| چکیده انگلیسی مقاله |
Introduction
The Marzroud- Nabijan intrusive rocks are located in the Alborz-Azarbaijan zone, NW Iran. The rocks intruded the Cretaceous volcanic and sedimentary rocks. The studies conducted in the study area were in the form of a master's thesis, and the study of mineral chemistry was not conducted.
Materials and methods
Folloing the field studies, and sampling of intrusions of Marzroud and Nabijan, four fresh (non-altered) samples were selected for Electron-probe microanalysis (EPMA) carried out at the Carleton University, Canada. The EPMA was performed using a Wavelength Dispersive X-ray (WDX)microprobe camera with an accelerating voltage of 20 kV and a beam current of 20 nA to determine the major elements in the minerals for thermal and pressure studies. The results of these analyses are presented in Tables 1 to 4. The obtained data were evaluated and analyzed using Excel software.
Results and discussions
EPMA results of clinopyroxene from the study area are represented in Table 1. The clinopyroxenes in the studied samples fall into the iron-magnesium-calcium pyroxenes Quad field (Morimoto et al., 1988). The compositions for the clinopyroxene are demonstrated in terms of mole fraction of enstatite, ferrosilite, and wollastonite (Morimoto et al., 1988). In this diagram, the resulting data yielded chiefly diopside. In the diagram of Na+Al(IV) versus Al(IV)+2Ti+Cr, all analyzed pyroxenes exhibit a high oxygen fugacity range (Schweitzer et al., 1979).
Magma series is subdivided into sub-alkaline, alkaline, and peralkaline, using Al2O3 and SiO2 values in the chemical composition of pyroxenes (Le Bas, 1962). The samples of the study area fall within the subalkaline range. In the diagram of Ca vs. Ti+Cr, pyroxenes, the samples were plotted in the volcanic arcs (Dorais, 1990). In the Si versus Al diagram, all samples under study lie above the saturation line of the tetrahedral position (Si+Al=2). This suggests that the tetrahedral sites in the clinopyroxene structure are completely occupied by Al and Si cations present in the pyroxene composition, and Ti cannot enter the pyroxene structure.
One of the ferromagnesian minerals accompanying the felsic and intermediate rocks is biotite. Based on the EPMA results, as seen in Figure A-6, the biotites in the gabbro-diorite of Marzrud belong to the primary biotite type. The composition of biotites in the gabbro-diorite of Marzrud is the range of biotite based on the Fe / Fe + Mg > 0.33. The magma series responsible for the formation of biotites are plotted within the calc-alkaline range in both study areas.
EPMA results of amphibole from the Marzroud and Nabijan are represented in Figure 7-B. It shows amphiboles are actinolitic hornblende in Marzroud monzogranitic body, actinolite in Nabijan granodioritic body, and tschermakitic hornblende in Nabijan gabbroic body. In the diagram of Al versus Fe/Fe+Mg, all analyzed amphiboles exhibit a high oxygen fugacity range (Helmy et al., 2004) indicating the intrusive masses of the region were formed concerning convergent plate boundaries (Anderson and Smith, 1995). Amphiboles belongs to the calcic type based on NaB<0.5, Ca>1.3 and (Ca+Na)B≥1 (Leake et al., 1997) pointing to the I – type granitoids nature. This is because type I granitoids contain a high content of CaO, leading to the crystallization of hornblende. All analyzed amphiboles are situated in the subalkaline range (Molina et al., 2009). In the Ti versus Al diagram, all amphiboles contain less than 0.5 cation Ti in their chemical formula. In the Ti versus Al diagram, all amphiboles contain less than 0.5 cation Ti in their chemical formula. The analyzed amphiboles from the Nabijan show that the amount of Ti and AlIV is higher in gabbro amphiboles than in granodiorite amphiboles. The number of cations of Ti and AlIV has a direct relationship with the rise in temperature in the formation time of minerals, also with the increase of Si in the crystallization system amount of AlIV in the amphibole structure decreases. In the Mg+Fe versus AlIV diagram, the analyzed samples exhibit a negative trend, indicating chemically controlled substitution.
The feldspar composition is andesine - labradorite in Marzroud gabbrodioritic rock, andesine and orthoclase in Marzroud monzogranitic rock. The feldspar is andesine and oligoclase in Nabijan granodiorite and labradorite to bytonite in the Nabijan gabbroic body.
The calculated emplacement pressure for the intrusive masses at the study areas, using the Al(total)value in the amphibole lattice, is approximately 0.8 kbar for Nabijan granodiorite and 0.5 kbar for Marzroud monzogranite (Schmidt, 1992). Due to the lack of paragenetic assemblage in Nabijan gabbro, the pressure corresponds to the crystallization depth of hornblende. The calculated pressure for the amphibole in gabbro is about 6.5 kbar, which corresponds to the pressure of hornblende crystallization during the hornblende gabbro formation. Thermometry of the intrusive masses was performed using the Ti content in amphibole (Otten, 1984) in granodiorite at Nabijan shows a temperature of 677°C, gabbro the Nabijan indicates about 992°C and monzogranite the Marzrud is 677.3°C.
Conclusion
Based on the mineral chemistry of mafic intrusive masses at study area, the composition of clinopyroxenes is in the diopside range. Amphibole minerals are calcic in two areas. The Nabijan plutons amphibole are actinolite in granodiorite and tschermakitic hornblende in gabbro. The hornblende in Marzroud monzogranite is actinolite. The Marzroud gabbrodiorite biotites with Fe/(Fe+Mg)>3 are enriched in Mg. The feldspar composition is andesine - labradorite in Marzroud gabbrodiorite, andesine and K- feldspars in Marzroud monzogranitic rocks. The plagioclase composition is andesine and oligoclase in Nabijan granodiorite and labradorite to bytonite in Nabijan gabbroic body. The chemistry nature of the biotite, amphiboles, and pyroxenes under study indicates that this intrusion is calc-alkaline affinity crystallized in a subduction zone setting. |