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Archive of SID Archive of SID IJST (2014) 38A3: 243-252 Iranian Journal of Science & Technology http://ijsts.shirazu.ac.ir An unusual assemblage of talc-phengite-chlorite-K-feldspar in quartz schists from the Nahavand area, Sanandaj-Sirjan zone, Iran J. Izadyar*, S. Mojab, O. Kuroshi and M. Zare Department of Geology, University of Zanjan, University Blvd., Zanjan, Iran E-mail: [email protected] Abstract For the first time, an unusual assemblage of talc-phengite-chlorite-K-feldspar was found in quartz schists from the Sanandaj-Sirjan zone in the Nahavand area in western Iran. The talc-bearing quartz schists occur as small bodies or lenses within pelitic schist layers and contain talc, phengite, chlorite, K-feldspar and quartz as major mineral constituents with subordinate amounts of calcite and graphite. Textural analysis revealed that talc, phengite, chlorite and K-feldspar are in sharp contact and no reaction rims between them were observed. Constructed petrogenetic gird in the K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH) model system containing talc, phengite, chlorite, K-feldspar, phlogopite and kyanite with excess quartz and H2O shows that divariant assemblage of talc- phengite-chlorite-K-feldspar is stable over a wide P-T range defined by the following two univariant reactions: phengite + talc + quartz = chlorite + K-feldspar + kyanite + H2O and chlorite + phlogopite + quartz = talc + phengite + K-feldspar + H2O. Constructed Al2O3-KAlO2-MgO+FeO (AKM) compatibility diagrams predict that phengite (XPh = 0.280, YPh = 0.860), chlorite (XChl = 0.570, YChl = 0.640), talc (XTlc = 0.160, YTlc = 0.02) and K- feldspar are stable at P = 11 kbar and T = 400°C. This relatively high-pressure assemblage could be formed during the subduction of the Neo-Tethys oceanic plate under Iranian microcontinent. Keywords: High-pressure metamorphism; Nahavand; Sanandaj-Sirjan zone; Talc-phengite-K-felds parassemblage 1. Introduction and Schreyer (1976) regarding the high-pressure origin for talc-phengite assemblages. This In natural rocks, talc-phengite assemblage was first assemblage was subsequently recognized in the found in the piemontite-quartz schist in Serbia western Alps (Chopin, 1981) and thereafter in a (Abraham and Schreyer, 1976). Abraham and number of areas around the world in high-pressure Schreyer (1976) suggested that talc-phengite metamorphic terrains such as the Sanbagawa assemblage is an indicator of high-pressure metamorphic belt, Japan (Izadyar et al., 2000; metamorphism based on theoretical grounds in the Hatanaka and Hirajima, 1999; Ubukawa et al., K2O-MgO-Al2O3-SiO2-H2O (KMASH) model system, 2007). Talc-kyanite and talc-omphacite although they also proposed an alternative assemblages have also been identified from ultra hypothesis that this pair could be formed due to the high-pressure metamorphic rocks (Reinecke, 1991; presence of small amounts of additional non- Mattinson et al., 2004; Liou and Zhang, 1995). KMASH components such as Fe2O3-MnO-CuO- However, the natural parageneses of talc-phengite NiO in phyllosilicates under low-pressure and talc-kyanite usually coexist with Fe-bearing conditions. Massonne and Schreyer (1989) assessed mineral phases such as garnet, biotite, chloritoid the stability field of talc-phengite assemblage by and chlorite (Chopin, 1981; Frank et al., 1987) and the synthetic work in the KMASH model system do not belong to the constituents of the simple containing kyanite, chlorite, talc, phlogopite and KMASH model system. Wei and Powell (2003) muscovite with excess quartz and H2O. They calculated the KFMASH model system for high- proposed that the following two reactions: chlorite+ pressure metapelites with mineral phases of garnet, phlogopite + quartz = talc + muscovite + H2O and chloritoid, chlorite, biotite, carpholite, talc, talc + muscovite = phlogopite + quartz + H2O staurolite, kyanite with phengite, quartz/coesite and intersect at an invariant point at 10.8 kbar and H2O in excess. They documented the typical 610°C, which supported hypothesis 2 of Abraham parageneses of talc-phengite and talc-kyanite for a variety of bulk compositions from several high- pressure terrains. Wei and Powell (2004) also *Corresponding author calculated the stable mineral assemblages of Na- Received: 4 February 2013 / Accepted: 7 May 2014 bearing metapelites in high-pressure conditions as www.SID.ir Archive of SID IJST (2014) 38A3: 243-252 244 they also occur widely in high-pressure types of the complexly deformed sub-zone are schist, metamorphic terrains (Chopin, 1981; Izadyar et al., marble, amphibolite, quartzite, dolomite-marble and 2000; Meyre et al., 1999; Theye et al., 1992). By metasandstone. All sub-zones are imbricated in a using Schreinemakers' analyses and thermodynamic complicated thrust system with out-of-sequnce thrusts calculation based on the dataset of Holland and that locally juxtaposes the younger units over the older Powell (1998), Izadyar et al. (2000) indicated that ones (Mohajjel et al., 2003). The Nahavand area the talc-phengite-albite assemblage from belongs to the complexly deformed sub-zone and is piemontite-quartz schists of the Sanbagwa located near the main Zagros reverse fault, which is metamorphic belt is stable at 580-600°C and 11.6- proposed as a suture zone between the Arabian plate and Eurasia (Sengör, 1984; Agard et al., 2006; 12 kbar. However, Ubukawa et al. (2007) obtained Ghasemi and Talbot, 2006) (Fig. 1B). Apart from a temperatures of 565-580°C and pressures of 9.5-10 thin ophiolite unit, the lithologic units in the Nahavand 3 kbar for talc-phengite assemblage in NCKFe region can be generally subdivided into two groups of MASH model system in talc-Na pyroxene metamorphosed and non-metamorphosed rocks (Alavi piemontite-quartz schist in the Sanbagawa and Mahdavi, 1994) (Fig. 2). The metamorphic rocks metamorphic belt. None of these studies, either include five different successions that are in fault experimental or calculated, have addressed a talc- contact with both the overlying and underlying rocks. phengite-K-feldspar assemblage. In fact, only The oldest one (PTrm) is a sequence of massive Massonne and Schreyer (1989) proposed high- marbles of Permian-Triassic age that are strongly pressure stability field for the talc-phengite-K- sheared and mylonitized near the contact with the feldspar assemblage purely on theoretical grounds underlying rocks. The next succession (TrJmv) but it is not reported in natural rocks from high- consists of an assemblage of Middle to Upper Triassic-Lowermost Jurassic metamorphosed basaltic pressure metamorphic terrains. During a and andesite-basaltic lava flows (with tholeitic petrological investigation in the Nahavand area of affinities). This assemblage contains locally occurring the Sanandaj-Sirjan zone, an unusual assemblage of pillow structures interlayered with well-foliated talc-phengite-chlorite-K-feldspar was recognized in marbles that have thin, upward increasing slate layers. quartz schists. This paper is the first to describe this Small plutons of gabbro and diorite (now assemblage, to characterize its mineralogy and metamorphosed) have locally intruded this assemblage texture and to explain its equilibrium condition (Alavi and Mahdavi, 1994) (Fig. 2). The third unit based on the petrogenetic grid in the KFMASH (TrJm) consists entirely of a sequence of fossiliferous model system containing K-feldspar. marbles of Late Triassic to Early Jurassic. The fourth succession (Jsv) is a sequence of Lower Jurassic 2. Geological setting slates, phyllites and metagreywakes that have intercalations of marbles and metamorphosed volcanic The Sanandaj-Sirjan zone, which is 1500km long and rocks in the lower parts. The fifth lithological unit (Jh) up to 200km wide, stretches from the northwest to the consists of an Upper Liassic succession of cordierite- southwest of Iran and is located between the central bearing hornfels which is the product of contact Iran block and Zagros Fold-thrust belt (Stoclin, 1968; metamorphism due to the intrusion of Late Cretaceous Berberian and King, 1981; Alavi, 1994) (Fig. 1A). The granitoid plutons (Alavi and Mahdavi, 1994) (Fig. 2). Sanandaj-Sirjan zone consists of mafic-ultramafic rocks, metamorphic complexes and Paleozoic- Mesozoic sequences that underwent amphibolite/greenshist-facies metamorphism during the Cretaceous-Tertiary continental collision between the Afro-Arabian continent and the Iranian microcontinent (Sengör and Natalin, 1996; Mohajjel and Fergusson, 2000; Mohajjel et al., 2003). From the southwest to the northeast it consists of several elongated sub-zones (Fig. 1B): (1) a radiolarite sub- zone consisting of Triassic-Cretaceous shallow marine limestone and dominant deep-marine radiolarite; (2) a Bisotun sub-zone that includes Late Triassic to Late Cretaceous limestone; (3) an ophiolite sub-zone that is the location of the Zagros suture zone; (4) a marginal sub-zone that contains abundant Late Jurassic-Early Cretaceous volcanic rocks in addition to shallow Fig. 1. (A) Simplified structural map of Iran showing the marine strata in the Cretaceous unit and (5) a location of the Sanandaj-Sirjan zone. (B) Tectonic map of complexly deformed sub-zone (Mohajjel et al., 2003). southwestern Iran showing the divisions of the Sanandaj- The last sub-zone is distinguished from the other sub- Sirjan zone (modified after Mohajjel et al., 2003) zones by abundant metamorphic rocks. The main rock www.SID.ir Archive of SID 245 IJST (2014) 38A3: 243-252 are observed (Figs. 4A, 4B, 4D and 4E). Chlorite is one of the major constituent
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