6.3. Petrogenetic relationships between plutonic and volcanic rocks
Establishing petrogenetic relationships between the Rio Boba plutonic sequence and the spatially related Puerca Gorda volcanic rocks are key to establish the nature of the mantle source of the magmas and to reconstruct the crustal section of the intra-oceanic Caribbean island arc. Field, petrographic and geochemical data described above provide strong evidence that the cumulate pyroxenites and gabbronorites are the product of partial crystallization of a magma whose remaining liquid was subsequently removed. A reasonable hypothesis is that this remnant liquid erupted as the volcanic rocks that make up the upper arc crust. This possibility can be tested by checking whether the cumulate pyroxenites and gabbronorites crystallized in equilibrium with liquids compositionally similar to the Puerca Gorda volcanic rocks and other regional volcanic units of the Caribbean island arc.
For this purpose, the composition of the ‘equilibrium melts’ was calculated using the trace-element composition of magmatic clinopyroxene in selected pyroxenites and gabbronorites, and appropriate clinopyroxene/melt partition coefficients (e.g. Bédard, 2005). Clinopyroxenes with petrographic evidence of deformation or recrystallization were not used in the calculation of equilibrium melts, since it may have changed the composition during metamorphic re-equilibrium at high-T. Further uncertainties in the equilibrium melt composition are due to the fact that clinopyroxene could have formed from a melt trapped in the interstices of cumulus minerals. In this case, the post-cumulus clinopyroxene may yield anomalously high concentration of incompatible elements due to closed-system crystallization (e.g., Bédard, 1999). In the analyzed clinopyroxenes, this effect is revealed by relatively high concentrations of HFSE and HREE. To avoid this effect, the samples selected in this study have a high clinopyroxene modal content and, in each sample, several large (0.2-10 mm) cumulus clinopyroxenes were analyzed. At thin section scale, no significant grain-to-grain variation in the incompatible elements composition of clinopyroxene was detected, suggesting that post-cumulus processes did not significantly affect its trace element characteristics. Calculated equilibrium melts are reported in Appendix F and plotted in the chondrite-normalized trace elements diagrams of the Fig. 11.
The melts modelled in equilibrium with the clinopyroxenites and websterites have low TiO2, HFSE and REE contents, where the HREE ratios are only 2 to10 times chondrite. Their patterns show variable LREE depletion and pronounced Nb and Zr-Hf negative anomalies (Fig. 11a, b, c). These characteristics are indicative of a strongly depleted mantle source and/or they result from high degrees of partial melting, with a variable, but generally small, subduction fluid component. Model melts are compositionally similar to the boninite and low-Ti IAT protoliths of the Puerca Gorda Schists, supporting a genetic relationship through crystal fractionation processes. They also show compositional affinities with the LREE-depleted IAT volcanic rocks of the Cacheal complex and Los Ranchos Fm, and the melts in equilibrium with the lower gabbronorites of the Puerto Plata complex. The model shows that melts in equilibrium with olivine websterite are similar to representative intermediate and high-Ca boninite lavas, suggesting that these cumulates derived from boninite-like magmas. Crawford et al. (1989) and Fallow and Crawford (1991) describe primitive high Ca boninite lavas with phenocrysts of olivine, orthopyroxene and clinopyroxene, which correspond to the cumulus phases found in the olivine clinopyroxenites and websterites.
The melts modelled in equilibrium with gabbronorites show a flat trace elements pattern with a strong positive Th and negative Zr-Hf and Ti anomalies (Fig. 11d). The LREE are generally slightly depleted and HREE absolute abundances are low (5-10 times chondrite), which also point to a depleted mantle source modified by a small component of subduction-related fluid. These model melts are similar to the low-Ti IAT and boninitic protoliths of Puerca Gorda Schists, the lavas of the Los Ranchos Formation, and melts in equilibrium with upper gabbronorites of the Puerto Plata complex. This suggests that the gabbronorites crystallized in equilibrium with melts that were extracted and erupted to produce these volcanic rocks. Crawford et al. (1989) describe evolved high-Ca boninite lavas with plagioclase phenocrysts associated with clinopyroxene, olivine and orthopyroxene, which correspond to the cumulus phases in the gabbronorites.
Although few data are available, the model of the melts in equilibrium with the troctolites also has low Ti contents and HREE absolute abundances (about 10 times chondrite), suggesting, as in the case of the pyroxenites, a depleted mantle source (Fig. 11e). However, model liquids show a distinctive flat trace elements pattern, with relatively high Th and Nb, indicating an additional melt component in the source, such as partial melted subducted sediments (Hochstaedter et al., 2001; Tollstrup et al., 2010). These modelled melts in equilibrium with the troctolite cumulate are compositionally similar to the boninite protoliths of the Puerca Gorda Schists and melts in equilibrium with intermediate troctolites of the Puerto Plata complex, suggesting that they are genetically linked. The nature of the troctolites indicate that the parental magma, if it was boninitic, was high-Ca type, which is the least depleted of the boninite subtypes of Crawford et al. (1989). This interpretation is supported by HFSE and REE in the troctolites, which are similar to those of the intermediate and high-Ca boninite lavas (Fig. 11d).
In summary, model melts provide a genetic link between the plutonic rocks (pyroxenite, gabbronorite, troctolite) and Puerca Gorda metavolcanic rocks (Fig. 11f). Thus, the ultramafic and mafic cumulates crystallized in equilibrium with melts in the lower crust. The melts were extracted and erupted to produce the volcanic sequence in the upper crust. The composition of model melts in equilibrium with more primitive clinopyroxenites and gabbronorites closely resemble those of LREE-depleted IAT and intermediate to high-Ca boninites. The crystallisation order of the Rio Boba mafic-ultramafic sequence with An-rich plagioclase after Mg-rich olivine, spinel and pyroxene is consistent with the phenocrysts mineralogy observed in primitive and SiO2-rich boninites (e.g. Taylor et al., 1994). The extremely low TiO2, HFSE and HREE in boninitic melts are commonly attributed to their derivation from a refractory mantle source (e.g. Pearce et al., 1992). The preserved remains of such refractory mantle are the basal harzburgite lenses found in tectonic contact with the underlying Cuaba unit (Fig. 2). The LILE enrichment characteristic of Rio Boba plutonic sequence and Puerca Gorda volcanic rocks is typical of boninites and has been related to the addition of a component produced by dehydration and eventually partial melting of a subducted slab and/or overlying sediments (Crawford et al., 1989; Pearce et al., 1992; Bédard, 1999; Falloon et al., 2008; Tollstrup et al., 2010; Pearce & Reagan, 2019).