Negligible surface uplift following foundering of thickened central Tibetan lower crust

This study used clinopyroxene (cpx) compositions and zircon Hf-O isotopes of Eocene adakitic rocks (EARs) from the Qiangtang block to resolve the mechanism(s) responsible for the formation of the central Tibetan Plateau. The two leading and opposing hypotheses for the origin of these rocks are (1) partially molten foundered lower crust, and (2) partial melting of continentally subducted upper crust. The consensus is that some crustal sources within the mantle have reached eclogite facies, while evidence remains insufficient. Reverse zonation for cpx in high Mg# andesitic samples shows a low Mg# core with lower Sr and Sr/Y than the high Mg# rim, suggesting derivation of parent magma by interaction between some eclogite-derived felsic melts and mantle peridotite. Overall, the mantle-like zircon delta O-18 (mean value of similar to 5.9 parts per thousand) and epsilon(Hf)(t) (up to +6.7) values argue for a mafic source rather than buried upper-crustal rocks. Given the EARs were formed within a short time span after the end of crustal shortening, the original felsic melts were most likely derived from the foundered and eclogitized lower crust. The foundering process explains the early Eocene low-relief topography and the intermediate, eclogite-free modern crustal composition of central Tibet. Surface uplift as a response to lithosphere removal, however, was likely negligible, based on various lines of evidence, including sediment provenance, isotope paleoaltimetry, and thermochronology, perhaps because the central Tibetan crust was weak.

Automated summary

This study used clinopyroxene compositions and zircon Hf-O isotopes to investigate the formation mechanism of Eocene adakitic rocks from the Qiangtang block in central Tibetan Plateau. Results suggest that the origin of these rocks is likely from foundered and eclogitized lower crust rather than subducted upper crust. The mantle-like zircon isotopes support a mafic source hypothesis. Surface uplift in central Tibet may not be significant due to weak crust as a response to lithosphere removal.