Identifying Tethys oceanic fingerprint in post-collisional potassium-rich lavas in Tibet using thallium isotopes

Abstract

Following the Indian-Asian continental collision, the Tibetan Plateau experienced continuous magmatism from south to north, most of which is characterized by high potassium (K) content and negative niobium‑tantalum‑titanium (Nb-Ta-Ti) anomalies. We performed the first exploration using thallium (Tl) isotopes as potential tracers of the source of post-collisional K-rich magmatism. A case study on Tl isotope systematics of K-rich lavas in Ashikule volcanic basin (AVB) in northwestern Tibet is provided, which represents the youngest volcanism in Tibet. Thallium abundances of the AVB lavas are significantly enriched compared to other magmatic settings, with concentrations from 148 to 8259 ng/g. Thallium isotope ratios (reported as ε205Tl) extend to some of the heaviest yet measured in igneous materials, with a range of ε205Tl between −2.7 and + 6.4 ε-units. We evaluate if secondary processes can account for the range in Tl isotope ratios, systematically examining the influences of alteration, degassing, fractional crystallization, and assimilation. Only two samples appear to be influenced by secondary processes, interpreted as kinetic fractionation during degassing, and one sample may be contaminated by sulfides with coupled elevated Tl and Pb contents. The remainder of the Tl isotope ratio range (−2.0 to +5.1 ε-units) we interpret to reflect the source region(s) from which the magmas are derived. The AVB lavas have restricted strontium and neodymium isotopes, suggestive of a homogeneous source, and therefore an apparent decoupling of radiogenic and stable isotope systems is apparent in these lavas. The variation in Tl isotope ratios is interpreted to reflect the contribution of oceanic sediments, which may be associated with ancient Paleo- and Neo-Tethys subductions.