Petrography, Geochemistry and Petrogenesis of Volcanic Rocks, NW Ghonabad, Iran

Introduction The study area is located in NW Gonabad, Razavi Khorasan Province, northern Lut block and eastern Iran north of the Lut Block. Magmatism in NW Gonabad produced plutonic and volcanic rock associations with varying geochemical compositions. These rocks are related to the Cenozoic magmatic rocks in Iran and belong to the Lut Block volcanic–plutonic belt. In this study, petrogenesis of volcanic units in northwest Gonabad was investigated. The volcanic rocks are andesites/trachyandesites, rhyolites, dacites/ rhyodacites and pyroclastics.These rocks show porphyritic, trachytic and embayed textures in phenocrysts with plagioclase, sanidine and quartz (most notably in dacite and rhyolite), hornblende and rare biotite. The most important alteration zones are propylitic, silicification and argillic.Four kaolinite- bearing clay deposits have been located in areas affectedby hydrothermal alteration of Eocene rhyolite, dacite and rhyodacite. Analytical techniques Five samples were analyzed for major elements by wavelength dispersive X-ray fluorescence (XRF) and six samples were analyzed for trace elements using inductively coupled plasma-mass spectrometry (ICP-MS) in the Acme Laboratories, Vancouver (Canada).Sr and Nd isotopic compositions were determined for four whole-rock samples at the Laboratório de GeologiaIsotópica da Universidade de Aveiro, Portugal. Results Petrography. The rocks in this area are consist of trachyte, andesite/ trachyandesite, dacite/ rhyodacite, principally as ignimbrites and soft tuff. The textures of phenocrysts are mainly porphyritic, glomerophyric, trachytic and embayed textures in plagioclase, hornblende and biotite. The groundmasses consist of plagioclase and fine-grainedcrystals of hornblende. Plagioclase phenocrysts and microlitesare by far the most abundant textures in andesite - trachyandesites (>25% and in size from 0.01 to 0.1mm). Euhedral to subhedral hornblende phenocrysts areabundant (3-5%)and 0.1 to 0.6mmin size. Trachyte is characterized by trachytic texture. Ninety percent of the rock consists of sanidine. In trachytes, 3 to 5% hornblende ( 0.3 mm) is replaced by carbonates. Rhyolites contain quartz, plagioclase, sanidine, and biotite phenocrysts in a microcrystalline to glassy groundmass. Rhyodacitehas phenocrysts, some glomerophyric, consisting of quartz, 2 to 3% (0.1-0.5 mm), plagioclase 7 to 10% (0.2- 0.8 mm), hornblende 5% and biotite 1%. Up to 15% of sanidineis altered to clay minerals. Crystal tuff and lithic-crystal tuff are distributed overa large area. Using the Zr/TiO2 and Nb/Y diagram of Winchester and Fold (1977), samples are designated as rhyolite, dacite and sub-alkaline basalt. In the Co vs. Th diagram of Hastie et al. (2007), samples plot in the shoshonitic and high calc-alkaline, rhyolite, dacite and andesite-basalt fields. The REE patterns and trace element contents of the volcanic samples show: (1) LREE/HREE enrichment ((La/Yb) N = 0.3 to 15.27), (2) Low negative Eu anomaly (ave.Eu*/Eu=0.2-0.85), (3) depletion in Ba, Sr, K2O, Zr and Ti (Lower continental crust-normalized spider diagram from Taylor and McLennan, 1985 and Chondrite-normalized diagram from Nakamura, 1974. Rhyolites show the most extreme negative Eu anomaly (Eu/Eu* = 0.2-0.3) compared with 0.65–0.85 for volcanic elsewhere and also show considerably differences in the contents of Rb,Sr,K,Ti,Zr,Hf,Ce. These differences are related to greater magmatic differentiation or derivation from the other sources. The Sr and Nd isotopic ratios of these volcanic rocks are: 87Sr/86Sr = 0.70699 to 0.71014 and 143Nd/144Nd =0.512144 to 0.512539. Assuming an age of 60 Ma, the initial 87Sr/86Sr ratios vary from 0.70671 to 0.71066 and initial 143Nd/144Nd values vary from 0.512098 0.51249 (εNdi = -9.1) to 0.51249 (εNdi = -1.4).In the εNdi versus (87Sr/86Sr)i diagram, the samples plot in the field typical of magmas that are of crustal origin or, at least, that underwent important processes of crustal assimilation/contamination. Andesitic rocks displays lightly lower rangesof87Sr/86Sr (0.7067-0.7068) and εNdi values from -1.44 to -2.34, than rhyolite. Distinct Sr and Nd isotopic compositions are seen between rhyolitic rocks and andesitic rocks. The geochemical data suggest that the rhyolitic magmas probably represent the final differentiates of parental magmas, resulting from partial melting of mafic lower crust. Generally, the magmas from this area have low Sr (less than 400 ppm), high K2O/Na2O and negative Eu anomalies. References Hastie, A.R., Kerr, A.C., Pearce, J.A. and Mitchell, S.F., 2007. Classification of altered volcanic island arc rocks using immobile trace elements: development of the Th-Co discrimination diagram. Journal of Petrology, 48(12): 2341- 2357. Nakamura, N., 1974. Determination of REE, Ba, Fe, Mg, Na, and K in carbonaceous and ordinary chondrites. Geochim, Cosmochim, Acta, 38(5): 757–775. Taylor, S.R. and McLennan, S.M., 1985. The continental crust, its composition and evolution, an examination of the geochemical record preserved in sedimentary rocks. Blackwell, Oxford, 312 pp. Winchester, J.A. and Floyd, P.A., 1977. Geochemical discrimination of different magma series and their differentiation products using immobile elements. Chemical Geology, 20(4): 325-343.

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