Magma source feature and eruption age of volcanic rocks in the Tram Tau district, Tu Le Basin

Abstract

A set of samples including porphyritic rhyolite, rhyo-trachyte, trachyte and basalt was collected in the Tram Tau district, Tu Le Mesozoic Basin, in NW Viet Nam for analysis for major, trace, elemental and Rb-Sr and Sm-Nd isotopic compositions. The volcanic rocks are alkaline and highly enriched in trace elements including rare earth elements (REEs). However, primitive mantle and chondrite trace element normalized patterns expose strong negative anomalies for Ba, Sr, Eu and Ti, possibly reflecting fractional crystallization of plagioclase (for Sr and Eu), pyroxene, and especially amphibole (for Ba and Ti). The measured ⁸⁷Sr/⁸⁶Sr isotopic ratios, varying between 0.8142 and 0.75283, are plotted against the corresponding ⁸⁷Rb/⁸⁶Sr forming an isochron that provides an age of 157± 2.9 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.708, corresponding to late Jurassic - early Cretaceous (J₃-K₁) in agreement with previously reported data. The corresponding Nd isotopic compositions expressed as e_{Nd (157Ma)} vary from -8,27 to -2.32 and the Nd model ages are Mesoproterozoic, ranging from 1 to 1.3 Ga. This highly enriched magma was postulated to have formed by mixing of a depleted mantle and Mesoproterozoic crustal rocks (for example, granite) following a continental extension event. Mixing between crustal rocks having strontium isotopic ratios of 0.715 and 0.730 and a depleted mantle with ⁸⁷Sr/⁸⁶Sr ratio of 0.703 would occur, respectively, at 3÷4% and 2÷5% to generate the observed strontium isotopic ratios in the Tram Tau volcanic rocks.

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