PETROLOGY
Year:2013 | Volume:3 | Issue:12|Papers Count:7

Plagiogranite intrusive rocks are outcropped in the gabbroic section of the Neyriz ophiolitic sequence in the south east of Shiraz. Petrographically, they are comprised of tonalite, trondhjemite and their contact with surrounding gabbros is either sharp or covered. Predominant textures are hypidiomorphic inequigranular, granophyry and micrographic. Mineralogically, they are consisting of plagioclase, quartz, sodium feldspars as major minerals and less than 10% amphibole±pyroxene, opaques and rarely titanite and zircon as minor minerals. Geochemical studies show that their magma is sub-alkaline type (calc-alkaline series) and metaluminous. Typologically, plagiogranites have characteristics of oceanic ridge granite (ORG) toward I-type granites. Chonderite normalized REE patterns of Neyriz plagiogranites show depletion in LREEs along with a flat HREE patterns and it seems that they were formed in supra-subduction zone environment by partial melting of mafic rocks which in turn were formed by the anatexis of a previously depleted harzburgitic mantle.

The Dalli area located northeast of Arak and lies in the Urumieh-Dokhtar magmatic belt. The mineralized area is hosted by volcanic-subvolcanic bodies comprising of diorite-quartz diorite andesite to dacite rocks that intruded into the andesit and pyroclastic rocks. Three main alteration zones including potassic (secondary biotite, magnetite and secondary orthoclase), phyllic (sericite, quartz and pyrite) and propylitic (calcite, chlorite and epidote) are recognized in this area. The geological, geochemical and fluid inclusion investigations show that mineralization in the studied area is of gold-rich porphyry copper type. Gold is observed as an inclusion within chalcopyrite and silicic veins. Fluid inclusion studies in silicic veins show that the homogenization temperature and the salinity is about 180 to 600oC and 26 to 70% wt NaCl respectively. The temperature and salinity of fluids decreases from potassic to phyllic alteration zone consistent with other Cu porphyry deposits in the world.

The IIeh Iron ore deposit is located about 250 km SE of Mashhad and 49 km SW of Taibad (Khorasan Razavi province). The oldest exposed rock unit belong to late Proterozoic and are meta volcano-sedimentary complexes with metamorphic grade of green schist facies. The other outcrops are granites with Upper Eocene age which cut the complex. The granitic bodies are calc-alkaline, high-K, peraluminous to meta aluminous, and characterized by negative Eu anomaly, the magnetite- series (I-Type) granites, are related to continental- margin subduction zones with the lower crust source. The homogenization temperatures (Th) obtained from fluid inclusions in calcite and quartz ranges from 200-385oC with mode of 300-320oC. Two salinity ranges of 4-6 and 28-38 NaCl eq. Wt% are observed in quartz and calcite, respectively, implying an possible fluid-mixing event. The occurrence of hematite could be taken as evidence for extremely oxidizing conditions and the possible derivation from a magmatic source during the trapping event. Based on XRD and thin- section studies, the altered rocks around ore deposits show sericitization, chloritization, illitization, kaolinitization, silicification and montmorillonitization are the main alteration types. This study suggestsa K-Fe-Mg-Si-Ca composition for the ore-bearing fluids. Further proof for the hydrothermal-origin of the ore-bearing fluids is indicated by the positive Eu anomaly in the respected REE abundance pattern of ore samples.

The Surk ophiolitic melange, as a narrow belt, is located in western part of Yazd province, between the southwest of Central Iran and Urumieh-Dokhtar magmatic arc, along Naein-Dehshir-Baft fault. This ophiolitic melange consists of mantle peridotites, gabbro, diabasic dykes, quartz andesite, dacite, serpentinite, listwaenite, rodingite, chert and Upper Cretaceous limestones. Petrography and mineral chemistry of mantle peridotites reveal incongruent melting of orthopyroxenes, the formation of excess olivines, the high value of Cr# in spinels of harzburgite and dunite and the occurrence of pargasitic amphibole in lherzolites in the studied ophiolite. All of these evidences suggest melt/oceanic lithosphere reaction. The reactive melt originated from a subducted slab that decreases the melting temperature of the lithospheric mantle and leads to widespread ascending melt/wall rock reaction. The peridotites of this ophiolitic melange consist of harzburgite, dunite and lherzolite in decreasing order. The ophiolite is a harzburgite type ophiolite (HOT) showing characteristics of supra subduction zone ophiolites (back-arc basin).

The Quaternary lamprophyres and related rocks from southwestern Salmas are laid on the old floodplain deposits in the border of Sanandaj-Sirjan and Urmia-Doukhtar zones. These rocks are all alkaline and include mostly camptonitic lamprophyres, contaminated Lamprophyres, basalt andesi-basalt and trachy-andesite. The rocks have been cropped out as prismatic and scoria structures with porphyritic and glomeroporphyritic textures. Lamprophyres are mostly composed of major minerals of augite, diopside, biotite, amphibole and tiny plagioclase in the matrix. Other rock types have augite, diopside and plagioclase as major phases. The presence of plagioclase and quartz xenocrystals with rounded margins, normative changes and variation in minor and trace elements content as well as their ratios are indicative of contamination with continental crust. Enrichment of LREE relative to HREE, the lack of Eu anomaly and the occurrence of Ta- and Nb-negative anomalies are important characteristics of all of the studied rocks. High ratios of Lan/Ybn, Lan/Smn and Smn/Ybn, the presence of spinel, augite and diopside and alkaline nature of all samples, indicate a melting process about 1 percent of a mantle with spinel lherzolite composition. The processes likely occurred following the Neotethys subduction beneath Central Iran ended. Slab break off in the remnants of the Neotethys oceanic crust occurred in location of the collision zone of Arabian-Eurasian plates. Arabian oblique tectonic pressure to the Eurasian plate and increasing activity of right-lateral strike-slip faults, which resulted of remnant slab break off the Neotethys, caused to decrease lithostatic pressure. As a consequence, the decompression melting occurred. The occurrence of these processes successively led to the rocks show tectonically within plate characteristic along with interacting of subduction elements. The mantle melts were erupted along these fault systems.

In the northern hillside of Central Alborz, south of Kamarbon village, there is an outcrop of alkaline gabbroic intrusion that its petrogenesis and tectonic environment have been studied in this research. The studied intrusion has fine grained theralite in the margins whichconverted to coarse grained teschenite toward the center. The Mg number is decreased from theralite towards teschenite. The occurrence of theralites in the fine grained margin can reveal early crystallization of the margins due to rapid fall of temperature. The studied rocks on the spider diagrams show the enrichment of LILE (Rb, Ba) and HFSE (Nb) and the depletion of P, K, HREE (Y, Yb). These trends are similar to those of interacontinental alkaline rocks. Low values (less than 1) of HFSE/LREE can represent the astenospheric mantle as the source of magma. The YbN<10, high values of (La/Yb)N (19.98-28.41) and high values of (Dy/Yb)N (1.39-1.68) may indicate the presence of garnet as residual phase in the mantle source of the magma that generated the Kamarbon gabbroic alkaline rocks. In gabbro classification diagrams, the studied rocks are located in the field of interacontinental rift gabbros. The development of this gabbroic intrusion can be related to the plutonism of interacontinental rift that had been activated in late Triassic time in Central Alborz.

The Arghash gold prospect occurs in the northeastern of Central Iranian domain. It is located in Sabzevar zone, north of Darouneh fault. The exposed rocks in the study area consist of volcanic rocks (andesite and dacite), plutonic rocks mostly diorite, quartz diorite, quartz monzodiorite, granodiorite and granite with minor sedimentary rocks (limestone, sandstone and conglomerate). We focus on a suite of hbl-bearing granitoid rocks consisting of diorite, quartz diorite, quartz monzodiorite and granodiorite. The rocks are metaluminous, medium K, calc-alkaline belonging to Magnetite-series granitoids (I-type). They are related to small numbers of iron bodies and veins. They show low TiO2, P2O5, Nb, La, Zr. Initial 87Sr/86Sr ratios and initial eNd are 0.703755 and+4.74, respectively. U/Pb zircon dating of this suite, using laser inductively coupled plasma mass spectrometry (LA-ICP-MS) yielded mean age of 92.8±0.9 Ma (Turonian). Their chemical compositions as well as initial isotope ratios of 87Sr/87Sr strongly suggest that the original magma formed in a depleted mantle in the subduction zone in Sabzevar Paleoocean.