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 |
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Authors: | G Vavra Dieter Gebauer Rolf Schmid William Compston |
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Institution: | Departement Erdwissenschaften, Eidgen?ssische Technische Hochschule Zürich, Sonneggstrasse 5, CH-8092 Zürich, Switzerland, CH Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia, AU
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Abstract: | Zircons from a metasedimentary and a meta-igneous quartz-feldspar granulite from the Val Sesia and Val Mastallone area of
the Ivrea Zone (Southern Alps) differ in their response to granulite facies metamorphism with respect to crystal morphology
and U/Pb ages. Detrital zircons in the metasediment developed an isometric overgrowth by the addition of Zr derived mainly
from co-existing minerals, most probably biotite, decomposing during anatectic melting. The overgrowth started in the pelitic
layer of the metasediment in the Late Carboniferous at approximately 296 Ma, significantly earlier than in the adjacent psammitic
layer where it started only at 261 ± 4 Ma (95% confidence level). These ages are ascribed to the differential initiation of
anatexis in the two layers. The delay of melting in the psammitic layer was probably due to the different position and less
steep slope of its solidus in P-T-space, as compared to the solidus in the pelitic layer. Accordingly, the melting in the psammitic layer at 261 Ma was initiated
by a thermal pulse and/or by a decompression event. Decompression melting is supported by a characteristic shell morphology
of the zircon overgrowth in the psammitic layer, which might have grown under shear movements during high-temperature extensional
faulting. The typically magmatic zircon population of the meta-igneous granulite crystallized at 355 ± 6 Ma (95% confidence
level). The morphology of the zircons and the chemistry of the rock suggest that the magma was calcalkaline. A minor subpopulation
of crystals is morphologically similar to the zircons in the pelitic layer of the metasediment. This points to the admixture
of a minor sediment component and, thus, to a largely volcaniclastic origin of the protolith. In contrast to the detrital
zircons in the metasediment, the magmatic zircons show rare and little overgrowth and, instead, have been strongly resorbed
by anatectic melt. In addition, they became partially recrystallized and the rejuvenated ages from the most thoroughly recrystallized
domains indicate that the rock was subject to prograde metamorphism after 279 Ma. This may correspond to the regional temperature
increase prior to the climax of metamorphism or to a local thermal pulse due to nearby mafic intrusions. An Upper Triassic
event at 226 ± 5 Ma is reflected by distinct peripheral zones in the overgrowths of some zircons in the metapelite. These
are interpreted as a second metamorphic pulse, possibly induced by the infiltration of fluids.
Received: 2 June 1995 / Accepted: 15 September 1995 |
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