PRECAMBRIAN ZIRCONS FROM THE FLORIDA BASEMENT - A GONDWANAN CONNECTION

A Gondwanan origin for the pre-Cretaceous basement of Florida is sugge sted by U-Pb ages of 515 to 2860 Ma for single zircons separated from subsurface samples of lower Paleozoic sandstone of the Suwannee basin (Alachua County, Florida) and Neoproterozoic Osceola granite (Osceola County, Florida). Forty individual grains analyzed by SHRIMP ion micro probe yielded ages from 515 to 2860 Ma; ages >1000 Ma were predominant ly concordant. Two principal populations are evident: (1) 515 to 637 M a (avg. = 574 Ma; Pb-206/U-238 ages) and (2) 1967 to 2282 Ma (avg. = 2 130 Ma; Pb-207/Pb-206 ages). Only six zircons were recovered from the granite; four are Pan-African and two are Archean. For the sandstone, the similarity between the Sm-Nd model age (1245 Ma) and the average o f the single zircon ages (1326 Ma) suggests that these zircons are chr onologically representative of the aggregate provenance of the sandsto ne. The two dominant zircon age groupings correspond chronologically t o the Pan-African and Birimian or Eburnian (Africa) and to the Brasili ano and Trans-Amazonian (South America) orogenic cycles. The presence of detritus from rocks of these two orogenic cycles clearly places the basement of Florida in Gondwanaland proximal to the West African and/ or Trans-Amazonian-San Luis cratons in the early Paleozoic, a location it probably shared with other circum-Atlantic exotic terranes (Avalon ian, Cadomian, and/or Carolina).