Early Miocene (21–23 Ma) high Sr/Y volcanic field in the Gangdese batholith, southern Tibet

Abstract

A detailed petrographic, geochemical and geochronological study reveals a suite of Miocene (~21–23 Ma) high Sr/Y and La/Yb volcanic rocks of dacite compositions and adjacent Late Cretaceous tonalite and granodiorite (~78–80 Ma) in the Qiewa of the southern Gangdese batholith. The analyses results show that (1) the dacites are characterized by Na-rich, peraluminous, high Sr/Y and La/Yb ratios, showing adakitic affinity, the granitic wall-rocks (tonalite and granodiorite) and enclave have the Na-rich and metaluminous features; and (2) the dacites have relatively radiogenic Sr ( 86 Sr/ 87 Sr(t) = 0.7049–0.7061), unradiogenic Nd (ε Nd (t) = −3.8 - +0.1), and positive Hf (ε Hf (t) = +3.0 − +9.5) isotope compositions, while its associated plutonic rocks have relatively lower Sr ( 86 Sr/ 87 Sr(t) = 0.7035–0.7039), but higher Nd (ε Nd (t) = +3.3 − +4.9) and Hf (ε Hf (t) = +11.1 − +14.7) isotope ratios. Data presented in this study demonstrates that Early Miocene dacites are generated by partial melting of thickened (~70 km) juvenile lower crust, whereas the Late Cretaceous granitic wall-rocks (tonalite and granodiorite) and enclave are derived from partial melting of juvenile crust with a normal crustal thickness (~40–44 km). Deposition of ~23 Ma dacitic volcanic rocks directly upon the ~80 Ma plutonic rocks suggests that the Gangdese batholith experienced an episode of rapid exhumation before ~23 Ma, possibly triggered by the delamination of thickened continental lithosphere. • The Qiewa dacites and the underlying plutonic rocks are formed at ~21–23 Ma and ~ 78–80 Ma, respectively. • The high Sr/Y dacites are derived from partial melting of a thickened (~70 km) enriched lower crust. • The plutonic rocks result from partial melting of a normal thickness (~40–44 km) of juvenile curst. • The Gangdese batholith experienced an episode of rapid exhumation before ~23 Ma.