Diagenesis of shallowly buried Miocene lacustrine carbonates from the Hoh Xil Basin, northern Tibetan Plateau: Implications for stable-isotope based elevation estimates

Abstract

The oxygen isotopic composition of lacustrine carbonate deposits has been frequently used to reconstruct paleoaltimetry of orogenic belts. However, the primary oxygen isotopic composition of lacustrine carbonate can be partially or wholly altered by post-depositional diagenetic processes, such as deep burial recrystallization and the interaction with geologically younger meteoric waters. To better understand how ancient lacustrine carbonates are diagenetically modified by meteoric waters, we present detailed mineralogy, petrography, stable carbon and oxygen isotopic compositions, and major and minor element geochemistry of lacustrine carbonates from the Miocene Wudaoliang Group, which underwent only shallow burial (<300 m) in the Hoh Xil Basin, northern Tibetan Plateau. Our observations indicate that the primary mineralogy in the Wudaoliang carbonate muds consists of a mixture of both low-Mg and high-Mg calcite. During or shortly after deposition, the carbonate muds experienced stabilization and associated carbonate cementation in a closed phreatic meteoric diagenetic system. Interaction with ^(18)O-depleted meteoric waters during diagenesis slightly reduced ^(18)O_c values, but overall the Wudaoliang carbonates largely retained primary oxygen isotope compositions. Our data support previous interpretations that δ^(18)O_c values of the Wudaoliang Group lacustrine carbonates are indicative of the primary δ^(18)O composition of Miocene meteoric water and can be used for paleoelevation estimate of the Miocene Hoh Xil Basin, northern Tibetan Plateau. By demonstrating the oxygen isotopic fidelity of the Wudaoliang carbonates, we are able to rule out open-system meteoric diagenesis from younger meteoric waters and subsequent isotopic alteration. This study highlights that integration of scanning-electron-microscope and cathodoluminescence petrography with stable isotopic and mineralogical data enables the determination of the extent of meteoric diagenetic alteration in lacustrine carbonates, particularly whether the diagenetic systems were open or closed with respect to the oxygen isotope.