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Phenocryst
About: Phenocryst is a research topic. Over the lifetime, 4132 publications have been published within this topic receiving 158441 citations.
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TL;DR: The Tuzgle volcanic center, along with several small shoshonitic centers, developed along extensional Quaternary faults of the El Toro lineament on the east-central Puna plateau, ≈275 km east of the main front of the Andean Central Volcanic Zone.
Abstract: The high-K Tuzgle volcanic center, (24° S, 66.5° W) along with several small shoshonitic centers, developed along extensional Quaternary faults of the El Toro lineament on the east-central Puna plateau, ≈275 km east of the main front of the Andean Central Volcanic Zone (CVZ). These magmas formed by complex mixing processes in the mantle and thickened crust (>50 km) above a ∼200 km deep scismic zone. Tuzgle magmas are differentiated from shoshonitic series magmas by their more intraplate-like Ti group element characteristics, lower incompatible element concentrations, and lower 87Sr/86Sr ratios at a given eNd. Underlying Mio-Pliocene volcanic rocks erupted in a compressional stress regime and have back-arc like calc-alkaline chemical characteristics. The Tuzgle rocks can be divided into two sequences with different mantle precursors: a) an older, more voluminous rhyodacitic (ignimbrite) to mafic andestitic (56% to 71% SiO2) sequence with La/Yb ratios 35. La/Yb ratios are controlled by the mafic components: low ratios result from larger mantle melt percentages than high ratios. Shoshonitic series lavas (52% to 62% SiO2) contain small percentage melts of more isotopically “enriched” arc-like mantle sources. Some young Tuzgle lavas have a shoshonitic-like component. Variable thermal conditions and complex stress system are required to produce the Tuzgle and shoshonitic series magmas in the same vicinity. These conditions are consistent with the underlying mantle being in transition from the thick mantle lithosphere which produced rare shoshonitic flows in the Altiplano to the thinner mantle lithosphere that produced back-are calc-alkaline and intraplate-type flows in the southern Puna. Substantial upper crustal type contamination in Tuzgle lavas is indicated by decreasing eNd (-2.5 to-6.7) with increasing 87Sr/86Sr (0.7063 to 0.7099) ratios and SiO2 concentrations, and by negative Eu anomalies (Eu/Eu* <0.78) in lavas that lack plagioclase phenocrysts. Trace element arguments indicate that the bulk contaminant was more silicic than the Tuzgle ignimbrite and left a residue with a high pressure mineralogy. Crustal shortening processes transported upper crustal contaminants to depths where melting occurred. These contaminants mixed with mafic magmas that were fractionating mafic phases at high pressure. Silicic melts formed at depth by these processes accumulated at a mid to upper crustal discontinuity (decollement). The Tuzgle ignimbrite erupted from this level when melting rates were highest. Subsequent lavas are mixtures of contaminated mafic magmas and ponded silicic melts. Feldspar and quartz phenocrysts in the lavas are phenocrysts from the ponded silicic magmas.
83 citations
TL;DR: In this article, a new approach for recognizing the origin of accreted basaltic rocks based on ocean plate stratigraphy (OPS), and on the petrology and geochemistry of basalts from mid-oceanic ridges (MORB) and oceanic islands (OIB) using examples from four accretionary complexes (AC) in SW Japan: Akiyoshi, Mino-Tamba, Chichibu and Shimanto.
83 citations
TL;DR: Fenn et al. as discussed by the authors showed that granophyric textures are common in epizonal granitic bodies, particularly those associated with volcanic rocks (Buddington, 1959; Dunham, 1965).
Abstract: vermicular to cuneiform (Barker, 1970). Typically, the Granophyric blocks within late-Pleistocene pyroclastic flow ejecta from the Alid volcanic center, northeast Africa, are the rapidly groundmass feldspars radiate off pre-existing feldspar crystallized, intrusive equivalent of pumice from the pyroclastic flow. phenocrysts, with which they are in optical continuity Phenocryst compositions and geochemical characteristics of the (Dunham, 1965). Some workers prefer the term pumice and granophyre are virtually identical. Silicate melt inclusions ‘micrographic’ for highly regular, interlocking arand other geochemical and geological constraints reveal those processes rangements of quartz and feldspar, as in the coarser leading to development of the granophyric texture. Rhyolitic (Agraphic granite (Fenn, 1986; Lentz & Fowler, 1992). type) magma with ~2·6 wt % dissolved H2O and a temperature Though Schloemer (1964) has shown that some graphic near 870°C was intruded to within 2–4 km of the surface, causing textures can grow by replacement [see also Augustithis deformation and structural doming of shallow marine and subaerial (1973)], most workers agree that phenocryst-bearing strata. Eruptions of crystal-poor rhyolite from this shallow magma granophyres typically form by rapid and simultaneous chamber caused degassing, which forced undercooling and consequent crystallization of quartz and feldspar from a melt (Smith, granophyric crystallization of some of the magma remaining in the 1974). Such crystallization is generally believed to be due intrusion. The most recent eruption from Alid excavated the cryto pronounced undercooling of the silicate liquid (Vogt, stallized granitic wall of the magma chamber, bringing the grano1930; Dunham, 1965); not necessarily at eutectic temphyric clasts to the surface. peratures or compositions (Fenn, 1986; London et al., 1989; Lentz & Fowler, 1992). Granophyric textures are common in epizonal granitic bodies, particularly those associated with volcanic rocks (Buddington, 1959; Dunham, 1965). On occasion, they erupt as comagmatic ejecta in pyroclastic deposits (e.g.
83 citations
TL;DR: The results of a complex study of melt inclusions in olivine phenocrysts contained in unaltered kimberlites from the Udachnaya-East pipe indicate that the inclusions were captured late during the magmatic stage, perhaps, under a pressure of <1 kbar and a temperature of ≤800°C as discussed by the authors.
Abstract: The results of a complex study of melt inclusions in olivine phenocrysts contained in unaltered kimberlites from the Udachnaya-East pipe indicate that the inclusions were captured late during the magmatic stage, perhaps, under a pressure of <1 kbar and a temperature of ≤800°C. The inclusions consist of fine crystalline aggregates (carbonates + sulfates + chlorides) + gas ± crystalline phases. Minerals identified among the transparent daughter phases of the inclusions are silicates (tetraferriphlogopite, olivine, humite or clinohumite, diopside, and monticellite), carbonates (calcite, dolomite, siderite, northupite, and Na-Ca carbonates), Na and K chlorides, and alkali sulfates. The ore phases are magnetite, djerfisherite, and monosulfide solid solution. The inclusions are derivatives of the kimberlite melt. The complex silicate-carbonate-salt composition of the secondary melt inclusions in olivine from the kimberlite suggests that the composition of the kimberlite melt near the surface differed from that of the initial melt composition in having higher contents of CaO, FeO, alkalis, and volatiles (CO2, H2O, F, Cl, and S) at lower concentrations of SiO2, MgO, Al2O3, Cr2O3, and TiO2. Hence, when crystallizing, the kimberlite melt evolved toward carbonatite compositions. The last derivatives of the kimberlite melt had an alkaline carbonatite composition.
83 citations
TL;DR: The Tsaolingshan lavas from Taiwan's Northern Taiwan Volcanic Zone as mentioned in this paper have been shown to have a similar composition to the Group I ultrapotassic orogenic lamproites from central Italy, Span and Tibet.
83 citations