Geology Reference
In-Depth Information
amph
þ
2NH
4
ð
Þ
Ca
2
Mg
5
AlSi
7
O
22
OH
ð
2
ð
Mg
2
SiO
4
Þ
Fo
þ
3
=
2O
2
ð
15
:
2
Þ
¼ð
MgAl
2
O
4
Þ
Sp
þ
ð
Þ
En
þ
4 CaMgSi
2
O
6
N
2
þ
7 MgSiO
3
6H
2
O
These ammonium-bearing minerals may be stable under low oxygen fugacity
and elevated water activity (Andersen et al. 1995).
Conticelli et al. (2007) investigated the cause of eruption of ultrapotassic mag-
mas during Neogene and Quaternary in the Mediterranean area. According to them
the Roman Magmatic Province is dominated by leucite-bearing rocks with variable
degrees of silica saturation, from undersaturated (leucitites and plagioleucitites) to
strongly undersaturated (kamafugites), but minor amounts of shoshonitic to high-
potassium calc-alkalic rocks are still present. The Lucanian Magmatic Province,
located at the southeasternmost edge of the volcanic belt, is dominated by foiditic
(hauynites and leucitites) and kamafugitic (melilitites) members, all strongly
undersaturated in silica. In spite of these petrologic differences, the Neogene Italian
potassic and ultrapotassic rocks display similar trace-element patterns. Depletion in
high
field strength elements with respect to large ion lithophile elements is a
common feature. Sr, Nd, and Pb isotopic compositions of ma
c high-MgO rocks
range widely, relating mainly to geographic location of eruption and to enrichment
in alkalies. The Os isotopic composition of these samples, however, does not
clearly correlate with eruption location, but is dependent on the amount of
“
con-
tinental crust component
added to the magmas. Some of the studied samples are
compatible with crustal contamination en route to the surface. In most cases,
however, there are several lines of evidence suggesting the possibility that crustal
components were added directly to the mantle source prior to partial melting. Large
amounts (many tens of weight percent) of
”
must be added to the
peridotitic mantle in order to obtain the
187
Os/
188
Os of the lamproites in Tuscany.
These large amounts of crustal components have been recycled into the mantle in
the form of either melts or
“
crustal component
”
fluids. The recycling can be reconciled with a veined
mantle in which the crustal component is concentrated. Partial melting of veins
would then produce the high-silica and high-magnesium lamproitic magmas from
Tuscany. Dilution of the crustal components by increasing partial melting of sur-
rounding mantle peridotite, or alternatively, a reduction of metasomatic veins, could
then produce shoshonitic and high-K calc-alkalic ma
c magmas.
Southeastward geochemical and isotopic variations are reconciled with
decreasing direct contributions from crustal components introduced into the mantle
by subduction, but an increasing role of subducted
fluids from dehydration of CO
2
rich sediments. The coupled isotopic and chemical characteristics of Italian magmas
cannot be reconciled with an ocean-island basalt (OIB)-like primary magma
composition due to the substantial overprinting by crustal- and or subduction-
related components.
While discussing potassic and ultrapotassic magmatism circum-tyrrhenian
region Avanzinelli et al. (2009) suggested pelitic sediment recycling at destructive
plate margin. They thought the central-western Mediterranean is one of the most
important areas on Earth for studying subduction-related potassic and ultrapotassic
Search WWH ::
Custom Search