MINERAL AND ROCK TEXTURES ACCOMPANYING THE ANTIGORITE BREAKDOWN TO FORSTERITE + TALC IN THE BERGELL CONTACT AUREOLE (ITALIAN ALPS)

The Malenco serpentinites (Central Alps, Italy) underwent regional Alpine greenschist facies metamorphism prior to contact metamorphism. The thermal aureole of the Bergell tonalite is characterized by successive metamorphic isograds in ultramafic rocks approaching the intrusion¹. The dominant dehydration reaction is represented by antigorite (atg) breakdown to forsterite (fo) and talc (tc)

A detailed field and petrologic study of the spatial distribution and textural characteristics of the fo+tc metamorphic assemblage reveals different textures for rocks with different modal abundance of tc produced. Protolith chemistry in turn is linked to the primary layering of the mantle rocks (dunite/lherzolite/clinopyroxenite). Jack-straw (spinifex-like) forsterite of up to 15cm size and aspect ratios of up to 20 developed at or near the isograde. Fo is homogeneous, but contains inclusions of atg, which is often extremely rich in aluminum. The external part of the fo contains tc. Fo crystal size decreases towards the intrusion. This is very similar to the carbonate-hosted olivines of the Ubehebe peak contact aureole (Death Valley, California²). The similarity between the contact aureoles indicates that this texture is due to limited nucleation at the isograd. The shape of the fo is an inherent growth form, rather than due to the specifics of the dehydration/decarbonation reaction. Overstepping of the reaction increases, as the intrusion is approached leading to abundant nucleation².

Initial atg breakdown can develop vein-like reaction zones composed of weakly foliated, fine-grained talc and equigranular olivine aggregates. Lithologies with abundant fo produced by serpentinite breakdown show often sub-parallel, wavy tc-veinlets of decimeter length and a fraction of a cm width. These tc veins likely acted as exfiltration channels for fluid produced by the atg breakdown.

 

¹Trommsdorff&Evans (1972), Amer. J. Sci 272, 423-437; ²Roselle et al., (1997), Geology 823-826