Detrital zircon geochronology in blueschist‑facies meta‑conglomerates from the Western Alps: implications for the late Carboniferous to early Permian palaeogeography
Résumé
In the Western Alps, the Money Complex of
the Gran Paradiso Massif, metamorphosed under blueschist
facies during the Alpine cycle, is considered to be Permo-
Carboniferous in age, but no palaeontological or radiometric
data constrain this interpretation. A revision of the
lithostratigraphy of the Money Complex allows recognizing
a polygenic (graphite-rich) and a monogenic (graphitepoor)
meta-sedimentary formation. 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 Lower Devonian,
and (iii) Carboniferous zircon grains are lacking. A
careful 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, the
Austroalpine and the Southalpine basements. The lack of
Carboniferous zircons in the Money Complex suggests that
the detritus was not shed from the Helvetic zone, which
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 Upper Missisippian to Upper Pennsylvanian.
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.