научная статья по теме BACK-ARC PALEO-TETHYS RELATED BLUESCHIST FROM CENTRAL IRAN, SOUTH OF CHUPANAN, ISFAHAN PROVINCE Геология

Текст научной статьи на тему «BACK-ARC PALEO-TETHYS RELATED BLUESCHIST FROM CENTRAL IRAN, SOUTH OF CHUPANAN, ISFAHAN PROVINCE»

BACK-ARC PALEO-TETHYS RELATED BLUESCHIST FROM CENTRAL IRAN, SOUTH OF CHUPANAN, ISFAHAN PROVINCE

© 2013 Ghodrat Torabi* and Shoji Arai**

*Department of Geology, University of Isfahan, Azadi Square, 8174673441, Isfahan, Iran;

e-mail: torabighodrat@yahoo.com **Department of Earth Sciences, Kanazawa University, 920-1192 Kanazawa, Japan; e-mail: ultrasa@kenroku.kanzawa-u.ac.jp Received May 3, 2012; in final form, October 22, 2012

Late Paleozoic blueschists present good exposures in the Pateyar metamorphic complex (south of Chupanan, Isfahan Province). They are formed by metamorphism of primitive basaltic lavas. Petrography and microprobe analyses show that the studied rocks are consist of glaucophane, actinolite, actinolitic hornblende, pla-gioclase (albite), sphene, magnetite, quartz and apatite. Secondary minerals are epidote, chlorite, pyrite, hematite and calcite. Mineralogical assemblages are consistent with blueschist facies metamorphism, which is followed by a retrograde metamorphism in greenschist facies. Estimation of the metamorphic conditions suggests 300—400°C and 7—11 kbar. Chemical signatures of the studied metamorphic rocks conclude that they retain main geochemical characteristics of the protoliths, which allow the petrochemical interpretations. Geochemical analyses of these blueschists show that they were originally tholeitic basalts. Evident negative anomaly of Nb, Ta and Ti spikes relative to Th, La and Ce, in the primitive mantle normalized spidergram, reveal subduction role in their petrogenesis. The studied metavolcanics exhibit an intermediate chemistry between the N-MORB (normal mid-ocean ridge basalt) and IATB (Island arc tholeitic basalt). Enrichment in LREE (light rare earth elements) and LILE (large ion lithophile elements) and relatively depletion in HFSE (high field strength elements) suggest a back-arc basin paleotectonic setting for the Chupanan samples. The primitive magma of the analyzed samples possibly have been produced by 8—13% melting of a spinel lherzo-lite. The field and petrological data propose that the studied Paleozoic metavolcanics were formed in a back-arc basin above the northward subduction of Paleo-Tethys oceanic lithosphere in Central Iran. The chemical criteria of the LILE/HFSE ratio, suggests that the subduction zone was young and immature during the vol-canism.

DOI: 10.7868/S0869590313040080

INTRODUCTION

Glaucophane-bearing metabasites and associated metasediments are the key index rocks of the blue-schist facies and possibly point to the paleo-subduction zones. Presence of the ophiolites in such terrains supports this hypothesis.

Late Paleozoic blueschist facies metabasites, metamsediments and metaophiolites are present in the western part of the Central-East Iranian Microcontinent (CEIM) (Torabi, 2009b; Torabi, 2011a, 2011b). The main exposures of Paleozoic blueschist are in the Anarak ophiolite and southern part of the Chupanan area (Arusan river valley). Reibeckite-bear-ing metabasites of the Anarak ophiolite are alkaline metabasalts resemble as oceanic intraplate magma-tism (Torabi, 2011b; Torabi, 2012). In some cases, they preserve the primary pillow structure. The Chu-panan area metavolcanics are associated with metasediments as interlayers.

The objective of this study is to determine the pet-rological and geochemical nature, P-T condition of metamorphism, as well as geotectonic setting of the

Paleozoic Chupanan area blueschists. Due to the geographical and stratigraphical position of these rocks, it hoped that this research will provide a tectonomag-matic approach to the Paleo-Tethys subduction-relat-ed episodes in Iran.

GEOLOGICAL SETTING

CEIM is one of the most important structural units of Iran with complex geological history. It comprises three main blocks from the east to the west: Lut block, Tabas (Kerman) block and Naein (Yazd) block (Fig. 1). This microcontinent is surrounded by Neot-ethys-related Mesozoic to Early Cenozoic ophiolites; but the Paleozoic Paleo-Tethys related ophiolites are present in the western part (Naein block). The main exposures of the Paleozoic ophiolites are in the Anar-ak, Jandaq, Bayazeh and Posht-e-Badam areas (Fig. 1). The western part of the Tukmeni-Ordib fault (in the Anarak area), is identified as the Paleo-Tethys suture zone of the Central Iran (Bagheri, 2007) (Fig. 2).

50° 54° 58° 62°

Fig. 1. The major structural units of Iran; KDF: Kopeh-Dagh fault; GKF: Great Kavir fault; NBF: Naiband fault; NF: Nehban-dan fault; SSZ: Sanandaj — Sirjan zone; PTSZ, Paleo-Tethys suture zone; UDMA: Urumieh — Dokhtar magmatic arc. An, Jn, By and PB present the Anarak, Jandaq, Bayazeh and Posht-e-Badam metaophiolites, respectively. Study area is marked as a rectangle.

The study area (Arusan-e-Kabudan, south of Chu-panan) is located on the western part of the CEIM and is a part of the Naein block with highly deserted geographical conditions. It is situated between the Chu-panan and Turkmeni-Ordib (Biabanak) faults (Fig. 2). These faults are among the longest and the most prominent faults of the Central Iran, and they play an important role in the regional tectonics. In the Chu-panan, jandaq and Anarak areas, the Paleozoic ophio-lites, as the lowermost rock unit of the area, has been covered by Paleozoic and Lower Mesozoic metamor-phic rocks (schist and marble). The Cretaceous limestone has been covered the Paleozoic metamorphic rocks. Eocene volcanic rocks and intrusive bodies, and Oligo-Miocene sediments are from the main rock units of the study area.

Rocks of the Pateyar complex crop out chiefly in the Arusan-e-Kabudan river valley. Metamorphic rocks of the Pateyar complex are mainly composed of schist, marble, metabasalt interlayers (the studied rocks), metasandstone and quartzite. The total thickness of the Pateyar complex in this area is estimated as 875 m (Aistov et al., 1984). A distinct feature of this metamorphic complex is a varying degree of meta-morphism of its rocks (Aistov et al., 1984). Apart from the metamorphism in the green schist facies, the rocks of this complex complicated locally by several parallel

gently dipping faults and additional deformation and foliation have been undergone high-P metamorphism in the glaucophane schist facies (Aistov et al., 1984). The exact age of the Pateyar metamorphic complex rock units is still a matter of debate. Presence of the studied metabasalts without pillow structure, in the metapelitic section of the Patyar metamorphic complex, reveal that the periodical underwater marine fissure eruption produced lavas above the carbonate compensation depth and below the pressure compensation depth (Shkol'nik et al., 2011). The latter concludes from the absence of metatuffites during the vol-canism.

40Ar-39Ar isotopic analyses on micas of the Anarak metamorphic rocks indicate 318.99 ± 1.63 and 333.87 ± 1.91 Ma (Middle and Late Carboniferous) (Bagheri, 2007). Furthermore, analysis of sodic amphibole in Anarak ophiolite blueschist by 40Ar-39Ar method presents 285.42 ± 1.65 Ma; which points to the Early Permian metamorphism of primitive lavas (Bagheri, 2007). K—Ar isotopic dating of the Chupan-an metabasites (blueschist) indicates 400 Ma (Early Devonian) (Aistov et al., 1984). Isotopic analyses of the chlorite schists, chlorite-muscovite schists and muscovite schists of the study area present 398 ± 14 (Early Devonian), 309 ± 9 (Late Carboniferous) and

Neogene sedimentary rocks

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Fig. 2. Simplified geological map of the Anarak-Khur-Jandaq-Bayazeh region (Isfahan province, Central Iran). Presence of two Paleozoic and Mesozoic ophiolites in this region is evident. The studied rocks are situated in the south of Chupanan city. The position of Paleo-Tethys suture zone is proposed by Bagheri (2007).

372 (Late Devonian) Ma, respectively (Aistov et al., 1984).

The Anarak to Arusan area is considered as remnant of a Permo-Triassic accretionary wedge was active in the Paleozoic to Lower Mesozoic (Bagheri, 2007). Detailed petrographical study of the Nakhlak Triassic succession (Alam, Baqoroq and Ashin formations) shows that it consists of pelagic sedimentary rocks including Rosso ammonitico facies, volcanic arenites, alluvial fan conglomerates and sandstones, nodular limestone and marl, and turbidites deposited in a volcanic arc setting along an active margin (Balini et al., 2009). This succession was deposited in a rapidly subsiding sedimentary basin (Zanchi et al., 2009). Previous authors (e.g. Soffel et al., 1996; Alavi et al., 1997; Besse et al., 1998) considered that the Paleozoic ophiolites of the CEIM are remnants of Paleo-Tethys, transferred to present position from NE of Iran, by 135° anti-clockwise rotation of CEIM. Whereas, new paleontological and paleomagnetic studies (e.g. Balini et al., 2009, Muttoni et al., 2009) do not support large anticlockwise rotations.

ANALYTICAL TECHNIQUES

Mineral chemical analyses were conducted by a wave-length-dispersive electron probe microanalyzer (EPMA) JEOL JXA-8800R, at the Earth Science Department of the Kanazawa University (Japan). The analyses were performed under an accelerating voltage of 20 kV and a beam current of 20 nA. The structural formula and Fe2+ and Fe3+ contents of amphiboles were calculated after Schumacher (1997). Leake et al. (1997) used for amphiboles naming. Table 1 and 2 include chemical analyses of minerals. The X-ray diffraction analysis by a Bruker D8 Adv

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