كاني‌شناسي زنوليتهاي موجود در آلكالي‌بازالت‌هاي اليگوسن جنوب غرب جندق تا شمال غرب عشين (استان اصفهان، ايران)

Oligocene alkali basalts in the northwestern part of the East-Central Iranian Microcontinent (CEIM) are well exposed in several locations, including Tovireh (southwest of Jan‎daq), Kafar Kuh (northwest of Ashin), Chah Ali-Khan (northeast of Zavareh), an‎d …. Field studies indicate that among the numerous outcrops of these basalts, only the Central an‎d Southern Tovireh alkali basalts contain xenoliths derived from the continental crust as well as mafic–ultramafic xenoliths. These alkali basalts are exposed near the Great Kavir Tovireh Faults. Rock-forming minerals of the Tovireh alkali basalts are phenocrystic olivine (chrysolite), chromian spinel, clinopyroxene (augite, diopside), an‎d plagioclase (labradorite) set within a groundmass composed of fine-grained crystals of the same minerals, along with sanidine an‎d ilmenite. Secondary minerals are serpentine, chlorite, zeolite, an‎d magnetite. The main textures of these rocks are porphyritic, poikilitic, an‎d microlithic-porphyritic textures. The rims of clinopyroxene phenocrysts exhibit elevated concentrations of Ti, Al, rare earth elements (REEs), high field strength elements (HFSEs), an‎d large ion lithophile elements (LILEs) compared to their cores, indicating rapid magma ascent due to rapid decompression. Geochemical analyses confirm that these rocks belong to the alkaline magmatic series, with REE patterns resembling those of continental alkali basalts. The parental magma of these basalts originated from low to moderate degrees (10–20%) of partial melting of an amphibole-bearing garnet lherzolite at depths of 80–110 km within the asthenospheric mantle. The geochemical characteristics of these basalts suggest that their source had previously undergone carbonate metasomatism an‎d enrichment, likely due to subduction-related processes associated with the Neotethyan oceanic crust surrounding the East-Central Iranian Microcontinent from the Triassic to the Eocene. A notable feature of the Tovireh alkali basalts is the presence of crustal granulitic an‎d mafic–ultramafic xenoliths. The granulitic xenoliths are Al-rich an‎d Si-poor, composed primarily of spinel an‎d plagioclase in nearly equal proportions, along with accessory minerals such as sillimanite, corundum, an‎d rutile. These crustal granulites contain high Al₂O₃ (32.8–38.6 wt%) an‎d low SiO₂ (26.0–38.0 wt%). Their chondrite-normalized REE patterns display a distinct positive Eu anomaly (Eu* = 1.6 to 4.8), characteristic of plagioclase-rich residual materials from the lower crust, which likely served as the source for S-type granitic magmas. The mafic–ultramafic xenoliths mainly consist of gabbro, norite, orthopyroxenite, an‎d dunite, with mineral assemblages including chromian spinel, olivine, Al-rich diopside, plagioclase, an‎d orthopyroxene. These xenoliths exhibit granoblastic an‎d poikiloblastic textures. Their mineralogical composition an‎d chemistry suggest derivation from a sub-alkaline an‎d tholeiitic magma. Pressure estimates for the granulitic an‎d mafic–ultramafic xenoliths indicate depths of 10–30 km an‎d 20–30 km, respectively, consistent with different levels of the crust beneath the region (with the Moho depth estimated at ~40 km). Field, petrographic, an‎d geochemical evidence suggests that the Tovireh alkali basalt magma ascended rapidly along the Tovireh Fault in a transtensional setting in the northwestern part of the East-Central Iranian Microcontinent during the Oligocene, incorporating fragments of the underlying crust during its ascent. In addition to the Tovireh basalts, granulitic xenoliths have also been identified in the Eocene volcanic rocks of Godar Siah. These Al-rich granulites likely resulted from the partial melting of metamorphosed sediments, generating S-type granitic melts. This melt may have contributed to the formation of the Paleozoic S-type granites in the Airkan an‎d Arousan regions.

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