جستجوی مقالات مرتبط با کلیدواژه « metamorphic » در نشریات گروه « زمین شناسی »

The regional and  contact metamorphic rocks of the studied area are located in the west of Hamedan city and Alvand plutonic body as a part of the northern part of the Sanandaj-Sirjan belt. This metamorphic complex consists of slate, garnet mica schist, amphibole schist and tremolitite in regional metamorphic rocks and spotted schists, mica hornfels, epidote amphibole hornfels and cordierite hornfels in contact metamorphic rocks. The analyzed minerals in these rocks include biotite, garnet, staurolite, amphibole and cordierite. EMP chemical analyses indicate that the composition of garnet is almandine, the composition of biotite is siderophyllite and amphibole is actinolite. Staurolite has iron-rich composition and cordierite has intermediate composition (iron-magnesian). Thermometric calculations based on biotite in garnet staurolite schists show a temperature of about 555°C, and the temperature of about 511°C was determined by the garnet-biotite pair method. Due to the presence of staurolite and the absence of kyanite and other medium and high pressure minerals, the pressure of 3 to 4 kilobars is possible. The type of metamorphism of the study area is suggested to be related to a  subduction tectonic environment. The stability of the garnet + staurolite assemblage requires the presence of water in the environment.

The Baghi-Abad skarn deposit is located south of Taft, Central Iran. Shirkuh granitoid batholite, marble and skarns are the most unit rocks in this area. The outstanding metamorphic zones are pyroxene-plagioclase skarn, melilite-fasaiite zone and finally spinel-clinopyroxene zone. The last zone of metamorphic halo is brucite marble. After this zone, marble limestone and then limestone of Taft Formation can be seen. The metamorphism and metasomatism caused by intrusion of the Shirkuh granitoid into the cretaceous carbonate rocks caused formation of calcite, wollastonite, clinopyroxene, garnet, vesuvianite, epidote, quartz and titanite. The main texture of these rocks is granoblastic, replacement and reaction, and poikiloblastic. Investigations confirm that the Baghi-Abad skarn is polygenetic in origin and evolved in two major stages, metamorphism and metasomatism (progressive and retrogressive). Prograde metamorphism characterized by anhydrous calc-silicates minerals such as wollastonite, diopside, pyroxene and garnets. The retrograde stage is revealed by the alteration of the calc-silicate minerals, the first skarn (clinopyroxene-garnet) and were formed epidote, vesuvianite. The Baghi-Abad skarn is a calcic- dolomitic and polygenic skarn and based on chemistry of clinopyroxene and garnet is comparable with Sn-W-Zn world mineralized skarns.

The Sartip Abad metamorphic complex in Qorveh (Kurdistan Province) is located in the Sanandaj-Sirjan zone. The complex is composed of regional metamorphic rocks including contact metamorphic rocks (hornfels), variety of schists (amphibole schist, actinolite schist, mica schist, and garnet schist), marble, metadiorite, metagabbro, and paragneiss. In schists and paragneiss, biotite is siderophilite and in hornfels and metadiorite is anite to phlogopite in composition, respectively. Paragneiss and metadiorites include hornblende and actinolite. The garnets, in the garnet-mica schist, are almandine type, and plagioclase crystals in the metadiorites and metagabbros are labradorite type in composition, as well as their paragneiss have andesine- type plagioclase. Petrography and thermobarometric studies indicate that the different types of regional metamorphic rocks are formed in green schist to epidote-amphibolite facies and in the interval between biotite and staurolite zones. But for contact metamorphic rocks, the maximum facies is assumed albite-epidote hornfels in isograd of zoisite to amphibole. Thermometry calculations, using biotite single and garnet-biotite exchange, show that the average temperature for the metamorphic rocks of this region is about 567°C and for the contact metamorphic rocks is about 647 °C. Also, we used thermo-barometer methods for M1 and M2 metamorphic stages. Data are yielded, M1 stage temperature and pressure are about 578°C and 5.16 kb and for M2 stage ~565 °C and 4.6 kbar. Based on field relationships, petrographic evidence and geochemical data, protoliths are sedimentary rocks including greywackes/arkoses and shales for paragneiss, hornfels and schists, respectively.

The area of this study is located near Hamadan within the Sanandaj - Sirjan tectonic zone. In the Hamadan area, consisting mainly of Mesozoic plutonic and metamorphic rocks, aplites and pegmatites locally contain garnets.(Baharifar et al., 2004, Amidi and Majidi, 1977; Torkian, 1995. Garnet-bearing schists and hornfelses in the area are products of regional metamorphism shown by slate and phyllite (Baharifar, 2004). In this investigation the distribution of elements in garnet in different rock type was studied to determine their mineral types and conditions of formation. Garnet samples from igneous and metamorphic rocks were analyzed by electron microprobe (EMPA), the results of which are presented in this article. Thirty-five samples were selected for thin section preparation and twenty thin-polished sections were prepared for mineralogical and microprobe analysis. Thin sections of garnet-bearing igneous (pegmatite) and metamorphic rocks (schist and hornfels) were studied by polarizing microscope. Chemical analysis was performed on the garnets (38 points) using a Caimeca SX100 electron microprobe at an acceleration voltage of 15 kV and electric current of 15 nA in the Mineral Processing Research Center, Iran. Separation of iron (II) and Fe (III) was calculated by Droop’s method (1987) and the structural formulas of the garnets were calculated using 24 oxygens to determine the relative proportions of the end-members using the mineral spreadsheet software of Preston and Still (2001). Based on the analyses, almandine (Fe - Al garnet) and spessartine (Mn - Al garnet) are the principal types of the (Kamari) metamorphic and (Abaro) pegmatitic garnets, that belong to the well-known pyralspite garnet group. Chemical zoning patterns of the garnets in the metamorphic rocks (schists) differ from those in the igneous rocks (pegmatite), showing different compositions from core to rim. Petrographic evidence such as: co-existing tourmaline with pegmatite garnets and andalusite with schist garnets; zoning in garnets (oscillatory zoning of Al in pegmatite garnet, Mn increasing in the cores of schist garnet contrasted with Mn decreasing in the cores of pegmatite garnets; the decrease of Mg in the cores of pegmatite garnets, contrasted with the increase of this element in the cores of schist garnets; and the linear trends of Al and Ca in hornfels garnets) Pyrope garnet composition in schist indicates a closed system for garnet formation condition in schist and a magmatic source for pegmatites. The compositions of garnets from schists change from Alm0.63, Prp0.07, Sps0.24, Grs0.05 in the cores, to Alm0.71, Prp0.09, Sps0.13, Grs0.05 in the rims. Garnets from pegmatites show a change from Alm0.73, Prp0.015, Sps0.24, Grs0.07 in the cores, to Alm0.71, Prp0.011, Sps0.28, Grs0.00 in the rims. Garnets from hornfelses showed changes from Alm 0.79, Prp0.14, Sps 0.06, Grs0.07 in the cores to Alm 0.8, Prp0.13, Sps 0.05, Grs0.01 in the rims. The percent of almandine and spessartine in the garnets of the schists and pegmatites are higher than that of garnets in the hornfelses. Almandine and spessartine in the pegmatite garnets from core to rim show a completely reversed trend. In the schist garnets from core to rim, the almandine trend is decreasing outward– increasing inward, while the spessartine trend is increasing – decreasing. In the hornfels garnets no specific trend could be determined, there is no zoning. This difference in trend between pegmatite garnets from that in schist garnets and hornfels garnets shows differences in their origin. Texture homogenization, rich in potassium, metaluminous to peraluminous magma of Hamedan granitoid intrusion (Aliani et al., 2012), Peraluminous biotite in this intrusion and lack of garnet zoning show that garnet pegmatites have been formed directly from granitic melt crystallization.

Surian volcano–sedimentary complex located on the eastern edge of the Sanandaj-Sirjan metamorphic zone, southwest Iran. Graphite schist is the most abundant lithological facies in this area. Due to the high grade of the graphite ore (up to 50%), the exploitation of the ore is economically valuable. Gangue minerals include quartz, muscovite, plagioclase, clinochlore, zircon and ilmenite. The occurrence of crystalline graphite with ordered structure is shown by the presence of a first order peak in Raman spectra (~1587 cm-1 Raman shift) and 3.35 ˚A d spacing. A linear relationship between metamorphic temperature and the Raman parameters R1 and R2 make graphite an accurate geothermometer. Using this geothermometer gives metamorphic Surian complex temperatures range from 234 to 258 ˚C, consistent with previous studies that give P-T conditions of green schist to lower amphibolite metamorphic facies.

Pyralespite garnet group (Almandine group) is common minerals in the metamorphic and igneous complex of Boroujerd area. Moreover, Almandin (Fe enriched garnet) is the most common mineral but the content of Spessartin (Mn enriched garnet) is high in this area. The changes of mineral composition in different grains show that these minerals are zoning chemical and core to rim composition is varied. The garnet composition in core to rim respectively in granodiorite changes to Alm0.65, Prp0.05, Sps0.264, Grs0.193 in core to Alm0.76, Prp0.05, Sps0.14, Grs0.04 in rim, in pegmatite, Alm0.71, Prp0.02, Sps0.26, Grs0 in core to Alm0.76, Prp0.03, Sps0.2, Grs0.007 in rim of pegmatite, and in hornfels, Alm 0.81, Prp0.07, Sps 0.09, Grs0.03 in core to Alm 0.63, Prp0.06, Sps 0.3, Grs0.002 in rim. Petrography (for exampl, crystallization of tourmaline to garnet pegmatites and Andalusite to granodiorites) and mineral chemistry (oscillatory zoning Al in pegmatite garnet, Mg decrease and Mn increase in granodiorites garnet and liner trend Al in garnet hornfelse) show that metamorphic source for garnet in granodiorite and hornfels and magmatic source for pegmatites.

The study area is a part of so-called Sanandaj-Sirjan zone or Zagros imbricate zone of the Zagros Orogen, Iran. The Hamedan region comprises a metamorphic sequence of low- to high-grade regional and contact metamorphic rocks intruded by mafic, intermediate and felsic plutonic bodies. The sequence comprises pelitic, psammitic, mafic, calc-pelitic and calc-silicate rocks.  Pelitic rocks are the most abundant rocks in the region which are mostly composed of slates, phyllites, mica schists, garnet-mica schists, garnet-andalusite-(±sillimanite/±kyanite) schists, garnet-staurolite schists, mica hornfelses, garnet hornfelses, garnet-andalusite-(± fibrolite) hornfelses, cordierite-(±andalusite) hornfelses, cordierite-K-feldspar hornfelses and sillimanite-K-feldspar hornfelses. Several interlayers of amphibolites and amphibole schists alternate with pelitic rocks in the sequence. Various types of Al2SiO5-bearing silicic veins occur in the region.  Isotope characteristics indicate metamorphic origin for some of them.  In this study, we have defined three different generations of kyanite according to their various geological characteristics, including: (1) metamorphic (randomly distributed and commonly co-existed with sillimanite/andalusite), (2) stress-(shear)-induced (pseudomorphs of kyanite after andalusite in the shear zones) and (3) late-stage (hydrothermal or metasomatic) kyanites.  The quartz-kyanite veins/pods are late structures that are created by interactions with wall rocks. Circulating Al- and Si-rich fluids may be responsible for formation of these veins/pods.