the evaluation of physico-chemical parameters of the nasrand plutonic complex by using mineral composition

introduction mineral composition is sensitive to variations in the composition of the magma and can be used to characterize the physical conditions of crystallization such as temperature, pressure, oxygen fugacity and water content. the studies have demonstrated that geobarometery by amphibole provides a tool for determining the depth of crystallization and knowledge of the depth of crystallization of hornblende through to solidification of calc-alkaline plutons (anderson and smith, 1995). the composition of pyroxene can be used as crystallization pressure and temperature indicators of pyroxene too. anlytical methods the mineral compositions of the nasrand intrusion were determined by electron microprobe, with special emphasis on the amphibole, feldspar, and pyroxene at the naruto university, japan, the epma (jeol- jxa-8800r) was used at operating conditions of 15 kv, 20 na acceleration voltage and 20s counting time. results the nasrand intrusion (33°13'–33°15' n, 52°33'–52°34'e) with an outcrop area of about 40 km2 is situated in the urumieh–dokhtar magmatic belt, se of ardestan. it is composed of granite and granodiorite and various dikes of diorite and gabbro which are intruded in it. it is intruded into eocene volcanic rocks, including andesite, rhyolite, and dacite. the petrographical studies indicate that the granitic and granodioritic rocks contain major minerals such as quartz, k-feldspar, plagioclase, and amphibole, which are in an approximate equilibrium state. the gabbroic-dioritic dikes usually show microgranular porphyric texture. they mainly consist of plagioclase, amphibole, and pyroxene. the plagioclase shows variable composition from albite to oligoclase in the granitoid rocks and from oligoclase to bytownite in dioritic and gabbroic dikes (deer et al., 1991). the amphiboles are calcic and their composition varies from hornblende to actinolite, whereas the composition of the basic dikes is inclined to hastingsite (leake et al., 1997). actinolitic probably crystallized as a subsolidus phase. pyroxene in the dikes is clinopyroxene with augite- diopside composition (morimoto, 1988). discussion the total al content of hornblende is a sensitive linear function of crystallization pressure and temperature (schmidt 1992; holland and blundy, 1994). however, the computed pressure may reflect the level at which the hornblende crystallizes rather than the pressure at which the granite consolidates. therefore, al content in hornblende geobarometer is only applicable in the presence of quartz and plagioclase; alkali feldspars, biotite, hornblende, clearly limit compositional influences (ague, 1997). oxygen fugacity has a marked effect on the mineral system, so only hornblendes with fe/(fe+mg) < 0.65, si ≤7.5 and ca ≥1.6 were used for geobarometry and are not applicable to subsolidus actinolite (stein and dietl, 2001). the average formation pressure in the intrusive rocks is evaluated to be 1.54 kbar by schmidt (schmidt, 1992) and anderson and smith (anderson and smith, 1995) equations, which is consistent with a depth of 5.9 km, whereas the average pressure of amphibole crystallization in the dioritic dikes is calculated to be about 2.96 kbar by the ridolfi equation (ridolfi et al., 2010), indicating 11.4 km depth. the estimated pressure for clinopyroxene crystallization in the dikes is calculated to be about 4–8 kbar by the soesoo (soesoo, 1997) and putirka (putirka, 2008) equations which is reflecting the initial crystallization pressure of pyroxene from magma which corresponds to depths of about 15-30 km. the average formation temperature of the intrusive rocks and amphiboles in dioritic dikes is estimated to be 700 and 940 °c respectively, by the holland and blundy (holland and blundy, 1994), vyhnal et al. (vyhnal et al., 1991), and ridolfi et al. (ridolfi et al., 2010) equations. the highest temperatures from pyroxene thermometry in the dikes is about 1150 – 1250 °c by soesoo (soesoo, 1997) and putirka (putirka, 2008) equations which are assumed to reflect the actual temperature of initial pyroxene crystallization and are usually higher than temperatures obtained by hornblende-plagioclase thermometry. oxygen fugacity in the granitoid rocks and dioritic dikes is above the ni-nio buffer and it is indicated to be -12.9 and 10.5 bars, respectively, by the ridolfi et al. (ridolfi et al., 2010) equation. water contents in the granitoid rocks and dikes are calculated to be about 3.6 and 4.6 wt. % respectively, by the ridolfi et al. 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