DIAGENETICALLY ALTERED OXYGEN ISOTOPE VALUES OF NONMARINE CARBONATE IN THE LHASA TERRANE WITH IMPLICATIONS FOR PALEOELEVATION STUDIES OF TIBET

Oxygen isotope values (δ¹⁸O, VPDB) of carbonate precipitated in nonmarine environments can be used to infer paleoelevation, and can therefore provide critical constraints for tectonic and geodynamic models. Inherent within such studies is the assumption that diagenetic alteration of the carbonate has been negligible, thus the measured oxygen isotope values reflect the meteoric water from which the carbonate was initially precipitated. We collected and analyzed carbonate from Cretaceous nonmarine and marine strata exposed in the Lhasa terrane of southern Tibet in order to determine if the oxygen isotope values of these rocks have been altered by recent fluid-rock interactions. Pedogenic and lacustrine carbonate from the southern portion of the Lhasa terrane have δ¹⁸O values of -12 to -17 ‰, suggesting they were precipitated in isotopically depleted meteoric water. However, interbedded marine limestone beds from the same locations also have low δ¹⁸O values (-12 to -20 ‰); values which are inconsistent with expected δ¹⁸O values of carbonate precipitated in marine water (~0 ‰). Individual petrofacies were micro-sampled but all yielded low δ¹⁸O values (-10 to -20 ‰). Micritic microfabrics, which are normally thought to be less susceptible to diagenetic alteration, were also pervasively altered. Our analyses indicate the δ¹⁸O values of carbonate within Cretaceous strata in southern Lhasa have been reset and should not be used in paleoaltimetry studies. Calculations indicate the depleted δ¹⁸O values measured in these carbonate samples can be explained by interactions with modern meteoric water in the Lhasa terrane at temperatures similar to the present mean annual temperature of the region. In contrast to southern Lhasa, many of the marine and nonmarine carbonate samples collected in the northern portion of the Lhasa terrane have δ¹⁸O values of -4 to -6 ‰, suggesting the oxygen isotope values within these rocks are relatively unaltered and that carbonate in this portion of the Lhasa terrane may prove more reliable for paleoelevation studies.