| چکیده انگلیسی مقاله |
Introduction The Sanandaj-Sirjan zone, one of the most critical areas for studying metamorphic events, originated by subduction of the Neotethys oceanic lithosphere beneath the Central Iranian microcontinent during Early Jurassic to Late Cretaceous (Berberian and King, 1981; Alavi, 1994; Hassanzadeh and Wernicke, 2016). The Qori metamorphic complex located in the southeastern part of the Sanandaj-Sirjan zone (Figure 1). Fazlnia et al. (2009), suggests a regional metamorphic phase (c.a. 187 to 180 Ma) concurrent with the orogeny activities, and another phase (c.a. 147 Ma) associated with arc magmatism. The main purpose of the present paper is to investigate the metamorphic evolution of Qori complex metapelites using the phase diagram calculations. This research can enhance the accuracy of previous studies and provide researchers with a better understanding of thermodynamic changes during progressive orogenic metamorphism related to the tectonic evolution of the southern Sanandaj-Sirjan zone. Geological setting The Qori metamorphic complex mainly comprises alternating actinolite schists, garnet amphibolites, and marbles interbedded with metapelites (garnet-kyanite-biotite schists) and metaultramafic rocks (olivine-orthopyroxene-spinel-hornblende schists) (Figure 2), subjected by Barrovian-type metamorphism (Fazlnia et al., 2009). The previous studies have reported peak metamorphic conditions of 9.2 ± 1.2 kbar and 705 ± 40°C attributed it to crustal thickening in the course of Early Cimmerian orogeny(180 - 187 Ma) (Fazlnia, 2007, 2017; Fazlnia et al., 2009). The rift propagating activity in Gondwana (Golonka, 2004; Sears et al., 2005) led to non-orogenic magmatism in northeastern Neyriz and the intrusion of the heterogeneous Talle-Pahlevani batholith into the semi-pelitic to pelitic metamorphic rocks of the Qori complex (Fazlnia et al., 2009). This caused intense contact metamorphism and migmatization at 700 to 750°C and P> 5 kbar (Fazlnia et al., 2023; Miri and Fazlnia, 2024). By the closure of the Neotethys, the study area underwent deformation, as the other parts of the Sanandaj-Sirjan zone. Material and methods About 50 metapelite samples we studied using polarizing microscope. The 6 selected samples were analyzed for their major oxides by a Philips PW1480 XRF instrument in University of Kiel, Germany. Petrography The major minerals include biotite, quartz, garnet, muscovite, kyanite, plagioclase, chlorite, staurolite, along with minor amount of magnetite, rutile and porphyro-lepidoblastic texture. Two foliation fabrics, S1 and S2, are traceable in the area (Figure 3a), leading to the preferred orientation of biotite and muscovite (Figure 3b). The main stage of the garnet and staurolite growth occurred during the second metamorphism stage along with S2 foliation. They were replaced by biotite, muscovite and chlorite through the retrograde metamorphism (Figures 3e, f). In higher P-T, the staurolite became unstable, and kyanite replaces it (Figure 3g) indicating the middle amphibolite facies condition (Bucher and Grapes, 2011). In the final stage of the metamorphic process and cooling, magnetite crystals formed post-tectonically, cutting through the rock foliation (Figure 3f). The lack of pressure shadows in these crystals point to post-tectonic growth. Geochemistry The chemical data of the whole-rock are represented in Table 1. The samples plot between the pelitic and mafic rock fields on a discrimination diagram (Figure 4a), although, the presence of kyanite and staurolite reveals their metapelitic natures (Bucher and Grapes, 2011). The FeO/K2O versus SiO2/Al2O3 diagram (Figure 4b) suggests a Fe-rich sandstone for their protolith. Discussion Phase diagram modeling The sample Af-220, containing peak P-T mineral assemblage and sufficient Al2O3, FeO, and MgO contents to form the desired minerals, was selected for calculations. The calculations were performed using Theriak-Domino software (de Capitani and Petrakakis, 2010), version 10.0.19044.1526, released in 2018, with the tcdb55c2d database in a K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) chemical system (Figures 5 to 7). The fluid was considered as pure water and in-excess. The solid solution models used in calculations include GARNET (White et al., 2007) for garnet, CHLORITE (Holland et al., 1998) for chlorite, PHNG (Coggon and Holland, 2002) for muscovite, BIO (White et al., 2007) for biotite, CORD (Holland and Powell, 1998) for cordierite, and LIQtc (White et al., 2007) for melt. Metamorphism conditions The stability fields of garnet + biotite + kyanite + sillimanite indicating peak metamorphic condition occur at T 650 to 780°C and P >7 kbar (Figures 5, the blue dash-line). At higher T, melt appears, suggesting that the sample did not experienced T>780°C. It should be noted that there is no field containing the kyanite and the sillimanite as well, but their coexistence in the samples display the polymorphic transformation P-T condition. However, the occurrence of sillimanite in the samples under study point to the higher T than that of kyanite, the field 1 is considered as the peak metamorphic condition prior to anataxis. This thermal shock occurred due to intrusion of the Talle-Pahlevani pluton into Qori metapelites (e.g. Fazlnia et al., 2023). Cordierite appears at P<7 kbar, thus, its absence in the samples indicates a minimum P of 7 kbar. Influence of protolith composition A MgO/(MgO+FeO) vs. T phase diagram at 8 kbar P (Figure 6) shows that the peak assemblages occur at MgO/(MgO+FeO) ratios of 0.2 to 0.5. Also, an Al2O3 vs. T phase diagram (Figure 7) suggesting that the assemblage requires at least 10 wt% Al2O3 to form kyanite and sillimanite at peak condition. Conclusions The metapelites of Qori complex originated from a Fe-rich sandstone protolith; The parageneses include (1) chlorite + muscovite + biotite (greenschist facies) → (2) biotite + garnet + staurolite (lower amphibolite facies) → (3) biotite + garnet + staurolite + kyanite + sillimanite (medium to upper amphibolite facies); Thermodynamic phase diagram calculations indicate that the peak assemblages formed at 650 to 780 °C and 7 kbar; The MgO/FeO and Al2O3 contents of the protolith affected the peak mineral assemblage. |