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

Abstract

Structural 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.