Pre-metamorphic melt infiltration in metasediments : geochemical, isotopic (Sr, Nd, and Pb), and field evidence from Serie dei Laghi (Southern Alps, Italy)
Articolo
Data di Pubblicazione:
2008
Citazione:
Pre-metamorphic melt infiltration in metasediments : geochemical, isotopic (Sr, Nd, and Pb), and field evidence from Serie dei Laghi (Southern Alps, Italy) / L. Pinarelli, M.A. Bergomi, A. Boriani, E. Giobbi. - In: MINERALOGY AND PETROLOGY. - ISSN 0930-0708. - 2008:3(2008), pp. 213-242.
Abstract:
Gradual transitions from K-feldspar free gneisses to K-feldspar bearing augengneisses
are sometimes observed in metamorphic terranes. They have been explained with
metasomatic porphyroblastic growth connected with regional metamorphism, or with
pre-metamorphic presence of magmatic megacrysts. A transition of this kind can be
observed in the Serie dei Laghi (Southern Alps, Italy), where coarse-grained metaarenites
(Cenerigneiss) grade into Ceneri augengneisses with large K-feldspar porphyroclasts,
and banded amphibolites of the ‘‘Strona Ceneri Border Zone’’ grade into Hbl
augengneisses rich in K-feldspar.
The Ceneri augengneisses are chemically indistinguishable from the Cenerigneiss, but
have higher 87Sr=86Sr (0.7256–0.7258 vs. 0.7215–0.7233), similar to those of the Ordovician
granites that were intruded, before the regional metamorphism, into the protoliths
of both Cenerigneiss and amphibolites. The Cenerigneiss contains two types of zircons:
(1) highly luminescent, rounded grains or fragments, yielding U–Pb SHRIMP
ages from 0.43 to 1.0 Ga; (2) euhedral grains with oscillatory zoning (magmatic), with
U–Pb SHRIMP concordant ages of 466 13 Ma. This age coincides with the Rb–Sr
whole rock emplacement age of the Ordovician granitoids (466 5Ma).The Hbl augengneisses form three groups with distinct geochemical patterns,
whose distributions on inter-element diagrams trend towards the Ordovician metagranites
and meta-aplites. In addition, the Hbl augengneisses have higher 87Sr=86Sr
(0.7132–0.7147 vs. 0.7031–0.7046) and lower 143Nd=144Nd (0.51214–0.51219 vs.
0.51273–0.51297) than the amphibolites, suggesting the addition of an isotopically
evolved component.
The observed chemical and isotope patterns, as well as the vicinity of the augen
gneisses to the Ordovician intrusions, lead us to conclude that the Ceneri augengneisses
and Hbl augengneisses are the result of infiltration of residual hydrous
magmas into the protolith of both the Cenerigneiss and the amphibolites at the time
of Ordovician granite emplacement, long before the regional metamorphism in the Serie dei Laghi.
Tipologia IRIS:
01 - Articolo su periodico
Keywords:
melt infiltration ; augengneiss ; amphibolites ; Cenerigneiss ; Serie dei Laghi
Elenco autori:
L. Pinarelli, M.A. Bergomi, A. Boriani, E. Giobbi
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