A new approach to the classification of igneous rocks using the basalt-andesite-dacite-rhyolite suite as an example
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Dacite
Basaltic andesite
Igneous petrology
Volcanology
The late Miocene Engaru volcanic field in northeastern Hokkaido contains basalts, Tomeoka basalt (TM) and Chiyoda-Kakurezawa basalt (CK), and rhyolite, Wakamatsu rhyolite (WK), with sobordinate amounts of andesite, Sakaeno andesite (SK), which erupted during 7-9 Ma.Both the TM and CK basalts have geochemical characteristics similar to those of back-arc basin basalt (BABB). Based on differences in major and trace element abundances and the initial values of Sr and Nd isotopic ratios (SrI and NdI), the CK basalt, SK andesite and WK rhyolite can be divided into two types (I and II), respectively. The SK type I andesite and WK type I rhyolite are of calc-alkaline series, whereas the SK type II andesite (including icclandite-like andesite) and WK type II rhyolite are of tholeiitic series.The WK type I rhyolite has remarkably high SrI and low NdI values compared with other volcanic rocks from the study area. The similarity in SrI of the rhyolite to S-type granitoids and pelitic-psammitic rocks of the Hidaka belt suggests the crustal origin of the rhyolite. The combined major- and trace-element and Sr- and Nd- isotopic data indicate that the main generation process of the SK type I andesite was the mixing of basaltic magma (the CK type I basalt) with some felsic magmas. The felsic magmas are the WK type I rhyolite magma and rhyolitic magmas having higher Sr and Nd contents, and higher SrI and lower NdI values than the WK type I rhyolite. On the other hand, the SK type II andesite and WK type II rhyolite may have been derived from the CK type II basaltic magma by fractional crystallization accompanied by a minor degree of assimilation of crustal materials.The genetic relationship of these coeval basalts, andesite and rhyolite can be attributed to spreading of the Kurile basin. BABB magma which is a partial melt in a hot asthenosphere uprising below the island are during the basin spreading, could have heated the crust to generate calc-alkaline rhyolitic magma. Andesitic rocks were derived both by mixing of BABB magma with the crust-derived rhyolitic magma and by fractional ctystallization of BABB magma.
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Abstract Late Pleistocene tephra deposits found from Sitka to Juneau and Lituya Bay are assigned to a source at the Mount Edgecumbe volcanic field, based on similarity of glass compositions to nearvent deposits and on thinning away from Kruzof Island. The sequence of near-vent layers is basaltic andesite and andesite at the base, rhyolite, and mixed dacite and rhyolite on top. The only breaks in the tephra sequence are two 1-mm-thick silt partings in a lake-sediment core, indicating a depositional interval from basaltic andesite to dacite of no more than about a millennium. Tephra deposits at sites >30 km from the vent are solely dacite and rhyolite and are 10,600 to 11,400 14 C yr old based on interpretation of 18 radiocarbon ages, including 5 by accelerator mass spectrometry (AMS). Basaltic andesite and andesite deposits nearer the vent are as much as 12,000 yr old. Discrepancy among radiocarbon ages of upland tephra deposits provisionally correlated as the same grainfall is resolvable within ±2 σ of analytical uncertainty. Comparison of bulk and AMS ages in one sediment core indicates a systematic bias of +600 to +1100 yr for the bulk ages; correlation of tephra deposits among upland and lacustrine sites implies an additional discrepancy of 200–400 yr between upland (relatively too young) and lacustrine ages. In any case, the Mount Edgecumbe tephra deposits are a widespread, latest Pleistocene stratigraphic marker that serves to emphasize the uncertainty in dating biogenic material from southeastern Alaska.
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Igneous petrology
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Abstract The ~5 km3, 4.54–4.09 Ma Caspana ignimbrite of the Altiplano-Puna volcanic complex (APVC) of the Central Andes records the eruption of an andesite and two distinct rhyolitic magmas. It provides a unique opportunity to investigate the production of silicic magmas in a continental arc flare-up, where small volumes of magma rarely survive homogenization into the regional magmatic system that is dominated by supereruptions of monotonous dacitic ignimbrites. The fall deposit and thin flow unit that record the first stage of the eruption (Phase 1) tapped a crystal-poor peraluminous rhyolite. The petrological and geochemical characteristics of Phase 1 are best explained by partial melting of or reheating and melt extraction from a granodioritic intrusion. Phase 2 of the eruption records the emplacement of a more extensive flow unit with a crystal-poor, fayalite-bearing rhyolite and a porphyritic to glomeroporphyritic andesite containing abundant plagioclase-orthopyroxene-Fe-Ti oxide (norite) glomerocrysts. The isotopic composition of Phase 2 is significantly more “crustal” than Phase 1, indicating a separate petrogenetic path. The mineral assemblage of the noritic glomerocrysts and the observed trend between andesite and Phase 2 rhyolite are reproduced by rhyolite-MELTS–based models. Pressure-temperature-water (P-T-H2O) estimates indicate that the main (Phase 2) reservoir resided between 400 and 200 MPa, with the andesite recording the deeper pressures and a temperature range of 920–1060 °C. Rhyolite phase equilibria predict an estimated temperature of ~775 °C and ~5 wt% H2O. Pressures derived from phase equilibria indicate that the rhyolite was extracted directly from the noritic cumulate at ~340 MPa and stored at slightly shallower pressures (200–300 MPa) prior to eruption. The rhyolite-MELTS models reveal that latent-heat buffering during the extraction and storage process results in a shallow liquidus during the extensive crystallization that produced a noritic cumulate in equilibrium with a rhyodacitic residual liquid. Spikes in latent heat facilitated the segregation of the residual liquid, creating the pre-eruptive compositional gap of ~16 wt% SiO2 between the andesite and the Phase 2 rhyolite. Unlike typical Altiplano-Puna volcanic complex (APVC) magmas, low fO2 conditions in the andesite promoted co-crystallization of orthopyroxene and ilmenite in lieu of clinopyroxene and magnetite. This resulted in relatively high Fe concentrations in the rhyodacite and Phase 2 rhyolite. Combined with the co-crystallization of plagioclase, this low oxidation state forced high Fe2+/Mg and Fe/Ca in the Phase 2 rhyolite, which promoted fayalite stability. The dominance of low Fe3+/FeTot and Fe-Ti oxide equilibria indicates low fO2 (ΔFMQ 0 − ΔFMQ − 1) conditions in the rhyolite were inherited from the andesite. We propose that the serendipitous location on the periphery of the regional thermal anomaly of the Altiplano-Puna magma body (APMB) permitted the small-volume magma reservoir that fed the Caspana ignimbrite eruption to retain its heterogeneous character. This resulted in the record of rhyolitic liquids with disparate origins that evaded assimilation into the large dacite supereruption-feeding APMB. As such, the Caspana ignimbrite provides a unique window into the multi scale processes that build longlived continental silicic magma systems.
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Evidence from several volcanic regions indicates that andesite and dacite of the basalt-andesite-dacite-rhyolite association form by contamination of primary basalt and rhyolite magmas by material of the volcanic pile. This evidence includes bulk composition, phenocryst assemblage, inclusion content of lava, and the association of different lava types in time and space. The dacite of Glass Mountain, formed by contamination of rhyolite magma as it intruded the Medicine Lake highland shield volcano, illustrates this process.
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Research subject. Zircons from the Saf’yanovskoe Cu-Zn deposit rhyolite (Middle Urals). For the first time, zircon U-Pb dating for the rhyolite of the ore-bearing volcanic-sedimentary rocks of the Saf’yanovskoe deposit was performed. The volcanites are characterized by an andesite-rhyodacite composition and are localized at the southern edge of the Rezhevskaya structural-formation zone (SFZ) of the Eastern Ural megazone. A number of publications assign these rocks either to the basalt-rhyolite formation of the Middle Devonian, or to the basalt-andesite-dacite-rhyolite formation of the Lower-Middle Devonian. Aim. To estimate the age of the ore-bearing volcanic rocks under study using the U-Pb SHRIMP-II isotop ic system of zircon from the rhyolite of the eastern side of the Saf’yanovskoe deposit. By its chemical composition, the rhyolite belongs to the silicic varieties of subvolcanic rocks. Methods and results. The U-Pb isotopic system of zircon was studied by 5-collector mass-spectrometer of high precision and emission of the secondary ions SHRIMP-II (ASI, Australia) in the VSЕGEI Institute. U-Pb relations were investigated by a procedure developed by I.S. Williams. The U-Pb data obtained based on 13 zircon grains showed the age of 422.8 ± 3.7 Ma. Conclusions. The U-Pb dating of zircon obtained previously from the lens-shaped andesite bodies of the western side of the Safyanovskoe deposit gave the age of 422.8 Ma, which corresponds to the Przydoli series epoch of the Upper Silurian. We established that, among the volcanic rocks of the Saf’yanovskoe deposit, the effusive formations of the Upper Silurian are present.
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Geochronology
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