3.2. Gabbroic rocks
The gabbroic rocks of the Rio Boba sequence form a stack of
kilometer-thick sills with the underlaying Quita Espuela gabbronorites
been compositionally different from the overlaying oxide-rich Matel
gabbronorites (Fig. 2). The Quita Espuela gabbronorite is a 0.8-1.6
km-thick unit, mainly composed of medium- to coarse-grained layered
spinel-bearing gabbronorite and olivine gabbronorite, with subordinate
troctolite and olivine gabbro. In the field, the contact between the
different gabbroic rocks is gradacional. The compositional layering is
defined by variations of the mafic mineral/plagioclase ratio at the
millimeter to meter scale (Fig. 3f), or by grain-size graded layers. The
layering is generally oriented WNW-ESE to NW-SE and dip a low angle
(<30ยบ) to the NE and SW. It is often sheared, boudinaged and
locally isoclinally folded, suggesting deformation and foliation
development at high-temperature (Fig. 3c).
The least deformed Quita Espuela gabbronorites exhibit in thin section
an orthocumulate to partly equilibrated granoblastic texture with
interpenetrated grain boundaries (2-10 mm) (Fig. 5f). They have an
anhydrous association of clinopyroxene (15-35 vol.%, hereinafter %),
brownish orthopyroxene (5-25 %), plagioclase (30-65 %), olivine (0-15
%) and green spinel (<8 %). Relict igneous cumulate textures
indicate ortho- to adcumulus growth. Clinopyroxene contains exsolution
lamellae of orthopyroxene, and vice versa. Zoning has not been detected
in any of the phases. Green spinel (hercynite) or Fe-Ti oxides
(magnetite-ilmenite) occurs interstitially. Some undeformed
gabbronorites are characterized by coronitic shells of orthopyroxene and
Fe-Ti oxide around olivine at the contact with plagioclase (Fig. 5d).
Deformed gabbronorite exhibits a penetrative grain-shape defined by
polycrystalline ellipsoidal clusters of pyroxene alternating with bands
of elongate plagioclase grains (Fig. 5e). The grain boundaries range
from straight to lobate. Lobate grain boundaries are indicative of
dynamic recrystallization at relatively high-temperatures (Passchier and
Trouw, 1996). In these deformed rocks, green-brown calcic amphibole
(0-15 %) poikilitically enclose both orthopyroxene and clinopyroxene.