Geochemistry and petrogenesis of Raviz-Shanabad intrusions, (SE UDMB): an evidence for Late Eocene magmatism

Abstract

The SE segment of the Urumieh-Dokhtar Magmatic Belt (UDMB), known as Dehaj-Sarduieh Magmatic Belt, includes a huge pile of Palaeogene volcano-sedimentary rocks and Eocene to Miocene mafic to felsic intrusions. In the middle of this belt (in Raviz-Shanabad, W Rafsanjan) mafic-intermediate intrusions (as stocks and/or dikes-sills) are abundant and have intruded into the volcano-sedimentary rocks of Dehaj-Sarduieh. The thickness of dikes range from 0.5 to >5 metres and the minimum diameter of the intrusions is ~10 m. These rocks are highly porphyritic and have gabbroic to dioritic composition. Petrographic data indicate that the main rock-forming minerals include plagioclase (labradorite to anorthite based on the new electron microprobe data), pyroxene (augite), and opaque (magnetite and titanomagnetite) minerals, set in a granular groundmass of the same minerals. New zircon U-Pb data show that the age of these rocks is 34.39 ± 0.31 Ma (Late Eocene). These rocks show calc-alkaline characteristics and their geochemical signatures are compatible with arcs with a derivation from subduction zones. Estimated temperature and pressure from Clinopyroxene of these rocks suggest a temperature in the range of 1050° to 1150°C and a pressure of >2 kbar. These data show the formation depth of these intrusions should be ~18 km. Initial 87Sr/86Sr ratio for these rocks varies from 0.70403 to 0.70409, whereas 143Nd/144Nd isotope ratio changes between 0.5128 and 0.5129, corresponding to εNd(i) values of +3.5 to +6.6. These geochemical and isotopic evidences show that the melts originated by 5% partial melting of amphibole spinel peridotite mantle source. Petrographic observations as well as mineral chemistry and isotopic studies indicate that assimilation-fractional crystallization (AFC) has probably played a dominant role in the evolution of the Raviz-Shanabad intrusions. We suggest these intrusions formed as a part of the Late Eocene high-flux magmatism in SE segment of the UDMB in an active continental margin, which was related to the subduction of Neo-Tethys oceanic lithosphere beneath the Central Iranian microcontinent.