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Features of the
formation of melilite-bearing rocks from Cupaello and Colle Fabbri,
Central Italy
A.T. Nikolaeva
Novosibirsk State
University, Novosibirsk, Russia
V.S. Sobolev Institute
of Geology and Mineralogy, SB RAS, Novosibirsk,
Russia
atnikolaeva@gmail.com
Intra-mountain
Ultra-alkaline Province (IUP) of Central Italy is
characterized by kamafugitic
and carbonatitic magmatism. This district consists of diatremes, maars
and tuff rings (Stoppa, Lavecchia, 1992). IUP magmatic centers, such as
the San-Venanzo volcano, the Colle Fabbri stock, and the Сupaello
volcano are found in the Pleistocene/Quaternary continental tectonic
depressions which cross cut the Pliocene Apennine thrust-fold system.
Genesis of Central Italy
kamafugites is still debatable. Some researchers (Peccerillo 2004;
Conticelli et al. 2002) relate the peculiar nature of Italian
kamafugites to the contribution of crustal rocks. A. Peccerillo thinks
that kamafugites and carbonatites of Italy resulted from the entrapment
of sedimentary carbonates by silicate melts during their ascent through
the thick layers of limestones and marls that occur along the peninsula
of Italy. Other authors refer the unique features of Italian rocks to a
magmatism related to subduction (Lustrino 2000; Serri 1997) or, vice
versa, consider it the result of intraplate magmatism and mantle
metasomatism (Cundari and Ferguson, 1991; Cundari 1994; Lavecchia and
Stoppa 1996; Bell et al. 2003). A plume model of formation of Italian
kamafugites and the presence of uncommon radiogenic mantle end-member
(ITEM) under peninsular Italy is popular nowadays (Bell et al. 2006;
Gasperini et al. 2002).
A detailed study of the
melilite-bearing rocks from Cupaello volcano and Colle Fabbri stock with
broad involvement of thermobarogeochemistry methods (melt inclusions
study in minerals) have provided direct evidence on the origin of some
IUP rocks, including information on the composition and evolution of the
parent melilitite magma, its temperature, physical and chemical
conditions of crystallization and differentiation, as well as on the
conditions of immiscibility of silicate and carbonatite melts.
The CUPAELLO volcano is mainly
represented by lava flow of the kalsilite-bearing meilitite about 700 m
long, 60-200 m wide, and up to 6 m thick (Gallo et al., 1984; Stoppa and
Cundari, 1995; Cundari and Ferguson, 1991).
Lenticular body of carbonatite tuff underlies the lava flow, its width
is up to 80 cm and a length is about several meters. According to
radiometric data, the age of the volcano is about 0.64 Ma (Stoppa and
Cundari, 1995). Kalsilite-bearing melilitite consists of major
phenocrysts of clinopyroxene, phlogopite and fine ones of melilite.
Groundmass is represented by clinopyroxene, melilite, kalsilite,
olivine, monticellite, perovskite, opaque minerals and glass. The
chemical composition of kalsilite-bearing melilitite is undersaturated
of SiO2 (~ 43,8 wt.%), characterized by low Al2O3
(~ 7,4 wt.%) and alkalis (4.4 wt.% K2O and 0.27 wt.% Na2O)
and high MgO (~ 11,3 wt.%), FeO (~ 6,7 wt.%), and CaO (~ 15.4 wt.%).
This composition is very close to that of olivine-bearing melilitites
from the San-Venanzo volcano (Cundari, Ferguson, 1991).
The COLLE FABBRI stock is
composed of leucite-, wollastonite-bearing melilitolite, containing
melilite, wollastonite, leucite, anorthite, Ti-garnet, apatite,
magnetite and Fe-Ni-sulfides. At the periphery of the melilitolite body
wollastonite - anorthite - pyroxene rocks exists in contact with the
pelite wall-rocks (Stoppa, Sharygin, 2009). The chemical composition of
leucite-, wollastonite-bearing melilitolite is strongly undersaturated
in SiO2 (~ 42 wt.%), characterized by relatively low Al2O3
(10,7-11,2 wt.%), and extremely high CaO (37.3-38.5 wt.%). In spite of
the presence of leucite in this rocks, the total amount of Na2O
+ K2O is low and is about 1.4-1.9 wt.%. The content of MgO
(1,6-2,4 wt.%) and FeO (3,3-3,7 wt.%) is low. Compared with
kalsilite-bearing melilitites Cupaello it has more Al2O3
and CaO.
In the leucite-,
wollastonite-bearing melilitolite of the COLLE FABBRI primary silicate
inclusions have been found in melilite and wollastonite. Inclusions are
single, partially crystallized, up to 50 μm in size. Their phase
composition is represented by glass, garnet, opaque phases and the gas
bubble. Based on the homogenization temperatures of inclusions, the
minimum crystallization temperature of melilite and wollastonite is at
least 1320 ± 15 °. The chemical composition of heated melt inclusions in
melilite corresponds to melilitite composition and contains (wt%) 35-39
SiO2, 13-25 Al2O3, 0.3-2.3 TiO2,
3-9 FeO, 1-2.8 MgO, 25-30 CaO, 0.4-1.1 Na2O, 0.7-7.4 K2O,
0.1-0.9 P2O5, up to 1.29 SO3.
In the kalsilite-bearing
melilitite of the CUPAELLO we have also found the primary
carbonate-silicate inclusions in clinopyroxene phenocrysts. They have a
rounded, irregular, or close to the prismatic form. Their sizes range
from 10-15 μm to 50 μm.
The content of inclusions is
represented by fine-grained aggregates of colorless, light green,
brownish phases and gas phase.
The scanning electron
microscope and Raman spectroscopy identified in inclusions:
biotite, kalsilit, pectolite - Ca2NaH [Si3O9],
combeite - Na2Ca2[Si3O9],
calcite, Ba-Sr carbonates, barite, alkali sulfates, and apatite. During
the heating and homogenization experiments in the inclusions it was
observed silicate-carbonate immiscibility in the 1025-1050 °C and was
set the minimum temperature of crystallization of clinopyroxene -
1150-1180 °C.
Homogenized inclusion
consist of a glassy silicate and fine-grained carbonate parts. The
chemical composition of the vitreous silicate part corresponds to
melilitite and characterized (wt%): 35.4-45.4
SiO2,
0.8-2 TiO2,
4.5-9.7 Al2O3,
4.3-6.9 FeO,
2.3-12.2 MgO,
7.6-16.8 CaO,
0.7-5.2 Na2O,
5.9-11.3 K2O,
0.2-1.7 BaO,
0.1-0.7 SrO,
0.6-1.9 P2O5,
0.05-0.3 Cl,
0.2-1.9 SO3.
Carbonate part of the inclusions contains (wt%): 1.7-4.81
SiO2,
0.1-0.2 TiO2,
0.7-1.1 FeO,
0.6-1.9 MgO,
24.9-40.9 CaO,
1.4-4.5 Na2O,
8.9-17 K2O,
3.8-5.1 BaO,
2.4-3.1 SrO,
0.3-0.5 P2O5,
0.06-0.24 Cl,
0.2-0.4 SO3.
Conclusion
The Colle Fabbri leucite-,
wollastonite-bearing melilitolite and the Cupaello kalsilite-bearing
melilitite are formed at the magmatic stage from the melilititic melts.
The crystallization of clinopyroxenes in the Cupaello kalsilite-bearing
melilitite occurred at 1150-1180 °C from melilititic melt enriched in
carbonates, sulfates, and alkalis. At 1025-1050 °C the
silicate-carbonate immiscibility was observed in the melt. The
crystallization of melilite and wollastonite in Colle Fabbri leucite-,
wollastonite-bearing melilitolite took place at 1320 ± 15 °C from the
melt drastically enriched in CaO. The high content of CaO in the melt
seems to be related with a particular mantle source which underwent
crust-like influence - the so-called abnormal mantle source ITEM.
Generation melilititic melts, in general, should be related with the
plume model (Panina and Nikolaeva, in press).
This work was supported
by a grant OPTEK.
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