Comparative K–Ar and Rb–Sr age determinations of retrograde processes on rocks from the KTB deep drilling project

 Conventional K–Ar and Rb–Sr age determinations were carried out on fine mineral fractions of the KTB rocks and of rocks from the surrounding area in order to get an idea of the timing of retrograde processes which the rocks had suffered in the Late Variscan and afterwards. Coarse-grained mica minerals were used as reference minerals. It can be demonstrated that conventional K–Ar dating on fine mineral fractions is a useful tool to date retrograde processes in crystalline rocks. For methodical reasons the Rb–Sr method is less suitable. A complex age pattern was found which could be divided into different age groups representing different periods of either penetrative retrograde overprint or formation and reactivation of distinct cataclastic fault zones. Whereas the rocks of the KTB have undergone a penetrative retrograde overprint during the Permo-Triassic without any visible deformation, the Jurassic brought a first reactivation of cataclastic fault zones. In Cretaceous time an overprint led to a pronounced second reactivation of cataclastic fault zones. This latter process turned out to be the important one for the stacking and a repetition of the drilled profile in the KTB. The data and interpretations obtained on the fine mineral fractions coincide excellently with fission track data on sphene from the drill site, with sedimentological records concerning a larger regional scale and with hydrothermal alterations and ore-forming events in mid-Europe.