Clinopyroxene chemistry based Petrogenesis of Triassic interlayer magmatism in the north of Shahreza (South of Isfahan, Sanandaj–Sirjan zone)

The Shahzade-Ali Akbar area located at 68 Km south of Isfahan is located in the southern zone of the Cimmerian Sanandaj-Sirjan block as a part of the northern shelf of the Neo-Tethyan Ocean (Stampli and Borel, 2002). This area lies in the Shahreza-Abadeh-Hambast belt and is well-known for its classic Permian-Triassic outcrops. The Lower Triassic carbonate rocks have hosted several interlayer igneous horizons. The composition of these rocks varies from olivine basalt to quartz basalt and their hypabyssal equivalents. Plagioclase (labradorite), clinopyroxene (augite), olivine, amphibole and quartz are major and ilmenite and titanomagnetite are minor minerals. The main objective of this research study is to investigate the geological, geochemical and petrogenesis of igneous rocks using mineral chemistry.

More than 50 samples representing whole units were selected in order to identify the geological setting of the igneous horizon, and their thin sections were prepared. Minerals and textures of rocks were studied by using polarizing microscope (Olympus BH-2). Then, 6 samples were selected for mineral chemistry and their mineral compositions were determined by electron microprobe at the Naruto University, Japan. The EPMA (Jeol- JXA-8800R) was used at operating conditions of 15 kV, 20 nA. Minpet software and spread sheet have been used for mineral chemistry studies and mineral formula calculations.

Based on the field observations, igneous units that are 120 centimeters to 10 meters thick have basaltic composition and are interlayered with Lower Triassic carbonate rocks. Microscopic study showed that these rocks are composed of plagioclase, clinopyroxene, olivine, amphibole, quartz and opaque minerals (ilmenite and titanomagnetite) and have porphyritic, ophitic, intersertal to intergranular textures. These rocks have undergone alterations and secondary minerals are widespread. EPMA analyses show andesine to labradorite composition, clinopyroxene (augite) and amphibole (edenite). In the Q-J diagram (Morimoto et al., 1988), all clinopyroxenes are located in the Mg-Fe-Ca (Quad) field. In the Wo-En-Fs diagram (Beccaluva et al., 1989), clinopyroxens show augitic with lessor amounts of diopside composition.  According to clinopyroxene chemistry diagrams such as Si O2 vs. Al2O3 and Ti vs. Al (Le Bas, 1962), the samples belong to sub-alkaline series. Discriminate diagrams such as Ti O2 vs. Al2O3 (Le Bas, 1962), Ti vs. Ca+Na and Ti vs. Al (Leterrier et al., 198a2) are used for identification of magma affinity. These diagrams show that the studied rocks are tholeiitic. The rocks under study demonstrate the MORB feature on tectonic discrimination diagrams (TiO2-SiO2/100-Na2O, Beccaluva et al., 1989) In the 2Ti+Cr+AlVI vs. Na+AlIV diagram (Morimoto et al., 1988) all clinopyroxenes are located below the Fe3+=0 line that indicates low oxygen fugacity during crystallization (Schweitzer et al., 1979). In Helz (1973) diagrams, the pressure and percentage of magma water estimated to be 2 to 10 Kbar pressure and about 10% water content. In YPT vs. XPT diagrams (Soesoo, 1997) the temperatures and the pressure of clinopyroxene crystallization are about 1150-1200 ◦C and 2-10 Kbar respectively.

The studied area had been a part of the Cimmeride microcontinent (Horacek et al., 2007) which had begun separating from the northern margin of Gondwana during Triassic time (Şengör, 1984), and traversed north to the southern Eurasian border (Stampli and Borel, 2002).In this area, several interlayer igneous rocks with basaltic composition are seen with Lower Triassic carbonate rocks. Based on the chemical composition of pyroxenes, the magma has sub-alkaline and tholeiitic affinity. The crystallization of ilmenite-titanomgenetite and diagram of clinopyroxenes crystallization conditions illustrate the low level of oxygen fugacity in the formation of the rocks under discussion. The pressure of magma crystallization is estimated to be between 2 and 10 kb, and the magmatic water content is about 10%. The studied rocks show MORB characteristics. Interlayering with lower Triassic sediments, sub-alkaline nature of the magma, low level oxygen fugacity during crystallization and geotectonic environment, suggest that the rocks have been formed in the early stages of the opening of the oceanic crust. A process that has led to the formation of the Neo-Tethys Ocean in the later stages.