Alan R. Hastie

Do Cenozoic analogues support a plate tectonic origin for Earth's earliest continental crust?

Archean continental crust largely comprises the trondhjemite, tonalite, and granodiorite/dacite (TTG/D) suite of igneous rocks. Formation of the earliest Archean (>3.5 Ga) TTG/Ds is controversial, being attributed to either subduction zone processes with active plate tectonics or thermochemical mantle convection with no plate tectonic processes. A suite of Cenozoic adakite-like lavas in Jamaica has geochemical compositions comparable to early Archean TTG/D. The data indicate that the adakites were generated by underthrusting (or subducting) and partial melting of oceanic plateau crust beneath Jamaica. This setting is analogous to proposed plate tectonic processes in the early Archean where hot, thick, and more buoyant Archean oceanic crust underthrusts adjacent plates. The new adakite data imply that earliest Archean TTG/D continental crust could have formed above primitive subduction zones.

Origin of the Aves Ridge and Dutch–Venezuelan Antilles : interaction of the Cretaceous ‘Great Arc' and Caribbean–Colombian Oceanic Plateau?

Abstract: In this paper we reassess the geochronology and geochemistry of three dredge hauls from the SE corner of the Aves Ridge (Caribbean Sea) originally sampled in 1968 by Duke Universitys R.V. Eastward. Two hauls consist of light rare earth element-enriched granitoids with a U–Pb zircon emplacement age of 75.9 ± 0.7 Ma. A further haul contains mostly calc-alkaline island arc basaltic andesites of uncertain age. Petrological, trace element and isotopic constraints indicate that the granitoids have an oceanic crustal source and were formed by melting of the lower arc, oceanic or oceanic plateau crust. The mafic rocks formed by partial melting of an incompatible trace element-enriched mantle wedge, which was probably composed of mantle plume material. Both the dredged rocks and data from the Dutch–Venezuelan Antilles indicate a period of west-dipping underthrusting and subduction beneath, or close to, the Caribbean–Colombian Oceanic Plateau between c. 88 and c. 59 Ma, concurrent with collision of part of the plateau with northwestern South America. Constraints from the geochemistry and geochronology of offshore southern Caribbean arc and plateau rocks suggest that in the southern Caribbean there was no pre-existing west-dipping subduction system during formation of the Caribbean–Colombian Oceanic Plateau, whereas long-lived SW-dipping subduction in the northern Greater Antilles is more probable. Supplementary material: Sample details, major and trace element data (file 1), cathodoluminescence images of analysed zircons (file 2) and whole-rock standards (file 3) are available at http://www.geolsoc.org.uk/SUP18438.

Geochemistry and tectonomagmatic significance of Lower Cretaceous island arc lavas from the Devils Racecourse Formation, eastern Jamaica

Abstract The Benbow Inlier in Jamaica contains the Devils Racecourse Formation, which is composed of a Hauterivian to Aptian island arc succession. The lavas can be split into a lower succession of basaltic andesites and dacites/rhyolites, which have an island arc tholeiite (IAT) composition and an upper basaltic and basaltic andesite sequence with a calc-alkaline (CA) chemistry. Trace element and Nd–Hf isotopic evidence reveals that the IAT and CA lavas are derived from two chemically similar mantle wedge source regions predominantly composed of normal mid-ocean ridge-type spinel lherzolite. In addition, Th-light rare earth element/high field strength element–heavy rare earth element ratios, Nd–Hf isotope systematics, (Ce/Ce*)n-mn and Th/La ratios indicate that the IAT and CA mantle wedge source regions were enriched by chemically distinct slab fluxes, which were derived from both the altered basaltic portion of the slab and its accompanying pelagic and terrigenous sedimentary veneer respectively. The presence of IAT and CA island arc lavas before and after the Aptian–Albian demonstrates that the compositional change in the Great Arc of the Caribbean was the result of the subduction of chemically differing sedimentary material. There is therefore no evidence from the geochemistry of this lava succession to support arc-wide subduction polarity reversal in the Aptian–Albian.

Geochemistry of Compositionally Distinct Late Cretaceous Back-Arc Basin Lavas: Implications for the Tectonomagmatic Evolution of the Caribbean Plate

The Cretaceous Blue Mountain Inlier of eastern Jamaica contains the Bellevue lavas, which represent a Mid- to Late Campanian back-arc basin succession of tholeiitic volcanic rocks. The lavas are composed of basic/intermediate and intermediate/acidic subgroups that can be related by intraformation fractional crystallization. Trace element and Hf radiogenic isotope data reveal that the mantle component of the Bellevue magmas is consistent with derivation from a mantle plume (oceanic plateau) source region. Modeling indicates that the magmas formed by 10%–20% partial melting of an oceanic plateau mantle source comprising spinel peridotite that had previously undergone approximately 5%–7.5% prior melt extraction in the garnet stability field. Trace element and radiogenic isotope systematics suggest that the Bellevue mantle source region was contaminated with a slab-derived component from both the altered basaltic slab and its pelagic sedimentary veneer. The data from the Bellevue lavas support the plateau reversal model of Caribbean tectonic evolution, whereby subduction on the Great Arc of the Caribbean was to the northeast until the Caribbean oceanic plateau collided with the southern portion of the Great Arc in the Santonian (85.8–83.5 Ma), resulting in subduction polarity reversal and thus southwest-dipping subduction. This polarity reversal allowed oceanic plateau source regions to be melted beneath a new back-arc basin to the southwest of the Great Arc.

The origin of Earth’s first continents and the onset of plate tectonics

The growth and recycling of continental crust has resulted in the chemical and thermal modification of Earth’s mantle, hydrosphere, atmosphere, and biosphere for ~4.0 b.y. However, knowledge of the protolith that gave rise to the first continents and whether the environment of formation was a subduction zone still remains unknown. Here, tonalite melts are formed in high P-T experiments in which primitive oceanic plateau starting material is used as an analogue for Eoarchean (3.6–4.0 Ga) oceanic crust generated at early spreading centers. The tonalites are produced at 1.6–2.2 GPa and 900–950 °C and are mixed with slab-derived aqueous fluids to generate melts that have compositions identical to that of Eoarchean continental crust. Our data support the idea that the first continents formed at ca. 4 Ga and subsequently, through the subduction and partial melting of ~30–45-km-thick Eoarchean oceanic crust, modified Earth’s mantle and Eoarchean environments and ecosystems.

Age and Petrogenesis of the Lower Cretaceous North Coast Schist of Tobago, a Fragment of the Proto–Greater Antilles Inter-American Arc System

The North Coast Schist of Tobago is part of the leading edge of the Caribbean Plate, which has been in oblique collision with northern South America for much of the Cenozoic. The North Coast Schist is dominated by two volcanic “formations” metamorphosed under greenschist-facies conditions during later deformation. The Parlatuvier Formation mostly consists of mafic to intermediate tuffs and tuff breccias with a U-Pb zircon ID-TIMS age of Ma. Trace element data and radiogenic isotopes reveal that the Parlatuvier Formation is derived from a heterogeneous subduction-modified, locally incompatible trace element–enriched, mantle source with some rocks containing the highest 176Hf/177Hf ratios found in the offshore Caribbean. The Mount Dillon Formation comprises silicified tuffs and tuff breccias that are derived from a more isotopically enriched mantle source with a significant slab fluid-related component. A thin belt of amphibolite-facies dynamothermally metamorphosed metavolcanic rocks lies in contact with a younger island arc pluton. Some of these amphibolites have an isotopically similar source to the Parlatuvier Formation but lack a clear subduction-related component. The age, geochemical heterogeneity, and proximal nature of eruption confirm that the North Coast Schist lay within an east-dipping proto–Greater Antilles arc. We propose that the arc system at the time of North Coast Schist magmatism was actively rifting, possibly during development of a back-arc basin. This arc system shut down during the Cretaceous, making way for southwest-dipping Greater Antilles subduction and relative eastward motion of the Caribbean Plate.

Is the Cretaceous primitive island arc series in the circum-Caribbean region geochemically analogous to the modern island arc tholeiite series?

Abstract The Early Cretaceous island arc lavas in the Caribbean region are frequently assigned to the primitive island arc (PIA) series and not to the island arc tholeiite (IAT) series. However, this review demonstrates that the Caribbean PIA rocks have immobile trace element abundances, trace element ratios and Nd–Hf isotope systematics which are indistinguishable from modern IAT lavas. Thus, it is proposed that the term PIA series be discarded and that the Early Cretaceous island arc rocks in the Caribbean be classified as IAT rocks.