New constraints on the tectono-magmatic evolution of the central Gangdese belt from Late Cretaceous magmatic suite in southern Tibet

Abstract

In the Gangdese magmatic belt of southern Tibet, widespread Late Mesozoic magmatism has been reported, although their petrogenesis and tectonic evolution remain ambiguous. Here we present new zircon U-Pb and geochemical data on a gabbro-diorite suite near the Namling County in the central Gangdese belt, that provide insights into the geodynamic processes in this region. Zircon U-Pb data from LA-ICP-MS and SHRIMP II analyses indicate that the granite crystallized at ca. 87 Ma, and the gabbro-diorite suite was emplaced at ca. 91–95 Ma, both suggesting an early Late Cretaceous magmatism in the central Gangdese belt. The gabbro-diorite suite displays wide ranges of SiO2 (57.21–44.25 wt%) and MgO contents (9.04–3.18 wt%), with high Al2O3 (21.47–16.14 wt%) and Na2O/K2O (1.10–5.84) values. Compared to these, the granite rocks show limited variations of SiO2 (76.97–76.40 wt%), high Na2O + K2O (8.84–8.04 wt%), K2O/Na2O (2.33–1.30) and Sr/Y (48.99–80.36), with low Mg# (30.77–20.39) values and significant Eu anomalies (Eu/Eu* = 1.20–0.24). The gabbro-diorite rocks are characterized by typical arc-type features including enrichment of light rare earth elements (LREE) (LaN/YbN = 11.52–5.06 > 1) and large ion lithophile elements (LILE) (e.g. Cs, K and Rb), but are depleted in high strength field elements (HSFE) (e.g. Nb, Ta, Ti and P). The granitic rocks are also enriched in LREE (LaN/YbN = 55.42–26.55 > 1) and LILE (e.g. Cs, Rb, K and Pb), but depleted in HSFE (e.g. Nb, Ta, Ti and P). The gabbro-diorite rocks show distinctly positive εNd(t) (+3.5 to +4.2) and εHf(t) values (+8.5 to +15.6) values with a restricted range, with relatively homogeneous and low (87Sr/86Sr)I ratios (0.704213–0.703820). The granitic rocks also show positive εNd(t) values (+4.1 to +4.4) and consistent (87Sr/86Sr)I ratios (0.703977–0.703180). Our data suggest that the gabbro-diorite rocks were mostly sourced from partial melting of the depleted mantle that was metasomatized by fluids released from the subducted Neo-Tethys oceanic slab. The granite was probably produced by partial melting of the juvenile crust with no participation of mantle material. We correlate the magmatism with tectonics associated with the roll-back of the Neo-Tethys oceanic slab during the early Late Cretaceous.