Mineral Chemistry and Thermobarometry of the Volcanic Rocks in Torud , Iran

This paper elucidates the compositional studies on clinopyroxene, plagioclase of basalts to andesitic rocks of Torud area to understand the geotectonic and geothermobarometry conditions. Early Eocene-Oligocene calc-alkaline volcanic rocks are exposed around Torud in the Central Iranian zone. Volcanic rocks consist of basaltic, andesite basalt, Tracyandesite, and andesite. Minerals in the volcanic rocks exhibit degrees of disequilibrium features. Plagioclase as dominant mineral in these rocks generally displays oscillatory zoning. Mineral chemistry studies show that clinopyroxenes in the volcanic rocks are diopside, augite and plotted in medium pressure field. The clinopyroxene composition yields the crystallization temperatures 900 ̊C 1000 ̊C. The mineral composition indicates that these rocks are formed in a tensional environment.


Introduction
Central Iran is one of the main and most complex geological zones of Iran (Figure 1).The oldest metamorphism (Precambrian) and the youngest semi-active volcanism occurred in this zone.In fact, this region is the oldest micro-continent plate in Iran which has suffered various geological processes.One of the most important and interesting geological events in Iran is the occurrence of orogenic movements comparable with the Katangan in Figure 1.Geographical location of the study area of (1:250,000) Troud map (Houshmandzadeh, Alavi, Right Pvr, 1357) [14].
Gondwanaland and Baikalian in the Eurasian continent.In Central East Iran the Microplate was more or less connected to the southern rim of Eurasia during the Paleozoic and early Mesozoic.A significant separation from Eurasia took place during the Jurassic (Soffel and Forster, 1984) [1].As a result of Neo-Tethys subduction beneath the Central Iran Microplate (CIM) and following the collision of Iranian and Afro-Arabian plates, various structural zones developed in Iran (Alavi, 2004, Nadimi, 2007, Sengör and Yilmaz, 1981 and Ahmadzadeh, Jahangiri, Lentz and Mojtahedi, 2010) [2]- [5].However, peak of this subduction-related calc-alkaline magmatic activity is thought to have been in the Eocene age (Stocklin, 1973, Farhoudi, 1978, Emami, 1981, Jahangiri, 2007, Berberian and King, 1981

Geology
Volcanic rocks and volcanic-sedimentary rocks of Middle to Upper Eocene in Torud area are composed of lava alternate with basic-intermediate pyroclastic rocks and they are deposited with the sedimentary layers (siltstone, nummulities bearing limestone) in a shallow marine environment.Combined spectrum of volcanic rocks consisting of basalt, andesite, trachyandesite, andesite and dacite were obtained by fractional crystallization and eventually contamination process, and clinopyroxene, amphibole and plagioclase, are the major minerals in the rocks.Porphyritic to mga-Porphyritic textures microlithic groundmass are the nature of these rocks.These features are along with its alkaline nature, and represent rocks formed in a back-arc tectonic position of the Middle Eocene-upper area.

Analytical Methods
In order to achieve the aims of this work, at the first field surveying and sampling was done, then thin and 38 thin polished sections were prepared.About the rocks of area, 5 rocks include andesitic basalt, basaltic 39 andesite and andesite were selected for microprobe analysis and their olivine, clinopyroxene and 40 plagioclase minerals by using XGT-7200 micro-XRF analyzer-Horiba with voltage of 50 kv and a 41 current of 3 mA in Kansaran Binaloud Laboratory were analyzed.A total of 80 points was analyzed 42 which results of this analysis are given in Tables 1-3.

Plagioclases
Plagioclase (PLAG) is largely unaltered in rocks from this site and shows a distinct difference in habit related to the type of occurrence and the magmatic affinity of the host lava.It occurs either as phenocrysts or microlites.Plagioclase compositions determined by microprobe are listed in Table 1.The overall and range is 97% -30%, so the majority of plagioclase crystals are oligoclase, andesine, labradorite, by townite and anorthite in composition between basalt and esite to andesite composition.PLAG plotting in the ternary classification diagram Ab-An-Or (Figure 2) shows that there may be some overlap between phenocrysts of each magma compositions.
The petrographic evidence that the PLAG was the liquidus phase is confirmed by the occurrence of individuals relatively rich in anorthite component (up to 95%).The most An-rich plagioclase phenocrysts occur in the Basalt rocks, while the most An-poor crystals are from the Andesitic rocks (Figure 2).Of albite, anorthite and alkali feldspar component is determined by the file Spreadsheet (Table 2).
The textural and compositional disequilibrium evidences in plagioclase are important.Sieve texture and zonation of plagioclase is noticeable in these volcanic rocks (Figure 3).This may be interpreted as resulting from either partial dissolution during the magma mixing process (Feeley and Dungan, 1996) [15], or a decompression effect (Nelson and Montana, 1992) [16].
Experimental and petrological studies have demonstrated that partitioning of minor elements (e.g., Ti and Al) in pyroxene is strongly growth-rate dependent, although equilibrium partitioning of Ca, Mg and Fe may not be seriously affected by cooling rates (Gamble and Taylor 1980) [23] and hence increase in Al/Ti ratios even in liquids fractionating plagioclase (Table 2 and Table 3).On the pyroxene classification of (Morimoto, 1988) [17] Figure 2. Ternary classification diagram of Ab-An-Or for felds pars (Deer, et al., 1997) [25].the pyroxenes plot in the augite field approaching diopside compositions, similar to pyroxenes from other orogenic volcanic rocks (Ewart, 1979) [24] (Figure 4).

Evaluation of Oxygen Fugacity
Oxygen fugacity plays an important role in changing of the liquidus temperature, melt and crystals composition The Fe 3+ in pyroxenes can be displaced 3-valence elements such as Al VI , Ti and Cr in the octahedral position.In the other hand, Fe 3+ in pyroxenes depends on the amount of Al VI which means it depends on the aluminum balance in tetrahedral and octahedral position.Figure 5 shows that the pyroxenes which crystallized at high oxygen fugacity, has been situated above the line of Fe 3+ .Furthermore, (Papike and Cameron, 1976) [30] have mentioned the distances of the samples from the Fe 3+ line and noted that further distances of the samples from this line were indicating more oxygen frugalities in their geological setting.In this diagram some samples are located above the line of Fe 3+ line and some are below that (Figure 5).

Tectonic Setting
Pyroxene composition depends on the chemical composition and tectonic setting of the host lava which can be used widely to determine geological setting of the rocks (Schweitzer, et al., 1979) [32], (Kushiro, 1960) [33] (Hout, et al., 2002) [34] (Figure 6).Ti + Cr vs. Ca and TiO 2 vs. Na 2 O + CaO diagrams define the pyroxenes which generated by tholeiitic and calc-alkaline magmas from those of alkaline.Plotting data on Ti + Cr vs. Ca and TiO 2 vs. Na 2 O + CaO diagrams show that the samples are subalkaline to alkaline tendency (Leterrier, et al., 1982) [35].Diagrams (Figure 6) indicate the extensional setting of the area when the rocks are formed.

Barometers
The ratios Al VI /Al IV , Ti + Al IV /Si and TiO 2 /(Mg + Mg + Fe) in pyroxene could be used as a pressure gauge.The diagram Al VI /Al IV samples are in the range of moderate pressures (Figure 7(a)).Also, the structure of clinopyroxene, chrome is in equilibrium with AlVi, The ratio of Cr*100/Cr + Al VI pressure is directly related to the pyroxene (Nimis and Taylor, 2000) [36].(Green and Ringwood, 1967) [37].The first reaction in the depth of about 120 km (containing garnet peridotite), and the second reaction occurs at depths less than 40 km.To determine the depth of the magma reservoir of aluminum in the pyroxene structure is used.Researchers, including (Helz, 1983) [38] have emphasized that the distribution of aluminum in the octahedral positions of tetragonal and clinopyroxene suitable criteria for estimating the water content of the magma and the pressure of the environment is the formation of igneous rocks.With this model, the pyroxene crystallized at a pressure of 5 kbar and water content of the magma is less than 10% (Figure 7(b)).

Conclusions
• The composition of feldspars and clinopyroxenes determined by XPMA in Torud intrusive bodies is consistent with the results obtained from determination of extinction angles in microscopic thin-sections.• types and P-T conditions of magma during crystallization can be calculated using Excel programs.• The composition of clinopyroxenes, magmatic series and tectonic environments can be determined using XPMA data.• The compositions of clinopyroxenes are predominantly augite and diopside according to the classification of Morimoto in 1989.• According to the distribution of aluminum in clinopyroxenes, these minerals have been formed at <5 Kb pressure and water content between 5 to 10 percent.• The mineral compositions of the rocks display an alkaline nature which indicates a tensional setting.

Figure 3 .
Figure 3. Photomicrographs of thin sections of lavas: (a) pyroxen phenocrysts in a andesite (XPL); (b) plagioclase phenocrysts in a basaltic andesite (XPL); (c) clinopyrpxene with microlitic texture in andesite basalt (XPL); (d) amphibole crystals extracted from a cavity filled by calcite in the andesite; (e) the mikrolite rounded olivine and plagioclase with clinopyroxene phenocrysts of the lavas tracks mikrolite andesitic basalt and porphyritic texture create Stream; and (f) plagioclase phenocrysts with reaction and sieve-textured, resorption-generated zone in mixed lavas (XPL).

Table 1 .
EPMA data of representative plagioclases from basalts to andesite composition of Torud area.

Table 2 .
Type of plagioclase using Spreadsheet.

Table 3 .
EPMA data of representative Clinopyroxene from basalts to andesite composition of Torud area.