Early Variscan I-type pluton in the pre-Alpine basement of the Western Alps: The ca. 360 Ma Cogne diorite (NW-Italy)


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Early Variscan I-type pluton in the pre-Alpine basement of the Western Alps: The ca. 360 Ma Cogne diorite (NW-Italy)
Guillot F., Bertrand J.M., Bussy F., Lanari P., Cosma L., Pin C.
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Located at the internal border of the Grand-Saint-Bernard Zone, the
diorite and its aureole lie on top of intensively studied Alpine
eclogitic units but this pluton, poorly studied yet, has kept locally
almost undeformed. The pluton intruded, at similar to 360 Ma,
country-rocks mostly composed of dark shales with Na2O > K2O and minor
mafic intercalations of tholeiitic basalt affinity. This association is
characteristic of the Vanoise (France) basement series, where available
age determinations suggest an Early Paleozoic age. Parts of the pluton,
and of its hornfels aureole that is evidenced here for the first time,
in the Punta Bioula section of Valsavaranche valley (NW-Italy), have
been well-preserved from the Alpine deformation. Syn-emplacement
hardening, dehydration-induced, probably prevented strain-enhanced
Alpine recrystallization. Magmatic rock-types range continuously from
subordinate mafic types at SiO2 similar to 48%, of hornblendite with
cumulative or appinite affinities, to the main body of quartz diorite to
quartz monzonite (SiO2 up to 62%). P-T estimates for the pluton
emplacement, based on the abundance of garnet in the hornfelses, using
also zircon and apatite saturation thermometry and Al-in-hornblende
barometry, suggest T similar to 800-950 degrees C and minimum P in the
0.2-0.5 GPa range, with records of higher pressure conditions (up to 1-2
GPa?) in hornblendite phlogopite-cored amphibole. The high-K, Na > K,
calcalkaline geochemistry is in line with a destructive plate-margin
setting. Based on major element data and radiogenic isotope signature
(epsilon Nd-360 Ma from -1.2 to + 0.9, Sr-87/Sr-86(360 MA) from 0.7054
to 0.7063), the parental magmas are interpreted in terms of deep-seated
metabasaltic partial melts with limited contamination from shallower
sources, the low radiogenic Nd-content excluding a major contribution
from Vanoise tholeiites. There is no other preserved evidence for
Variscan magmatism of similar age and composition in the Western Alps,
but probable analogs are known in the western and northern parts of
French Massif Central. Regarding the Alpine tectonics, not only the age
of the pluton and its host-rocks (instead of the Permo-Carboniferous age
previously believed), but also its upper mylonitic contact, suggest
revisions of the Alpine nappe model. The Cogne diorite allegedly
constituted the axial part of the E-verging ``pli en retour [backfold]
du Valsavaranche'', a cornerstone of popular Alpine structural models:
in fact, the alleged fold limbs, as attested here by field and
geochemical data, do not belong to the same unit, and the backfold
hypothesis is unfounded. (C) 2012 Elsevier B.V. All rights reserved.
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