The framework compositions of sand-stones from the Costa Rican forearc provide a compositional norm for detrital modes from basins associated with intraoceanic arcs, a poorly documented tectonic setting. Nicoya Peninsula sandstones are made up of mainly volcanic lithic fragments, plagioclase, sedimentary lithic fragments, and mafic accessory minerals. Modal point counts of 70 samples average Q3F32L65, Qm2F32Lt66, Qp1Lvm72 Lsm27, and Qm6P94K0, defining these sand-stones as less quartzose than sandstones from most other tectonic settings and less lithic than most quartz-poor sandstones from intraplate oceanic islands.

Four newly defined modal parameters quantify temporal changes in composition. These parameters are (1) Vi/V′, the ratio of volcanic fragments with microlitic or felsitic textures to those with lathwork, microlitic, or felsitic textures; (2) Mh/M, the ratio of hydrous to total mafic silicate mineral grains; (3) Po/P, the ratio of oscillatory-zoned to total plagioclase grains; and (4) Sc/S, the ratio of reworked carbonate detritus to total sedimentary lithic fragments. All four parameters increase from negligible values in the basement unit to moderate or high values in cover units. These increases are followed by generally low values in the youngest (Miocene) sandstones. Significant temporal changes are not revealed by previously defined modal parameters that are commonly used to analyze sandstone composition.

Petrography of these sandstones provides strong evidence for the initial construction of an andesitic arc edifice by Campanian time (Late Cretaceous). Increases in Vi/V′, Mh/M, and Po/P ratios through most of the section record increased detrital input from evolved volcanic sources. A parallel increase in Sc/S ratios reflects the progressive construction and erosion of a carbonate-rich sedimentary are apron. The Miocene reversal of these trends reflects subaerial erosion of up-lifted basement rocks at the forearc structural high. Miocene sandstones also contain minor plutonic detritus that includes potassium feldspar, signaling incipient unroofing of the arc. Sandstones of the Nicoya Peninsula thus preserve a relatively complete record of the evolution and erosion of the Upper Cretaceous through Miocene intraoceanic arc of southern Central America.

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