Process of lithospheric delamination beneath the Lhasa–Qiangtang collision orogen: Constraints from the geochronology and geochemistry of Late Cretaceous volcanic rocks in the Lhasa terrane, central Tibet

Abstract

The geochemistry of post-collisional granitoids can be used to reconstruct crustal thickness and constrain deep geodynamic processes. Two series of volcanic rocks of the Jiangba Formation in the Xiongmei area, central Tibet, record the process of lithospheric delamination in the Lhasa–Qiangtang collisional orogen. The andesites and dacites crystallized at c. 87 and 85 Ma, respectively, and have positive zircon εHf(t) values (2.7–5.4 and 2.7–7.1, respectively). They have calc-alkaline compositions, with relatively high Mg# (100 × molar Mg2+/[Mg2+ + Fe2+]; 39–66), Sr contents, and Sr/Y and (La/Yb)N ratios (subscript N signifies normalization to chondrite), low Y and Yb contents, and no clear negative Eu, Sr, or Ti anomalies, similar to adakites. The andesite–dacite series likely formed by partial melting of foundering juvenile mafic lower crust with residual garnet, followed by the interaction of this melt with peridotite. The rhyolite series was emplaced at c. 81 Ma, and has positive zircon εHf(t) values (1.0–3.5). The rhyolites are characterized by non-adakitic signatures and have high-K calc-alkaline to shoshonite compositions, with low MgO contents (Mg# = 21–34) and Sr/Y and (La/Yb)N ratios, coupled with flat HREE patterns and marked negative Eu, Sr, and Ti anomalies. Geochemical data indicate that these rhyolites were generated by partial melting of a much shallower crustal root, with a plagioclase-rich residue. Zircon U–Pb ages and different compositional trends for the two series indicate that the lithospheric mantle keel was partially removed by small-scale mantle convection that triggered the Late Cretaceous high-Mg# adakitic magmatism in the Lhasa–Qiangtang collisional orogen. Partial lithospheric delamination increased heat flow at the base of the crust and caused the thinned crust to melt, producing the low-Mg# non-adakitic rocks.