| چکیده انگلیسی مقاله |
Introduction The Urmia-Dokhtar Magmatic (volcanic and plutonic) Arc (UDMA) with 2000 Km length in Iran is a small section of the Alpine-Himalayan orogenic belt (Glennie, 2000; Alavi, 2004; Ilbeyli, 2004). The studied area is located in the northeast of Ashtian and composed of two area of Feshk and Master. These areas are situated at 49°44' and 49°55' longitude and 34°31' and 34°44' latitude. Based on the structural zone, this area is a small part of UDMA (Hajian, 1970). Wrinkles and big faults mechanisms such as Tafresh, Talkhab, and Tabarteh have been effective in the formation and morphology of this area that particularly control the sedimentary basins and volcanic processes via the approximate procedure of northwest-southeast. Geology Rock outcrops in the study zones are associated with the Mesozoic and Cenozoic Eras with the oldest ones made of carbonate rocks belonging to the Middle Triassic. The first group is attributed to Eocene, mainly includes acidic tuffs to gray basic, tuffs containing the crystal glass pieces and trachyte lava, trachyandesite and dacitic- rhyodacite types in the form of a semi-dense, gray dark to bright pink dikes and dark lavas from andesites- trachyandesite, quartz latite andesite and basaltic andesite. The Eocene rocks are divided into six units (Lithozone E1 to E6) in this zone consisting of E1 (conglomerate, marl, and sandstone), E2 (ignimbrite, tuff, and nummulite-bearing sandy limestone), E3 (green rhyolitic to dacitic tuff accompanied by marl, shale, sandstone, and limestone), E4 (red to dark brown tuff and dark lava), E5 (green tuff, sedimentary layers, and rhyolite), E6 (basalt, andesite, and dark tuff interlayered with nummulitic limestone) (Hajian, 2001). Sampling and Analytical Techniques The survey was conducted by selecting the proper locations and taking 300 samples. They were exported to the Geological Survey of Iran (GSI) for whole rock chemical analysis with ICP-MS and XRF methods. The microprobe analyses were conducted by Cameca SX100 microprobe (France) with carbon coating at the Iranian Mineral Processing Research Center on 10 polished sections at the Iran Minerals Processing Research Center after detailed petrography studies. Discussion The microscopic study of samples reveals that the rhyolite, dacite, andesite, trachy-andesite and andesi-basalt rocks dominantly have porphyritic, microlite porphyritic and microlite glomeroporphyritic textures. In the dacite rocks, plagioclases have oscillatory zoning. If the magmatic chamber is attacked by the basic magma while cooling the dacite melts, first, the plagioclase crystals melt partially and then become recrystallized. Therefore, these crystals can adsorb dusty textures (Gills, 1981). Amphibole phenocrysts with 3 to 5 Vol.% reveal the effects of burns and decay in most samples of dacite, trachy-andesite, andesi-basalt, and especially the andesite. Amphiboles in the form of inclusion in the plagioclase, are almost anhedral and have a green-blue multiplicity which can represent their sodic composition (Tabatabai Manesh et al. 2010). Pyroxene is another detectable mineral in andesite and andesibasalt rocks. Olivine crystals are observed in approximately 5 Vol% in basaltic andesitic rocks, damaged completely by the decomposition pressure with only a small piece remaining intact. In some thin sections of the dacite rocks, quartz with 3-5% modal frequency is observed with anhedral and neomorph margins in many cases. Geochemistry According to the TAS and AFM Diagram (Le Bas et al., 1986; Irvine and Baragar, 1971), as to lithology combinations, where: 1) the samples vary within dacite to andesite types in the Shahvaroogh and Sadabad area; 2) from trachy-andesite, trachy-basalt, basaltic andesite, andesite and dacite types in Feshk area; 3) from rhyolite, dacite and basaltic andesite composition in Kordabad area. Conveying the obtained results from samples analysis to A/NK to A/CNK, Ti/100-Zr-Y.3, Ti/100-Zr-Sr/2, Ti–Zr (Pearce and Can, 1973), and Nb/Th on Nb (Boztug et al., 2006) and Hf/3-Th-Nb/16 diagram (Wood, 1980; Maniar and Piccolo, 1992), based on Al value reveals that most samples have a meta-alumina nature while others possess per-aluminus affinity. According to many researchers, enrichment of LILE elements (e.g., K, Rb, Sr, Ba, Th,) and LREE and the increase in negative anomaly to great amounts of elements including Ti, Eu, Nb, and HREE (Yb) contribute to the magma contamination by the upper crust during the magmatic evolutions or the presence of intrusive compounds such as the fluids or melts obtained from intrusive sediments in addition to adaptation to the dominant characteristics on the beneath shell (Temel et al., 1998; Kurkcuoglu, 2010). According to the same researchers, in normalized spider figures, the depletion of elements from left to right of the figure indicate that the significant features of faulting zones and LREEs enrichment (between 10-100 times more) in calc-alkaline series are normal. In the Ab-An-Or diagram (Deer et al. 1992) the plagioclase in the trachybasalt and andesibasalt samples lies within the labradorite to bytownite range (only one case of anorthite), in andesitic and trachyandesitic type is oligoclase to andesine, and in dacite- rhyodacite and rhyolite samples are of albite type in Ab-An-Or chart. Based on the Wo-En-Fs ternary plot (Morimoto et al., 1988), all pyroxenes are of augite type. Conclusion The studied volcanic rocks are of basalt andesite types. According to petrographic studies and microprobe analysis, the plagioclase minerals include albite and oligoclase in dacite, and rhyodacite, and rhyolite rocks, oligoclase-andesine in the trachy-andesite and andesite rocks, bytownite-labradorite to anorthite in the trachy-basalt and andesite basalt rocks, and also clinopyroxene (augite and clinoenstatite), amphibole (hornblende and oxihornblende), quartz, biotite, apatite and opaque minerals. The geochemical and petrological results indicate that the source magma of these rocks is a combination similar to the melts derived from the enriched mantle with a fractional 10-20% melting degree of a spinel lherzolite source to garnet-spinel lherzolite. The fluids released from the subducted oceanic slab were effective in the mantle metasomatism. The appearance of the rocks in this area with the volcanism related to the subduction of Neo-Tethys oceanic crust under Central Iran microcontinent. |