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.