Geochemical and geochronological constraints on distinct Early-Neoproterozoic and Cambrian accretionary events along southern margin of the Baydrag Continent in western Mongolia

Abstract

New geological, geochemical and geochronological data characterize architecture and geodynamic evolution of the southern Baydrag continental margin in Mongolia. The structurally deepest Zamtyn Nuruu Complex is composed of highly deformed orthogneisses and amphibolites intruded by syn-tectonic diorites and gabbros. Geochemical affinity of the amphibolites varies from continental-arc to within-plate tholeiites, while the Grenvillean (948±6 and 941±11Ma) orthogneisses show features typical of magmatic-arc origin. The syn-metamorphic gabbros and diorites intruded coevally with the regional thermal event at 542±4Ma. Zircon Hf isotope and whole-rock geochemical data point to a primitive source of mafic rocks, most likely formed in Early Cambrian arc developed on Grenvillean basement. The overlying Alag Khadny subduction mélange contains eclogites and amphibolites with MORB affinity while lenses of orthogneisses (953±12Ma: zircon, 939±5Ma: monazite) reveal late- to post-collisional signature. The subduction event is dated by zircon at ca. 538±20Ma. The matrix of the mélange consists of metapelites and carbonates, most likely fragments of Neoproterozoic to Early Cambrian carbonate platform scrapped off the subducting plate during the mid-Cambrian accretion. The hanging-wall Khan-Taishir–Erdene Uul ophiolite is formed by Neoproterozoic pillow lavas, red cherts and associated ultramafic rocks, with gabbros dated at 973±12Ma. Taken together, the new data allow placing south Mongolian continental fragments into a geodynamic model of Rodinia formation, fragmentation and Palaeo-Pacific subduction initiation. The Zamtyn Nuruu Grenvillean arc is interpreted as a result of Mirovoi Ocean subduction beneath Rodinian margin, while the Early Cambrian diorites and gabbros testify initiation of supra-subduction magmatism of Palaeo-Pacific plate beneath continental fragment of Gondwanan, most likely Tarim affinity. These two events were separated by the development of Neoproterozoic passive margin probably associated with Rodinia fragmentation.