Plain Language Summary
The process of intra-oceanic subduction brings an oceanic slab under an
overriding oceanic slab resulting in the formation of a convergent plate
margin. Consequently, an oceanic island arc is formed in the upper
plate, as is the case of the magmatically active arcs of southwest
Pacific. Unlike continental magmatic arcs, intra-oceanic arcs are less
studied because a large part of them is located below sea level,
emerging as chains of small islands that constitute just the tops of
large submarine volcanoes. In the northern Dominican Republic, recent
geochemical studies of the Caribbean volcanic and plutonic rocks
indicate that older tholeiitic and boninitic melts were successively
replaced by younger island arc tholeiitic melts. This change in the
compositional magmas, as well as related mantle sources, places
important constraints on the magmatic and tectonic processes associated
with the initiation and evolution of the Caribbean island arc. In this
sense, the results presented in this work allow to be compared with the
chemical stratigraphy observed in actual oceanic arcs and with the
predictions of models for the initiation of intra-oceanic subduction,
which constitutes one of the main questions not completely resolved of
the global plate tectonics.
1. Introduction
The process of intra-oceanic subduction brings an oceanic slab under an
overriding oceanic slab resulting in the formation of a convergent plate
margin. Consequently, an oceanic island arc is formed in the upper
plate, as is the case of the magmatically active arcs of
Izu-Bonin-Mariana, South Sandwich and Lesser Antilles (Leat & Larter
2003; Stern, 2010; Arculus et al., 2015). Unlike continental magmatic
arcs, intra-oceanic arcs are less studied because a large part of them
is located below sea level, emerging as chains of small islands that
constitute just the tops of large submarine volcanoes. Despite these
difficulties, the magmatic processes in intra-oceanic arcs have been
directly and indirectly studied from: (1) lower crust and upper mantle
xenoliths erupted in active volcanoes (McInnes et al, 2001; DeBari &
Green, 2011); (2) diving, dredging and drilling partial crustal
exposures on the deep sea floor (Pearce et al., 1992; Taylor et al.,
1994; Ishizuka et al., 2006; Reagan et al., 2010, 2019); and (3) from
geophysical surveys of the island arc crust (Takahashi et al., 2008;
Calvert, 2011).
Direct evidence of the processes controlling the evolution and formation
of volcanic arcs also comes from the obducted sections of intra-oceanic
arc lithosphere that form ophiolitic sequences in orogenic belts
(Pearce, 2003; Stern et al., 2012). However, examples of well-preserved
exhumed arc sections, complete from their mantle roots to upper
volcano-sedimentary levels are very scarce. The best studied arc
sections probably are: the Jurassic Talkeetna arc in south-central
Alaska (Green et al., 2006; DeBari & Green, 2011; Kelemen et al.,
2014); and the Cretaceous Kohistan arc in northern Pakistan (Garrido et
al., 2006, 2007; Jagoutz et al., 2007, 2011, 2018; Dhuime et al., 2007;
Burg, 2011; Bouilhol et al., 2015). Both Talkeetna and Kohistan
paleo-arcs are compositionally stratified and contain a lower section
made up of a basal ultramafic sequence of peridotite and pyroxenite,
overlain by a mafic sequence of gabbroic rocks. To explain the genetic
link between the ultramafic and mafic sequences two main hypotheses have
been proposed.
The first hypothesis suggest that the ultramafic-mafic sequence,
composed of dunites, wehrlites, pyroxenites, hornblendites and
gabbronorites, may have crystallized in the upper mantle and lower crust
from a single type of primitive arc magma [Mg#>60; where
Mg# = molar 100×Mg/(Mg+Fetotal)] (Greene et al.,
2006; DeBari & Green, 2011; Kelemen et al., 2014). The existence of
primitive gabbronorites and the complementary compositions of the more
evolved plutonic and volcanic rocks, together with the rather homogenous
Nd-isotopic compositions of diverse igneous units of the arc, are put
forward to argue for a common origin (magmatic or cumulative) for the
ultramafic and mafic rocks in the crustal section through (simple)
fractional crystallization (Greene et al. 2006; Kelemen et al. 2003;
Rioux et al. 2007; DeBari & Green, 2011). Therefore, the gabbronorites
would represent the crystallized cumulate pile and the erupted volcanic
rocks the residual liquid following differentiation. This hypothesis is
supported by experimental studies (e.g. Müntener et al., 2001; Villiger
et al., 2004, 2007; Müntener & Ulmer, 2018), which successfully
reproduced the formation of high-Mg# pyroxenites and complementary
low-Mg# melts during the crystallization of anhydrous primitive magmas
at lowermost arc crust conditions.
In the Kohistan paleo-arc, however, the scarcity of rocks with
intermediate Mg# values between high-Mg# dunites-wehrlites-pyroxenites
and overlying gabbros, as well as the existence of significant
variations in the Sr-Nd-Pb isotope data between these groups of rocks,
rule out a simple fractional crystallization relationship between the
ultramafic and mafic sequences. These petrological characteristics and
REE numerical modeling suggest a second hypothesis for the origin of the
ultramafic sequence by melt-rock reaction at the expense of the sub-arc
oceanic mantle. (Garrido et al., 2006, 2007; Dhuime et al., 2007; Burg,
2011). Although predicted by crystal fractionation models, a thick
ultramafic layer of cumulates is nevertheless absent in the crustal
section of both arcs. This absence has been interpreted as a consequence
of delamination of dense, unstable lower crust and/or convective
thermomechanical erosion of the sub-arc lithosphere (Jull & Kelemen,
2001; Garrido et al., 2006, 2007; Dhuime et al., 2007; Kelemen et al.,
2014). Later studies establish a more complex magmatic evolution for the
Kohistan arc that includes different mantle sources for the ultramafic
and mafic rocks throughout an extended period of ca. 30 Ma. This
evolution includes a first stage of extensive boninitic magmatism
connected with initiation of subduction, followed by a tholeiitic
magmatism second stage associated with the building of a mature arc.
This last stage culminates with granitic magmatism that produces
intra-crustal differentiation (by fractionation process), associated
with delamination and/or erosion of the lower arc crust (Dhuime et al.,
2007; Jagoutz et al., 2011, 2018; Jagoutz & Schmidt, 2012; Stern, 2010;
DeBari & Green, 2011).
A multi-stage tectono-magmatic evolution has also been proposed to
explain the characteristics of the mantle and crustal sections of the
Puerto Plata ophiolitic complex (PPC), which constitutes a segment of
the Caribbean, intra-oceanic island arc (Escuder-Viruete et al., 2006,
2014). Currently preserved at several places in the Greater Antilles,
the Caribbean island arc contains volcanic rocks as old as Late Aptian
to Lower Albian in northern and central-eastern Dominican Republic
(Kesler et al., 2005; Lewis et al., 2002; Escuder-Viruete et al., 2006,
2014; Jolly et al., 2006; Proenza et al., 2006; Marchesi et al., 2006;
Rojas-Agramonte et al., 2011, 2016; Hastie et al., 2013; Torró et al.,
2017). Following Draper et al. (1994), the arc is generally interpreted
to have formed in a supra subduction zone (SSZ) setting at the leading
edge of the Caribbean plate by SW-directed subduction (present-day
coordinates) of the proto-Caribbean lithosphere.
In the northern Dominican Republic, geochemical studies of the Caribbean
volcanic rocks indicate that older LREE-depleted tholeiitic and
boninitic melts were successively replaced by younger island arc
tholeiitic (IAT) melts (Escuder-Viruete et al., 2006, 2014). This change
in the compositional magmas, as well as related mantle sources, places
important constraints on the magmatic and tectonic processes associated
with the initiation and evolution of the Caribbean island arc. These
changes coincide with the chemical stratigraphy observed in actual
oceanic arcs (Ishikawa et al., 2002; Ishizuka et al., 2006, 2011; Reagan
et al., 2010, 2019) and with models for the initiation of intra-oceanic
subduction (see review in Stern & Gerya, 2018). Recent advances in
regional geological knowledge have made it possible to identify the
plutonic rocks that constitute the lower crust of the Caribbean arc and
their complementary volcanic rocks in the upper crust, which have been
very little studied.
Here, we combine field mapping, petrological, mineralogical and
geochemical data in order to characterize the lower crust of the
Caribbean island arc exposed in the Rio Boba mafic-ultramafic plutonic
sequence in the northern Dominican Republic. The main objective is to
establish the petrogenetic relationships among the cumulate pyroxenites
and gabbronorites of the plutonic complex, and the structurally adjacent
mafic metavolcanic rocks of the Puerca Gorda Schists. These
relationships allow us to (1) constraint the main differentiation
processes in the magmatic system, (2) reconstruct the crustal section of
the intra-oceanic Caribbean island arc, (3) place constraints on the
nature of parental magmas during subduction zone infancy, and (4)
propose regional correlations based on a spatial/temporal evolution in
stages for the arc magmatism.