Late Cretaceous calc-alkaline and adakitic magmatism in the Sistan suture zone (Eastern Iran): Implications for subduction polarity and regional tectonics. (December 2020) - Jentzer, Michael
Graphical abstract: Highlights: Arc magmatism reveals Late Cretaceous (~86–71 Ma) NE dipping subduction of the Sistan ocean. Low-K calc-alkaline series derive from mixing of mantle DMM and slab-derived fluids. High silica-adakites (HSA) require an additional slab melt component. HSA are best explained by slab breakoff-related asthenosphere upwelling after eclogite formation. Abstract: The N-S trendingSistan orogen (E Iran) stretches along ~700 km at a high angle compared to other Alpine-Himalayan ranges marking the Neotethyan suture (including the nearby Zagros, Makran or Alborz ranges). Both the geometry and timing of closure of the Sistan ocean are currently debated. We provide geochemical data on Late Cretaceous (~78 ± 8 Ma) magmatic samples collected on the eastern side of the Sistan suture zone. Petrography and major element compositions reveal two coexisting groups: a low-K calc-alkaline series with basaltic to rhyolitic composition, and a set of calc-alkaline intermediate to felsic samples. The low-K calc-alkaline series reflects classical arc magmatism and is characterized by negative anomalies in high field strength elements and positive anomalies in large ion lithophile elements. The calc-alkaline intermediate to felsic samples correspond to high-silica adakites characterized by strong positive anomalies in Sr and higher La/Yb ratios. Sr and Nd isotopic compositions of the low-K calc-alkaline series support partial melting of a DMM-like source contaminated by sediment-derived fluids, consistent with slab-dehydration in a juvenile subduction setting. An additional fraction of slab-derived melt is necessary to model trace element patterns of our adakites. Altogether, results indicate formation of a Late Cretaceous magmatic arc associated with NE-dipping subduction of the Sistan ocean below the stretched continental Afghan margin. The emplacement of adakites postdate the formation of the suture zone eclogites by a few Ma at most. Upwelling of hot asthenosphere following slab break-off would best explain the necessary warming-up of the subduction thermal regime.