Abstract
Abstract 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 εNdt 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.
Highlights
The whole Wilson cycle for the formation of orogenic belt involves oceanic opening, subduction and closure, and collision between continents and/or arcs [1]
Slab windows associated with the slab roll-back are rarely reported and the responses and records of magmatism generated by this geodynamic process are still not well understood
The isotopically most depleted sample of the gabbros in this study shows Sr–Nd isotopes similar to those of the Paleozoic ocean island basalts (OIBs)-like rocks and some of E-MORBlike rocks in the OPS (Ocean Plate Stratigraphy) sections of the South Tianshan (Figure 9) [37], indicating that their parent magmas were probably derived from relatively enriched asthenosphere
Summary
The whole Wilson cycle for the formation of orogenic belt involves oceanic opening, subduction and closure, and collision between continents and/or arcs [1]. For the accretionary orogenic belts that underwent long-time subduction-accretion processes, the occurrences of slab window are common [2,3,4]. The opening of slab window provides a pathway for the upwelling of subslab asthenosphere materials, which will significantly affect the thermal structure of the overlaying lithosphere and result in the extensive extension and uplift of the overriding plate, in particular the generation of geochemically unusual magmatism with a regular distribution in space. Considering the complexity of their sources, these slab window magmatic rocks can provide a rare opportunity to investigate the chemistry of different mantle reservoirs and subducting oceanic slab and their interactions. Slab windows associated with the slab roll-back are rarely reported and the responses and records of magmatism generated by this geodynamic process are still not well understood
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