic evolution of the western Jinsha Paleo-Tethys Ocean

Abstract

The western Jinsha suture, representing a branch of the Paleo-Tethys, makes the junction between the Songpan-Ganzi Complex and the North Qiangtang Block (NQB). However, the Triassic tectonic evolution of the western Jinsha Paleo-Tethys Ocean is poorly constrained. This paper shows new LA–ICP–MS zircon U-Pb geochronology, major and trace elements, and Sr–Nd–Hf isotopic compositions of the Zhiduo dacitic volcanic rocks in the northern margin of the NQB. LA–ICP–MS zircon U-Pb dating displays that they erupted between 227 ± 2 and 228 ± 1 Ma. These rocks exhibit high silica, medium-K calc-alkalic signature, and low MgO, TiO2, P2O5, Cr, and Ni compositions. They are enriched in light rare earth elements and large-ion lithophile elements with moderate to strong negative Eu anomalies (Eu/Eu* = 0.24–0.56), and depleted in heavy rare earth elements and high field strength elements. They possess relatively low initial 87Sr/86Sr ratios (0.7055–0.7084), positive whole rock εNd(t) (+0.2 to +1.4), and magmatic zircon εHf(t) values (focus on 9.2 ± 2.7) with young Nd and Hf model ages (T2DM (Nd) = 885–983 Ma; T2DM (Hf) = 467–1035 Ma). These features are comparable to those of typical arc volcanic rocks and indicate that they were probably derived from partial melting of a juvenile crust source. The eruption of these volcanic rocks could be attributed to southward subduction of the western Jinsha Paleo-Tethys Ocean. In combination with previous studies, we propose that the subduction of the western Jinsha Paleo-Tethys Ocean initiated at the beginning of the Late Triassic (ca. 228 Ma) and closed at the end of the Late Triassic (ca. 208 Ma). In addition, there is a universal northeastward-younging trend of arc magmas in the northern margin of the NQB, which may indicate a model involving the initial shallow subduction and subsequent slab roll-back.