PETROLOGY
Year:2011 | Volume:2 | Issue:6|Papers Count:8

The Kalut area is located at about 70 km northeast of Ardakan (Yazd), which is a part of the structural zone of Central Iran (Yazd block). The igneous rocks of the area belong to Tertiary and are subdivided into intermediate (microdiorite) and basic rocks (basalt and microgabbro). The majority of these rocks show intergranular textures. Plagioclase (albite to bytownite), clinopyroxene (diopside) and olivine are the main minerals of basic rocks. Mica (biotite), apatite and Fe-Ti oxides are accessory minerals. Albite, calcite, chlorite, epidote, amphibole, opaque, sphene, prehnite formed during alteration process. The main textures of intermediate rocks are intergranular. Plagioclase (oligoclase to labradorite), amphibole and clinopyroxene (diopside) are the significant minerals of the aforementioned rocks. The chemistry of clinopyroxenes and the existence of pillows in exotic magmas of mafic rocks, imply that during formation of magma it was mixed with another basic magma, which the nature and the sources of these magmas were likely similar. The results of petrogenetic studies by using mineral chemistry of clinopyroxene indicate that the composition of magma is tholeiitic to calcalkali basalts which is related to volcanic arc zone.

The Mesr granitoid complex situated in the NE of Isfahan Province and from structural viewpoint belongs to Central Iran. In this area, three granitoid phases and one pegmatite phase with the age of Middle Jurassic, cross cut the ophiolite and metamorphic rocks.The phases 3 and 1 present the maximum and the minimum volume of intrusive bodies, restectively. Also, the phases 4 and 1 present the youngest and the oldest intrusive bodies respectively. There are quartz, K-feldspar, plagioclase, and muscovite in all the studied rocks. Biotite exists in only phases 1, 2 and 3. Tourmaline belongs to the phases 2 and 4 and garnet is limited in phase 3. The geochemistry of the major elements in the granitoid complex points to the descending trend of MgO, Al2O3, Fe2O3, FeO, TiO2, P2O5, MnO, CaO and ascending trend of K2O vs. silica, which can be related to plagioclase crystallization (Al2O3, CaO) and precipitation of titanite (TiO2, FeO). The magma that produced these rocks have calc-alkaline nature with weakly to strongly peraluminous to metaluminous. According geochemical diagrams, the samples of the study area are recognized syncollision and the type of magma is recognized as hybrid.Geothermometric investigations were carried out using two-feldspar composition and the given temperatures are lower than the temperature of igneous environments. Thus, we can conclude that the feldspars composition have changed by the subsolidus effects and mylonitization. Biotite thermometry yields a temperature of 642-718 oC for biotites in phase 1 suggesting the magmatic environment temperature.

The quaternary alkali olivine basalts exposed in the east and northeast of Sanandaj in Qorveh, Takab and Bijar axis. These rocks are mainly olivine basalts with normative nepheline and olivine. The existence of gneissic xenolith, quartz xenocrysts and phenocrysts of biotite are evidences of crustal contamination. From geochemical point of view, negative anomaly of P, Zr, Nb, Ti and Pb positive anomaly as well as enrichment of LIL elements indicate crustal contamination. In tectonic setting diagrams, the samples plot in intra-plate alkali basalts. The low ratios of Y/Nb, Zr/Nb, high ratio (La/Yb)N ~24 and the similarity of REE pattern with OIB, show that of magma generated in an enriched mantle. Also, geochemical studies indicate that the studied basalts are derived from enriched mantle OIB-like source with spinel facies in the pressure of 10-15 kbar, depth of less than 60 km and less than 15% partial melting.

The Astaneh granodiorite intrusion, located in the southwest of Arak, is characterized by rounded and rarely ellipsoid microgranular enclaves ranging in composition from gabbro to diorite and quartz diorite. The diameter of enclaves varies from a few millimeters to 40 centimeters. In the marginal part, the enclave forming-minerals are finer than that of the central parts. The fine grained enclaves are an evidence of rapid magma cooling which these rocks were surrounded by host felsic magma. Petrographical evidences such as ocellar quartz, acicular apatite, bladed biotite, poikilitic textures and pargasitic amphibole in all of the enclaves show that magma mingling occurred. The enrichment of the studied enclaves in CO, Ni, Cr and HREE display that these rocks are globules of mafic magmas mingling to felsic type.

The Zafarghand granitoid pluton ranging in composition from gabbro to tonalite and age of early-middle Miocene crops out about 35 km of southeast Ardestan. The pluton has intruded into the Eocene volcanic and volcanosedimentary rocks of Orumieh-Dokhtar structural zone. The pre, syn and post-plutonic dikes with mostly andesitic composition are associated with this pluton. The presence of abundant microgranular enclaves and interrupted syn-plutonic dikes are remarkable evidences of magma mixing in Zafarghand granitoid pluton. Based on the geochemical characteristics, there is a limited gap between mafic-intermediate and felsic rocks which may be the result of differences in their source rock regions. Enrichment in both LILE and Pb are indication of crustal contamination. Considering the typical negative anomaly of Ti and Nb, it can be said that this pluton is originated and produced in a volcanic arc of an active continental margin. Based on the discrimination diagrams of tectonic setting, the Zafrghand pluton can be classified as I-type, volcanic arc and continental arc granitoids (VAG and CAG).The melting of subducted Neothytian oceanic slab, it's overlay metasomatized mantle wedge and partly continental lower crust as well as fractional crystallization of generated magma, led to the formation of the studied pluton.

The study area is located 50 km southwest of Birjand, in the northwest margin of Sistan suture zone. In Hosseinabad area, the Tertiary volcanic rocks cross-cut the upper Cretaceous ophiolitic sequences. These rocks are andesite and display porphyric and glomeroporphyric textures. Plagioclase, hornblende, augite and biotite are the main phenocrysts. Sieve texture and selective alteration in plagioclase, opacitization of hornblende and the presence of mica-schist and amphibolitic enclaves are very common in these rocks. High ratio of LREE/HREE, Sr/Y (average: 38.55) and SiO2 (average: 59.09), and lack of Eu negative anomalies indicate that these rocks are similar to silicarich adakites suggesting that this magma could have been originated from a garnet amphibolite source, resulting from metamorphism of thickened lower crust.

The study area is a part of the western Alborz-Azarbaijan magmatic belt. The main outcrops in Ghaflankuh are including alternative andesite lava, andesite-basalt, trachyandesite and related tuffs. This sequence is coverd by acidic domes and in some areas by acidic lavas and ignimbrites. Olivine, clinopyroxene, hornblende, biotite, plagioclase, quartz and sanidin are the major minerals of the studied rocks, displaying hyalo-porphyric, microlithic and porphyry and glomero-porphyry textures. The mafic to intermediate lavas have andesi-basalt and felsic rocks have rhyolithic composition. The mafic to intermediate magmas are low-K and metaluminous with significant enrichment in LREE in compare to HREE, as well as Sr, K and display depletion in Pb, P, Pr, Zr, Y, Nb and Ti. The negative anomaly of Ta, Nb, and Ti in these rocks is similar to subduction related rocks. These lavas are produced from differentiation of magma derived from enriched- mantle with 1-5 precent partial melting of garnet-lehrzolite. The variations of Rb/Sr, Nb/Th imply for the presence of phologopite (glimeritic veins) in the source. The high ratios of K/P (6.5-43) and La/Nb (1.7-2.8) point to contamination of magma with crustal materials. This sequence is evolved in an active continental arc environment.