Early Paleozoic subduction in the Indochina interior: Revealed by Ordo-Silurian mafic-intermediate igneous rocks in South Laos

Abstract

The early Paleozoic tectonic nature and subsequent orogenic process remain poorly known in the Indochina interior. This study introduces the general geology of South Laos and presents a set of new zircon UPb and HfO isotopic analyses, and whole-rock elemental and Sr-Nd-Pb isotopic data for the mafic-intermediate igneous rocks in South Laos. The representative samples from the previously-mapped Ordo-Silurian strata in the southern Truong Son zone give the formation ages of 434–438 Ma. Five samples from the Tam Ky-Phuoc Son tectonic zone yield zircon UPb ages of 447–485 Ma. The corresponding zircon εHf(t), δ18O and TDM values are in range of +1.6~ −6.5, 6.1–8.6‰, and 1.02–1.79 Ga, respectively. Forty-five mafic-intermediate igneous rocks in South Laos with mg-number = 42–69 have SiO2 = 53.04–61.66 wt%, Al2O3 = 12.13–20.92 wt% and MgO = 2.73–9.55 wt%, and are characterized by enrichment in LILEs and depletion in HFSEs with NbTa negative anomalies. Their (87Sr/86Sr)i ratios range from 0.7044 to 0.7109 and εNd(t) values from −6.2 to −2.6, with (206Pb/204Pb)i = 18.08–18.68, (207Pb/204Pb)i = 15.50–15.77, and (208Pb/204Pb)i = 38.04–38.97. Ten gabbroic samples, with mg-number = 77–79, show similar incompatible elemental ratios and Sr-Nd-Pb-Hf-O isotopic compositions with the other mafic-intermediate igneous rocks, but have lower SiO2 and higher MgO, Cr, Ni and V contents, reflective of a primary mafic magma. Such characteristics suggest a newly-modified wedge source by recycled sediment-derived components. In combination with other geological observations in Central Vietnam, we propose a northerly-younging early Paleozoic igneous event in response to the on-growing accretionary orogenesis in the Indochina interior. The southward consumption of the early Paleozoic Tam Ky-Phuoc Son Ocean in the Indochina interior continued until at least ~430 Ma and the assemblage of the Northern Indochina with the Kontum terrane likely terminated at the early Devonian.