Online

Pirrie, Gavin George

Geology

2022

M.S.

The Mongol-Okhotsk suture is integral in understanding the closure of the Mongol-Okhotsk Ocean, an ocean basin that once separated the Siberian craton from terranes of the Central Asian Orogenic Belt in southern Mongolia. However, the timing of closure and exact location of the suture are poorly known. This study examines two areas in northeastern Mongolia adjacent to the suture, the Ereendavaa Metamorphic Core Complex (EMCC) and the Duch Gol Basin, to aid in this understanding. The EMCC is an Early Cretaceous metamorphic core complex with an associated top-to-the-NW extensional detachment fault. Two sets of mineral lineations have previously been documented in the core complex--one NW-SE-trending set in the shear zone associated with exhumation and one NE-SW-trending set located in the high-grade core. The Duch Gol Basin is the remnant of an accretionary wedge that contains Devonian-Triassic marine sediments and volcanic rocks that have been metamorphosed. Together, these two areas potentially record information on the timing of collision and subsequent extension. Oriented samples and structural data were obtained from transects across the EMCC, the Duch Gol Basin, and the area between. Satellite imagery was used to confirm large-scale structural trends and create inferences where field data were absent. Orthogonal thin sections were created for microstructural analysis including determining kinematics, temperatures of deformation, mineral assemblages, and metamorphic grade. The EMCC exhibits primarily greenschist grade metamorphism across the previous mapped extent. Additionally, structural and microstructural data supports the previous interpretation of a top-to-the-NW detachment fault on the NW flank of the EMCC. Samples proximal to the previously mapped extent of the EMCC exhibit similar qualities to samples in the previously mapped extent. Results of this study indicate that the EMCC extends further to both the SW and the NE than previously mapped. Further, the rocks studied from the Duch Gol Basin record deformation at lower metamorphic grade compared to the EMCC, although still greenschist facies in localized areas. This study also highlights an extensive amount of future work that can be completed in this area. This work includes using 40Ar/39Ar and U-Pb geochronology in determining the timing of deformation and protolith age, respectively. Two additional areas that are poorly understood include the boundary of the shear zone and the high-grade core and the true lateral extent of the EMCC. Additional transects across the core complex are suggested to aid in constraining these facets.