THE SOURCE OF PUMICE FOUND AT PREHISTORIC ARCHAEOLOGICAL SITES IN FLORIDA – PETROGRAPHIC AND GEOCHEMICAL EVIDENCE

Pumice is found as a relatively scarce drift material on the Atlantic beaches of Florida. Pumice is also found at many (32) archaeological sites near the Atlantic coast and at one site near the Gulf of Mexico in northwest Florida. Most of these sites are associated with the Glades I period (Woodland), dating to approximately 2000 years BP. The absence of other types of volcanic material at these sites, such as basalt or obsidian, supports a hypothesis that the pumice at these sites arrived via ocean drift similar to the modern mode of occurrence - rather than by long-distance trade. Pumice samples from excavations at the Miami Circle-Brickell Point archaeological site located near downtown Miami, Florida were used along with selected samples from other sites in a study to determine the possible sources for this igneous material. A majority of the pumice is characterized by high silica (74-76 wt. %) and high K2O contents (3.5-4.5 wt. %); strong light REE enrichment with La 50-70 times chondritic abundance. These samples also exhibit moderate to strong (0.5-0.25) negative Eu anomalies. A limited number of pumice artifacts have medium-K, rhyolitic to andesitic compositions, characterized by moderate light REE enrichment and no significant Eu anomalies. All rhyolitic samples have Rb versus (Y + Nb) contents that are characteristic of a calc-alkaline, volcanic-arc setting. Rare mafic pumice and scoria artifacts have an alkaline composition, characterized by elevated concentrations of K, Ti, and Nb, relative to cal-alkaline basalts. The calc-alkaline composition of most of the pumice artifacts is compatible with derivation from two nearby sources - the Lesser Antilles of the southern Caribbean or the easternmost portion of the Trans Mexico Volcanic Belt (TMVB), which terminates near the Gulf of Mexico. The overall high silica and high-K composition of the Florida pumice artifacts has not been observed in volcanics of the Lesser Antilles, but is common in the TMVB. Medium-K rhyolitic to andesitic pumice artifacts are similar to volcanics of the central Lesser Antilles. Young (< 2000 years BP), felsic volcanic eruptions are not well documented in either the Lesser Antilles or the eastern TMVB. Pumice was probably introduced into the Gulf of Mexico by mass wasting and river transport, rather than by direct volcanic eruptions.