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Melt movement and the occlusion of porosity in crystallizing granitic systems

Published online by Cambridge University Press:  05 July 2018

D. N. Bryon ,
M. P. Atherton ,
M. J. Cheadle  and
R. H. Hunter
D. N. Bryon
Affiliation:
Department of Earth Sciences, University of Liverpool, Liverpool, L69 3BX, UK
M. P. Atherton
Affiliation:
Department of Earth Sciences, University of Liverpool, Liverpool, L69 3BX, UK
M. J. Cheadle
Affiliation:
Department of Earth Sciences, University of Liverpool, Liverpool, L69 3BX, UK
R. H. Hunter
Affiliation:
Department of Earth Sciences, University of Liverpool, Liverpool, L69 3BX, UK
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Abstract

Porosity occlusion has been studied in a granodiorite rock from the Peruvian Coastal Batholith. The texture of the granodiorite is characteristic of Cordilleran I-type rocks, and the textural relations and modelled crystallization path within the quaternary An-Ab-Or-Qz system indicate alkali feldspar was the last major phase to start crystallizing. In thin section, alkali feldspar crystals occur both as large anhedral ‘plates’ containing numerous inclusions, and small interstitial cuneiform ‘pockets’. The alkali feldspar pockets are interpreted as late stage nucleation and growth of new crystals in pores that became isolated from the larger crystals during the latter stages of crystallization. Their geometry therefore mirrors that of the pores immediately after isolation.

From the modal abundance of the interstitial pockets, and taking into account contemporaneous growth of the other major phases, it is suggested that crystallization in isolated pores involved solidification of the final 3–4% of liquid. Alkali feldspar growth on the rims of the large anhedral plates prior to pore isolation is evidence for the localised (mm-cm scale) diffusion of chemical species within the interconnected melt phase. However, Rayleigh number calculations indicate that the separation of melt from crystals by compositional convection is unlikely to have occurred during interstitial crystallization.

Keywords

granitetextural developmentporosity occlusioncompositional convection
Type
The 1995 Hallimond Lecture
Information
Mineralogical Magazine , Volume 60 , Issue 398 , February 1996 , pp. 163 - 171
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1996

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