Different Localities of Potassium-Rich Silica-Undersaturated Igneous Rocks and Their Silica-Rich Variants - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 173

incompatible elements, evidence of modi cation of such element contents during

low pressure fractionation has been observed.
According to Cox et al. (1976), if the incompatible element concentration is not
related to low pressure fractionation then the correlation with 87 Sr/ 86 Sr ratio is
possibly a primary magmatic feature, which was inherited from the parental liquids
in equilibrium with mantle host rocks. They concluded that absolute incompatible
element content obtained in the five most calcic samples is possibly not much

different from their primary values. Even if the primary magmas were much more
magnesian than those erupted (i.e. the magma lost 50 % of the ferromagnesian
phases during its ascent to the surface), then the incompatible element content should
have increased by a factor of two. According to Cox et al. in case of Roccamon

na,
not all members were erupted and a compositional gap exists between the low-K and
high-K series of lavas. It is however noted that the composition of the lavas of Monte
Somma located about 50 km south of Roccamon

na fills the compositional gap

between the low- and high-K series of Roccamon na. The K, Rb, Sr, Ba and Zr

contents of lavas of Roccamon ed greatly from their pri-

mary magma values by fractionation. Thus, the Sr isotopic ratios and concentration
of the above trace elements should provide ample evidence regarding the mantle
source region. They thought that the Sr-isotope and trace element variations may
re

na have not been modi

ect either (1) disequilibrium melting of a chemically homogeneous whole rock, or
(2) melting of an inhomogeneous mantle source rock on a large scale with respect to
Sr-isotopes and bulk composition. They argued that magma with highest Sr isotopic
ratio could then be explained by selective incorporation of a phase by the liquid with
high Rb/Sr ratio. Magma with lowest Sr isotopic ratio should be produced by a
greater incorporation of a phase with low Rb/Sr ratio. High Sr isotopic ratio can be
produced by disequilibrium melting of phlogopite-rich source. In this model high Sr
isotopic ratio can be obtained if partial melting takes place. Melts however, formed
by more advanced partial meltings have lower 87 Sr/ 86 Sr ratios and Sr content. This
model however does not explain the linear correlation between Sr isotopic ratios and
Sr contents. Likewise, the other linear correlation between Sr isotopic relation and
K 2 O, Rb, Sr and Ba are also dif

cult to be produced by disequilibrium melting
models unless a small degree of partial fusion took place. Chemical compositions of
some leucite-bearing rocks from this area are given in Table 4.16 .
Contichelli et al. (2009) have studied the rocks of Roccamo na volcano The

Roccamon na volcano is characterized by two stages of volcanic activity that are

separated by volcano-tectonic caldera collapses. Ultrapotassic leucite bearing rocks
are con

ned to the pre-caldera stage and display geochemical characteristics similar
to those of other volcanoes in the Roman Province. After the major sector collapse
of the volcano, occurred at ca. 400 ka, shoshonitic rocks erupted from cinder cones
and domes both within the caldera and on the external flanks of the pre-caldera

Roccamon na volcano. On the basis of new trace element and Sr - Nd - Pb isotope

data, they show that the Roccamon na shoshonitic rocks are distinct from shosh-

onites of the Northern Roman Province, but are very similar to those of the Nea-
politan volcanoes. The last phases of volcanic activity erupted sub-alkaline magmas
as enclaves in trachytic domes, and as lavas within the Monte Santa Croce dome.