VITRINITE REFLECTANCE AND SHEAR-INDUCED GRAPHITIZATION IN OROGENIC BELTS - A CASE-STUDY FROM THE KANDERSTEG AREA, HELVETIC ALPS, SWITZERLAND

Vitrinite reflectance (VR) is an important indicator of incipient meta morphism and it is generally assumed that temperature and, to a lesser degree, time, are the two main variables. The role of stress in VR is controversial and will be addressed in this study. In the Kandersteg area, north of Lake Oeschinen, four different Helvetic tectonic units are present, comprising mainly limestones, marls and greywackes of Mes ozoic and Lower Tertiary age. VR (R-r, R-max, R-min) and illite crysta llinity (IC) were determined from a very steep section at elevations b etween 1700 and 2900 m a.s.l. In general, R-r, R-max and IC increase f rom tectonically higher to lower units, i.e. from upper diagenetic (2- 3 to 4-5% R-r) to upper anchizonal conditions (3-4 to 5-6% R-max). Sev eral samples contain, in addition to coaly fragments, abundant semigra phitized and graphitized particles that are believed to be of authigen ic origin. 'Transitional matter' shows R-max values ranging from 7 to about 11% and R-min of 1.3-3.4%; 'optical graphite' shows R-max of abo ut 15-16% and R-min of 1.6-1.8%. There is good textural evidence that tectonic deformation has played a major role in the development of the se strongly coalified particles. The first tiny needle-like crystals o f graphitic material occur within narrow shear zones transecting the r ock matrix. The beginning of graphite formation parallels the onset of plastic deformation and dynamic recrystallization of quartz. Elevated vitrinite reflectance (R-max) and bireflectance values are found near thrust planes, where simple optical observation on quartz grain morph ology indicates an increase in strain intensity. It appears that frict ional heating associated with thin shear zones may have been responsib le for the formation of authigenic graphite in the Kandersteg samples. (C) 1997 Elsevier Science B.V.