Late Triassic ridge subduction of Paleotethys: Insights from high-Mg granitoids in the Songpan-Ganzi area of northern Tibet

Abstract

The time of final closure of the Paleotethys Ocean is still hotly debated in particular regarding the Jinsha Ocean in northern Tibet. This study reports new geochronologic, geochemical and zircon Hf isotopic data of six granodiorites-diorites, which intrude the Mid-Late Triassic Songpan-Ganzi complex (SGC). LA-ICP-MS zircon U-Pb analyses of three quartz diorites (214.9 ± 2.2, 211.0 ± 1.3, 205.8 ± 1.4 Ma) and three granodiorites (210.8 ± 2.7, 206.7 ± 3.3, 205.9 ± 3.5 Ma) gave a Late Triassic age range (214.9–205.8 Ma). These granodiorite-diorite rocks have relatively high MgO (up to 9.1%) and Mg# (up to 70.2), Al2O3, Cr, Ni, but low TiO2, and high magnesium andesite/diorite (HMA) chemical signatures with low Sr and Y content, and slight-moderate negative Eu anomaly. Also, the dioritic enclaves and Late Triassic detrital zircons (as young as 208 Ma) were captured by studied rocks. Hf isotope data show overall slight to moderate negative zircon εHf(t) values (−1.97 to −5.76, −4.12 to −6.72) and heterogeneous TDMC ages (1.38−1.61Ga, 1.51−1.68Ga) for quartz diorites and granodiorites respectively, together with other published Hf isotopic data in SGC, are in highly concert with those around ~240 and ~440Ma magmas (as potential sources for SGC turbidites) in east Kunlun orogenic belt. Taken together, the HMA magmas were probably derived from a reaction between subducted marine sedimentary siliceous melts and peridotites from a mantle wedge.The linear distribution of Late Triassic HMAs and their eastward-younging trend (from northern Qiangtang to southern SGC), in combination with the contrasting distribution of volcanic rocks in the northern Qiangtang terrane, lead us to suggest that a slab window opened in the downgoing oceanic crust with two diverse slab angles during oblique subduction. Accordingly, the final closure of the Paleotethys Ocean was later than 205 Ma, and most likely in the end of the Late Triassic to Early Jurassic.