Metamorphic evolution of Neoproterozoic - Lower Cambrian Schists in NW of Sarve-Jahan (Soltanieh belt NW of Iran) using equilibrium phase diagrams

Mineral assemblage of metamorphic rocks forms by various factors including temperature, pressure, protolith chemical composition, and fluids. Although, P-T variation is the main factor controlling mineralogical evolution and assemblages, chemical differences of protoliths and metamorphic fluids also play an important role in the formation of final mineral assemblages (e.g. Spear, 1993). Therefore, mineral assemblages can be used in the study of thermodynamic and chemical conditions of metamorphic events. The studied area is located in the northwest of Sarve-Jahan village. For the purpose of the present paper, we investigate the metamorphic evolution of Sarve-Jahan schists (Zanjan province) and the effect of important factors on the formation of their mineral assemblages are through calculating phase diagrams. The study area is located in the northwest of Sarve-Jahan from Soltanieh structural zone and hosts Upper Neoproterozoic - Lower Cambrian schists (Fig. 1). The Soltanieh strip is a narrow and long structural strip trending northwest-southeast with a length of over 150 km and a width of 10-12 km. This belt as an uplifted tectonic bedrock lies in the structural zone of Central Iran. It comprises a complete set of thick clastic, carbonate and pyroclastic sediments (Kahar, Bayandor, Soltanieh, Barut and Lalon Formations) (Stöcklin and Eftekharnezhad, 1969) belonging to Precambrian-Paleozoic.. In the northwest of Sarve-Jahan and the vicinity of Sarve-Jahan granite, there is a metamorphic complex trending northwest-southeast, composing pelitic schists, metacarbonate (chalk schist) and sandstone (quartz schist and quartzite). The complex belongs to the Kahar Formation (Stöcklin and Eftekhar-nezhad, 1969; Babakhani and Sadeghi, 2004). In the southwest of metamorphic complex, there is a row of slates gradually replaced by phyllites and schists towards the northeast. The exposed Mesozoic deposits in the studied area include the sediments of Shemshak Formation and Lar limestones (Fig. 1) (Babakhani and Sadeghi, 2004, Stöcklin and Eftekhar-nezhad, 1969). Tertiary deposits include a sequence of detrial sediments as well as igneous and volcanic rocks (Stöcklin and Eftekhar-nezhad, 1969).

Seventy microscopic thin sections were studied by a polarizing microscope with transmitted light. Five samples with the highest number of phases in equilibrium were selected for microprobe analyze. The samples were analyzed in Iranian Mineral Processing Research Center using a CAMECA model SX 100 device with an electron beam acceleration voltage of 15 kv, current intensity of 3 nA and analysis time of 40 seconds for each point. Whole-rock compositions of these samples, also were determined by X-ray Fluorescence (XRF) spectroscopy method using a PHILIPS PW1480 instrument in the Kansaran Binaloud laboratory.

Petrography:

The studied rocks include garnet and staurolite porphyroblasts lie in a well-oriented matrix composed of biotite + muscovite + chlorite + plagioclase + quartz. Three phases of deformation (D1, D2, D3) and two phases (M1, M2) of metamorphism occurred in the metamorphic rocks. The M1, could be related to the D1 and characterized by garnet + staurolite + chlorite + biotite + muscovite + plagioclase + quartz assemblage. The M2, could occur in association with D2 and formed chlorite + biotite2 + muscovite2 + quartz. The third deformation phase (D3) caused crenulation cleavage on D2 schistosity but with no new mineral phase. Garnet and staurolite porphyroblasts formed during M1. Retrogressive metamorphism replaced these minerals with chlorite and muscovite in their rims. Whole-rock and mineral chemistry The samples studied contain high SiO2, Al2O3 and K2O corresponding to a Fe-rich shale protolith (Table 1). The biotites and white-micas have high annite and muscovite end-members, respectively. Garnets and staurolites are also Fe-rich and plagioclase are of albite to oligoclase compositions (Table 2).

Thermobarometry Various thermobarometry calculations based on garnet-biotite (Bhattacharya et al., 1992), muscovite-chlorite (Vidal and Parra, 2000) and garnet-plagioclase-biotite-quartz (Hoisch, 1990) methods resulted c.a. 600 °C and 5-7 kbar for the M1 and 500°C and 6 kbar for the M2.

Equilibrium phase diagram:

The equilibrium phase diagrams were calculated using GeoPs software (version 3.3) and Bi(w), Chl(W), Crd(W), Ctd(W), St(W), Gt(W), Mica(W), Opx(W), melt(W) and Fsp(C1) (Holland and Powell, 2003; White et al, 2014) solid solution models for average composition of the studied samples. The P-T pseudosection diagram (Fig. 5) shows that the M1 assemblage garnet + staurolite + biotite + muscovite + chlorite + quartz + plagioclase is stable in 550 – 650 °C and 5 – 7.2 kbar, while the assemblage biotite2 + muscovite2 + chlorite2 + quartz + plagioclase corresponding to M2, is stable in T<550 °C and P<5 kbar. The calculated isopleths for pyrope in garnet and phlogopite in biotite intersect at the stable filed of M1 assemblage (Fig. 6) confirming the thermobarometry results.

Protolith composition:

The Mg# [Mg# = (MgO/MgO+FeO)] versus T variations phase diagram (Fig. 7) shows that the stability of garnet and staurolite is affected by Mg and Fe contents of the protolith. They leave the system with increasing Mg# at constant T. In other words, the high FeO contents of the Sarve-Jahan schist protolith had a significant role in the formation and the stability of the peak metamorphism mineral assemblage.

Fluid composition:

The CO2 contents of metamorphic fluids is important in the stability of minerals (e.g. Miri et al., 2023). A T-XCO2 phase diagram was calculated for the sample assuming a H2O-CO2 fluid to investigate the role of CO2 in the metamorphic of the samples (Fig. 8). The diagram indicates that the M1 mineral assemblages was stable at XCO2 < 0.3, but the M2 was stable only in the presence of pure H2O fluid.

Microstructures show that the deformation phases (D1, D2, and D3) and the two metamorphic phases (M1 and M2) occurred in the area. The M1 and M2 were contemporaneous with D1 and D2, respectively, while D2 was not in association with a metamorphic phase.The mineral assemblage garnet + staurolite + chlorite + biotite + muscovite + plagioclase + quartz formed in M1 and peak of metamorphism (amphibolite facies) and biotite2 + muscovite2 + chlorite2 + quartz + plagioclase formed in the retrogressive metamorphism M2 phase (greenschist facies).The traditional (mineral pairs) and modern (phase diagram) thermobarometry calculations show obtain 550 – 650 °C and 5 –7 kbar for M1 and < 500 °C and < 5kbar for M2. On this basis, a clockwise P-T-t path can be considered for metamorphism of the Sarve-Jahan schists. Which could have occurred in a subduction-collision tectonic site.The calculated T-Mg# and T-XCO2 phase diagrams show that the composition of the protolith and the metamorphic fluid played a significant role in the minerals assemblage event occurring during peak metamorphic conditions.