An extensive study of the central part of the Sesia Lanzo Zone has been
undertaken to identify pre-Alpine protoliths and to reconstruct the
lithologic and tectonic setting of this part of the Western Alps.
Three main complexes have been defined: 1) the Polymetamorphic Basement
Complex, corresponding to the lower unit of the Sesia Lanzo Zone after
COMPAGNONI et al. (1977), is further subdivided into the three following
units: a) an Internal Unit characterized by eo-Alpine high pressure (HP)
assemblages (DAL PIAZ et al., 1972) (Eclogitic Micaschists); b) an
Intermediate Unit where HP parageneses are partially re-equilibrated
under greenschist conditions and c) an External Unit where the main
foliation is defined by a greenschist paragenesis (Gneiss Minuti auct.).
2) the Monometamorphic Cover Complex, subdivided into the followings: a)
the Bonze Unit, composed of sheared metagabbros, eclogitized metabasalts
with MORB geochemical affinity and related metasediments (micaschists,
quartzites and Mn-cherts) and b) the Scalaro Unit, containing
predominantly metasediments of supposed Permo-Triassic age (yellow
dolomitic marbles, calcschists and conglomeratic limestones, micaschists
and quartzites with thin levels of basic rocks with within plate basalts
[WPB] geochimical affinity). Multiple lithostratigraphic sequences for
the Monometamorphic Cover Complex are proposed.
The contact between the Bonze and Scalaro Units is defined by
repetitions of dolomitic marbles and metabasalts; the ages of the
metasediments have been assigned solely by analogy with other sediments
of the Western Alps, due to the absence of fossils. The Monometamorphic
Cover Complex can be considered as the autochthonous cover of the Sesia
Lanzo Zone because of the primary contacts with the basement and because
of the presence of preAlpine HT basement blocks in the cover sequences.
3) The pre-Alpine high temperature (HT) Basement Complex (or `'Seconda
Zona Diorito-Kinzigitica''), comprises HT Hercynian rocks like
kinzigites, amphibolites, granulites and calcite marbles; this Complex
is always located between the Internal and the External Units and can be
followed continuously for several kilometers south of the Gressoney
Valley to the Orco Valley.
A schematic evolution for the Sesia Lanzo Zone is proposed; based on
available data together with new geochronological data, this study shows
that the internal and external parts of the polymetamorphic basement of
the Sesia Zone experienced different cooling histories .