Geol Rundsch (1997) 86 : 141 — 155 ( Springer-Verlag 1997 ORIGINAL PAPER F. Hauff · K. Hoernle · H.-U. Schmincke R. Werner A Mid Cretaceous origin for the Gala ´ pagos hotspot : volcanological, petrological and geochemical evidence from Costa Rican oceanic crustal segments Received: 29 July 1996 / Accepted: 22 September 1996 Abstract The Quepos, Nicoya and Herradura oceanic igneous terranes in Costa Rica are conspicuous features of a Mid to Late Cretaceous regional magmatic event that encompasses similar terranes in Central America, Colombia, Ecuador and the Caribbean. The Quepos terrane (66 Ma), which consists of ol-cpx phyric, tholeiitic pillow lavas overlain by highly vesicular hy- aloclastites, breccias and conglomerates, is interpreted as an uplifted seamount/ocean island complex. The Nicoya ( & 90 Ma) and Herradura terranes consist of fault-bounded sequences of sediments, tholeiitic volcanics (pillow lavas and massive sheet flows) and plutonic rocks. The volcanic rocks were emplaced at relatively high eruption rates in moderate to deep water, possibly forming part of an oceanic plateau. Major and trace element data from Nicoya/ Herradura tholeiites indicate higher melting temperatures than inferred for normal mid-ocean-ridge basalts (MORB) and/or a different source composition. Sr—Nd—Pb isotopic ratios from all three terranes are distinct from MORB but resemble those from the Gala´pagos hotspot. The volcanological, petrological and geo- chemical data from Costa Rican volcanic terranes, combined with published age data, paleomagnetic re- sults and plate tectonic reconstructions of this region, provide strong evidence for a Mid Cretaceous ( & 90Ma) age for the Gala´ pagos hotspot, making it one of the oldest known, active hotspots on Earth. Our results also support an origin of the Caribbean Plate through melting of the head of the Gala´pagos starting plume. F. Hauff ( ) · K. Hoernle · H.-U. Schmincke · R. Werner Department of Volcanology and Petrology, GEOMAR, Wischhofstr. 1-3, D-24148 Kiel, Germany Fax: (49) 431-600-2978 E-mail: fhauff@geomar.de Key words Costa Rica · Ophiolites · Oceanic crust · Hotspot · Gala´pagos islands · Volcanology · Petrology · Geochemistry · Isotopes Introduction The origin, age and emplacement of numerous uplifted oceanic crustal segments along the Pacific Coast of Central America (Fig. 1) is a matter of ongoing debate, with models of in situ formation contrasting with mod- els of terrane accretion. Age estimates of these com- plexes range from Jurassic to Paleocene (Schmidt- Effing, 1979). The term ophiolite is commonly used for these complexes, but we prefer oceanic igneous terranes because 1) crucial portions of the ophiolite inventory (Coleman, 1977), such as a sheeted dike complex, are generally missing and 2) the segments occur as fault- bounded blocks of different ages. At least six such terranes are exposed along the Pacific coast in Costa Rica: Santa Elena Peninsula, Nicoya Peninsula, Her- radura, Quepos, Osa Peninsula, Golfito and Burica Peninsula (Fig. 1). Here we integrate volcanological, petrological and geochemical data from the Nicoya, Herradura and Quepos terranes in order 1) to assess the petrogenesis of the igneous suites and 2) to deter- mine the origin of the convergent plate margin of Costa Rica. A wide variety of models have been proposed for the origin of the Central American oceanic terranes. Their present location at a convergent plate margin have led some workers to favour accretion of Pacific crust (Galli-Olivier, 1979), while others prefer uplift of in situ Caribbean basement (Donnelly et al., 1990). More complex, multistage geotectonic models based on bio- stratigraphic and geochemical studies invoke terrane formation at a mid-ocean ridge (north Nicoya) and subsequent overprinting in intra-plate and primitive island arc settings (south Nicoya, Herradura, Osa) (Schmidt-Effing, 1979; Frisch et al., 1992). Still others