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).