TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

whole-rock basalt data. Cumulate xenoliths and phenocrysts generally are pieces
of disrupted solidification fronts. Solidification fronts would have been ubiquitous
throughout the OJP magma chamber system. The OJP magma chamber system
was composed of interconnected dikes and sills and consisted of regions dominated
by liquid (magma chamber interior) and regions dominated by crystal-liquid mush
(along magma chamber floors and walls, within dikes and conduits). Crystals and
crystalline debris were extensively recycled throughout the OJP magma chamber
system. This recycling accounts for the wide compositional spectrum of magmas
parental to plagioclase xenolith crystals and phenocrysts with An content out of
equilibrium with their Kwaimbaita host basalts. Ironically, this process also ac-
counts for the dominance of a single magma composition erupted on to the OJP,
as it represents the homogenized body of magma in the main part of the up-
per chamber. The compositions of melts periodically flushed from the interstices
of the crystal-mush network were affected by their residence times in the mush.
Depending upon the extent of the crystal-rich portions of the magma chamber
system, residence time may have had a large effect on overall basalt chemistry.
The shallow crustal OJP magma chamber system corresponded with a zone of
neutral buoyancy in the 0-7 km depth range, as in other volcanic systems [122].
As OJP volcanism progressed the plateau gained elevation, and the zone of neutral
buoyancy gradually migrated upward leading to slow yet pervasive assimilation of
overlying seawater altered basalt.
The greatest diversity of OJP basalts is along the margins of the plateau
where it is thinner. During formation, magmas ascending into the marginal and
thinner magma chamber system around the plateau margins were subject to less
density filtration, which allowed ascent and eruption of relatively dense Kroenke
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