A NEW TERRANE SUBDIVISION FOR MONGOLIA: IMPLICATIONS FOR THE PHANEROZOIC CRUSTAL GROWTH OF CENTRAL ASIA

Abstract

We present a new terrane synthesis for Mongolia that incorporates geological, geochemical and geochronological data from more than 60 years of Mongolian, Russian and joint international studies. Forty-four terranes are distinguished and classified into cratonal, metamorphic, passive margin, island arc, forearc/backarc, accretionary complex, or ophiolitic types. New detailed stratigraphic columns for all terranes are presented which summarize the geological evolution of each terrane. Our analysis reveals that small Precambrian cratonic blocks in the Hangay region acted as a central nucleus around which Paleozoic arcs, backarc/forearc basin assemblages, associated subduction complexes and continental slivers were accreted. The temporal and spatial order of accretion and amalgamation was complex and probably not simply from north to south with time. The timing of terrane accretion is partly constrained by sedimentary overlap assemblages and post-amalgamation intrusive complexes. The main stages of amalgamation occurred during the Neoproterozoic, Cambrian–Ordovician, Devonian, Pennsylvanian–Permian and Triassic. The arcuate trends of terranes around the central Hangay region provide the first-order structural grain for Mongolia. This crustal anisotropy has played a major role in controlling the geometry and kinematics of all subsequent Phanerozoic deformation and reactivation of structures in the region, including the Cenozoic development of the Altai and Gobi Altai. Our results provide the most detailed synthesis to date of the basement geology of Mongolia which should provide an important crustal framework for interpreting the Phanerozoic tectonic evolution of a large part of Central Asia. In addition, our synthesis allows the economic resources of Mongolia to be placed in a modern tectonic context.