Petrology of Andesitic, Spilitic, and Keratophyric Flow Rock, North-Central Puerto Rico
Chemical, mineralogical, and petro-graphical variations in a thick accumulation of andesitic, spilitic, and keratophyric flow rock result mainly from compositional differences and crystallization history. Together, the clinopyroxene-rich and plagioclase-rich andesitic rocks (1) range from andesitic basalt to dacite, (2) show a calc-alkalic chemical trend, (3) contain plagioclase and clinopyroxene rich in Ca, (4) generally contain no olivine, and (5) show no Fe enrichment. Their origin is attributed to partial fusion under hydrous conditions: magmas of plagioclase-rich rocks formed at depths where basaltic phases existed; magmas of clinopyroxene-rich rocks where eclogitic phases existed. Plagioclase-rich spilitic rocks generally differ from their andesitic counterparts in having (1) higher Na and K, and lower Ca, Mg, and Al; (2) lower phenocryst content; (3) greater alteration; (4) greater cloudiness of plagioclase; (5) slightly lower inferred Ca content in some clinopyroxenes; and (6) no actinolite. Clinopyroxene-rich rocks show the same differences but are more variable chemically. High-K andesitic rocks otherwise resemble normal andesitic rocks. Spilitic rocks resemble andesitic rocks in matrix textures and in apparent simultaneous crystallization of plagioclase and clinopyroxene. Calcic plagioclase is relict in some spilitic rocks, and Ca-rich secondary minerals occur in both. The spilitic rocks are interpreted as having formed from andesitic rocks during the late-magmatic or deuteric stage, although they may have resulted from postconsolidation metasomatism. Keratophyric rocks occur only in plagioclase-rich associations. Quartz content suggests that most keratophyres are felsic magmatic rocks; other felsic rocks in the map area contain excess quartz and are products of hydrothermal silicification.