جستجوی مقالات مرتبط با کلیدواژه « میگماتیت » در نشریات گروه « زمین شناسی »

The Zanjan-Takab complex is a metamorphic belt with NW-SE trend that includes gneiss, amphibolite and gneissic amphibolite, old metagranites, pelitic schists with migmatites and metaophiolites. Takab mafic migmatites, based on field evidence and partial melting degree, are divided into two main groups of metatexites with patchy, ophthalmitic, diktyonitic, agmatic, stromatic structures and diatexites with schollen, ptygmatic, folded, stictolithic, phlebetic, schliren, nebulitic structures. These migmatites consist of mesosome part with textures of porphyroblastic, xinoblastic, granoblastic and nematoblastic and main minerals plagioclase, hornblende, biotite, melanosome part with nematoblastic, xinoblastic, granoblastic and oriented textures and main minerals hornblende and plagioclase. The leucosome part is composed of granular, sympletic and myrmekitic textures and the main minerals are plagioclase, quartz, k-feldspare, titanite, hornblende and biotite. Investigating the shape of the crystal size scatter diagrams (CSD) shows the physical conditions and petrological processes are effective in the studied rocks. In order to investigate these processes, plagioclase crystals in 4 leucosome samples were quantitatively analyzed with the help of Jmicro vision and CSD Corrections software, and then the results obtained from the analysis of different leucosome samples were compared. The crystal size scatter diagrams of plagioclase crystals show two stages of growth with different speeds in the studied samples, so that the larger crystals (the end part of the curved diagram on the right) belong to a melt that is at greater depths or it cooled in a calmer environment. Nevertheless, the initial part of the diagram on the left side crystallized in more superficial areas and at a higher speed.

Migmatites are silisic rocks form in middle to high degree metamorphic territories, at lower to middle continental crust and through water producing reactions (Makrygina, 1977; Brown, 1979; Ashworth, 1985). During intrusion of Chah-Bazargan batholith in the Barrovian-type Qori-Neyriz regional metamorphic rocks at 170 Ma (Fazlnia, 2017), some pieces of these rocks were trapped in the magma and endured intense metamorphism and anatexis. They show stromatic and nebulitic magmatic structures. In the present study, the stable mineral assemblages, conditions, and factors affected partial melting in the Chah-Bazargan migmatites are investigated using phase diagrams and petrographic evidence.

The nebulitic migmatites representing the highest metamorphic degree and close and gradational relationships with peraluminous granites in the area were sampled. Petrographic studies were performed on the thin sections to determine mineral assemblages of peak metamorphic condition. The phase diagrams were calculated using whole-rock analyses reported in Fazlnia (2017) and TheriakDomino software (de Capitani, 2010).

PetrographyThe main textures of nebulitic migmatites are granoblastic and poikilioblastic, although porphyroblastic texture can be observed in places. Stabe mineral assemblage cordierite + spinel + garnet + prtithic K-feldspar + plagioclase together with dissolved silimanites and biotites inclusions represent the peak metamorphic condition. Corundum + spinel sympetectic around the cordierites also form in high metamorphic degrees (Whittington et al, 1998). With increasing metamorphic degree, the biotites and cordierites become less and the more, respectively.Phase diagramsThe P-T phase diagram was calculated in Na2O-CaO-K2O-FeO-MgO-SiO2-H2O (NCKFMASH) chemical system with fluid as pure water and in excess. According to it, mineral assemblage feldspar + chlorite + biotite + muscovite + quartz was stable at temperatures below 550 °C and represents hornblende hornfels zone of the area metapelitic rocks. Partial melting occurs at 700 that increases up to 750 °C with pressure increasing. The assemblage feldspar + garnet + biotite + cordierite + quartz melt + H2O observed in the xenoliths occurs a pressures below 5 kbar. Although, comparing calculated almandin isopleth for garnet and phlogopite for biotite with chemical compositions of these minerals represented in Fazlnia (2017) indicates that anatexis could onset in lower temperatures.According to the calculated T-XCO2 diagram, the partial melting occurs in lower temperatures with increasing in CO2 content of the fluid. Also, T-XO2 diagram show that presence of sufficient O2 in the environment could decrease the partial melting temperatre to below 700 °C. Moreover, the T-Mg# diagram (Mg# = MgO/MgO+FeO) reveals that low Mg# contents of these rocks (0.2 – 0.3) could trigger anatexis in temperatures below 700 °C.

Falling of metapelitic xenoliths in Chah-Bazargan batholith producing magma caused intense contact metamorphism and partial melting. In the peak condition, the assemblage cordierite + spinel + K-feldspar + garnet + palgiocasle together with corundum + spinel intergrowth on cordierite formed. Based on the calculated phase diagrams, anatexis occured in temperatures 700 – 750 °C and pressures less than 5 kbar, however, chemical compositions of the garnets and biotites from the xenoliths show that it could onset in lower temperatures. The T-XCO2, T-XO2, T-Mg# diagrams reveal that increasing CO2 and O2 and high FeO contents of the protolith probably triggered anatexis in lower temperatures.

The process of migmatitization in the Ghareh Naz has resulted in the formation of various rate of partial melting and eventually the formation of a large spectrum of metatexite and diatexite migmatites. Mineral chemistry studies by EPMA method shows that amphibole composition is magnesio-hastingsite and has a metamorphic origin and feldspar composition is andesine to oligoclase. Effective reactions in the formation of the melt and Leucosome part of migmatites are include fluid-present reactions and fluid-absent reactions. High grade mineral assemblages are probably formed by the decomposition of lower-grade minerals by fluid absent reactions. The mineralogical composition of the leucosome is similar to that of tonalite to granodiorite resulting from partial melting and corresponds to the crystallization origin of the melt. Based on the thermobarometry of these rocks. the temperature is about 767 to 868 °C and the pressure is about 5.7 to 7.7 Kbar.

Migmatites have formed an important part of high-grade metamorphism in the Boroujerd area aureole. Leucosome of migmatites are mainly composed of plagioclase, quartz, potassium feldspar and biotite. Migmatites mesosome are mainly composed of plagioclase, quartz, potassium feldspar, biotite, garnet, andalusite, sillimanite, spinel, corundum and cordierite. Geochemical studies of migmatites show that composition of old meta-sedimentary rocks (source of migmatites) are shale and pelitic. Discrimination diagrams indicate an active continental margin tectonic setting for the source of Boroujerd migmatites. Sources of Boroujerd migmatites are intermediate and felsic igneous rocks (andesite –dasite and ryodacite) based on geochemistry of immobile elements. Field observations, microscope and geochemical data show that the migmatites are composed of the metaplites partial melting. Decreasing and increasing trace and rare earth elements in the metaelites and migmatites were resulted of stability or instability in the metamorphic minerals during peak of the metamorphism, which caused migmatization. Based on the partition coefficients of elements in different minerals, light rare earth elements and high rare earth elements (LREE, HREE) were mostly controlled by garnet and apatite (but not a lot) during the partial melting of the metaelites. Related to the LREE, heavy rare earth elements (HREE) and Y were controlled by garnet. Elements with high field strength (HFSE), such as Zr, Nb, Ta and Th were controlled and distributed by biotite and ilmenite. Large ionic lithophile elements (LILE) such as Sr, Ba and Rb showed that plagioclase and biotite are the main minerals that control and distribution the elements. Based on the P–T pseudosection, Crd+Kfs+Spl+Crn minerals paragenesis, the maximum temperature and pressure for metamorphism estimated approximately 750 ° C and 2.7 kbar respectively. Therefore, the intrusion of mafic intrusions in the metapelite caused the formation of pelitic hornfels and partial melting derived migmatites.

1-Introduction Corundum (Al2O3) is one of the rare metamorphic minerals that its formation requires particular chemical condition (low SiO2 and high Al2O3) in combination with high temperature (Simonet et al., 2008). Therefore, one of the most critical cases in the study of corundum-bearing rocks is the accurate determination of protolith and metamorphic conditions (Yakymchuk and Szilas, 2018). In the Broujerd area, Berthier et al. (1974) reported the corundum in migmatites for the first time. Despite the many studies in metamorphic rocks, these corundum-bearing rocks never reported in later works, until (Ghaffari, 2010) and then (Papi, 2015) again reported corundum-bearing rocks in the area and both concluded that they are a kind of migmatites, evidence of granulite facies metamorphism. Neither Berthier et al. (1974) nor Gaffari (2010) and Papi (2015) reported albitite rocks that are in direct relations with corundum-bearing rocks. Also differences between whole rock composition in migmatites and corundum-bearing rocks and its possible role in corundum formation, never discussed. 2-Materials and Methods Different samples collected for petrographic studies and finally, five samples had chosen for whole rock analysis (Table 1). Bulk rock compositions of selected samples were determined using an X-ray fluorescence spectrometer at the Zarazma Co., Tehran, Iran. Chemical compositions of minerals were obtained using a JEOL W-EPMA JXA8900-R electron microprobe in the Institute of Earth Sciences, Academia Sinica, Taiwan, at an acceleration voltage of 15 kV, a current of 15 nA, and a beam size of 2 nm. (Droop, 1987). The method used for Recalculation of Fe as Fe3+ and Fe2+. All mineral abbreviations are from (Whitney and Evans, 2010). 3-General geology and field relations Borujerd area located in Sanandaj-Sirjan Zone in the western part of Iran. Intrusive rocks, as main granitoids and rare norite, gabbro, and pegmatites, with NW – SE trend, injected along dominant schistosity in metapelites (Berthier et al., 1974), during middle Jurassic (175-170Ma; (Ahmadi-Khalaji et al., 2007; Mahmoudi et al., 2011). Contact metamorphism developed during magmatism, especially in the northern part of Broujerd batholith. Highest metamorphic grade restricted either to the northern part of granitoids or as large enclaves. Migmatites in Ab-Bakhshan area, show one of the most significant outcrops and located between granitoids and hornfelses. Based on Fig. 1, albitites dikes injected in migmatites. In general, towards albitites, migmatites first start to metasomatized and formed metasomatized-migmatites, then corundum-bearing rocks will appear either in contact or inside albitites. Migmatites are metatexite with the low-melt portion, generally with patchy structure. In petrographic studies, they include relatively uniform mineralogy: quartz, plagioclase and potassium feldspar are main minerals in the leucosomes with granoblastic texture, while biotite, cordierite, andalusite, sillimanites, spinel, and Fe-Ti oxides, are major minerals in mesosome, generally with lepidoblastic or grno-lepidoblastic texture. Their chemical composition (Table 1) with high Al2O3 and SiO2, and low alkali and calcium show that they have the pelitic origin. Metasomatized migmatites in the field are the same as migmatites, but under the microscope, andalusite and sillimanites start to replace by sericites, and biotites with chlorite. Near the corundum-bearing rocks (based on Figure 1) all andalusite and sillimanites completely replaced by sericites, all biotites with chlorites and also, feldspars altered. Comparing to migmatites, they have higher SiO2 and less Al2O3 (Table 1). Corundum-bearing rocks have different mineralogy. They mainly composed of corundum, chlorite, white mica (tin muscovites) and feldspar (albite), with rare rutile, ilmenite, and apatite. Albitites are mainly composed of albite (more than 80%) and quartz, sericites, potassium feldspars as minor minerals. Their chemical composition is entirely different from migmatites, with a low concentration of SiO2 and higher Al2O3 and MgO (Table 1). Albitites are deformed and make dikes and small patches in migmatites. They mainly composed of albite with rare muscovite, quartz, and k-feldspar. Their chemical composition is following high–Na plagioclases (Table 1. Figure 1. Schematic illustration of field relations between migmatites, corundum-bearing rocks and albitites (without scale) Table 1. Major elements analysis of selected samples, based on Figure 1 Sample BM-96.5 BM-96.8 BM-96.9 BM-96.102 BM-96.7 SiO2 63.85 71.08 43.25 43.73 61.45 Al2O3 17.35 14.65 28.38 26.55 19.54 TiO2 0.82 0.73 0.69 0.48 0.67 Fe2O3 7.49 3.6 4.57 6.94 2.02 MgO 2.2 1.95 8.7 8.68 2.75 MnO 0.16 0.05 0.05 0.07 CaO 0.64 0.8 0.54 0.92 0.51 K2O 3.35 2.39 5.48 2.79 0.32 Na2O 1.53 1.74 1.82 3.59 10.09 P2O5 0.22 0.1 0.06 0.09 0.16 LOI 2.31 2.91 6.46 6.09 2.48 4-Mineral chemistry Selected mineral analysis from corundum-bearing rocks is represented in Table 2. Corundum crystals in a mica matrix, have a relatively pure chemical composition (Table 2) and their aluminum content is above 1.99 a.p.f.u, only iron is a unique element that it reaches up to 0.006 a.p.f.u. Chlorite is one of the most abundant minerals in corundum-bearing rocks, which forms a large part of the rock's matrix. Chlorites usually contain rutile and or ilmenite inclusions. Chlorite are Mg-rich (Mg/Mg+Fe from 0.75 to 0.76; Table 2). In white micas, iron and magnesium are low and close to a pure muscovite composition. Feldspars are sodic in chemical composition, in which the XAb is 0.99 (Table 2). In other words, feldspars are pure albite. Cordierite is a rare mineral between feldspars and chlorites, and due to petrographic similarities, it is difficult to detect under the microscope. They are Mg-rich (Mg/(Mg+Fe)=0.81; Table 2). Rutile as inclusions in chlorite are relatively pure, and their iron and manganese are deficient (Table 2). Ilmenite is also found in some parts in the chlorite or the rock matrix, and its chemical composition is close to the ideal ilmenite, with little impurities (Table 2). Table 2. Chemical analysis of minerals in corundum-bearing rocks of Broujerd area. Only selected analyzes are provided Mineral Chl Chl Ms Ms Crn Crn Crd Crd Fsp Fsp Rt Ilm Point 1 2 1 2 1 2 1 2 1 2 SiO2 27.31 27.29 47.70 45.98 0.02 0.01 48.21 48.18 68.67 68.03 0.42 0.00 TiO2 0.05 0.06 0.09 0.00 0.01 0.05 0.00 0.00 0.00 0.00 98.78 53.97 Al2O3 21.50 22.51 34.86 37.72 99.82 99.71 33.04 33.12 19.42 19.41 0.25 0.00 Cr2O3 0.04 0.15 0.00 0.00 0.06 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe2O3 0.06 0.00 0.00 0.00 0.00 0.00 1.21 1.23 0.00 0.00 0.00 0.00 FeO 13.34 14.08 0.65 0.33 0.16 0.20 4.35 4.44 0.00 0.00 0.17 43.67 MnO 0.08 0.06 0.00 0.00 0.01 0.00 0.65 0.61 0.00 0.00 0.00 1.85 MgO 24.14 23.36 1.22 0.39 0.00 0.00 10.36 10.45 0.00 0.00 0.18 0.10 CaO 0.00 0.01 0.00 0.60 0.00 0.00 0.00 0.00 0.15 0.09 0.05 0.00 Na2O 0.04 0.00 0.61 1.04 0.00 0.00 0.00 0.00 11.23 11.58 0.00 0.00 K2O 0.00 0.00 9.76 9.22 0.00 0.00 0.00 0.00 0.05 0.04 0.00 0.00 Totals 86.56 87.52 94.89 95.28 100.08 99.97 97.82 98.03 99.52 99.15 99.85 99.59 Oxygens 14.00 14.00 11.00 11.00 3.00 3.00 18.00 18.00 8.00 8.00 2.00 3.00 Si 2.73 2.70 3.15 3.03 0.00 0.00 4.94 4.92 3.01 2.99 0.00 0.00 Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.99 1.02 Al 2.53 2.63 2.72 2.93 2.00 2.00 3.99 3.99 1.00 1.01 0.00 0.00 Cr 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe3 0.01 0.00 0.00 0.00 0.00 0.00 0.09 0.10 0.00 0.00 0.00 0.00 Fe2 1.12 1.17 0.04 0.02 0.00 0.00 0.37 0.38 0.00 0.00 0.00 0.92 Mn 0.01 0.01 0.00 0.00 0.00 0.00 0.06 0.05 0.00 0.00 0.00 0.04 Mg 3.60 3.45 0.12 0.04 0.00 0.00 1.58 1.59 0.00 0.00 0.00 0.00 Ca 0.00 0.00 0.00 0.04 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 Na 0.01 0.00 0.08 0.13 0.00 0.00 0.00 0.00 0.95 0.99 0.00 0.00 K 0.00 0.00 0.82 0.78 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Sum 10.00 9.97 6.93 6.96 2.00 2.00 11.02 11.03 4.97 4.99 1.00 1.98 XMg 0.76 0.75 0.79 0.79 Mg/(Mg+Fe) 0.76 0.75 0.81 0.81 Or 0.31 0.20 Ab 98.96 99.40 An 0.73 0.40 5- Phase equilibria in the corundum-bearing rocks Phase equilibria for corundum-bearing rocks were modeled in the system Na2O-K2O- CaO-FeO-MgO-Al2O3-SiO2-H2O- TiO2 (Ti-NCKFMASH) using the THERIAK-DOMINO software v.03.01.12 (Capitani and Petrakakis, 2010) with the internally consistent thermodynamic dataset of (Holland and Powell, 1998). The activity models of (Baldwin et al., 2015) are used for feldspar, those of (White et al., 2002) for corundum, and (Coggon and Holland, 2002) for micas. The water content is taken from the 'loss of ignition' (LOI) during XRF analyses, following (Sarkar and Schenk, 2014), although the effects of variation in the amount of water calculated. The fluid phase is assumed to be pure H2O. The final calculated pseudo section is represented in Figure 2. Figure 2. Calculated pseudosection for corundum-bearing rocks in Broujerd area (Sampled BM-96.102 in Table 1 and Fig. 1). Gray region is following the mineralogy of corundum-bearing rocks. Dashed red and green lines represent equal values of Mg/Mg+Fe for chlorite and cordierite, respectively and their intersection show 605 °C - 3.3 kb as T and P, based on the mineral analysis in Table 2 6- Conclusion In the Ab-Bakhshan area, corundum-bearing rocks appeared as small patches, located in albitites or albitite-migmatite contact. Based on calculated pseudosection for migmatites, their composition is not suitable for corundum formation, even in high T. Using whole rock composition of corundum-bearing rocks, T and P estimated as 605 ℃ in 3.3 Kbar. Field relation between metasomatic rocks (albitites) and corundum-bearing rocks show that metasomatism was effective during corundum formation. The albitites occur most commonly in conjunction with other types of metasomatic rocks including scapolitised metagabbros and Mg-Al-rich lithologies such as orthoamphibole-cordierite schists (Engvik et al., 2014; Engvik et al., 2018). During Na metasomatism and albitite formation, Mg-Al rich fluids generated and cause chemical changes in migmatites, lead to appropriate whole rock composition for corundum formation. Na metasomatism could generate Mg-Al rich rocks, as corundum-bearing rocks of Broujerd area. So Mg-metasomatism and corundum formation either in migmatites or albitites, are consequences of Na-metasomatism in the area or are not evidence of very high T metamorphism. References Ahmadi-Khalaji, A., Esmaeily, D., Valizadeh, M., Rahimpour-Bonab, H., 2007. Petrology and geochemistry of the granitoid complex of Boroujerd, Sanandaj-Sirjan Zone, Western Iran. Journal of Asian Earth Sciences 29(5-6), 859-877. Baldwin, J.A., Powell, R., White, R.W., Štípská, P., 2015. Using calculated chemical potential relationships to account for replacement of kyanite by symplectite in high pressure granulites. Journal of Metamorphic Geology 33(3), 311-330. Berthier, F., Billiaul, H.P., Halbroronn, B., Marizot, P., 1974. Etude Stratigraphique, petrologique et structural de La region de Khorramabad (Zagros, Iran). These De 3e cycle, Grenoble. Capitani, C.d., Petrakakis, K., 2010. The computation of equilibrium assemblage diagrams with Theriak/Domino software. American Mineralogist 95(7), 1006-1016. Coggon, R., Holland, T., 2002. Mixing properties of phengitic micas and revised garnet‐phengite thermobarometers. Journal of Metamorphic Geology 20(7), 683-696. Droop, G.T.R., 1987. A General Equation for Estimating Fe3+ Concentrations in Ferromagnesian Silicates and Oxides from Microprobe Analyses, Using Stoichiometric Criteria 51, 431-435. Engvik, A.K., Taubald, H., Solli, A., Grenne, T., 2018. Dynamic Metasomatism: Stable Isotopes, Fluid Evolution, and Deformation of Albitite and Scapolite Metagabbro (Bamble Lithotectonic Domain, South Norway). Geofluids 1, 1- 22. Engvik, A.K., Ihlen, P.M., Austrheim, H., 2014. Characterisation of Na-metasomatism in the Sveconorwegian Bamble Sector of South Norway. Geoscience Frontiers 5(5), 659-672. Ghaffari, M., 2010. Petrology of metamorphic rocks in the southeast of Boroujerd. MSc thesis, Geological Survey of Iran, Tehran, Iran (in Persian). Holland, T., Powell, R., 1998. An internally consistent thermodynamic data set for phases of petrological interest. Journal of metamorphic Geology 16(3), 309-343. Mahmoudi, S., Corfu, F., Masoudi, F., Mehrabi, B., Mohajjel, M., 2011. U–Pb dating and emplacement history of granitoid plutons in the northern Sanandaj–Sirjan Zone, Iran. Journal of Asian Earth Sciences 41(3), 238-249. Papi, N., 2015. Petrogenesis of granulite facies rocks in the contact aureole of boroujerd complex. M.Sc Thesis, Kharazmi University (In persian). Sarkar, T., Schenk, V., 2014. Two-stage granulite formation in a Proterozoic magmatic arc (Ongole domain of the Eastern Ghats Belt, India): Part 1. Petrology and pressure–temperature evolution. Precambrian Research 255, 485-509. Simonet, C., Fritsch, E., Lasnier, B., 2008. A classification of gem corundum deposits aimed towards gem exploration. Ore Geology Reviews 34(1), 127-133. White, R.W., Powell, R., Clarke, G.L., 2002. The interpretation of reaction textures in Fe-rich metapelitic granulites of the Musgrave Block, central Australia: constraints from mineral equilibria calculations in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3. Journal of Metamorphic Geology, 20(1), 41-55. Whitney, D.L., Evans, B.W., 2010. Abbreviations for names of rock-forming minerals. American mineralogist 95(1), 185. Yakymchuk, C., Szilas, K., 2018. Corundum formation by metasomatic reactions in Archean metapelite, SW Greenland: Exploration vectors for ruby deposits within high-grade greenstone belts. Geoscience Frontiers 9(3), 727-749.

Intrusion of the Alvand plutonic complex into the pelitic and sim-pelitic rocks of the Tuyserkan area has caused the development of a thermal aureole in the Sarabi area. The rocks within the aureole are thermally metamorphosed and formed different types of hornfelses. High-grade metamorphic rocks with chemically suitable compositions were melted adjacent to the contact due to the heat from the pluton. Heat derived migmatization in high-grade metamorphic and adjacent to the contact is observable. Scale of partial melting and volume of produced melt are very small. Main minerals in the light colored parts of the migmatites (leucosomes) include quartz and K-feldspar with an igneous texture specially euhedral to subhedral texture of K-feldspar, in which interstitial – textured – quartz filled their interspaces, Textural differences between light leucosomes and dark melanosomes, mineralogical composition of the leucosomes, existence of igneous textures within the leucosomes and restriction of the leucosome formation to the pelitic rocks all are distinct evidence for occurrence of partial melting in the Sarabi area aureole. The liable reactions for melting include fluid-present reactions and fluid-absent reactions. High-grade metamorphic assemblages contain corderite, sillimanite and garnet. Mineralogical compositions of leucosomes resemble leucogranites resulting from partial melting. This indicates crystallization of the leucosomes from a silicate melt.