PETROLOGY
Year:2010 | Volume:1 | Issue:2|Papers Count:7

The Upper Eocene sub-volcanic rhyolitic rocks of Nasir-Abad area, in south-west of Rayen, (placed in Kerman province -Iran) are under the pyroxene-andesite rocks of Lower Eocene volcano-sedimentary complex of Razak formation, which is a part of Uromieh-Dokhtar Volcanic Belt. One of the main characteristics of these sub-volcanic rhyolitic rocks, is their columnar joints structure, in which most parts are vertical and occasionally with a 50-degree to 70-degree tilting to the east. The general shape of the these joints, are often 5 to 6 sided volumes, but rarely 3, 4 and 7 sided volumes are also visible. These structures have been formed during the chilling process of the magma mass. The sub-volcanic rhyolitic rocks have micro porphyry texture, but according to some thin sections, glomeroporphyry and felsophyric textures have been observed. It is worth noting that, alkali feldspar, plagioclase, quartz, biotite and hornblende are the main mineral constituents. The pyroxene-andesitic rocks consist of porphyritic textures with plagioclase, amphibole and pyroxene as the main mineral constituents. Secondary minerals such as chlorite, calcite, sericite and epidote exist in both types of rocks.Investigation of the results of geochemical data, and tectonic discrimination diagram indicates that: the sub-volcanic rhyolitic rocks have characteristics of metaluminous, calk-alkaline, I-type granitic rocks and that the andesitic rocks derived from a rich mantle. Moreover, they were formed in a volcanic arc setting in an active continental margin environment.

In Borooni area (SW of Ardestan), which is a part of Uromiyeh – Dokhtar magmatic belt, the Miocene basaltic columnar-jointed dikes cross cut the Eocene volcanic rocks.These rocks contain the granitoid enclaves of Oligocene age and possibly, they are Miocene in age. The studied rocks are composed of chloritized olivine, clinopyroxene, plagioclase, chlorite, ilmenite and magnetite. Textures of these rocks are porphyritic, microlitic, microlitic porphyritic, glomerophyric, and sieved texture of plagioclases.Clinopyroxenes are augite to diopside in composition, and plagioclases range from labradorite to bytownite. Chlorites show diabantite composition and most of them produced by olivine alteration. Some chlorites are present in groundmass. Petrography and chemistry of minerals show that the parent magma subjected to a magma mixing with xenoliths of granitoid rocks during the ascending. These rocks are similar to the volcanic arc basalts.

The Khoy ophiolite with Precambrian basement rock is located in north western of Iran.The tectonite peridotites are most formed by clinopyroxene harzburgites. These harzburgites characterized by protogranular, porphyroclastic and transformation textures, and composed of olivine (with kink band), orthopyroxene (with wavy extinction and exsolution clinopyroxene), spinel (like Holly – Leaf and shaped) and less than 10% of clinopyroxene. Two stages of alteration occur in these rocks, including static and dynamic. These peridotites are partial melting residual of spinel lherzolites.The Khoy taconite periodontitis show the U-form pattern in the REEs. These patterns are probably related to the partial melting procedures and the process following the partial melting such as influence of felsic magmas after emplacement of ophiolites.

The plutonic complexes of Marivan area, composed of two types of plutonic rocks of different genesis, are situated in the northwest of Sanandaj-Sirjan metamorphic belt.The stratigraphical units of the area are phyllite, limestone, volcanic rocks of the Cretaceous-Paleocene age, marl, sandstone and shale of the Oligo-Miocene age. Two plutonic complexes of the area are: (1) the granitoid complex of NW Marivan which is composed of syenogranite, monzogranite and granodiorite. Geochemically, these rocks are magnesian alkali-calcic to magnesian calc-alkali, sub-alkaline (calc-alkaline), peraluminous and are S-type. From geochemical and mineralogical aspects, this complex resembles the syn-collision granites. (2) The gabbro-diorite-quartz diorite complex of SE Marivan is composed of various plutonic rocks including gabbro, diorite and quartz diorite. Geochemically, these rocks are sub-alkaline (calc-alkaline), metaluminous, magnesian calcic and are I-type. Rocks of these complex show characteristics of plutonic rocks derived from upper mantle sources. It is worth to note that blocks of I-type complex are surrounded by rocks of S-type complex.

The Jahagh anticline is located at 40 km south of Kashan and east of Ghohrud village.The Early Paleozoic volcanic rocks are outcropped in the Jahagh anticline core. Despite low degree alteration and metamorphism that are identified by mineralogical studies, the primary textures are well preserved whereas mineralogical composition has changed.The major minerals including of plagioclases and rare Fe-Mg minerals have experienced alteration. The presence of secondary minerals such as albite, chlorite, epidote and calcite shows that the low-grade metamorphisms have occurred under greenschist facies. Following the intrusion of Ghohrud granitoid, the adjacent volcanic rocks have been metamorphosed under hornblende - hornfels facies. This phenomenon is evidenced by the association of plagioclase (oligoclase - andesine) and amphibole (Ferro -Tschermakite) in the rocks of western parts (approaching the intrusive mass).

The composition of Boroujerd granitoid varies from quartz diorite, granodiorite to monzogranite associated with acidic dykes intruded into the Hamedan phyllite and developed a metamorphic aureole. On the basis of mineral chemistry the biotites have different compositions and their parent magma has sub alkaline to calc-alkaline nature.The studied amphiboles are classified as calcic (magnesio-hornblende) which point to the I-type nature of the Boroujerd granitoid. Plagioclases vary from andesine-oligoclase to labradorite, alkali feldspars occur as orthoclase and those in pegmatites are sodic (albite) and potassic (orthoclase). The changes in feldspar composition in different units are normal indicating magma differentiation. In all samples, garnet displays zoning and is almandine-spessartine in composition. Also, garnet in hornfelses and granodiorites is cogenetic and nonmagmatic, but in pegmatites it has magmatic nature. Al-barometer accounts that the pressure of amphibole crystallization is 0.43 to 1.61Kbar and Altotal versus Fetotal/ (Mg+Fetotal) accounts the pressure between 1 to 3 Kbar. Hornblende -plagioclase thermometer shows 653 to 732 °C for equilibrium of these two minerals and garnet-biotite thermometer shows 472 °C for garnet-biotite in hornfelses. The estimated oxygen fugacity (logƒO2 varies between -18.3 to -15.6) imply an oxidation magma (confirms magnetite and enrichment of Mg in amphibole) and its formation in convergent plate boundary.

The Oligocene felsic outcrops are located in the south of Ardestan (NE of Isfahan). The area is a part of Uromieh –Dokhtar structural zone. These outcrops are composed of rhyolite and rhyodacite rocks. Geochemically, these rocks are sub-alkaline, calc-alkaline composition with high-K and per aluminous. Although the whole rock composition of the felsic rocks corresponds to S-type granites (i.e. high K, Al, large ion lithophile elements, and low Ca and Sr) but the studied rocks have remarkably primitive and igneous sources. The geochemical data suggest that mantle wedge is partly metasomatized with rhyolitic materials from subducted slabs; it is more likely that the rhyolite magma developed by very low degree partial melting of the metasomatized mantle wedge. The initial reason for direct eruption of the mantle-derived rhyolitic magmas would be a tensional condition of the Ardestan region during late Eocene-Oligocene time. If mantle-derived rhyolitic magmas ascended within a compression crust, the magmas should easily react with crustal materials and therefore it would be indistinguishable from felsic magmas produced by crustal fusion. The Petrological and geochemical evidences as well as the tectonic discrimination diagrams show that rhyolitic magma formed in an active volcanic arc. It seems that these rocks are formed following the subduction of Neo-Tethys oceanic crust beneath the central Iranian micro continent.