BASEMENT INFRASTRUCTURE OF NORTHWEST FLORIDA REVEALED BY 39AR/40AR AGE SPECTRA OF K-FELDSPAR AND MUSCOVITE

We present new 40Ar/39Ar age spectra of K-feldspar and muscovite from granitic rocks recovered from deep wells in northwest Florida that, together with new geophysical maps, refine tectonic models of the coastal-plain basement geology of the southeastern U.S. Previous workers attributed these rocks to a single suite of Neoproterozoic Gondwanan or peri-Gondwanan intrusions, the “Gaskin intrusive complex.” However, K-feldspar from only one of three wells in this study returns a cooling history consistent with Neoproterozoic granitoids in the Appalachian orogen. The others record significantly younger cooling ages that are hard to reconcile with Neoproterozoic crystallization.

Step-heating of euhedral muscovite grains from the “type” Gaskin granite, recovered from a depth of 3.96 km, produces a 653.8±3.4 Ma plateau age. K-feldspar grains from the same rock indicate cooling through Ar closure in the latest Neoproterozoic to earliest Cambrian and were not reset by reburial or other heating. These data confirm the presence of Gondwanan components in the crust of northwest Florida, limit the size of Mesozoic basins in the area, and constrain the extent of subsequent intrusive activity.

In contrast, step-heating of K-feldspar recovered from a well 57 km to the NW at a depth of 3.75 km records Triassic cooling. Such a prolonged cooling history is not present in the Neoproterozoic Gaskin granite nearby, and near-complete resetting of the feldspar Ar system is unlikely in the absence of associated metamorphism. This cooling history is, rather, common among Alleghanian granitoids. Gravity and aeromagnetic data do not support contiguity with the Fox Creek granite which occurs ~90 km to the E. Therefore, this sample probably belongs to a previously unidentified Permian intrusion.

Most unusual is a third sample recovered 35 km further west at a depth of 4.42 km, from which K-feldspar grains return early Cretaceous cooling ages, younger than the presumed ages of overlying sediments. A Late Jurassic granite is documented in southern GA, >400 km to the ENE. If this sample is contemporaneous, rapid exhumation in the early Cretaceous is required to produce the observed age spectra. An alternative explanation is Cretaceous crystallization, which would significantly extend the duration of magmatic activity known in the area.