Detrital zircons in blueschist-facies meta-conglomerates : Implications for the Early Permian palaeo-topography of the Western Alps
Abstract
In theWestern Alps, the Money Unit is a monocyclic unit, metamorphosed
at blueschist facies during the Alpine cycle. It crops out as a tectonic
window below the overthrusted polycyclic, eclogite-bearing unit of the
Gran Paradiso and its present position is the result of the Alpine tectonics.
The Money Unit is considered to be Permo-Carboniferous in age,
but no palaeontological or radiometric data constrain this interpretation.
The Money unit exposes a sequence of clastic sediments and volcanics
(Money Complex) that are intruded by a granitoid body (Erfaulet metagranite).
In the Money Complex a polygenic (graphite-rich) and a monogenic
(graphite-poor) meta-sedimentary formation have been recognized.
The two sequences are separated by fine-grained biotite-amphibole
gneisses, and by albite-bearing gneisses and amphibolites.
Detrital zircon U-Pb geochronology in both meta-sedimentary formations
shows that (i) the main population is Cambrian and Ordovician in
age, (ii) the youngest grains are Silurian and Early Devonian, and (iii)
Carboniferous zircons are lacking. A study of the age distributions in the
Alps suggests that potential source for the detrital material in the Money
Complex is the Briançonnais basement. Late Carboniferous magmatism
is widespread in the Helvetic Zone of the Alps. Permian magmatism is
dominant in the Briançonnais, in the Austroalpine, and in the Southern
Alps. The lack of Carboniferous zircons in the Money Complex suggests
that the detrital material did not derive from the erosion of the
Helvetic Zone. The Helvetic Zone was separated from the Money basin
by the Zone Houillère basin, where the main drainage pattern was developed
from south to north and where the depocenters migrated northwards
from the Namurian to the Stephanian.We suggest that the Money
Complex may had been located to the east of the main river drainage
inside the Zone Houillère basin or alternatively may represent a small
basin, located on the east of the Zone Houillère.