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.