The Early Cretaceous gabbro in Yare area, middle Lhasa Block: Magmatism response to the slab break-off of the southward subduction Bangong-Nujiang Ocean lithosphere

In order to understand the Early Cretaceous tectonic-magmatic process in the central and north Lhasa Block, the Early Cretaceous gabbro in the Yare area of western part of middle Lhasa Block were studied.The zircon U-Pb age of gabbro is 115.5±0.5 Ma.The rocks belong to tholeiite series, and their characteristics of rare earth elements are similar to those of oceanic island basalt (OIB).Mg^# value (46.07~48.05) and contents of Cr (6.97×10^-6~18.5×10^-6), Ni (6.87×10^-6~11.2×10^-6) are relatively low.The large ion lithophile elements such as Rb, Ba, K, Sr of gabbro samples are relatively enriched, Th、U and Pb show positive anomalies, and the high field strength elements such as Nb, Ta, Zr, Hf, P and Ti show negative anomalies, indicating that the gabbro samples have some "arc volcanic rock" properties.The gabbro show positive zircon ε_Hf (t) values (+4.3~+7.9) and younger Hf-depleted mantle model ages (T_DM1) of 489~614 Ma.By comprehensive analysis, it is proposed that the Early Cretaceous gabbro in the Yare area is most likely triggered by the slab break-off of the southward subducting Bangong-Nujiang Tethyan Ocean lithosphere, and can be considered as the product of partial melting of the asthenosphere mantle which was metasomatized by recent melts or supercritical fluids from the subduction slab, and subsequently experienced varying degrees of fractional crystallization.