A multi-stage history of refertilization and melt-impregnation in peridotites from the Santa Elena Ophiolite, NW Costa Rica

The Santa Elena Ophiolite (SEO), located on the northwestern coast of Costa Rica, consists primarily of preserved oceanic mantle and crustal rocks thrust above an accretionary complex. The SEO is predominantly characterized by mantle peridotites (i.e., primarily spinel lherzolite with minor amounts of harzburgite and dunite) cut and intruded by minor pegmatitic gabbros, layered gabbros, plagiogranites, and doleritic and basaltic dykes. Previous studies have concluded that the complex formed in a suprasubduction zone (SSZ) setting based on the geochemical nature of the layered gabbros and plagiogranites, as well, as the peridotites [1]. Sixty-three ultramafic samples collected during the winter 2009/2010 & 2010/2011 field seasons exhibit abundant evidence for melt-rock reaction (e.g., disseminated plagioclase and plagioclase-spinel, clinopyroxene-spinel, and plagioclase-clinopyroxene symplectites) and provide a unique opportunity to characterize the textural and chemical nature of melt-rock reaction in the SEO. We present the results of a petrologic investigation (i.e., petrography, EPMA, and LA-ICP-MS) of 78 thin sections derived from the extant sample set. Major element analyses of spinels in the peridotites exhibit chemistries characteristic of abyssal peridotites that overlap with forearc peridotites and subduction-related chromitites (e.g., Cr#s 7-93, Mg#s 24-86, TiO₂ 0-0.77). Petrographic observations have confirmed the presence of fresh plagioclase (An 87-90) in 10 of the peridotite thin sections. The intimate intermingling of both abyssal and subduction-related chemistries in the SEO suggests that it may preserve a full record of its history from mantle upwelling and melt generation to its eventual exposure on land.

[1] Denyer and Gazel, 2009, Journal of South American Earth Sciences 28:429-442.