U–Pb zircon age and geochemistry of the Cuocun gabbro in the southern Lhasa Terrane: Implications for Early Cretaceous rollback of the Neo‐Tethyan oceanic slab

Abstract

The geodynamic mechanism of the Early Cretaceous magmatic rocks in the southern Lhasa Terrane remains controversial. To constrain the Early Cretaceous petrogenesis and tectonic evolution of the Neo‐Tethyan oceanic slab, we have undertaken zircon U–Pb dating, whole‐rock major and trace element analyses, and Sr–Nd–Pb–Hf isotopic studies of the Cuocun gabbro from the southern Lhasa Terrane. The zircon U–Pb dating yielded an Early Cretaceous crystallization age of 146.9 ± 0.6 Ma (MSWD = 0.77, 2σ). The Cuocun gabbro is enriched in large‐ion lithophile elements (e.g., Rb, Sr, and Ba) and depleted in high‐field‐strength elements (e.g., Nb, Ta, and Ti). The initial 87Sr/86Sr value are low (0.70361–0.70460), εNd(t) and εHf(t) values are high and positive (from +5.1 to +6.6 and from +11.0 to 17.0, respectively), and Pb isotope specific value are homogeneous (initial 206Pb/204Pb = 18.163–18.457, 207Pb/204Pb = 15.541–15.632, and 208Pb/204Pb = 38.262–38.673). The gabbro originated from the partial melting of a depleted mantle wedge that had reacted with slab‐derived fluids, and the ascending magmas underwent fractional crystallization without significant crustal contamination. Combining our data with those from previous studies of Early Cretaceous magmatism in the southern Lhasa Terrane, we suggest that the Early Cretaceous magmatic rocks (147–130 Ma) in the southern Lhasa Terrane were generated within a continental marginal arc setting related to the rollback of the northwards subducting Neo‐Tethyan oceanic slab.