Minor and Rare Earth Element Geochemistry of K-Rich Silica-Undersaturated Igneous Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 224

0.717

TORRE

GNEISS
XENOLITH

VICO
S.VENANZO
VULSINI

ALFINA

M.SABATINI

0.716

0.715

RADICOFANI

M. CIMINI

0.714

MANTLE
MIXING
LINE ?

0.713

M. AMIATA

0.712

CUPAELLO

C

0.711

A

ALBAN

HILLS B-100 0.71024

B-2

0.710

B

ERNICI

0 4 8 12 16 20 24 28 32 36 40

-1 4

1/Sr (ppm ) x 10

Fig. 5.11 A plot of 87 Sr/ 86 Sr ratios versus [1/Sr (ppm)] X 10 4 of various K-rich Tuscan lavas of
Mt Cimini, Radicofani and Mt. Amiata (Hawkesworth and Volmer 1979; Volmer 1976; Poli et al.
1984) including data from San Venanzo, Vico, Mt Sabatini, Alban Hills (Stippled pattern) and Mt.
Ernici (diagonal line pattern) (from various sources, see Ferrara et al. 1986). The lines are related
to mixing. A and B refer to the highest and lowest K 2 O content, respectively. C refers to rocks of
transitional characteristics; B-2 and B-100 represent tephritic leucitite and a drill core sample
beneath Latera Caldera

primary high K magma of central Italy. They further thought that because of high
initial Sr contents of the Mount Vulsini lavas, the 87 Sr/ 86 Sr probably were not
affected by wall rock exchange reactions or by low temperature processes that
raised the

18 O values. They thought that the 87 Sr/ 86 Sr ratios of K-rich magma of
Mount Vulsini, Cupaello and San Venanzo are quite high, though they are mantle-
derived. They thought that these magmas were derived from a zone in the upper
mantle which was previously metasomatically enriched in 87 Sr. According to them,
it is possible that the high K-series parent magma at Mount Vulsini had a uniform
87 Sr/ 86 Sr ratios (0.7102

δ

0.7103), which are identical to those of the parant magma
of Alban Hills volcanic centre, located 120 km to the south. Even though the Sr
isotopic data at Vico and Mount Sabatini are sparsely determined, they thought that
same conclusion could be derived for the rocks of these intervening centres also.
Some of the low K-series rocks however, were erupted at Mt. Vulsini and were
derived from a mantle source with a lower 87 Sr/ 86 Sr ratio than the rocks of high K-
series rocks (0.7097). They observed that from Mt. Vulsini to Naples, the rocks
belonging to low-K series end members of all the localities have lower 87 Sr/ 86 Sr
ratio compared to high K-series end members. They further observed that the lavas
that erupted at Mt. Vulsini have

-

18 O values (+ 6.5

13.8) and 87 Sr/ 86 Sr ratios δ

-

(0.7097 0.7168) that are in general higher and have a much broader range than any

-