5th Swiss Geoscience Meeting, Geneva 2007
Folded isotherms at the northeastern rim of the
Lepontine thermal dome: Evidence from Raman
spectroscopy of carbonaceous material in metasedimentary units of the Valaisan and Ultra-Helvetic
domains
Michael Wiederkehr*, Romain Bousquet**, Martin A. Ziemann**, Stefan M.
Schmid*, Alfons Berger***
*Geosciences Institutes, University of Basel, Switzerland (m.wiederkehr@unibas.ch)
**Institut für Geowissenschaften, Universität Potsdam, Germany
***Institute of Geological Sciences, University of Bern, Switzerland, contact:
Carbonaceous material is a common constituent of meta-sedimentary rocks. It
derives form organic material originally present in the host rock. This
transformation during metamorphism is generally called “graphitization”, a
process whereby the amorphous organic matter is converted into ordered
anisotropic, crystalline structures, graphite being the end-member. The degree
of crystallization of organic material is mainly temperature dependent and can
therefore be related to the grade of metamorphism (Buseck & Bo-Jun, 1985).
Raman microspectroscopy is a powerful tool for determining the crystalline
structure of carbonaceous material. The Raman spectra of metamorphic
carbonaceous material exhibit a significant evolution with metamorphic grade
(Pasteris & Wopenka, 1991; Wopenka & Pasteris, 1993). The relationship
between grade of crystallisation and metamorphic conditions has been
calibrated and can be used as an empirical geothermometer (Beyssac et al.,
2002). In contrast to many mineral transformations, graphitization of organic
matter is strictly an irreversible process during metamorphism; hence this
geothermometer always records the peak temperature reached along the P-T
loop of a given metamorphic rock. For the first time this study investigates the
evolution of carbonaceous material in the metasedimentary units of the
Valaisan and Ultra-Helvetic domains at the northeastern rim of the Lepontine by
Raman microspectroscopy. The resulting temperature distribution map will be
discussed together with geologic structures in order to decipher the
metamorphic evolution.
The metasediments of the Ultrahelvetic and Valaisan sedimentary units around
Olivone/TI show a remarkable metamorphic gradient: Carpholite-bearing
assemblages indicate pressure dominated metamorphism under blueschist
facies conditions in the east, while the units further west are characterized by a
temperature dominated Barrow event that took place under amphibolite facies
conditions (Lepontine metamorphism). Three deformation phases have been
distinguished: D1, D2 and D3, corresponding to three regional deformation
events, also referred to as the Ferrera, Domleschg and Chièra phases,
respectively (Schmid et al., 1996). Over 140 samples have been collected
between the Lucomagno/Pizzo Molare area in the West and the Safien valley in
the East for investigating the transition from high-pressure to amphibolite facies
conditions. From each sample between 15 and 25 Raman spectra have been
recorded in order to get a statistically well-defined temperature.
5th Swiss Geoscience Meeting, Geneva 2007
The peak temperatures derived from the Raman spectra show a continuous
increase from 350 °C in the Safien valley in the east up to 570 °C at the Pizzo
Molare in the west. Most of this increase in temperature occurs in the Luzzone
valley, i.e. between Piz Terri and Olivone. Further east a fairly homogeneous
temperature between 350 and 400 °C, with only a moderate increase, has been
observed. The comparison of the resulting temperature distribution pattern with
the geologic structures yields the following very interesting observation: In the
west the “isothermes” clearly cut all D1 nappe contacts und D2 nappe refolding
structures, while in the east the “400 °C isotherm” appears to be folded around
a large scale D2 structure (Lunschania antiform). This shows that the peak
temperatures were achieved at different times and under completely different
metamorphic conditions, in turn related to different geodynamic stages during
the evolution of the Central Alps.
The resulting temperature distribution pattern can be interpreted as resulting
from a superposition of the following two events: (1) Subduction of the Valaisan
and adjacent Ultrahelvetic units under blueschist facies conditions during
Tertiary Alpine collision led to the pressure dominated metamorphism observed
in the eastern part of the working area, where the temperatures range between
350 and 425 °C. (2) A late thermal event, probably due to tectonically accreted
or thickened heat-producing crustal material, was active under more or less
static conditions. It produced the temperature distrbution observed in the west
characterized by a strong field thermal gradient starting around 425 and
increasing up to 570 °C. This onion shaped pattern is also known as the
Lepontine thermal dome and it represents a classical Barrow-type amphibolite
facies overprint, clearly postdating HP metamorphism and subsequent
decompression. Combined with structural data and the analysis of
microstructures in thin sections we interpret this thermal event as a separate
heating pulse that occurred after nappe stacking (Ferrera phase) as well as
after a first nappe refolding stage (Domleschg phase), but before and or during
the initial stages of a second nappe-refolding event that produced the Northern
Steep Belt of the Pennine nappes (Chièra phase).
REFERENCES
Beyssac, O., Goffé, B., Chopin, C. & Rouzaud, J. N. (2002): Raman spectra of
carbonaceous material in metasediments: a new geothermometer., J.
Metamorphic Geol., 20, 859-871.
Buseck, P. R. & Bo-Jun, H. (1985): Conversion of carbonaceous material to
graphite during metamorphism. Geochim. Cosmochim. Acta., 49, 2003-2016.
Pasteris, J. D. & Wopenka, B. (1991): Raman spectra of graphite as indicators
of degree of metamorphism. Can. Mineral., 29, 1-9.
Schmid, S.M., Pfiffner, O.A., Froitzheim, N., Schönborn, G., and Kissling, E.,
1996: Geophysical-geological transect and tectonic evolution of the SwissItalian Alps. Tectonics, 15, 1036-1064.
Wopenka, B. & Pasteris, J. D. (1993): Structural characterization of kerogens to
granulite-facies graphite: applicability of Raman microprobe spectroscopy. Am.
Mineral., 78, 533-557.