Geochemistry and geochronology of the Mesozoic Lanong ophiolitic mélange, northern Tibet: Implications for petrogenesis and tectonic evolution.

The Lanong ophiolitic mélange is a typical ophiolitic mélange in the middle section of the Bangong–Nujiang suture zone in northern Tibet. It mainly consists of ultramafic and mafic rocks, and its tectonic setting and formation age remain poorly constrained. In this paper, new geochemical and LA–ICP–MS (laser ablation–inductively coupled plasma mass spectrometer) zircon U–Pb age data obtained from gabbro, gabbro–dolerite, dolerite and basalt of the Lanong ophiolitic mélange are provided. The pillow basalts exhibit N-MORB (normal mid-ocean ridge basalt)-like geochemical features with a zircon U–Pb age of 147.6 ± 2.3 Ma. They were generated by ~ 20–30% partial melting of a depleted mantle source composed of spinel lherzolite. The gabbro, massive basalt and gabbro–dolerite samples are characterised by more depleted and “V”-shaped REE (rare earth element) patterns, and they exhibit variable degrees of boninite-like geochemical characteristics, with a zircon U–Pb age of 149.1 ± 1.2 Ma (gabbro-dolerite). They were derived from the remelting of a significantly refractory mantle source following one or more episodes of previous basaltic melt extraction. Geochemical data of these mafic rocks indicate that they were developed in a continental fore-arc setting, and magmas were derived from depleted mantle sources modified by subducted slab-derived fluids and melts with minor crustal contamination. On the other hand, the dolerites show distinct OIB (oceanic island basalt)-like geochemical features, with a zircon U–Pb age of 244.1 ± 3.0 Ma. They were formed in a rift setting on a continental shelf-slope and originated from a low degree of partial melting of a depleted asthenospheric magma source mixed with some ancient sub-continental lithospheric mantle materials. The signatures presented here, combined with the results of previous studies, suggest that the Lanong ophiolitic mélange probably developed in a convergent plate margin under the southward subduction of the Bangong–Nujiang Tethys Ocean beneath the Lhasa terrane during the Middle Triassic–Early Cretaceous. Namely, the OIB-like dolerites likely reflect an extensional rift setting featuring thin continental crust in the Middle Triassic, and the gabbros, gabbro–dolerites and basalts represent a later stage of a fore-arc basin during the Late Jurassic–Early Cretaceous. [ABSTRACT FROM AUTHOR]