GEOLOGIC UNITS OF THE AAR MASSIF AND THEIR PRE-ALPINE ROCK ASSOCIATIONS - A CRITICAL-REVIEW

While the evolution of the Mesozoic rocks belonging to the Helvetic re alm in the Central Alps has been investigated in detail with respect t o deposition, metamorphism, and deformation, much less is known about the pre-Mesozoic history of the basement rocks in the Helvetic domain. The present study is an attempt to summarize the petrography and geol ogy of the pre-Variscan portions of the Aar massif. Often used subdivi sions are critically discussed. The importance of considering the Vari scan and Alpine retrograde alteration for any petro-tectonic subdivisi on of the Aar massif is demonstrated. The following main geologic unit s with similar lithologies and metamorphic evolution are distinguished : (1) The migmatitic Innertkirchen-Lauterbrunnen Crystalline Zone (ILC ). (2) a zone comprising the Erstfeld Gneiss Zone (EGZ) with high-grad e to anatectic metasedimentary rocks and minor orthogneisses, the Gutt annen Unit (GU) consisting of a layered series of metasediments with s trong retrogression, and the Ofenhorn-Stampfhorn Unit (OSU) with amphi bolite-facies to anatectic metasediments and major mafic and ultramafi c inclusions. (3) The Southern Gneiss Zone (SGZ) with high-grade to an atectic quartzofeldspatic and mafic rocks of igneous as well as of sed imentary origin. This subdivision is primarily based on the tectonic a nd metamorphic situation in the Western and Central Aar massif. Howeve r, lithologic units of the Eastern Aar massif can be attributed to the main units as outlined before. Recent studies indicate a Caledonian a ge for the amphibolite-facies metamorphism and subsequent anatectic ev ent in the Innertkirchen-Lauterbrunnen Crystalline Zone and the more i nternal units. No age data are available for the Southern Gneiss Zone but a similar metamorphic evolution is likely despite a probable diffe rent paleotectonic origin.