A particular feature of biotite grains from some of the Canadian Appalachian granitic rocks is the presence of lenses of Ca-Al silicates developed along cleavage planes. These from the more to less common are prehnite, pumpellyite and grandite garnet, an intermediate composition between andradite and grossular. Rarely all three minerals can be observed together in one single biotite grain. Microprobe analyses show that Al2O3 contents of prehnite vary from 17.52 to 24.55 and Fe2O3 from 2.38 to 8.88 wt. %, which can be reflected in a substantial and variable substitution of Fe for Al. Furthermore, a fairly positive correlation of MnO values of both prehnite and host biotite may indicate the role of biotite replacement by prehnite. Fe2O3 content in nine analyzed pumpellyite samples varies between 10 and 23 wt%. Fe2O3 contents of more than 10 wt% in pumpellyite are indicative of zeolite and prehnite-pumpellyite facies conditions.
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.
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.
This study reports the earliest presumably known collisional magmatism of the NW Iran–eastern Turkey–Caucasus region from the north of Sarab city in East Azarbaijan province, NW Iran. Volcanic rocks of the area comprise alternating lava flows of mainly alkalibasalt, andesite, trachyandesite, dacite and pyroclastics (tuff and ignimbrite) affiliated with high-K calk-alkaline to shoshonitic magmatic series. Ar–Ar geochronology of the glass, matrix and plagioclase laths revealed crystallization time restricted to Late Eocene to Early Oligocene (34.90–30.69 Ma) which is coincident with the onset of the Arabian-Eurasian continental collision. Geochemical data show LREE enrichment compared with HREE and Nb and Ti depletion, indicating that they are subduction related. The Sr-Nd-Pb isotopic data reflect the incorporation of oceanic sediment into the magma source. We suggest that the volcanic rocks were generated from a metasomatised subcontinental lithospheric mantle source through 5 to 10 vol% partial melting in an extensional back arc basin by the Neotethys slab roll-back under the Iranian plateau.
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.
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
Investigation of variations in the micro-texture and chemical composition of plagioclases (core to rim) allows the sequencing of the magma chamber processes and helps interprete and associate textures to specific processes. In this contribution, the micro-textures and chemical zoning of the plagioclases, significant recorder of magma chamber processes, from the west of Torbat-e-Heydariyeh andesitic rocks (WTHAR) are considered to decipher the physical and chemical parameters of magma evolution. The rocks are cropped out in the northern branch of Neotethyan magmatic belt and considered as the products of Eocene magmatic activities of the Sabzevar zone (N-NE Iran). The rocks show vitrorphyric and vitroglomeroporphyric textures with main phenocrysts, plagioclase (andesine, labradorite, and bytownite), clinopyroxene (augite), orthopyroxene and magnetite scattered in a glassy matrix. The recognized micro-textures of the WTHA plagioclases can be divided into two categories: (i) growth- related textures in the form of coarse-sieved (CS), fine-sieved (FS), core sieved and intact margin (CSIM), core intact and sieved margin (CISM) and entirely sieved (ES) morphologies, oscillatory zoning (OZ), rounded zone corner (RZC) and resorption surfaces (RS) formed due to the changes in temperature, melt H2O content, pressure, composition of the melt, equilibrium at the crystal-melt interface and (ii) morphological textures such as glomerocrysts (GLO), synneusis (SY), swallow-tail (ST) crystals, broken crystals (BC), formed by the effect of dynamic behavior of the crystallizing magma (convection, degassing, etc.) and magmatic differentiation. Also, the occurrence of these changes can be related to the self-mixing process in the magma chamber. The self-mixing with recharge event can be the reason for the dynamic activities in the magma chamber.
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