222 THE FLORIDA ANTHROPOLOGIST

2006 VOL. 59(3-4)

 

Qz

 

Ab

Or

Figure 7. Triangular plot of the normative minerals quartz (QZ), albite (Ab) and orthoclase (Or) for Florida pumice samples
that have a rhyolitic composition. Open squars represent high siliica (>74% SiO,) samples; filled squares are low silica (70-75
% SiO,) samples; open circle is a pumice sample from the Wynnhaven Beach site in northwestern Florida. The outlined field
represent the dominant composition field for 1269 chemically analyzed rhyolite and granite samples (Tuttle and Bowen

1958).

alteration, such as silicification, potassic or sodic metasomatism
of granitic-rhyolitic rocks, will produce samples that plot
outside of this central field. The high-silica, rhyolitic pumice
samples from this study all plot within the center of Tuttle and
Bowen’s “average granite” field (Figure 7). This relationship
suggests the observed chemical composition of the felsic
pumnice reflects a primary, rhyolitic signature that has not been
affected by alteration.

A second measure of the chemical character and possible
alteration of volcanic rocks is the relative abundance of alumina
(AI,0,) relative to the alkali content of the rock. Shand (1927)
established that igneous rocks may be subdivided into four
categories based upon alumina saturation: alumina saturated
(Al,O,>Na,0+K,0+Ca0) or peraluminous (corundum norma-
tive); metaluminous and subaluminous (no normative corun-
dum or acmite in norm) and peralkaline rocks in which
Al,O,<Na,O+K,O and normative acmite or sodium metasilicate
appear in the norm.

Unaltered, felsic calc-alkaline igneous rocks are normally
metaluminous or weakly peraluminous. Igneous rocks that have
been hydrothemally altered, with clays replacing the primary
silicates such as feldspars, will be highly peraluminous. The 15
felsic pumice samples analyzed for this study are either
metaluminous or weakly peraluminous (Figure 8). A single
sample from the Deering Estate site (FBAR 00.14.47.1) has a

moderate peraluminous value of 1.19. The overall alumina
saturation values of all samples are within the normal range of
calc-alkaline rocks. This also is reflected by the low values of
normative corundum (Table 5), which is associated with a
peraluminous composition.

Trace Element Chemistry

Trace element studies can be combined with the major
element information for a suite of cogenetic igneous rocks to
refine the model for magma generation and evolution or the
trace element composition of individual samples can be used as
a “fingerprint,” to infer the potential tectonic setting of the
sample. For the felsic pumice samples selected for this study
the variation in major element chemical composition is limited,
suggesting that most of the materials originated and evolved
under similar conditions. In this case, trace element character-
ization of the pumice samples can be useful for “tectonic
discrimination” (characterizing the geological setting associated
with the volcanic source of the pumice) and as a means of
looking for additional geochemical variations between samples
that may indicate separate sources. A major problem in using
trace elements studies for SiO,-rich, volcanic material such as
rhyolite, is that relatively large-scale trace element variations
can be present within a single eruptive unit that has a relatively