Zircon U–Pb and garnet Lu–Hf geochronology of eclogites from the Lhasa Block, Tibet

Abstract

The Tibetan Plateau is the result of several collision events between a series of Gondwana-derived blocks (e.g., Qiangtang, Lhasa and India) and the Asian continent since the early Paleozoic. The occurrence of high-pressure eclogites has a significant bearing on the Paleo-Tethys subduction and plate suturing processes in this area. A quartzite, together with its host eclogite from the Jilang County in the Lhasa Block of northern Tibet, was investigated in terms of petrology, geochemistry, and combined Lu–Hf and U–Pb geochronology. The composition of the Jilang eclogite is similar to supra-subduction-zone basalts found in back-arc basins (LREE depleted, negative Nb–Ta anomalies, positive Pb anomalies, εNd(t)=~+8). The observations of coesite/coesite pseudomorph in the eclogites and the estimated P–T conditions of 3.4–3.8GPa and 753–790°C for the eclogites imply that the Jilang eclogites have undergone ultra-high-pressure metamorphism. The Lu–Hf age of 265.9±1.1Ma (2σ, 14 points, MSWD=1.8) of the Jilang eclogite most likely reflects garnet growth and is interpreted to reflect eclogite–facies metamorphism due to the occurrence of omphacite inclusions from core to rim in garnets and the spherical geometry effect despite the well-preserved prograde zoning in the garnets. U–Pb analyses of the metamorphic rims of zircons from eclogites indicate that two episodes of zircon growth occurred at ~261Ma and ~238Ma. The identical zircon 206Pb/238U age of 291±4Ma (2σ, MSWD=2.2, n=12) for the quartzite and 290.6±6.2Ma (2σ, MSWD=1.1, n=11) for the magmatic cores of the eclogite suggest that the detrital zircon in quartzite has the same origin as the protolith zircon in the eclogite. The initial opening of the back-arc basin in the Lhasa Block was no later than ca. 290Ma; the final closure of the back-arc basin in the Lhasa Block, led by the arc–continent collision between the Gangdese island arc and the northern Lhasa Block, was no earlier than ca. 266Ma.