Augen gneisses of debated origin and unknown protolith age form much of the Hercynian basement of the Peloritani Mountains of Sicily (Calabria-Peloritani Orogen). Petrogenetic studies based on field, petrographic and geochemical features have yielded inconclusive results. The augen gneiss protoliths have variously been interpreted as ignimbrites, Ordovician granitoids, or mixed felsic volcanoclastic-sands. Three augen gneisses from different sectors of the Peloritani Mountains have been studied by SHRIMP zircon U-Pb to assess their ages and geological significance. On the basis of zircon morphology, internal structure and composition, we now consider that the augen gneiss protoliths were granitoids. The zircon consists mostly of igneous crystals or overgrowths with oscillatory zoning, and some texturally distinct cores. Igneous zircon overgrowths from the three samples gave latest Proterozoic ages of ~565, ~545 and ~545 Ma respectively. These rocks are much older than expected. The augen gneiss protoliths were produced in the latest stages of the Cadomian orogeny leading to the final amalgamation of Gondwana. Late-Archean and Paleo- to Neoproterozoic zircon cores, mostly ~2.5, ~1.0 and <0.8 Ga, were present in the two younger samples, whereas only cores younger than 0.6 Ga occured in the older sample. The presence of large amounts of inherited zircon cores with a wide range of ages indicates a dominantly metasedimentary magma source for the two younger granitoids, and a different source for the older one. Most of the younger zircon cores (<0.6 Ga) in all samples cannot be interpreted as detrital inherited zircon given that their ages are similar to those of the melt-precipitated rims. They are better explained as zircon crystallized under anatectic conditions and/or during magma evolution. The patterns of inherited core ages, and especially the occurrence of Grenvillian ages, suggest that, unlike suggested for similar augen gneiss cropping out in Calabria, the sediments in the sources of the Peloritani augen gneisses did not have a West-African provenance.
Identifying Cadomian events through SHRIMP geochronology: Ediacaran magmatism in the Peloritani Mountains (S Italy).
FIANNACCA, PATRIZIA;CIRRINCIONE, ROSOLINO;LO GIUDICE, Antonino;PEZZINO, Antonino
2009-01-01
Abstract
Augen gneisses of debated origin and unknown protolith age form much of the Hercynian basement of the Peloritani Mountains of Sicily (Calabria-Peloritani Orogen). Petrogenetic studies based on field, petrographic and geochemical features have yielded inconclusive results. The augen gneiss protoliths have variously been interpreted as ignimbrites, Ordovician granitoids, or mixed felsic volcanoclastic-sands. Three augen gneisses from different sectors of the Peloritani Mountains have been studied by SHRIMP zircon U-Pb to assess their ages and geological significance. On the basis of zircon morphology, internal structure and composition, we now consider that the augen gneiss protoliths were granitoids. The zircon consists mostly of igneous crystals or overgrowths with oscillatory zoning, and some texturally distinct cores. Igneous zircon overgrowths from the three samples gave latest Proterozoic ages of ~565, ~545 and ~545 Ma respectively. These rocks are much older than expected. The augen gneiss protoliths were produced in the latest stages of the Cadomian orogeny leading to the final amalgamation of Gondwana. Late-Archean and Paleo- to Neoproterozoic zircon cores, mostly ~2.5, ~1.0 and <0.8 Ga, were present in the two younger samples, whereas only cores younger than 0.6 Ga occured in the older sample. The presence of large amounts of inherited zircon cores with a wide range of ages indicates a dominantly metasedimentary magma source for the two younger granitoids, and a different source for the older one. Most of the younger zircon cores (<0.6 Ga) in all samples cannot be interpreted as detrital inherited zircon given that their ages are similar to those of the melt-precipitated rims. They are better explained as zircon crystallized under anatectic conditions and/or during magma evolution. The patterns of inherited core ages, and especially the occurrence of Grenvillian ages, suggest that, unlike suggested for similar augen gneiss cropping out in Calabria, the sediments in the sources of the Peloritani augen gneisses did not have a West-African provenance.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
