EXCESS RADIOGENIC ARGON IN METAMORPHIC MICAS FROM EASTERN ALPS, AUSTRIA

Potassium-argon determination on 23 muscovite-biotite pairs from the Upper East Alpine Altkristallin of the Eastern Alps yield apparent ages ranging between 79 and 107 m.y. for the muscovite and between 78 and 430 m.y. for the co-existing biotite. The micas appear to be of the same generation and neither differential leaching of potassium nor abnormal blocking temperature relationships are able to explain this discordance. It is concluded that excess radiogenic 40Ar entered the micas in a zone at least 1 1 2 km thick and 200 km2 in area. The amount of excess argon in biotite increases with depth up to six times the volume of in situ radiogenic 40Ar as estimated from the co-existing muscovite KAr ages and some RbSr determinations on mica pairs. The excess 40Ar was acquired by absorption under a high partial pressure of 40Ar in the host rock between 80 m.y. and 65 m.y. ago. Biotite was most strongly affected but the co-existing muscovite has also been affected at least to the extent that in situ radiogenic argon was prevented from outgassing under conditions in which this would normally have occurred. © 1969.