Diagenesis and Metamorphism of Clay Minerals in the Helvetic Alps of Eastern Switzerland

Hejing Wang1, 2, Martin Frey1 and Willem B. Stern1
1 Mineralogisch-Petrographisches Institut der Universität, Bernoullistrasse 30, CH-4056 Basel, Switzerland
2 Present address: Department of Geology, Peking University, Beijing 100871, P.R. China

Abstract: Helvetic sediments from the northern margin of the Alps in eastern Switzerland were studied by clay mineralogical methods. Based on illite “crystallinity” (Kübler index), the study area is divided into diagenetic zone, anchizone and epizone. Data on the regional distribution of the following index minerals are presented: smectite, kaolinite/smectite mixed-layer phase, kaolinite, pyrophyllite, paragonite, chloritoid, glauconite and stilpnomelane. Isograds for kaolinite/pyrophyllite and glauconite/stilpnomelane are consistent with illite “crystallinity” zones. Using the ordering of mixed-layer illite/smectite, the diagenetic zone is subdivided into three zones. The illite domain size distribution was analyzed using the Warren-Averbach technique. The average illite domain size does not change much within the diagenetic zone, but shows a large increase within the anchizone and epizone. The average illite b0 value indicates conditions of an intermediate-pressure facies series.

The Helvetic nappes show a general increase in diagenetic/metamorphic grade from north to south, and within the Helvetic nappe pile, grade increases from tectonically higher to lower units. However, a discontinuous inverse diagenetic/metamorphic zonation was observed along the Glarus thrust, indicating 5–10 km of offset after metamorphism. In the study area, incipient metamorphism was a late syn- to post-nappe-forming event.

Key Words: Clay mineralogy • Diagenesis • Helvetic Alps • Incipient metamorphism • Switzerland

Clays and Clay Minerals; February 1996 v. 44; no. 1; p. 96-112; DOI: 10.1346/CCMN.1996.0440109
© 1996, The Clay Minerals Society
Clay Minerals Society (www.clays.org)