The Cenerigneiss, the main component of the Strona Ceneri Zone (Serie dei Laghi - Southern Alps), is a medium- to coarse-grained gneiss with Variscan metamorphism, composed by Qtz+Bt+Pl+Kfs+Ms+Ky+Grt. The Cenerigneiss is associated and marginally interfingered with fine-grained paragneisses (Gneiss Minuti), with similar mineral composition (except for Kfs) and the same Ca-silicate nodules, but showing relict sedimentary structures (graded bedding, cross lamination). Cenerigneiss also have similar major and trace element composition, but without magmatic differentiation trends. The geochemical characters of Cenerigneiss are typical of wackes (HERRON, 1988), and their distribution in various plots (TiO2 vs Ni, Th/Sc vs Zr/Sc, Th/U and La/Th) is compatible with a source dominated by intermediate to felsic magmatic rocks. This main source of the sediment is also supported by the dominance, in the detrital zircon populations of Cenerigneiss, of slightly abraded crystals typologically similar to those from diorites - granodiorites (Caironi, 1995). Cenerigneisses have PAAS-type chondrite-normalized REE patterns that resemble those of "Active Continental Margin" wackes (BHATIA, 1985). Also in the diagrams of BHATIA & CROOK (1986) Cenerigneiss samples plot in the field of "continental island arcs", or between this field and that of "active continental margins". Normalized to the upper continental crust (FLOYD et al., 1991), the Cenerigneiss shows negative Nb and positive V-Cr-Ni spikes, suggesting a source largely composed of subduction-generated rocks (continental arc-active margin). Their isotopic composition shows chemical affinity with modern turbidites from back-arc basins. SHRIMP ages for the detrital zircon populations (BERGOMI & CAIRONI, 2004; PINARELLI & BORIANI, 2007; PINARELLI et al., 2008) range from 520 to 1000 Ma, with a peak around 600-700 Ma. Ordovician ages (weighted mean at 465+14 Ma, PINARELLI et al., 2008) have been obtained on euhedral zircon crystals, which occur in the Cenerigneiss in increasing proportions with increasing amount of K-feldspar. For a better understanding of Cenerigneisses, they have been compared with the closely associated Gneiss Minuti, which still preserve graded bedding, show variable proportions of pelitic and psammitic fraction in the different layers. The chemistry of pelitic fraction indicate a certain contribution of intermediate to mafic igneous rocks, while the more Qtz-rich fraction, rich in Zr and Th and poor in Sc, suggest recycled sediments enriched in heavy minerals. The above mentioned characters may be well explained by interpreting Cenerigneisses as a mass-flow turbidite and Gneiss Minuti as well sorted deposits from turbidity currents (BORIANI et al., 1997).
The protolith of the Cenerigneiss (Serie dei Laghi - NW Italy) and its paleogeographic significance
Pinarelli L
2008
Abstract
The Cenerigneiss, the main component of the Strona Ceneri Zone (Serie dei Laghi - Southern Alps), is a medium- to coarse-grained gneiss with Variscan metamorphism, composed by Qtz+Bt+Pl+Kfs+Ms+Ky+Grt. The Cenerigneiss is associated and marginally interfingered with fine-grained paragneisses (Gneiss Minuti), with similar mineral composition (except for Kfs) and the same Ca-silicate nodules, but showing relict sedimentary structures (graded bedding, cross lamination). Cenerigneiss also have similar major and trace element composition, but without magmatic differentiation trends. The geochemical characters of Cenerigneiss are typical of wackes (HERRON, 1988), and their distribution in various plots (TiO2 vs Ni, Th/Sc vs Zr/Sc, Th/U and La/Th) is compatible with a source dominated by intermediate to felsic magmatic rocks. This main source of the sediment is also supported by the dominance, in the detrital zircon populations of Cenerigneiss, of slightly abraded crystals typologically similar to those from diorites - granodiorites (Caironi, 1995). Cenerigneisses have PAAS-type chondrite-normalized REE patterns that resemble those of "Active Continental Margin" wackes (BHATIA, 1985). Also in the diagrams of BHATIA & CROOK (1986) Cenerigneiss samples plot in the field of "continental island arcs", or between this field and that of "active continental margins". Normalized to the upper continental crust (FLOYD et al., 1991), the Cenerigneiss shows negative Nb and positive V-Cr-Ni spikes, suggesting a source largely composed of subduction-generated rocks (continental arc-active margin). Their isotopic composition shows chemical affinity with modern turbidites from back-arc basins. SHRIMP ages for the detrital zircon populations (BERGOMI & CAIRONI, 2004; PINARELLI & BORIANI, 2007; PINARELLI et al., 2008) range from 520 to 1000 Ma, with a peak around 600-700 Ma. Ordovician ages (weighted mean at 465+14 Ma, PINARELLI et al., 2008) have been obtained on euhedral zircon crystals, which occur in the Cenerigneiss in increasing proportions with increasing amount of K-feldspar. For a better understanding of Cenerigneisses, they have been compared with the closely associated Gneiss Minuti, which still preserve graded bedding, show variable proportions of pelitic and psammitic fraction in the different layers. The chemistry of pelitic fraction indicate a certain contribution of intermediate to mafic igneous rocks, while the more Qtz-rich fraction, rich in Zr and Th and poor in Sc, suggest recycled sediments enriched in heavy minerals. The above mentioned characters may be well explained by interpreting Cenerigneisses as a mass-flow turbidite and Gneiss Minuti as well sorted deposits from turbidity currents (BORIANI et al., 1997).I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
