PETROLOGY
Year:2020 | Volume:10 | Issue:40|Papers Count:6

The cratonic basement in the Kabul district is dominated by gneiss, migmatite, schist, and amphibolite and is characterized by extensive outcrops. Orthopyroxene and garnet are the index minerals of some of the gneisse, hence, they are considered as charnockite. Biotite, quartz and feldspar are the rock-forming minerals of the migmatites and their modal abundances vary between melanosome and leucosome. The predominant minerals of the schistic samples are garnet, kyanite, quartz, feldspar, muscovite and biotite. Amphibole (pargasite), garnet and plagioclase are the essential minerals of amphibolitic samples. Based on thermocalc software, petrogenetic grade and conventional thermobarometers the calculated average pressure and temperature for charnockitic, schistic and garnet-amphibolitic samples are 7. 03 kbar and 590 ℃ , 9. 94 kbar and 518 ℃ and 9. 24 kbar, 664 ℃ respectively. The mineralogical paragenesis and the geothermal gradient resulted from thermobarometry calculations of metamorphic basement of the Kabul block is in accordance with metamorphic gradient of collisional orogenic belts. The cratonic basement of the Kabul block is possibly representing metamorphic rocks of the deep level of Proterozoic collisional orogenic belts during the formation of Columbia and Rodinia supercontinents.

The rocks of the Robaie area located in the Torud-Chahshirin belt and south of Damghan, includes the Eocene andesite and trachyandesite rocks in which subvolcanic igneous rocks as stoke and dyke with diorite, monzonite and monzodiorite porphyry composition are intruded. The main textures of volcanic rocks are porphyritic characterized by plagioclase, hornblende and biotite phenocrysts as well as apatite and zircon as minor minerals. The rocks studied are mainly of shoshonitic nature and only one sample is considered as high-K calc-alkaline. Several line of evidence including LILE and LREE enrichment, HREE and HFSE depletion, high Th/Yb with negative anomalies of Ti, Nb and the position of the samples on the tectonic discrimination diagrams indicate that the volcanic rocks in discussion were emplaced into the subduction zone related to an active continental margin setting. The initial 87Sr/86Sr, 143Nd/144Nd ratios and (ε Nd)i value of andesite are 0. 704445, 0. 512691 and 2. 29, respectively. All of these evidences confirm that the studied volcanic rocks were generated from partial melting of the mantle wedge above the subduction zone. Petrographic observations along with geochemistry of rare earth and trace elements suggest that the calc-alkaline affinity of the rocks studied and their parent magma from a subducted-related environment as well as crustal assimilation and fractional crystallization.

The Cheshmeh-ghassaban area plutonic rocks are located in NW of the Hamedan city and is a part of the Alvand plutonic complex. These bodies intruded in the Hamedan phyllites in Mesozoic. They range in composition from olivine gabbronorite, hornblende gabbro to biotite gabbro together with some quartz-diorite and granodiorite. The olivine gabbrosnorites are composed of orthopyroxene, clinopyroxene, plagioclases and olivines with corona texture. Chemical analyses of the minerals show that the olivine forsterite contents range from 66 to 77 and increase toward the crystal cores. The olivines also were replaced with orthopyroxene and hornblende in their rims due to reaction with the surrounding melt and its associated fluids that formed the corona texture. The clinopyroxenes are of diopside type and orthopyroxene have pigeonite composition. Moreover, MgO and Cr2O3 contents of the clinopyroxenes indicate that the parent magma of the olivine gabbronorite was tholeiitic and formed in a subduction zone that endured rifting and asthenospheric magma upwelling. Geothermobarometry calculations based on proxene composition show 900 to 1200 oC and 7 to 10 kbar for this rock. Also, petrological and mineralogical properties of these rocks are similar to those of the ultramafic-mafic Alaskan type complexes that form above subduction zones. The results of the present study indicate the role of mantle-originated magmas in formation of the Alvand complex.

The Mirabad dacitoid dykes exposed in the Sistan and Baluchestan province, Sistan suture zone and South of Zahedan city, near Mirabad village. These dykes, with mainly dacitoid in composition, are hosted by the Eocene flysch assemblages and the Cretaceous ophiolitic mé lange. They are composed of plagioclase, quartz, hornblende and biotite minerals and the dominant texture is porphyry. The studied dykes have sharp boundary with host rocks and sometimes have metapelitic enclaves. They are calc-alkaline, meta-aluminous and I-type nature with LREE and LILE enrichment and HREE and HFSE depletion. Europium does not show any distinct anomaly reflecting the oxidation state of magma. The rocks under study have a common origin on the base of similar REE trends and spider diagram and are related to subduction environments on tectonomagmatic diagrams. Based on the Ta/Yb versus Th/Yb tectonomagmatic diagram, it is possibly that the parent magma derived from the mantle wedge above the subduction plate, which is usually affected by the fluids released from the subducted plate and their elements (including silica, potassium and sodium). The overall results indicate that the investigated dykes are synmagmatism occurred during subduction of Neothytean oceanic crust between the Lut and the Afghan blocks.

Eocene volcanic and plutonic rocks exposed in the 90 km south of Khusf, south Khorasan Province. The texture of these rocks is porphyritic, glomeroporphyritic, seriate and poikilitic. Plagioclase, Alkali feldspar, quartz, pyroxene and hornblende are abundant mineral in these rocks. Plagioclase crystals show sieve texture and zoning. These rocks are calc-alkaline to tholeiiti. They characterized by enrichment of LREE and LILE, depletion of HREE and HFSE. Negative anomalies of Ti, Nb, P, Zr, and positive anomalies of Pb, K, U reflect emplacement of volcanic rocks in an active continental margin. In order to investigation of magma crystallization, crystal size distribution (CSD) technique also used for 10 andesite and basalt rock samples. Increasing of plagioclase crystal size in a semi-logarithmic plot which demonstrate population density versus size, represents fractional crystallization. Concavity of the CSD diagrams resulting from combination of two populations of crystals, that one of them formed during slow cooling and the other formed during a period of pressure reduction and rapid cooling. Additionally, chemical features such as variation trends of Nb/Y vs. Nb and MgO vs. Fe2O3 and two different trends of volcanic and plutonic samples in Harker and dendrogram diagram, regarded as an indicator of magma mixing processes.

The occurrence of doleritic, microgabbroic, hornblende-gabbroic, pegmatoidic and gabbronoritic dikes is the conspicuous characteristics of mantle sequences of the ophiolitic complex from south Soltan Abad. Dikes trend is mostly western-eastern and because of deformation, some display boudin-domino structure. Pegmatoidic and doleritic dikes contain clinopyroxene and plagioclase minerals. Magmatic amphibole is observed in addition to clinopyroxene and plagioclase in the hornblende-gabbroic dikes. There are plagioclase, clinopyroxene and orthopyroxene in the gabbronoritic dikes. Microgabbroic dikes include plagioclase and secondary amphibole. Clinopyroxene is replacing by amphiboles in the most dikes. Occurrence of orthopyroxene and magmatic amphibole, pegmatoidic texture, pyroxene crystallization prior to plagioclase and anorthite rich composition of the plagioclase indicate water participation in the magma genesis. The pattern of the multi-element spider diagram of the most investigated samples shows noticeable resemblance to the suprasubduction ophiolites and Cenozoic fore arc basalts. Hornblende-gabbroic and doleritic dikes are alkali basalt, so derived from enriched lower mantle source rock. However, the others exhibit geochemical signatures of tholeiitic basalt with arc island affinity. Hence based on these documents, mafic-ultramafic complex of south Soltan Abad represents oceanic lithosphere of forearc from suprasubduction zone setting. According to the classification and tectonic setting discrimination diagrams, most dikes exhibit geochemical characteristics of tholeiitic basalt with boninitic and island arc affinity. Hornblende-gabbroic and doleritic dikes encompass alkali nature, so they have derived via melting of source rock from lower enriched mantle. The compositional sequence, probably represents incorporation of various mantle source rock during the evolutionary trend of the subduction zone at late Cretaceous.