Paleo-Pacific subduction in Southeast Vietnam: Evidence from Late Cretaceous intrusive rocks in the Dalat zone

Abstract

The Dalat zone in Southeast Vietnam forms a key component of the Paleo-Pacific subduction system, which stretches from the Coastal South China to West Borneo. Petrology, zircon UPb geochronology and in-situ LuHf isotopes, and whole-rock geochemical analyses for gabbros and granitoids in this zone might define details of its role in the Pacific subduction. The gabbros in this region yield an age of ca. 93 Ma, and are enriched in large-ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs) with remarkable negative anomalies in NbTa and Ti, similar to the Cretaceous mafic rocks in the Coastal South China. Their initial 87Sr/86Sr ratios range from 0.70531 to 0.70615, εNd(t) values range from −0.8 to +1.4, (206Pb/204Pb)i ratios range from 18.58 to 18.67 and zircon in-situ εHf(t) values range from +0.4 to +9.7. These characteristics suggest that the gabbros originated from a mantle wedge that had modified by sediment-derived melts. The granitoids found in the Deoca, Dinhquan, Ankroet (Cana) suits of the Dalat zone formed at 103–93 Ma. They exhibit significant variations in major oxide content (SiO2 = 58.84–78.61 wt%; MgO = 0.04–2.66 wt%), classifying them as high-K calc-alkaline I-type granites. In comparison with coeval gabbros, the granitoids show similar whole-rock Sr-Nd-Pb and zircon in-situ LuHf isotopic compositions, arguing for a juvenile crust with arc-like geochemical affinities. The Cretaceous igneous rocks in the Dalat zone of Southeast Vietnam give collectively two age-clusters of ca. 122–112 Ma and ca. 108–75 Ma, resembling those in the Coastal South China and West Borneo. Consequently, these rocks are considered the part of the Cretaceous Paleo-Pacific subduction system, providing a connection between South China in the north and West Borneo in the south.