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

The chemical composition of biotite in mineralization associated with granitoids and copper porphyry deposits is sensitive to several chemical and physical factors. It is also related tomagmatic and hydrothermal activities including water concentration, halogen and metal deposits, oxidation-sulfidation equilibrium, volatility (in melt-fluid-vapor equilibrium), elemental distribution relationships, and temperature and pressure of economic deposits (Webster, 1997, 2004).

Detailed field studies have been done, and several thin sections and polished thin sections were studied by conventional petrographic methods. Thirty points of biotite grains were selected and analyzed by a CAMECA SX Five electron probe micro-analyzer with 15 kV accelerator voltage and 20 nA beam current (5 μm beam size) at the Institute of Geology and Geophysics in the Chinese Academy of Sciences (IGG-CAS). The results were processed using MICA + software (Yavuz, 2003a, 2003b).

The Geysour granitoid pluton (Lower Cretaceous) consists of granodiorite, mafic microgranular enclaves, and micro-granite sill. The granodioriticrocks are mainly composed of plagioclase, quartz, K-feldspar and biotite along with accessory minerals of zircon, apatite and magnetite. Mafic microgranular enclaves are composed of quartz diorite, granodiorite and biotite granite, with fine-grained to porphyry texture and large eyes of quartz and plagioclase assemblages. The microgranite has porphyry texture with a fine-grained groundmass. Its phenocrysts are plagioclase, quartz and biotite along with accessory minerals of allanite, needle like apatite, epidote and calcite.Biotite is the only ferromagnesian mineral in theGeysour granitoid which falls into the category of real trioctahedral mica. The biotites of granodiorite and enclave samples are in group I and group of ferrous biotites. The biotitesof microgranite samples are in group I and group of magnesium biotites (Tischendorf et al., 1997). In the 10*TiO2-(FeOtot+MnO)-MgO ternary diagram (Nachit et al., 2005) all the analyzed biotites fall into the field of reequilibrated primary biotite. The formation temperatures of biotites in granodiorite, enclave and microgranite are 653-732 oC, 631-724 oC and 689-732 oC, respectively (Luhar et al., 1984; Henry et al., 2005). The mean pressure values are about 4 Kbar ​​for granodiorite and enclave and 2 Kbar for microgranite (Uchida et al., 2007). Biotites of granodioriteand enclave biotites are located on top of the NNO buffer, which correspond to biotite compositions of magnetite series magmas, and biotites ofmicrogranite lie below the NNO buffer line and within the QFM buffer range. Biotite composition based discriminant diagrams cannot be used to determine the tectonic setting of the Geysour granitoids because they are low temperature I-type granites. The mean logarithmic ratios of fH2O to fHF and fHCl, and fHF to fHCl for the rocks studied are as follows: log(fH2O/fHF)fluid=4.56, log(fH2O/fHCl)fluid=4.47 and log(fHF/fHCl)fluid=-0.53. The first two values ​​are much larger than 1 indicating that the fluids are rich in water. Also, all biotites have high angles with linear trends of log(fHF/fHCl), log(fH2O/fHCl) and log(fH2O/fHF) indicating changes in fugacity conditions and halogen content of the fluid due to wall-rock reaction (Boomeri et al., 2009). Hydrothermal fluid fugacity ratio has been calculated for biotites of granodiorite, enclaves and microgranite samples at mean temperature of 661 oC, 654 oC and 703 o C, respectively, which indicate that hydrothermal fluids are of potassic type, because the log(fH2O/fHCl) is high, the log(fHF/fHCl) is slightly negative and the log(fH2O/fHF) is lower than that of phyllic alteration (Selby and Nesbitt, 2000). Meanwhile the magmatic fluid is significantly different from porphyry-type fluids (Baldwin and Pearce, 1982; Mason and Feiss, 1979; Selby and Nesbitt, 2000).

The Geysour area is located east of Gonabad and is a part of the northern Lut block. The rocks in this area include granitoid and metamorphic rocks. Based on field and mineralogical evidence, the protolith of amphibolite rocks are calc-silicate. The metamorphic rocks of this region are divided into metapelites and amphibolites. Textures, such as post-tectonic garnet and random amphiboles, are indicative of contact metamorphism in amphibolites. Garnets behave from ideal to non-ideal. The end members of the garnet are varied in these rocks and include Al-schorlomite, uvarovite, spessartine, pyrope, almandine, grossular, andradite, morimotoite, and majorite, but their profile patterns are flat. The amphibole is of the ferrohornblende type to the ferrochermakite and chermakite, and has zoning. Biotite has converted to chlorite. The presence of titanite, hematite and epidote are an indication of the effect of oxygen fugacity. Temperature and pressure were calculated in the range of 534-609 oC and 2-<4 Kbar, respectively.  Geysour amphibolites have progressed to the mid-amphibolite facies in the metamorphic aureole. Also, these rocks were undergone a retrograde metamorphism where the temperature and pressure were 489oC and 2.8 kbar, respectively, and the chloritizied biotites exhibit temperatures below 150 oC.

Geysour area, east of Gonabad, is a part of the northern Lut block. The rocks in this area include granitoid and a series of metamorphic rocks. In metapelitic rocks, the presence of S1 schistosity and the mineral chloritoid are indicative of regional metamorphism. The presence of random textural evidence in minerals such as andalusite, staurolite, muscovite and post-tectonic garnet and silimanite growth within the andalusite margin is characteristic of contact metamorphic rocks. The mineral garnet has two stages of growth, and mainly contains the end-members of Almandine, Grossular and spessartine. Chloritoid and staurolite are ferrous. Muscovite has little paragonite replacement. Chloritization of biotite and garnet, sericitization of andalusite and the occurrence of radial micro-cracks in garnet all indicate a retrograde metamorphism and uplift. Based on conventional geothermobarometric methods, temperature and pressure were calculated in the range of 550-608 oC and 3.8-4.5 Kbar, respectively. The existence of radial micro-cracks around quartz inclusions in garnet is due to a large component of isothermal decompression.