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| Titel |
Phase equilibria constraints on the pressure and temperature evolution of metapelites in the Ivrea Zone |
| VerfasserIn |
Charlotte Redler, Tim E. Johnson, Richard W. White |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250045844
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| Zusammenfassung |
| Over the last few decades the Ivrea Zone in northern Italy has been the focus of numerous
petrological, geochemical and structural studies principally because it is inferred to represent
an almost complete section through the mid to lower continental crust. This study
re-examines the field, petrological and mineral chemical characteristics of metapelites
exposed within Val Strona di Omegna, Val Sesia and Val Strona di Postua with an
aim to better constraining their metamorphic evolution. Quantitative constraints on
metamorphic conditions are based on phase equilibrium modelling calculations
using THERMOCALC and the internally consistent datatset of Holland & Powell
(1998).
In Val Strona di Omegna, the metapelites show a structural and mineralogical change from
mica-schists with the common assemblage bi-mu-sill-pl-q-ilm ± liq at the lowest grades,
through metatexitic migmatites (g-sill-bi-ksp-pl-q-ilm-liq) at intermediate grades, to complex
diatexitic migmatites (g-sill-ru-bi-ksp-pl-q-ilm-liq) at the highest grades. The section records
a transition from, amphibolite to granulite facies conditions, within which several mappable
isograds occur, including the first appearance of K-feldspar and orthopyroxene. The inferred
onset of melting in the metapelites occurs around Massiola and is consistent with
melting via the breakdown of first muscovite then biotite. Modelled melt fractions of
30–40% are predicted at the highest-grade. The metamorphic field gradient in Val
Strona di Omegna is constrained to range from conditions of 3.5–6 kbar at T -
650-C to around 10–12 kbar at T > 900-C. The peak P–T estimates, particularly
for granulite facies conditions are significantly higher than those of most earlier
studies.
In Val Sesia and Val Strona di Postua only amphibolite facies mineral assemblages are
observed. The metapelites, especially those located next to the contact with the Mafic
Complex, contain cordierite (± spinel) as a stable phase, implying lower pressures than those
recorded in Val Strona di Omegna, and are interpreted as a contact metamorphic effect
related to intrusion of the Mafic Complex that overprints regional metamorphism.
P–T estimates for the cordierite-bearing rocks are between 750–850-C and 5–6.5
kbar in Val Sesia and approximately 800–900-C and 5.5–7 kbar for Val Strona
di Postua. The lower pressures are consistent with some decompression from the
regional metamorphic peak prior to the intrusion of the Mafic Complex. In contrast,
metapelites in Val Strona di Omegna show no clear mineralogical evidence for any such
overprinting or decompression reaction textures caused by the Mafic Complex. The
extrapolated mineral isograds are cross-cut by the Mafic Complex to the south of
Val Strona di Omegna, consistent with the intrusion of the Mafic Complex having
postdated the high-grade regional metamorphism and anatexis in Val Strona di
Omegna.
Reference
Holland, T. J. B. & Powell, R., 1998. J. Met. Geol., 16 (3), 309–343. |
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