Layered Rhyolite Bands in a Thick North Mountain Basalt Flow: The Products of Silicate Liquid Immiscibility?

John D. Greenough* and J. Dostal
Department of Geology, Mount Allison University, Sackville, New Brunswick, E0A 3C0
Department of Geology, St. Mary's University, Halifax, Nova Scotia, Canada, B3H 3C3
*Present address: Department of Geological Sciences, University of British Columbia, Okanagan College, 1000 K.L.O. Road, Kelowna, British Columbia, Canada V1Y 4X8.

Abstract: The upper 35 m of a thick (⩽175 m) Early Jurassic North Mountain Basalt flow at KcKay Head contains 25 cm thick differentiated layers that are separated by 130 cm sections of basalt. The lower layers are mafic, pegmatitic, and contain thin (2 cm), fine-grained ‘rhyolite’ bands. Evidence that the rhyolite represents a Si-rich immiscible liquid includes: (1) textures such as fine-grained globules of Ferich pyroxene (once Fe-rich liquid) bordering pegmatite feldspar grains; (2) structureless, microcrystalline, interstitial, polygonal patches of Si-rich minerals and similar areas of Fe-rich stilpnomelane surrounding skeletal Fe-Ti oxide grains, with bulk chemical compositions (to a first approximation), relative proportions and total modal percentages suggesting they were once Si-rich and Fe-rich glasses respectively; (3) basalt and pegmatite compositions (particularly their Fe, and Ti contents) similar to rocks known to contain immiscible liquids; (4) rhyolite major element compositions generally consistent with formation from an immiscible Si-rich liquid; (5) mineral compositions and temperature of pegmatite formation compatible with immiscibility; (6) the inability of mass balance calculations (crystal fractionation) to explain rhyolite formation unless mesostasis stilpnomelane (representing the Fe-rich liquid) is included in the caculations. If, as we suggest, these rocks are the result of immiscibility, they shed light on the incipient formation of granophyres in mafic intrusions and support liquid immiscibility as an important rock-forming process.

Keywords: rhyolite • immiscibility • silicate liquid • North Mountain Basalt

Mineralogical Magazine; September 1992 v. 56; no. 384; p. 309-318; DOI: 10.1180/minmag.1992.056.384.02
© 1992, The Mineralogical Society
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