TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

Yamashita, [123], and thus did not significantly change bulk magma chemistry,
unless extensive resorption of crystals occurred.
A laterally and vertically extensive system of interconnected dikes and sills
with regions dominated by liquid and other regions dominated with a crystal-liquid
mush can account for the zoning and compositional features recorded in OJP xeno-
lith and phenocryst plagioclase crystals (see Fig. 2.11). Oscillatory zoned crystals
may have formed in a quiet environment where feeDBack between the rate of crys-
tal growth and diffusion in a boundary layer at the crystal-melt growth interface
yielded small and frequent variations of melt composition near the growing crystal
[1, 115, 141] (Figs. 2.2, 2.3, 2.5). Oscillatory zoned crystals may have also formed
where small and frequent variations in melt composition and/or temperature were
produced by convection of the main magma body at the inward edge of the solid-
ification front e.g., [60]. Indeed, where magma recharge is frequent, solidification
front growth can be arrested or reversed placing deeper zones of the front in re-
peated contact with fresh primitive melt (Marsh, 1996). I have noted evidence
that magma recharge was both vigorous and frequent to have generated the most
of the volume of the OJP around ∼122 Ma. In the case of a shallow OJP magma
chamber, frequent and vigorous recharge would favor production of oscillatory-
zoned An-rich plagioclase crystallizing from relatively primitive parental magmas,
whereas a stagnant environment would favor production of oscillatory zoned An-
poor plagioclase crystallizing from more evolved parental melts. Thick cumulate
mush layers along the floors of OJP magma chambers were also likely important in
the widespread genesis of Kwaimbaita basalt (e.g., Fig. 2.11). An important factor
in basalt differentiation in the crystal-mush dominated portions of the magmatic
system would have been the residence time of melts within the vertically and lat-
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