Document Type

Dissertation

Date of Award

1969

Keywords

Spilites, United States Virgin Islands, Petrological studies, Geochemical studies, Caribbean Island arc

Degree Name

Doctor of Philosophy (PhD)

Department

Geological Sciences

First Advisor

Thomas W. Donnelly

Second Advisor

Herman E. Roberson

Third Advisor

Raymond E. Smith

Series

Science and Mathematics

Abstract

A continuous section of extrusive and intrusive rocks have been recovered through a drill core of about 2h30 feet that was taken at St. John, U. S. Virgin Islands. The present study is confined to the upper 1180 feet and comprises a cummulative total of 950 feet of albitized mafic extrusive rocks (spilite). This portion of the core is also intruded by four dikes: two amphibole porphyryes (129-13h and 732-7&2 feet): an albite-diabase (177-226 feet) and an andesine-diabase (8h7-1003 feet).

The spilite is a fine—grained rock with pilotaxitic and intersertal textural features. Throughout the flow, the major mineralogical assemblages consist of partly albitized Ca-plagioclase, chlorite, epidote and quartz. The upper part of this flow shows a concentration of dark colored and fine-grained patches set in a green, fine-grained, and abundantly recrystallized matrix. This green matrix is characterized by glassy textural features and lower temperature assemblages (epidote, quartz, chlorite) than the dark patchy material. The patchiness decreases with depth and below 650 feet disappears, being replaced by a homogeneous, fine-grained spilite. ’Below the non-patchy spilite, there is a sheared and recrystallized spilitic zone which contains epidote, quartz, chlorite and breccia fragments.

After extrusion of the spilite flows onto the surface, the upper part of the core solidified faster than the underlying non-patchy zone. The dark patches represent centers of crystallization which solidified before the surrounding green matrix. Once the lava flow solidified, it was intruded by several dikes and a later faulting gave rise to the bottom brecciated zone.

Alkali migrations occur throughout the rocks studio. The K20 content in the upper part of the spilite flow is higher and the Na2O content lower than the bottom of the flow. Local variations of alkalis show also that the dark patches are higher in Na20 than the surrounding green matrices. The alkali migration throughout the different lithological units of the core could have been caused by both magmatic and metamorphic processes. During the last stage of crystallization, hydrothermal solutions might have concentrated some of the K20 in the green matrix and the Na2O in the adjacent dark patches. Burial metamorphism also affected the core and caused. the disappearance of prehnite and pumpellyite with depth.

The parental magma of the spilite is believed to be derived from the partial melting of amphibolite tapped in the lower Crust or upper Mantle (extrapolated from experimental data) beneath the Virgin Islands.

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