Abstract

Both shoshonites and calc-alkaline andesites of Late Cretaceous age occur in south-central Puerto Rico. These rocks are chemically and mineralogically related in many respects, but the potassium-rich varieties display extreme enrichment in K2O (more than 6 percent), Rb (up to 150 ppm), and Cu (up to 300 ppm). All of these Late Cretaceous rocks contain relatively high U (from 0.6 to 1.9 ppm) and Th (from 1 to 5 ppm) compared to typical andesites. The potassic magmas may have been generated in dehydrated oceanic crustal materials along the Benioff zone at relatively high pressures.

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