Ages and character of igneous rocks of the Da Lat Zone in SE Vietnam and adjacent offshore regions (Cuu Long and Nam Con Son basins)

The Da Lat Zone in SE Vietnam consists of a Mesozoic orogenic belt that is characterised by abundant Cretaceous igneous rocks that were intruded into or deposited on Permo-Triassic rocks (represented by the Chau Thoi Formation) and Jurassic metasedimentary rocks. The three Cretaceous magmatic phases of the Dinh Quan, Deo Ca, and Ankroet suites of the Da Lat Zone were previously subdivided on the basis of geochemical and isotopic differences and related to within-plate and continental arc magmatism during Paleo-Pacific subduction. Limited dating gave varied estimates for crystallisation ages of the different suites. Here we present new whole-rock chemical data and LA-ICP-MS U-Pb zircon analyses for Cretaceous plutonic and volcanic rocks. The results indicate volcanic arc magmatism started in the Early Cretaceous, represented by the Dinh Quan suite at c. 113–135 Ma, and continued into the mid to Late Cretaceous (Deo Ca, Nha Trang, Dak Rium and Don Duong formations) with magmatism from c. 90 to 109 Ma. Granitoid basement rocks of the Cuu Long Basin yielded similar ages of c. 90 to 92 Ma, representing offshore equivalents. The Ankroet suite represents the youngest magmatic phase and is interpreted as within-plate magmatism from c. 75 to 87 Ma. The Cretaceous rocks generally have very few older zircons and their rarity is interpreted to indicate magmas were generated predominantly by melting of juvenile arc crust.