NEW GEO- AND THERMOCHRONOLOGICAL CONSTRAINTS ON THE ARCHITECTURE AND EVOLUTION OF THE SUWANNEE TERRANE, FLORIDA, USA

Geochronology and thermochronology from basement rocks penetrated by deep wells in Florida constrain the Neoproterozoic through Paleozoic history of the Gondwanan Suwannee terrane (ST) buried beneath the Coastal Plain in the southeastern U.S. Components of the ST include the Gaskin and Osceola intrusive complexes (GIC and OIC, respectively), the North Florida volcanic series (NFVS), and sedimentary rocks of the Suwannee basin (SB).

U-Pb zircon (656±38 Ma; n=6) and ⁴⁰Ar/³⁹Ar magmatic muscovite (653.8±3.4 Ma) ages establish the Cryogenian crystallization of a granite of the GIC in northwest Florida. A K-feldspar ⁴⁰Ar/³⁹Ar cooling age of 575-550 Ma from the same sample further distinguishes the GIC from the younger OIC of peninsular Florida, from which we report U-Pb zircon ages of 554±13 Ma (n=14) and 544±11 Ma (n=8). These ages corroborate earlier ages for the OIC and confirm its temporal coincidence with some rocks of the NFVS. Prolonged NFVS deposition is indicated by the intercalation of the upper NFVS with the Cambrian to Ordovician basal sedimentary rocks of the SB. ⁴⁰Ar/³⁹Ar age spectra of sericitized dacite showing recrystallization from ~550-470 Ma and a K-feldspar cooling age of ~470-460 Ma from a granite in the OIC suggest that the static sub-greenschist facies alteration in some OIC, NFVS, and SB rocks coincides with NFVS deposition. ⁴⁰Ar/³⁹Ar age spectra of muscovite from Middle Ordovician strata in the SB indicate sediment sourcing from rocks older than 650 Ma.

The new ages permit an alternative reconstruction of the ST in the early Paleozoic involving north- or northwest-dipping subduction (at current latitude and configuration) to produce the OIC-NFVS arc and a back-arc SB. The ST was unaffected by early to middle Paleozoic orogenesis that preceded its accretion to Laurentia. During the Alleghanian orogeny, the GIC and associated Gondwanan rocks more proximal to Laurentia may have shielded distal parts the ST from deformation and metamorphism. Alleghanian effects on the ST are limited to the intrusion of granitoids along the northwestern margin. A U-Pb zircon age of 282±13 Ma (n=3) for a granite in southern Alabama and ⁴⁰Ar/³⁹Ar spectra of K-feldspar from a granite in northwest Florida recording Triassic cooling are interpreted to extend Alleghanian plutonism to the Gulf Coast.