Abstract:
The mechanism of fractionation in a sheet-like magma body was investigated based on observations from the Nosappumisaki intrusion, northern Japan. This is one of a number...Show MoreMetadata
Abstract:
The mechanism of fractionation in a sheet-like magma body was investigated based on observations from the Nosappumisaki intrusion, northern Japan. This is one of a number of late Cretaceous shoshonite sills in the Nemuro peninsula and has a compound structure comprising a bottom cumulate zone containing ∼50 vol. % cumulus crystals and a overlying middle zone free of such cumulus crystals. These zones are sandwiched between porphyritic rocks of upper and lower chilled and marginal zones. It is inferred that massive crystal settling or flotation from a magma initially laden with ∼20 vol. % of crystals (primocrysts) in a sheet-like magma body formed a cumulus pile and an overlying crystal-free melt layer. After the formation of the compound structure with partial freezing of the upper and lower marginal zones, the internal part of the intrusion slowly solidified. During this period the melt in the central part of the intrusive sheet underwent fractional crystallization. This is shown by patterns of vertical variation in the composition of the interstitial melt. The overall patterns are inverted (mirror-image) S-shaped and S-shaped for incompatible and compatible elements, respectively. The incompatible element abundances show maxima near the bottom of the middle zone, which corresponds to the final solidification horizon. They show minima in the cumulate zone, which suggests discharge of an evolved melt from the crystal pile. Fractionation is inferred to have taken place via compositional convection without settling of crystals grown after the intrusion event or collapse of the upper boundary layer. The melt in the central melt layer descended into the permeable crystal pile counterbalancing discharge of buoyant evolved melt formed by crystallization of the pore melt in the crystal pile. This conclusion is based on the following observations. (1) There is no accumulation of crystals grown after the intrusion of the magma above the cumulate zone. (2) Leucocratic pipe-li...
Published in: Journal of Petrology ( Volume: 52, Issue: 10, October 2011)
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- Index Terms
- Northern Japan ,
- Magma Body ,
- Compositional Convection ,
- Chemical Composition ,
- Number Density ,
- Structure Of Compound ,
- Boundary Layer ,
- Cumulated ,
- Closed System ,
- Marginal Zone ,
- Lower Zone ,
- Cumulus ,
- Average Composition ,
- Olivine ,
- Fractional Crystallization ,
- Middle Zone ,
- Variation In Chemical Composition ,
- Part Of Zone ,
- Upper Zone ,
- Downwelling ,
- Top Zone ,
- Alkali Feldspar ,
- Clinopyroxene ,
- Euhedral ,
- Layering ,
- Magnetite ,
- One-dimensional Model ,
- Zeolite ,
- Partial Dissolution ,
- Intrusion ,
- Magma Composition ,
- Function Of Time ,
- Total Mass ,
- Step Function ,
- Lower Layer ,
- Infinitesimal ,
- Zone Boundary ,
- Entire Zone ,
- Melt Fraction ,
- Melt Layer
- Author Keywords
Keywords assist with retrieval of results and provide a means to discovering other relevant content. Learn more.
- Index Terms
- Northern Japan ,
- Magma Body ,
- Compositional Convection ,
- Chemical Composition ,
- Number Density ,
- Structure Of Compound ,
- Boundary Layer ,
- Cumulated ,
- Closed System ,
- Marginal Zone ,
- Lower Zone ,
- Cumulus ,
- Average Composition ,
- Olivine ,
- Fractional Crystallization ,
- Middle Zone ,
- Variation In Chemical Composition ,
- Part Of Zone ,
- Upper Zone ,
- Downwelling ,
- Top Zone ,
- Alkali Feldspar ,
- Clinopyroxene ,
- Euhedral ,
- Layering ,
- Magnetite ,
- One-dimensional Model ,
- Zeolite ,
- Partial Dissolution ,
- Intrusion ,
- Magma Composition ,
- Function Of Time ,
- Total Mass ,
- Step Function ,
- Lower Layer ,
- Infinitesimal ,
- Zone Boundary ,
- Entire Zone ,
- Melt Fraction ,
- Melt Layer
- Author Keywords