Dating of orogenic phases in the central Alps by K-Ar ages of hornblende

Abstract

The Gotthard massif in central Switzerland, part of the autochthonous basement of the Alps, has been metamorphosed during the Alpine orogeny. Structural studies of the metasedimentary hornblende rocks along the southern border reveal the existence of several mineral generations which were formed at different times and under different conditions. The sequence of formation can be deduced from their mutual intergrowth and orientation relative to the fabric of the rock. Steiger [1962] concluded that the Alpine orogeny in this region consisted of two distinct tectonic phases separated by a thermal phase. The main tectonic phase was the dragging along of the Gotthard massif by the overlying northward-moving nappes. Most of the pre-Alpine mineral assemblage was crushed and new minerals were formed which show preferential N-S orientation (direction of the nappe movements). The subsequent thermal metamorphism was linked with anatectic processes in the Lepontinic region south of the Gotthard massif. At this time porphyroblastic minerals of random orientation were formed along the southern margin of the massif. A weaker tectonic phase consisting of an E-W contraction caused N-S oriented small folds and wrinklings in mica-rich schists, and produced cross biotites. Hornblende was formed in pre-Alpine times as well as during both the main tectonic and the thermal phase of the Alpine orogeny. K-Ar ages were determined for 17 hornblendes. N-S oriented hornblendes show exclusively ages of 46 m.y.; random hornblendes yield ages of 23 to 30 m.y. Partially oriented hornblendes give apparent ages of 23 to 112 m.y. The hornblende ages are definitely higher than Rb-Sr ages of bioties (16 m.y.) from this region [Jager, 1962]. It is concluded that the N-S oriented hornblendes give a minimum age Rb-Sr ages of the biotites (16 m.y.) appear to indicate the time of the E-W contriction of proximately date the period (23 to 30 m.y.) of the anatexis in the Lepontinic region. The Rb-Sr ages of the biotites (16 m.y.) appear to indicate the time of the E-W constriction of the massif and the Lepontinic region. Partially oriented hornblendes cannot be related to any particular event. They may in part represent relics of pre-Alpine origin which suffered differential loss of argon during the Alpine metamorphism.