232 THE FLORWA ANTHROPOLOGIST

2006 VoL. 59(3-4)

 

Wynnhaven Beach site (near Pensacola).
Potential Provenance Locations

An evaluation of the nature and composition of volcanic
rocks within the circum-Atlantic and Gulf of Mexico limits the
potential provenance for most of the Florida pumice artifacts.
Extensive pumice deposits have been found on strand lines
along the northern coastlines of Europe, Greenland and on
Ellesmere and Devon islands in the eastern Canadian Arctic
(Figure 12). These pumice clasts are associated with
postglacial, raised beaches that are approximately 6,700-4,000
years old. This pumice is dark brown and has a medium-K,
andesite to dacitic composition (Blake 1970; Binns 1972).
Modern geochemical studies (Newton et al. 2000) have
identified the Katla volcanic center in Iceland as the source for
most of the north Atlantic pumice. The absence of high-K,
felsic pumice and the distinct, non calc-alkaline trace element
signature for Iceland volcanic material (Lacasse and Garbe-
Schinberg 2001) rule out this location out as a source for
Florida pumice samples. Although there are voluminous, felsic
pumaceous deposits present on the Canary Islands (Wolff et al.
2000; Edgar 2002), Azores (Guest et al. 1999) and Cape Verde
Islands (Plesner et al. 2003), these deposits are dominated by
alkaline, silica undersaturated trachytes and phonolites that
exhibit high to extreme enrichment in trace elements such as Zr,
Nb and the REE. These trace element signatures are distinctly
different from the low Nb and moderately high REE, calc-
alkaline signatures of the Florida pumice samples. Mafic rocks
from the same Atlantic islands also have alkaline signatures,
which are characterized by elevated trace element concentra-
tions including Zr, Nb and REE. The mafic scoria and pumice
artifacts from Florida have similar trace element signatures and
may be related to the Atlantic island sites. Additional discus-
sion of the mafic samples is covered at the end of this section.

Potential Provenance Locations - Felsic Materials

There are two potential locations of young, calc-alkaline
volcanic activity that are relatively near the southeast Florida
coast. The locations include the Lesser Antilles islands in the
southern Caribbean and stratovolcanoes, domes and monogenic
volcanoes in the easternmost portion of the Trans Mexico
Volcanic Belt (Figure 13). The frequent eruption activity of
volcanoes in the Lesser Antilles, and the ease by which pumice
can reach the ocean from these relatively small islands, make
the Lesser Antilles a highly favorable candidate as the source
for the Florida pumice. There are two potential routes for drift
pumice to move from the Lesser Antilles to Florida. Relatively
weak, northward flowing currents, pass through the northern
Caribbean Islands and merge with the Gulf Stream off the coast
of southern Florida (Figure 13). The drift distance by that route
would be approximately 2500 km. The more dominant Atlantic
Equatorial current flows westward through the Lesser Antilles
and into the Gulf of Mexico where it merges with the Florida
Current and then the inner portion of the Gulf Stream, which
flows near the Atlantic coastline of south and central Florida
(Figure 13) - a total distance of 4500 km.

The Lesser Antilles form a narrow volcanic arc composed

of 15 small islands. The islands are located the southernmost
Caribbean, extending approximately 600 km in a north-south
direction, from near the coastline of Venezuela to the Virgin
Islands (Figure 13). The arc can be divided into three petro-
genetic domains that have moderately different chemical
characteristics (Baker 1984). A northern group of islands (Saba
to Redonda) is characterized by voluminous andesitic volcanics
and minor basalts and dacites (Brown and Holland 1977).
Volcanic rocks of rhyolite composition are rare but have been
reported from St. Kitts and St. Eustatius (Gunn et al. 1980).
The central group of islands (Montserrate to St. Vincent) are
characterized by a similar proportion of volcanic lithologies,
but the overall K,O content for a given rock type is more
elevated than the northern group of islands (Figure 14). A
limited number of islands (Grenada and the Grenadines) form
a southern petrogentic group. Volcanic rocks in the southern
group are dominated by basalt and basaltic andesite. Andesite,
dacite, and rhyolite are rare. Volcanic rocks of the southern
group have been described has having an alkaline character
(Arculus 1976; Thirlwall and Graham 1984), however, the
actually alkaline characteristics are more a function of the rocks
being silica-undersaturated due to high CaO and low SiO,
contents. The total alkali contents of these rocks are not
elevated (MacDonald et al. 2000).

The SiO, and K,O contents for the volcanic rocks of the
Lesser Antilles are distinctly lower than the Florida high-K,
rhyolite pumice artifacts (Figures 14 and 15). While there have
been numerous pyroclastic eruptions in the Lesser Antilles that
produce andesitic and dacitic pumice in the last 2000 years
(Robol and Smith 1980), there are no records of young
rhyolitic volcanism. The preserved rhyolitic material is quite
limited (<5%) and is associated with geologically older units
(71.0 Ma). Gunn et al. (1980) noted that the most felsic
volcanics of the northern Lesser Antilles were Na-rhyolites,
which were probably formed from the fractional crystallization
of more mafic magma. Extensive deposits of medium-K dacite,
exposed on the southern portion of the island of St. Lucia
(Jackson et al. 2002), are composed of biotite-rich pumice
somewhat similar to the medium-K rhyolitic pumice artifacts
found at some sites in southern Florida. The SiO, (<68 wt. %)
and K,O (<2 wt. %) concentrations for St. Lucia pumice
(Wright et al. 1984), however, appear to be lower that the
medium-K, rhyolitic pumice found at Florida archaeological
sites. The most silica-rich unit, the 20-34 Ka Belford pumice’
(Wright et al. 1984), has a low porosity and high crystal content
(35 %); most samples have a density greater that 1.0 (Wohletz
et al. 1986). These physical characteristics do not match any of
the Florida pumice samples. While there is not a close similar-
ity between the rhylotic pumice samples from Florida and
volcanic units in the Lesser Antilles, the two andesite samples
from southern Florida are similar to the medium-K andesites
from the central island group (Figure 15).

A major region of active, dormant and extinct volcanoes is
located in eastern Mexico between Mexico City and Veracruz
(Figure 13). These volcanoes form the easternmost portion of
the Trans-Mexican Volcanic Belt (TMVB), which extends for
approximately 1000 km in an east-west trending belt across
central Mexico. The actual tectonic setting of the TMVB is still
debated, however, the overall petrologic and geochemical