Geochemical and zircon U-Pb geochronological constraints on late mesozoic Paleo-Pacific subduction-related volcanism in southern Vietnam

Abstract

Late Mesozoic volcanic rocks comprising mainly basalt and basaltic-andesite to dacite occur in south-central Vietnam (Dalat zone) and to a lesser extent in southwestern Vietnam (Bay Nui area). Mineral and whole-rock chemistry indicate a calc-alkaline affinity for samples in the Dalat zone and a high-K calc-alkaline to shoshonitic affinity for rocks in the Bay Nui area. Mineral characteristics and variation diagrams of selected elements suggest that fractional crystallization dominated during magma differentiation. The Bay Nui volcanic rocks generally are more enriched in potassium and LILEs (large-ion lithophile elements) than volcanic rocks from the Dalat zone, which may indicate a more evolved nature or crustal assimilation. The similar chemical characteristics and eruption/emplacement age range of volcanic and plutonic rocks (ca. 90–110 Ma) of equal silica concentration indicate that the magma feeding the volcanic eruptions had the same source as that of the plutonic rocks. The observed mineral and whole-rock compositions with enrichment in LILEs, depletion in HFSEs (high field strength elements), and noticeably negative Nb, Ta, and Ti anomalies are characteristic for arc signatures. Zircon U-Pb geochronological data for the volcanic rocks indicate an age range of 95–105 Ma for the eruption. These geochemical and geochronological data link this Late Mesozoic volcanism with continental arc magmatism driven by the subduction of the Paleo-Pacific beneath eastern Indochina. Zircon xenocrysts with a likely magmatic origin cluster around 350 Ma and 250 Ma, indicating two earlier magmatic events most likely related to the subduction of the Paleo-Tethys beneath western Indochina and the subsequent Indosinian orogeny.