Cocos Plate Seamounts offshore NW Costa Rica and SW Nicaragua: Implications for large-scale distribution of Galapagos plume material in the upper mantle

摘要

The origin of intraplate volcanism not directly part of a hotspot track, such as diffuse seamount provinces, and the extent of mantle plume influence on the upper mantle remain enigmatic. Here we present new Ar-40/Ar-39 age data and geochemical (major and trace-element and Sr-Nd-Pb isotopic) data from seamounts on the Cocos Plate presently located offshore of NW Costa Rica and SW Nicaragua. The seamounts (similar to 7-24 Ma) require mixing of an enriched ocean island basalt composition, similar to that of the Northern Galapagos Domain, with two depleted components. One of the depleted components is similar to East Pacific Rise normal mid-ocean ridge basalt and the other has more depleted incompatible elements, either reflecting secondary melting of MORE or a depleted Galapagos plume component Seamounts with ages significantly younger than the ocean crust formed in an intraplate setting and can be explained by northward transport of Galapagos plume material along the base of the Cocos Plate up to 900 km away from the hotspot and 250-500 km north of the Galapagos hotspot track. We propose that melting occurs due to decompression as the mantle upwells to shallower depth as it flows northwards, either due to changes in lithospheric thickness or as a result of upwelling at the edge of a viscous plug of accumulated plume material at the base of the lithosphere. The tholeiitic to alkaline basalt compositions of the Cocos Plate Seamounts compared to the more silica under-saturated compositions of Hawaiian rejuvenated and arch (alkali basalts to nephelinites) lavas are likely to reflect the significant difference in age (<25 vs similar to 90 Ma) and thus thickness of the lithosphere on which the lavas were erupted. (C) 2014 Elsevier B.V. All rights reserved.