Cretaceous 40 Ar/39 Ar detrital mica ages in tertiary sediments, solving the debate on the Eo-Alpine evolution?

Abstract

Clastic sediments deposited in the syn-orogenic Tertiary Piedmont Basin in northwest Italy represent the depositional counterpart of the cooling/exhumation and erosion of Western Alpine rocks over the last 30- 35 Myr. 40 Ar/ 39 Ar analyses of detrital white micas from Oligocene-Miocene sediments and present-day river sands show, in addition to younger Tertiary age groups, a wide range of Cretaceous ages. Pronounced well defined Late Cretaceous (~70-90 Ma) and Early Cretaceous (~105-120 Ma) age clusters are recorded in Lower to Upper Miocene sediments, forming discrete age groups with a contribution to the total detrital population of as high as 58%. This age pattern of discrete age peaks is remarkably constant and can be followed up-sequence through different formations spanning a time interval for sedimentation of >20 Myr. Our new detrital mica ages may be the result of either excess Ar in the source rock, as commonly assumed for HP mica Ar ages from the internal western Alps, or of inherited Ar, and consequently be representative of real geological events. The first scenario would imply that incorporation of excess Ar in minerals can lead to non-random detrital age populations which could then mistakenly be interpreted as representative of real geological events. The second scenario would imply that during the last Eocene thermal event, pre-existing micas were only partially overprinted and the presence of older ages are the result of real Cretaceous metamorphic events of the Western Alpine orogen. We argue that our new data derived from the sedimentary record, in particular from the time interval from Serravallian to Present, cannot easily be explained as simply being due to incorporation of excess argon. We therefore interpret these ages to be representative of cooling following major metamorphic events in the Alpine orogen. The new argon data from the sediments in combination with the data from the rocks exposed in the orogen today point to a complex Mesozoic history of the internal Alpine orogen.