The Altai-Mongolia terrane in the Central Asian Orogenic Belt (CAOB): A peri-Gondwana one? Evidence from zircon U-Pb, Hf isotopes and REE abundance

摘要

Understanding the tectonic nature of the Altai-Mongolia terrane is significant in tectonic reconstruction of the Central Asian Orogenic Belt (CAOB), a Paleozoic accretionary one similar to the present-day SW Pacific archipelago. Uncertainty in stratigraphic frame of this terrane has led to many controversies in tectonic interpretation. New zircon U-Pb ages, along with previously published geological data help us to build a simplified stratigraphic frame, consisting of (1) the Middle to Upper Cambrian clastic turbidite intercalated with bimodal volcanics, and (2) the Devonian arc-like volcanics and shallow marine sediments. New geochemical data for the meta-basalts suggests that the bimodal volcanics are rift-affinity. Detrital or xenocrytic zircon U-Pb data, combined with zircon e{open}_(Hf)(t) results reveals that the Altai-Mongolia terrane has a similar Neoproterozoic to Early Paleozoic tectonothermal history in comparison with the Cadomian-Avalonian orogenic belt. This similarity suggests that the Altai-Mongolia terrane likely is a peri-Gondwana one separated from the northern margin (at present geographic coordinate) of Gondwana during the Middle to Late Cambrian. The rifting might have led to the generation of a branch of the Paleoasian Ocean, an eastern equivalence of the Rheic Ocean. The Silurian to Devonian northward subduction (at present geographic coordinate) of the Paleoasian oceanic crust has given rise to an Andean type arc along the southern margin of the Altai-Mongolia terrane. New magmatic zircon U-Pb ages combined with zircon e{open}_(Hf)(t) and REE results suggest that the Silurian to Devonian arc-like magma were generally derived from the depleted mantle. Our findings shed some light on tectonic reconstruction of central Eurasia and allow its comparison with some pre-Variscan massifs in Europe.