Middle Jurassic MORB-type gabbro, high-Mg diorite, calc-alkaline diorite and granodiorite in the Ando area, central Tibet: Evidence for a slab roll-back of the Bangong-Nujiang Ocean

Abstract

Mesozoic intrusions, including MORB-type gabbros, high-Mg diorites, calc-alkaline diorites and granodiorites, were exposed in the Ando microcontinent that is bounded between the Qiangtang and Lhasa terranes. Discoveries of these Mesozoic intrusions have provided new petrogenetic constraints on our understanding of Bangong-Nujiang ocean evolution. Zircon U–Pb dating shows that these intrusions formed in the early-middle Jurassic (174–177Ma). The gabbros have relatively flat REE distribution patterns, which is analogous to the geochemical features of MORB. Their positive εNd(t) values (εNd(t)=4.4–5.5) are consistent with those of ophiolites along the Bangong-Nujiang suture zone. These gabbros are also characterized by enrichments of fluid-sensitive elements and negative to positive Nb anomalies, indicative of the influence of subduction-related compositions in their mantle source. These features suggest that the gabbros were most likely originated from asthenosphere-derived melts metasomatized by enriched lithospheric mantle during the upwelling. The high-Mg diorites are characterized by typical features of high compatible elements (MgO=8.3–10.24wt%, Cr=400–547ppm, Ni=120–152ppm), high Mg# (70–74) and low Sr/Y ratios. Their high initial 87Sr/86Sr isotopic ratios and negative εNd(t) values (−10.5 to −10.8), together with their sanukitic characteristics, imply that the high-Mg diorites were probably produced by partial melting of mantle peridotites metasomatized by slab-derived melts and aqueous fluids. The calc-alkaline diorites have relatively high MgO (4.04–5.50wt%), Cr, Ni contents and Mg# (56–59), as well as high (86Sr/87Sr)i ratios and negative εNd(t) values (−7.5 to −7.3), suggesting that they were most likely formed by partial melting of the Ando basement rocks with significant input of mantle components. The granodiorites are peraluminous and have higher (86Sr/87Sr)i ratios and more negative εNd(t) values (−10.6 to −10.8), similar to those of the granitic gneisses of the Ando basement, suggesting partial melting of the Ando basement. Taking into account the spatial and temporal distribution of the Mesozoic magmatic rocks in the Southern Qiangtang terrane, we suggest that the formation of the Early-Middle Jurassic magmatic “flare-up” in the Southern Qiangtang terrane was related to the asthenospheric upwelling triggered by the roll-back of the subducted Bangong–Nujiang oceanic slab.