5th Swiss Geoscience Meeting, Geneva 2007
Preliminary observations on the Fitz Roy Plutonic
Complex.
Ramírez de Arellano Cristóbal*, Putlitz Benita*, Kosmal Andrés** & Dessimoz
Mathias*
*Institut de Minéralogie et Géochimie, Université de Lausanne. Batiment L`Anthropole
CH-1015 Lausanne, Suisse. (cristobal.ramirez@unil.ch)
**Costanera Norte 46, El Chaltén, Provincia de Santa Cruz, Argentina.
(andreskosmal@gmail.com)
The Fitz Roy Plutonic Complex (FRPC; 18+/-3 MY, K-Ar whole rock, Nullo et
al., 1978) is one of the Miocene plutons located between the volcanic arc and
the Patagonian plateau basalts. These formed in the context of the subduction
of the Nazca and the Antartica plates beneath the South American plate (Cande
& Leslie, 1986). It is in a similar tectonic position as the Torres del Paine
Plutonic Complex (TPC).
The FRPC is formed by at least three rock types: a main central granodioritic to
granitic body (GD) surrounded by a Mafic Complex (MC) (Kosmal, 1997)
composed of a tonalite (TN) and a gabbro (GB). The similarity of this igneous
association to that of the TPC has led some researchers to speculate on a
common genesis of the FRPC and the TPC (Michael, 1991).
The central GD has abundant schlieren structures and aplitic dikes with
miarolitic cavities, some of which are filled with centimetric smoky quartz
crystals. Its contacts with the MC and the host rock are sharp and everywhere
steep.
The contact between the TN and the GB can be sharp, but undulating. In other
places a brecciated contact zone with meters sized angular gabbroic blocks in
the TN was found. Ductile and brittle deformation at macro and microscopic
scale are ubiquitous. The GB is very strongly deformed; in some areas, a
penetrative foliation with metamorphic mineralogy (amph, bt, fsp, qz) was found,
indicating that these rocks experienced a deformation under at least upper
greenschist facies conditions. No evident metamorphic mineralogy has been
observed so far in the TN, but abundant low temperature hydrothermal
alteration is present. We propose that the TN intruded into the GB in a ductile
environment. The observed metamorphism might be related to the Magallanes
Fold and Thrust Belt.
The absence of deformation in the GD and its sharp contacts with the MC
clearly demonstrate that it intruded after cooling and deformation of the MC.
There are some minor quantities of mafic rocks related to the GD intrusion.
There are a few decimeter wide vertical mafic dikes that cut the felsic rocks.
Our field results clearly show that the FRPC can not be considered a simple
bimodal intrusive complex, in contrast to the TPC.
5th Swiss Geoscience Meeting, Geneva 2007
REFERENCES:
Cande, S.C. & Leslie R.B. 1986 : Late Cenozoic tectonics of the southern Chile
Trench. Journal of Geophysical Research, 91, 471-496.
Kosmal, A. 1997: Nuevos aportes a la geología de la zona del Cerro Fitz Roy,
Departamento Lago Argentino, Provincia de Santa Cruz. Trabajo final de
licenciatura. Departamento de Geología de la Facultad de Ciencias Exactas y
Naturales de la Universidad de Buenos Aires.
Michael, P.J. 1991: Intrusion of basaltic magma into a crystallizing granitic
magma chamber: The Cordillera Paine pluton in southern Chile) by in situ
fractional crystallization. Contributions to Mineralogy and Petrology, 108, 396418.
Nullo, F., Proserpio, C., Ramos, V. A. & Rabassa, J. 1978: Estratigrafía y
Tectónica de la vertiente este del Hielo Continental Patagónico, ArgentinaChile. Actas del VII Congreso Geologico Argentino, Neuquén, Tomo1, 455-470.