Lithospheric sources of North Florida, USA tholeiites and implications for the origin of the Suwannee terrane

Abstract

Sr, Nd, and Pb isotopic data for Mesozoic tholeiites from six drillholes in northern Florida (Suwannee terrane) indicate derivation of the magmas from a source that was slightly incompatible-element enriched with respect to bulk earth, probably continental lithospheric mantle variably contaminated by more enriched crust. Initial isotopic compositions of the Florida tholeiites show a considerable range: 87 Sr/ 86 Sr =0.7059 to 0.7097, ε Nd=−4.6 to +0.1, 206 Pb/ 204 Pb =18.30 to 18.65, 207 Pb/ 204 Pb =15.57 to 15.66, and 208 Pb/ 204 Pb =38.22 to 38.35. These compositions are distinctly different from those of Mesozoic alkali basalts from the subsurface of southwestern Florida [Heatherington, A.L., Mueller, P.A., 1991. Geochemical evidence for Triassic rifting in southwestern Florida. Tectonophysics 188, 291–302]. It is suggested that the North Florida tholeiites were derived from within the continental lithosphere, in contrast with Mesozoic volcanic rocks from southwestern Florida, which have been previously suggested to be related to a mantle plume. Sm–Nd isotope systematics suggest derivation of the tholeiites from a ∼1-Ga source, a similar age to that proposed by Pegram [Pegram, W.J., 1990. Development of continental lithospheric mantle as reflected in the chemistry of the Mesozoic Appalachian tholeiites, USA Earth and Planetary Science Letters 97, 316–331] for the source of tholeiitic basalts within the Carolina terrane. This supports Proterozoic tectonic models which suggest that both the Suwannee and Carolina terranes had affinities with South American portions of Gondwana containing ∼1 Ga crust, specifically, the Sunsas-Rondonian province [Nance, Murphy, 1996; Mueller, P.A., Kozuch, M., Heatherington, A.L., Wooden, J.L., Offield, T.W., Koeppen, R.P., Klein, T.L., Nutman, A.P., 1996. Evidence for Mesoproterozoic basement in the Carolina terrane and speculations on its origin. In: Nance, R.D., Thompson, M.D. (Eds.), Avalonian and Related Peri-Gondwanan Terranes of the Circum-North Atlantic, GSA Special Paper 304, pp. 207–218].