Journal of Economic Geology (Nov 2017)

Geochemistry and Mineral Chemistry of Zeolites Bearing Basic Volcanic Rocks from the Boumehen-Roudehen Area, East of Tehran

  • Amir Ali Tabbakh Shabani,
  • Morteza Delavari Kooshan ,
  • Mahsa Hajiabdolrahim Khabbaz

DOI
https://doi.org/10.22067/econg.v9i2.49478
Journal volume & issue
Vol. 9, no. 2
pp. 397 – 418

Abstract

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Introduction The Upper Eocene basic volcanic rocks that have cropped out in Karaj formation in the Boumehen and Roudehen area in the east of Tehran are characterized by fibrous zeolites filling their vesicles, cavities and fractures creating amygdale texture. The study area is located structurally in the Central Alborz orogenic belt. The presence of large volumes of shoshonitic magma during the Middle to Late Eocene in southern–central Alborz implies that partial melting to produce shoshsonitic melts was not a local petrological event. Thus, their ages, formation processes, and interpretations are of regional tectonic significance. In this study, we present a detailed petrography, mineral chemistry, and whole-rock geochemistry of high-K (shoshonitic) basic rocks to understand the petrogenesis and source region and to deduce the nature of the tectonomagmatic regime of the Alborz. Materials and methods In this study, we present new major and trace element data for a selection of 4 of the least altered samples by a combination of X-ray fluorescence (XRF) and ICP-OES techniques at the Zarazma Mineral Studies Company. Mineral analyses were obtained by wavelength dispersive X-ray spectrometry on polished thin sections prepared from each rock sample described above for 12 elements using a Cameca SX-50 electron microprobe at the Istituto di Geologia e Geoingegneria Ambientale, C.N.R., University La Sapienza of Rome, Italy. Typical beam operating conditions were 15 kV and probe current of 15 nA. The accuracy of the analyses is 1% for major and 10% for minor elements. A total of 24 point analyses were collected. Results and Discussion The extent of alteration in the study rocks varies from slight to severe and shows porphyritic to glomeroporphyritic textures. Pyroxenes are generally subhedral to euhedral and occur as discrete crystals as well as aggregates. Olivine may occur only as relics filled with iddingsite, chlorite and calcite. Plagioclase is subhedral to euhedral and occurs both as pheocrysts and microliths in the glassy groundmass. The plagioclase crystals are variably sassuratised and sometimes replaced by zeolites. Microprobe data indicate a restricted range of chemical composition for pyroxene falling in diopside and augite fields of ternary pyroxene classification diagram (Morimoto, 1988). The plagioclase composistions have been plotted in the fields of labradorite and bytownite in the orthoclase–albite–anorthite ternary diagram (Deer et al., 1992). On the F1-F2 tectonic discrimination diagram of Nisbet and Pearce (1977), pyroxene compositions plot mainly in volcanic arc basalt field consistent with their whole rock geochemistry. Thermobarometry based on pyroxene composition (Soesoo, 1997) displays a range of temperatures from 1150 to 1250 0C and pressure from 3 to 8 kbar for its crystallization. Whole rock compositions show that the variations of SiO2 contents are narrow (47.08 – 47.47 wt%) and TiO2 (1.1 – 1.24 wt%). Relatively higher contents of K2O show a shoshonitic affinity in the K2O–SiO2 diagram (Peccerillo and Taylor 1976). Trace element and rare earth element (REE) distribution patterns for the basaltic samples normalized to the primitive mantle (McDonough et al., 1992) and chondrite values (Sun and McDonough, 1989) show similar patterns. The samples are all enriched in large-ion lithophile elements (LILEs), such as Rb, Ba, and K, and light rare earth elements (LREEs) ((La/Sm)N= 2.3–3.2) relative to the more immobile elements (e.g., Hf, Ti and Y). The plot of analyzed samples in a series of different tectonic discrimination diagrams shows that the Boumehen-Roudehen alkaline basalts are consistent with characteristics of subduction related (active continental margins) tectonic environments. In addition, enrichment in LILE and depletion in HFSE on spidergram create patterns which are very similar with the pattern of Andean counterparts indicating an arc setting. Acknowledgments Marcello Serracino is thanked for microprobe analyses. The authors are grateful to Journal Manager and reviewers who critically reviewed the manuscript and made valuable suggestions for its improvement. References Deer, W.A., Howie, R.A. and Zussman, J., 1992. An Introduction to the Rock Forming Minerals. Longman, London, 696 pp. McDonough, W.F., Sun, S.-S., Ringwood, A.E., Jagoutz, E. and Hofmann, A.W. 1992. Potassium, Rubidium and Cesium in the Earth and Moon and the evolution of the mantle of the Earth. Geochimica et Cosmochimica Acta, 56(3): 1001-1012. Morimoto, N., Fabries, J., Ferguson, A.K., Ginzburg, I.V., Ross, M., Seifert, F.A., Zussman, J., Aoki, K. and Gottardi, G., 1988. Nomenclature of pyroxenes. American Mineralogist, 73)9-10:( 1123–1133. Nisbet, E.G. and Pearce, J.A., 1977. Clinopyroxene composition in mafic lavas from different tectonic settings. Contributions to Mineralogy and Petrology, 63)2:(149-160. Peccerillo, A. and Taylor, S.R.,1976 . Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastamonu area, Northern Turkey. Contributions to Mineralogy and Petrology, 58(1): 63-81. Soesoo, A. 1997. A multivariate statistical analysis of clinopyroxene composition: Empirical coordinates for the crystallisation PT- estimations. Journal of the Geological Society of Sweden, 119(1): 55-60. Sun, S.S. and McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts; implications for mantle composition and processes. In: A.D., Saunders and M.J., Norry (Editors), Magmatism in the ocean basins. Geological Society, London, Special Publications, 42. 313-345.

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