The Bangong Lake ophiolite (NW Tibet) and its bearing on the tectonic evolution of the Bangong-Nujiang suture zone

The Jurassic Bangong Lake ophiolite, NW Tibet, is a key element within the western part of the Bangong-Nujiang suture zone, which marks the boundary between the Lhasa and Qiangtang blocks. It is a tectonic melange consisting of numerous blocks of peridotite, mafic lavas and dikes. The mantle peridotites include both clinopyroxene-bearing and clinopyroxene-free harzburgites. The Cpx-bearing harzburgite contains Al-rich spinel with low Cr#s (20-25), resembling peridotites formed in mid-ocean ridge settings. On the other hand, the Cpx-free harzburgite is highly depleted with Cr-rich spinel (Cr# = 69-73), typical of peridotites formed in subduction zone environments. Mafic rocks include lavas of N-MORB and E-MORB affinity and boninites. The N-MORB rocks consist of pillow lavas and mafic dikes, whereas the E-MORB rocks are brecciated basalts. The boninites have high SiO2 (53.2-57.9 wt%), MgO (6.5-12.5 wt%), Cr (166-752 ppm) and Ni (63-213 ppm) and low TiO2 (0.22-0.37 wt%) and Y (5.34-8.10 ppm), and are characterized by having U-shaped, chondrite-normalized REE patterns. The N-MORB and E-MORB lavas probably formed by different degrees of partial melting of primitive mantle, whereas the boninites reflect partial melting of depleted peridotite in a suprasubduction zone environment. The geochemistry of the ophiolite suggests that it is a fragment of oceanic lithosphere formed originally at a mid-ocean ridge (MOR) and then trapped above an intraoceanic subduction zone (SSZ), where the mantle peridotites were modified by boninitic melts. The Bangong-Nujiang suture zone is believed to mark the boundary between two blocks within Gondwanaland rather than to separate Gondwanaland from Eurasia. (C) 2007 Elsevier Ltd. All rights reserved.