Document Type

Dissertation

Date of Award

1970

Keywords

Petrology, Puerto Rico, Robles Formation

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

Thomas W. Donnelly

Second Advisor

Don L. Kissling

Third Advisor

William D. MacDonald

Series

Science and Mathematics

Abstract

The Robles Formation of south central Puerto Rico is composed primarily of andesitic lavas and pyroclastic rocks. Two important andesitic lava members are present within the formation. One, the Lapa Lava Member, is enriched in potassium. Another, the Las Tetas Lava Member, is poor in potassium. Mineralogy of the flows was strongly governed by crystal settling both in the magma chamber before eruption and within individual flows at the time of eruption. All of the igneous rocks associated with the Robles Formation belong to the calc-alkaline series.

Plagioclase and olivine phenocrysts contain a series of metamorphic alteration phases as follows: (1) analcime, celadonite, chlorite, sericite; (2) laumontite and albite; (3) prehnite; (4) pumpellyite; (5) epidote; (6) actinolite. Assemblages containing analcime, celadonite, and laumontite belong to the zeolite facies of metamorphism. Remaining assemblages belong to the prehnite-pumpellyite facies. Secondary mineralogy of the rocks is consistent with residual chemistry after removal of relic plagioclase and metastable clinopyroxene fractions. The degree of alteration seems related to permeability: phases appear in greater quantities in porous pyroclastics.

Absolute quantities of potassium. rubidium. and copper in the Lapa Lava Member are independent of phenocryst or mafic content. and decrease toward the south. This may represent original inhomogeneity of the magmas, or it may be the result of metasomatism through either hydrothermal activity or metamorphism.

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