Kish GEOCHEMICAL STUDIES OF PUMICE 227

 

1000

100

Rb (ppm)

10

1 10

 

100 1000

Y + Nb (ppm)

Figure 11. Trace element composition of rhyolitic Florida pumice samples plotted on the (Y+Nb)-Rb discrimination diagram
of Pearce et al. (1984). Filled squares: low silica, high — K rhyolites; open squares: high silica, high — K rhyolites; open circles:
medium — K rhyolites. Abbreviations: VAG, volcanic are granitoids; Syn-COLG, syn-collisional granitoids; WPG, within-

plate granitoids; and ORG, oceanic ridge granitoids.

approximately 7-9. Heavy REE have nearly uniform values of
5-8 times chondritic abundance and Gd,,/Yb,, ratios that are
approximately equal to one. Low-silica, high-K pumice samples
have similar REE patterns, but have overall higher REE
concentrations, especially in the heavy REE (Figure 9), with
values 15 times chondritic concentrations. Both types of
pumice exhibit moderate to high depletion of the element
Europium (Eu) relative to other “middle” REE. Europium
depletion is expressed as a negative “Europium anomaly” (see
footnote in Table 8). Low-silica, high-K pumice has moderate
negative Eu anomalies (~0.5), while high-silica, high-K pumice
has moderate (0.5-0.6) to high (0.25-0.35) negative Eu anoma-
lies. The Eu anomalies for the high-silica samples appear to fall
into two separate groups (Figure 9); however, there does not
appear to be a strong correlation between the magnitude of the
Eu anomaly and major element composition. Europium
anomalies are often produced by the preferential partitioning of
Eu” into residual plagioclase and potassium feldspar during the
generation of a magma (partial melting) or by crystal fraction-
ation of the same mineral species during the subsequent
crystallization ofa magma. Preferential retention of Eu in these
solid phases would produce negative Eu anomalies in the
resulting melt. The large negative Eu anomalies observed in the
high-silica pumice samples is characteristic of most high-silica
granites and rhyolites.

Pumice samples with medium-K rhyolitic composition have
REE patterns (Figure 10) that exhibit less variation between the
light and heavy REE compared to high-K samples. Medium-K
pumice has moderately elevated light REE (90-100 times
chondrite values) and no significant Eu anomalies (Table 7).
Pumice with an andesitic composition has nearly “flat” REE
patterns with La.,/Ybcy = 3.5, weak Eu anomalies (0.9) and
total REE concentrations that are lower th (La = 35-40 times
chondritic values) at more felsic material. A single dacite
sample (EGLIN 056) has a REE pattern similar to andesite but
with an overall higher REE concentration, with La = 110 times
chondritic values. The REE patterns for the two mafic samples
are characterized by elevated REE concentrations that are not
characteristic of subalkaline, tholeiitic basalts, which normally
have “flat” REE patterns that are 10 times chondritic values.
Rather, these samples have REE patterns that are similar to
alkaline basalts (Wilson 1989).

Trace Element Discrimination Diagram

Geochemical discrimination diagrams have been developed
to identify the potential tectonic setting of igneous rocks. The
diagrams, in reality, categorize igneous rocks that have formed
by similar geochemical processes (Rollinson 1993). However,