TSK 11 Göttingen 2006 Kühn & Austrheim
 Metastability and HP meta-
 morphism at fluid deficient
 conditions, an example from
 the Bergen Arcs (Western
 Norway) Poster
 Alexander Kühn1 Haakon Austrheim2
 In the Lindæs Nappe, Bergen Arcs
 Western Norway, Precambrian gran-
 ulites facies anorthosites (P<10 kbars,
 T=800°C) and their Caledonian eclogite
 (P<17 kbars, T=700°C) and amphibo-
 lite facies equivalents alternate on me-
 ter scale. It has recently been sug-
 gested by Camacho et al. (2005) that the
 granulite facies anorthosites, remained
 at low temperatures (350°C) and were
 only locally heated to 700°C and reacted
 to eclogites by spasmodic hot fluids.
 This is in contrast to previously pub-
 lished models (Austrheim 1987) where
 the fluid-triggered mineral reactions in
 a terrain that was at 700°C during
 the Caledonian Orogeny. In the lat-
 ter model the dry granulites metastably
 survive the Caledonian HP/HT meta-
 morphic event at 425Ma.
 In pristine granulites, occurring adja-
 cent (less than 1m distance) to the com-
 pletely eclogitised parts, visible Cale-
 donian metamorphism is very limited.
 Reactions are confined to local (15m
 wide kyanite, zoisite, garnet and K-
 feldspar aggregates and feathery inter-
 growth of K-feldspar and zoisite growing
 on plagioclase-plagioclase grain bound-
 aries. In addition 0.1 to 0.5mm thick
 coronas are observed around spinel and
 corundum that is embedded in pla-
 gioclase (Fig. 1). During progres-
 1 Institut für Geowissenschaften, Becher-
 weg 21, Universität Mainz, D-55099 Mainz,
 Germany 2 Physics of Geological Processes
 (PGP), University of Oslo, PO Box 1048 Blin-
 dern, 0136 Oslo, Norway
 Figure 1: Eclogite facies garnet-omphacite-
 kyanite corona around spinel/corundum
 embedded in a plagioclase groundmass
 within an otherwise unreacted granulite fa-
 cies anorthosite. The grain boundaries be-
 tween plagioclase grains show tiny growth
 of K-feldspar and kyanite.
 sive reaction/fluid availability these re-
 action rims (coronas) become thicker
 and change their mineral assemblage
 from garnet, omphacite, kyanite to om-
 phacite, amphibole, kyanite. In addi-
 tion to these changes, plagioclase trans-
 forms to an increasing amount of fine-
 grained zoisite and phengite needles.
 The little H2O needed to facilitate these
 transformations is provided from the
 breakdown of primary hydrous phases
 like scapolite and hornblende that are
 typically found in the granulites.
 Based on the above described textural
 and chemical characteristics (e.g. the oc-
 currence of omphacite and garnets rich
 in Alm and Grs component, distinctly
 different to the granulite facies mineral
 compositions), that can be observed in
 the most pristine granulites we suggest
 that these coronas and can be assigned
 to the Caledonian metamorphism. This
 in turn means that also the least re-
 acted granulites record the same P–T
 conditions as are recorded in the ad-
 jacent eclogites but not the same fluid
 1
Kühn & Austrheim TSK 11 Göttingen 2006
 conditions. The little water needed to
 balance the zoisite formation on the mi-
 cro scale can be balanced by the break-
 down of hydrous primary phases (horn-
 blende and scapolite). These ‘granulite
 facies’ anorthosites remained fluid de-
 ficient compared to their fully equili-
 brated eclogite facies equivalents. Fluid
 deficiency and meta-stability at meta-
 morphic conditions as recorded by the
 nearby eclogites explain the observed
 close occurrence of granulites and eclog-
 ites.
 The granulites from the eclogitised ar-
 eas in the Lindæs Nappe contain suf-
 ficient amounts of fluid to record the
 high-pressure event on the microscale.
 However, these signs of metamorphism
 are easily miss-interpreted as of lower
 grade origin. Extending our observa-
 tions to a completely dry bulk composi-
 tion it is inferred that such a rock will
 miss all signs of the Caledonian meta-
 morphism and that the first metamor-
 phic mineral assemblage will first form
 when fluid is introduced.
 References
 Austrheim A (1987) Eclogitization of lower
 crustal granulites by fluid migration through
 shear zones. Earth and Planetary Science
 Letters 81, 221–232
 Camacho A, Lee JKW, Hensen BJ & Braun
 J (2005) Short-lived orogenic cycles and the
 eclogitisation of cold crust by spasmodic hot
 fluids. Nature 435, 1191–1196
 2