پترولوژی (Oct 2022)
Geometric properties, petrography, geochemistry and tectonic setting of columnar andesites of Goorid quarry rubble mine, west of Sarbisheh city (South Khorasan)
Abstract
IntroductionIn the Goorid quarry rubble mine 5km west of Sarbisheh city (Southern Khorasan) outcrops of andesitic lavas with columnar structures exist and from the view of geological subdivisions, located in the eastern part of the Lut block. The magmatic activity in the Lut block began in the middle Jurassic (165-162 Ma) and reached its peak in the Tertiary. Volcanic and subvolcanic rocks of the Tertiary age cover over half of the Lut block with up to 2000 m thickness and formed due to subduction before the collision of the Arabian and Asian plates (Camp and Griffis, 1982). In the northern parts of the Lut block (eastern Iran), andesitic volcanic rocks along with dacite and rhyodacite have erupted in the interval of 50 million years from the late Cretaceous to the lower Neogene (Jung et al., 1983). In the northwestern and the western parts of Sarbisheh, the outcrops of Tertiary volcanic rocks (Eocene-Oligocene to Pliocene) with basic, intermediate, and acidic compositions along with pyroclastic deposits are observed that andesitic lavas are widespread displaying columnar structure around the village of the Goorid, shows. In this research, the geometrical characteristics, origin, and tectonic setting of the columnar lavas of the Goorid mine have been investigated.Regional GeologyThe studied area is located in the southern part of Sarbisheh 1:100000 geological map. The major rock units in the studied area include Tertiary volcanic and pyroclastic rocks. The oldest rock unit of the area under study consists of turbidite deposits consisting of alternating dark green to gray shales and brown sandstone belonging to the Paleocene-Eocene, which are observed in the west and the southwest of Goorid-e-Paein village. Dark-colored units with basaltic andesite and pyroxene andesite compositions cover a large part of the studied area. These rocks are exposed in the form of single and connected hills around the village of Goorid, which shows a unique columnar structure on the western margin of the village.Materials and MethodsThis research is based on field studies, sampling of rock units, measurement of geometric features of columns, the study of thin sections, and the results of chemical analyses of 11 rock samples. Some 11 samples of volcanic rocks from the Goorid quarry rubble mine were selected and analyzed in Zarazma company, Tehran, Iran by alkaline melting method for major elements (code AF-01) and ICP-MS (for rare and rare earth elements) (code MMS-01).Petrography and GeochemistryBased on petrographic studies, the volcanic rocks of Goorid area have andesitic (pyroxene andesite) composition. Lava-forming minerals including abundant plagioclase, pyroxene, and sometimes opaque minerals. The main texture of these rocks is porphyry with vitreous microlitic groundmass, glomeroporphyry and poikilitic. Zoning, sieve texture, and embayment in plagioclases are signs of disequilibrium conditions during magma crystallization. Investigation of geometric properties of columnar jointing in the Goorid mine shows that columns are a mixture of 4, 5, 6, and 7-side polygons and belong to the irregular group. Based on measurements in three separate blocks of Goorid mine andesitic columns (each block with an area of about one square meter), the average hexagonality index is 0.83. Geochemically, these rocks have an andesite composition with high-K calc-alkaline nature point to subduction zones. Total REE in the Goorid quarry rubble mine columnar lavas show a range from 185.36 to -204.23ppm and a uniform pattern with LREE enrichment relative to HREE similar to calc-alkaline rocks with (La/Yb)N=8.26-9.76, (Ce/Yb)N=6.76-7.85 and weak negative Eu anomalies (average: Eu/Eu*=0.89). Enrichment in LREE relative to HREE with enrichment in LILE (except Ba) and depletion in HFSE (Nb, Ti, P) in studied lavas suggests active continental margin volcanic arc magmatism.Discussion and ResultsA thorough understanding of the occurrence of volcanic rocks in continental orogenic belts, as well as of their origin and source material, is an important component of studies on continental dynamics. Such rocks provide a window into the mantle and aid our understanding of the formative mechanisms of heat–stress anomalies, crust-mantle interaction, material exchange, and geological processes in the deep earth (Liu et al., 2012). Based on the different tectonic environment discriminant diagrams, Goorid andesitic lavas are located in the active continental margin and mantle-enriched areas. Geochemical characteristics of Goorid volcanic rocks, such as LILE and LREE enrichment and depletion of HFSE (negative anomaly of Nb, Ti, and P) and HREE ((Tb/Yb)N=1.40-1.88), indicating the absence or the presence of a small amount of garnet at the origin. The low Dy/Yb ratio (1.78 to 2.32) in these rocks confirms the spinel lherzolitic source. Using the (La/Yb)N (LREE/HREE) and (Gd/Yb)N (MREE/HREE) ratios it was also found that the samples are plotted in the spinel stability field and adjacent to the spinel lherzolite melting curve with about 90% spinel (Açlan et al., 2020).
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