US EPA

6861119 

Journal Article 

The ultramafic contact aureole about the Bregaglia (Bergell) tonalite: Isograds and a thermal model 

Trommsdorff, V; Connolly, K; , 

1996 

0 

Schweizerische Mineralogische und Petrographische Mitteilungen
ISSN: 0036-7699 

STAUBLI VERLAG AG 

ZURICH 

76 

3 

537-547 

English 

WOS:A1996VY50600011 

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Thermodynamic calculations for the Iherzolitic bulk composition of the Malenco serpentinite reproduce the observed mineral compositions and assemblages with great fidelity. The resulting petrogenetic grid predicts narrow temperature intervals for the serpentinite ''isograd'' reactions, in agreement with sharp isograds observed in the Bergell aureole. The thermodynamic calculations indicate that large enthalpic (> 400 MJ/m(3)) and volume (> 25%) effects are associated with the serpentinite metamorphism. The solid volume reduction associated with the terminal antigorite dehydration reaction alone is ca. 18%, and probably caused the olivine + talc-filled extensional veins observed on the low temperature side of the corresponding isograd. Incorporation of latent heat effects and variation in thermal conductivity due to metamorphism in a simple heat conduction model results in consistency between geothermometers and the computed thermal profile. It would not be possible to obtain the same degree of consistency if the slab-like Bergell tonalite was emplaced in an orientation more horizontal than indicated by its present-day exposure. The pre intrusive ambient temperature is constrained to be in the vicinity of 350 +/- 20 degrees C, requiring a geothermal gradient of 28 +/- 2 degrees C/km. As peak regional metamorphic temperatures (ca. 450 degrees C) occurred at 65 Ma prior to the Bergell emplacement at 32 Ma (JAEGER, 1973; VON BLANKENBURG, 1990) this implies an average cooling rate of 3 degrees C/Ma over this time interval. It is unnecessary to invoke free advection, i.e., buoyancy driven, of a fluid phase to explain the temperature distribution in the aureole, suggesting that fluid movement was probably forced during metamorphism. Thermal modeling does not predict temperatures adequate to drive talc and anthophyllite decomposition reactions in a pure H2O fluid. That these reactions occurred sporadically near the contact is most likely related to decarbonation reactions in the contact zone that lowered the thermodynamic activity of water in the metamorphic fluid phase. This implies that the fluid phase present in the aureole was not well-mixed. and therefore also supports the conclusion that fluid flow was forced. 

contact metamorphism; regional metamorphism; heal conduction; thermal model; antigorite; Malenco serpentinite; Bergell (Bregaglia) tonalite; Central Alps