Forearc lava stratigraphy of the Beila Ophiolite, north-central Tibetan Plateau: Magmatic response to initiation of subduction of the Bangong-Nujiang Meso-Tethys Ocean

Abstract

It has been suggested that boninites and associated volcanic rocks found in supra-subduction zone (SSZ) ophiolites form in a forearc setting, meaning that these rocks could provide crucial constraints on understanding the magmatic response to subduction initiation. Here, we report newly documented forearc basalts (FAB) and boninites from the Beila Ophiolite of the Bangong–Nujiang Suture Zone, north-central Tibetan Plateau. The lava stratigraphy, from bottom to up, includes FAB, basaltic boninites, and boninites, similar to lavas from the Izu–Bonin–Mariana (IBM) forearc. The FAB has geochemical characteristics similar to those of IBM forearc basalts, with nearly flat REE patterns, and shows enrichment in Rb, Ba, Pb, and Sr and depletion in Nb, Ta, and Zr, suggesting an origin by decompression melting of a depleted mantle source with minor input of subducting plate materials. The basaltic boninites and boninites show similar petrological features, as well as porphyritic textures with phenocrysts of olivine, clinopyroxene, and minor orthopyroxene. The basaltic boninites and boninites display uniform major- and trace-element characteristics that are similar to those of typical boninites, with high MgO (>10.78 wt%), Cr, and Ni contents, extremely low TiO2 (<0.21 wt%), rare earth element (REE), and high-field-strength element (HFSE) contents, and U-shaped REE patterns. In addition, the basaltic boninites and boninites have lower ɛNd(t) values than those of the FAB and display variable degrees of influence by slab melts and fluids. Geochemical modeling further suggests that the boninites were generated later during residual highly depleted mantle fluxing by the subducting plate. Together with contemporary boninitic and arc-related magmas along the Bangong-Nujiang Suture Zone and adjacent terranes (the North Lhasa and South Qiangtang), it suggests that the whole Meso-Tethys Ocean was subducted since the Middle Jurassic. Moreover, the Beila lava stratigraphy with FAB overlain by boninite is similar to that found in the IBM forearc and in many ophiolites (e.g., Troodos, Oman, and Xigaze), and represents a continuous geochemical transition from MORB-like to boninitic affinities. This similarity suggests that initiation of subduction might be commonly involved in the formation of ophiolites.