Thermochronology and Geochemistry of Lower Crustal Xenoliths, Central Mongolia: Formation and Evolution of the Deep Crust in an Intracontinental Setting

Lehigh University, Bethlehem PA

Investigators

Abstract

One thing we do not yet understand about Earth is why and how high elevations can form far from the boundaries of the tectonic plates. Within one such region of anomalous topography, young lavas from Tariat. Mongolia carry in them rare samples of the lower crust in the form of what are called ?xenoliths?. These xenoliths can help answer questions about how changes in the lower crust might in turn control surface topography. The formation and destruction of high topography can serve as a control on the evolution of ecosystems and biodiversity, and can alter climate patterns. In particular, understanding when high topography formed in Mongolia has significant ramifications for understanding northern-hemisphere paleoclimate evolution because high-elevation regions in central Mongolia are currently important for the nucleation of northern-Pacific storms. To understand the timing, formation and thermal evolution of the lower continental crust in central Mongolia this multidisciplinary project of Tariat, lower-crustal xenoliths will use thermobarometry, geochronology, thermochronology, and major-element, trace-element, and isotope geochemistry. Thermobarometry will be used to constrain the equilibration temperatures pressures of the xenoliths. Abundant zircon observed in the samples will be dated by U-Pb laser ablation ICP-MS and ID-TIMS to determine the timing of lower-crustal formation and any metamorphism. To the extent that the Hangay region is supported by a crustal root, constraining the age of the root will help place constraints on the timing of rock uplift and, by inference and by modeling, surface uplift. Whole-rock major- and trace-element geochemistry as well as Sr, Nd, Hf, and Pb isotope analyses will help constrain what processes formed the lower-crust. The post-orogenic, long term thermal evolution of the lower crust will be assessed using U-Pb analyses of a number of trace phases likely to be present in these samples, including monazite and apatite. Together with ongoing studies of Mongolian geodynamics, this work, which is part of a Ph.D. dissertation, will determine the timing and evolution of the high topography in central Mongolia, and more broadly, advance our knowledge about geodynamic processes that can produce high topography in continental interiors.