Geochemical and Radiogenic Isotopes Measured from Cenozoic Basalts from Vietnam

Widespread mafic volcanism initiated onshore Vietnam ~17 Ma, but has occurred mainly over the last ~5 Ma. Two eruptive episodes have been observed, first large volume tholeiites followed by less voluminous alkali basalts. Previous studies identified heterogeneous geochemical and radiogenic isotope ratios in the basalts, inferred to reflect mixing of melts from distinct reservoirs. This includes mixing between an EM1 and N-MORB asthenosphere, which was then mixed with an N-MORB and A-MORB (anomalous MORB) mantle with varying contributions from a local EM2 source (Hoang et al., 1996; Hoang et al., 2019). It remains unclear, however, whether the radiogenic 206Pb/204Pb and 87Sr/86Sr with unradiogenic 143Nd/144Nd isotope ratios and enrichment in incompatible elements observed are in fact derived from the lithosphere or related to crustal contamination. To better investigate these questions, here we contribute new Sr-Nd-Hf results for basalts from Dalat, Pleiku, and Quang Nam field locations in south-central Vietnam, and compare the results to previous literature values and recent lithospheric mantle xenolith measurements. Basalts from this study are primarily tholeiites with moderately radiogenic to unradiogenic 143Nd/144Nd and 176Hf/177Hf and radiogenic 87Sr/86Sr. These samples also have elevated large ion lithophile elements over high field strength elements (e.g., Ba/Nb, Rb/Nb, and Th/Ta), suggesting a possible lithospheric origin for the previously observed EM2 signature. One sample from Dalat is andesitic with intermediate Mg# = 60, 87Sr/86Sr = 0.7063, and 143Nd/144Nd = 0.5125, suggesting possible crustal melting, magma mixing, and/or fractional crystallization effects. Analyzed basalts are likewise relatively evolved with moderately high SiO2 wt.% (45-53) and relatively low Mg# (56-61). We suggest that lavas with radiogenic 87Sr/86Sr and moderately radiogenic to unradiogenic 144Nd/143Nd and 176Hf/174Hf may indicate that the EM2 signatures in Cenozoic lavas from south central Vietnam primarily record variable amount crustal assimilation. Further testing will include analysis of additional samples, as well as Pb and 187Os/188Os isotope measurements.