PETROGENESIS OF EARLY MESOZOIC THOLEIITE IN THE FLORIDA BASEMENT
AND
AN OVERVIEW OF FLORIDA BASEMENT GEOLOGY
By
Jonathan D. Arthur
INTRODUCTION
Basalts are extrusive igneous rocks characteristically associated with major tectonic events such as intraplate or "hot spot" vulcanism, plate subduction, sea-floor spreading and continental rifting. Diabase (or dolerite), an intrusive chemical equivalent of basalt, is primarily associated with rifting events. In eastern North America (ENA), diabase is exposed within the Appalachian orogen from Alabama to Newfoundland as sheets (e.g., sills and lopoliths) and subparallel dikes. The sills, as well as associated basalt flows, crop out within a series of Mesozoic basins that parallel the Appalachian orogen. Geophysical and corehole data reveal the presence of diabase sills beneath the ENA Coastal Plain province (Gottfried et al., 1983; Chowns and Williams, 1983). Numerous studies have suggested that the ENA diabase-basalt suite is genetically related to Early Mesozoic rifting of the Pangean supercontinent (King, 1961; May, 1971; DeBoer and Snider, 1979). In Florida, 39 oil test wells have encountered diabase and basalt within pre-Cretaceous sedimentary rocks. Available geochemical and mineralogical data reveal that these igneous rocks are tholeiitic in composition (see also Part I, "Classification" section). Based upon stratigraphic, geochronologic, and very limited geochemical data, Chowns and Williams (1983) have proposed that Florida tholeiite belongs to the ENA tholeiitic suite. It is possible, however, that additional magmatic systems may have been associated with Florida tholeiite genesis due to the continental plate configuration immediately prior to the rifting event. Plate reconstructions (Bullard et al., 1965; Van der Voo et al., 1976), lithologic similarities (Smith, 1982) and isotopic data (Odom and Brown, 1976; Dallmeyer et al., 1987) suggest that northwest Africa, northeast South America and southeast North America were juxtaposed during the Early Mesozoic.
Part I of this study provides new geochemical data in order to determine the petrogenesis of Florida tholeiitic magma(s). Furthermore, this study will investigate the possibility of a genetic relationship between Florida and Georgia magmas and those magmas represented by tholeiitic dike systems of Africa and South America which were emplaced during the break-up of Pangea. Existence of such a relationship would further support theories regarding the Early Mesozoic rifting event and provide further insight into the geochemical nature of tholeiite prior to generation of mid-Atlantic ridge transform and oceanfloor basalts.
Part Two of this report summarizes Florida basement geology. Included in this section is a presentation of nomenclature currently recognized by the Florida Geological Survey as well as unpublished radiometric data on file at the Survey.
Metric Conversion Factors
The Florida Geological Survey, in order to prevent duplication of parenthetical conversion units, inserts a tabular listing of conversion factors to obtain metric units.
Multiply By To Obtain
feet 0.3048 meters
miles 1.6090 kilometers
inches 25.40 millimeters
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