Petrogenesis of high-Mg# andesite lavas from Ruapehu volcano, New Zealand
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The majority of arc-type andesites in the Tongariro Volcanic Centre are highly porphyritic, hornblende-free, two-pyroxene andesites. An exception is tephras from the c. 10,000 ka Pahoka-Mangamate event. Magmas of these Plinian eruptions bypassed the extensive crustal mush columns under the central volcanoes and sequentially derived a series of almost aphyric rocks spanning a compositional range from dacite to basaltic andesite. Mineral composition, trace element and isotopic data suggest that this eruptive series tapped a mid-crustal magma reservoir, resulting in the initial eruption of an hydrous dacitic magma and several following eruptions characterised by less-evolved and less-hydrous compositions at progressively higher temperatures and substantially lower 87Sr/86Sr ratios. Systematic changes in magma chemistry are also reflected in a sequential change in phenocryst content starting with an early hornblende–plagioclase-dominated assemblage to a late olivine–plagioclase-dominated assemblage.
Phenocryst
Andesites
Porphyritic
Dacite
Hornblende
Magma chamber
Fractional crystallization (geology)
Volcanic arc
Igneous differentiation
Basaltic andesite
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Andesites
Lava dome
Geochronology
Magma chamber
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Hornblende
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We conducted textural and chemical analyses of melt inclusions and their host plagioclase crystals in the scoria of the Izu–Omuroyama monogenetic volcano, erupted at ~ 4 ka in the Higashi–Izu monogenetic volcanic field, Japan. The groundmass melt was andesitic with ~ 59–61 wt% SiO2, and it contained abundant microphenocrysts of olivine and plagioclase. In contrast, ~ 59% of the plagioclase–hosted melt inclusions have rhyolitic compositions with ~ 70–75 wt% SiO2. The host plagioclase phenocrysts have cores with An# of 44.7 ± 4.2 [An# = 100Ca/(Ca + Na) in mol] and rims with An# of 68–78, and the calcic rims have compositions similar to the microphenocrysts. The cores of the host plagioclase phenocrysts have FeO* and K2O contents that are in equilibrium with the rhyolitic melt inclusions. Using the plagioclase–melt geohygrometers and assuming temperatures of 790–850 °C, we estimated the H2O contents of the rhyolitic melt inclusions to be ~ 4.4–10.2 wt%, indicating H2O–saturation depths of >4.5 km. Our results suggest that an inhibited reservoir of plagioclase–bearing rhyolitic melt existed beneath the monogenetic volcano at the time of the scoria eruption, which was ~ 800 years earlier than the first rhyolitic eruption in the volcanic field. Plagioclase content in the silicic reservoir is estimated to be less than 35.8%, suggesting the magma was eruptible. Our results demonstrate the potential usefulness of plagioclase–hosted melt inclusions for indicating the existence of such an inhibited silicic magma.
Phenocryst
Silicic
Melt inclusions
Scoria
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Arc volcanoes often erupt andesite that appears to have been stored in reservoirs at shallow depth for protracted periods. As crystal-rich andesite is close in density to upper crust, such storage may be quite stable. Petrological evidence, and occasionally geological and geophysical evidence as well, suggests that the immediate trigger for eruption of the stored magma is injection of new magma into the reservoir, presumably through dykes rising from depth. When the dyke magma is more mafic than the stored andesite, effusive eruption typically results. When the dyke magma is voluminous and more silicic, the results are catastrophic, with production of discontinuously zoned tephra deposits and caldera collapse. Contrasting end-members are illustrated by the eruptions of Karymsky Volcano in 1996 and of Mt Katmai in 1912.
Silicic
Caldera
Basaltic andesite
Dense-rock equivalent
Lateral eruption
Magma chamber
Strombolian eruption
Effusive eruption
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Lava dome
Dome (geology)
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Volcan Tepetiltic is located in the northwestern segment of the Mexican Volcanic Belt, about 40 km SW of the city of Tepic. The structure is a calc-alkaline stratovolcano composed primarily of andesite and dacite lava flows topped by an elliptical caldera measuring approximately 5 by 2.5 km. At least two cycles of andesite volcanism followed by rapid differentiation into volumetrically subordinate dacite flows and dikes built the majority of the complex. The second pulse of andesitic lavas were more basic than the first and appear to have been the result of reinjection of mafic magma into the shallow andesitic magma chamber. This was closely followed by the emplacement of two rhyolite domes and associated ash deposits on the eastern flank of the volcano. Finally, two small hornblende andesite domes were erupted on the floor of the caldera, and a lake formed in the northeastern corner of the caldera. Cinder cones on the flanks of the volcano have erupted alkaline lavas of mugearitic affinity. These are chemically unrelated to the calc-alkaline lavas erupted from Tepetiltic itself. The latest activity of Tepetiltic was the emplacement of a crystal rich rhyolite domes on the southern flank, which has blocked stream drainages to formmore » a coulee lake. This last event has occurred within the last several thousand years. The rocks erupted from Tepetiltic form a chemically continuous suite which could have been derived through crystal fractionation of andesitic magma. No basic parental magmas, however, have erupted throughout the area.« less
Dacite
Stratovolcano
Caldera
Magma chamber
Lava dome
Silicic
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The Jinlongdingzi active volcano erupted before 1600a,and it is the latest basaltic explosive volcano at Longgang Volcano.Its volcanic products include the Jinlongdingzi Volcanic cone(elevation 999.4m),the lava flow and the widely-spread volcanic pyroclastic sheet(sihai Pyroclastic Sheet),Jinlongdingzi volcanic rocks are trachybasalts with very similar REE patterns and incompatible element patterns,and their 87Sr/86Sr and 143Nd/144Nd ratios range from 0.704846 ot 0.704921 and from 0.512619 to 0.512646,respectively.It is revealed that the trachybasalt has the character of primary magma derived directly from mantle sources with very little evolution and crust contamination during its ascending.The younger mantle xenoliths demonstrate that the mantle source of the Jinlongdingzi Volcao is hydrous,with relatively low temperature.
Lava dome
Peléan eruption
Xenolith
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The Journal of the Japanese Association of Mineralogists Petrologists and Economic Geologists (1974)
Kayo-dake volcano is located in Akita Prefecture and is one of the volcanoes of Nasu Volcanic Zone. The main stage of volcanic activity was in the Early Quaternary. The volcano consists of lavas and a small amount of pyroclastic rocks. They are basalt, mafic andesite and andesite which belong to the tholeiite series and andesite of the calc-alkali rock, series. The rocks of the tholeiite series surpass those of the calc-alkali rock series in quantity. In the beginning of volcanic activity, basalt and mafic andesite of the tholeiite series were effused, and then, andesite belonging to the same rock series was erupted. After an intermission of activity, andesite of the calc-alkali rock series forming parasitic volcanoes, lava flows from a central crater, and a central cone, was effused. It may be a distinctive feature of this volcano that a comparatively large amount of andesite of the tholeiite series is present.
Basaltic andesite
Alkali basalt
Dacite
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