Chemical characteristics of biotite from Boroujerd Granitoid complex (Middle Jurassic), Western Iran
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Biotite samples from different units of Boroujerd Granitoid Complex (BGC) of the Sanandaj-Sirjan Zone, western Iran, have beenanalyzed by electron microprobe for major elements. Biotite analyses from three units of quartzdiorite, granodiorite and monzograniteof BGC have their own distinct non-overlapping compositional fields in the annite – siderophyllite – phlogopite – eastonitequadrilateral (ASPE), reflecting their host rock compositions. Biotite from each rock unit has an increasing trend of Al contents atalmost fixed Fe/(Fe+Mg) values. In quartzdiorite it shows an approximately constant range of Fe/(Fe+Mg) with a low to moderate Alcontent from 2.5 to 3 atoms per formula unit (apfu). Biotite from granodiorite exhibits a fairly wide range of Al values reaching up to3.32 apfu, at Fe/(Fe+Mg) from 0.6 to 0.7, whereas biotite from monzogranite have a relatively narrow range of Fe/(Fe+Mg) and totalAl values of limited range of 3.1 to 3.3 apfu. Biotite compositions from these two latter units considered to be derived entirely fromcrustal material, characterized by a remarkable increase in total Al at relatively high Fe contents. Biotite samples of quartzdioritesdefine a distinct and non-overlapping trend from those of granidiorites and monzogranites and hence interpreted to be derived from aparental magma with different composition. Calculation of log(XMg/XFe) ranges from -0.09 to -0.02 and most of samples fromquartzdiorite fall within weakly and moderately contaminated I-type field of log(XF/XOH) versus log(XMg/XFe) diagram, whereasthe other two units, containing biotites with log(XMg/ XFe)< -0.21, classified as strongly contaminated reduced I-type. Oxygenfugacity (log ƒO2) of -15.4 to -17.5 bars and ƒH2O of 200 to 560 bars were calculated for quartzdiorite. Likewise, log (ƒO2) of –17.66bars and water fugacity (ƒH2O) of 400 and 700 bars were also calculated for granodiorite and monzogranites respectively. In theFeO*–MgO–Al2O3 biotite discrimination diagram, biotite compositions from BGC are distributed between the calc-alkaline andperaluminous fields, i.e., biotite from the qaurtzdioritic rocks fall principally in the calc-alkaline field, whereas those from thegranodioritic and monzogranitic units plot almost exclusively in the peraluminous field consistent with their host rock natureKeywords:
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Igneous biotite has been analyzed from three I-type calc-alkaline intrusives of the Shah Jahan Batholith in NW Iran, which host several Cu-Mo-Au prospects.The X Mg (Mg/Mg+Fe) value of biotite is the most significant chemical factor and the relatively high value of X Mg corresponds to relatively high oxidation states of magma (estimated ƒ O2 is mostly 10 -12.5 to 10 -7.5 bars), which is in good agreement with their host intrusions' setting and related ore occurrences.Based on criteria of Al IV and Al VI values, all studied biotites are primary (Al VI = 0), and based on Al total values (2.23-2.82apfu) are in distinctive ranges of mineralized granitoid (Al total =3.2 apfu).The maximum F content of biotite from the Shah Jahan intrusions is moderately higher than those from some other calc-alkaline intrusions related to Cu-Mo porphyries in the world, and in contrast, Cl content is relatively lower.It is likely a result of primary magmatic vs. secondary hydrothermal origin, as well as the Mg-rich characteristics of the biotite in Shah Jahan.X Mg values do not correlate with F and Cl contents of biotite, suggesting that biotite records changes in the F/OH and Cl/OH ratios in coexisting melt/fluids.It is consistent with F-compatible and Cl-incompatible behavior during fractional crystallization of wet calc-alkaline I-type granitoid magma generated at subduction related arc settings.The fugacity ratios of (H 2 O/HF), (H 2 O/HCl) and (HF/HCl) magmatic solutions coexisting with biotite illustrate similar trends in the three intrusions, which can be due to parental magma sources and/or indicate occurrence of similar magmatic processes prior to or contemporaneous with exsolution of fluids from melt.The observed trends caused F-depletions and Cl-enrichments within developed magmatic-hydrothermal systems which are one of the essential characteristics of potential Cu-Mo-Au mineralized I-type granitoids.
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Biotite, the dominant ferromagnesian mineral in Paleozoic granitic rocks of the Canadian Appalachians, has been analyzed with an electron microprobe (wavelength dispersion) for major elements and by 57Fe Mossbauer spectroscopy. We sampled a wide variety of rock types, ranging from gabbro, diorite, syenite to granite, but by far mostly granitic ( sensu lato ). The most pronounced variations are in total Al contents and Fe/(Fe + Mg) values. In the biotite quadrilateral (annite –siderophyllite –phlogopite – eastonite), biotite from A-type granites of the Humber and Avalon zones in Gaspe (Quebec) and New Brunswick is characterized by low mean Al contents, ~1.15 atoms per formula unit ( apfu ), and variable Fe/(Fe + Mg) values in the range 0.4 to 0.9. In the granites of the Notre Dame arc of the Dunnage zone in Newfoundland, biotite has moderate mean values of Al (~1.40 apfu ) and Fe/(Fe + Mg) (~0.58). In granites of the Gander zone of New Brunswick and Newfoundland, biotite has a mean Fe/(Fe + Mg) value of 0.6 and shows a pronounced trend of increasing total Al (1.05 to 1.75 apfu ), confirming significant contributions of aluminous supracrustal material to the magmas, either by assimilation or anatexis. Finally, in granites of the Meguma zone, derived entirely from metasedimentary material, biotite exhibits a remarkable increase in total Al (1.30 to 2.00 apfu ) and considerable iron-enrichment [Fe/(Fe + Mg) in the range 0.4 to 1], with compositions nearing the siderophyllite end-member. The biotite from most zones plots on or above the NNO buffer, indicating moderately oxidizing conditions, whereas that from the Meguma zone plots mainly between the QFM and NNO buffers, implying fairly reducing conditions during crystallization. Assuming a reasonable range of crystallization temperatures of 750 to 900°C, oxygen fugacities ranged from 10−10 to 10−16.9 bars during crystallization. The composition of biotite reflects primarily the nature of the host magmas. It cannot readily be used for tectonomagmatic characterization of these rocks without the aid of other types of data.
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Compositions of biotite from three different rock types of Mashhad granitoids, i.e., granodiorite, monzogranite and leucogranite in NE of Iran have been documented by electron microprobe and wet chemistry for Fe3+ and Fe2+. Mashhad granitoids have been geochronologically and petrologically grouped into G1 and G2 phases. Microprobe data show that the total Fe contents in biotite from G2 leucogranite are higher than those in biotite from G1 granites. In addition, the oxidation state of iron determined by wet chemistry shows that Fe3+/(Fe2+ + Fe3+) ratio in biotite from G2 leucogranite is 0.10 indicating relatively reducing whereas, in G1 ones is 0.18 and 0.23 suggesting more oxidizing conditions. The most outstanding compositional characteristics of Mashhad biotite are differences in total Al contents and Fe/(Fe+Mg) ratios. In the annite-siderophylite-phlogopite-eastonite (ASPE) quadrilateral, represented based on the above parameters, biotite samples from G1 and G2 granites define two distinct and non-overlapping trends. Each trend is characterized by a pronounced trend of increasing total Al at relatively narrow Fe/(Fe+Mg) values. The total Al contents of G1 biotite are in the range of 2.8 to 3.1, whereas, in G2, 3.3 to 3.6 (apfu). Fe/(Fe+Mg) values of G1 biotite are in the range of 0.52 to 0.59 which is considerably lower than those from G2 biotite, 0.67 to 0.72. The trend of increasing Al contents at constant Fe/(Fe+Mg) is relatively common and observed in biotite from several locations worldwide and attributed to considerable contributions from aluminous supracrustal material, either by assimilation or anatexis.
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Tourmaline occurs in a variety of pegmatites, leucogranites, migmatites, metasedimentary rocks and quartz veins in the anatectic Variscan Tormes Dome (TD) in the central Iberian Zone, Spain. In general, these tourmalines belong to the alkali group and to the hydroxy subgroup. The ratios X □/( X □ + Na) and Mg/(Fe + Mg) range from 0 to 0.6 and 0 to 0.7, respectively, where X □ represents vacancies at the X site. Metasomatic tourmaline precipitated in the metasedimentary rocks reflects the composition of the host rocks. Elbaite is characteristic of the most evolved pegmatites. Except for the Li-rich tourmalines, where the substitution mechanisms LiAlR 2+ −2 and X □ Y Al 0.5 X Na −1 Y Li −0.5 account for most of the Li incorporation, the chemical variations in tourmaline from the TD can be described in terms of a combination of the FeMg −1 , Al X □(NaR 2+ ) −1 and AlO(R 2+ ,OH) −1 exchange vectors, where R 2+ represents (Fe + Mg + Mn). The substitution AlO(R 2+ ,OH) −1 becomes more important from leucogranites, through metasedimentary units, migmatites to metasomatized granite. In tourmaline associated with pegmatites, the influence of the proton-loss substitution increases with the degree of evolution of the pegmatites. Biotite occurring with tourmaline has VI Al in the range of 0.26–0.75 cations apfu and Mg/(Mg + Fe) values of 0.1 to 0.4. These values correlate well with those of the associated tourmaline, the K D tur/bt depending on the lithology. The F intercept values of biotite decreases in the order leucogranites, BP and IP, which is consistent with a normal evolutionary trend. Low Fe 3+ /∑Fe values in biotite and tourmaline from migmatites, leucogranites and pegmatites reflect conditions of low oxygen fugacity. In contrast, biotite from metasediments and metasomatized granites, in which zinnwaldite occurs, are presumed to have significant amounts of Fe 3+ , suggesting relatively oxidizing conditions.
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The late Eocene-early Oligocene Sarnowsar granitic rocks and many dacitic to rhyolitic dykesintruded in metamorphosed and metasomatized shale, sandstone, calcite and dolomite marbles andvolcaniclastic and pyroclastic rocks. As a result several million tons of iron ores formed within carbonaterocks in the Sangan area. The Sarnowsar granitic rocks are mainly granite to granodiorite, metaluminous, Itype and high calc-alkaline.Fluorine contents in the biotite range from 0.37 to 4.4 wt. % and Cl contents range from 0.1 to 0.30 wt.%. Fluorine contents in the biotite are negatively correlated with XTi, while Cl contents positively correlatedwith XMg and XSi.Values of the calculated log (ƒH2O/ƒHF) and log (ƒH2O/ƒHCl) of fluid in equilibrium with thechemical composition of biotite range from 3.38 to 4.43 and 0.57 to 1.2, respectively. The contour linesrepresenting log (ƒH2O/ƒHF) and log (ƒH2O/ƒHCl) values are different with the slope of the trend of biotitecomposition suggesting that, in addition to chemical structure of biotite, the fluid composition also playssome role in the incorporation of F, and especially Cl in biotite. The F intercept values for biotite in theSarnowsar granitic rocks are similar to those of igneous rocks and porphyry Cu ore deposits. The Cl interceptvalues of biotite in the Sarnowsar granitic rocks are similar to those of hydrothermal and ore forming systems.The data suggest that chlorine intercept values for biotite from the Sangan deposit tend to be more Cl richthan comparable values from biotites in common igneous rocks. F-rich biotites and F-poor biotites from theSangan deposit show similar and narrow ranges of F/Cl intercept values corresponding to Cl-rich and oreformingsystems such as porphyry copper deposits. Therefore, the chemical composition of biotite from theSarnowsar granitic rocks interacted with hydrothermal solutions.
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Hornblende
Fractional crystallization (geology)
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