GEOCHEMICAL AND STABLE ISOTOPIC EVALUATION OF FENGHUOSHAN GROUP LACUSTRINE CARBONATES, NORTH-CENTRAL TIBET: IMPLICATIONS FOR THE PALEOALTIMETRY OF THE MID-TERTIARY TIBETAN PLATEAU

A sedimentologic, petrologic, and geochemical evaluation of lacustrine carbonates from the Eocene-Oligocene Fenghuoshan Group of north-central Tibet provides insight into the paleoenvironmental and paleolimnological setting of the Hoh Xil basin during the mid-Tertiary. Fenghuoshan lacustrine rocks consist primarily of carbonate mudstones and wackestones. These carbonates are generally less than 3 m in thickness and are intercalated with fluvial and alluvial deposits. Individual limestone beds coarsen and contain increasing amounts of siliciclastic material upward. Fenghuoshan carbonates also contain abundant ostracode, bivalve, and charophyte fossils. Sedimentologic evidence suggests carbonate deposition occurred in shallow, relatively short-lived lacustrine systems that were subsequently filled by alluvial deposits of coeval fluvial systems.

Stable isotope analysis of Fenghuoshan carbonates indicate a lack of covariance between C and O isotopic systems with d¹⁸O values ranging from –11.7‰ to –9.6‰ , and d¹³C values between –7.4‰ to –2.2‰. The relationship between the two isotopic systems support the sedimentological interpretation of a shallow, open system lacustrine setting during the time of deposition. This interpretation is reinforced by the major element composition from the Fenghuoshan lacustrine rocks. K, Mg, Na, and Sr concentrations indicate non-evaporative conditions while the relationship between the concentrations of Ca, Sr, and Fe signify well oxygenated lake waters. In addition, relationships between Ca, Mg, and Sr suggest that Fenghuoshan lacustrine rocks have not undergone significant diagenetic alteration. Collectively, these data indicate that the isotopic composition of Fenghuoshan carbonates may serve as an accurate proxy for the isotopic composition of meteoric water in the basin at the time of deposition, from which the paleoaltimetry of central Tibet during the mid-Tertiary can be estimated. Model results using the isotopic data from Fenghuoshan carbonates indicate the hypsometric mean elevation of the drainage basins feeding Hoh Xil lakes were about 2 km high and that these lakes were situated at elevations below 1 km.