6.7. Tectono-magmatic model for the Caribbean island arc in northern Hispaniola
Much of the plutonic and volcanic rocks of the Caribbean island arc in northern Hispaniola have a depleted geochemical signature, in particular the boninitic rocks. This depleted nature results from melting of a refractory mantle source, from which melts had previously been extracted (i.e. they are “second-stage melts”; Crawford et al., 1989; Pearce et al., 1992; Bédard, 1999; Falloon et al., 2008, Pearce & Reagan, 2019). The temperatures required for melting a refractory mantle to produce boninites (1100-1550 ºC) are higher than those expected in a typical sub-arc mantle wedge. Several processes, in specific tectonic settings, have been proposed to explain such elevated temperatures (see review in Pearce and Reagan, 2019). Among these geodynamic contexts, a possible scenario for the generation of boninites in the Caribbean island arc involves subduction initiation (Escuder-Viruete et al., 2006, 2014). The absence of a previous intra-oceanic arc indicates that boninitic magmas did not form by arc or fore-arc rifting or propagation of a spreading center into an arc.
Boninite magmatism is commonly linked to embryonic arc volcanism following intra-oceanic subduction initiation, as has been proposed for the Eocene boninites in the Izu-Bonin-Mariana fore-arc (Taylor et al., 1994; Stern, 2010; Dobson et al., 2006; Reagan et al., 2015, 2019). In this area, subduction initiation was followed by the creation of oceanic crust by a seafloor spreading, where compositions evolved from tholeiitic basalt (“fore-arc basalt”) to (low-Si) boninite (Ishizuka et al., 2006, 2011; Reagan et al., 2010, 2019). This was followed by construction of a protoarc of predominantly boninitic (high-Si boninite) composition, as the residual mantle from the spreading event undergoes second-stage melting induced by flux of fluids and melts from the newly formed subducting plate (e.g., Taylor et al., 1994; Pearce & Reagan, 2019). Stabilization of subduction and advection of more fertile mantle to the fusion zone gives rise, via transitional compositions, to the beginning of normal tholeiitic arc magmatism (Ishizuka et al., 2011; Leng et al., 2012; Stern & Gerya, 2018).
In this context, a tectono-magmatic model for the evolution of the Caribbean island arc is proposed in Fig. 13, inspired by the geometry for subduction initiation driven by internal vertical forces of Maunder et al. (2020). Subduction was initiated in the Pacific realm during the Lower Cretaceous, probably along a weak zone in the oceanic crust (Fig. 13a). This caused extension and stretching in the overriding plate, leading to eventual breakup. During this stage (Fig. 13b), decompression melting was probably minor, due to a low geothermal gradient and the scarcity or absence of fluids (no subducting slab). These magmas generated new crust now preserved as the pyroxenites and gabbronorites of the Rio Boba sequence and the lower gabbronorites of the Puerto Plata ophiolite complex. Complementary volcanic rocks are the LREE-depleted IAT of Puerca Gorda. Cacheal and Los Ranchos Formation. These rocks lack a significant geochemical subductive component because the transfer of trace elements from the subducting slab to the mantle wedge must have been limited during the arc infancy (e.g., Dhuime et al., 2009). Extension in the upper Caribbean plate produced sub-horizontal ductile stretching and mid-P upper amphibolite to granulite-facies metamorphism in the lower arc crust, recorded in the heterogeneous deformation fabrics and recrystallization microstructures preserved in the gabbronorites. In the Puerto Plata ophiolite complex, the volcanic upper crust is structurally disrupted probably, by low-angle detachment faulting similar to that occurring along mid-ocean ridges.
Once subduction started (Fig. 13c), the associated rollback led to an immediate influx of hot mantle from below (Stern, 2010). At this stage, boninitic magmas would have formed when the depleted mantle reached a level where it was fluxed with fluids and/or melts derived from the subducted slab. These magmas continue to form crust in the form of the gabbronorites and troctolites of the Rio Boba and Puerto Plata ophiolite complex. Regionally related volcanic rocks are the boninite protoliths of the Puerca Gorda Schists and the boninite lavas of the Los Ranchos Formation and Cacheal complex. This change in magmatism is not abrupt, since there is a continuous compositional transition between LREE-depleted IAT and boninite. Subduction initiation must have occurred prior to 126 Ma, the age of the intermediate troctolites of boninitic affinity. This scenario is consistent with the undeformed nature of the troctolites and their late placement at pressures of approximately 0.4 GPa, suggesting a vertical uplift of 6-9 km of the host pyroxenites and gabbronorites, related to extensional tectonics, prior to the troctolite intrusion.
As extension proceeded, the fertile mantle may have decompressed enough to initiate melting. This effect would have been amplified if the rising fertile mantle entered the region of the mantle wedge that was fluxed by fluids expelled from the subducting slab (Fig. 13d). As the convergence rate and subduction angle stabilized, reorganization of the asthenospheric circulation caused the fore-arc to cool and forced the magmatic axis to retreat (Ishizuka et al., 2006, 2011; Reagan et al., 2010, 2019; Stern, 2010). This process may have yielded ‘normal’ tholeiitic SSZ magmas, which generated the upper olivine gabbros and gabbronorites in the Puerto Plata ophiolitic complex. Regionally related volcanic rocks are the IAT of the Puerca Gorda, Los Caños and Los Ranchos Formations, and El Cacheal complex. This magmatic stage is apparently not recorded in the Rio Boba sequence, probably due to its position close to the trench and far from the volcanic front, located to the southwest (∼200 km from the trench in the Izu-Bonin-Mariana arc). The presence of more evolved andesites and dacites-rhyolites in the upper stratigraphic levels of the Los Ranchos Formation suggests that the Caribbean island arc matured during this magmatic stage (Kesler et al., 2005; Lewis et al., 2002; Escuder-Viruete et al., 2006).
Experimental data show that large ultramafic cumulates can form by fractional crystallization of up to 50% of primary, mantle-derived melts, crystallizing as pyroxenites prior to plagioclase saturation at the base of the crust (e.g. Villiger et al., 2004). However, this sequence of ultramafic cumulates is missing at the exposed base of the Caribbean island arc. The relatively small ultramafic bodies intruded into the lower crustal gabbronorites of the Rio Boba sequence only represent ~5% of the outcrop area. The lack of the expected cumulate sequence indicates that the base of the Caribbean island arc was significatively disturbed during, or slightly after, the main stage of arc crustal building. This may reflect delamination of dense, unstable lower crust comprising ultramafic cumulates (Jull & Kelemen, 2001), or convective thermomechanical erosion of the sub-arc lithosphere (Kelemen et al., 2014). As shown schematically in Fig. 13d, mantle corner flow enhanced by pervasive hydration of the mantle wedge may account for upper plate thinning (down to 30 km thick) in a relatively short time span of 15-25 Ma, from the beginning of arc building to cessation. Both processes, however, would account for the high temperature conditions required for dehydration/melting of the lower arc section. Hornblende tonalite melts produced during this melting event were intruded at shallow crustal levels into the volcanic rocks of Los Ranchos Formation at 116-115 Ma (Escuder-Viruete et al., 2006). 40Ar/39Ar plateau ages of hornblende in most tonalites are Albian (109–106 Ma) and interpreted as final cooling ages, prior to unroofing and erosion of the inactive Caribbean arc, which is unconformably covered in the upper Lower Albian by the reef limestones of the Hatillo Formation.
Finally, the basal part of the Rio Boba plutonic sequence experienced ductile deformation, mylonitization and amphibolite facies retrograde metamorphism in the 88-84 Ma interval, before tectonic juxtaposition to the Cuaba unit along the Jobito detachment zone in the 82-70 Ma interval. The surface exposure and erosion of the sequence in the Maastrichtian-lower Eocene is related to collision of the Caribbean plate with the North American continental margin, which took place at about 60±5 Ma (see Escuder-Viruete et al., 2011a, b).