The San Pedro and San Pablo volcanoes of northern Chile and their hot avalanche deposits
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Hornblende
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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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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1961)
Phenocrystic hornblende in an andesite from the Iki islands was chemically analysed. This hornblende is very high in TiO2 content, attaining to 4.18%, and is intermediate between common hornblende and kaersutite. Some petrogenetic considerations are discussed.
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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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An unusual andesitic suite from the Miocene volcanic arc in Northland, New Zealand, comprises pyroxene andesite and garnet-bearing hornblende–pyroxene, hornblende and biotite–hornblende andesites. Garnet crystals occur as 1–10 mm single crystals or more commonly as two or more annealed crystals and as garnetite lenses. The andesitic rocks also contain enclaves of high-MgO pyroxenite, hornblendite, and pyroxene–hornblende gabbro as well as high-Al2O3 hornblende gabbro, garnet–hornblende gabbro, and anorthosite. Garnet crystals in the andesitic volcanic rocks and in the enclaves show comparable compositional ranges, zoning patterns and inclusions, which indicate that they share a common petrogenetic history. They can be grouped into four distinct types on the basis of mode of occurrence, chemical composition and zoning patterns, which leads to their interpretation as antecrysts rather than orthocrysts. The compositions of the garnets, as well as their included mineral assemblages, reflect a petrogenetic trend from high-temperature pyroxene-bearing high-Mg garnet to low-temperature Fe-rich garnet at relatively constant pressure. Well-preserved zoning patterns, in particular those of the Ca- and Mg-rich garnets, reflect processes within a deep crustal arc environment. Later assimilation is suggested by some zoning patterns that show decreasing Ca and increasing Fe and Mn contents. The garnets are interpreted as being derived by disintegration of discrete but closely related cumulate material that formed at pressures of 8–10 kbar. The host volcanic rocks and their garnet crystals together with the enclaves thus represent a consanguineous mixture of liquid and solid components that developed where subduction-related magmas ponded and interacted at or near the base of the crust. Together they represent a rare snapshot of the processes and components that produce arc-type rocks.
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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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