TEXTURAL AND MICROANALYSIS OF IGNEOUS ROCKS: TOOLS ...

erally extensive crystal-mush network (Marsh, 1996; Fig. 2.11). Near steady-state
supply of magma into the system (e.g., [120]) would have resulted in tightly con-
strained basalt compositions consistent with Kwaimbaita-type basalts that make
up the bulk of the OJP. Given the size of the OJP, persistent melting over the
lifetime of the causative thermal disturbance is amenable to near steady-state melt
infiltration of the mush-column-type OJP magma chamber system as illustrated
in Figure 2.11 e.g., [98]. Shallow OJP magma evolution was thus influenced by a
balance of several processes that included: 1) Formation of evolved melts within
crystal-mush dominated regions; 2) Flushing of variably evolved melts from the
crystal mush and homogenization of these melts within the main magma body;
3) Entrainment of phenocrysts and other crystal debris (e.g., An-rich xenoliths)
during magma recharge; 4) Resorption (partial or complete) of crystal debris; 5)
Crystal settling; 6) Input of new, primitive magma (see Fig. 2.11). Partial crystal-
lization of magmas at depths of 0-7 km, within the range suggested for the OJP,
coincide with zones of neutral buoyancy as discussed by Ryan [122]. Studies have
suggested the OJP crust contains significant gabbro and/or dolerite intrusions
that would have corresponded to zones of neutral buoyancy and a large shallow
magma chamber system during active OJP volcanism e.g., [45, 53, 54, 73, 110].
Widespread gabbro and dolerite intrusions are consistent with my interpretation
that the OJP magma chamber system consisted of interconnected crystal-mush-
rich magma chambers. Ryan [122] suggested as crust is thickened by erupted
lavas, the zone of neutral buoyancy will slowly migrate upward. Slow upward
migration of the magma chamber system would favor slow yet pervasive assim-
ilation of overlying rock, potentially seawater altered basalts in the case of the
OJP. Indeed, Michael [103] and Roberge et al. [121] suggested assimilation of sea-
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