2008 Joint Annual Meeting (5-9 Oct. 2008): Devonian-Carboniferous Evolution of the Bodonch Metamorphic Complex in Southwestern Mongolia and Its Implications for Reconstructing the Central Asian Orogenic System (CAOS)

The Central Asian Orogenic System (CAOS) is the largest Phanerozoic accretionary orogen and consists of magmatic arcs, large flysch complexes, and high-grade metamorphic terranes. Whether the CAOS was a significant site of continental growth has been debated due to uncertainty about the importance of juvenile arc construction, provenance of the flysch complexes (continental vs. oceanic), and nature of the high-grade terranes (deformed juvenile arcs vs. Precambrian continental fragments). To address the question with regard to the high-grade terranes, we conducted an integrated study of the Bodonch metamorphic complex in SW Mongolia that employs field mapping, thermochronology, P-T history analysis, and geochemistry. The Bodonch complex comprises greenschist- to amphibolite-facies meta-pelites, meta-basite, and gneiss thrust over a Devonian-Carboniferous arc sequence. Initial observations reveal that the protolith of the gneiss is largely meta-greywacke, suggesting a depositional setting associated with an active plate margin. In situ U-Th-Pb dating of monazite inclusions in garnet reveals the timing of upper greenschist- to amphibolite-facies metamorphism in the Middle to Late Devonian. Quantification of P-T conditions associated with this event is on-going. The cooling history of the metamorphic complex will be constrained by 40Ar/39Ar thermochronology, while the protolith age and provenance of the gneiss complex will be determined by combined U-Pb dating and Hf analysis of detrital zircons. The results of our study will have implications for whether the Bodonch metamorphic complex is a fragment of a microcontinent or a metamorphosed arc complex and how it has been exhumed from a middle to lower crustal depth during the construction of the CAOS.