Application of Clinopyroxene Chemistry to Interpret the Physical Conditions of Ascending Magma, a Case Study of Eocene Volcanic Rocks in the Ghohrud Area (North of Isfahan)

Volcanic rocks with a porphyritic texture have experienced two crystallization stages. The first is slow, resulting in phenocrysts, and the second, which took place at, or near the surface, or during intrusion into a cooler body of rock, result in a groundmass of glass, or fine crystals. The pressure and temperature history of a magma during crystallization is recorded in the chemical composition of the phenocrysts during both stages. These phenocrysts provide valuable data about the physicochemical conditions of the parent magma during the process of crystallization. The composition of clinopyroxene (cpx) reflects not only the chemical condition and therefore the magmatic series, but also the physical conditions, i.e., temperature and pressure of a magma at the time when clinopyroxene crystallized.
The Ghohrud area lies in the middle part of the Urumieh-Dokhtar Magmatic Arc , which is part of a much larger magmatic province extending in a vast region of convergence between Arabia and Eurasia north of the Zagros-Bitlis suture zone (Dilek et al., 2010). In the Ghohrud area, north of Isfahan, exposed Eocene volcanic rocks belong to the first pulse of Cenozoic volcanism of Iran (Sayari, 2015), ranging in composition from andesitic basalt to basalt. The basaltic rocks of the Ghohrud area are composed mainly of plagioclase phenocrysts surrounded by smaller crystals of clinopyroxene in a groundmass of microlites, glass and opaques. In this study, the clinopyroxene and plagioclase of these rocks were analyzed in order to estimate the physicochemical conditions of the parent magmas.
Clinopyroxene and plagioclase phenocrysts of nineteen samples were analyzed with the electron microprobe. The chemical compositions of the clinopyroxenes were used to estimate both the chemical evolution and temperature and pressure conditions of the magmas during crystallization, using SCG, a specialized software for clinopyroxene thermobarometry (Sayari and Sharifi, 2014). Microprobe analyses show that plagioclases in the Eocene basaltic rocks are labradorite-bytownite (An85-58Ab15-41) and clinopyroxenes are augite (En41-49Di29-38Fs17-26). The compositions of the clinopyroxenes indicate a tholeiitic affinity for the magma. After plotting the cpx thermobarometry results on a P-T diagram, and applying a linear regression, an equation of P-T describing the physical conditions of the ascending magma was obtained.
Several complex thermobarometry equations used to estimate T and P of cpx have been introduced to the Society of Petrology by different researchers (e.g., Sayari, 2012; Sayari and Sharifi, 2014Putirka et al., 1996; Nimis and Ulmer, 1998; Putirka, 2008). Ten well-known barometric and six thermometric equations developed for clinopyroxene were tested for the analyzed samples with the aid of SCG, and then the equations giving the best match were selected and integrated to estimate contemporaneous P and T. According to the systematic cpx-thermobarometry calculations done with SCG software, it was inferred that the clinopyroxenes crystallized over a range of 1120-1170 °C and a range of pressure of 2-6 kbar.
The results of the cpx-thermobarometry were then plotted on a P-T diagram and a linear regression was used to find a function describing P and T for clinopyroxenes. The equation of the regression line is: P (kbar) = 0.0846T (°C)-93.128
The equation has a high coefficient of determination parameter (R2), making it reliable to determine the rate of T-loss against P-reduction. By assuming that the pressure on the magma was lithostatic due to the weight of overlying rocks, and considering the density of continental crust of about 2.7 gr/cm3, this equation shows that while magma was ascending while the clinopyroxenes were crystalizing, pressure was decreasing at a rate of about 84.6 bar per 1 °C temperature loss. This pressure loss indicates a rise of about 320 m in the continental crust.