nETPOmma, 2014, m0M 22, № 6, c. 665-680
EOCENE CONTINENTAL DYKE SWARM FROM CENTRAL IRAN (KHUR AREA)
© 2014 Ghodrat Torabi*, Shoji Arai**, and Hamideh Abbasi*
*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.kanazawa-u.ac.jp Received February 18, 2014; in final form May 10, 2014
Abstract—The Eocene dyke swarm with east-west general trend intrudes the Cretaceous sedimentary rocks in ~25 km north of the Khur city (Central Iran). Some of the studied dykes can be followed for over 7 km, but the majority of exposures in the area are less than 5 km long. The dykes commonly exhibit a chilled contact with the wall rocks. These dykes are trachybasalt and basalt in composition. The trachybasalt dykes are much more abundant. The basaltic dykes cross cut the trachybasalt dykes in some locations, indicating that trachy-basalt dykes are older than the basaltic ones. Primary igneous minerals of the basaltic dykes are olivine (chrysolite), clinopyroxene (diopside, augite), plagioclase (labradorite), sanidine, magnetite, orthopyroxene (enstatite), spinel and phlogopite, and secondary minerals are zeolite (natrolite and mesolite), chlorite (dia-bantite), calcite and serpentine. The trachybasalt dykes are composed of clinopyroxene (diopside), plagio-clase (labradorite), sanidine, mica (biotite and phlogopite), amphibole (magnesio-hastingsite) and magnetite as primary minerals, and chlorite and calcite as secondary ones.
Whole rocks geochemical data of the studied dykes indicate their basic and calc-alkaline nature and suggest that these two set of dykes were derived from the same parental magma. The chondrite-normalized REE patterns and the primitive mantle-normalized multi-elemental diagram of the Khur dykes show enrichment of light rare earth elements (LREE) relative to heavy rare earth elements (HREE), and negative anomalies of high field strength elements (HFSE) (e.g. Ti, Nb and Ta). These rocks show enrichment of the large ion li-thophile elements (LILE) (e.g. Cs, Ba, Th and U) and depletion of the HREE and Y relative to MREE, Zr and Hf. In the chondrite-normalized REE diagram, the basalts show elevated REE abundances relative to the trachybasalt samples. Geochemical analyses of the studied samples suggest a spinel lherzolite from the mantle as the source rock and confirm the role of subduction in their generation. The chemical characteristics of the Khur dykes resemble those of continental arc rocks, and they were possibly formed by subduction of the Central-East Iranian microcontinent (CEIM) confining oceanic crust and decompression melting of a litho-spheric subcontinental mantle spinel lherzolite enriched by subduction.
DOI: 10.7868/S0869590314060065
INTRODUCTION
Dykes represent the major conduits of magma transfer from the mantle to the upper crust and constitute a common expression of crustal extension. They occur in various geotectonic settings (Baer and Heimann, 1995) and provide useful data about the geological history of the region (Srivastava, 2011). Mantle-generated magma pulses are typically short-lived (less than a million year) and are commonly associated with continental break-up (Hanski et al., 2006).
The Eocene magmatic rocks of Iran represent a voluminous pulse of arc-related magmas and are exposed in all parts of Iran, except for the Zagros and Kopeh-Dagh zones. These magmatic rocks form volcanic, subvolcanic (dyke) and intrusive exposures in central Iran.
Dyke swarms of various ages (Paleozoic, Mesozoic and Cenozoic) and trends occur widely throughout
the studied region (Anarak, Jandaq, Khur and Bayazeh areas). In central part of Iran, about 25 km north of the Khur city, a number of basic dykes with a general east-west trend cross-cut the Cretaceous sedimentary rocks. Petrography and geochemical data indicate that the Khur area dykes comprise two predominantly trachybasalt and basalt pulses. In this article, the petrology, whole-rock geochemistry and mineral chemistry of Eocene dykes north of the Khur are discussed. We hope that the study of these two set of Eocene dykes will be useful for understanding the Cenozoic geological evolution of Central Iran.
GEOLOGICAL SETTING
The Khur area is situated in the western part of the Central-East Iranian Microcontinent (CEIM) (Fig. 1), which is bordered by the Doruneh (Great Kavir) fault to the north, by the Dehshir-Baft fault to
Fig. 1. The main structural units of Iran (Torabi and Hemmati, 2011; Slightly changed). The study area is marked as a rectangle.
the west and southwest, by the Bashagard fault to the south, and by the Nehbandan Fault to the east. It is surrounded and limited by major faults and by the Me-sozoic to Lower Cenozoic ophiolites and ophiolitic melanges that are remnants of Neo-Tethys. This microplate comprises three major crustal domains from east to west: the Lut Block, the Kerman (Tabas) Block, and the Yazd (Naein) Block. The study area is situated in the eastern part of the Yazd (Naein) block (Fig. 1). This microplate has experienced severe fracturing manifested by a network of fractures creating a mosaic of blocks, which moved against each other and created several basins by vertical movements. The most important structural characteristic of the CEIM is the occurrence of distinct horst (e.g. Lut block) and graben (e.g. Tabas region) structures (Reichert, 2007).
The Turkmeni-Ordib (Biabanak), Chupanan and Doruneh faults, which are among the longest and most prominent faults of Iran, are very close to the study area. They play an important role in regional tectonics (Almasian, 1997) (Fig. 2).
The Eocene dykes of the Khur area strike nearly east-west; their thickness varies from 15—20 cm to 5—
6 m. The length of these dykes occasionally reaches up to the 8 km (Fig. 3). Most of these dykes are vertical to subvertical and show sharp contacts with the country rocks with chilled margins. The country rocks (marls) were affected by the heat transfer and display a sharp baked zone (Fig. 4). Two subsets of trachybasaltic and basaltic dykes can be recognized. The trachybasalt dykes are much more abundant. In some locations, the basaltic dykes cross cut trachybasaltic ones, which indicates the younger age of the basalts.
The K-Ar analysis of dykes in the Khur area yielded 54 and 48 Ma (Aistov et al., 1984) for the trachybasaltic and basaltic dykes, respectively. Both of these ages confirm the Eocene epoch and are consistent with the field relationships.
Xenoliths are present in some samples of the basaltic dykes and typically represent broken fragments of the wall-rock.
ANALYTICAL PROCEDURE
The chemical compositions of minerals were determined at Kanazawa University (Kanazawa, Japan) us-
J Neogene sedimentary rocks I Eocene sedimentary rocks Eocene intrusive Eocene volanic rocks Paleocene limestone Cretaceous sedimentary rocks I Mesozoic ophiolite Jurassic sedimentary rocks I Nakhlak Triassic formation
q Farm O City ^ Fault Road 20 km
I Paleozoic sedimentary rocks
N
Paleozoic metamorphic rocks Paleozoic ophiolite Precambrian alkali-granite
ft
Jandaq'
M
Nakhlak A
Chupanan
Ashin
^Turkmeni-; Ordib Fault
Naein ophiolite
.'.Anarakf fj
Bayazeh
' Posht-e-Badam Complex
33°00'
53°00' E
55°30'
Fig. 2. Simplified geological map of the Anarak-Khur-Jandaq-Bayazeh area (Isfahan province, Central Iran) (Torabi and Arai, 2013; Slightly changed). The studied dykes are situated in the north of Khur city, which is marked as rectangle.
ing a wavelength-dispersive electron probe microana-lyzer (EPMA) (JEOL JXA-8800R), with 15kV accelerating potential, 15 nA beam current, and a counting time of 40 seconds. Natural minerals and synthetic materials were used as standards. The ZAF program was used for data correction. The representative chemical compositions of minerals from the studied dykes are listed in Tables 1 and 2. The Fe3+ content of minerals was estimated by stoichiometry. The Cr#, Mg#, Fe# and Fe3+# parameters of minerals were calculated as 100[Cr/(Cr + Al)], 100[Mg/(Mg + Fe2+)], 100[Fe2+/(Fe2+ + Mg)] and 100[Fe3+/(Cr + Al + Fe3+)] atomic ratios, respectively.
The X-ray diffraction analysis with a Bruker D8 Advance XRD machine at the Central laboratory of the University of Isfahan was used to identify the minerals.
Trace elements were analyzed in clinopyroxenes by LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) using an ArF 193 nm Excimer Laser coupled to an Agilent 7500S at the Earth Sci-
ence Department of the Kanazawa University (Japan). The diameter of the analyzed spot was 60 ^m. The trace and major elements analyses of clinopy-roxenes were obtained by LA-ICP-MS and EPMA, respectively, and are presented in Table 3.
Ten whole-rock samples have been analyzed for major and trace elements (Table 4). The analyzed rocks were collected from the middle part of the dykes to minimize alteration and contamination. These samples have been analyzed at the SGS Laboratory (Canada) by a combination of Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) methods. The precision of the whole rocks chemical analyses is less than 5% for major and trace elements. The FeO and Fe2O3 concentrations are recalculated from Fe2O*, using recommended ratios of Middlemost (1989). Chemical analyses of two samples (B-17448 and B-17461) are taken from Aistov et al. (1984).
Fig. 3. Simplified geological map of the Arusan area (north of Khur city, Central Iran).
Mineral abbreviations in tables and photomicrographs are from Whitney and Evans (2010).
PETROGRAPHY AND MINERAL CHEMISTRY
The studied dykes are fine to medium-grained and porphyritic in hand specimen. Some primary mag-matic minerals are partly replaced by secondary ones (Fig. 5). Main textures are porphyritic, trachytic,
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