Open Session - SWISS GEOSCIENCE MEETINGs

In the two regions investigated the shortening due to Alpine N-S compression was accommodated in two different ways: In
the Nufenen Pass area the shortening took place through a leading imbricate fan where the structures have been ramped up
in a sub-vertical position by the development of listric thrusts in the frontal part of the imbricate zone (Burg et al., 2002). In
the Lukmanier Pass area, however, the shortening was a consequence of a system of synchronous conjugate thrusts, in which
the vertical position of the units is due to the frontal thrust movement and the antithetic backthrusting.
REFERENCES
Burg, J.-P., Sokoutis, D., & Bonini, M. 2002: Model-inspired interpretation of seismic structures in the Central Alps: Crustal
wedging and buckling at mature stage of collision. Geological Society of America, 30 (7), 643 – 646.
Kamber, B. S., 1993: Regional metamorphism and uplift along the Southern margin of the Gotthard massif: results of the
Nufenenpass area. Schweiz. Mineral. Petrogr. Mitt., 73, 241 – 257.
Klaper, E. M., Bucher-Nurminen, K., 1987: Alpine metamorphism of pelitic schists in the Nufenen Pass area, Lepontine Alps.
J. Metamorphic Geol., 5, 175 – 194.
2.18
Cooling and crystallization of natrocarbonatitic lava flows: Reconciling
laboratory experiments with field observations
Mattsson Hannes B.*, Caricchi, L.*
*Institute for Mineralogy and Petrology, ETH Zurich (hannes.mattsson@erdw.ethz.ch)
Cooling and syn-emplacement crystallization of lava flows have a significant impact on the emplacement dynamics and also
on the resulting surface morphologies of the lava flows. In the summer of 2007 we sampled natrocarbonatitic lavas during
emplacement. Our samples comprise: (1) a lava lake feeding an aa-type lava flow, (2) the central part of the flow channel, and
(3) the distal flow front.
As experimental starting material we used rock-powders from the lava lake. The powders were placed in Ag 70 Pd 30 crucibles
and heated well-above the natrocarbonatite liquidus (750°C) for 30 minutes to homogenize the starting material. The samples
were subsequently cooled at different rates (ranging from instant quenching to slow cooling, e.g. 0.1°C/min). The textures
in the experimental samples were compared with natural lavas collected during field-campaigns in 2006 and 2007. Our experimental
data clearly show that the crystallization of natrocarbonatitic lavas is an extremely rapid process. Although
gregoryite is the first mineral to start crystallizing at 1 atm, the growth of nyerereite laths is much faster. Even relatively
rapidly cooled experimental samples (>3 °C/min) produces textures that overlap with natural samples from Oldoinyo Lengai
in both the types of crystals formed but also in their overall size. Field observations suggests that many natrocarbonatitic
lava flows remain very fluidal over steep slopes (even at high crystal contents >60 vol.%), but at lesser slopes (lower strainrates)
the crystals interact and as a result of this the viscosity increases dramatically. This may force the transition from
pahoehoe to aa-type flow morphologies in crystal-rich natrocarbonatitic lavas.
Symposium 2: Mineralogy-Petrology-Geochemistry
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