Polycyclic paleozoic tectono-metamorphic evolution of the southern Chinese Altai, Central Asian orogenic belt

Abstract

The Chinese Altai, NW of the Altai Orogen represents a crucial geological unit to understand the accretionary-collisional processes. Our results, combined with published data allow reviewing the units of the Chinese Altai in detail. During the last decades the Chinese Altai was regarded as a complex range of 5 tectono-stratigraphic terranes separated by faults. However review of the distribution of granitoids, stratigraphy, metamorphism, deformation and 40Ar/39Ar ages showed that the Chinese Altai evolved as a unique accretionary wedge. Granitoids are mainly intruded in a syn-orogenic setting in the Ordovician to Carboniferous. The sediment sequence is composed of Ordovician, Devonian and Carboniferous sequences, and the Ordovician sequence (Habahe group) is present in four terranes. Metamorphism recorded 3 successive events with an Early Devonian MP-MT event and two HT-LP to UHT-HT metamorphic events which were dated in the Devonian and in the Permian, respectively. Consequently, the Chinese Altai can be considered as a Devonian magmatic arc built upon a single Cambro-Ordovician accretionary wedge affected by polycyclic metamorphism and deformation. Structural and geochronological approach have been applied in the Kalasu area to fully comprehend the accretionary history and the timing of deformation. Based on metamorphism and structural pattern the area was defined as upper, middle and lower crustal levels. Zircon U-Pb isotopic study of the UHT-HT cordierite-bearing migmatites of the lower crust revealed Devonian and Early Permian metamorphic ages, defined by zircon cores and rim respectively. Structural analysis showed that the Permian metamorphism was associated with compressive tectonics due to crustal scale folding during a D3 event. This event is characterized by NW-SE trending fabric steeply inclined, which reworked the previous NE-SW trending Devonian sub-horizontal S1 fabric. Geochronological study of the magmatic rocks demonstrate that this Permian metamorphic event was associated with exhumation of gabbros and post-orogenic granitoids. Phase equilibria modelling combined with deformation study and precise geochronology enable us to draw the P-T-t-D (Pressure-Temperature-time-Deformation) evolution of the different orogenic crustal levels. The upper crustal level is characterized by garnet-biotite metamorphic grade and pseudosection shows burial to 3.5 kbar at 650 °C during D1 and exhumation to 1.2 kbar at 600 °C during D3. The middle crustal level is characterized by sillimanite-mica-garnet-rutile assemblage and sillimanite-mica-garnet-ilmenite assemblage and pseudosection shows burial to 6.8 kbar at 650-750 °C during D1 and exhumation to 4.5 kbar at 620 °C during D3. The lower crustal level is characterized by kyanite-staurolite-garnet grade and sillimanite-cordierite-k-feldspar-biotite grade and mineral assemblage suggest burial to 6-8 kbar during D1 and pseudosection shows exhumation from 8.5 kbar at 795 °C to 6-5 kbar at 780-800 °C during D3. The monazites were dated from ca. 297 Ma to ca. 268 Ma consistent with published zircon U-Pb metamorphic ages. 40Ar/39Ar biotite age shows cooling at 247.5 ± 0.5 Ma.