WHAT IS THE ORIGIN OF THE OIB-LIKE SIGNATURE IN PLIOCENE-RECENT COSTA RICAN ARC VOLCANIC ROCKS?

Major and trace element and Sr-Nd-Pb isotope geochemistry show a transition from mid ocean ridge basalt (MORB)-type compositions in volcanic arc rocks from Nicaragua to ocean island basalt (OIB)-type compositions in arc rocks from Central Costa Rica (CCR). The OIB-type component is consistently observed in samples from the volcanic front to the backarc of the CCR arc segment. The main question in our investigation is whether the OIB-signature is derived from changes in the mantle wedge or from the subduction input. There are two possibilities for a change to OIB-type chemistry in the mantle wedge beneath CCR outlined here: 1) the change from MORB- to OIB-type chemistry occurs near the boundary between the Chorortega and Chortis Blocks. The mantle underlying the Chorortega Block may have been contaminated by OIB-type material during the Caribbean Large Igneous Province (CLIP) event. However, the Nd isotope composition of CLIP is too radiogenic to serve as a source of the OIB-type component in CCR volcanics. 2) The OIB-type component could reflect a mixture of Galápagos plume material, entering the mantle wedge through a slab window, with a MORB-type mantle wedge (e.g. Abratis & Wörner, 2001, Geol. 29: 127-130), but it is not clear how plume material can be transported ~1500 km from the Galapagos hotspot to CCR. There are two main sources of input into the CCR subduction zone: 1) the subducting slab consisting of oceanic crust formed at the Galapagos Spreading Center overprinted by magmatism from the Northern Galápagos Domain (Hoernle et al., 2000, Geol. 28: 435-438), and 2) tectonic erosion of the CCR margin, consisting of CLIP basement and accreted Galápagos terranes. Nd and Pb isotope compositions of CCR volcanics are not consistent with the subducting slab or CLIP basement being the source of the OIB-type component, although the composition of accreted Galápagos terranes is very similar to the composition of CCR volcanic rocks. The onset of Cocos Ridge subduction beneath Costa Rica (~5-6.5 Ma) resulted in major erosion of the continental margin (Vannucchi et al., 2003, Geoph. Res. Abstr. 5: 05168), coincident with the change to more OIB-type compositions of CCR arc rocks. Thus, erosion of accreted Galapagos terranes may serve as a feasible source component for the OIB-type compositions observed in CCR arc volcanics.