Dating the longevity of ductile shear zones: insight from 40Ar/39Ar in situ analyses
Résumé
We attempt to improve temporal constraints on the longevity and the termination of ductile shear zones by performing texturally-controlled in situ 40Ar/39Ar analyses of pre-kinematic muscovite, biotite and K-feldspars, of syn-kinematic phengite and K-feldspar, and of post-kinematic phengite within the same samples of sinistral shear zones from the western Tauern Window (Eastern Alps). Additionally two samples were dated by the Rb/Sr method (microsampling). Relative sequences of mineral formation based on microstructural, cross-cutting relationships were confirmed by in situ 40Ar/39Ar analyses, showing that syn-kinematic minerals are, in general, younger than pre-kinematic minerals and older or of equal age than the post-kinematic minerals of the same sample.
From the rim to the core of the western Tauern Window syn-kinematic phengite and K-feldspar reveal a set of formation ages varying between 33 and 15 Ma for the northernmost and peripheral shear zone (Ahorn Shear Zone), between 24 and 12 Ma for the intermediate shear zone network (Tuxer Shear Zones), and between 20 and 7 Ma for the southernmost and central shear zone (Greiner Shear Zone). The age variation of syn-kinematic phengite and K-feldspar analyses is larger than the analytical error of each age obtained. In addition, isochron calculations of the syn-kinematic minerals reveal atmospheric-like 40Ar/36Ar intercepts. Therefore, the obtained age values of the syn-kinematic minerals are interpreted as formation ages which date increments of a long lasting deformation period. The time range of deformation of each shear zone system is bracketed by the oldest and youngest formation ages of syn-kinematic phengite and K-feldspar.
Post-kinematic phengite laths show the youngest formation ages and overlap with the youngest syn-kinematic formation ages. This relationship indicates that post-kinematic growth occurred immediately after syn-kinematic mineral formation at the end of ductile sinistral shear. Hence, the termination of deformation is dated by the ages of these post-kinematic phengite blasts.
Pre-kinematic minerals are characterized by break down and exsolution reactions and their age values are heterogeneous and often affected by the presence of extraneous Ar. These age values are usually older than, but sometimes overlapping with, ages of the syn-kinematic minerals.
Using the temporal constraints obtained by the ages of pre-, syn-, and post-kinematic minerals, we could assess partly overlapping time intervals of syn-kinematic mineral formation of 19 Myr (33–15 Ma) in the Ahorn Shear Zone, 13 Myr (24–12 Ma) in the Tuxer Shear Zones and 14 Myr (20–7 Ma) in the Greiner Shear Zone. This indicates successive localization and propagation of ductile shear zones in the western Tauern Window from lower metamorphic sites at the rim towards higher metamorphic sites in the center.