پترولوژی (Jun 2024)
Petrology and mineral chemistry of the Hamaneh granitoid, West of Yazd (Central Iran zone)
Abstract
IntroductionMineral chemistry has a significant role in determining the stages of magma evolution. Amphiboles, Biotite, plagioclase, and chlorite are minerals whose chemical composition show the temperature and pressure conditions of magma. These minerals are often used for the petrogenesis of granitoid bodies and record the characteristics and geochemical features as well as tectonic conditions of the producing magma (Abdel-Rahman, 1994; Nachit et al., 2005). The granitoid bodies of west Yazd have wide outcrops with different compositions. These rocks are part of the volcano-plutonic belt of Central Iran (Aftabi and Atapour, 2000) with calc-alkaline nature, meta luminous, and mostly of I-type (Hassanzade et al., 2002). The Hamaneh monzogranite bodies, with light color and dark enclaves, are located in 45 km at E53°48ˊ to E53°58ˊ longitude and N31°50ˊ to N31°56ˊ latitude. This area is part of the Shirkuh Mountain in Yazd with a northeast-southwest trend. Hamaneh monzogranite belongs to the Oligocene magmatic activity. In the present research, the mineral chemistry of amphibole, biotite, plagioclase, and chlorite has been investigated to determine the origin and stages of formation and evolution of the magma.Regional GeologyThe studied area geologically located in the middle part of the Urmia-Dokhtar magmatic zonewith a wide collection of Cenozoic magmatic rocks and is characterized by a length of about 2000 km, and a width of approximately 50 to 150 km, parallel to the metamorphosed zones of Sanandaj-Sirjan and Zagros, has a wide collection of Cenozoic magmatic rocks. This area has witnessed extensive magmatic activity in the Cenozoic, especially in the Eocene, which during the Oligocene-Miocene hosted numerous intrusive masses with different ages and compositions (Hassanzadeh et al., 2002). The oldest formation in the region is the green siltstone shale and sandstone of the Kahar Formation. The dolomitic rocks of the Soltanieh Formation are unconformably on the Kaher Formation with purple and cream-colored sand shale layers. The Permian sediments (Jamal Formation) composed of limestone and dolomite layers and the Mesozoic sediments started with a dark unit of volcanic laterite rocks with the Lower Triassic followed by dolomitic rocks belonging to the Otri Formation. The shale and sandstone of the Shemshak Formation are upper Triassic sediments. Cretaceous sediments are exposed in the form of destructive rocks. Upper Triassic (Sangestan Formation) has a variety of conglomerate and conglomerate sandstones with round to semi-round pieces of sandstone and Shirkuh granite. The Paleogene begins with the Kerman Formation conglomerate. Then, there are alluvial cones, alluvial plains, and alluviums of young rivers.Analytical methodsFor the lithology and chemistry of minerals in Hamaneh monzogranite, 20 thin-section samples were selected for chemical analysis of amphibole, plagioclase, biotite, and chlorite minerals and sent to Oklahoma City University laboratory. Microprobe analyses were performed with the Cameca model SX100 device with an accelerating voltage of 15 KV and current intensity of 15 nA. The results of chemical data of amphibole, biotite, feldspar, and chlorite minerals are given in Tables 1 to 4.Petrography and Minerals ChemistryIt is often fine to medium grain texture, granophyric and perthitic texture. The main minerals are orthoclase, plagioclase, and quartz. Orthoclase with an abundance of 25 to 32% is amorphous to semi-amorphous, and plagioclase is a frequency of 24 to 44% that was often as shaped to semi-shaped with zoning. The quartzes are often amorphous and intergranular, and some have wave extinction and fractures with a frequency of 19 to 31. The minor minerals were amphibole, biotite, sphene, zircon, and opaque minerals. Amphibole is green with an abundance of 2.5 to 4.5%, shaped to semi-shaped with a simple twinning. Biotite crystals with an abundance of 2 to 4%. Mineral chemistry of amphiboles was shown as calcic magmatic amphiboles, ranging from magneiso-hornblende and actinolite. Which formed at 530-890℃ and up to 4.3 kbars. That was shown depth 3 to 5.40 km and fO2 (0.5). <Fe#, which corresponds to the calc-alkaline nature in a subduction environment (Anderson, 1996; Rieder et al., 1998). The Hamaneh amphiboles were formed in active continental margins related to subduction. Mineral chemistry of biotites point to a primary biotites that are magmatic and emplaced between annite and phlogopite in the Mg-biotite range. The Hamaneh biotites are I-type indicating the tectonic setting of the calc-alkaline granitoid magmatic series in the subduction zone. Biotites under study were formed at 650-730°C and a pressure of 10-11 to 10-14 kbar. Also, feldspar are (An21 to An32) (Or45 to Or59) orthoclase. The feldspars are of the oligoclase-andesine type, as well as in the feldspar thermometry diagram (Elkins and Grove, 1990; Deer et al., 1991) that was formed in 700 to 800ºC. Based on mineral chemistry, the chlorites are ripidolite and pycnochlorite in composition. Regarding the relatively high iron amount, they have formed at 330 and 360 ºC by alteration of biotite and amphiboles.Discussion and Conclusion The Hamaneh monzogranite, with the main minerals of quartz, orthoclase, plagioclase, amphibole, biotite and secondary minerals of sphene, zircon, and apatite lies in the west of Yazd. The main textures are medium-fine-grained, granophyric, and myrmekite. This body is calc-alkaline and type I. Based on mineral chemistry data, the crystallization of minerals was according to the Bowen series. The calcic amphiboles of magnesio-hornblende to actinolite nature crystallized at 890-530 ℃. Simultaneously with this mineral, oligoclase-type plagioclase to andesine crystallized at 700-800 ℃ and magnesium biotites at 730-650 ℃. Finally, due to secondary alteration, chlorite was formed from the biotite and amphibole developed at a temperature of 330 to 360 ℃ All these minerals point to the mantle nature of the parent magma produced these mineral and have suffered crustal contamination during the ascent. This magma originated at a depth of 3 to 5.5 kilometers, pressure of 0.5 to 4.5 kbars, temperature of 530 to 890 ℃ and at oxidizing conditions. The parent magma of the rocks under study belongs to the subduction structural ground position related to the active continental margin.
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