Kish GEOCHEMICAL STUDIES OF PUMICE 235

 

7.0

Florida pumice

6.0

0
O
© Antilles rhyolite
x

Los Humerous Volcanic Center

¢ Quetzalapa Pumice

*
oO

K,O [wt %]
wo
©

1.0 O

 

0.0 1
40.0 90.0

60.0

    

©

Low-K

OT

70.0 80.0

SiO, [mt%]

Figure 15, SiO, versus K,O diagram illustrating potential sources for rhyolitic pumice artifacts for southern and northwestern
Florida. The Quetzalapa pumice (Rodriguez et al. 2002) and volcanic rocks of the Los Humerous volcanic center (Ferria and
Manhood 1987; Verma and Lépez 1983; Rodriguez 2005) are located in eastern Mexico. Antilles rhyolites are taken from

Gunn et al (1980) and Turner et al. (1966).

diagram) and have distinct depletion of Nb, Ta, Ti. The
depletion of Nb, Ta, Ti is often interpreted to be the product of
a residual phase in the source for the magma that retains these
elements. Alkalic basalts that are associated with “within-plate”
magmatism typically have moderate to highly elevated LIL and
moderately enriched rather than depleted Nb, Ta, Ti concentra-
tions. The only alkaline basaltic rocks present in the Lesser
Antilles arc are found on the southernmost island of the chain-
Grenada (Archulus 1976; Thirlwall and Graham 1984). Basalts
with MgO content similar to the Florida samples have moderate
enrichment of LIL cations (Rb, Ba, K) and a relative depletion
of the high-valency cations (Nb, Ta, Ti). This pattern is not
similar to those of the Florida mafic material (Figure 16a).
Medium MgO (evolved alkaline) basalt from the Tuxtla
area, which borders the Gulf of Mexico, have trace element
patterns that are moderately similar to the mafic Florida pumice

(Figure 16b), but exhibit depletion of Nb, Ti and Ti, which is
characteristic for most of the volcanic rocks at that location
(Nelson et al. 1995). The trachybasalts of Toxtlacuaya lava
flow associated with the Naolinco volcanic field (Siebert and
Niifiez 2002), located near Xalapa, Mexico, have a within-plate
geochemical signature with minor negative depletion in Nb, Ta,
and Ti (Figure 16b), Both the Tuxula and Naolinco volcanic
rocks have relatively elevated Zr/Nb ratios (9-10) relative to the
lower ratios (<5) measured in the Florida mafic pumice.

The greatest similarity between the Florida basaltic pumice
and a potential alkaline source appears to be basaltic pumice
from mafic lavas from the volcanic islands of the eastern
Atlantic. Pumice and scoria for the 0.17-0.37 Ma Hernandez
Formation associated with the Las Canada volcanic edifice on
Tenerife, the largest of the Canary Islands (Figure 12), have
trace element signatures (Wolff et al. 2000) that are nearlv