TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

spectrum of magmas parental to plagioclase xenolith crystals and phenocrysts
with An content out of equilibrium with their Kwaimbaita host basalts. Ironi-
cally, this process also accounts 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 upper 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 magmas and slightly less dense Singgalo
magmas (see Fig. 2.1 for magma types and distribution across the OJP). Slight
variations in paths of magma ascent, depths of magma pooling and crystallization,
as well as rates and amounts of assimilation of seawater altered basalt would have
affected the isotopic and incompatible trace element characteristics of known OJP
basalts. I suggest that these physical factors may account for both the homogene-
ity of basalts in the central portion of the OJP and the relative heterogeneity of
basalts around the plateau margin.
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