The Santa Elena peninsula is located on the Pacific coast of northern Costa Rica. Most of the peninsula is formed of the serpentinised, peridotitic Santa Elena Nappe composed of spinel harzburgites, and a variety of gabbroic rocks and mafic dykes. Underlying the basal thrust of the nappe is the Santa Rosa Accretionary Complex, exposed along the southern coast around Playa Santa Rosa and in the Protero Grande tectonic window to the east. This complex represents an autochthonous basement of the region, comprising pelagic sedimentary (radiolarites, turbidites, cherts, and shales), and alkaline mafic rocks affected by a low-grade metamorphic overprint. Alkaline rocks are present in several localities both within and cutting through the stratigraphy of this unit, including alkaline basalts and basanites to phonolites. Best exposed at Playa Santa Rosa, alkaline basaltic rocks are present within the sequence of sedimentary rocks as sills, flows, tuffs, and dykes, thought to represent a small, accreted seamount. Of note is their potassic nature uncommon for seafloor basalts, which suggests this seamount formed as a petit-spot volcano. Descriptions of regional geology can be found in Tournon (1994). A database of collated geochemical data including whole rock major and trace element and Sr, Nd, Pb data can be found in Gazel et al., (2006), with further whole rock and amphibole data being found in Buchs et al., (2013).