Jean Hernandez

Multiple plume events in the genesis of the peri‐Caribbean Cretaceous oceanic plateau province

The oceanic crust fragments exposed in central America, in north-western South America, and in the Caribbean islands have been considered to represent accreted remnants of the Caribbean-Colombian Oceanic Plateau (CCOP). On the basis of trace element and Nd, Sr, and Pb isotopic compositions we infer that cumulate rocks, basalts, and diabases from coastal Ecuador have a different source than the basalts from the Dominican Republic. The latter suite includes the 86 Ma basalts of the Duarte Complex which are light rare earth element (REE) -enriched and display (relative to normal mid-ocean ridge basalts, NMORB) moderate enrichments in large ion lithophile elements, together with high Nb, Ta, Pb, and low Th contents. Moreover, they exhibit a rather restricted range of Nd and Pb isotopic ratios consistent with their derivation from an ocean island-type mantle source, the composition of which includes the HIMU (high 238U/204Pb) component characteristic of the Galapagos hotspot. In contrast, the 123 Ma Ecuadorian oceanic rocks have flat REE patterns and (relative to NMORB) are depleted in Zr, Hf, Th, and U. Moreover, they show a wide range of Nd and Pb isotopic ratios intermediate between those of ocean island basalts and NMORB. It is unlikely, on geochemical grounds, that the plume source of the Ecuadorian fragments was similar to that of the Galapagos. In addition, because of the NNE motion of the Farallon plate during the Early Cretaceous, the Ecuadorian oceanic plateau fragments could not have been derived from the Galapagos hotspot but were likely formed at a ridge-centered or near-ridge hotspot somewhere in the SE Pacific.

Mesozoic sequence of Fuerteventura (Canary Islands): Witness of Early Jurassic sea-floor spreading in the central Atlantic

The Fuerteventura Jurassic sedimentary succession consists of oceanic and clastic deposits, the latter derived from the southwestern Moroccan continental margin. Normal mid-oceanic-ridge basalt (N-MORB) flows and breccias are found at the base of the sequence and witness sea-floor spreading events in the central Atlantic. These basalts were extruded in a postrift environment (post–late Pliensbachian). We propose a Toarcian age for the Atlantic oceanic floor in this region, on the basis of the presence higher up in the sequence of the Bositra buchi filament microfacies (Aalenian–Bajocian) and of clastic deposits reflecting tectono-eustatic events (e.g., late Toarcian to mid-Callovian erosion of the rift shoulder). The S-1 sea-floor oceanic magnetic anomaly west of Fuerteventura is therefore at least Toarcian in age. The remaining sequence records Atlantic-Tethyan basinal facies (e.g., Callovian–Oxfordian red clays, Aptian–Albian black shales) alternating with clastic deposits (e.g., Kimmeridgian–Berriasian periplatform calciturbidites and a Lower Cretaceous deep-sea fan system). The Fuerteventura N-MORB outcrops represent the only Early Jurassic oceanic basement described so far in the central Atlantic. They are covered by a 1600 m, nearly continuous sedimentary sequence which extends to Upper Cretaceous facies.

Bimodal magmatism as a consequence of the post-collisional readjustment of the thickened Variscan continental lithosphere (Aiguilles Rouges-Mont Blanc Massifs, Western Alps)

High Precision U-Pb zircon and monazite dating in the Aiguilles Rouges-Mont Blanc area allowed discrimination of three short-lived bimodal magmatic pulses: the early 332 Ma Mg-K Pormenaz monzonite and associated 331 Ma peraluminous Montees Pelissier monzogranite; the 307 Ma cordierite-bearing peraluminous Vallorcine and Fully intrusions; and the 303 Fe-K Mont Blanc syenogranite. All intruded syntectonically along major-scale transcurrent faults at a time when the substratum was experiencing tectonic exhumation, active erosion recorded in detrital basins and isothermal decompression melting dated at 327-320 Ma. Mantle activity and magma mixing are evidenced in all plutons by coeval mafic enclaves, stocks and synplutonic dykes. Both crustal and mantle sources evolve through time, pointing to an increasingly warm continental crust and juvenile asthenospheric mantle sources. This overall tectono-magmatic evolution is interpreted in a scenario of post-collisional restoration to normal size of a thickened continental lithosphere. The latter re-equilibrates through delamination and/or erosion of its mantle root and tectonic exhumation/erosion in an overall extensional regime. Extension is related to either gravitational collapse or back-arc extension of a distant subduction zone.

Accreted fragments of the Late Cretaceous Caribbean–Colombian Plateau in Ecuador

The eastern part of the Western Cordillera of Ecuador includes fragments of an Early Cretaceous (c123 Ma) oceanic plateau accreted around 85–80 Ma (San Juan unit). West of this unit and in fault contact with it, another oceanic plateau sequence (Guaranda unit) is marked by the occurrence of picrites, ankaramites, basalts, dolerites and shallow level gabbros. A comparable unit is also exposed in northwestern coastal Ecuador (Pedernales unit). Picrites have LREE-depleted patterns, high eNdi and very low Pb isotopic ratios, suggesting that they were derived from an extremely depleted source. In contrast, the ankaramites and Mg-rich basalts are LREE-enriched and have radiogenic Pb isotopic

Is the Lower Duarte Igneous Complex (Hispaniola) a Remnant of the Caribbean Plume-Generated Oceanic Plateau?

Eleven samples of metapicrites, metaankaramites, diabases, and cumulates of the lower Duarte Complex in central Hispaniola were analyzed for major, trace element, and Nd-Sr isotopic compositions. The picrites are plagioclase-free and rich in phenocrysts of clinopyroxene and olivine pseudomorphs. The ankaramites differ from the picrites by the presence of abundant and large clinopyroxene phenocrysts. The diabases consist of plagioclase laths embedded with clinopyroxene. These rocks show E-MORB affinities. Relative to N-MORB, they exhibit high concentrations in Nb, Ta, Th, Ti, light and medium rare earth elements, and low Y contents. They are associated with olivine-clinopyroxene cumulates and gabbros. The olivine-clinopyroxene cumulates differ from the lavas by lower trace element contents and a lesser LREE enrichment. The gabbros exhibit flat REE patterns. All these clinopyroxene-rich rocks show homogeneous

Late Jurassic Oceanic Crust and Upper Cretaceous Caribbean Plateau Picritic Basalts Exposed in the Duarte Igneous Complex, Hispaniola

\epsilon_{Nd(T = 150 Ma)}

Short-term metasomatic control of Nb/Th ratios in the mantle sources of intraplate basalts

ratios ( + 5.3 to +7.1), which plot within the range of Ocean Island basalts. Their