PETROLOGY
Year:2011 | Volume:1 | Issue:4|Papers Count:7

The rhyolitic and trachytic domes in south of Dastgerd in Qom province, contain garnet phenocrysts. Based on the micro-probe analysis, garnets are almandine rich, and poor in other components. Considering the whole rock chemistry, the magmas of the area have continental origin or at least contaminated with a continental source. Based on chemical composition of biotites, the magmas are per aluminous with continental origin. Several evidences such as lack of any metamorphic xenocrysts or xenoliths, homogeneous composition of garnets, compositional equilibrium between garnet and biotite, lack of metamorphic inclusions in garnet and lack of reaction rims, in addition with the origin of host rocks and whole rock composition, indicate that garnets in the area could be crystallized from a per aluminous magma and they are igneous garnets.

Basalts, andesites, trachyandesites and latites with shoshonitic affinity are the main volcanic rocks of Eocene age in the Lahrud region (Ardabil). Plagioclase, K-feldspar, biotite and amphibole associated with clinopyroxene are the main constituents of trachyandesites (shoshonites) while clinopyroxene, plagioclase and biotite are the rockforming minerals of basalts (absarokites). The Lahrud volcanic rocks show enrichment in LREE and are characterized by enrichment in LILE and depletion in HFSE.Petrographical observations along with geochemistry of rare earth and trace elements of these lavas suggest shoshonitic affinity and derivation from a subduction zone. The geochemical behavior of the Lahrud lavas reveals the role of the trench sediments in the source region and genesis of these volcanic rocks. These lavas exhibit low degree of partial melting from a garnet-spinel lherzolite source. The comparison between the Lahrud volcanic rocks with Plio-Quaternary lavas of Sabalan and Eocene lavas of Hashtjin region highlights the different mantle source and degree of partial melting for the genesis of these volcanic rocks. The formation of these lavas is linked to slab steepening and breakoff in a post subduction collision zone.

The Kohroyeh Pb-Zn ore deposit is located about 25 Km SW of Shahreza. The ore deposit belongs to Esfahan-Malayer metallongenic belt. Mineralization occurred as veins and veinlets consisting of galena and slight amount of sphalerite, calcite and quartz gangue lying between Jurassic formation (containing shale, siltstone and sandstone) and Cretaceous limestones. The geochemical maps define anomaly, the site of mineralization and concentration of Pb and Zn elements in this area. The C, O and S isotopic studies on calcite and galena show a non-marine and non-sedimentary provenance, and influence of meteoric waters in the genesis of ore deposit as well. On the base of field observations, there is a close relationship between ore bearing veins and faults, tectonic processes and deformation. According to geometric situation of deformed twins of large calcite crystals, the formation temperature of veins and ore deposit is estimated over 250oC. The overall data indicate that the provenance of the Kohroyeh Pb and Zn ore deposit may be considered as epigenetic and categorized as Mississippi valley type similar to Pb and Zn deposit of the SE Missouri.

The extensive spectrum of intrusive and extrusive igneous rocks with bimodal nature have exposed in the south of Bahabad area, northeast of Bafq. The intrusive igneous rocks are gabbro to granitic composition. The extrusive rocks compose of volcanosedimentary complex, known as Narigan series, alternation of basalt and rhyolite associated with pyroclastic rocks, sandstone, dolomite and evaporates (gypsum and halite). This complex displays features of shallow sedimentary and within plate environments. The presence of Archeocyatide fossil in the Limestone - dolomitic horizons and radiometric dating confirm Lower Cambrian age for this complex. The intrusive rocks mainly emplaced in the form of stock, dyke or sill intruded into the volcano sedimentary complex (Lower Cambrian) and Middle-Upper Cambrian and Ordovician deposits, therefore, these rocks are referred to Lower Paleozoic. According to the geochemical data, gabbros and diorites are enriched in LREE and depleted in HREE and indicate alkaline within plate nature. The felsic rocks (leucogranites and rhyolites) indicate typical characteristics of crustal rocks. The leucogranites belong to I, H and A-Type granites. The magnetite, apatite, barite, lead and zinc mineralization in this area and adjacent regions (Bafq-Bahabad) are resulted from these magmatic evolutions.

The Fesharak and Shirkuh skarns in the Cenozoic magmatic belt of Central Iran formed when the granitoid intruded into the Lower Cretaceous dolomitic-carbonates. The main mineralogical assemblage concerned is Clintonite-spinel-garnet in Fesharak and clintonitespinel in Shirkuh. Clintonite is either adjacent to garnet and spinel, or it contains the inclusion of these minerals displaying a stable relationship with them. Resorbed boundaries as well as development of corrosion textures in these two minerals are textural evidences suggesting spinel and garnet act as the source of Al in the genesis of clintonites according to the following reaction: 7 Spinel+Grossular+CaMg (CO3)2+SiO2+4H2O=4 Clintonite +2CO2.

In order to identify the provenance of met sediments of the Shahindezh Metamorphic Complex (SMC), NW Iran, we have studied the bulk chemistry of the metapelites in the mentioned area. The geochemistry of these rocks suggests that the parent sedimentary rocks are classified as shale. Major and trace elements concentration (e.g. TiO2, K2O, Ni, Rb) indicates that the primary igneous rock was acidic (andesite to rhyodacite). The chemical index of alteration (CIA) show that the source rocks have undergone moderate to high degree of chemical weathering and plagioclase composition varies from andesine to oligoclase. Using major oxides elements, the continental active margin is considered for SMC metapelites source. In comparison with average upper continental crust, the SMC metapelites show strong negative Ca and Sr anomalies and slight depletion in Nb and Mn but moderate enrichment in Y, Ba, Al and transitional elements.The similarity in the trends of multi-elements diagrams for all studied samples shows that bulk chemistry factor was constant variant at the different mineral par agenesis appearance process. Based on the standard PT pseudo sections drown by scientists for average metapelitic composition, Ms+Bt+St and Ms+Bt+St+Ky par agenesis were in equilibrium at the PT conditions estimated, previously.

The Jurassic magmatic sequences in the southern part of Sanandaj-Sirjan Zone show significant geochemical variations during their compositional evolution. These magmatic sequences have exposed in the Hoseinabad region with Early to Middle Jurassic age and in Hajiabad region with Late Jurassic to Early Cretaceous age. The parent magma of the first sequence in Hoseinabad region, with island arc tholeiite (IAT) affinity have been originated from the spinel lherzolite mantle source with primitive mantle (PM) composition. This mantle source is affected by liquids and sediments resulted from the sub ducting slab. The parent magma of the second sequence in Hajiabad region with island arc tholeiite to transitional affinity have been resulted from the garnet - spinel lherzolite mantle source with E-MORB composition. These compositional changes may be related to high sediment and hydrothermal fluxes which resulted from the deep subducted slab into the mantle wedge.These magmatic sequences are originated during the subduction of the Neo-Tethyan oceanic lithosphere under the Sanandaj-Sirjan Zone, during the Early Jurassic to Early Cretaceous time, in the island arc tectonic environment.