Growth of the continental crust induced by slab rollback in subduction zones: Evidence from Middle Jurassic arc andesites in central Tibet

Abstract

Andesitic rocks have similar compositions to average continental crust and have played a central role in improving our understanding of the formation and evolution of the continental crust. Here we present data for andesites from the Jiaqiong area of the Bangong–Nujiang suture zone (BNSZ), Central Tibet. LA–ICP–MS zircon U–Pb ages demonstrate that these andesites were generated during the Middle Jurassic (169–164 Ma). The Jiaqiong andesites have variable Mg# (molar 100 × Mg/[Mg + Fe] = 35–63) values, and Cr (3.02–175 ppm) and Ni (5.70–47.7 ppm) contents. These andesitic rocks are enriched in light rare earth elements (REEs) ([La/Yb]N = 5.2–14.6) and yield relatively flat heavy REE patterns ([Gd/Yb]N = 1.4–2.1) with negative Ta, Nb, and Ti anomalies. They also have moderately high initial 87Sr/86Sr (0.7077–0.7092) ratios and variable negative εNd(t) (−11.0 to − 3.3) and zircon εHf(t) (−13.7 to + 2.1) values. These geochemical characteristics suggest that the Jiaqiong andesites were most probably produced by partial melting of mélange rocks. Combining our data with the regional geology, particularly that of nearby ophiolites and Jurassic andesitic rocks, we conclude that the formation of these andesites was triggered by asthenospheric upwelling and hot corner flow caused by the rollback of the northward-subducting Bangong–Nujiang Tethyan oceanic lithospheric slab during the Middle Jurassic. The mélanges provided the material for the formation of andesites, and probably make a substantial contribution to the vertical growth of the continental crust in subduction zones.