Different Localities of Potassium-Rich Silica-Undersaturated Igneous Rocks and Their Silica-Rich Variants - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 182

(1) phonolitic pumice,
(2) brown fibrous pumice,

(3) vesicular green pumice and
(4) and (5) two types of tephri-phonolitic scorias.

The scoria includes dense to moderate vesicular leucite spatter and aphyric glass
with fine vesicules. The tephriphonolitc F-Tuffs contain porphyritic scoria frag-

ments in which leucite is often replaced by analcite. Turbville described several
metres of white phonolitic pumices (PF-4) grading upward to black tephriphonolitic
spatter and a welded vitriophyric Pitigliano tuffs. The PF-4 pumice contains Ba-rich
sanidine phenocrysts (Ba content up to 8 wt%). Plagioclase crystals are anorthite-
rich (An 77 - 87 ). Chemical analyses of some rocks from Latera Caldera are given in
Table 4.16 .
Conticelli et al. (2002) discussed about the genesis of potassic volcanism along
the Tyrrhenian border of the Italian peninsula, which has been the site of intense
magmatism from Pliocene to recent times. Although calc-alkaline, potassic and
ultrapotassic volcanism overlaps in space and time, a decrease of alkaline character
towards south is observed. Alkaline ultrapotassic and potassic volcanic rocks are
characterized by variable enrichment in K and incompatible elements, coupled with
consistently high LILE/HFSE values, similar to those of calc-alkaline volcanic
rocks from the nearby Aeolian arc. On the basis of mineralogy and major and trace
element chemistry, two different arrays can be recognized among primitive rocks; a
silica- saturated trend, which resulted in the formation of leucite-free ma c rocks,

and a silica undersaturated trend, characterized by leucite-bearing rocks. According
to Conticellli et al. (2002), initial Sr 87 /Sr 86 and Nd 143 /Nd 144 values of Italian ul-
trapotassic and potassic ma

c rocks range from 0.70506 to 0.71672 and from
0.51173 to 0.51273, respectively. The Pb 206 /Pb 204 values range between 18.50 and
19.15, Pb 207 /Pb 204 values range between 15.63 and 15.70, and Pb 208 /Pb 204 values
range between 38.35 and 39.20. The general epsilon (Sr) versus epsilon (Nd) array,
along with crustal lead isotopic values, clearly indicates that a continental crustal
component has played an important role in the genesis of these magmas. The main
question is from where this continental crustal component has been acquired by the
magmas. Volcanological and petrologic data indicate continental crustal contami-
nation to be a leading process along with fractional crystallisation and magma
mixing. Considering, however, only the samples thought to represent primary
magmas, which have been in equilibrium with their mantle source, a clearer picture
emerges. A large variation of epsilon (Sr) versus epsilon (Nd) is still observed, with
epsilon (Sr) from 12. A bifurcation of this

array is observed in the samples that plot in the lower right quadrant, with ma

− 2 to +180 and epsilon (Nd) from +2 to −

c
leucite-bearing Roman Province rocks buffered at epsilon (Sr) = , + 100, whereas
the ma c leucite-free potassic and ultrapotassic rocks point towards strongly

radiogenic Sr compositions. Conticelli et al. argued that ma c leucite-bearing

Roman Province rocks have epsilon (Sr) and epsilon (Nd) values similar to those of
Miocene carbonate sediments, whereas mafic leucite-free potassic and ultrapotassic
rocks owe their genesis to mixing with silicate upper crust end-members with the