LIMITED FLUID FLOW IN THE TORRES DEL PAINE CONTACT AUREOLE (PATAGONIA, CHILE)

Fluid-rock interaction is a common process in contact aureoles. It plays an important role in mass and heat transport during metamorphism and partially controls the stability field of mineral assemblages. Fluid flow in contact aureoles has been described often in meta-carbonates and -marls but rarely in metapelites. Here we present hydrogen and oxygen isotope analyses and Cl content in biotite (bt) in the Torres del Paine contact aureole (Chile).

Oxygen isotope whole-rock values of metapelites scatter by several permil; but neither homogenization nor infiltration fronts are observed. In contrast, hydrogen isotopes delineate an infiltration and a dehydration front. Infiltration is limited to ca. 100m. Chlorine content of metamorphic biotite shows a similar evolution. A constant value (0.01 – 0.06 wt%) is observed in the whole aureole. But some samples reach values of up to 0.2wt% close to the intrusion. The fact that not all of the samples close to the intrusion show a high Cl content is interpreted as the evidence of limited, channelized fluid infiltration.

Hydrogen and chlorine exchange in biotite is facilitated by recrystallization. In addition, retrograde muscovite (after prograde K-feldspar) is present in all high chlorine content samples. This indicates infiltration of fluids just after the peak of metamorphism. A second fluid infiltration event is recorded in the Torres del Paine by muscovite (ms) +chlorite (chl) intergrowth replacing biotite at lower temperature, later in the cooling history. This second event is very localized, in some small (few to tens µm-sized) pockets.

The contact aureole experienced small amounts of fluid flow, as demonstrated by the absence of oxygen isotopic variations. Fluid flow was highly channelized, as shown by local variations in hydrogen isotopes and chlorine composition.

The first fluid flow event is related to the end of the crystallization of the oldest granite (Almirante granite) at the border of the pluton. This granite is rich in miarolitic cavities. Intrusion of subsequent granites within the next 100ky²resulted most likely in the second, faint alteration of the metapelites. Surprisingly, no large hydrothermal system was established in the case of the Torres del Paine complex.

¹Roselle et al. (1999) Amer. J. Sci, 299, 93–138.

²Leuthold et al. (2012) EPSL, 325-326, 85-92