Late Paleozoic Accretionary and Collisional Processes along the Southern Peri-Siberian Orogenic System: New Constraints from Amphibolites within the Irtysh Complex of Chinese Altai

Abstract

The Irtysh shear zone is one of the largest strike-slip systems within the Central Asian Orogenic Belt, recording the accretionary-collisional processes between the peri-Siberian and Kazakhstan-Junggar orogenic systems. This study focuses mainly on amphibolites from the Irtysh Complex, the best-preserved exposure of the shear zone in northwestern China. Their protoliths and metamorphic ages/conditions are investigated to constrain the tectonic affinity of this complex and to explore the above-mentioned tectonic processes. Three types of amphibolites are identified geochemically. The geochemical compositions of type I suggest that their protoliths represent N-MORB (normal mid-ocean ridge basalt)-type basalts that could be produced by ca. 10% partial melting of depleted spinel lherzolites. Type II amphibolites are characterized by enriched-MORB-type trace-element signatures. Their protoliths were possibly generated by <5% partial melting of a similar mantle source. Type III amphibolites show typical arc affinity and have a more depleted mantle source. We infer that ankaramites from the Dulate arc of East Junggar, part of the Kazakhstan-Junggar orogenic system, possibly served as their protoliths. The available data suggest that the Irtysh Complex probably represents an accretionary complex built along the southern Chinese Altai (part of the peri-Siberian orogenic system), incorporating crustal relicts from both the Ob-Zaisan Ocean and the Dulate arc. Metamorphic investigation shows that the amphibolite facies metamorphism occurred at ca. 284 Ma, recording a high geothermal gradient (30°–45°C/km) along the Irtysh shear zone. Data compilation suggests that East Junggar probably collided with Chinese Altai between ca. 313 and 299 Ma, which was followed by an episode of high- to ultrahigh-temperature metamorphism possibly related to syn-/postcollisional extension in the early Permian.