پترولوژی (Oct 2022)
Investigating the tectonomagmatic origin of tourmaline in the felsic dikes of the Deh-Salm Metamorphic Complex, East of Lut block
Abstract
IntroductionGranites are among the most abundant rocks associated with orogenic regions that have a wide range of compositions (Atherton and Tarney, 1979; Pitcher, 1993). Among the main environments related to orogeny are continental arcs and subduction zones (Brown et al., 1995). Knowing the composition and the nature of granites helps to understand the evolution stages of the continental crust (Barbarin, 1999).Among the methods of knowing the nature, the composition, and the petrogenetic origin of granites is the precise determination of the chemical composition of their constituent minerals. The chemical composition of minerals indicates the temperature, the pressure, and the nature of granite at the time of emplacement (Kaygusuz et al., 2008).Sometimes it may not be possible to accurately determine the nature and composition of the stone with regard to the main minerals due to the influence of alteration and some other processes that may have caused changes in their values, so it is possible to determine the conditions of stone formation by relying on the composition of some secondary minerals such as tourmaline Restored the original environment. Since tourmaline is formed in a wide range of geochemical conditions, it can contain petrogenetic information from two different metamorphic and magmatic conditions (Henry and Guidotti, 1985; Henry and Dutrow, 1996).Regional GeologyOn the geological maps of the region, the distribution of rocks in the Deh-Salm metamorphic complex is limited to the lower and upper parts (Naderi Mighan and Akram, 2005; Akrami and Naderi Mighan, 2005; Hamzehpour, 2005), which is probably equivalent to the formations Nayband and Shemshak Formations belonging to the late Triassic-Jurassic (Stöcklin et al., 1972).New dating results have introduced the age of the Deh-sal-m metamorphic complex as 168-163 million years based on the U/Pb ratio on single grains of zircon, monazite, and xenotime (Mahmoodi et al., 2009). Such age is consistent with the published data of Shah-Kuh granite by the K-Ar method of 158-168 Ma (Esmaeily et al., 2005). The small age range related to the cooling of the Shah-Kuh granite and its marginal metamorphic complex has been attributed to rapid tectonic changes in a magmatic back-arc environment (Mahmoodi et al., 2009). From west to east, five tectonic and lithological units, from A to E, are recognized in the Deh-Salm metamorphic complex. Also, five metamorphic phases, from D1 to D5, have been identified in the region, subjected to the existing tectonic units with different intensities and quality (Arefnejad, 2010; Bahramnejad, 2015).Materials and MethodsFor mineralogical and petrological studies, 50 thin sections of the dykes were prepared and studied by a polarizing microscope. To determine the chemical composition of the tourmalines in the studied dykes, chemical analysis was carried out by electron microscanning on the main elements in the tourmalines in selected samples by the JOEL-JXA-8600M automatic superprobe with an accelerating voltage of 15KV took place at the Department of Earth and Environmental Sciences, Yamagata University, Japan.Petrography and Mineral ChemistryBased on the microscopic observations, in the dykes, quartz with about 35 to 40 volume percent has direct extinction. Euhedral and subhedral orthoclase is 25 to 30 volume percent. Some samples contain coarse-grained and subhedral plagioclase (about 20 volume percent). Tourmaline and muscovite with 10 to 15 volume percent of the rock are secondary minerals. Muscovites are usually flaky minerals with a bunch of parallel faces. Zircon occurs as an inclusion in orthoclase. The main texture of the studied rock is granular.Tourmalines are euhedral. In the longitudinal thin section, they are elongated crystals, and in the transverse sections, they show tetra- and hexagonal shapes and inverse blue-to-pink pleochroism. The tourmalines of dykes with high Fe/(Fe+Mg) ratio are schorle in composition and they are classified as alkaline type because of their high Na2O content.Discussion and ConclusionThe deformation phases caused felsic dykes injection in the Deh-Salm metamorphic complex, especially in the migmatites which are widespread in the eastern parts of the Deh-Salm metamorphic complex. The dykes are characterized by the presence of quartz, plagioclase, orthoclase, muscovite as well as tourmaline. The high Fe/(Fe+Mg) ratio, the absence of chemical zoning, and the euhedral shapes are evidence of the magmatic origin of tourmalines.Based on tourmalines composition, the dykes in the Li-poor granites resulted from a late magmatic event concurrent with the extensional tectonics.Several deformation events of D1, D2, and D3 are recognized in this area. These dykes formed during an extensional deformation event of D3 that caused them to crosscut the major schistosity of the metamorphic rocks. Therefore, the dikes occurred after the migmatization and probably simultaneously with the uplift of the Deh Salam metamorphic complex.AcknowledgmentsWe sincerely appreciate Prof. Nakashima Kazuo at the Department of Earth and Environmental Sciences, Yamagata University, Japan, for conducting chemical analyses
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