Journal of Economic Geology (Aug 2024)
Petrogenesis of melanite-garnets in monzodiorites from SW of Jandaq (NW of Central-East Iranian Microcontinent)
Abstract
The Godar-e-Siah Eocene monzodiorite stock is located in the southwest of the Jandaq area (NE of Isfahan) and northwest of the Central-East Iranian Microcontinent (CEIM). The main minerals in these monzodiorites are plagioclase, K-feldspar, clinopyroxene, phlogopite, and garnet, which are set in a fine-grained groundmass of feldspars. The main textures are granular, porphyritic, and poikilitic. In some cases, these rocks contain euhedral to subhedral garnet crystals with inclusions of the igneous clinopyroxene and groundmass minerals including feldspar and graphite. These garnets exhibit Ti-andradite and Ca-melanite composition from the solid solution series of andradite-grossular. The chemical zoning patterns of the studied garnets confirm that these garnets have a non-magmatic origin and metamorphic nature. In the investigated monzodiorites, the presence of euhedral garnet crystals with inclusions of igneous clinopyroxenes metasomatic scapolite, and metasomatic phlogopite shows that these garnets are of metasomatic origin, which formed due to the alteration of igneous clinopyroxenes. All geochemical and petrographic evidences from the studied garnets indicate that they have formed as a result of the intrusion of Eocene monzodiotites into the carboniferous limestones (or dolomites), leading to the creation of endoskarn or reactions skarn that can be distinguished at the millimetric scale in microscopic studies. Introduction Garnet is a general mineral forms in metamorphic rocks derived from the sedimentary and igneous protoliths and at all metamorphic grades above the greenschist facies (Baxter et al., 2017). However, the presence of garnet in certain types of igneous rocks, such as peraluminous granite and ultramafic rocks in the upper mantle, introduces complexities in unraveling the petrogenesis of garnets in igneous and metamorphic rocks (Rong et al., 2018). Titanium-rich garnets are enriched in andradite, occur in various types of rocks, including a variety of igneous rocks, encompassing trachytes and phonolites (Dingwell and Brearley 1985), syenites and carbonatites, nephelinites and tephrites (Gwalani et al., 2000), as well as ultramafic lamprophyres, rodingites (Schmitt et al., 2019); high temperature metamorphic rocks and skarns. The composition of titaniferous garnets besides their paragenetic relationships is one of the significant petrology factors (Chakhmouradian and McCammon, 2005). The study area is situated in the northwestern part of the CEIM (northwestern part of the Yazd block), and southwest of the Jandaq City. The rock units of the Jandaq area are mainly include Paleozoic metamorphic rocks, Upper Paleozoic sedimentary rocks, Cretaceous and Paleocene sediments, Eocene intrusive rocks, Eocene subvolcanic (dikes) and volcanic rocks (Jamshidzaei et al., 2021), the Pis-Kuh upper Eocene sedimentary rocks (flysch), and Early Oligocene lamprophyric rocks, and alkali basalts (Torabi, 2010; Berra et al., 2017; Sargazi et al., 2019; Jamshidzaei et al., 2021). In this paper, the chemical characteristics of the monzodiorite stock and origin of garnet mineral are discussed. Material and methods To determine the chemical compositions of minerals, JEOL JXA-8800 WDS at the Department of Earth Science, Kanazawa University, Kanazawa, Japan was used. Chemical analyses of minerals was performed under an accelerating voltage of 20 kV, a probe current of 20 nA, and a focused beam diameter of 3μm. The ZAF program was used for data correction. Natural minerals and synthetic materials with well-characterized compositions serve as standards for calibration and validation purposes. The Fe2+# and Mg# parameters of minerals are represented by the atomic ratios of Fe2+/(Fe2++Mg) and Mg/(Mg+Fe2+), respectively. To recalculate the FeO and Fe2O3 concentrations from Fe2O3*, recommended ratios of Middlemost (1989) is used. The mineral abbreviations used in this context are derived from Whitney and Evans (2010). Results and discussion The monzodiorites of the Godar-e-Siah area mostly show fine to coarse-grained granular, porphyritic and poikilitic textures. These rocks are mesocrate in color, displaying massive and mineralogically homogeneous nature in their outcrops. K-feldspars are the primary minerals and garnet mineral is imposed on these rocks. The main minerals of this monzodiorite stock are plagioclase, K-feldspar, clinopyroxene, phlogopite, and garnet, set in a fine-grained matrix of feldspars. These rocks have mainly granular, porphyritic, and poikilitic textures. In some cases, these rocks contain euhedral to subhedral garnet crystals with inclusions of igneous clinopyroxene and groundmass minerals including feldspar and graphite. These garnets have a composition of Ti-garnet and Ca-melanite from the solid solution series of andradite-grossular. Based on the EPMA data, the clinopyroxenes show diopside to hedenbergite compositions, indicating that two types of clinopyroxene are in these rocks. The first group of this mineral contain MgO (6.9-10.20 wt.%), FeO*(11-16.88 wt.%), Al2O3 (2-5.84), and Na2O (1.1-1.9 wt.%), is reactive pyroxenes. The second category contains MgO (9.98-12.89 wt.%), FeO* (9.13-13.87 wt.%), Al2O3 (1.82-3.11 wt.%), and Na2O (0.7-1.42% Wt.%), is igneous pyroxenes. Chemistry of the pyroxenes reveals that reactive pyroxenes have higher concentrations of FeO* and Al2O3 than igneous pyroxenes. Chemistry of the feldspars indicates that the K-feldspars is orthoclase in composition. Also, chemical analyses of mica show that these minerals contain high concentrations of MgO (21.54-22.60 wt.%) and low values of Al2O3 (12.89-13.30 wt.%). The mica from the studied rocks of the Jandaq area plots in the phlogopite field. The garnet grains in these rocks contain 61.94-66.39 mol.% almandine (Fe2Al2Si3O12), 18.60-23.40 mol.% grossular (Ca3Al2Si3O12), 10.06-15.11 mol.% pyrope (Mg2Al2Si3O12), and 1.09-4.32 mol.% spessartine (Mn2Al2Si3O12). These garnets have a composition of Ti-garnet and Ca-melanite from the solid solution series of andradite-grossular. The chemical zoning patterns of the studied garnets confirm that these garnets have a non-magmatic origin and metamorphic nature. The presence of discontinuous chemical zoning and the pattern of variations in the end-member compositions of these garnets indicate that they were formed under disequilibrium conditions accompanied by changes in the environmental oxidation conditions. In the studied monzodiorites, the presence of euhedral garnet crystals with inclusions of igneous clinopyroxenes, metasomatic scapolite, and metasomatic phlogopite shows that these garnets are of metasomatic origin which formed due to the alteration of igneous clinopyroxenes. The geochemical characteristics and petrographic evidences from the studied garnets; including the presence of euhedral crystals with distinct boundaries to contact minerals, the occurrence of inclusions of background minerals and igneous clinopyroxenes in the garnets, as well as the presence of discontinuous chemical zoning, confirms that they have formed as a result of the intrusion of Eocene monzodiotites into the carboniferous limestones (or dolomites), leading to the creation of endoskarn or reactions skarn.
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