2.2. Main structural subdivision of the Rio San Juan complex
The Rio San Juan complex consists of three elements (Fig. 2): (1) a core
of Mesozoic igneous and metamorphic rocks; (2) a peripheral folded and
faulted unconformable cover of Paleocene to middle Miocene sedimentary
rocks; and (3) a subhorizontal Miocene to Pleistocene reef limestone.
The large-scale internal ductile deformation of the core consists of a
SW-dipping nappe pile (Escuder-Viruete et al., 2013a). In ascending
structural order, it is made-up of (see Appendix A for more detail): the
Gaspar Hernández and Helechal peridotites, the Jagua Clara
serpentinite-matrix mélange, the Morrito and Cuaba units, and the Rio
Boba mafic-ultramafic plutonic sequence. The last three units belong to
the Caribbean upper plate of the subduction-accretionary prism.
In the NE sector, the Gaspar Hernández peridotite forms km-scale
tectonic blocks of harzburgite and lherzolite, variably replaced by a
low-T chrysotile-lizardite assemblages, intruded by gabbro sills of
N-MORB chemistry and Lower Cretaceous age. In the SE sector, the
Helechal peridotite forms a tectonic slice of peridotites of a similar,
abyssal-like composition, which have structural continuity along a
km-scale synform under the Cuaba and Morrito units. Both peridotite
units have been interpreted as fragments of the proto-Caribbean oceanic
lithosphere, consumed by subduction below the Caribean island arc
(Escuder-Viruete et al., 2011c).
The Jagua Clara mélange consists of foliated antigorite mainly, which
warps around blocks of high-P rocks (Krebs et al., 2011; Escuder-Viruete
& Pérez-Estaún, 2013). The mélange contains mafic blocks plucked from
both the upper plate (arc-like protoliths, Caribbean island arc) and the
lower plate (N-MORB protoliths, proto-Caribbean Ocean), suggesting that
the Jagua Clara serpentinite-matrix mélange represents the deep
subduction channel, formed during intra-oceanic subduction.
The Morrito unit is composed of the Puerca Gorda Schists in the lower
structural levels and the El Guineal Schists in the upper ones (Draper
& Nagle, 1991). The mafic protoliths of Puerca Gorda Schists were
heterogeneously deformed and metamorphosed to blueschist- and
greenschist-facies conditions during arc-continent convergence. However,
towards the upper structural levels, the strain intensity decreases and
the unit consists of porphyritic, aphyric and vesicular (amygdaloidal)
mafic-intermediate volcanic flows. On a microscopic scale, these
volcanics rocks preserve pyroxene-phyric/microphyric and variolitic
quench volcanic textures. Euhedral/subhedral orthopyroxene and
clinopyroxene are the most abundant phenocrysts followed by subhedral
plagioclase. Mafic volcanic protoliths derived from boninite, low-Ti IAT
and IAT type magmas (Escuder-Viruete et al., 2011c). The Guineal Schists
derived from dacitic to rhyolite protoliths and have provided SHRIMP
zircon core ages of 122.2 and 121.7 Ma (unpublished). In less deformed
domains, quartz and feldspar-phyric volcanic textures are preserved.
Based on major and trace element compositional data, Escuder-Viruete et
al. (2011c) concluded that the metavolcanic rocks of the Morrito unit
represent the volcanic part of the Caribbean fore-arc. The Morrito basal
thrust juxtaposes the Puerca Gorda Schists northward onto the Jagua
Clara serpentinite-matrix mélange. This juxtaposition took place in the
latest Maastrichtian to Paleocene, at the onset of the arc-continent
collision (Escuder-Viruete et al., 2013a, b).
The Cuaba unit is composed of two coherent tectonometamorphic
assemblages (Fig. 2). The structurally uppermost Jobito assemblage
consists of foliated and mylonitized metabasites metamorphosed to low-P
amphibolite-facies conditions. The underlaying Guaconejo assemblage is
made up of garnet-epidote amphibolites, mafic eclogites and
heterogeneous coarse-grained garnet-bearing and garnet-free orthogneises
(metaultramafic cumulates, metagabbros and metadiorites) metamorphosed
to upper amphibolite and eclogite-facies conditions (Abbott et al.,
2007; Escuder-Viruete & Pérez-Estaún, 2013). Blocks of garnet-bearing
ultramafic rocks are a distinct component of the Guaconejo assemblage.
Their magmatic mineral assemblages record a liquid line of descent (by
fractional crystallization) consistent with mantle conditions
(>3.2 GPa; Gazel et al., 2011; Abbott & Draper, 2013). ).
A low-pressure alternative origin for the garnet-bearing ultramafic
rocks has also been proposed (Hattori et al. 2010a, b). Mafic protoliths
of the Cuaba unit originated from boninite, low-Ti IAT and IAT type
magmas (Escuder-Viruete et al., 2011c; Escuder-Viruete &
Castillo-Carrión, 2016), suggesting that this unit represents part of
the subducted fore-arc of the Caribbean island arc. Abbott & Draper
(2013) describe eclogites derived from related N-MORB protoliths in the
Cuaba unit, probably derived from subduced oceanic lithosphere and later
exhumed in the subduction channel. The Jobito and Guaconejo assemblages
are tectonically juxtaposed by a Campanian to Maastrichtian
(~75-70 Ma) late retrograde detachment zone, which is
marked by several rootless bodies of serpentinized peridotites,
compositionally similar to supra-subduction zone (SSZ) mantle (Fig. 2).
A basal section of mafic–ultramafic cumulates is lacking. The Guaconejo
assemblage is also tectonically juxtaposed against the underlaying
Helechal peridotites.
The uppermost Rio Boba mafic-ultramafic plutonic sequence includes three
main cartographic units (Fig. 2): Quita Espuela layered gabbronorites;
Matel oxide gabbronorites; and La Manaclá hornblende gabbros, diorites
and tonalities. Outcrop conditions under a tropical climate are
generally very poor. Gabbroic rocks are in occasions deformed and
recrystallized to a two-pyroxene granulite, but the meta- prefix is
omitted hereafter for simplicity. The metamorphic evolution of the
plutonic complex will be presented in a separate publication. The Cuaba
unit, the Puerca Gorda Schists and the Rio Boba plutonic sequence were
intruded by syn-kinematic hornblende-bearing tonalites during the Late
Cretaceous (90.1±0.2 Ma; U-Pb in zircon; Escuder-Viruete et al., 2013b).