Orientations of Phenocrysts in Hornblende-Andesite Mass of Siroyama, Kanbara-mati, Sizuoka Prefecture
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Phenocryst
Hornblende
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Basaltic andesite
Igneous differentiation
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A study of amphiboles and associated minerals in samples of Soufrière Hills andesite erupted from 1995 to 2002 shows significant compositional variations within hornblende phenocrysts, a separate set of small pargasitic crystals in the groundmass, and two types of reaction rims on the phenocrysts. The composition of the amphiboles and coexisting phases defines the thermal history of the erupting magma. As many as seven zones (<200 µm wide) in the hornblende phenocrysts begin with a sharp increase in Mg and Si, and then change gradually to a more Fe- and Al-rich hornblende, a transition that is consistent with a temperature rise. Analyses of the hornblende phenocrysts and associated Fe–Ti oxides verify previous conclusions that the pre-eruption magma was at 130 MPa and 830 ± 10°C, but was variably heated before eruption. The heating occurred within ∼30 days of eruption for all magmas erupted, based on the width of Ti-rich rims on titanomagnetite phenocrysts. Experimental phase equilibria for the andesite confirm that the natural hornblende phenocrysts would be stable between 825 and 855°C at a PH2O of 130 MPa, and would be even more Al rich if crystallized at higher pressure. Pargasite is not stable in the andesite, and its presence, along with high-An plagioclase microphenocrysts, requires mafic magma mingling and hybridization with pre-existing andesite. Experimental melts of the andesite at 130 MPa and 830 and 860°C compare well with melt inclusions in quartz and plagioclase, respectively. Reaction rims on a few hornblende crystals in each andesite sample are rich in high-Ca pyroxene and are produced experimentally by heating the andesite above the stability limit for hornblende. Decompression-induced breakdown rims occur in some samples, and the rate of this reaction has been experimentally calibrated for isothermal andesite magma ascent at 830–860°C. The average ascent rate of magma during much of the 1995–2002 eruption has been >0·02 m/s, the rate that allows hornblende to erupt free of decompression-induced reaction rims.
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Hornblende
Amphibole
Igneous differentiation
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We found high-Mg andesite (56.5 wt.% SiO2 and 7.2 wt.% MgO) from Mikasayama in Wassamu town, northern Hokkaido. Its K-Ar age is 11.1±0.8 Ma. The high-Mg andesite is characterized by co-existence of Fo-rich olivine (Fo90-85) and An-poor plagioclase (An64-38) phenocrysts. The mineralogical evidence suggests that the high-Mg andesite from Mikasayama was produced by mixing of primitive basalt magma, containing Mg-rich olivine and clinopyroxene phenocrysts, and hornblende dacite magma.
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Dacite
Basaltic andesite
Igneous differentiation
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Hornblende
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1983)
The fission track dating has been carried out on zircon separated from six specimens of the Hyugami Lava which outcrops in Oita Prefecture. The results are as follows: HW-20 Two-pyroxene hornblende andesite Kameishi-yama 33°1l′30″N, 131°03′45″E 0.42±0.09 Ma HW-3 Altered hornblende andesite (borecore) Noya 33°14′28″N, 131°16′21″E 1, 56±0.41 Ma HW-5 Altered hornblende andesite (borecore) Noya 33°14′03″N, 131°15′42″E 1.80±0.29 Ma HW-7 Biotite-bearing hornblende andesite Yoko-yama 33°15′28″N, 131°14′03″E 1.78±0.32 Ma HW-12 Biotite-bearing hornblende andesite Karuto-yama 33°15′51″N, 131°17′26″E 2.2110.29 Ma HW-19 Two-pyroxene hornblende andesite Hyugami 33°10′26″N, 131°46′59″E 2.85±0.49 Ma Considering both these fission track ages and the results of modal analysis of the rocks, HW-20 may belong to the middle Pleistocene Bungo Volcanic Rocks, and HW-3, HW-5 and HW-7 to the early Pleistocene Hohi Volcanic Rocks. However, HW-12 and HW-19 erupted in the Pliocene and come under none of known volcanic groups in central Kyushu.
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We found high-Mg andesite (56.5 wt.% SiO2 and 7.2 wt.% MgO) from Mikasayama in Wassamu town, northern Hokkaido. Its K-Ar age is 11.1±0.8 Ma. The high-Mg andesite is characterized by co-existence of Fo-rich olivine (Fo90-85) and An-poor plagioclase (An64-38) phenocrysts. The mineralogical evidence suggests that the high-Mg andesite from Mikasayama was produced by mixing of primitive basalt magma, containing Mg-rich olivine and clinopyroxene phenocrysts, and hornblende dacite magma.
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Dacite
Basaltic andesite
Igneous differentiation
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The Half Dome Granodiorite, Yosemite National Park, California, is recognized in the field by euhedral, fresh-looking, black hornblende phenocrysts up to 2 cm in length. This variety of granodiorite typifies intermediate-age hornblende-phyric units of Cretaceous nested plutonic suites in the Sierra Nevada batholith. Although only inclusions of feldspar are evident in hand samples, the phenocrysts are riddled with up to 50% inclusions of every major mineral found in the host granodiorite plus metamorphic minerals formed during cooling. Amphibole compositions within single phenocrysts vary from actinolite with less than 1 wt% Al2O3 to magnesiohornblende with over 8 wt%. Elemental zoning within the amphibole is highly irregular on the micrometer scale, showing patches and polygonal zones with dramatically different compositions separated by sharp to gradual transitions. The chemical compositions of entire phenocrysts are equivalent to hornblende plus a small proportion of biotite, suggesting that the non-biotite inclusions are the result of metamorphism of the phenocrysts. Backscattered electron imaging shows evidence of brecciation that may have been the result of volume changes as hornblende was converted to actinolite. Pressure calculations using the Al-in-hornblende barometer show unreasonably wide variations on the micrometer scale that cannot have been produced by temperature or pressure variations during crystallization. These hornblende phenocrysts would thus be unsuitable for geobarometry, and caution must be used to avoid similarly zoned phenocrysts in the application of the Al-in-hornblende geobarometer.
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Hornblende
Actinolite
Batholith
Amphibole
Greenschist
Paragenesis
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Melt inclusions
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Abstract Thirty-three new partial or complete chemical analyses are presented for pyroxene, amphibole, and biotite phenocrysts separated from three andesitic, three dacitic and thirteen rhyolitic volcanic rocks. The orthopyroxenes are mostly of hypersthene composition, with the rhyolitic hypersthenes generally being slightly more Fe-rich compared to the andesitic types. No detailed correlation, however, is evident between the Fe/Mg ratio of the rhyolitic orthopyroxenes and their parent rocks or co-existing groundmasses. A correlation between Ca content and the type of co-existing ferromagnesian assemblage was found. The clinopyroxenes are augites, and occur most commonly in the andesites and dacites. Two amphiboles are found, a calcic amphibole and cummingtonite. The former range from a tschermakitic hornblende (andesitic) to magnesio-hornblende in the rhyolites. One dacitic ferro-hornblende was found. The data illustrate the increasing substitution of Al and increasing Na in the andesitic hornblende. A close correlation of the Niggli mg ratio, and also MnO, exists between the analysed co-existing orthopyroxenes and hornblendes. Again, however, no detailed correlation between Fe/Mg ratio of hornblende and parent rock (or co-existing groundmass) was found to exist.
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Hornblende
Amphibole
Andesites
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