234 THE FLORIDA ANTHROPOLOGIST

2006 VoL. 59(3-4)

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between rivers flowing eastward into the Gulf of Mexico and a
westerly drainage that , in part, flows into the closed Serdan-
Oriental intermontane basin in the southernmost portion of the
Mexican Alltiplano or plateau (Rodriguez et al. 2002).
Numerous rhyolite domes and basaltic pyroclastic cones also
are present within the same area. The Tuxtla Volcanic Field is
a separate collection of mafic alkaline to calc-alkaline volca-
noes and pyroclastic cones that are located near the Gulf of
Mexico, 50 km south of Veracruz, Mexico (Thorpe 977s

Nelson et al. 1995).

Numerous pyroclastic eruptions of Pico de Orizaba have
taken place during the Holocene. A major collapse of the
volcano’s summit area took place approximately 33,000 years
ago. (Héskuldsson and Robin 1993). The resulting 11 km’
debris avalanche flowed 60 km eastward, down the Tliapa and
Tlachhuatl valleys to the Cordoba plains. The debris flow
contains mostly basaltic andesite and dacite (Héskuldsson and
Robin 1993). Several major pyroclastic deposits of scoria and
pumice flow deposits formed between 8,500-9,000 years ago.
These pyroclastic deposits include both dense, andesitic scoria
and andesitic to dacitic hornblende-bearing, pumice (Carrasco-
Niifiez and Rose 1995), plus very limited quantities of rhyolitic
pumice.

Rhyolitic tuff and pumice are more common in the more
northerly portion of the eastern TMVB. Extensive high-silica
pumice deposits and rhyolite are exposed on the flanks of the
Los Humeros Caldera (Verma 1984; Ferria and Manhood 1987;
Rodriguez 2005) and the Las Derrumbadas thyolite, domes
(Siebe and Verma 1988). The felsic rocks in this area are
characterized by total alkali contents (8-9 wt. %) that are
somewhat higher than Florida pumice samples (7-7.8 wt. %).

The Quetzalapa Pumice, the most voluminous pumice unit
in the eastern TMVB, has a rhyolitic composition. It is exposed
on the western slopes of the Las Cumbres Volcanic Complex.
The pumice is present as a thick (up to 20 meters), extensive
unit of white, highly vesiculated pumice containing abundant
biotite and plagioclase phenocrysts (Rodriguez et al, 2002).
The Quetzalapa Pumice has SiO, and K,O contents that are
very similar to the high-silica Florida pumice (Figure 15).
While the mineralogy and major element composition of the
Quetzalapa Pumice appears to be very similar to the high-silica
pumice of Florida, there is only a limited amount of trace
element data is available for this unit that can be used for
comparison with Florida pumice. Both Ba (965-1099 ppm) and
Zr (119-169 ppm) concentrations are similar to Florida pumice.
Radiocarbon dating of charcoal fragments within the pumice
indicates an eruption age of approximately 20,000 years ago
(Rodriguez et al. 2002). While the Quetzalapa Pumice has
mineralogic and bulk chemical (Figure 15) similarities to high-
silica pumice artifacts from Florida the pumice, it is almost
exclusively exposed on the westward drainage divide of the
region. Thin (0.3- 0.05 m), distal reworked beds derived from
the Quetzalapa Pumice can be detected in the Gulf of Mexico
drainage region (Rodriguez et al. 2002), however, it is uncertain
if any well-preserved pumice is actually present within the
reworked beds. Rodriguez (2005;189-190) has measured
moderately thick (1 m), multiple layers of pumice on the east
flank of the Las Cumbres Volcanic Complex. These deposits
are exposed near the east flowing Huitzilapan, Chichiquila, and

Jamapa rivers. Specific information covering the physical and
chemical nature of this pumice is not available. Radiometric
dating of charcoal in associated pyroclastic deposits span a
wide time range (40,000 - 2,000 B.P). If this pumice was the
source of Florida pumice, the probably transport mechanism
would have been simple erosion along river exposures, or large
scale mass wasting and subsequent river transport. A possible
modem analog for such an event was the extensive amount of
landslide activity produce by Hurricane Mitch in 1988, which
devastated a large area in Central America (Vallance et al.
2001). The travel distance for the pumice from the Veracruz
region to southern Florida would be approximately 2000 km
and would include transport by the Florida Loop Current, the
Florida Current and the Gulf Stream.

Potential Provenance Locations - Mafic Materials

The two mafic pumice-scoria samples found at archacologi-
cal sites in Florida are chemically distinct from other pumice
artifacts in that they have a moderate alkaline signature com-
pared to the calc-alkaline signature for felsic samples. This is
evident by their moderately elevated total alkali and K,O
contents (Figures 5 and 6). Both of these mafic samples have
strong light REE enrichment (Figure 10) and enrichment of
several other trace elements including Ba, Nb, Ta and Ti. The
enrichment of Nb, Ta and Ti is a trace element signature of
within-plate alkaline igneous activity compared to Na-Ta-Ti
depletion that is observed in basaltic lavas associated with a
calc-alkaline volcanic arc environment (Wilson 1989). The
Zr/Nb ratios for both samples are low (<5) — a relationship also
associated with oceanic-island (or within plate) basalts (Wilson
1989). Four potential locations (Figure 12) that could be the
source of the basaltic pumice include: (1) alkaline basalts from
volcanic islands of the eastern Atlantic, including the Canaries
and Cape Verde Islands; (2) weakly alkaline volcanic rocks on
the island of Grenada in the Lesser Antilles; (3) alkaline
basaltic rocks in the Tuxtla volcanic field boarding the Gulf of
Mexico in east-central Mexico and (4) basaltic rocks of the
Naolinco volcanic field on the eastern flanks of the Cofre de
Perote volcanic center, located approximately 50 kilometers
inland from the Gulf of Mexico near Veracruz, Mexico.

Mafic rocks that are similar in their major element composi-
tion may have distinct differences in trace element composition
that can provide significant information about the possible
differences in tectonic setting between samples. Trace element
data can be readily compared by the use of normalized, multi-
element “spider” diagrams, which are similar to chondrite-
normalized REE patterns, however, a wide range of “incompat-
ible” trace elements are used in this type of diagram. Incompat-
ible trace elements are those elements that either because of
their large or small ionic size, or because of high ionic charge,
will not readily be incorporated into common igneous minerals
- their concentrations are sensitive indicators of the source ofa
magma and its subsequent modification. Spider diagrams for
tholeiitic basalts typically have a flat or positive pattern with
respect to the high field strength elements, which are plotted on
the right-hand side of the spider diagram. Island arc, calc-
alkaline basalts typically have elevated large ion, lithophilic
element (LIL) values (plotted on the left-hand side of the spider