نشریه پترولوژی
پیاپی 47 (پاییز 1400)

The Tarom- Masuleh area (western Alborz) exposes considerable Late Paleozoic volcanic rocks. Stratigraphic relationships indicate that the igneous units are of Carboniferous- Permian age. Based on whole- rock chemical data, the samples are compositionally basic (SiO2= 45.3- 50.7 wt.%) and alkaline (TiO2= 1.1- 4.4 wt.%; Nb/Y=0.9- 2.3). Furthermore, these rocks are characterized by nearly primitive to evolved compositions (MgO= 14.5- 2.5 wt.%). Major and trace elements data show that chemical variations were predominantly controlled by crystal fractionation and, mantle source processes or crustal contamination was not so effective. Trace element theoretical modeling of crystallization process also shows that fractional crystallization of olivine + clinopyroxene has played a major role in the chemical evolution of the melt. The chemical characteristics of the samples in terms of HFSE enrichment and elevated LREE/HREE ratio ((La/Yb)N= 4.8- 17.5) are identical to those of oceanic island basalts (OIB). Thus, the Late Paleozoic magmatism of the Tarom-Masouleh region took place in an intraplate setting (passive continental margin) influenced by the extensional tectonic regime. Coeval magmatism in other parts of Iran, including Central Iran, Sanandaj- Sirjan and Azerbaijan, as well as other areas of the northern margin of Gondwana (Oman, North Africa) and China, similarly represent intraplate magmatic signature attributed to the mantle plume and/or hot spot activity or extensional tectonics and rifting concurrent with the early stages of the appearance of the Neotethys Ocean.

The magmatic complex of Azadegan, composed of basalt, andesitic basalt, andesite, gabbro and diorite, located 45 km northwest of Shahrekord and in the middle part of the Sanandaj-Sirjan zone.. The study rocks are dominated by plagioclase, olivine, pyroxene, hornblende as well as biotite and calcite as minor minerals. Geochemically, the rocks under study are calc-alkaline in nature and are characterized by Ce, Zr, Ba, and Sr depletion. Chondorite- normalized REE pattern illustrates LREE enrichment relative to HREE. Geochemical data of the studied samples indicate partial melting of the mantle source with the influence of fluids and sediments of the subducted crust during subduction. The ratio of Th/Yb (2-2.75) values are in consistent with those of the mature arc islands magma. Also, Ce /Y and Sr/Y ratios indicate the thickening of arc islands along with orogenic processes and maturation of arc islands in the study area. The for mentioned ratios also represent the thickness of crust up to 50 km., during orogenic processes at the end of subduction stages of the Neotethys.

Dorojin area in the Southwest of Ardestan city is part of the Urmia-Dokhtar magmatic zone. In the area of study, the intrusion of the Early Miocene granodiorite into the Middle-late Eocene sedimentary-volcanic complex caused the formation of hydrothermal propylitic alteration zones with green color. The vlcanics range in composition from basalt-andesite, trachyandesite, andesite, to rhyolite. These rocks under study are characterized by LREE and LILE enrichment compared to HFSE, calc-alkaline nature, and meta-aluminous composition related to the subduction zone. The most abundant mineral in the alteration zones is epidote formed by saussuritization of plagioclase. Average Pstacite [Fe3+/(Fe3++Al)×100] composition is 22% (PS22). Comb and radial textures are its remarkable textural features. Calcite is the second most abundant mineral in alteration zones. The nodules tourmaline is chemically dravite and alkaline. They have FeO/ (FeO+MgO)<0.6 and lower Al amount, high Mg, and low X site vacancy. Dravite composition and the Mg abundant reflect their alteration origin. Hence, they are regarded as hydrothermal tourmalines.  Titanite with CaO<25wt%, Al2O3<2wt% as well as Al2O3+TiO2 versus CaO, and Al/Fe on variation diagrams are indication of hydrothermal alteration. Ore mineral potential in the region points to the effect of hydrothermal fluids.

Naqadeh intrusive pluton is composed of two distinct sections of felsic (40 Ma age) and mafic (99 Ma age) rocks. The granitoid rocks of Naqadeh pluton have different types of mafic enclaves, including mafic microgranular enclaves (MME) and quartz + amphibole bearing enclaves. The MME has the same age to its host granitoids and could be divided into two types. The first type has similar Sr-Nd composition to the host rock (87Sr/86Sri = 0.7062 and εNdi= -0.89), and their trace amphibole composition is like host granitoids. These enclaves are cogenetic with the granitoid magma and formed by early crystal-melt segregations. Another type of MME shows the textural evidence of magma mixing and is characterized by different isotopic ratios from granitoids (87Sr/86Sri = 0.7042 and εNdi= +2.98). The trace element composition of this type of enclave is completely different from the other enclaves and the host rocks. The quartz + amphibole bearing enclave formed contemporaneous with mafic rocks of Naqadeh pluton (99 Ma), and displays the same isotopic ratios and amphibole composition.

The Harris granitoid body, as a part of Alborz-Azarbaijan zone, located in the northwestern Iran and west of Shabestar city. The intrusive body cut the Kahar formation and covered by non-conformably Permian sediments. This body is granite- alkali granite in composition with peraluminous to metaluminous nature. Granular and perthite are dominant textures of the studied samples. The rocks under investigated are marked by quartz, K-feldspar, plagioclase as well as biotite, amphibole, pyroxene, apatite, titanite and zircon as minor minerals. The strong Eu negative in the REE diagram points to either the presence of plagioclase in the source or differentiation process of the plagioclase during evolution of the parent magma. As the petrochemical data display, the studied rocks are A-type granitoids, and due to Nb poor, they are compatible with A2- type. In addition, Ba negative anomaly and Rb, Th enrichment relative to Ta and Nb represent the crustal origin of Harris intrusive body. In these granites, Ba shows more negative anomalies than post collision granites and the Zr, Ta, Nb and Yb amounts are somewhat higher than those of post collision granites. The Harris granite body can be regarded as intraplate and non-orogenic granites generated in a post collision tensile environment. Overall, the Harris granitoid possibly originated by partial melting of lower crust with tonalite-granodiorite composition.

Chamatoo intrusive body with monzonite to granite composition located south of Amlash city and east of Guilan in the Western Alborz. Petrographically, the Chamatoo intrusive body is mostly made of alkali feldspars and plagioclase with biotite, pyroxene, opaque minerals, and different textures from granular to porphyry granular as the main textures and perthitic, graphic, poikilitic as the accessory textures. In the chondrite-normalized spider diagram, the granitoid rocks display subparallel, linear and homogeneous REE profiles with LILE enrichment as well as Ta and Ti negative anomalies. In addition, all samples have the characteristics of WPG. Overall, the Chamatoo body was formed during the subduction of the oceanic crust of the Southern Caspian Sea and owing to the activity of an subducted mid oceanic ridge towards south, in an extensional regime above the supra-subduction zone a Slab Window was formed. Finally, Slab Window tectonic regime caused the rise of asthenosphere and the alkaline OIB magmatism in the region, followed by the generation of A1- type granite and the per-alkaline Chamatoo granitoid from fractional crystallization of OIB magma source.