Petrogenesis of Middle Triassic intermediate-mafic igneous rocks in East Kunlun, Northern Tibet: Implications for the crust growth and Paleo-Tethyan orogeny

Abstract

Triassic intermediate-mafic igneous rocks are sporadically exposed in the East Kunlun Orogen Belt (EKOB), and their petrogenesis and geological implications for the Paleo-Tethyan orogeny are not well constrained. Geochronologic and geochemical data for the gabbro, gabbroic diorite and quartz diorite in Qingshuihe pluton are presented here to constrain their ages, petrogenesis and geodynamic settings. LA-ICP-MS zircon U-Pb dating indicates that the gabbro, gabbroic diorite and quartz diorite were formed at 239.8 ± 2.1 Ma, 238.4 ± 1.1 Ma and 235.7 ± 1.1 Ma, respectively. The studied samples span a wide range in geochemical compositions with SiO2 content of 47.55–63.93 wt%, exhibiting low-K tholeiitic to high-K calc-alkaline affinities. All samples have trace element compositions resembling typical arc magmas, however, from gabbro, gabbroic diorite to quartz diorite, the fractionation degree of rare earth elements gradually increases ((La/Yb) N = 2.08–2.52, 2.20–9.23 and 7.15–17.95, respectively), and the enrichment in large ion lithophile elements (e.g., Rb, Ba, Th and K) and the depletion in high field strength elements (e.g., Nb, Ta, and Ti) are also more obvious. These geochemical features, combined with their enriched Sr-Nd-Hf isotopes ((87Sr/86Sr)i = 0.709694–0.71445, εNd(t) = -7.9 – -4.7, εHf(t) = -3.65–3.79, respectively), unanimously indicate their parental magma was derived by partial melting of an enriched lithospheric mantle, followed by cumulation of tabular plagioclase and prismatic amphibole generating the appinitic gabbro, and subsequent certain degree of magma mixing and fractional crystallization forming the evolved gabbroic diorite and quartz diorite. This study therefore not only demonstrates that mantle-derived arc magma underplating followed by crust-mantle mixing and fractional crystallization is an important mechanism of continental crust growth, but also confirms that the East Kunlun Paleo-Tethyan oceanic subduction lasted at least until 235 Ma.