The composition and evolution of an Oligocene regolith on top of the Sesia–Lanzo Zone (Western Alps) |
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| Authors: | Notburga Kapferer Ivan Mercolli Alfons Berger |
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| Institution: | 1.Institute of Geological Sciences,University of Bern,Bern,Switzerland;2.Institut for Geografi og Geologi,K?benhavns Universitet,K?benhavn K,Denmark |
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| Abstract: | An Oligocene paleosurface (regolith) lies on top of the high-pressure metamorphic rocks of the Sesia–Lanzo Zone near Biella,
NW Italy. Only the saprock, the lowermost part in a regolith profile, is preserved. No evidence for any paleosoil can be observed.
Field observations indicate that the regolith developed through in situ mechanical fracturing of the rocks of the Sesia–Lanzo
Zone in a continental environment. Density estimations of the regolith and the underlying rocks of the Sesia–Lanzo Zone confirm
the field observations and imply that a relatively small amount of alteration minerals was formed. The main detected alteration
phases are chlorite, various carbonates, quartz, clay minerals, Fe-oxides, and Fe-hydroxides. Chlorite differing in chemistry
and crystallographic ordering demonstrates different stages of alteration. Oxygen and carbon isotopic composition of carbonates
suggests temperatures higher than surface conditions. Illite and chlorite thermometry indicates temperatures related to the
anchizone (~250–300°C). These data are considered as a robust indication of the re-burial of the regolith together with its
substrate and its volcanic cover. The burial is closely related to the tilting of the preserved stratigraphic sequence formed
by the rocks of the Sesia–Lanzo Zone, the regolith, and the rocks of the Biella Volcanic Suite (Lanza, in Schweiz Miner Petrogr
MItt 57: 281–290, 1977; Lanza, in Geologishe Rundschau 68: 83–92, 1979). Furthermore, the burial is consistent with this sequence of subaerial rocks being very close to the intrusion depth of
the Valle del Cervo Pluton at the time of its emplacement (4–7 km; Zanoni et al., in Rend Online SGI Note Brevi 1: 199–202,
2008; Zanoni et al., in Int Geol Rev 52: 1244–1267, 2010 and references therein). |
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