Geodynamics and Crust-Forming Processes in the Early Caledonides of the Bayanhongor Zone, Central Mongolia

摘要

The Caledonian Bayanhongor Zone is an element of the South Hangay group of terranes in central Mongolia. This group comprises (from south to north) (1) the pre-Riphean Baydrag Block with the Riphean slaty cover (Burdgol Group); (2) the zone of the Cambrian island-arc basalt-andesite-dacite volcanism; (3) the Bayanhongor Zone of the Upper Riphean-Cambrian rocks that consists of the Delbkhairhan terrigenous-carbonate subzone, the ophiolitic Bayanhongor proper subzone, and the Khaluutbulag terrigenous-carbonate sub-zone with subordinate volcanics; and (4) the Dzag Zone of the predominantly Cambrian terrigenous-slaty sequences that corresponds to the southern Hangay Terrane with the Riphean crust. The rocks typical of oceanic lava plateaus or oceanic islands dominate among igneous rocks of the ophiolitic Bayanhongor Zone. No sedimentary rocks with isotopic signatures of ancient continental crust are known in this zone; thus, the sedimentation proceeded within an oceanic basin at a great distance from the paleoconti-nents. The composition of sedimentary rocks in the Dzag Zone of the Hangay Terrane fits the mean composition of the continental Earth's crust. The shelf segments situated far offshore and partially composed of fragments of the Late Riphean island arcs that exerted an rejuvenating effect on the two-stage model Nd isotopic age t_(Nd)(DM-2st) ～ 1.6 Ga were favorable for the formation of such rocks. The sedimentary cover (Burdgol Group) of the Baydrag Block was characterized by a sharp prevalence of local provenances. The sediments were deposited at the shelf close to the ancient continental basement that underwent erosion. The juxtaposition of these heterogeneous fragments of the Late Riphean-Cambrian lithospheric shell of the Earth as a common composite terrane was related to the collision of fragments of the Rodinia supercontinent and its Riphean shelf with a large oceanic lava plateau or a group of oceanic islands of the Hawaiian type. This collision accompanied by subduction and obduction occurred about 500 Ma ago in the process of general accretion completed by formation of the Early Caledonian superterrane of the Central Asian Foldbelt.