Abstract

The Utuado pluton is emplaced among Upper Cretaceous volcanic and sedimentary rocks in north-central Puerto Rico. It consists mainly of granodiorite and quartz diorite; minor amounts of quartz porphyrite and gabbro occur marginally. Present evidence favors the conclusion that these rocks were formed by transformation of pre-existing rocks, without, in the main, passing through a liquid state.

The age of the pluton is not yet accurately fixed but is probably late Cretaceous, although it may be early Eocene. The pluton probably was formed at not more than 3 miles depth and probably very much less.

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