abdolsamad pourmohammad

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).

Geysour area is located in the east of Gonabad city and is part of the north of the Lut Block. The rocks in this area include granitoid rocks and high temperature - low pressure metamorphic rocks. Granitoid rocks are composed of granodiorite, enclave (metamorphic and igneous) and microgranites. These rocks belong to medium to high potassium calc-alkaline series, low temperature I type granite and are poorly peraluminous. The Chondrite-normalized REE patterns show that the rocks are enriched incompatible elements, with negative anomalies in Nb, Ta, Sr, P, Ti and Ba and strong enrichment in Rb, K and Th. These patterns are in perfect harmony in granodiorite, microgranular enclave (MME) and microgranite specimens. This harmony also has in upper, middle continental crust and greywackes. Positive anomalies in Rb, Th, Sm and negative anomalies in Ba are prominent in the composition of the crust. Based on the integration of these patterns with the pattern of upper continental crust (UCC) elements and greywackes and adaptation to laboratory work, the Geysour granitoid originated from crustal materials and a little mantle component. The temperature of granitoid formation was estimated based on the Zircon saturation temperature of 748-790 C. Microgranular enclaves have rounded and oval shapes, mixed areole around them, fine grained texture, quartz and plagioclase eyes, bladed biotite, needled apatite, oxide phases in biotite, and the presence of a simple mixed - hyperbolic curve between MME and granodiorite. Field, petrography and geochemistry of the major and rare earth elements data suggest mixing/ mingling (partial melting) processes for the origin of enclaves and the rare earth elements patterns indicates the relationship between Geysour granitoid with the subduction system. Analysing its data, based on logarithmic ratios, show collision tectonic environment. Also, the tectonic-chemical distinctive diagrams suggest a syn-collision to post-collision tectonic type that is interpreted in connection with the collision of the Afghan Block with the Lut Block.