پترولوژی (Oct 2022)
Whole rock and mineral chemistry of intermediate intrusive rocks in the northern side of Shahroud River near Zardkooh in Guilan province (Iran)
Abstract
IntroductionZardkooh is located in Western Alborz zone in the structural divisions of Iran. According to many published researches, most of the magmatic activities in the north and northwest of Iran are related to the Neotethys orogeny process, which are the result of the subduction of the Neotethys crust, the collision of the Arabian plate with Iran, and the tectonic regimes that occurred following the collision. For the purpose of the present study, the nature, the genesis and the tectonic setting of intermediate intrusive rocks were determined using whole rock and mineral chemistry. Incidentally, Microprobe analyses results were used to document the precise chemical composition of the rocks studied and the formation conditions of the minerals as well.Geology of the areaThe exposed rock units in the studied area mostly include the Eocene (lava and pyroclastic rocks belonging to the Karaj Formation), but locally the Paleocene (the detrital sedimentary rocks of Fajan Formation), the Permian (the detrital sediments of Dorood and the limestones of Ruteh Formations) and the Carboniferous (the limestone of Mobarak Formation) outcrops and the Oligo-Miocene intrusive bodies are also present.Most of the intrusive masses are composed of gabbroic rocks, but some of them also have intermediate compositions, which are the goal of this research. These rocks are mostly seen as dykes and relatively small stocks.Research MethodIn order to investigate the geochemical properties, five samples were sent to Zarazma company in Iran for whole rock chemical analysis. In the laboratory of Zarazma company, the amounts of oxides of the major elements were determined using the ICP-OES method. The abundance of rare earth elements and refractory elements were measured by ICP-MS method. A monzonite sample with freshest minerals was analyzed by EPMA at the Iran Mineral Processing Research Center in Karaj (I M P R C), in order to understand the chemical compositions of minerals and their application in petrogenesis.PetrographyAccording to petrographic studies, the intermediate intrusive bodies exposed in the study area are mostly microdiorite, monzonite and monzodiorite. The textures of monzonites are porphyroid and their minerals are zoned. The principal minerals are plagioclase (An17-An45) and orthoclase, and the minor minerals are mostly biotite (phlogopite to magnesium-rich biotite), amphibole (ferropargasitic hornblende to pargasitic hornblende), clinopyroxene (augite and salite) as well as opaque minerals set in a groundmass consisting of kaolinized orthoclase, some quartz and opaque minerals as well. Microdiorite has porphyroid texture and the phenocrystals include kaolinized plagioclase, biotite, hornblende and opaque minerals, and the groundmass is composed of altered plagioclase, few quartz and opaque minerals. The textures of monzodiorite are various in different samples, so that in some it is medium granular and in others is intergranular. All the samples are characterized by the presence of the abundant plagioclase (An31) and orthoclase, clinopyroxene, biotite, opaque minerals and a small amount of quartz, apatite and zircon are also occurred in some samples.Mineral ChemistryThe chemical compositions of biotites and clinopyroxenes indicate that the magmatic series of intrusive igneous rocks of the studied area is sub-alkaline and calc-alkaline type.Based on the composition of clinopyroxenes, the tectonomagmatic environment of the magma formation of the investigated rocks is volcanic arc and active continental margin type. Based on the microprobe data obtained from the biotites, the studied granitoids are classified as the ilmenite series and were derived from the mantle material or a mixture of mantle –crust.Whole Rock GeochemistryThe high potassium calc-alkaline and shoshonitic nature of the rocks under study are displayed by magmatic series determination diagrams, Moreover, the REE similar pattern point to their common origin. The strong negative anomaly of Nb, Ta, and Ti elements in the spider diagrams are of the prominent characteristics of subduction-related continental arc magmas. The positive anomaly of Pb, K and in general the enrichment of LILE elements are also attributed to crustal contamination of magmas. The geochemical data on the tectonomagmatic discrimination diagrams are placed in the realm of volcanic arc environment located on the continental crust. Their parent magma has a compositional similarity with melts derived from an enriched mantle, and according to various diagrams, it was derived from about 10-20% melting of a garnet-spinel peridotite source enriched by mantle metasomatism as a result of the addition of products derived from the subducting slab at depths of 100 to 110 km.Discussion and ConclusionThe geochemical data in combination with mineralogical study indicate that the investigated rocks belong to a subduction environment and were originated from an enriched source, but due to the great distance between the Cenozoic magmatic belt of Alborz and Urmia Dokhtar magmatic arc (and the subduction site of the Neotethys oceanic crust) a back-arc basin can be considered for the region, which was in the early stages of evolution but was completely affected by the characteristics of the arc magmas (Teimouri, 2011; Asiabanha and Foden, 2012). Like magmatic arcs, the formation of magma in these environments was also caused by the inflow of fluids resulting from the dewatering of the subducting oceanic slab and the melting of the metasomatized mantle wedge on it. During magma ascent through the continental lithosphere, the magma suffered contamination and some of its chemical properties have been dictated to the magma.
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