Radiolarian petrographic preservation types in Jurassic to Lower Tertiary cherts of Costa Rica

Abstract

Radiolarian cherts are locally intercalated, mostly as up to 45 m thick sequences of rhythmically bedded chert, in the Jurassic to Lower Tertiary Nicoya Ophiolite Complex of western and southern Costa Rica (Central America). Additionally, bedded chert is present in some Campanian and Paleocene/Eocene sedimentary series that overly the Nicoya Complex. The ophiolitic cherts may be thermally recrystallized to different grades due to post depositional basaltic magmatism. The radiolarian skeletons mainly consist of microquartz; complete dissolution of the skeletons, i.e. exclusive preservation of the fillings, is rare. The fillings are variable in composition: quartz, length-fast chalcedony, and quartz-pigment mixtures are dominant; opal-CT, clay minerals, chlorite, zeolites, baryte, manganese and iron oxides, epidote and calcite are subordinate. The non-siliceous minerals may locally replace the siliceous phases and/or intergrow with them. Thus, approximately 40 preservation types have been observed in the Costa Rican cherts. In combination with the different preservation grades, a large number of preservation variants is present. However, in the cherts of the Nicoya Complex quartz-rich preservation types are strongly dominant, whereas opal-CT, zeolites and calcite are more important in the cherts of the overlying sedimentary series. Types and grades of radiolarian preservation represent an important aid in the reconstruction of the postdepositional diagenetic and thermal history of the Costa Rican cherts.