Origin and tectonic implications of the Shiquanhe high-Mg andesite, western Bangong suture, Tibet

Abstract

This study presents geochronological, petrological and geochemical data for the newly discovered Shiquanhe high-Mg andesite (HMA) in the Shiquanhe area, central Tibetan Plateau. It is the first report for high-Mg andesites in the west section of the Bangong Meso-Tethys suture zone. The Shiquanhe high-Mg andesite is in contact with the Shiquanhe ophiolite peridotites and has pillow structures indicating underwater eruption. LA-ICP-MS zircon U-Pb dating yields Late Jurassic ages (160.8 ± 2.3 Ma and 160.7 ± 2.9 Ma). The Shiquanhe high-Mg andesite is characterized by moderate SiO2 (54.07–63.89 wt%), high MgO (4.12–7.24 wt%), Cr (up to 251 ppm), Mg# (58–70), Th (6.7–12.1 ppm), (La/Sm)N (3.2–5.4), (87Sr/86Sr)t (0.713513 to 0.715180), and low Ba/Th, U/Th, and Sr/Y, similar to the well-documented Cenozoic high-Mg andesite in southwest Japan and elsewhere in Tibetan Plateau. Negative whole-rock εNd(t) (−12.5 to −13.4) and zircon εHf(t) (−8.7 to −9.8) values of the Shiquanhe high-Mg andesite are similar to those of Middle Jurassic or older sediments and magmatic rocks in the northern Lhasa terrane. We argue that the Shiquanhe high-Mg andesite may be derived from partial melting of subducted sediments and their subsequent interaction with the overlying mantle peridotite in a high-temperature and low-pressure setting. The southward subduction of the young and hot Shiquanhe back-arc oceanic crust may melt the mantle wedge and northern Lhasa terrane-derived sediments, leading to the formation of the Shiquanhe high-Mg andesite. We suggest an analogous example of the sanukite in the Setouchi Volcanic Belt (SVB), SW Japan, which was argued to be produced by the subduction of the young and hot Shikoku back-arc oceanic crust.