Journal of Economic Geology (Jun 2022)
Petrology and geochemistry of Eocene volcanic rocks from southeast of Khur (Isfahan province, Central Iran)
Abstract
Introduction Subduction-related magmas are characterized by enrichment of large ion lithophile elements (LILEs), light rare earth elements (LREEs) and depletion in high field strength elements (HFSEs) (Harangi et al., 2007). These geochemical signatures of magmatic rocks are commonly explained by the addition of hydrous fluids from subducting oceanic lithosphere combined with the flux of melts from subducted sediments to the mantle wedge, lowering the mantle solidus and leading to magma generation (Aydınçakır, 2016). Asthenospheric mantle, subcontinental lithospheric mantle and/or lower crust may be the principal source of these rocks (Eyuboglu et al., 2018). In addition, magma differentiation processes, such as fractional crystallization, crustal contamination, and magma mixing may also play an important role in the genesis of these rocks. This research study presents new petrological and geochemical data from the volcanic rocks with NW–SE trending, which are situated in the northwestern margin of the Central –East Iranian Microcontinent (CEIM) (south-east of Khur, Isfahan Province) which have been formed during the peak activity of Eocene. Study of this typical small volume subduction- related magmatism will be useful in understanding the origin and geological evolution of the Central Iran in Cenozoic. Analytical Methods The petrographic investigations on Eocene volcanic rocks from the SE of Khur area were carried out with an optical microscope (Olympus-BH2) in the petrology Laboratory of the University of Isfahan, Iran. Major and trace element concentrations of samples from whole- rocks were obtained by a combination of inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) at the Als Chemex Laboratory of Ireland. The chemical compositions of 4 samples (B865, B866, B867, and B868) were determined by Neutron Activation Analysis (NAA) in the Isfahan Activation Center. The detection limit was 0.01% for all major element oxides and 0.01 ppm for rare earth elements. Mineral abbreviations were adopted from Whitney and Evans (2010). Results and Discussion Eocene volcanic rocks with trachy-basalt and trachy-basaltic andesite composition are exposed in the northwestern part of the Central-East Iranian Microcontinent (CEIM) (SE of Khur, Isfahan Province, Central Iran). These rocks which have a dominant northwest-southeast trend crosscut the Cretaceous sedimentary rocks. Petrography and mineral chemistry analyses indicate that the predominant rock-forming minerals of volcanic rocks are olivine, plagioclase, clinopyroxene and orthopyroxene. Phenocrysts set in a fine to medium grained matrix of the same minerals plus sanidine with minor amounts of opaque minerals. Secondary minerals are chlorite and calcite. The most common textures of these rocks are porphyritic, microlitic porphyritic, poikiolitic and glomeroporphyritic. Geochemical analyses of whole rock samples show that these rocks have been enriched in alkalies and large ion lithophile elements (Cs, K, Rb, Sr, Ba,), and have been depleted in high field strength elements (HFSE) (Ta, Nb, Ti). All samples indicate moderate to high fractionation in LREE patterns. These geochemical signatures point out to the subduction-related calc-alkaline nature of these rocks and their similarity to volcanic rocks of continental arcs or convergent margins (Yu et al., 2017). Pb enrichment and low values of Nb/La, Nb/U and Ce/Pb ratios reveal that crustal contamination has played an important role in magma evolution (Srivastava and Singh, 2004; Furman, 2007). The large volume of hydrous fluids coming from the subducted slab rather than sediments have caused enrichment and metasomatism of the subcontinental lithospheric mantle source. The geochemical characteristics of the studied rocks suggest that the parental magma have been derived from partial melting of a metasomatized spinel lherzolite of lithospheric mantle, which was previously modified by dehydration of a subducting slab. The tectonic environment, in which these rocks were formed has probably been a volcanic arc. Subduction of oceanic crust around the Central-East Iranian Microcontinent (CEIM) is the most reasonable mechanism which can be used to explain enrichment in volatiles of the mantle, and the calc-alkaline magmatism of the study area in Eocene times. Acknowledgments The authors thank the University of Isfahan for financial supports.
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