Fig. 3. Field features of pyroxenites and gabbronorites from the Rio Boba plutonic sequence. (a) Cumulate texture in olivine websterite. Width of view=35 cm. (b) Alternating layers at centimeter scale of clinopyroxenite and olivine clinopyroxenite. The coin is 2.5 cm in diameter. (c) High-temperature foliation (Sm) in deformed gabbronorites defined by the preferential mineral orientation, ductile stretching and microboudinage of the pyroxene and plagioclase aggregate. Note the high-angle intrusion of an undeformed gabronorite dike. (d) Cumulate texture in coarse-grained gabbronorite, which is intruded by a pyroxenite vein. (e) Layered gabbronorite intruded by anatamosing dikes of undeformed oxide gabbronorite (f) Modal layering in gabbronorites (Sm), defined by variations of the mafic mineral/plagioclase ratio at the millimeter to decimeter scale. (g) Matel oxide gabbronorite characterized by development of a penetrative magmatic to solid-state deformative foliation (Sm). Width of view=2.5 m. (h) Massive troctolite with cumulate igneous texture containing centimetre-size enclaves of foliated gabbronorite.
Overlaying and interleaved with the layered gabbronorites there is a 75-200 m-thick unit of coarse-grained, layered troctolite, subordinate olivine gabbro and rare gabbroic anorthosite (volume <10%, approximately). These rocks preserve cumulate igneous textures and contain rare centimetre-scale enclaves of foliated gabbronorite (Fig. 3b). Undeformed troctolite has orthocumulate texture, and commonly exhibits layer-parallel, preferred orientations defined by plagioclase laths and elongated olivine. Troctolite has variable proportions of plagioclase (45-90 %), but rocks with around 65 % plagioclase and 35 % olivine are particularly abundant. Plagioclase is subhedral, 0.2-1.5 cm sized and locally recrystallized into polygonal aggregates. It contains inclusions of idiomorphic olivine (Fig. 5d). Olivine is 0.5-5 mm long, sub- to euhedral, variably serpentinized and locally surrounded by coronitic shells of orthopyroxene and Fe-Ti oxide. Some undeformed troctolites are characterized by clinopyroxene-spinel and amphibole-spinel symplectites at the olivine-plagioclase interface (Fig. 5c). Associated olivine gabbros are cumulates dominated by plagioclase and clinopyroxene, with minor olivine. In these rocks, olivine (<25 %) forms 0.5-1.5 mm grains dispersed between dominant subhedral plagioclase (45-70 %) and clinopyroxene (20-35 %). Orthopyroxene (<15 %) is generally interstitial between plagioclase laths and clinopyroxene. Magnetite is the only oxide present.
The Matel gabbronorites is 0.6-1.2 km-thick unit composed of medium- to fine-grained oxide gabbronorite, oxide gabbro and subordinated diorite. With respect to the underlying Quita espuela gabbronorites, these rocks are characterized by a higher modal abundance of Fe-Ti oxides and a smaller grain size. Also they are often characterized by the development of a penetrative magmatic to solid-state foliation, which is flat-lying or dip a low-angle to the NE or SW (Fig. 3g). This foliation is sub-concordant to the layering in the underlying Quita Espuela gabbronorites. Less deformed oxide gabbronorites contain rare centimetre-scale enclaves of gabbronorite.
Granoblastic textures in thin section indicate that ductile flow and recrystallization occurred at high-T. Recrystallization of clinopyroxene, orthopyroxene and plagioclase results in elongate to equant polygonal textures. Black oxides (5-15 %) occur as tabular shaped grains and are made of magnetite-ilmenite pairs. In the more deformed oxide gabbronorites, a strong foliation is defined by alternating plagioclase-rich and plagioclase-poor bands on a millimeter scale, reinforced by the preferred orientation of Fe-Ti oxide grains. In some samples, spinel is green, anhedral, and occurs interstitially. Amphibole (0-10 %) rims and poikilitically encloses ortho and clinopyroxene. Oxide gabbronorites are characterized by clinopyroxene-spinel, amphibole-spinel and amphibole-Fe-Ti oxides symplectites around elongated plagioclase and orthopyroxene, suggesting that cooling took place after high-T deformation. The Fe-Ti oxide gabbros are characterized by an elongated granular texture, composed by subhedral 2-5 mm-scale tabular plagioclase and 0.5-8 mm-scale anhedral clinopyroxene.