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Fig. 5.13 Spidergram plots
of chondrite-normalized REE
concentration in selected
samples of Latera pumice,
syenite and lava (after
Turbeville 1993)
1000
Trachyte
Phonolite
Tephriphonolite
Garnet Syenite
Quartz Syenite
Tephrite Lava
100
10
1
La
Ce
Pr
Nd
Sm Eu Gd Tb
Dy
Ho
Er
Tm Yb
differences in Rb/Th, Nb/Ta, Zr/Hf and La/Yb ratios suggest ef
cient fractionation
of incompatible trace element within the evolved liquids. He observed that Na/Ta
ratio increases with differentiation from tephriphonolite to phonolite and trachyte.
This is related to progressive fractionation of sphene that is common in evolved
pumices and syenites. In his chondrite-normalized REE plots, negative Eu anom-
alies are very much prevalent in the felsic tuffs (Fig.
5.13
), which indicate that they
are derivative liquids after plagiocase fractionation.
The tephritic phonolite tuffs and tephritic lavas do not have Eu anomaly, which
according to Rogers et al. (1985) are inherent source characteristic of unevolved
Vulsini magmas. Turbeville found that the syenites have widest range in trace
element content amongst Latera rocks and their REE content spans the entire
concentration range of these elements in the rocks of Vulsini complex. He observed
that syenites have especially high Th, Zr, and Rb contents with more variable REE
patterns and there is stronger decoupling among the LIL elements than is displayed
by the composition of the pumice. He thought that the elemental abundances of the
syenites re
ect high modal concentration of sphene, apatite, zircon and amphibole.
Turi and Taylor (1976) measured the
18
O/
16
O ratios of leucite-bearing lavas, py-
roclastics, and other related rocks from the Roman Province of Italy. They observed
a general increase northward in the
18
O values in the following sequence: Ischia
δ
(5.8
7.0), Somma-Vesuvius and Phlegrean Fields (7.3
8.3), Mount Sabatini
-
-
(7.3
11.7). The Sr isotopic
ratios of the rocks of the Roman Province also show a similar northward increase. A
marked increase in the
9.7), Vico Volcano (7.4
10.2) and Monti Vulsini (8.7
-
-
-
18
O value was noted by them just north of Rome, where the
rocks of the Roman Province just begin to overlap the Tuscan calc-alkaline rocks.
This observation led them to believe that this increase in the
δ
18
O contents and
87
Sr/
86
Sr ratios might be due to direct mixing of the parent magma with the Tuscan
rocks, which are high in the
δ
18
O country
rocks underwent heating during 2 million years of the Tuscan igneous activity.
They estimated that the strongly undersaturated magma, which was probably
derived from the upper mantle had a
18
O content, or to the fact that the high
δ
δ
18
O value of + 6, and the Sr isotopic ratio of
δ
0.704
-
0.705 and SiO
2
content of less than 44 wt%.
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