Age and Petrogenesis of the Gabbros from Tajik South Tianshan: Implications for Early Paleozoic Geodynamic Evolution of the Southwestern Central Asian Orogenic Belt

Identification of slab window process is important for understanding the nature of the accretionary orogenesis. In this study, we report detailed petrological, geochronological, geochemical, Sr-Nd isotopic, and mineral chemical data for two dyke-like gabbroic intrusions from the South Tianshan belt of Tajikistan, southwestern margin of the Central Asian Orogenic Belt. Both intrusions are composed of coarse- and fine-grained gabbros. U-Pb zircon dating shows that they were emplaced at 431 +/- 5 Ma. The gabbroic rocks show relatively large variation in elemental and isotopic compositions, with SiO2 of 40.62-53.97 wt.%, Sr of 333-1261 ppm, and epsilon(Nd) (t) of +2.5 to +5.8. Especially, the fine-grained gabbros show lower SiO2 and higher MgO but more evolved isotopes than the coarse-grained gabbros for each of the intrusions. All the rocks display OIB-like or transitional OIB-/E-MORB-like geochemical characteristics with no obvious Nb-Ta depletion, indicative of an intraplate affinity. Combined with their mineral chemical compositions, we suggest that these gabbroic rocks were generated by partial melting of asthenospheric mantle in the transitional spinel-garnet stability field, followed by different degrees of fractional crystallization of olivine, clinopyroxene, and plagioclase and mixing with carbonatitic melts. The available data indicate that roll-back of the subducting Turkestan oceanic slab occurred during the Late Ordovician to Early Silurian period. Asthenosphere upwelling due to the opening of slab window resulted from localized slab tearing during slab roll-back may have been responsible for the generation of the studied dyke-like gabbroic intrusions.