Genesis and evolution of subduction-zone andesites: evidence from melt inclusions
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Abstract:
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.Keywords:
Phenocryst
Melt inclusions
Andesites
Igneous differentiation
Basaltic andesite
Felsic
Magma chamber
Fractional crystallization (geology)
Phenocryst
Melt inclusions
Igneous differentiation
Fractional crystallization (geology)
Incompatible element
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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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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
Andesites
Basaltic andesite
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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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Petrographical and geochemical characteristics of calc-alkaline andesites on Shodo-Shima Island, SW Japan, having bulk compositions largely identical to the continental crust, are presented. The following petrographic observations suggest a role for magma mixing in producing such andesite magmas: (1) two types of olivine phenocrysts and spinel inclusions, one with compositions identical to those in high-Mg andesites and the other identical to those in basalts, are recognized in terms of Ni–Mg and Cr–Al–Fe3+ relations, respectively; (2) the presence of orthopyroxene phenocrysts with mg-number >90 suggests the contribution of an orthopyroxene-bearing high-Mg andesite magma to production of calc-alkaline andesites; (3) reversely zoned pyroxene phenocrysts may not be in equilibrium with Mg-rich olivine, suggesting the involvement of a differentiated andesite magma as an endmember component; (4) the presence of very Fe-rich orthopyroxene phenocrysts indicates the association of an orthopyroxene-bearing rhyolitic magma. Contributions from the above at least five endmember magmas to the calc-alkaline andesite genesis can also provide a reasonable explanation of the Pb–Sr–Nd isotope compositions of such andesites.
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Andesites
Igneous differentiation
Basaltic andesite
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Basaltic andesite
Pyroxene
Igneous differentiation
Magma chamber
Silicic
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Andesites
Dacite
Igneous differentiation
Basaltic andesite
Fractional crystallization (geology)
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Andesites
Silicic
Basaltic andesite
Fractional crystallization (geology)
Igneous differentiation
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Pumice
Melt inclusions
Igneous differentiation
Silicic
Magma chamber
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