The Wilson Cycle of the North Altyn Ocean: A Record of Subduction Initiation, Slab Rollback, and Final Closure

Abstract

The North Altyn ophiolitic belt was formed by the subduction–closure of the Altyn segment of the Proto–Tethys Ocean and recorded the accretion of Tarim to northern Gondwana, but the timing of initial subduction and detailed tectonic transition processes remain poorly constrained. This study reports the oldest Phanerozoic magmatic record of 537 Ma juvenile I–type granodiorite for the Altyn belt, which was formed under high oxygen fugacity (−14.51 ≤ log f O 2  ≤ −10.96) and water–rich (6.92–9.50 wt%) arc condition. The southward subduction of the North Altyn Ocean thus initiated a priori to about 537 Ma. Coupled I– and S–type granitoids dated at ca. 505–478 Ma, which were generated in subduction ‐ related but relatively reduced and reworked conditions, have also been observed. Moreover, a review of zircon Hf isotopes, crustal thickness, geobarometry and oxygen fugacity of magmas across the belt also argues for the advancing and retreating subduction of the North Altyn Ocean at ca. 537–500 Ma and ca. 500–470 Ma, respectively. The slab rollback event at 500–470 Ma triggered upwelling of reduced low oxygen fugacity mantle materials and also led to crustal thinning, which overlapped well with regional extensional magmatic activities. In addition, renewed increases in these proxies suggest resumed advancing subduction until ocean closure at ca. 450–440 Ma. Thus, magmatic records across the belt suggest a prolonged 100 Ma subduction cycle from initial, advancing and retreating subduction to final closure for the North Altyn Ocean.