nETPOmma, 2010, m0M 18, № 3, c. 328-336
P-T CONDITIONS OF THE JANDAGH METAPELITIC SCHISTS, NORTHEASTERN ISFAHAN PROVINCE, IRAN
© 2010 S. M. Tabatabaei Manesh*, M. Sharifi*, A. Romanko**
*University of Isfahan, HezarJerib st., Isfahan, Iran, 81746-73441; e-mail: Tabataba@sci.ui.ac.ir ** Geological Institute, Russian Academy of Sciences, Pyzhevsky per. 7, Moscow, 119017, Russia;
e-mail: a-romanko@ya.ru Received July 5, 2009, in final form November 25, 2009
Abstract—The metapelitic schists of Jandagh or simply Jandagh metapelites can be divided into four groups based on mineral assemblages: (1) quartz—muscovite schists, (2) quartz—muscovite—biotite schists, (3) gar-net—muscovite—chlorite schists, and (4) garnet—muscovite—staurolite schists. The Jandagh garnet-musco-vite-chlorite schists show the first appearance of garnets. These garnets contain 58-76% almandine, 1-18% spessartine, and 8-20% grossular. Microprobe analysing across the garnets demonstrates an increase in Mg# from core to rim. This is a feature of the prograde metamorphism of metapelites. Well-preserved garnet growth zoning is a sign that metapelites were rapidly cooled and later metamorphic phases had no effect here. The appearance of staurolite in garnet-muscovite-chlorite schists signifies a beginning of the amphibolite facies. The absence of zoning in staurolite suggests that its formation and growth during prograde metamor-phism occurred at a widely spaced isograde. Thermobarometric investigations show that the Jandagh metapelites were formed within a temperature range of 400-670°C and pressures of 2.0-6.5 kbar. These results are in agreement with the mineral paragenetic evidence and show the development of greenschist and amphibolite facies in the area studied.
INTRODUCTION
The Jandagh area is located in the northeast of Isfahan and is a unique field laboratory for studying regional metamorphism in Iran. This region surrounds a very large section of the southern parts of the low lying salt plain (Fig. 1).
Metapelites1 are the best known metamorphic rocks of Jandagh, in which the main constituent minerals show a regular distribution pattern that may be related to the grade of metamorphism.
The first geological studies of the Jandagh region including the preparation of geological maps started in 1955. Researchers who studied the geology of Iran, such as A.F. Stahl and von Zur (1897), A. Ganser (1955), and J. Stoklin (1968a) have made references to Jandagh. In 1975 Russian researchers made preliminary studies of the Anarak-Khur-Jandagh region (Romanko et al., 1984) and published a report in 1984
The following mineral abbreviations and thermodynamic symbols are used: Alm - almandine; And - andalusite; Ann - an-nite; Bt - biotite; Chl - chlorite; Crd - cordierite; En - ensta-tite; Grt - garnet; Grs - grossular; Hbl - hornblende; Kfs - K-feldspar; Ms - muscovite; Pl - plagioclase; Prp - pyrope; Qtz -quartz; Sil -sillimanite; St — staurolite.
Xfs = ¿/(Fe + Mg + Al(VI)) i = Fe, Mg, Al(VI);
xf = j/(Fe + Mg + Al(VI) +Ti) j = Fe, Mg, Al(VI), Ti;
xf = k/(Fe + Mg + Ca + Mn) k = Fe, Mg, Ca, Mn;
Nj = 100X;; aa, - activity of the component i of the mineral a; T - temperature, (K/C); P - pressure, kbar.
in compliance with the "Techno-Export" contract. Other studies of this region include Hatefs M.S. thesis entitled "The Geology and Petrology of Igneous and Metamorphic Rocks of the Khur-Jandagh region (Central Iran)" (Hatef, 1996).
METHODOLOGY AND GEOLOGY
Minerals were analysed using a Cameca electron microprobe at the Iran Center of Technological Research, and the whole rock analysis was carried out by the ICP-MS method at the Amdel Laboratory, Australia (Table 1). Also, a series of current petrologic methods such as texture and structure examination, paragenetic analysis, regional categorization of coexisting minerals based on microprobe profiles and, finally, geothermobarometrical calculations were used. In this study the main issue centered on Ms, St, Chl, Grt, Bt as a collection of minerals which, in the presence of H2O and Qtz are sensitive to the conditions of metamorphism.
PETROGRAPHY AND MINERAL CHEMISTRY
The main purpose of this section is to characterize mineral assemblages and their compositions in the rocks from the Jandagh area and show the relationships between the minerals in different rock types.The metamorphic rocks in this area are serpentinized peri-dotite, mylonitic granite, amphibolite, marble and alternating layers of schist, quartzite and marble (Fig. 2). The general feature of Metapelitic schists in the Jandagh area is the presence of lepidoblastic, porphyo-
Asphalted Road , ■" - Non-Asphalted Road^Main City ] Study Area
54°30'
5 10 km _l_I
0 Farm or Village
20 km
Scale
54°45'
Fig. 1. Geological map of the area studied and access roads (Technoexport, 1974).
blastic, lepido-porphyroblastic and lepido-porphyro-poikiloblastic textures. The whole rock compositions of Jandagh Metapelits fall within the pelagic shale zone in the AFM-diagram, having higher Al and Fe and lower Ca than those of platformic shales (Fig. 3).
The Metapelitic rocks of Jandagh are subdivided into four types:
(1) Quartz—muscovite schists (Qtz + Ms + Kfs + Pl).
These rocks contain substantial amounts of organic matter and have very distinct lepidoblastic textures. Small grains of quartz with smooth boundaries and an average size of 0.1 to 0.2 mm and aligned muscovite sheets, 0.1 to 0.2 mm in average size, account for 95%
of these rocks. The xMg of these rocks varies between 0.49—0.59. Biotite, chlorite and Fe oxide are the minor minerals. Muscovites embedded in opaque minerals are sometimes present in these rocks. The presence ofvery fine crenulations is another feature in the structures of these rocks (Fig. 4).
(2) Quartz—muscovite—biotite schists (Grt + Qtz + + Bt + Ms + Kfs + Pl) have larger grains in comparison with the quartz—muscovite schists and are not often seen as layers enriched in quartz, muscovite, and bi-otite accompanied by organic matter (Fig. 4). Lepido-blastic textures are a distinct feature of these rocks. Their essential minerals are grains of smooth surfaced quartz with an average size of 0.1 to 0.2 mm and aligned muscovite and biotite sheets with an average size of 0.1—0.2 mm. The simultaneous formation of these two minerals indicates reaction (1) Chl + Kfs = = Bt + Ms + Qtz + H2O. Chlorite, garnet and Fe oxide
are the accessory minerals. The xMg varies from 0.41 to 0.54 in these rocks.
(3) Garnet—muscovite—chlorite schists (Grt + + Qtz + Ms + Kfs + Pl + Chl) have much less biotite
(about 2 to 3%) but more garnet (about 10%) in comparison with the previous group. Garnet forms irregular broken phenocrysts surrounded by smaller grains of
garnet scattered within the rock (Fig. 4). Its xM
Mg
var-
ies between 0.4 and 0.2, and the end member composition is as follows: almandine from 59 to 76%, spessartine from 1 to 18%, and grossular from 8 to 20%. The mineral assemblage of these rocks and the presence of muscovite and, sometimes, biotite inclusions in the garnet show that chemical reaction (2) Ms + Bt + + Qtz = Grt + Kfs + H2O has taken place. The presence of a very fine crenulation fabric is another feature
in the make-up of these rocks. The X"Mg varies from 0.41 to 0.46. Figure 5 shows the position of Bt and Grt in the AFM-diagram compared with the whole rock composition. Large cores of Grt contain more Mg in comparison with small Grt, and this shows that they must have formed after prograde metamorphism; this difference is clearly evident in the AFM-diagram.
The microprobe profile of Grt in contact with Chl shows an increase in NMg near Chl. This is important because the NMg of Chl during prograde metamor-phism is always higher than that of adjacent Grt; therefore, this observation may signify limited stability of Fe-chlorite and an increase in the amount of Grt and accompanying Mg-chlorite (Fig. 6 and Tabatabaei Manesh, 2006).
(4) Garnet—muscovite—staurolite schists (Grt + + Qtz + St + Bt + Ms + Kfs + Pl + Chl) have coarse grains; the matrix is composed of layers rich in Ms accompanied by very small amounts of Bt with lepido-blastic, porphyroblastic, and poikiloblastic textures.
The xMg in these rocks varies from 0.39 to 0.60. The Grt present in the staurolite schists are both small and large in size, with large Grt containing more Mg com-
Table 1. Selected microprobe analyses of the schist minerals, wt %
Component Sample Mjc11 Sample JM5d
Grt Grt Grt Grt Grt Grt Grt Grt Grt Grt Grt
1 2 3 4 5 1 2 3 4 5 6
SiO2 TiO2 A12O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Si Al Al(IV) Al(VI) Cr Ti Fe3+ Fe2+ Mn Mg Ca Na K XMg XCa Alm Sps Prp Grs 38.534 0.11 20.88 0.00 25.96 6.12 0.84 6.45 0.00 0.00 Cation ra 3.10 1.98 0.00 1.98 0.00 0.01 0.00 1.75 0.42 0.10 0.56 0.00 0.00 0.04 0.20 0.62 0.15 0.04 0.20 38.363 0.08 20.71 0.03 24.63 7.32 0.96 6.74 0.00 0.00 itios calcu 3.09 1.97 0.00 1.97 0.00 0.00 0.00 1.66 0.50 0.11 0.58 0.00 0.00 0.05 0.20 0.58 0.18 0.04 0.20 38.867 0.09 21.10 0.00 28.50 2.65 1.50 6.72 0.00 0.00 ated per 1 3.10 1.98 0.00 1.98 0.00 0.01 0.00 1.90 0.18 0.18 0.57 0.00 0.00 0.08 0.20 0.67 0.06 0.06 0.20 38.653 0.08 20.55 0.00 32.31 0.35 1.71 5.16 0.00 0.00 2 oxigens 3.11 1.95 0.00 1.95 0.00 0.00 0.00 2.17 0.02 0.20 0.45 0.00 0.00 0.09 0.16 0.76 0.01 0.07 0.16 38.113 0.14 20.91 0.04 32.92 0.37 2.70 4.10 0.00 0.00 3.05 1.97 0.00 1.97 0.00 0.01 0.00 2.20 0.03 0.32 0.36 0.00 0.00 0.13 0.12 0.76 0.01 0.11 0.12 38.14 0.08 21.53 0.05 31.36 0.29 1.81 6.99 0.00 0.00 3.03 2.01 0.00 2.01 0.00 0.00 0.00 2.08 0.02 0.21 0.59 0.00 0.00 0.09 0.20 0.72 0.01 0.07 0.20 38.23 0.07 20.78 0.00 31.50 0.27 1.67 7.23 0.00 0.00 Cation ra 3.06 1.96 0.00 1.96 0.00 0.00 0.00 2.11 0.02 0.20 0.62 0.00 0.00 0.09 0.21 0.72 0.01 0.07 0.21 37.86 0.04 21.07 0.00 33.80 0.00 4.07 2.97 0.00 0.00 tios calcul 3.02 1.98 0.00 1.98 0.00 0.00 0.00 2.25 0.00 0.48 0.25 0.00 0.00 0.18 0.08 0.75 0.00 0.16 0.08 37.74 0.09 21.05 0.02 32.21 0.31 1.6
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