PART I PETROGENESIS OF EARLY MESOZOIC THOLEIITE IN THE FLORIDA
BASEMENT
Background
AGE
A comparison of geochronological studies of Florida diabase suggests that the average age of
crystallization is about 192 million years (Ma) (Milton and Grasty, 1969; Barnett, 1975; Mueller and Porch, 1983). Results of these studies are summarized in Table 1. Only those age determinations from unaltered diabase and basalt with uncertainties less than + 15 Ma were considered for this average.
Several studies have noted a lack of reproducibility in K-Ar ages for ENA diabase due to K or Ar inhomogeneity, radiogenic 40Ar loss (Armstrong and Besancon, 1970) or an excess of radiogenic 40Ar (Dooley and Wampler, 1983). Sutter (1985) notes that 40Ar/39Ar ages are more reproducible than K-Ar ages. For these reasons, the 40Ar/39Ar age determinations reported by Mueller and Porch (1983) for Florida diabase (averaging 194 + 9 Ma) may be closer to a true age of crystallization. A compilation of age determinations for diabase beneath the Coastal Plain of the southeastern United States can be found in Chowns and Williams (1983, Table 6). For diabase in the subsurface of Georgia, their tabulated
Table 1. Reported radiometric age determinations from Florida tholeiite. (WR whole rock, MS mineral separates)
Florida
Geol. Survey Rock
County Well Number Depth (Ft.) Description Method Age (Ma) Reference
Dixie Gainesville 12,450 Altered Basalt K-Ar WR 244 + 10 Standard Oil, (offshore) BIk. 707, #1 unpublished data
Franklin W-8487 14,275 Diabase K-Ar WR 203 + 12 Barnett (1975) 182 + 11
186 + 9
195 + 12
Franklin W-8487 14,275 Diabase K-Ar MS 153 + 61 Barnett (1975) (Pyroxene) 181 + 72
129 + 60
Hardee W-1655 11,853 Highly Altered K-Ar WR 147 + 3 Milton and Grasty Basalt 143 + 7 (1969)
Hardee W-1655 11,870 Basalt 40Ar/39Ar 192 + 7 Mueller and Porch, WR 196 + 6 (1983)
Highlands W-966 12,664 Slightly Altered K-Ar WR 183 + 10 Milton and Grasty Basalt (1969)
Lee W-10566 15,708 Basalt K-Ar WR 163 + ? Barnett (1975) (Composite)
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