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.