Multi-episode metamorphism and magmatism in the Paleozoic Altyn Orogen, West China: Implications for the tectonic evolution of the Proto-Tethys Ocean

The Proto-Tethys Ocean plays a significant role in the processes of supercontinent breakup-assembling and Eurasia's formation history. The Paleozoic Altyn Orogen marks the northern boundary of the Proto-Tethys Ocean realm and holds the deepest records of continental subduction, but the detailed tectonic evolution of this orogen remains controversial. We present a systematic overview of recent studies of metamorphism and granitoid magmatism within the Paleozoic Altyn Orogen. This Orogen includes the North Altyn Tectonic Belt (NAB), Central Altyn Block (CAB) and South Altyn Tectonic Domain (SAD), in which four-episode metamorphism (M-I to -IV) and five-episode granitoid magmatism (G1–5) are recognized. The M-I metamorphism is marked by high-pressure and low-temperature (HP-LT) eclogites with ages of >522 Ma in the NAB, which, together with the arc-related G1 granitoids of 536–510 Ma, indicates an early Cambrian cold subduction. The M-II metamorphism is characterized by ultrahigh-pressure (UHP) rocks in the SAD. It shares clockwise P–T paths characterized by the prograde to the P_max (peak pressure) (M-IIa) stage at ∼500 Ma and post-P_max decompression to high amphibolite facies or HP-(U)HT (ultrahigh- to high-temperature) granulite facies conditions (M-IIb) at ∼480 Ma. The P_max conditions of M-IIa vary from the stability field of quartz to that of coesite and stishovite, registering continental subduction to different mantle depths with the maximum up to 300 km. The M-IIa metamorphism was accompanied by the G2 adakitic granitoids with ages of 510–500 Ma, and M-IIb metamorphism was associated with the G2 A-type granitoids aged 495–480 Ma, related to the subduction and exhumation of a microcontinent. The M-III metamorphism is represented by LP-(U)HT granulites with ages of 462–441 Ma and LP overprinting on the exhumed HP-UHP rocks in the SAD. This LP metamorphism, associated with the G3 I/A-type granitoids (475–450 Ma), may indicate an arc/backarc extension. The M-IV metamorphism occurred in the SAD involves an earlier HP eclogite stage at ∼430 Ma (M-IVa) and a later granulite facies overprinting stage at ∼400 Ma (M-IVb). The M-IVa, together with the G4 adakitic granitoids (450–420 Ma), suggests closing of the backarc basins with crustal thickening. While the M-IVb, along with the G5 A-type granitoids (420–370 Ma), indicates the post-collision extension. Summarizing the regional metamorphism, granitoid magmatism and other geological data, we propose an alternative model to address tectonic evolution of the Altyn Orogen. It involves: (i) the early subduction of the Proto-Tethys Ocean (>522–510 Ma), (ii) subsequent micro-continental subduction and exhumation (510–480 Ma), (iii) late oceanic subduction and arc-backarc extension (475–450 Ma), (iv) closing of the backarc basin and continental subduction-collision (450–420 Ma) in response to the final closure of the Proto-Tethys Ocean, and (v) post-collisional extension (<420 Ma). This model indicates that the tectonic evolution of the Proto-Tethys realm includes two cycles from oceanic subduction to continental subduction-collision with two episodes of UHP metamorphism, intervened by a phase of arc/backarc extension with LP metamorphism. The similar tectonic scenarios may have also occurred in the North Qinling region in the east Proto-Tethys realm, but have not been reported from other UHP terranes worldwide.