Petrography, mineral chemistry and thermobarometry of the Sinavand intrusive body and its enclaves (NE-Sonqor -Kermanshah)

Document Type : Original Article

Authors

Department of Geology, Faculty of Science, Bu-Ali Sina University, Hamedan, Iran

Abstract

Introduction
Enclaves provide valuable information for geologists, which includes the location of magmas, the nature of the host rock and the relative age of the masses, the dynamics of the magma chamber and cooling plutons, and various reactions inside the magma. The studied mass body (Sinavand Mountain) is located in the northeast of the Sonqor and in the Sanandaj-Sirjan zone with a relative age of Upper Eocene to Lower Oligocene. The purpose of this research is to investigate the field relationships and microscopic studies as well as the chemistry of amphibole crystals of the host rock and mafic microgranular enclaves (MMEs), for calculating of temperature and pressure of both the enclave and the host. The studied area consists of granodiorite-granite and diorite. The diorite rocks are exposed in the form of small and large enclaves with sharp edges and variable sizes. These enclaves are mostly angular, oval and shapeless with the contact margin often curved.
 
Materials and Methods
This research is based on studies of geological maps, field relationships, thin sections and geochemical data. Plagioclase and amphibole minerals have been analyzed in Iran Mineral Processing Research Center (Karaj) using EMP analysis, in the time 15-20 seconds and a current of 20 Na.
 
Results and Discussion
Petrography
Host rocks: Granodiorite rocks are characterized by high abundance, medium grain and granular texture, and granites are medium to coarse grained and have granular texture. The host rock is composed of plagioclase (30-42%) with zoning and orthoclase (18-25%) and quartz (23-30%) and some secondary minerals of amphibole, titanite, biotite, apatite, zircon and metal oxides. Myrmikite, granophyric, poikilitic and perthitic textures are also observed. Enclaves: The composition of diorite enclave (up to quartz diorite) includes plagioclase (50-60%), amphibole (15-20%), potassium feldspar (less than 5%), quartz (3-15%) and secondary minerals apatite, sphene and Metal minerals.
The texture is microgranular and sometimes there is chemical zoning that indicates chemical imbalance in the environment of their formation.
Chemistry of crystals
Amphibole: It is present in almost all compositions of host and enclave rocks, and 4 samples from each (8 samples in total) were selected and analyzed. According to the diagram of Si against Ca+Na+K, amphiboles in host rocks have igneous nature. Based on the charts of Lake et al., the amphiboles of the host rock are magnesio-hornblende and hastingsite, and in the enclaves, they are magnesio-hornblende and edenite. Plagioclase: The composition of plagioclase in granite is albite with a variation of anorthite content of 4 to 5% and albite granodiorite to oligoclase, but in diorite enclaves from oligoclase to labradorite and anorthite content is 20 to 55%. In both enclave and host, the composition of plagioclase is zoned and it will reflect the change caused by magmatic mixing/mixing. The dissolution reaction of more calcic plagioclase with a more sodic lava forms plagioclase that has less anorthite than the main types (i.e. core composition), but may be more calcic than its coexisting magmatic liquid. The significant textural and compositional imbalance between the calcic core and its sodic cover strongly indicates magmatic mixing and, of course, the durability and preservation of the signs requires rapid crystallization and incomplete mixing. The average pressure calculated for granite and granodiorite host rocks is 4.23 and 1.31 kbars, respectively, and the average pressure at the time of host mass replacement is 2.77 kbars. But the enclaves have a pressure of 2.46 kbars. Temperature has an effect on pressure calculation, and at high temperatures, the increase of tetrahedral aluminum in hornblende increases, and as a result, it increases the total aluminum and increases the pressure of mineral crystallization and magma replacement, which is true for enclaves.
 
Conclusion
The macroscopic and microscopic characteristics of these enclaves, including grain size, morphology, presence of contact surface and different shapes of enclaves, the presence of elongated, shaped plagioclase crystals, needle apatite crystals and finally microgranular and poikilitic textures, indicate that these enclaves are the result of rapid cooling and mingling/ mixing of felsic and mafic magmas.

Keywords

Main Subjects

References

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Researches in Earth Sciences
Volume 15, Issue 3 - Serial Number 59
September 2024
Pages 18-38

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