Petrology and tectono-magmatic setting ofZorrati granitoid, southwest of Birjand (East of Lut block)

The Zorrati granitoid (ZG) pluton exposed on the eastern edge of the Lut block trending north-south. The Lut block surrounded in the north by the Darone fault, in the south by the Jazmurian fault and in the east and  the  west by Nahbandan and Naiband faults respectively (Naderi Miqan and Akrami, 2006). A number of  investigations have been carried out  by various researchers regarding  how and when the magmatism and volcanism of the Lut Block was initiated among which Eftekharnezhad (1980) can be notable, who predicted the subduction of the oceanic crust towards the west and under the Lut Block based on the volume and time distribution. Saccani et al. (2010) suggested that the subduction of the oceanic crust took place towards the east and under the Afghan block. The closest intrusive bodies to the Zorrati granitoid are the Shah Kouh granitoid, the Deh Salam granitoid, the Chahar Farsakh granitoid, and the Sefidkoh granitoid.

Regional Geology:

This dominant rocks of the pluton under study are  biotite tonalite and syenogranite-alkali feldspar granite together with biotite and tourmaline porphyritic granodiorite, and granite. The ZG consisting of igneous and metasediment enclaves as well as felsic veins and dykes. According to several studies including field observations, petrology, and geochemical studies the rock units make up the studied pluton have possibly different origins.

For lithological and geochemical investigations of the Zorrati granitoid pluton and its tonalitic enclaves, after microscopic studies, 11 fresh samples were selected and sent to the Institute of Geology and Geophysics of the Chinese Academy of Sciences for XRF and ICP-MS analyses.

Petrography:

The constituent minerals of the granitoid rocks are quartz, plagioclase, microcline, orthoclase, biotite, muscovite and tourmaline.

Whole Rock Chemistry:

The Zorrati granitoid (ZG) is a high potassium calc-alkaline, peraluminous, and S-type granitoid. Trace element plots show at least two trends, which probably point to  different origins for the rock units forming this pluton. The studied rocks are Rb, Th, U, K, and Pb enrichment and depleted in Nb, Sr, P, Ti, and Zr, indicative of the crustal origin of the relevant magmas in a collision zone. Tourmaline porphyritic granodiorite and syenogranite-alkali feldspar granite were formed by melting a clay-rich metapelitic protolith with upper crust origin due to muscovite dehydration without the intervention of the mantle in a continental collision zone. Porphyritic biotite granodiorite along with granite, biotite tonalite, and igneous enclaves were formed by melting of a metagreywacke-poor clay protolith with upper crustal origin due to biotite dehydration without the intervention of the mantle in a collision and a post-collision settings, respectively. Water pressure of ≥5 kbar and temperature of 650 to 700 °C were estimated for the tourmaline-biotite porphyritic granodiorites and the syenogranite-alkali feldspar granite. Likewise, temperature of ~775 °C was determined for the granite, biotite tonalite and igneous enclaves.

Biotite tonalite and syenogranite to alkali feldspar granite units are the two main and large granitoid units  covering most of the area. The other units including granite, biotite, and tourmaline porphyritic granodiorite along with dikes and aplitic and pegmatite veins show high potassium and peraluminous calc-alkaline series related to S-type granites. The remarkable features of these rocks are  of the Rb, Th, U, K, Pb enrichment and the Nb, Sr, P, Ti, and Zr depletion as well as having  different origins and different tectonic environment. For example, the tourmaline-bearing porphyritic granodiorite and syenogranite to alkali feldspar granite were originated by melting of a metapelitic clay-rich source in the upper crust without mantle intervention, in a collisional tectonic environment while  muscovite dehydration plays a significant role. Biotite-bearing porphyritic granodiorite along with granite, biotite-bearing tonalite, and its enclaves formed by melting of a plagioclase-rich metagreywacke in the upper crust without mantle interference, in a collisional and post-collisional tectonic environment, respectively, while dehydration of biotite was involved in their formation. According to their lithological, geochemical, tectonic characteristics and different origin, like the other granitoids in this part of the Lut block (Moradi Noghondar et al., 2012), two different ages of the Middle Jurassic (biotite-bearing tonalite) and the Eocene - Oligocene (syenogranite - alkali feldspar granite) can be suggested for the Zorrati granitoid (ZG) pluton.