O. T. Tobisch
1968
<p><span>The basal complex of </span>Puerto<span> </span>Rico<span> consists principally of serpentinite, minor amounts of chert and spilite, and locally small blocks of </span>amphibolite<span>. A detailed structural and metamorphic study of a relatively large block of </span>gneissic<span> </span>amphibolite<span> </span>at<span> </span>Las<span> </span>Palmas<span> reveals that the rock has undergone repeated deformation and regional metamorphism prior to contact metamorphism by intrusive serpentinite. The first event recorded by the </span>amphibolite<span> is </span>amphibolite<span> facies regional metamorphism. Foliation and some hornblende lineation were formed </span>at<span> this time. An increase </span>in<span> the intensity of regional metamorphism then produced quartzo-feldspathic segregations from the host rock, and was accompanied by small-scale folding and development of several types of lineation oblique (o the </span>early<span> hornblende lineation. The metamorphic intensity then decreased slightly and tectonic movement radically decreased, as nonfoliated but metamorphosed mafic dikes cut the </span>gneissic<span> </span>amphibolite<span>. As the metamorphic grade decreased further, late-stage kink-bands were developed </span>at<span> high angles to the earlier small-scale folding. Later, the </span>amphibolite<span> was intruded by serpen-tinite, causing incipient low-grade contact metamorphism and some calcium metasomatism. H. H. Hess has maintained that </span>Puerto<span> </span>Rico<span> and other islands </span>in<span> the </span>Greater<span> </span>Antilles<span> rest directly on oceanic crust, and that the intrusive serpentinite is derived from the bottom layer of the crust. </span>In<span> the </span>Las<span> </span>Palmas<span> area, the </span>amphibolite<span> formed prior to intrusion of serpentinite, indicating a complex preserpentinite </span>history<span>. Three interpretations of the </span>gneissic<span> </span>amphibolite<span> are considered. It may represent: (1) metamorphosed gabbroic rocks genetically related to the serpentinite, (2) regionally metamorphosed mafic rocks that are part of the oceanic crust, or (3) older mafic rocks regionally metamorphosed as part of an extensive pre-Late Cretaceous orogeny. The evidence from </span>Las<span> </span>Palmas<span> is not conclusive, but when considered </span>in<span> the regional framework, either of the last two interpretations seems reasonable.</span></p>
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10.1130/0016-7606(1968)79[557:GAALPP]2.0.CO;2
en
Geological Society of America
Gneissic amphibolite at Las Palmas, Puerto Rico, and its significance in the early history of the greater antilles island arc
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