Contrasting TiO₂/MgO ratios in the Namwon granitic complex
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We report adventages of employing MgO as a differentiation index for the Namwon granitic complex. It is shown to be much more sensitive than the usual Harker index. The complex can be divided into two groups on the basis of /MgO ratio. The low /MgO group consists of hornblende biotite tonalite-granodiorite, porphyritic hornblende biotite granodiorite (PHBGd) and part of biotite granite (loBG). PHBGd shows its own distinct variation in the low group. This group is characterized in most cases by the presence of hornblende, even if it occurs as a trace amount. The high /MgO group consists of part of biotite granite (hiBG) and two mica granite. The major element differences between rock types are also apparent in biotite chemistry. These chemical data indicate that at least two distinct origins of magma are rquired for the complex. Two kinds of biotite granite revealed in this study show distinct geographic distribution, suggesting that a new geologic map should be made.Keywords:
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1977)
Chemical compositions of coexisting biotites and hornblendes and their host rocks of the Shishigawa-type of the Okueyama granodiorite batholith were investigated. Compositional difference which can be seen between granitic rocks of the southern part and those of the northern part in the batholith indicates that the rocks of the latter were affected by a residual liquid due to differentiation of magma of the main body of the Okueyama granodiorite. Both biotites and hornblendes are poor in Al, and low in Mg/Fe+2 ratio. The distribution coefficient for Mg and Fe+2 of the coexisting biotite and hornblende pairs is 0.75 in average. Comparing with chemical compositions of coexisting biotite and hornblende pairs from granitic rocks in Kitakami and Ryoke regions, those of coexisting biotite and hornblende pairs from the Okueyama granodiorite suggest that rocks of the Okueyama main body would have been formed as a volcano-plutonic complex.
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Research Article| June 01, 1983 Tin granites of Seward Peninsula, Alaska TRAVIS HUDSON; TRAVIS HUDSON 1U.S. Geological Survey, 1209 Orca Street, Anchorage, Alaska 99501 Search for other works by this author on: GSW Google Scholar J. G. ARTH J. G. ARTH 2U.S. Geological Survey, Reston, Virginia 22092 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1983) 94 (6): 768–790. https://doi.org/10.1130/0016-7606(1983)94<768:TGOSPA>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation TRAVIS HUDSON, J. G. ARTH; Tin granites of Seward Peninsula, Alaska. GSA Bulletin 1983;; 94 (6): 768–790. doi: https://doi.org/10.1130/0016-7606(1983)94<768:TGOSPA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Seven granite plutons, spatially and genetically related to tin metalization, are exposed in a 170-km-long belt across northwestern Seward Peninsula, Alaska. These plutons are cupolas and epizonal composite stocks that consist of several textural varieties of biotite granite, including medium- to coarse-grained seriate biotite granite, porphyritic biotite granite with an aplitic groundmass, and fine- to medium-grained equigranular biotite granite. The common accessory minerals are fluorite, allanite, apatite, and zircon. Other accessory minerals that are locally present include tourmaline, sphene, opaque oxide minerals, and late-forming (deuteric) muscovite and chlorite. The granites range in major-element contents as follows: SiO2, 72.5% to 76.6%; A12O3, 12.7% to 14.3%; Na2O, 2.9% to 4.0%; K2O, 3.9% to 5.6%; and CaO, 0.6% to 1.2%. The sum of FeO + Fe2O3 + MgO ranges from 0.3% to 2.4%; and the K2O to Na2O ratio from 1.1 to 1.8. The 0.1% to 0.9% F and 0.01% to 0.2% Cl reflect the over-all volatile-rich nature of the granites. The granites contain average or below-average concentrations of Co, Sc, Cr, and Zn, and generally above-average to distinctly high concentrations of Th, U, Hf, and Ta. The large cations emphasize the evolved nature of the granites; the Rb/Sr ratio is as high as 90 in some samples. Initial 87Sr/86Sr ratios range from 0.708 to as high as 0.720. The three Rb-Sr isochrons defined by the data agree with K-Ar age determinations and show that the stocks were emplaced during the Late Cretaceous, between about 70 and 80 m.y. ago.The field, petrologic, and geochemical data indicate that the plutons had a multistage origin that involved large-scale melting of sialic crust, emplacement of magmas derived from batholithic fractionation at depth, and subsequent evolution of these magmas to generate small volumes of more highly evolved residual magmas. Although evolution of the granite complexes was largely governed by crystal-melt fractionation, some minor-element variations in the highly evolved granites cannot be explained by this process. For example, the distribution of rubidium and the light rare-earths appears to have been influenced by volatile depletion at the final stages of crystallization. The field data, petrologic data, and variation trends, such as distinct shifts toward higher albite contents in the residual granites, suggest that the coexistence of a volatile phase was important in their evolution. These results require that models seeking to explain compositional gradients in high-level granite (rhyolite) systems fully consider the role of a coexisting volatile phase. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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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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The Moslavacka Gora (MG) granite body located in the southwestern part of the Pannonian Basin (Croatia) shows marks of a complex evolution history during Cretaceous time. Field, petrographic and chemical studies reveal existence of several types of granitic rocks (andalusite-, sillimanite- and tourmaline- bearing granite, leucogranite, biotite granite, monzogranite, granodiorite). These granites penecontemporaneously occur together with medium-grade metamorphic rocks (amphibolite, marble, metapelite) and often comprise metapelitic xenoliths (biotite, quartz, feldspar, sillimanite p ; andalusite). K-feldspars megacrysts, up to 10 cm in size, locally occur inside the Cretaceous peraluminous biotite granite (quartz, plagioclase, biotite, K-feldspar and muscovite) from Srednja Rijeka locality together with peculiar tourmaline nodules (see Balen & Broska, this volume). Chemical analysis of the hosting granite yields 73.9 wt.% SiO2, 14.2 wt.% Al2O3, low concentrations of Fe2O3 (0.93 wt.%), MgO (0.25 wt.%), CaO (0.48 wt.%) and TiO2 (0.14 wt.%), and relatively high K2O (6.01 wt.%) and Na2O (2.71 wt.%). Strong peraluminosity (ASI=1.2) and high SiO2 content are coupled with low concentrations of ferromagnesian elements together with high LIL trace elements Ba (330 ppm), Sr (70 ppm), Cs (26ppm), Rb (277 ppm). The host granite is characterized with moderately flat and fractionated REE chondrite normalized patterns, slight enrichment in the LREE and strongly negative Eu anomaly ((La/Yb)N = 4.3 ; Eu/Eu* = 0.4 ; low Σ REE=70.3 ppm). The X-ray powder diffraction (XRPD) patterns together with selected area electron diffraction (SAED) patterns of K-feldspar megacrysts obtained on b* and directions and b* and c* directions indicate monoclinic symmetry of K-feldspar, which defines it as orthoclase. However, slight continuous streaking parallel to b* on some SAED patterns reveals that the orthoclase structure is modulated on the nano scale. If reflections are splitted parallel to b*, they show mirror symmetry which is proper of albite twinning. The thickness of the twin lamellae varies between 30 and 200 nm. K-feldspar megacrysts, chemical and petrographic features of hosting peraluminous biotite granite including existence of tourmaline nodules point to temperature, pressure and compositional fluctuations in the magma during crystallisation. The megacrysts nucleated and grew in an environment of increasing undercooling, probably during the ascent of the magma. The low population density of the megacrysts has been ascribed to nucleation difficulties i.e. they develop at conditions of unusually low nucleation to growth ratio in granitic melt until the temperature is close to the solidus. Though K-feldspar is commonly among the last minerals crystallizing in granitic magmas, abundant melt is still present at that stage. Igneous microstructures of the megacrysts as crystal shape, simple twinning, zonal growth (compositional zoning) are consistent with growth from melt.
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