Petrostructural and Geochronological Constraints on Devonian Extension‐Shortening Cycle in the Chinese Altai: Implications for Retreating‐Advancing Subduction

Abstract

AbstractStructural and metamorphic effects of a Devonian extension‐shortening cycle in the NW Chinese Altai were investigated using combined structural‐petrological analysis and zircon and monazite U‐Pb geochronology. Structural observations revealed a ubiquitous, sub‐horizontal metamorphic fabric (S1), which was originally sub‐parallel with horizontal bedding (S0) of supracrustal Devonian volcanic‐sedimentary basins. This metamorphic fabric is defined by a sillimanite‐bearing migmatitic gneissosity associated with extensional shear bands and a cordierite‐bearing schistosity in the orogenic lower and middle crust, respectively. Staurolite relics preserved in the S1 fabric are interpreted as relics of older, higher pressure Barrovian‐type metamorphism. These structural‐metamorphic features are correlated with decompression and heating and interpreted as reflecting a crustal‐scale extensional phase. S1 was folded to form N‐S‐oriented upright antiforms cored by migmatites and locally transposed by steep S2 foliation during a ∼W‐E‐directed shortening (in current coordinates). Andalusite and cordierite overgrew the migmatitic S2 foliation attesting to decreasing pressure associated with vertical extrusion of migmatites in cores of F2 antiforms. New U‐Pb age data from subhorizontally foliated migmatites suggest that the D1 extension started at least before ∼410 Ma. Age data of syn‐D2 granite intrusions and dykes constrain the D2 shortening starting soon after D1 extension and ending at ∼378 Ma. Combined with regional data, cycles of extensional and contractional tectonic regimes can be defined, which also show eastward migration along‐strike of the Altai range. Such extension‐shortening cycles could result from alternating retreating and advancing subduction, which governed the evolution of the Altai accretionary system from the Ordovician until the Carboniferous.