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).