Intra-continental boninite-series volcanic rocks from the Bangong-Nujiang Suture Zone, Central Tibet

Abstract

The boninitic volcanic rocks from Rutog, western Bangong-Nujiang Suture Zone (BNSZ), are characterized by high MgO, SiO2 but low TiO2 contents and interpreted as the products of intra-oceanic subduction in earlier studies. Recently, we discovered new breccia-like boninitic volcanic rocks (mainly trachyandesite and trachyte) that overlie the Rutog ophiolitic peridotites in BNSZ. These volcanic rocks not only show typical boninitic petrographical features of porphyritic texture in which euhedral augite phenocrysts (ca. 5 vol%) are embedded in fine-grained groundmass composed of pyroxene, plagioclase, quartz and minor glass, but also display strong boninitic geochemical affinities with high Mg# (61.59–67.90), Al2O3/TiO2 (40.31–55.97) ratios and low TiO2 (0.26–0.35 wt%) contents. In addition, their rare earth elemental patterns show a slight right-dip pattern with (La/Gd)N values of ~1.28 and (Gd/Yb)N values of ~1.31. Given the evolutionary trend from magnesium-rich intermediate to acid series, the boninite-series are therefore used to describe the vlocanic rocks. Zircon U-Pb isotopic dating on these boninite-series volcanic rocks yielded an age of ca. 79.3 ± 2.0 Ma, after the closure of the Bangong-Nujiang Tethyan Ocean (BNTO) that occurred before 96 Ma. The newly recognized boninite-series volcanic rocks probably formed more than 20 Ma after the latest regional magmatism related to subduction process and thus the subduction setting is not suitable for these boninite-series volcanic rocks. The enrichment in compatible elements and depletion in high field elements indicate the Rutog boninite-series originated from a severely depleted mantle while several lines of evidence (e.g., the enrichment in light rare earth elements, large iron lithophile elements) imply that the mantle source has been modified by continental crust. Given the widespread Cretaceous high Sr/Y magmatic rocks within BNSZ, the Rutog boninite-series rocks may provide an example of the crust-mantle interaction as a result of the delamination in a partially thickened and cratonized convergent margin during the late Cretaceous within BNSZ, central Tibetan Plateau.