Geology Reference
In-Depth Information
ago. This was preceded by another explosive event resulting in the outpouring of
phonolite magma on the southern part of the plateau. Culmination in the initial
period of caldera collapse began around 232 ka ago, when trachytic Plinian
(A-tuffs) pumice eruption and ignimbrite emplacement took place. This was fol-
lowed by eruption of thick phonotrachytic and phonolitic tuff sequence (ignimbrite-
X and B tuffs). Turbeville further described that after roughly 25,000 year, there
was emplacement of several coarse-grained phonolitic ignimbrite designated by the
Italian volcanologists as Y and C tuffs. This was followed by phreatomagmatic
eruptions that produced trachytic ignimbrites and quite a few ash
flow units, the age
of which is placed at 191 ka (D-tuffs). This was followed by eruption of wide
spread phonolitic tuffs, tephritic phonolitic spatter and scoraceous ignimbrites
(e-series ignimbrite and E-tuffs) from the northern caldera rim (Nappi et al. 1991).
This event was followed by 40,000 years of ma
c surge (all in vol%). Their study
was also related to the development of scoria cone around the caldera rim (Poggio
Pinzo Tuffs). There was also replacement of zoned tuffs around a vent northwest of
the Latera Caldera. A major explosive eruption from the Latera volcano took place
around 156 ka and this may be considered as the last volcanic activity. After drilling
more than 350 m, a zoned syenitic pluton of 5 km width, was discovered. The
seismic re
ection pro
les across Latera Caldera suggest that the pluton may contain
melt fractions.
Turbeville (1993) described more than six different types of pumice character-
ized by different colours: white, pink, light gray, dark gray, green, and brown to
black. According to him, the more felsic fragments contain phenocrysts of sani-
dine > leucite > plagioclase > clinopyroxene > biotite > magnetite with or without
apatite and sphene. In some clasts he found that leucite is more abundant than
sanidine. Although mineralogically different pumice types are similar, they vary in
vesicularity, crystallinity and composition. Glommero-phenocrysts of plagioclase,
clinopyroxene and biotite are present almost always in the trachytic and phonolitic
pumice fragments. Clinopyroxenes are of two varieties: colourless diopside and
dark green salite. The trachytic A and D tuffs are moderately vesicular, pink to light
gray in colour and they are mainly pumice clasts. They are usually poorly por-
phyritic and lack leucite. Plagiocalse and sanidines crystals are unzoned containing
<0.13 wt% BaO. Some diopside crystals have brown glass inclusions. The X and B
tuffs are phonolitic and display large difference in colour and texture. The pheno-
cryst content of the pumice clast is <5 to >50 vol%, but on an average it is 15
20 %.
-
Leucite is usually absent from pumice clast in ignimbrite X, but in a few
ows
leucite-bearing clasts are very common at the top of a
flow unit. Apatite intergrown
with plagioclase and diopside is also common. Ignimbrite Y and the C-tuff contain
black phonolitic pumice with
fibrous translucent brown matrix. The clasts are often
porphyritic and are very rich in leucite (leucite
≫
sanidine in most clasts).
Aggregates of plagioclase, diopside and magnesite are embayed and show reaction
relationship.
In the stratigraphically lower e-series
flow units, phonolitic pumice clast lack
leucite, whereas the gray green and brown clasts in the overlying deposit are rich in
leucite. The
flow deposits of E-Tuffs contain
five different types of vesicular clasts:
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