GARNET TEXTURES AND DIFFUSION OF BIVALENT CATIONS IN GARNETS IN POLYMETAMORPHIC BASEMENT ROCKS OF THE ADAMELLO CONTACT AUREOLE

The Adamello batholith (42-29 Ma) is located in the Northern Italian Alps. It intruded into the South Alpine basement and its cover series. Garnet bearing assemblages were found in the polymetamorphic basement, which is composed of pelitic schists and minor acidic gneisses. Pre-Alpine garnet is partially retrogressed and subsequently recrystallized during contact metamorphism.

Approaching the pluton contact, the following textures were observed (distances from contact given). A) >3 km: pre-Alpine garnets were broken into fragments and replaced by chlorite and/or clay minerals; B) 3 km: garnets are replaced partially by white mica and chlorite. The patches are in places rimmed by biotite; C) 1.5 km: crystallization of new, idiomorphic garnets and overgrowth on pre-existing fragments; D) 0.75 km: a continuous rim surrounds accumulations of small roundish garnet crystals; E) 0.35 km: single garnet crystal, or clusters of a small number of individual crystals. These observations, together with spectacular chemical zoning maps, suggest that contact metamorphism led to multiple garnet nucleation and growth around fragments, which subsequently grow together to form individual garnet crystals.

Temperature estimates obtained from Grt/Bt thermometry (P=3 kbar) yield about 630 °C at 0.35 km distance of garnet (E). This estimate agrees with the observation of partial melting (at ca. 0.2 km from contact). TitaniQ temperatures of about 500 °C in the leucosomes (0.2 km distance) are likely too low, as are simple thermal model (2D) predictions (<600 °C at the contact).

Fe, Mg and Mn distributions in X-ray element maps document significant relaxation of the zoning patterns at 0.35 km from the contact, while Ca-zoning retains information of the above growth process. The required diffusion distances are approx. 150 microns. For Mn, using published diffusion data, this would require a diffusion time of >2 Ma at 630 °C. Alternatively, temperature would have to be around 700 °C to obtain the necessary diffusion distance in ca. 150 ka, which is reasonable for this aureole.

These observations suggest that garnet diffusion coefficients are probably underestimated by approx. 2 orders of magnitude. Improved thermometry estimates and thermal models (3D) will better constrain these values.