Adakite genesis and plate convergent process: Constraints from whole rock and mineral chemistry, Sr, Nd, Pb isotopic compositions and U-Pb ages of the Lakhshak magmatic suite, East Iran

Abstract

The genesis of Eocene to Early Oligocene plutons and dykes emplaced in the suture zone between two colliding continental blocks in southeastern Iran has been the subject of debate concerning the processes that formed magmas and influenced their intrusive history. We report geological, geochemical, mineralogical and isotope evidence that constrains the origin of the Lakhshak plutonic complex. SiO2 contents in the granodioritic pluton and dacite dykes average 67 wt%, and in the monzodiorite dykes 53 wt%. Based on (La/Yb)N vs. YbN the rocks correspond to adakites, originating from 10 to 25% melting of garnet amphibolite. The high A12O3 (14.95–17.15 wt%), Sr (442–1385 ppm) and Ba (601–1935 ppm) contents and relatively high La/Yb (25–45) and Sr/Y (35–57), but low Yb (mostly <1.6 ppm) and Y (mostly <18.2 ppm) are also features typical of adakitic rocks. Zircon and titanite from two samples of the pluton yield U-Pb ages of 29.98 ± 0.08 and 29.90 ± 0.05 Ma, a monzodiorite dyke yields 29.02 ± 0.10 Ma, and a dacite dyke 29.39 to 27.96 Ma. These ages confirm that the magmatism was post-collisional. The monzodiorite and dacite dykes have (87Sr/86Sr)i = 0.7053–0.7056 and εNdi = −1.1 to 2.2 (except one sample with εNdi = −8.6), which limit the extent of crustal assimilation and plot along a mixing array between Sistan ophiolites and the country rocks of the pluton. This is consistent with an origin from oceanic slab melts variously mixed with melts from flysch sediments. Isotope compositions of Sr, Nd and Pb, xenocrystic zircons, and magma mixing and mingling textures support such a process. Thermobarometry on various minerals indicates a continuum of crystallization stages during magma ascent, with pyroxene formed at 8 kbar, followed by amphiboles, which record conditions decreasing from 1166 °C / 7.8 kbar to 933 °C / 4.0 kbar. Tectonically, the Lakhshak adakites were most likely the consequence of post-collisional delamination that led to melting of Sistan oceanic lithosphere and its interaction with melts from pelagic sediments.