Devonian subduction retreat recorded in the Chinese Altai Orogen: Petro-structural and geochronological constraints from orogenic lower crust migmatite-granite complexes

Abstract

Crustal extension due to the retreat of subduction system has been assigned as an important mechanism governing the Devonian tectonic evolution of the Chinese Altai Orogen in central Asia. While the extensional regime has been repeatedly inferred from petro-structural (petrographic and structural) records preserved in the orogenic upper crust sedimentary covers and orogenic middle crust schists, such records have rarely been documented in the deep crustal migmatitic and/or magmatic rocks. This gap is filled in this study by applying combined petro-structural analysis in conjunction with zircon and monazite U-Pb geochronology on representative orogenic lower crustal migmatite-granite complexes. These complexes are characterized by the development of a sub-horizontal foliation associated with extensional melt‐filled shear bands. The foliation commonly contains garnet-sillimanite ± K-feldspar ± cordierite-bearing metamorphic assemblages that are compatible with a low-pressure/high-temperature thermal regime. In addition, such mineral assemblages are usually found to replace pre-existing middle-pressure/middle-temperature Barrovian-type ones, reflecting a tectonic switch from thickening to extension. New monazite and zircon U-Pb age data, together with regional pre-existing ages, suggest that the sub-horizontal high-temperature foliation formed during a period of 420–390 Ma. Combined with previously published data, our data reinforce that a crustal-scale extensional tectonic regime developed in response to retreating of subduction system governed the geodynamic evolution of the Chinese Altai Orogen during Devonian times. This process led to regional anatexis of the pre-existing Ordovician accretionary wedge complex, facilitated the formation of stabilized and vertically stratified continental crust, and promoted the metallogenic evolution in the region during the Devonian.