Origin of Ba-rich Sanidine Megacrysts in a Porphyry from Papua New Guinea

P. H. Nixon*, N. A. Chapman and J. V. Smith
Department of Geology, Box 4820, University P.O., Papua New Guinea
Institute of Geological Sciences, Harwell, Oxfordshire OX11 0RA
Department of Geophysical Sciences University of Chicago, Illinois 60637, USA
*Present address: Department of Earth Sciences, The Unversity, Leeds LS2 9JT, U.K

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.

Mineralogical Magazine; September 1980 v. 43; no. 331; p. 845-850; DOI: 10.1180/minmag.1980.043.331.04
© 1980, The Mineralogical Society
Mineralogical Society (www.minersoc.org)