Lolita Campos

Chromite and platinum group elements mineralization in the Santa Elena Ultramafic Nappe (Costa Rica): geodynamic implications

Chromitites associated with strongly altered peridotite from six distinct localities in the Santa Elena ultramafic nappe (Costa Rica) have been investigated for the first time. Santa Elena chromitites commonly display a compositional variation from extremely chromiferous (Cr/(Cr+Al)=0.81) to intermediate and aluminous (Cr/(Cr+Al)=0.54). This composition varies along a continuous trend, corresponding to calculated parental liquids which may have been derived from the differentiation of a single batch of boninitic magma with Cr-rich and (Al, Ti)-poor initial composition. Fractional precipitation of chromite probably occurred during differentiation of the boninitic melt and progressive metasomatic reaction with mantle peridotite. The distribution of platinum group elements (PGE) displays the high (Os+Ir+Ru)/(Rh+Pt+Pd) ratio typical of ophiolitic chromitites and, consistently, the platinum group minerals (PGM) encountered are mainly Ru-Os-Ir sulfides and arsenides. Textural relations of most of the platinum group elements suggest crystallization at magmatic temperatures, possibly under relatively high sulfur fugacity as indicated by the apparent lack of primary Os-Ir-Ru alloys. The chemical and mineralogical characteristics of chromitites from the Santa Elena ultramafic nappe have a strong affinity to podiform chromitites in the mantle section of supra-subduction-zone ophiolites. Calculated parental melts of the chromitites are consistent with the differentiation of arc-related magmas, and do not support the oceanic spreading center geodynamic setting previously proposed by some authors

Sedimentary Basins of Costa Rica: Late Mesozoic-Cenozoic Evolution and Hydrocarbon Potential

The Upper Cretaceous/Cenozoic basins of Costa Rica are underlain by oceanic crust and located along the western margin of the Caribbean Plate. A primitive intraoceanic arc formed along the western margin of the Caribbean Plate since Albian time, with associated anoxic pelagic sedimentation. This arc marked the beginning of subduction of the Pacific Plate in this region and of the first elements of the land bridge joining North and South America. Continued subduction of the Pacific Plate from Campanian until the present created a convergent marginal tectonic setting with, from southwest to northeast, a trench, an outer arc, a forearc region, a volcanic arc, and a backarc region. Localized platforms of shallow water carbonates, overlain by extensive cyclical turbidite systems, characterized the sediments of most of the basins up to the Eocene, when a major tectonic compressive event affected the area. Also about this time, a major sinistral east-west fault system (Costa Rica TranscurrentFaultZone) divided the region into north and south segments, and transtensional or pull apart basins developed along the fault system. During this pre-Miocene time, the north segment was emergent and subject to erosion, whereas in the south segment sedimentation continued. From Miocene on, shallow marine deposition was widespread in both segments. In the north segment, extension in the backarc region created a structural depression filled with Neogene sediments. The south segment experienced compressional tectonics due to north-directed movement of the Panama block. Later, in the Pleistocene, northeasterly-directed shallow subduction of the thick Cocos Ridge resulted in additional thrusting and the development of a foreland basin.