The evaluation of physico-chemical parameters of the Nasrand Plutonic complex by using mineral composition
Author(s):
Abstract:
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)
Language:
Persian
Published:
Journal of Economic Geology, Volume:8 Issue: 2, 2017
Pages:
307 to 323
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