TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

equivocally the roles of H2O, pressure, and temperature on the genesis of low
MgO, An-rich OJP plagioclase based solely upon major element data. Plagioclase
parent magma trace element compositions allow us to further test the contrasting
hypotheses that An-rich crystals grew from relatively primitive magmas versus
evolved H2O-rich magmas.
2.6.1.4 Trace Elements
The relatively low incompatible trace element abundances in the calculated
parental magmas, relative to the whole-rock basalt compositions, indicates that
crystal fractionation had not occurred to any significant degree and, thus, cannot
be invoked as a process to elevate the water content of the magma. Therefore,
if water-rich magmas were responsible for An-rich plagioclase crystallization, the
magma(s) would need to have been derived from a relatively water-rich source.
However, the data from Roberge et al. [121] indicate this was not the case. As
an alternative to growth in an H2O-rich magma, I suggest An-rich plagioclase,
whether xenolith or phenocryst, crystallized from relatively primitive magmas
in the shallow portions of the OJP magma chamber system. This interpreta-
tion reconciles the relatively primitive parent magma compositions (i.e., lower
incompatible element abundances at a given Mg#; see Fig. 2.9a) of the An-rich
portions of OJP xenolith crystals and phenocrysts. Extensive plagioclase crys-
tallization depletes a residual melt of Sr, as DSr in plagioclase is generally > 1.2
(see Table 2.2). In an environment where plagioclase is the dominant crystallizing
phase, early formed crystals will contain higher Sr/Ti and lower Ba/Sr ratios,
since DBa and DTi < 1 in plagioclase. In a confined, nonturbulent environment
components rejected by growing plagioclase crystals spawn local oversaturations
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