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45 50 55 60

THE FLORIDA ANTHROPOLOGIST

2006 VOL. 59(3-4)

 

     
  
 

oO Rhyolite

Dacite

   

80

75

65 70

SiO, [wt%]

Figure 5. Total alkali-silica (TAS) classification for Florida pumice artifacts. Explanation for abbreviations: T — Trachyte;
TA - Trachyandesite; BT — Basaltic trachyandesite; TBA — Trachybasalt; PH — Phonlite; TPH — Tephriphonolite; PTA —
Phonotephrite and FD --Foidite. Classification diagram from LeMaitre et al 2002. Heavy line represents the boundary be
subalkaline (SA) and alkaline (A) rocks (Irvine and Baragar 1971). Open square symbols represent pumice samples from
southern Florida. Filled circles represent pumice samples from the Wynnhaven Beach site located in northwestern Florida.

samples also can be classified by their position within
compositional fields plotted on bivariate oxide-oxide major
element diagram (Le Maitre 2002). For most geochemical
classification schemes, the total major element composition of
a sample is recalculated to an anhydrous total of 100 percent,
without the inclusion of volatile compounds such as water and
carbon dioxide. This adjustment places all samples in the same
relative comparison frame, eliminating the dilutional effects of
the volatile components (Irvine and Baragar 1971). A. small,
moderately weathered sample from the Whitebelt 1 site FBAR
99.65.7.1) had an unacceptably high LOI value (10.8 wt. %)
and was not included for classification.

For this study, the total alkali-silica diagram (TAS) of Le
Bas et al. (1986) was used as the primary means for chemical
classification of Florida pumice artifacts. The sampies also
were characterized using the K,O versus SiO, diagram of
Peccerillo and Taylor (1976). This classification has additional
subdivisions of the calc-alkaline basalt-rhyolite series -- a low-
K or tholeiite series, a medium-K calc-alkaline series and a
high-K calc-alkaline series. This classification scheme can be
used to infer some information about the tectonic setting of the

volcanic material. High-K calc-alkaline volcanic activity
normally takes place in a more mature volcanic arc setting
relative to tholeiitic or medium K calc-alkaline volcanics.

Major element, whole rock analyses for Florida pumice
artifacts are reported in Table 4 and the calculated normative
minerals for the samples are reported in Table 5. All felsic
pumice samples have high normative quartz, orthoclase, and
albite, and the total normative mafic components (diopside +
hypersthene + magnetite + ilmenite) make up less than 10
weight percent of the normative composition.

Results - Major Element Classification

The TAS classification diagram for the Florida pumice
samples is shown in Figure 5. Fifteen out of nineteen pumice
samples from southern Florida fall within the field of
subalkaline rhyolites. Eleven of the 15 samples also have
uniform, high SiO, concentrations (75-76 wt. %) and are
classified using the informal term “high-silica rhyolites.” Four
samples have SiO, concentrations in the range 71-72 weight
percent and are classified using the informal term “low-silica