ABSTRACT

In this paper, tectonic and geothermobarometric environments have been studied with respect to the combination of pyroxene, olivine and plagioclase in volcanic rocks in the southeast of Bam. The combination of volcanic rocks in the region consists of olivine basalt, basalt, alkaline basalt, andesite, trachyandesite and pyroxene andesite. This combination is the result of the processes of crystallization and sometimes contamination. Plagioclase, clinopyroxene, olivine, and amphibole constitute the major minerals (rock forming minerals) in these rocks. Porphyritic to mega-porphyritic textures with microlithic, glumero-porphyritic and amigdaluidal matrix are observed. Based on the thermometric calculations, plagioclase, pyroxene, and olivine minerals and the rocks of this region are crystallized at a pressure of 1.5 to 7 kb and temperatures ranging from about 700˚C to 1250˚C.

Keywords:

Plagioclase, Pyroxene, Geochemistry, Thermo-Barometry, Volcanic Rocks, Bam

1. Introduction

The investigated area in 120 km southeast of Bam covering a region of approximately 800 km² is situated between 28˚03'E - 28˚24'E and 58˚49'N - 59˚00'N. The study area is, structurally, located in the southern sector of the Dehaj-Sarduiehmetallogenic belt as the southeastern part of the Urmieh-Dokhtar volcanic zone (Figure 1). Two types of tectono-magmatic models are proposed for the Urmieh-Dokhtarvolcanic zone, namely, subduction of the neo-tethys oceanic crust beneath the central Iranian continental crust from cretaceous to tertiary [1] [2] [3] [4] and/or intermittent opening and closing of intra-continental rifts [5] [6] . Sampling and microscopic studies on the volcanic rocks were carried out after the field studies, in this research. After that and given the composition of the volcanic rocks, minerals including plagioclase, clinopyroxene and olivine were used in determination of the origin, pressure and temperature of magma.

2. Study Method

About 150 microscopic sections of rock units were studied using polarizing microscope to identify the texture of the rocks, and their mineralogical composition. In order to conduct thermobarometric studies on the studied basaltic andesites, 5 thin-polished sections were selected and analyzed at the mineralogy division of the Iranian Mineral Processing Research Centre (IMPRC) using a Cameca SX-100 electron microprobe through which 90 point analysis were carried out on olivine, pyroxene and plagioclase minerals. The results obtained were used in a software called Excell software Spread sheet to determine temperature, pressure and also the type of minerals (Tables 1-4).

3. Geology of the Region

The area studied is located east of 1:100,000 Nagisan sheet and southeast of 1:250,000 Jahan Abad sheet, 120 km SE of Bam. Eocene alternative sequences of volcanic rocks (andesite, dacite) and volcanoclastics (tuff and ignimbrites) form

the oldest geological units in the area (Figure 2(a)). Intrusion of a multistage Batolithic mass with the combination of diorite, tonalite and granite is the result of the Miocene magmatic activity. From Oligocene to Pliocene, various sediments consist of conglomerate, silty and marl sandstones with gypsum and lava deposits in bright gray to black color are formed. In the southeastern part of the Nagisan region, quaternary magmatic phases, especially the Pliocene-Pleistocene, are volcanic blocks that discontinuously cover all conglomerate deposits or Neogene time andesites and subordinate units. Quaternary deposits are mostly of olivine-bearing basalts with associations of pyroclastic rocks. The lava flows of these rocks have distinct compression blades. One of the characteristics of the quaternary lava is the relative preservation of the cone and its volcanic crater (Figures 2(b)-(d)).

4. Petrography

In terms of petrography, the volcanic rocks of the region have an olivine basalt, basalt, alkaline basalt, trachybasalt, andesite, trachyandesite and pyroxene andesite mineralogy.

4.1. Olivine Basalts, Basalts

These rocks show porphyritic textures with microlithic to vitreous matrix, however, glumero-porphyritic, amigdaluidal and afitic textures are sometimes observed (Figure 3(b)). The rock-forming minerals are plagioclase, clinopyroxene and olivine, which are hosted by a matrix of plagioclase, clinopyroxene (augite),

olivine, and opaque minerals or glass. Opaque minerals are of accessory minerals while iddingsite (or bulangite), epidote, serpentine and chlorite form the main secondary minerals. Matrix is formed from glass, plagioclase microlites, opaque minerals and secondary minerals.

4.2. Alkali Olivine-Basalt

These rocks are dark green to black in color (Figure 3(a)). microlitic-porphyric and porphyritic-amigdaluidal are dominant textures in these rocks. Ca-plagioclase (labradorite-bitonite) forms course phenocrysts. Plagioclase founds also as fine grain microlites within the vitreous matrix. Olivine and, to a lesser extent, clinopyroxene are among the other rock components found either as phenocrysts or microlite. Chlorite, calcite, iddingsite, serpentine and iron oxides, as the result of alteration of various minerals, are also found within these rocks. Opaque minerals are among the accessory minerals. Cavities are often filled with calcite.

4.3. Trachybasalts

These rocks have generally porphyritic texture with microlithic to vitreous matrix and in some parts show glomero-porphyritic to trachytic texture. Phenocrysts include plagioclase, clinopyroxene and olivine. Matrix is made of glass, plagioclase, clinopyroxene, olivine and opaque minerals (magnetite and titanomagnite). Chlorite, iddingsite and iron oxides are also found in these rocks. opaque minerals are among the accessory minerals in these rocks.

4.4. Andesites

The andesitic rocks of the area include basaltic andesite, andesite, hornblende- pyroxene andesite, pyroxene andesite and trachy-andesite. These rocks represent a variety of porphyrytic, flow, glomeru-porphyrytic, and sieve textures.The rock forming minerals of these rocks are plagioclase and one or more mafic minerals such as hornblende and pyroxene (Figure 3(c)). Plagioclase is often crystallized in the form of zoned phenocrysts, oligoclase-andesinein composition. In all of these rocks, plagioclase has been decomposed into sericite, chlorite, and clay minerals. Secondary quartz crystals are also seen as fine grains in the matrix and sometimes as phenocrysts. The secondary minerals in the andesitic rocks of the area are mostly magnetite, titanumgentite (opaque minerals) and apatite. Trachy- andesites exhibit porphyritic texture with microlithic and trachytic and in some cases mega-porphyritic matrix. The mineralogy of these rocks contains plagioclase crystals and one or more mafic minerals such as hornblende and pyroxene. One of the most prominent features of these rocks is bay-like corrosion at the margin of plagioclase and clinopyroxene, thieve texture in coarse and fine grain plagioclase, and also longitudinal and transversal sections of auto-morph hornblende with burned margins. There are inclusions of opaque and apatite minerals in these crystals. In some samples, limited occurrences of olivine are also observed (Figure 3(d)).

5. Determination of Plagioclase, Clinipiroxone and Olivine Minerals Based on Chemical Composition

a) Plagioclase

On Or-Ab-An diagram [8] , plagioclase minerals in the igneous rocks of the region are within the boundaries of labradorite and bitonite (Figure 4(a)). Chemical composition of Coarse plagioclase crystals differs from An_61.98 Ab_37.83Or_0.19 to An_79.85 Ab_19.83Or_0.32, in trachy andesites, from An_65.22 Ab_34.24Or_0.51 to An_68.28 Ab_31.22Or_0.5 in pyroxene andesites, An_80.81 Ab₁₉Or_0.19 in basalts, and from An_63.66 Ab_35.34Or_1.00 to An_76.95 Ab_22.45Or_0.6 in olivine basalts (Table 1).

b) Pyroxene

According to the nomenclature [9] pyroxenes fall mainly in the clino-enstatite and augite fields (Figure 4(b)). Chemical composition of Coarse pyroxene crystals differs from En_41.4 Fs_11.4 Wo₄₆ to En_54.2 Fs₂₆ Wo₁₉ in trachy andesites, from En_54.6 Fs_24.8 Wo_2.0 to En_58.4 Fs_24.8 Wo_14.4 in pyroxene andesites, En_51.8 Fs_8.5 Wo_38.8 in basalts, and from En_52.94 Fs₁₂ Wo_34.5 to En_77.6 Fs_18.3 Wo_3.8 in olivine basalts (Table 1).

c) Olivine

Olivines, on different discrimination diagrams, [10] [11] for example, lie within the field of hyalosiderite and crysolite rather than fyaliteindicating low Fe contents (Figure 4(c) and Figure 4(d)). Primary olivine minerals certainly have a higher magnesium content but the edingzitation and oxidation of these minerals has led to the stabilization of Fe in place of Mg and the placement of Olivine’s to the Faylite pole.

6. Thermobarometry

In this section, using the total analysis of the rock and analyzing the plagioclase, pyroxene, and olivine samples, temperature and pressure using the excel spreadsheet used by several researchers are estimated for rocks in the region. In these tables, temperature (TC) and pressure (kbr) are approximately estimated and Eqn refers to the equations used in the spreadsheets. The results of thermobarometry of clinopyroxene in basalt and andesite rocks, the temperature is estimated to be about 967.20˚C to 1143.20˚C and the pressure is estimated about 0.28 kb (Table 2) and based on the results thermobarometry of plagioclase,the temperature and pressure is estimated to be about 1162˚C and 31.98 kb in andesite, but in basalt, temperature and pressure is 1244˚C and 27 kb (Table 3). The results of thermobarometry of olivines also indicate a temperature of about 1089˚C to 1270˚C.

In the following, various models have been used to measure the temperature of the rock in the area. Using the Or-Ab-An ternary system, the temperature was estimated to be around 650˚C to 750˚C for different rocks (Figure 5(a)). Clinopyroxene thermometer was also used for rock in the region. Based on this thermometer, the formation temperature estimated was about 1100˚C to 1250˚C (Figure 5(b)).

7. Conclusion

The Quaternary volcanic rocks of the area include olivine basalt, basalt, alkali basalt, hialopyroxene andesite, pyroxene andesite and trachy andesite. The main minerals of these rocks are plagioclase, olivine and pyroxene. According to the chemistry of these minerals, the composition of the plagioclase minerals in these igneous rocks lies within the field of Andesin and Labradorite. The composition of clinopyroxenes is of augite type and some of pyroxenes are Ca-poor minerals. Similarly, the combination of olivine is of chrysolite-hyalociderite type. According to thermobarometry calculations of plagioclase, pyroxene and olivine

minerals, the rocks of the region have been crystallized in a temperature ranging from 650˚C to 1250˚C and a pressure between 1.5 to 7 kb.

Cite this paper

Yazdi, A., Ashja-Ardalan, A., Emami, M.-H., Dabiri, R. and Foudazi, M. (2017) Chemistry of Minerals and Geothermobarometry of Volcanic Rocks in the Region Located in Southeast of Bam, Kerman Province. Open Journal of Geology, 7, 1644-1653. https://doi.org/10.4236/ojg.2017.711110

References