Compositional dependence of the Mg/Fe2+ distribution coefficient between biotite-hornblende pairs from calc-alkaline granitic rocks.
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Hornblende
Granitic rock
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Chlorite flakes, as a product of alteration of biotite, the dominant ferromagnesian mineral in the Paleozoic granitic rocks of the Canadian Appalachians, have been analyzed by electron microprobe for major elements and by 57Fe M?ssbauer spectroscopy for the coordination and oxidation state of Fe. Comparison of M?ssbauer Fe3+/Fe ratios obtained from chlorite and its host biotite indicates that chloritization might have occurred under relatively oxidizing conditions. Based on 54 analyzed samples, Si cation totals of these sheet silicates are less than 6.25 atoms per formula unit (apfu), and the sum of octahedral cations is very close to 12 both an indication of trioctahedral chlorite. The calculated mole fraction of chlorite in interlayered phase, Xc, ranges from. 0.72 to 0.98 confirming that the chlorites are completely free of any smectite layers. Compositional variations in chlorite are strongly controlled by host biotite and rock type. Fe/(Fe+Mg) ratio ranges from 0.35 to 0.93 and Si contents from 5.18 to 6.11 apfu lead to the classification of chlorites mainly as ripidolite and brunsvigite. All major elements in the chlorite are strongly correlated with each other. Fe/(Fe+Mg) ratio in biotite is well preserved by chlorite. Chlorite thermometry based on the variation in tetrahedral Al content within the chlorite structure shows a large variation in temperatures from 200 to 390 °C with an average of 340 °C. The chlorite from igneous rocks could also be used to detect reheating events and reveal the thermal history of the rocks.
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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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Batholith
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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.
Leucogranite
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Chemical characteristics of biotite from Boroujerd Granitoid complex (Middle Jurassic), Western Iran
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 nature
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Phlogopite
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Muscovite
Petrogenesis
Hornblende
Fractional crystallization (geology)
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Mg/(Mg + Fe) = 0.35 to 0.95. Strong correlations among Mg, Fe, Ti, VI Al, and IV Al. Principal components analysis shows that 98% of the total variance in compositions of biotite from the muscovite-bearing assemblage can be ascribed to two substitutions with phlogopite. Biotite compositions from a particular assemblage might be sensitive indicators of the thermal gradient.--Modified journal abstract.
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Muscovite
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Granitic rocks in Cambodia are divided into two groups, i.e. ilmenite-series in southern Cambodia and magnetite-series in northern Cambodia. Both groups belong to high-K calc-alkaline series, metaluminous to slightly peraluminous and display typical features of I-type granitic rocks. The ilmenite-series granitic rocks are characterized by high SiO2 contents (70-75 wt. %) with abundance of quartz and K-feldspar, enrichment of LILEs, and strong negative anomalies of Ba, Sr, Eu (Eu/Eu*=0.1-0.5), Nb, and Ti. Chondrite-normalized REE patterns exhibit enrichment of LREE ([La/Yb]N=1.4-17.6) with flat HREE patterns and flat to concave-upward MREE patterns. These granitic rocks have high CaO+FeO+MgO+TiO2 and intermediate Al2O3/(FeO+ MgO+TiO2) with intermediate magnesium number (Mg# = 22-38) (Mg# = 100 x MgO/[MgO+Total FeO]). Geochemical features of the granitic rocks suggest partial melting of crustal igneous rocks of intermediate composition where plagioclase was a major fractionating and/or residual phase. On the other hand, the magnetite-series granitic rocks show wide range of SiO2 contents (59-70 wt. %), higher TiO2, Al2O3, CaO, and MgO contents than ilmenite-series granitic rocks, and smallnegligible negative anomalies of Sr and Eu. The high CaO+FeO+MgO+TiO2 and low Al2O3/(FeO+ MgO+TiO2) coupled with high Mg# (32-48) suggest partial melting of amphibolite-type source with presence of plagioclase. The granites of Cambodia were formed in subduction-related tectonic setting. The ilmenite-series granites range from volcanic-arc granites to syn- and postcollision granites while the magnetite-series granitic rocks belong to volcanic arc granites.
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Abstract The Llano Uplift of central Texas is a gentle structural dome exposing ~1.37 Ga to 1.23 Ga metaigneous and metasedimentary rocks of Grenville affinity intruded by ~1.119 to 1.070 Ga post- to syntectonic granites collectively known as the Town Mountain Granite (TMG). The Lone Grove batholith (LGB), one in a series of TMG plutons, is chemically and texturally zoned consisting of four mappable units: 1) porphyritic coarse-grained, 2) coarse-grained, 3) medium-grained, and 4) fine-grained granite. Principal minerals include perthitic microcline, zoned plagioclase (An 1–20), quartz, biotite, and hornblende, with accessory Fe-Ti oxides, zircon, apatite, titanite, and fluorite. The LGB suite has mineralogical, textural, and chemical characteristics of A-type granites: i.e., 1) accessory fluorite, 2) rapakivi texture, 3) A/CNK 0.93–1.02, 4) K2O/Na2O 1.21–1.97, 5) high Fe/Mg, 6) high Sr, Zr, and Y, 7) low Ca, Cr, Ni, Eu, and Zn, and 8) enrichment in Ba, K, LREE, and HREE. Additionally, they are sub-solvus, metaluminous to marginally peraluminous, high-K, calc-alkaline biotite-hornblende granites (Fe/(Fe+Mg)= 0.71–0.91 and 0.78–0.91 for biotite and hornblende, respectively), that display distinct variation trends with increasing silica content. Variation diagrams suggest that the four mappable units lie along a common liquid line of descent and may represent sequential fractionation products. Estimated crystallization T-P ranges of 750–850° C and 0.2 to 0.5 GPa respectively are based on thermal metamorphic mineral assemblages, normative Q-Ab-Or plots, and Q-Ab-Or-H2O experimental data (Johannes and Holtz, 1996). The assemblage of titanite + magnetite + quartz suggests crystallization occurred at low fO2 [confirmed by Fe/(Fe+Mg) vs[4] Al microprobe analyses of hornblende] and a water content of less than 1.5 wt. % (Wones, 1989). Compared to other Town Mountain-type plutons, the LGB granites display a comparable iron content at similar alkali and silica enrichments. Melting models suggest the LGB evolved from the partial melting of lower crustal rocks of tonalitic composition. On tectonic discrimination diagrams (e.g., Rb vs Yb+Ta, Nb vs Y) the LGB granites plot in the “within plate” and “syn-collisional granite” fields. However, with few intermediate and virtually no mafic rocks, and no coeval volcanic rocks in the Llano district, the LGB’s tectonic setting, in comparison to its A-type chemistry, remains unclear.
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Hornblende
Titanite
Allanite
Porphyritic
Petrogenesis
Pegmatite
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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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