Tectonic implications of polyphase magmatism in the Rushan-Pshart suture zone



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The Rushan-Pshart tectonic complex lies along the suture zone between the Central and Southern Pamir terranes within the Pamir salient. The southward subduction of the Rushan Ocean basin is not well documented due to the absence of significant magmatism in the Southern Pamir. Estimates of Rushan Ocean basin closure in the Late Triassic are primarily based on stratigraphic relationships and detrital zircon analyses. Only two plutonic samples from the eastern Rushan-Pshart zone have been reliably dated, yielding earliest Jurassic and Middle Jurassic ages, postdating the estimated closure of the Paleotethys. New petrographic, geochronologic, and geochemical analyses of three igneous rocks from the Rushan-Pshart accretionary provides new information on the tectonic evolution of the Rushan-Pshart suture zone. Two samples collected from the Rushan-Pshart plutonic complex are geochemically classified as a granite and a granodiorite. A volcanic sample from the Rushan complex is geochemically ultrapotassic and has a shoshonite-absarokite magmatic association. Zircon grains from felsic plutonic samples document significant inheritance with two main age distributions in the latest Triassic to earliest Jurassic (206-186 Ma) and the Middle Jurassic (180-168 Ma). The Middle Jurassic peak is interpreted as the age of the plutonic rocks. The older, Triassic-Jurassic population overlaps with the youngest zircon from the volcanic sample and the age of eruption is tentatively interpreted as Late Triassic (202 Ma). Geochemically, all three igneous samples exhibit OIB REE trends with positive Eu anomalies and are generally enriched in HFSE. I interpret the source of the ultrapotassic volcanics to be an enriched, lithospheric mantle. The Middle Jurassic plutons are interpreted to be sourced from magma chambers in the lower and upper crust remobilized by aesthenospheric upwelling. I suggest that the Rushan-Pshart arc formed in three primary stages: 1) Late Triassic formation of an arclogite root in the lithospheric mantle, 2) Post-collisional slab breakoff in the earliest Jurassic, mobilizing the arclogite into a series of fractionated magma chambers in the crust, resulting in the eruption of ultrapotassic volcanics, and 3) Densification and foundering of the arclogite root, inducing Middle Jurassic partial melting in the lower crust and generating the granitic complex.



Geochronology, rushan-pshart, suture zone