Collision-induced basalt eruptions at Pleiku and Buôn Mê Thuột, south-central Viet Nam

Abstract

Neogene-Quaternary basalts occur as dispersed volcanic clusters in the vicinity of the Tethyan tectonic belt, possibly representing ‘far-field’ effects of the Early Tertiary collisions of Gondwana fragments with the southern margin of Eurasia. In Indochina, such a ‘Diffuse Igneous Province’ post-dates the 45–42Ma ‘hard’ India-Asia collision and southeastward, collision induced (c. 30–17Ma.), extrusion of Indochina. Extrusion was accommodated by left-lateral strike-slip shearing on the Ailao Shan-Red River Fault, coeval with seafloor spreading in the East Viet Nam (South China) Sea. The Indochina basalts mostly comprise shield-building tholeiites capped by small-volume undersaturated types, the latter often bearing mantle xenoliths and ‘exotic’ xenocrysts such as sapphire, zircon. They appeared at c. 17Ma, more-or-less coinciding with the cessation of both continental extrusion and seafloor spreading. At this point extensional stress appears to have shifted westwards to continental Indochina, with magmatic activity appearing, characteristically, at ‘pull-apart’ basins. However, the relationship of mantle melting beneath this region to its geodynamic setting is controversial, being variously attributed to mantle plumes, extreme lithospheric stretching, and lateral asthenospheric displacement. There is little or no definitive evidence for regional mantle upwelling while lithosphere stretching alone appears to be insufficient to allow for melting, Here, we present geochemical and Sr, Nd, and Pb isotopic (and paleomagnetic data), for cored sections from the Pleiku and Buon Mê Thuột plateaus in south-central Viet Nam, representative in most respects of the Indochina province as a whole. In the Pleiku shield olivine tholeiite flows are intercalated with quartz tholeiites while, in contrast, alkali basalts predominate over olivine tholeiite in the Buon Mê Thuột (BMT) shield. The first of these features (in Pleiku) probably reflects crustal wall-rock reaction while the second (at BMT), suggests an atypical magma supply system, possibly reflecting a contemporaneous change in the regional stress field. In common with most Indochina shields, tholeiites at Pleiku and BMT show slightly higher Mg – for equivalent MgO contents than those of later-stage undersaturated magmas, suggesting the former may have interacted with lithospheric mantle, depleted by prior melting. On the other hand, because the xenolith- and xenocryst-bearing post-shield magmas show near-primitive, uncontaminated character, and probably tap the more fertile asthenosphere, realistic potential temperatures (Tp) may be interpolated, estimates ranging between c. 1440° and 1660°C, as compared to expected ‘normal’ values (1280–1300°C). The new data are used to re-examine earlier postulates that thermally anomalous asthenosphere was displaced laterally prior to and during the Early Tertiary India–Asia collision, serving as a potential driver of lithosphere extrusion while allowing for localized, transtensional mantle melting. New paleomagnetic data confirm indications of existing data that there has been little or no tectonic rotation, as predicted by the extrusion model. However, variable asthenospheric flow paths are suggested by the distribution of Pleiku, BMT, and other volcanic centers, matched by geodetic data, suggesting minimal traction between Indochina lithosphere and the underlying ductile mantle.