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

Pham Trung Hieu, ,Pham Minh, Wang Xiao Lei, Anh Thi Quynh Nong, Kenta Kawaguchi,  Truong Chi Cuong

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.

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