Mineral chemistry and ages of the Eocene Gapdan granitoid pluton and related dykes (Sistan suture zone, eastern Iran): multi-stage emplacement of a zoned pluton during progressive deformation and exhumation

Abstract

• The Eocence Gapdan pluton in the Sistan suture zone is syn -tectonic and calc-alkaline. • Mineral chemistry and thermo-barometry reveal a multistage evolution. • Biotite granite crystallized first at 44.3 Ma and 4–5.4 km depth. • It was exhumed to 2.5 km, where it was intruded by granodiorite at 42.2 Ma. • Crosscutting diorite dykes contain amphibole antecrysts crystallized at 23 km depth. Mineral compositions provide key information on the petrogenesis and tectonic settings of granitoid rocks. These relationships are addressed in the present paper, which is focused on the Eocene Gapdan pluton, near Zahedan in eastern Iran. The pluton is part of a granitoid suite intruded into Late Cretaceous to Middle Eocene flysch of the Sistan suture zone. It is zoned, consisting of granodiorite in the center and biotite granite at the margin, and is cross-cut by late diorite dykes. The country rocks are low to medium grade biotite schists with subordinate amphibolite. Biotite is the only ferromagnesian mineral in the Gapdan granitoids, consistent with calc-alkaline and I-type affinities, and with formation in an arc-related tectonic setting. Thermo-barometry and geochronology, combined with lithological observations, establish the following sequence of events: (i) melting of mantle related to subduction of the Sistan oceanic slab and closure of the oceanic basin, generation of basaltic magma, and finally crustal melting and hybridization with crustal melts during ascent. (ii) At 44.31 ± 0.21 Ma the resulting granitic magma crystallized at 4–5.4 ± 1 km depth; biotite and feldspar thermometers indicate crystallization temperatures of ~ 850˚C and ~ 730˚C, respectively. (iii) This stage was tectonically controlled and the pluton was exhumed to shallower crustal levels of ~ 2.5 km, imparting a mylonitic fabric on the granite. (iv) At 42.15 ± 0.55 Ma granodiorite was emplaced in the center of the pluton at a depth of ~ 2.5 km. Crystallization depths are supported by the pressure estimates from Al-in-hornblende in rocks of the contact aureole. (v) Cross-cutting diorite dykes were emplaced at ~ 2.5 km, but they contain phenocrysts of magnesio-hastingsite that had crystallized at 894 ± 55 °C and ~ 23 km depth during ascent of the magma, followed by kaersutite crystallization at temperature decreasing to 744 ± 15 °C at ~ 12 km depth.