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Magmatic conditi ons inferred from the composition of the FishCanyon Tuff by Whitney &Stormer (J. Petrology, 26, 726–62(1985)) and Stormer & Whitney (Am. Miner. 70, 52–64(1985)) differ from studies of other ash-flow tuff and calderasystems, in that they consider the magma to have been unzonedand to have resided at {small tilde}9 kb pressure just priorto eruption. We find these conclusions unconvincing becauseof incomplete sampling of the Fish Canyon Tuff, alteration ofthe tuff after emplacement, and, in particular, serious limitationsof coupled Fe-Ti oxide and two-feldspar geothermobarometry. 相似文献
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Physical and Chemical Models of Zoned Silicic Magmas: The Loma Seca Tuff and Calabozos Caldera, Southern Andes 总被引：1，自引：3，他引：1
The late Pleistocene Calabozos ash-flow and caldera complexlies in central Chile in a section of the Andean cordillerathat is transitional between dominantly andesitic-to-rhyoliticvolcanism to the north and mafic andesitic and high-aluminabasaltic volcanism to the south. The Calabozos rocks range incomposition from basaltic andesite to rhyodacite and definea high-K calcalkalic suite. They contain 2–25% phenocrystsof plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides,and apatite, ? minor biotite or amphibole. More than 1000km3 of rhyodacitic to dacitic magma erupted atthe Calabozos caldera complex as three major compositionallyzoned ash-flow sheets, Unit L (0?8 Ma), Unit V (0? 30Ma), andUnit S (0?15 Ma) of the Loma Seca Tuff. Phenocryst modes, trace-elementcontents, inferred magmatic volatile contents, and oxygen fugacitiesvary systematically with major-element composition in the tuffs.In the cases of Units V and S, it is possible to reconstructcompositional, thermal, and volatile gradients that existedin density-stratified magma chambers shortly prior to theireruption. The magma graded from crystal-poor, water-rich, andbiotite-bearing rhyodacite in the upper reaches of the chamberto more crystal-rich, water-poor, and amphibole-bearing daciteat deeper levels. Fe-Ti oxide equilibration temperatures are800 to 900?C for rhyodacite and 900 to 950?C for dacite. Magmathat erupted as Unit S was slightly hotter and more oxidizedthan magma that gave rise to Unit V. More mafic magmas wereassociated with the voluminous rhyodacitic to dacitic magmareservoir, as indicated by the presence of andesite and basalticandesite lava flows and by scoria inclusions in Unit V. The compositional suite from basaltic andesite to rhyodacitecan be simulated satisfactorily by crystal-fractionation calculationsbased on major-element phenocryst and rock compositions, andis consistent with modes of the Calabozos rocks. Rhyodacitesof Units V and S, however, are enriched in elements such asRb, Ba, and Zr relative to trace-element contents predictedby crystal-fractionation models. The enrichment can be achievedby assimilation of wall rock or a partial melt of the wall rock.The latter requires that the ratio of assimilation rates tocrystallization rates be between 0?1 and 0?3. Rates of assimilationversus crystallization were greater for Unit S than for UnitV, which is consistent with the lower Fe-Ti oxide temperaturesand less oxidized state of the latter. The Loma Seca Tuff is similar in bulk composition to sanidine-bearingash-flow sheets erupted on ‘mature’ continentalcrust, but it is mineralogically akin to ash-flow tuffs eruptedon ‘immature’ crust. The difference is attributed,in part, to the effect of the density of the crust on the rateof magma ascent at shallow levels. The ascent of large bodiesof silicic magma is slower in silicic (less dense) crust thanin mafic crust, causing the magmas to be erupted at a laterpoint in the crystallization history. 相似文献
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Textural and Thermal History of Partial Melting in Tonalitic Wallrock at the Margin of a Basalt Dike, Wallowa Mountains, Oregon 总被引：2，自引：1，他引：2
Columbia River Basalt Group dikes invade biotite–hornblendetonalite to granodiorite rocks of the Wallowa Mountains. Mostdikes are strongly quenched against wallrock, but rare dikesegments have preserved zones of partial melt in adjacent wallrockand provide an opportunity to examine shallow crustal melting.At Maxwell Lake, the 4 m thick wallrock partial melt zone containsas much as 47 vol. % melt (glass plus quench crystals) aroundmineral reaction sites and along quartz–feldspar boundaries.Bulk compositional data indicate that melting took place underclosed conditions (excepting volatiles). With progressive melting,hornblende, biotite, and orthoclase were consumed but plagioclase,quartz, and magnetite persisted in the restite. Clinopyroxene,orthopyroxene, plagioclase, and Fe–Ti oxides were producedduring dehydration-melting reactions involving hornblende andbiotite. Reacting phases became more heterogeneous with progressivemelting; crystallizing phases were relatively homogeneous. Progressivemelting produced an early clear glass, followed by light (high-K)and dark (high-Ca) brown glass domains overprinted by devitrification.Melts were metaluminous and granitic to granodioritic. Thermalmodeling of the partial melt zone suggests that melting tookplace over a period of about 4 years. Thus, rare dikes withmelted margins represent long-lived portions of the ColumbiaRiver Basalt dike system and may have sustained large flows. KEY WORDS: Columbia River Basalt dike; crustal melting; dehydration-melting; tonalite–granodiorite; thermal model 相似文献
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