External geodynamic evolution of the Mongolian Accretionary Collage: Insights from petro-chronology of zircon

Abstract

Evolution of internal or external oceanic domains can bring decisive indications into the mode of formation of supercontinents, which is key to understand the supercontinent cycle. Defining the type of ocean evolving in the Central Asian Orogenic belt (CAOB) is thus critical to better understand the Earth’s supercontinental evolution. In this contribution, we provide systematic and combined U-Pb, trace-element in zircon and hafnium-in-zircon analyses of four turbidites of the Altai accretionary wedge. These turbiditic rocks provide constraints into the early Paleozoic subduction-accretionary stages of the Mongolian Accretionary Collage (MAC) and show a major detrital age peak in the Cambro-Ordovician (∼490–500 Ma), with minor Neo- and Paleo-Proterozoic to Archean peaks. The zircon trace-elemental patterns are consistent with derivation mostly from magmatic arc (with granitoids affinity), interpreted as detritus sourced from the Ikh-Mongol continental arc system further north-east (present coordinates). The main detrital zircon peak together with the zircon magmatic arc origin suggest that the subduction-accretionary stage was active from 550 to 430 Ma showing mixed εHf(t) values (-20 to + 15). A summary of the detrital zircon εHf(t) values from 550 to 430 Ma suggests a continuous increase of εHf(t) mean values from + 0.2 to + 6.1 towards younger ages. The younging juvenile trend can be compared to the evolution of external orogeny, therefore consistent with a Pacific-type accretionary system facing an external ocean. At last, the results combined with two paleo-geographic reconstructions from the literature imply that the MAC may have been part of the Peri-Siberian accretionary orogeny in the early Paleozoic.