PETROLOGY
Year:2014 | Volume:4 | Issue:16|Papers Count:7

In this study, we investigate the geochemical evolution and the age of intrusive rocks from central part of the magmatic arc of the northeastern Iran in the south of Sabzevar (Khorasan Razavi province). The geochemical evidences point to magmatism related to volcanic arc and subduction zone. The intrusive rocks vary in composition from granite to diorite and gabbro. Based on mineralogy and the high values of magnetic susceptibility [(>400) × 10-5 SI], the intrusive rocks are classified as magnetite-series of oxidized I-type granitoid. Chemically, they are meta to per-aluminous, enriched in LILE (K, Th and Rb) and depleted in HFSE (Nb, P and Ti) and belong to tholeiitic series. Low (La/Yb) N, low Sr/Y ratios and the negative anomaly of Eu indicate a classic island arc type magmatism. Trace elements behavior in acidic and intermediate intrusive rocks show nearly flat pattern (La/SmPm =0.97-2.57) and lie outside the garnet stability field. Thus, the source rocks should not be eclogite or garnet amphibolite. The geochemical signature of rare and trace elements suggests derivation from melting under relatively low pressure condition (shallow depth). The results of U-Pb zircon dating of the studied are 97-100 Ma (mid-Cretaceous time). The initial 87Sr/86Sr ratios and the initial eNd are 0.7049 and +4.54, respectively.

In the northwest of the Central-East Iranian Microcontinent (CEIM), southwest of Jandaq, the Toveireh Oligocene alkaline basalt with NW-SE to W-E trend is outcropped. This alkaline basalt with porphyritic, poikilitic and microlithic porphyritic textures have olivine (chrysolite), clinopyroxene (diopside and augite), plagioclase (labradorite) and spinel as primary minerals and titanomagnetite, serpentine and zeolite as secondary phases. The rock in question is enriched in alkalies (Na2O+K2O), TiO2, LILE (Cs, Rb and Ba), HFSE (Ti, Nb, Hf and Zr) with high ratio of LREE/HREE (Light Rare Earth Elements/Heavy Rare Earth Element) (La/Yb=9.64-12.68). The chemical composition of theses rocks indicates that the primary magma of the Toveireh alkaline basalt is produced by partial melting of carbonated garnet lherzolite of asthenospheric mantle. The geological situation of the study area suggests that the subduction of oceanic crust along the Great Kavir Fault from the Triassic to the Eocene caused carbonate metasomatism and mantle enrichment. The presence of abundant xenoliths, xenocrysts and the reaction textures show fast magma rising. The activity of Great Kavir and the Toveireh faults in an extensional system in the NW of CEIM can be accounted for the Oligocene alkaline magmatism.

Sistan suture zone ophiolitic rocks are remnants of Tethyan oceanic lithosphere between Lut and Afghan continental blocks. This oceanic basin has been existed at least during the Cretaceous. Mineral chemistry from crustal sequence ultramafic-mafic associations and petrographical evidences from parts of these ophiolitic complexes (the ophiolitic rocks located between the south of Birjand and Tchehel Kureh ophiolites) suggest distinct geodynamic setting for the formation of oceanic lithosphere. Plutonic rocks from crustal sequence mainly include ultramafic cumulates (dunite and pyroxenite), cumulate gabbro, gabbronorite and isotropic gabbro. The crystallization trend in these rocks is variable including earlier crystallization of pyroxene relative to plagioclase and vice versa which implies SSZ (supra-subduction zone) and MORB (Mid-Oceanic Ridge Basalt) - type trends, respectively. The composition of plagioclase in cumulate gabbros and gabbronorite is bytownite and anorthite, respectively. Moreover, the contents of compatible and incompatible elements of clinopyroxene and orthopyroxene in ultramafic cumulates, cumulate gabbros and gabbronorite represent significant differences. The chemical composition of these minerals and crystallization trends indicate that pyroxenites-gabbronorite and troctolite-cumulate gabbros-isotropic gabbros have been formed from two different magma series with SSZ and MORB affinities, respectively. In the study area, it seems that the MORB-type magmatism resulted from partial melting beneath mid ocean ridge produced a depleted mantle. Subsequently, consequence of intra-oceanic subduction the MORB mantle has been suffered higher depletion due to fluid-induced melting in supra subduction zone setting and produced SSZ-type magmatism.

In the west of Zanjan, east of Halab, Gabbroic bodies with tholeiitic nature injected into the Eocene rocks. The main minerals are plagioclase, pyroxene ± olivive and epidots, chlorite, termolite-actinolite and the opaque minerals are accessory minerals. The rocks studied have SiO2: 45.1-47 wt% and are classified as gabbro. The normalized trace element patterns show enrichment in LREE and depletion in HREE. The positive anomalies of K, Th, Rb and the negative anomalies of HFSE (Zr, Nb and Ti) indicate the rocks contamination with continental crust. On the tectonic setting diagrams the studied rocks are plotted in the continental intraplate field. Based on all these data, it seems that, the studied gabbroic rocks are formed by the initial stages of the magmatism related to an Eocene pull apart basin and formed due to partial melting of upper asthenosphere risen under continental lithosphere and contaminated with crustal rocks during the Eocene.

The skarns of the Ochestan area are located in the Sanandaj- Sirjan zone, about 60 Km south of Mahallat. The rock complex in this area consist of intrusive bodies and metamorphic rock such as: schist, phyllite, slate, hornfels, amphibolite and metavolcanic. The injection of magma in the Cretaceous-Paleocene with syenogranite and monzogranite composition into the Paleozoic limestone and dolomite led to formation of skarn and marble. Clinopyroxene (diopside), olivine (forsterite), tremolite, talc, serpentine, phlogopite, margarite, wollastonite, opaque minerals, calcite and quartz are observed minerals in contact metamorphism rocks. According to the mineralogical studies, the Ochestan skarns are exoskarn-type and the presence of minerals such as: forsterite, diopside, tremolite, antigorite and phlogopite show that the host rock is dolomitic the in study area. These skarns show zoning of intrusive body into the host rock: wollastonite skarn, pyroxene skarn, forsterite- serpentine marbles, mica marbles, tremolite- talc marbles, calcite and dolomite marbles.

The granitoid bodies of Yazd belonging to the Oligo-Miocene in age located in the central part of Urumiaeh-Dokhtar volcanic belt. These bodies intruded into the Cretaceous limestone of the Taft formation and pyroclastic rocks of the Eocene. These rocks generally show granular texture; and porphyroid, granophyric, graphic, perthite, myrmekitic and anti-rapakivi textures are occasionally identified. On the basis of field observations and petrographic and geochemical studies, the Yazd intrusive bodies fall within the range of alkali-granite to tonalite in composition and belong to high temperature I-type (Cordilleran) ACG, VAG, Post-COLG and HSS groups. Various geochemical diagrams show the enrichment of LILE, LREE and depletion of HREE and HSFE. These are the characteristics of calc-alkaline magma developed in subduction regime of active continental margin (ACM). Therefore, it is suggested that the magma was generated from the subducted oceanic crust (Neothety) and evolved mantle wedge above it. The magma was generated at the base of crust owing to crust melting and subsequent differential crystallization, assimilation and contaminations (AFC), all are involved in magmatic evolution. Proposed geotectonic model of the area could be similar to the central Andes model, in which magma generated by polygenic and polystages phenomena.

The study area is located 50 km northeast of Nayband in the easternmost part of Yazd province. The area is a portion of the Tertiary volcanic-plutonic rocks in the west of the Lut block. The geology of the region includes latitic to trachyandesitic lavas, which are intruded by subvolcanic rocks with monzonitic, syenitic and granitic compositions. The texture of all the intrusive rocks is porphyric to glomeroporphic. Plagioclase, K-feldspar, biotite and quartz are the main minerals of biotite quartz monzonite porphyry, biotite syenite porphyry and biotite granite porphyry. Clinopyroxene, besides these minerals is present in biotite-pyroxene monzonite porphyry. Based on geochemical studies, the biotite-pyroxene monzonite porphyry shows metaluminous and shoshonitic nature. This rock belongs to the magnetite series (oxidized) granitoid and is allocated to I-type. The relative enrichment of LREE versus HREE and enrichment of LILE and HFSE impoverishment indicate magma formed in subduction zone. The magma exhibits low degree of partial melting (>0.1 to <3) from a garnet-spinel lherzolite source, which is contaminated by continental crust. The biotite granite porphyry is peraluminous and ultrapotassic. The rock belongs to ilmenite series (reduced) granitoid and is related to S-type. Based on Nb, Yb, Rb, Y and Ta contents, the magma formed in syn-collisional tectonic setting. The granitic magma is generated by partial melting of continental crust and clay-poor sediments source. The relative enrichment of LREE and depletion of HREE, highly negative Eu anomaly and low ratios of (La/Yb) N and (Ce/Yb) N can be attributed to the presence of residual plagioclase in the source area. The intrusion of this rock into the monzonitic unit indicates a collision occurred after the Eocene in the west of Lut block in which access to accurate geochronological and radiogenic isotopes data is required.