Open Session - SWISS GEOSCIENCE MEETINGs

100
Symposium 2: Mineralogy-Petrology-Geochemistry
Geodynamic interpretation: The ages around 210 Ma coincide with the beginning of crustal thinning and rifting of the
Europe-Adria continent at the locus of the future Piemont ocean. The extensional processes led to low degrees of decompressional
melting of the subcontinental mantle, which produced the alkali-rich pegmatoids. The advection of heat caused
overprinting of isotopic systems in many different mineral phases in the lower crust (granulite-facies rocks of the Ivrea zone),
such as zircon and monazite. Published biotite K-Ar ages range down to 180 Ma (e.g. Hunziker 1974), indicating cooling to
≈300°C during extension-related exhumation of the lower crust. The Ivrea Zone may therefeore not be the ideal place to
study the behaviour of isotopic systems during granulite facies, since many of those are overprinted and disturbed by late
Triassic heat advection during the first stages of rifting following the continental breakup of the European and Adriatic
plates.
REFERENCES
Girlanda F., Antognini M., Weiss S. & Praeger M. (2007) Zirkon aus Nephelin-pegmatiten im Peridotit Finero-Centovalli
(Schweiz). Lapis 32/6, 13-23
Grieco G., Ferrario A., von Quadt A., Koeppel V. & Mathez E.A. (2001) The zircon-bearing chromitites of the phlogopite
peridotite of Finero (Ivrea Zone, Southern Alps): Evidence and geochronology of a metasomatized mantle slab. J. Petrol.
42, 89-101
Lu M., Hofmann A.W., Mazzucchelli M. & Rivalenti G. (1997) The mafic-ultramafic complex near Finero (Ivrea-Verbano Zone),
II. Geochronology and isotope geochemistry. Chem. Geol. 140, 223-235
Oppizzi, P. and Schaltegger, U. (1999) Zircon-bearing plagioclasites from the Finero complex (Ivrea zone): dating a Late
Triassic mantle hic-up Schweiz. Mineral. Petrogr. Mitt. 79, 330-331
Stähle, V., Frenzel, G., Kober, B., Michard, A., Puchelt, H. & Schneider, W. (1990) Zircon syenite pegmatites in the Finero
peridotite (Ivrea zone): evidence for a syenite from a mantle source. Earth Planet. Sci. Lett. 101, 196-205
Vavra G., Gebauer D., Schmid R. & Compston W. (1996) Multiple zircon growth and recrystallization during polyphase Late
Carboniferous to Triassic metamorphism in granulites of the Ivrea zone (Southern Alps): an ion microprobe (SHRIMP)
study. Contrib. Mineral. Petrol. 122, 337-358
Vavra G., Schmid R. & Gebauer D. (1999) Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite
facies zircons: geochronology of the Ivrea zone (Southern Alps). Contrib. Mineral. Petrol. 134, 380-404
Vavra, G. & Schaltegger U. (1999) Post-granulite facies monazite growth and rejuvenation during Permian to Lower Jurassic
thermal and fluid events in the Ivrea Zone. Contrib. Mineral. Petrol. 134, 405-414
Weiss S., Fehr T., Ansermet S. & Meisser N. (2007) Zirkonführende Nephelin-pegmatite im Centovalli, Südschweiz: Struktur,
Mineralogie und Kristallisationsabfolge. Lapis 32/6, 24-30
2.28
Shocked quartz in Ticino, and beyond
Franz Schenker
SCHENKER KORNER + PARTNER GmbH Geologische Beratung Büttenenhalde 42 6006 Luzern (franz.schenker@fsgeolog.ch)
The Breccia of Arzo (Wiedenmayer 1963) consists mainly of brecciated clasts. Such “breccia within breccia” may be formed
by “normal” geological mechanisms such as neptunic dykes, dilatation tectonics or volcanic activities. Today, polymict lithic
breccia additionally are considered as one of many clues to recognize terrestrial impact structures (e.g. Masaitis 2005).
Definite evidence for shock deformation is, among other, the incidence of planar deformation features (PDFs) in quartz
(French 1998). Because our samples from the Breccia of Arzo did not contain any quartz, some quartz bearing formations
from Ticino were sampled with regard to find shock-produced features in minerals and rocks.
The Bernardo Gneiss (Reinhard 1953) shows intense fracturing and PDFs in quartz (Figure 1). Further indication of shock
metamorphism are coarse flakes of mica (biotite and muscovite) with kink-bands, and glassy spots in feldspars (?maskelynite)
and within or at the border of quartz (?diaplectic quartz glass).
PDFs in quartz occur in the Upper Carboniferous Manno Conglomerate, and in the porphyries and granophyres of the
Luganese, too (Figure 2). Petrography, geochemistry and spatial occurrence of the “Porfido Luganese” (Buletti 1985) show
exiting similarities with impact melt sheets from Sudbury and other impact structures (Therriault et. al. 2002). Years ago,
Ernst Niggli pointed to the chemical and mineralogical disaccord between common volcanic rocks and the Porfido Luganese
(Niggli 1953).