6.4. Origin of the pyroxenite bodies
Pyroxenites have been described from a number of arc crust and mantle
environments (e.g. Berly et al., 2006). Arc crustal pryroxenites are
interpreted as medium to high-pressure, ultramafic cumulates formed in
mid to lower crustal magma chambers, some spanning the crust-mantle
boundary at the base of an arc (e.g. DeBari & Green, 2011).
Mantle-derived pyroxenites differ from arc crustal pyroxenites in that
they generally include a large variety of rock types ranging from
orthopyroxenite through websterite to clinopyroxenite (Garrido &
Bodinier, 1999; Berly et al., 2006).
In the Rio Boba plutonic sequence, the lithological contact between
pyroxenite bodies and gabronorites could not be observed due to the
absence of outcrops. However, the magmatic layering in the pyroxenites
suggests that the layering was originally sub-horizontal. Likewise,
magmatic layering in the adjacent, overlying gabbronorite was originally
horizontal to subhorizontal, both at the outcrop and regional scales.
Therefore, the layering in the pyroxenite is parallel to the layering in
the gabbronorite. These relationships suggest that the pyroxenites form
as sub-horizontal sills, whose upward transition to the gabronorites was
controlled by gravity settling during magmatic crystal fractionation.
The subhorizontal arrangement of the pyroxenite sills is therefore
magmatic and represents the intrusion geometry of the sills during their
emplacement in the lower arc crust. The observed centimeter-thick
subvertical intrusions of pyroxenites in the gabbronorites represent
magmatic conduits or feeder dikes (Fig. 3f).
The clinopyroxenites and websterites of the Rio Boba sequence are
characterised by a mineralogy similar to that of arc-crustal
pyroxenites. Although their olivine compositions are primitive, they do
not correspond to the higher Mg# and NiO-rich compositions observed in
the olivine of the SSZ mantle peridotites of La Cuaba unit and the
Puerto Plata ophiolite complex. (Fig. 6). The Mg# values from
orthopyroxene and clinopyroxene are lower than in mantle peridotites
(Mg#>90), but similar to those of the more primitive
gabronorites. The absence of replacement textures precludes an origin
through reaction between a peridotite and a circulating metasomatic
agent (aqueous fluid and/or melt). These relationships suggest that the
pyroxenite bodies were magma conduits along which primitive
mantle-derived melts had risen through the crust-mantle transition into
the lower crust and the basal part of large gabbroic sills. The gabbroic
sills would form the lower crust of the arc, through multiple pulses of
magma injection and fractionation.