Zircon U–Pb geochronology and Sr–Nd–Hf isotopic compositions of the felsic dykes from the Dalat zone, southern Vietnam: petrogenesis and geological significance

Abstract

ABSTRACT Felsic dykes from the Dalat zone (southern Vietnam) are commonly NE–SW oriented and penetrate early cretaceous volcanic-plutonic rocks. These felsic dykes consist of predominant porphyritic granite, minor granitic aplite and aplite. The primary rock-forming minerals are plagioclase, K-feldspar, quartz and subordinate biotite, and the main secondary mineral is epidote. Geochemically, these felsic dykes are characterized by I-type affinity and high SiO2 content (>71 Wt%), low to moderate Al2O3, and variable total alkali (Na2O+K2O). Trace and rare-earth elements show the enrichment of LILEs such as Rb, U, Th, and K and the depletion of HFSEs such as Nb and Ti and a strongly negative Eu anomaly, indicating fractional crystallization of K-feldspar, plagioclase and titanite. Zircons obtained from the felsic dykes are prismatic and exhibit oscillatory zoning with Th/U ratio > 0.1 typical for magmatic origin. In situ zircon LA-ICP-MS zircon U-Pb dating yielded a weighted mean 206Pb/238U age of 91 Ma, temporally coeval with Early Cretaceous magmatism, which has been widely reported to originate from Paleo-Pacific subduction. Their (87Sr/86Sr)i ratio (0.709917 to 0.715288), whole-rock εNd(t) values (−1.3 to +0.9), zircon εHf(t) values (+3.1 to +9.5), and whole-rock Nd and zircon Hf model ages (TDM2) (999–552 Ma) of the studied felsic dykes indicate that they might derive from partial melting of neoproterozoic crustal materials with a input of mantle-derived melt during magma formation and differentiation. The obtained geochemical and isotopic characteristics suggest that these felsic dykes are genetically related to I-type granite and might have formed during the late stage of the Late Mesozoic Paleo-Pacific subduction beneath Indochina or shortly prior to the subsequent extensional regime due to slab retreat.