SHOSHONITIC MAGMATISM WITHIN THE YARLUNG ZANGBO SUTURE ZONE, TIBET: A WINDOW THROUGH THE DEEP UNDERLYING CRUST

The Neo-Tethys Ocean has been consumed into north-dipping intra-oceanic and active margin subduction zones. Post-collisional potassic to ultrapotassic magmatism occurred since about 14 Ma. No such magmatism was reported within the YZSZ. The purpose of this presentation is to document, for the first time, the discovery of Miocene magmatic rocks within the YZSZ. The igneous rocks crop out in the Saga and Sangsang areas about 600 and 450 km west of Lhasa respectively. They consist of intrusions cutting through the ophiolitic upper mantle at Sangsang and the ophiolitic mélange and ophiolitic crust at Saga. The phenocrysts are made of amphibole and F-bearing biotite (Mg# 0.3-0.6; F up to 2.56 wt. %), K-feldspar (Or85-61), plagioclase (An77-40), garnet (Al79-59Gr5-26Sp5-12Py11-3, and pleonaste set in quartz-rich albite-oligoclase, apatite, and zircon fine-grained matrix. Partly replaced xenocrysts of garnet (Al67-74G r5-7Sp0.4-2Py28-19) and paragneiss xenoliths are observed. The geochemical data reveal that the rocks are trachyandesites. The average composition is: 60.8 wt. % SiO2, 17 wt. % Al2O3, 4.6 wt. %, MgO 3.3 wt. %, CaO, 2.8 wt. % K2O, and 4.9 wt. % Na2O. The intrusive rocks belong to the shoshonitic clan in terms of K2O vs Na2O and K2O vs SiO2 relationships and to ultrapotassic in the CaO vs Al2O3 space but have K2O/Na2O <2 (0.7-1.2). Trace elements show high contents in Ba (243-934 ppm), Ce (38-134 ppm), La (up to 80 ppm), Rb (up to 183 ppm), Sr (262-973 ppm), and Zr (146-223 ppm). The rocks show large variations in Ba/Nb (34-117), Rb/Ba (5-15), Sr/Y (20-105) but uniform Rb/Sr (0.14-0.18) and La/Ce (0.5-0.6). Strongly fractionated Zr/Y ratios (11-18), high concentrations of LaPM (42-117), CePM (21-75), Ti and other incompatible elements, and their similarities to the average of the upper continental crust, suggest these rocks were derived from a continental source. They are likely derived from post-collisional partial melting of lower to middle crustal material underlying the YZSZ. 40Ar-39Ar geochronology on magmatic amphibole points to a Middle Miocene age, making them the youngest igneous rocks reported within the YZSZ. The presence of magma at depth south of the Gangdese belt in the Miocene could have an impact on further modeling of crustal thermal behaviour. The trachyandesites provide a unique window allowing a probe into the deep Indian crust underlying Tibet Plateau.