Geology Reference
In-Depth Information
30,000 years ago in the Phlegrean Fields. This
field lies about 50 km
south
na.
This is one of the Plio-Pleistocene volcanic centres situated to the west of the
Appenines. The rocks of this locality fall distinctly into two series (Appleton 1972):
(1) a highly potassic series, consisting of nepheline
southeast of Roccamon
-
and leucite
normative lavas. (2)
-
-
a potassium-poor series, consisting of nepheline or quartz
normative basalts, trachy-
basalts, and biotite augite latites. Series (1) is richer in K, P, Ba, Ce, Rb, Sr, Th, than
Series (2). According to Appleton (1972) the highly potassic lavas of Roccamon
-
na
can be further subdivided into the following groups:
(a) Clinopyroxene-microphyric lavas,
(b) Leucite-macrophyric lavas,
(c) Aphyric or microphyric lavas,
(d) Biotite-clinopyroxene-plagioclase(An
72
-
79
) magnetite-phyric lavas, and
(e) Lavas containing sanidine as the main phenocrystal phase.
Group (c) lavas contain leucite, clinopyroxene, and plagioclase (An
66
-
75
)as
phenocrysts. Group (b) lavas contain phenocrysts of clinopyroxene, plagioclase and
magnetite with leucite ranging up to 40 vol%. Sanidine forms only in those lavas,
which contain more than 55 % SiO
2
. Some of the leucite-macrophyric lavas contain
glommero-aggregates of clinopyroxene, plagioclase and magnetite with or without
apatite, sometimes surrounded by leucite phenocrysts indicating an early period of
crystallization of the glommero-aggregates prior to that of leucite. Appleton con-
cluded that an intermediate or high pressure process produced the parent magma
with speci
c levels of enrichments in potassium and associated elements; and
fractionation of biotite gabbro (which is found as nodules) from such a liquid at low
pressure would produce a chemical variation towards salic derivatives.
Appleton suggested that the evolutionary history of Roccamon
na rocks has two
parts. In the
first phase there was a wide variation in the level of incompatible
element enrichment on a series of rather similar primitive magmas (Cox et al.
1976). Later, due to crystal fractionation at low pressure a large variety of more
salic derivatives were formed. In the later stage highly potassic magmas were
evolved due to the fractionation of biotite pyroxenites followed by biotite gabbro. It
was argued that K-de
cient magmas were formed by the precipitation of olivine
gabbro. Gianneti found that apatite and opaques are usually present along with the
major phases (quoted in Cox et al. 1976). They argued that if isotopic variation in
the erupted magmas is related to the early event associated with fractionation of
incompatible elements in the primitive magmas, then correlation between isotopic
composition and absolute concentration of incompatible elements should take into
account variation related to low pressure fractionation at a later stage. If K, Rb, Sr,
Ba and Zr content of the samples are correlated with
87
Sr/
86
Sr ratios, then good
positive correlation is obtained from most primitive volcanic series. They however,
noted that Ti showed no correlation with respect to Sr isotopic ratio, suggesting that
the most primitive lavas might have fractionated suf
cient amount of titaniferous
phases to eliminate any relationship that might have existed. They emphasized that
although there is a good correlation between Sr
isotopic ratio with other
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