The pioneer work of Bernard Kübler and Martin Frey in very low-grade metamorphic terranes: paleo-geothermal potential of variation in Kübler-Index/organic matter reflectance correlations. A review
Résumé
Low-temperature metamorphic petrology occupies
the P–T field between sedimentary and metamorphic
petrology. Two important pillars of low-temperature metamorphism
are coal petrology and claymineralogy. When low
temperature petrology was established bridging a hiatus
between the two classical geological disciplines of sedimentary
geology and metamorphic petrology, geologists faced a
need for the usage of different terminology tenets.Martin Frey
and Bernard Ku¨bler were two pioneers in low-grade metamorphic
petrology. They focused their research on clarifying
the relationships of clay mineralogy and organic petrology to
metamorphic pressure (P) and temperature (T) conditions.
The ultimate aim of M. Frey and B. Ku¨bler was to establish a
correlation between clay indices and organic parameters for
different geodynamic setting and therefore for various pressure–
temperature (P–T) conditions occurring in low grade
metamorphic terranes. For this purpose, a special attention
was addressed to the correlation between the Ku¨bler-Index
(KI) and vitrinite reflectance (VR). All these efforts are dedicated
to estimate the P–T conditions and thus to gain insight
into the geodynamic evolution of low-grade metamorphic
terranes. B. Ku¨bler and M. Frey honored here concentrated
their studies to the Helvetic Central Alps area. The very lowgrade
Helvetic domain is therefore of basic interest of this
paper. Ensuing the extensive compilation of data from the
Helvetic domain, a reinterpretation of Ku¨bler and Frey’s
research is presented in the light of last decade’s scientific
progress. A comprehensive dataset available enables to discriminate
many factors influencing the Ku¨bler-Index and
organic-matter reflectance alongside to time, temperature and
pressure. The correlation is restricted to the KI and organic
matter reflectance (mostly VR) because most of the studies
used both methods. Organic matter reflectance (OMR)
includes data from vitrinite reflectance and bituminite
reflectance measurements. Geodynamics has important control
on the KI/VR (OMR) correlation. Tectonic units having a
similar geodynamic evolution are featured by the comparable
KI/OMR trends, related to the particular paleo-geothermal
conditions. Obviously the KI/OMR correlations provide a
mean to characterise geothermal gradients and metamorphic
very-low-grade pressure–temperature conditions. In terranes
where high deformations rates are reported, exceeding the
high anchizone conditions, strain promotes the kinetic effects
of temperature and pressure on the KI versus OMR ratio.