ALBERT

Description: In contrast to the long narrow volcanic chains in the Pacific Ocean, Atlantic hotspot tracks, in particular in the South Atlantic (e.g., Tristan-Gough, Discovery, Shona, and Bouvet), are irregular and, in some cases, diffuse and discontinuous. An important question is whether this irregularity results from tectonic dismemberment of the tracks or if it represents differences in the size, structure, and strength of the melting anomalies. Here we present new age and geochemical data from volcanic samples from Richardson Seamount, Agulhas Ridge along the Agulhas-Falkland Fracture Zone (AFFZ), and Meteor Rise. Six samples yielded ages of 83–72 Ma and are 10–30 m.y. younger than the underlying seafloor, indicating that they are not on-axis seamounts associated with seafloor spreading. The incompatible element and Sr-Nd-Pb-Hf isotopic compositions range from compositions similar to those of the Gough domain of the nearby Tristan-Gough hotspot track to compositions similar to samples from the Shona bathymetric and geochemical anomaly along the southern Mid-Atlantic Ridge (49°–55°S), indicating the existence of a Shona hotspot as much as 84 m.y. ago and its derivation from a source region similar to that of the Tristan-Gough hotspot. Similar morphology, ages, and geochemistry indicate that the Richardson, Meteor, and Orcadas seamounts originally formed as a single volcano that was dissected and displaced 3500 km along the AFFZ, providing a dramatic example of how plate tectonics can dismantle and disseminate a hotspot track across an ocean basin.

Description: It is commonly accepted that large igneous provinces are formed through catastrophic volcanic events occurring over vast areas within a few million years at the initiation of hotspots (mantle plumes). New 40Ar/39Ar ages (111–139 Ma) and geochemical results from the Nicoya Peninsula, Costa Rica, extend the age range of volcanism in the Caribbean large igneous province to 70 m.y. (69–139 Ma). Our results are not consistent with the formation of this vast igneous province through a single plume head at the initiation of a mantle plume such as the Galápagos. Instead we propose that multiple oceanic intraplate igneous structures, such as plateaus and hotspot tracks, were accumulated through the subduction process. The igneous structures could be remnants of the earlier history of the Galápagos hotspot, making it one of the oldest active hotspots on Earth. Alternatively they could have been derived from several spatially distinct mantle-melting events that sampled similar source material, e.g., oceanic lithosphere of similar age.

Description: More than 500 trachytic to phonolitic cone sheet dikes, hypabyssal syenite stocks, and subordinate radial dikes form a 20-km-diameter intrusive complex in the volcaniclastic fill of the Miocene Tejeda caldera (20 x 35 km) on Gran Canaria (Canary Islands). The dikes intruded concentrically around a central axis of radial symmetry and dip uniformly an average of similar to 41 degrees toward the center. Single-crystal Ar-40/Ar-39 ages of dikes and syenite stocks ranging from 12.3 +/- 0.1 to 7.32 +/- 0.05 Ma suggest more or less continuous intrusive activity during both volcanically active and inactive phases. Intrusions were emplaced at average intervals of similar to 5-10 k.y., while explosive eruptions took place at 〉 50 k.y. intervals. The estimated aggregate volume of evolved cone sheet magma added at shallow level (〈 2000 m below sea level) amounts to similar to 250 km(3) compared to 〉〉 500 km(3) of evolved extrusive volcanics erupted during the same period. Formation of the Tejeda cone sheets most likely resulted from deformation processes due to resurgent doming, initiated by the recurrent replenishment of a flat, laccolith-like shallow magma chamber. Magma supply exceeding the volume that could be compensated for by up-doming of the overlying caldera fill resulted in the formation of cone-shaped fractures.

Description: Volcanic rocks were dredged from the Cocos and Fisher ridges and seamounts along a 250 km profile parallel to the Pacific coast of Costa Rica. The composition and laser 40Ar/39Ar ages of the Cocos Ridge and Seamounts are consistent with their formation above the Galápagos hotspot 13.0–14.5 Ma. The reconstructed paleoenvironment and chemistry of the Fisher Ridge are consistent with it having originated at a mid-oceanic ridge system. Laser 40Ar/39Ar dating of fresh basalt glass from the Fisher Ridge yielded isochron ages of 19.2 ± 0.3 Ma and 30.0 ± 0.5 Ma. The Fisher Ridge is along a lithospheric fault that may represent an extensional fracture formed when the oceanic floor rode over the Galápagos hotspot. Even though the younger structures are currently at water depths of 〉1000 m, volcanological, geochemical, and geophysical observations indicate that they once formed an emerged archipelago very similar in morphology to the Galápagos islands. The diversity of the biota on the isolated Galápagos islands, as first described by Charles Darwin, has had an important influence on the development of the theory of evolution. The existence of a now-drowned Galápagos archipelago 14.5 Ma considerably increases speciation times for the Galápagos biota and provides a complete solution to a long-standing controversy concerning the divergence of the Galápagos marine and land iguanas from a single ancestral species.

Description: We present the results of volcanological, geochemical, and geochronological studies of volcanic rocks from Malpelo Island on the Nazca plate (15.8–17.3 Ma) belonging to the Gala´pagos hotspot tracks, and igneous complexes (20.8–71.3 Ma) along the Pacific margin of Costa Rica and Panama. The igneous complexes consist of accreted portions of ocean island and seamount volcanoes and aseismic ridges, representing the missing (primarily subducted) history of the Gala´pagos hotspot. The age and geochemical data directly link the Gala´pagos hotspot tracks on the Pacific Ocean floor to the Caribbean large igneous province (ca. 72–95 Ma), confirming a Pacific origin for the Caribbean oceanic plateau from the Gala´pagos hotspot. We propose that emplacement of this oceanic plateau between the Americas and interaction of the Gala´pagos hotspot tracks with the Central American Arc played a fundamental role in the formation of land bridges between the Americas in Late Cretaceous–Paleocene and Pliocene-Holocene time. The land bridges allowed the exchange of terrestrial faunas (e.g., dinosaurs, mastodons, saber-tooth cats, and ground sloths) between the Americas and served as barriers for the exchange of marine organisms between the central Pacific Ocean and the Caribbean Sea and the central Atlantic Ocean.