6.6. Magmatic evolution of the Rio Boba plutonic sequence and
Puerca Gorda metavolcanic rocks
The field, structural, petrological and geochemical data presented
suggest a magmatic evolution in three stages for the Rio Boba plutonic
sequence and Puerca Gorda metavolcanic rocks. The first stage is the
formation of an arc crustal substrate as the result of melting a
refractory mantle source, represented by the cumulate sequence of
pyroxenites and gabbronorites. Modelling suggests that melts in
equilibrium with these rocks would have erupted as the variably
LREE-depleted and low-Ti IAT and boninitic volcanic protoliths of the
Puerca Gorda Schists, among which there is probably a compositional
transition. Low LREE contents, small negative Nb, and positive Th
anomalies indicate that the subduction component was, if present, small
in this initial stage. Sub-horizontal ductile stretching, deformative
fabrics and recrystallization microstructures indicates that this
mafic-ultramafic substrate was deformed at mid-P granulite to upper
amphibolite metamorphic facies conditions.
The second stage included the volumetrically subordinate troctolites,
which preserve igneous cumulate textures, have a boninitic geochemical
affinity and are not penetratively deformed. According to modelling,
these would be asociated with some of the Puerca Gorda boninitic
protoliths. The mantle source is refractory and enriched by a LILE-rich
hydrous fluid, and possibly by a LREE-rich melt, derived from a
subducting slab and/or overlying sediments (Pearce et al., 1992; Bédard,
1999; Falloon et al., 2008). Regionally, the troctolites have provided a
U-Pb zircon age of 126.1 ± 1.3 Ma, therefore constraining the high-T
deformation to pre-126 Ma times.
The third stage is recorded in the supra-crustal section of the arc by
the Puerca Gorda Schist, no record of this latter stage has been found
in the Rio Boba gabbroic rocks. The third stage encompassed the ‘normal’
IAT volcanic protoliths with higher Th and higher LREE and a pronounced
negative Nb anomaly. These volcanic rocks indicate that the source of
tholeiitic magmas became enriched by a strong subduction component.
In summary, the magmatic evolution of the Rio Boba sequence is
multi-stage, and involves the formation of magmas from melting of
different mantle sources in a supra-subduction zone with a progressive
involvement of a subduction component. The evolution constitutes the
basis for a tectono-magmatic model for the Caribbean island arc proposed
below.