The transition
from oceanic subduction to continental collision is a key stage in the evolution o
f ancient orogens. We present new data
for Early Cretaceous diorite and granite porphyry
from northecentral Tibet to constrain the evolution o
f the BangongeNujiang Tethyan Ocean (BNTO). The diorites have moderate SiO2 and high MgO contents, similar to high-Mg andesites. Zircon grains yield UePb ages o
f 128e124 Ma and positive εH
f(t) values between þ13.2 and þ 16.3, corresponding to H
f depleted-mantle model ages (TDM) o
f 281e131 Ma. The high-Mg diorite was probably
formed by partial melting o
f hydrous mantle wedge
fl
fluxed by slab-derived
fl
fluids in an oceanic subduction setting. The granite porphyries yield zircon UePb ages o
f 117e115 Ma and zircon εH
f(t) values ranging
from þ0.1 to þ4.5. Most samples have high SiO2 and Fe2O3T contents, variable FeOT /MgO and Ga/Al ratios, and are depleted in Ba, Sr, P, and Ti, similar to I- and A-type granites. The granite porphyries were most likely derived
from partial melting o
f juvenile dioritic or granodioritic crust due to break-o
ff o
f the BNTO lithosphere
following collision between the Lhasa and Qiangtang blocks. The Early Cretaceous high-Mg diorite and Atype granite porphyry thus record the Early Cretaceous transition
from oceanic subduction to continental collision along the BangongeNujiang suture zone (BNSZ).
© 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.