The granite pluton of Zahedan (SE Iran): a petrological and magnetic fabric study of a syntectonic sill emplaced in a transtensional setting

The ∼32 Ma, NW-SE elongate, 75 km-long granite pluton of Zahedan is one of the most conspicuous geological features of southeast Iran. It is composed of metaluminous to weakly peraluminous rocks, corresponding to magnetite- and ilmenite-bearing I-type granitoids, ranging from diorites to granites. All rocks, including the late andesitic to dacitic dykes, belong to the high-K calk-alkaline series. Field and structural relationships, as well as evidence for mixing-mingling, can be used to unravel the magmatic history of the different rock types. Microstructural examinations and magnetic fabric measurements were performed on a collection of oriented specimens from the northern part of the pluton. Ubiquitous flat-lying foliations argue in favour of a sill-like body, except for the diorite. The biotite-bearing granitic facies, at the pluton's periphery, was the first batch of magma which initiated the sill. This sill probably behaved as a N-S-directed décollement zone, marked by conspicuous 'magmatic-cataclastic' microstructures and N-S treading lineations. The intrusion of the hornblende-bearing granodiorite, with typical magmatic structures, inflated the sill. Deformation occurred at the same time, with E-W to NW-SE stretching, marked by lineations. The main dioritic stock forming the floor of the sill is considered to be a remnant of the feeder-zone. An abundant dyke swarm marks the end of the syntectonic emplacement of the Zahedan sill. A three-dimentional model of emplacement is proposed in the context of the latest episodes in the closure of the Neo-Tethys during which the Lut Block ended its northward migration with respect to the Afghan blocks.