Cannibalization of an amphibole-rich andesitic progenitor induced by caldera-collapse during the Matahina eruption: Evidence from amphibole compositions
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Abstract:
The diverse range of calcic-amphibole compositions in eruptive products from the ca. 330 ka Matahina eruption (ca. 160 km3 rhyolitic magma) of the Okataina Volcanic Complex, Taupo Volcanic Zone, including crystal-rich basalt-dacite pumice from post-collapse deposits, reveals several pre- and syn-eruption magmatic processes. (1) Amphibole phenocrysts in the basaltic-andesite and andesite crystallized at the highest pressures and temperatures (P to 0.6 ± 0.06 GPa and T to 950 °C), equivalent to mid-crustal depths (13-22 km). Inter- and intra-crystalline compositions range from Timagnesiohastingsite → Ti-tschermakite → tschermakite → magnesiohornblende and some display gradual decreases in T from core to rim, both consistent with magma differentiation by cooling at depth. (2) The largest amphibole crystals from the basaltic-andesite to andesite display several core to rim increases in T (to 70 °C), indicating that new, hotter magma periodically fluxed the crystal mush. (3) The dominant population of rhyolite amphibole is small and bladed (magnesiohornblende) and crystallized at low P-T conditions (P = 0.3 GPa, T = 765 °C), equivalent to the eruptive P-T conditions. Dacitic and low-silica rhyolitic amphibole (tschermakite-magnesiohornblende) form two distinct populations, which nucleated at two different T (high: 820 °C and low: 750 °C). These compositional variations, governed primarily by differences in T conditions during crystal growth, record the mixing of two distinct amphibole populations that approached a thermal equilibrium at the eruptive temperature. Therefore, the diversity in amphibole compositions can be reconciled as an exchange of crystals + liquid between the basaltic-andesite to dacite from the mid-crust and rhyolite from the upper crust, which quenched against one another, modifying the dacite to low-silica rhyolite compositions as the eruption progressed.Keywords:
Amphibole
Basaltic andesite
Phenocryst
Igneous differentiation
Strombolian eruption
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We found high-Mg andesite (56.5 wt.% SiO2 and 7.2 wt.% MgO) from Mikasayama in Wassamu town, northern Hokkaido. Its K-Ar age is 11.1±0.8 Ma. The high-Mg andesite is characterized by co-existence of Fo-rich olivine (Fo90-85) and An-poor plagioclase (An64-38) phenocrysts. The mineralogical evidence suggests that the high-Mg andesite from Mikasayama was produced by mixing of primitive basalt magma, containing Mg-rich olivine and clinopyroxene phenocrysts, and hornblende dacite magma.
Phenocryst
Dacite
Basaltic andesite
Igneous differentiation
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Phenocryst
Silicic
Magma chamber
Igneous differentiation
Basaltic andesite
Pumice
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Abstract Barujari volcano is an active volcano located in Lombok Island Indonesia. The volcano is a part of post caldera stage of Rinjani volcano. The volcano shows strombolian to vulcanian type eruption and produces mainly lava with basaltic-andesitic composition (54-56 wt% SiO 2 ). The volcano actively erupts several times during past 70 years. However, temporal change of its petrological characteristic has not been discussed. This paper discusses petrological changes in Barujari volcano erupted material through time from AD 1944 to 2015. Total 31 samples erupted from AD 1944, 1966, 1994, 2004, 2009, and 2015 are observed and analyzed with XRF method. Our study shows that most of samples can be classified into basaltic trachy-andesite with 53-56 wt% SiO 2 . All samples show nearly linear trend in harker diagram and shows no systematic change through time. However, AD 1966 shows curved trend and more silicic composition compared to other samples. We suggest that magma mixing with repeated injection of more mafic magma play an important role in producing most of basaltic-andesitic Barujari lava as suggested by linear trend of harker diagram and appearance of heterogeneity in groundmass. AD 1966 samples also contain evidence of magma mixing but curved trend does not suggest simple mixing processes. Appearance of olivine-poor samples in SiO 2 rich AD 1966 samples might suggest more close system differentiation process that mixed with common Barujari products.
Strombolian eruption
Silicic
Basaltic andesite
Igneous differentiation
Phenocryst
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We found high-Mg andesite (56.5 wt.% SiO2 and 7.2 wt.% MgO) from Mikasayama in Wassamu town, northern Hokkaido. Its K-Ar age is 11.1±0.8 Ma. The high-Mg andesite is characterized by co-existence of Fo-rich olivine (Fo90-85) and An-poor plagioclase (An64-38) phenocrysts. The mineralogical evidence suggests that the high-Mg andesite from Mikasayama was produced by mixing of primitive basalt magma, containing Mg-rich olivine and clinopyroxene phenocrysts, and hornblende dacite magma.
Phenocryst
Dacite
Basaltic andesite
Igneous differentiation
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Phenocryst
Dacite
Silicic
Igneous differentiation
Basaltic andesite
Lava dome
Magma chamber
Felsic
Melt inclusions
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Andesite magmatism plays a major role in continental crustal growth, but its subduction-zone origin and evolution is still a hotly debated topic. Compared with whole-rock analyses, melt inclusions (MIs) can provide important direct information on the processes of magma evolution. In this article, we synthesize data for melt inclusions hosted by phenocrysts in andesites, extracted from the GEOROC global compilation. These data show that melt inclusions entrapped by different phenocrysts have distinct compositions: olivine-hosted melt inclusions have basalt and basaltic andesite compositions, whereas melt inclusions in clinopyroxene and othopyroxene are mainly dacitic to rhyolitic. Hornblende-hosted melt inclusions have rhyolite composition. The compositions of melt inclusions entrapped by plagioclase are scattered, spanning from andesite to rhyolite. On the basis of the compositional data, we propose a mixing model for the genesis of the andesite, and a two-chamber mechanism to account for the evolution of the andesite. First, andesite melt is generated in the lower chamber by mixing of a basaltic melt derived from the mantle and emplaced in the lower crust with a felsic melt resulting from partial melting of crustal rocks. Olivine and minor plagioclase likely crystallize in the lower magma chamber. Secondly, the andesite melt ascends into the upper chamber where other phenocrysts crystallize. According to SiO2-MgO diagrams of the MIs, evolution of the andesite in the upper chamber can be subdivided into two distinct stages. The early stage (I) is characterized by a phenocrystal assemblage of clinopyroxene + othopyroxene + plagioclase, whereas the late stage (II) is dominated by crystallization of plagioclase + hornblende.
Phenocryst
Melt inclusions
Andesites
Igneous differentiation
Basaltic andesite
Felsic
Magma chamber
Fractional crystallization (geology)
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Phenocryst
Igneous differentiation
Magma chamber
Basaltic andesite
Lava dome
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Abstract Many andesites erupted at arc volcanoes are the products of magma mixing, and this process could be important in triggering their eruption. Throughout the life of Ngauruhoe volcano (∼2.5 ka), erupted andesites lack macroscopic enclaves or lithological features of mingled magma. However, intricate microscopic features of magma mixing and mingling occur. Plagioclase phenocrysts have a wide range of textures. Their diverse 87 Sr/ 86 Sr values (0.7039–0.7060) are mostly discordant with the groundmass (∼80%), and many phenocrysts (>40%) are isotopically zoned. Resorbed calcic cores overgrown by sodic rims are common, and record a gradient of increasing 87 Sr/ 86 Sr, the result of progressive mixing with a radiogenic melt. Clinopyroxenes and orthopyroxenes have relict cores that nucleated in silicic melts (∼Mg# 30–40), and their rims record cycles of growth in mafic melts (∼Mg# 50–60). Olivine‐bearing, mafic glassy blebs (mostly <500 μm) that occur in the groundmass are the relicts of the intruding magma that disintegrated during the mixing/mingling process, and liberated crystals. Thus, some andesites that lack banding or enclaves are in fact the product of thorough mixing of contrasting magmas. This may require a regime of frequent intrusion leading to hybridization with the stagnant resident magma. Thus, frequent eruptions during Ngauruhoe's historic episode ending in 1975 CE are likely to have been a manifestation of frequent replenishment. A mixing origin for intermediate rocks at volcanic arcs is likely to be more prevalent than previously documented: further evidence for the rarity of andesite liquids in such settings.
Phenocryst
Igneous differentiation
Andesites
Silicic
Basaltic andesite
Scoria
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Abstract Amphiboles in dykes from the dominantly silica-undersaturated Monteregian series range from pargasitic megacrysts and xenocrysts to kaersutitic, pargasitic and hastingsitic phenocrysts, groundmass prisms and reaction rims. Amphiboles in dykes and plutons from the silica-oversaturated White Mountain Magma Series range from kaersutite, through hornblende, hastingsite and edenite, to sodic-calcic and sodic varieties. This contrast between the amphiboles from the two series is probably a reflection of differing melt silica activity and is a useful petrologic discriminant. In most cases, pargasitic amphibole megacrysts from Monteregian monchiquites reflect the Mg numbers of their host rocks and are considered cognate. The megacrysts are lower in Ti and higher in Mg and Al than their phenocryst mantles and rims. This is probably a result of higher pressures of formation.
Amphibole
Phenocryst
Alkali basalt
Hornblende
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