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Crystallization and textural evolution of a closed-system magma chamber: insights from a crystal size distribution study of the Lilloise layered intrusion, East Greenland

Published online by Cambridge University Press:  08 March 2010

C. MAGEE ,
B. O'DRISCOLL  and
A. D. CHAMBERS
C. MAGEE*
Affiliation:
School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK
B. O'DRISCOLL
Affiliation:
School of Physical and Geographical Sciences, Keele University, Keele, UK
A. D. CHAMBERS
Affiliation:
School of Geography, Earth and Environmental Sciences, University of Birmingham, Birmingham, UK
*
†Author for correspondence: [email protected]
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Abstract

The recognition that postcumulus processes significantly modify primary textures in layered mafic intrusions has thrown into question many early observations on which classical crystallization theories are based. Petrographic observations combined with quantitative textural analysis of samples from various stratigraphic levels of the Lilloise intrusion, East Greenland, demonstrate that postcumulus textural modification of cumulates, formed during the solidification of a closed system magma chamber, may be detected. Crystal size distribution (CSD) measurements of Lilloise cumulates and the resulting CSD profiles are compared to predicted theoretical closed system CSD profiles. Similarities between the measured CSD profiles and published predicted CSD profiles support Lilloise magma evolving in a closed system chamber and indicate that primary crystallization processes can be distinguished using quantitative textural techniques. Textural coarsening driven by syn-magmatic deformation is suggested to be the dominant postcumulus process affecting CSD plot morphology. CSD slope values and profiles (plot shapes) remain relatively constant for a given liquidus mineral (particularly olivine and clinopyroxene), so that the number of phases on the liquidus at any one time affects mineral modal abundances. As a result, CSDs generally exhibit overall smaller grainsizes and progressively lower nucleation densities at higher levels in the intrusion.

Keywords

Lilloiselayered mafic intrusioncrystal size distributionclosed systemsolidification
Type
Original Article
Information
Geological Magazine , Volume 147 , Issue 3 , May 2010 , pp. 363 - 379
Copyright
Copyright © Cambridge University Press 2010

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