Paleozoic sedimentation, volcanism and tectonics of southern Mongolia

Central Asia is a geologically complex region that records much of the Phanerozoic tectonic growth of Asia. Paleozoic rocks of southern Mongolia record a key part of this growth but are very poorly understood. The goal of this study was to collect well-documented, reproducible data and begin to build a much-needed database to reconstruct the tectonic history of Mongolia and central Asia.; Sedimentary and volcanic strata of southern Mongolia record erosion of older continental material during the Ordovician and Silurian, development and accretion of volcanic arc systems during the Devonian to Early Permian, and a transition from marine to non-marine depositional systems during the Carboniferous to Permian. Ordovician and Silurian facies preserved in the northern part of the study area record marine depositional environments. Sandstones of this age are primarily compositionally mature quartzites which suggest a continental source. This source was either a continental block or an eroded, remnant arc and may be represented by older rocks preserved in central Mongolia outside of the study area.; Devonian strata record marine conditions as well as the onset of arc volcanism. Shallow-marine, fossiliferous limestones, volcaniclastic sandstones, pyroclastic deposits, lava flows and porphyry copper deposits all record components of a large arc system. This system may have extended for over 2000 kilometers and was probably a complex combination of island arc components, continental regions, and backarc basins, analogous to regions of the modern western Pacific. Carboniferous strata record similar environments and indicate continued arc activity built upon portions of the Devonian arc. Carboniferous deposits suggest the existence of two arc systems: one arc localized in southern Mongolia that may correlate with the Bogda Shan Arc in China and another centered in southwestern Mongolia that may include portions of the Kelameili Arc, also in China. These interpretations are further supported by geochemical analyses of Devonian and Carboniferous basalts and basaltic andesites which suggest formation within arc-related tectonic settings.; Permian non-marine deposits most likely accumulated in foreland and intermontane basins which may have formed as the North China Block and portions of western China collided with southern Mongolia during closure of the Paleoasian ocean.