Géologie de l'Antépermien de Vanoise septentrionale (zone ...

tel-00011367, version 2 - 27 Dec 2010
Pm-Pemiun termnes in northern Vanoise massif, internat Brian-
çonnais zone, French Atps, by François GUILLOT.
ABSTRACT.- The BellecGte massif ("département de SavoieM) has a Pennlan and Trlasslc
cover. Locally allochthonous because of alpine age decollenients, thls Briançonnais - type
cover may also have reinalned stratigraphically tled to lts basement, on whlch lt rests
unconfomabl y wi th frequentl y reduced thicknesses : *krfënienm (Lower Trlasslc) and
Pemlan sandstones are often mlsslng between the pre-Pennian basement and the carbonate
Trlassic cover.
The pre-Pennlan terranes remai n undated , but might be of pre-Upper Carboniferous
age. 1/10 000 - scale mapplngs have allowed to recognlze a 2 km - thlck serles. Chernical
data, petrological ly control led, have revealed a chimlostratigraphic zonation, whlch
concerns mainly the abundant metainorphosed greenstones, of magmatlc orlgln, and magmatic-
related, hydrothermally al tered levels.
Pre-Pennlan series (overlaying an hypothetical, polymetamorphic basement, made
of gamet micaschists) includes :
- 1 km of banded greenstones, prevlously spi1 ltic basal ts, wi th thinner beds of sodic
quartz-keratophyrs. Their spi1 1 tizatlon took place earl y, but later than thelr mgmatic
cool lng. The last acldic-basic alternances incl ude gabbro-diori tic laccol lths, and non-
sodic, minly siliceous levels (Si02 > 902 ; long-lasting hydrothermal slliclflcations ?).
Most greenstones have revealed a low - TlO2 (0.5% - 1.52). tholelitic composition. Taking
into account their probably subnarlne spilitlzation, those chemlcal features could
indlcate a marginal basin context.
- 1 km of black slates (thln-grained, organic matter and pyrite-rlch metasediments).
Alblte is abundant (Na20 > 4%. K20 < 1.52). and attests prevalllng volcanlc Influences,
beslde a low proportion of slallc detritals. The black slates contain many dark-green
sllls, having a TIOZ-rlch (2% - 2.5%). still tholelltlc, chemlcal composltlon. Those
last greenstones are therefore chemically simllar to mld-oceanic tholelites.
Slnce lts main characters suggest an oceanic si tuatlon, the Bel lec6te pre-Pennian
serles qulte dlffers from the neighbouring Upper Carbonlferous (aged Namurlan to
Stephanlan) and from the Stephano-Pennian formations, both typlcally continental.
Moreover, lt could have been folded before the Pennlan. Hence, the northern Vanoise
would dlsplay a pre-Namurian basement, without Upper Carbonlferous, locally unreached
by sedlmentatlon untll the Trias. Such pre-Namurian series do exlst ln the contlguous
massifs (Mont Pourri, Grand Bec), and in the External Crystalllne Massifs (located
50-100 km to the W). Al1 those similar series might be of an Ordovlcian to Silurian
age .
Fol lowing a presusned pre-Permian folding , three major alpine tectonlc events have
been separated. During the flrst event, a great, kilometrlc-scale, flat-lying penninic
fold have been generated dipping N to NE, the core of which lncludes metamylonltes.
The second event would have been the so-called backfoldlng, wlth E to SSE dipplngs.
The third event has been characterized by a nroderate lithostatlc overburden, and by
high fluid pressures along the thrusts and faults, creating tectonic, mlneralized breccias
(amng whlch the so-cal led cargneules, issued from dolml tic Trîassic formations). During
that third event, northern Vanoise has been thrust over the "zone houill&ren to the
North ; i ts displacement niay be related to a greater scale, senestral, N-S décrochement,
whlch runs along the frontier between interna1 Briançonnais zone (the Vanolse, to the
E) and external Briançonnais zone (the *zone hou1 1 lère", to the W) .