Late Cretaceous back‐arc extension and arc system evolution in the Gangdese area, southern Tibet: Geochronological, petrological, and Sr‐Nd‐Hf‐O isotopic evidence from Dagze diabases

Back‐arc extension and asthenosphere upwelling associated with oceanic lithospheric subduction affect the structure and thermal regime of the arc lithosphere, which often triggers widespread extension‐related mafic magmatism. Although it is commonly accepted that the Neo‐Tethyan oceanic lithosphere subducted beneath the southern Lhasa block, resulting in the well‐known Late Mesozoic Gangdese magmatic arc, the possible role of contemporary back‐arc extension and asthenosphere upwelling has been disputed due to a lack of evidence for extension‐related mafic magmatism. Here, we report detailed petrological, geochronological, geochemical, and Sr‐Nd‐Hf‐O isotopic data for the Dagze diabases located in the north of the Gangdese district, southern Lhasa block. The zircon U‐Pb analyses indicate that they were generated in the Late Cretaceous (ca. 92 Ma) instead of the Eocene (42–38 Ma) as previously believed. These mafic rocks are characterized by variable MgO (4.0–12.2 wt %) and Mg # (42 to 71) values combined with flat to slightly enriched ([La/Yb] N  = 1.87–5.23) light rare earth elements (REEs) and relative flat heavy REEs ([Gd/Yb] N  = 1.36–1.87) with negative Ta, Nb, and Ti anomalies (e.g., [Nb/La] PM  = 0.16–0.51). They also have slightly variable ε Nd ( t ) (−1.25 to +4.71) and low initial 87 Sr/ 86 Sr (0.7045–0.7058) values with strong positive igneous zircon ε Hf ( t ) (+8.0 to +12.1) and low δ 18 O (5.31–6.12‰) values. The estimated primary melt compositions are similar to peridotite‐derived experimental melts. Given their high melting temperature (1332 to 1372°C) and hybrid geochemical characteristics, we propose that the Dagze mafic magmas likely represent mixtures of asthenospheric and enriched lithospheric mantle‐derived melts that underwent minor crustal assimilation and fractional crystallization of clinopyroxene. Taking into account the spatial and temporal distribution of Mesozoic mafic‐felsic magmatic rocks and regional paleomagnetic and basin data, we suggest that the Dagze mafic rocks resulted from asthenospheric upwelling associated with intracontinental back‐arc extension during the rollback of subducted Tethyan oceanic lithosphere in the Late Cretaceous.