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Origin of Ba-rich sanidine megacrysts in a porphyry from Papua New Guinea

Published online by Cambridge University Press:  05 July 2018

P. H. Nixon ,
N. A. Chapman  and
J. V. Smith
P. H. Nixon
Affiliation:
Department of Geology, Box 4820, University P.O., Papua New Guinea
N. A. Chapman
Affiliation:
Institute of Geological Sciences, Harwell Oxfordshire OX11 0RA
J. V. Smith
Affiliation:
Department of Geophysical Sciences University of Chicago, Illinois 60637, USA
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Summary

A Miocene porphyry belonging to the sheshonite association contains 7 cm sanidine megacrysts in a groundmass of microphenocrysts of labradoritebytownite, augite, sporadic hastingsite, magnetite, sphene, K feldspar, apatite, Ca zeolite and calcite. The megacrysts (Or78–94) are enriched in BaO (≃ 2.0 wt%) and SrO relative to the groundmass. Although mineralogr and texture suggest that the megacrysts were suspended in a liquid, now represented by the groundmass, calculated liquid densities except for dry melts are less than those observed for the megacrysts and it is concluded that the original magma contained very little water. It possibly originated in a subduction zone with the main hycration taking place at shallow levels of intrusion during contact with groundwater. This produced zeolitization of the feldspars and may have played a part in the K enrichment at the margins of the sanidine megacrysts.

Type
Research Article
Information
Mineralogical Magazine , Volume 43 , Issue 331 , September 1980 , pp. 845 - 850
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1980

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Footnotes

*

Present address: Department of Earth Sciences, The Unversity, Leeds LS2 9JT, U.K.

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