A PRELIMINARY EVALUATION OF THE PETROGENESIS OF THE ACCRETED MESOZOIC ISLAND ARC
OF THE SOUTHERN CARIBBEAN
A.W. SNOKE
Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071-3006 USA
Many tectonic models for the southern Caribbean postulate the accretion of Mesozoic oceanic island-arc rocks during
the Late Cretaceous and/or Tertiary. Parts of this accreted arc include the Concha schists near Santa Marta (Colombia),
the ArubaBlanquilla island chain, the Villa de Cura klippe (Venezuela), and Tobago, West Indies. A southern segment
of the Mesozoic oceanic arc was accreted in the northern Andes and is manifested as parts of the Western Cordillera
of Colombia and Ecuador.
These Mesozoic island-arc fragments were part of the "Great Arc of the Caribbean" of Burke (1988) that
is inferred to have entered the Caribbean region from a Pacific realm. Major parts of the Great Arc are exposed
throughout the northern Caribbean and include oceanic basement complexes (e.g., ophloliticfragments of Cuba, Hispaniola,
and Puerto Rico) as well as primitive and mature island-arc volcanicplutonic suites. In the southern Caribbean,
the Great Arc experienced a complex post-magmatic evolutionary history involving subduction of frontal parts of
the arc (blueschist fades metamorphism of the Villa de Cura Group), continentward obduction
(emplacement of the Villa de Cura klippe), and Cenozoic strike-slip faulting (Tobago terrain).
To a first approximation, at least three distinct phases of arc magmatism constitute the composite igneous evolution
of the accreted southern Caribbean Mesozoic oceanic island arc: 1) Early Cretaceous-Late Jurassic (?),2) mid-Cretaceous,
and 3) Late Cretaceous. The igneous suites associated with magmatic phases 1 & 2 generally correspond with
the primitive island-am suite of Donnelly and Rogers (1967) and phase 3 to younger, locally superimposed calc-alkaline
magmatism. At no one locality is a single island-arc crustal cross-section exposed that would allow direct examination
of the inferred complete magmatic history. Consequently, such a cross-section must be assembled from several key
localities and represents a generalized view of cumulative arc evolution through time. Nonetheless such a reconstruction
provides an unusually complete view of the petrotectonic evolution of an intraoceanic island arc as well as a model
to interpret Mesozoic arc magmatic rocks throughout the southern Caribbean.
Tobago, West Indies, provides an excellent natural cross-section across rock units associated with magmatic phases
1 & 2. The oldest oceanic arc rocks exposed on Tobago, and perhaps in the southern Caribbean, are the North
Coast Schist (NOS). This composite volcanogenic unit is multiply deformed and has experienced pervasive lower greenschist
facies metamorphic conditions. Two basic facies units constitute the NCS: Parlatuvier Formation= basaltic andesite
lapilli turf and crystal-lapilli turf and associated hypabyssal mafic intrusives, and Mt. Dillon Formation=dacitic
crystal-lapilli turf, volcanogenic argillite and siliceous argillite, and siliceous hydrothermal replacement deposits
(volcanogenic chart). The basement upon which this volcanogenic sequence was deposited is not exposed; but Nd isotopic
data from Mt. Dillon Formation siliceous argillite indicate virtually no continental detritus in this sediment
(Frost and Snoke, 1989), consequently, a basement of oceanic crust is inferred (i.e., Jurassic ophiolite).
The NOS was intruded and dynamothermally deformed by an Albian plutonic suite. The plutonic suite consists of ultramafic
to tonalitic rocks that evolved in an island-arc magmatic reservoir that experienced a complex history of crystal
fractionation and periodic replenishment. Cumulates associated with the crystal differentiation history are manifested
by ultramafic-mafic rocks chiefly including dunite, wehrlite, olivine+/- hornblende clinopyroxenite, hornblendite,
and hornblende +1- augite gabbro. Similar Cretaceous arc-root ultramafic-mafic suites are also well documented
in Venezuela (El Chacao ultramafic complex) and in the West Cordillera of
Such intrusive plutonic complexes provide our best "window" into the complex magmatic processes that
occurred in the subvolcanic magma chambers of the mid-Cretaceous arc.
Consanguineous Albian voicanic rocks on Tobago are chiefly volcaniclastic breccias, but locally lavas, sometimes
pillowed, are an important component of the volcanic pile. The lithic clasts in the breccias are commonly porphyritic,
and conspiciously clinopyroxene-phyric lithic clast turf breccia is a very distinctive lithology in this primitive
island-arc volcanic sequence. The Bacolet Formation consists nearly exclusively of this volcanic lithotype, and
clinopyroxene-plagioclase-phyric volcanic rocks are widespread throughout the Tobago Volcanic Group (TVG). In Venezuela,
clinopyroxene-rich lavas and turf breccias also form important components of the Villa do Cura Group (e.g., El
Carmen Formation). I also include the Curacao Lava Formation as part of the mid-Cretaceous primitive island-arc
suite, although others interpret these mafic volcanic rocks as an uplifted exposure of an inferred extensive Cretaceous
oceanic plateau.
On Tobago, the last igneous event associated with the Albian magmatic evolution was the emplacement of a widespread
mafic dike swarm. These dikes intruded the NCS, TVG, and ultramafic to tonalitic plutonics suite and perhaps constitute
the best representatives of parental liquid compositions in the completed Albian magmatic association.
A well-known example of the late Cretaceous caic-alkaline magmatic suite is the Aruba batholith, and analogous
plutonic rocks are also exposed on several of the Venezuelan offshore islands. The Aruba batholith includes gabbroic
rocks but consists chiefly of various tonalitic rocks and is therefore a generally more silicic plutonic suite
than the mid-Cretaceous Tobago pluton. Interestingly, mafic dikes petrographically similar to some representatives
of the Tobago dike swarm cut the Aruba batholith.
In summary, the oldest elements of the accreted Cretaceous oceanic island arc of the southern Caribbean consist
of Late Jurassic(?) to Early Cretaceous penetratively-deformed, greenschist-. facies volcanogenic rocks that presumably
evolved on oceanic crust. These oldest arc rocks were plastically deformed prior to mid-Cretaceous magmatism, but
were subsequently further deformed in dynamothermal aureoles surrounding the mid-Cretaceous ultramafic to tonalitic
plutonic suites. Presumable these oldest arc volcanic rocks were also locally unconformably overlain by the mid-Cretaceous
volcanic rocks. The late Cretaceous magmatic history is characterized by the evolution of more silicic plutonic
and volcanic rocks that may reflect the greater involvement of continental detritus during magma genesis.
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