Abstract La Désirade in the Lesser Antilles contains one of the rare fragments of Jurassic oceanic crust known on Caribbean islands. Others in the northeastern Caribbean occur on Puerto Rico and Hispaniola. These fragments each include radiolarian-bearing chert that has been linked to an origin in the Pacific Ocean. Of these, a fragment in Sierra Bermeja, Puerto Rico is clearly of Pacific origin as it contains Lower Jurassic radiolarians that predate the opening between North and South America. Red ribbon chert at El Aguacate, Dominican Republic is essentially identical to widespread radiolarites found in accreted material of the Pacific basin and from Pacific Ocean ODP Site 801. Extensive sampling in the Atlantic basin has produced no Jurassic radiolarites. Thus, based on age (the older of the Sierra Bermeja outcrops) and lithology (El Aguacate), two of these fragments are definitely of Pacific origin. Re-evaluation of the chert/pillow lava sequence on La Désirade in light of recent discoveries at spreading ridges has resulted in a revised interpretation of their probable origin. A wide range of features of these cherts indicate pelagic and hydrothermal sedimentation at an Upper Jurassic spreading ridge, one that almost assuredly existed in the eastern Pacific realm. These features include: the types of chert found on the island, lack of argillaçeous partings, small outcrop size, discontinuous chert bodies, presence of limestone squeeze-ups into pillow lavas and indications of hydrothermal activity, including epidotization of basalt migrating outward from pillow margins with chert rinds that record pelagic and hydrothermal sedimentation at an Upper Jurassic spreading ridge, one that almost assuredly existed in the extreme eastern Pacific realm.

La Désirade Island in the Cenozoic Lesser Antilles forearc region exposes a pre-Tertiary complex of oceanic volcanic, plutonic, and dike rocks. Previous work has established the stratigraphy and structure of the La Désirade igneous complex and also its late Mesozoic age. Dredge hauls from the nearby submerged Désirade fault scarp consist of similar volcanic and dike rocks plus greenstone, diabase, and gabbro. The composite section from island and submarine escarpment resembles upper oceanic crust but of controversial origin, original tectonic setting, and geodynamic significance. More precise ages for the La Désirade igneous complex and its individual members provide important constraints on proposed tectonic models. We reanalyzed Radio-laria from intralava sediments in basaltic pillow lava and zircon from trondhjemite to pinpoint their age. The radiolarian assemblage correlates with those of formations in east-central and west-central Mexico. The Mexican radiolarian faunas are chronostratigraphically calibrated by co-occurring ammonites and Buchia. Abundant Mexican biostratigraphic and chronostratigraphic data (ammonites, Radiolaria, and Buchia) constrain the composite radiolarian assemblage from six localities on La Désirade to zone 4, upper subzone 4β (mid-upper Tithonian). Using the new chemical abrasion (CA) thermal ionization mass spectrometry zircon method of Mattinson, the results from three zircon fractions from trondhjemite provide a 143.74 ± 0.33 Ma U-Pb age for the La Désirade igneous complex. Combined biostratigraphic, chronostratigraphic, and geochronometric data put the geochronologic age for the mid-late Tithonian near 143.74 Ma, a maximum for the latest Jurassic.