Evidence for palaeo-Tethyan oceanic subduction within central Qiangtang, northern Tibet

Abstract

The mechanism of formation of blueschist–eclogite belts and their space-time distribution are important in understanding the tectonics associated with convergent plate boundaries. Here we investigate the garnet-bearing blueschists from Rongma area of central Qiangtang in northern Tibet. The mineral assemblage in these rocks is characterized by porphyroblastic garnet set within a matrix of fine-grained amphibole, white mica, epidote, chlorite, albite and quartz with accessory rutile, titanite and apatite. The garnet porphyroblasts exhibit core and rim portions, and the cores carry abundant inclusions of Na amphibole, quartz and rutile, as well as rhomb-shaped inclusions of paragonite and epidote which are interpreted as pseudomorphs after lawsonite. The rims are characterized by coarse-grained inclusions of epidote as well as the absence of paragonite and epidote aggregates, clearly suggesting that the transition from garnet core to rim marks a metamorphic transformation from lawsonite- to epidote-stability field. The Mn content of the garnet porphyroblasts decreases from core to rim, whereas the Fe and Mg contents show an increasing trend. In the matrix, we identify two stages of Na amphibole rimmed by Na-Ca amphibole and albite. Retrograde chlorite is rimmed by fine-grained biotite. Based on microstructural observations and pseudosection modelling, we trace the P–T path for the Rongma garnet blueschist from 1.92GPa and 490°C (lawsonite eclogite field) to about 1.68GPa and 535°C (epidote eclogite field), marking an initial increase in temperature and decrease in pressure. This stage is followed by a decrease of pressure through the blueschist facies down to P–T conditions of about 0.6GPa and 530°C. In combination with previous work including the available isotopic age data, the P–T path obtained in the present study suggests the deep subduction of palaeo-Tethyan oceanic crust between southern and northern Qiangtang blocks, supporting the model that the blueschist belt defines the location of the palaeo-Tethyan suture zone within northern Tibet. T-X(O) pseudosection modelling further reveals that the transformation of lawsonite to epidote occurred under high O contents. Our results provide important clues to better understand the subduction-related late Permian to early Triassic magmas in northern Tibet.