A MID CRETACEOUS ORIGIN FOR THE GALAPAGOS HOTSPOT - VOLCANOLOGICAL, PETROLOGICAL AND GEOCHEMICAL EVIDENCE FROM COSTA-RICAN OCEANIC CRUSTAL SEGMENTS

The Quepos, Nicoya and Herradura oceanic igneous terranes in Costa Ric a are conspicuous features of a Mid to Late Cretaceous regional magmat ic event that encompasses similar terranes in Central America, Colombi a, Ecuador and the Caribbean. The Quepos terrane (66 Ma), which consis ts of ol-cpx phyric, tholeiitic pillow lavas overlain by highly vesicu lar hyaloclastites, breccias and conglomerates, is interpreted as an u plifted seamount/ocean island complex. The Nicoya (similar to 90 Ma) a nd Herradura terranes consist of fault-bounded sequences of sediments, tholeiitic volcanics (pillow lavas and massive sheet flows) and pluto nic rocks. The volcanic rocks were emplaced at relatively high eruptio n rates in moderate to deep water, possibly forming part of an oceanic plateau. Major and trace element data from Nicoya/Herradura tholeiite s indicate higher melting temperatures than inferred for normal mid-oc ean-ridge basalts (MORE) and/or a different source composition. Sr-Nd- Pb isotopic ratios from all three terranes are distinct from MORE but resemble those from the Galapagos hotspot. The volcanological, petrolo gical and geochemical data from Costa Rican volcanic terranes, combine d with published age data, paleomagnetic results and plate tectonic re constructions of this region, provide strong evidence for a Mid Cretac eous (similar to 90 Ma) age for the Galapagos hotspot, making it one o f the oldest known, active hotspots on Earth. Our results also support an origin of the Caribbean Plate through melting of the head of the G alapagos starting plume.