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| Titel |
A mid-crustal metamorphic field gradient in Val Strona di Omegna, Ivrea Zone, Italy: constraints from metabasic rocks |
| VerfasserIn |
Barbara E. Kunz, 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 |
250045843
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| Zusammenfassung |
| The Ivrea Zone has been the subject of several studies in the last decades and is commonly
interpreted as a section through the lower continental crust. Most existing P–T estimates have
used conventional thermobarometry on metasedimentary rocks, in which peak metamorphic
conditions of 600–800-C and 4–8 kbar (e.g. Henk et al., 1997) show a discrepancy with
conditions expected in the lower crust. In particular, the pressure estimates of 4–8 kbar are
too low, and the Ivrea Zone is better interpreted as a section through the mid continental
crust.
This study focusses on metabasic rocks that are interlayered with the metapelitic rocks in the
classic section through the Kinzigite Formation in Val Strona di Omegna. Although the
metabasic rocks are volumetrically abundant, they have received relatively little
attention. Field and petrographic observations are combined with mineral chemical data
and thermodynamic phase equilibria modelling to provide independent constraints
on the metamorphic evolution of this part of the Ivrea Zone. Peak metamorphic
conditions are constrained via P–T pseudosections calculated in the NCFMASHTO
(Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O) system using the software
THERMOCALC (Powell & Holland, 1988) and the Holland & Powell (1998) data
set.
Mineral assemblages are dominated by hb+pl+q+bi+ilm at lower grades, through those
dominated by di+hb+pl+ilm±bi±g at intermediate grades, to assemblages containing
opx+di+hb+pl+ilm±bi±g±q at the highest grades, and record the transition from
amphibolite to granulite facies conditions. Close to the amphibolite–granulite facies
transition, the rocks contain small leucocratic segregations that are interpreted as evidence for
in situ partial melting of the metabasic rocks. The abundance of leucosome increases with
metamorphic grade. Euhedral clinopyroxene porphyroblasts within the leucosomes are
consistent with the fluid-absent breakdown of hornblende and plagioclase to produce
clinopyroxene and melt. Although partial melting of the metabasic rocks has previously been
proposed (Reinsch 1973a, 1973b), more recent studies specifically preclude the possibility
(Harlov & Wirth, 2000).
Systematic sampling along Val Strona di Omegna allows reconstruction of a near-linear
regional metamorphic field gradient. The modelled P–T estimates range from a
minimum of 500-C and 4 kbar at the lowest grades to in excess of 900-C and 9
kbar at the highest grades. Maximum temperatures are about 100-C higher than
most previous studies have reported and exceed the experimentally determined
fluid-absent solidus for amphibolite (e.g. Wyllie & Wolf, 1993), consistent with
the field evidence for partial melting. Field evidence combined with petrographic
identification of peak metamorphic assemblages provide the framework for a detailed
characterisation of a consistent metamorphic evolution via mineral equilibrium
modelling.
References
Harlov, D., & Wirth, R., 2000, CMP, 140 (2), 148-162
Holland, T.J.B., & Powell, R., 1998, J. Met. Geol., 16 (3), 309-343
Henk, A., Franz, L., Teufel, S., & Oncken, O., 1997, J. Geol., 105 (3), 367-377
Powell, R., & Holland, T.J.B., 1988, J. Met. Geol., 6 (2), 173-204
Reinsch, D., 1973, N. Jb. Miner. Abh., 118 (2), 190-210
Reinsch, D., 1973, N. Jb. Miner. Abh., 119 (3), 266-284
Wyllie, P., & Wolf, M., 1993, Geol. Soc. Lond. Sp. Pub., 76 (1), 405-416 |
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