Reaction Textures and Metamorphic Evolution of Sapphirine-bearing Granulites from the Gruf Complex, Italian Central Alps

Abstract

Mineral chemistries and textures are described from a suite of sapphirine-bearing granulites from the Gruf Complex of the Italian Central Alps. The granulites contain combinations of garnet, orthopyroxene, sapphirine, sillimanite, cordierite, biotite, quartz, spinel, corundum, staurolite, plagioclase, K-feldspar, ilmenite and rutile, in assemblages with low (usually negative) variance. They are outstanding in that they preserve a textural and chemical record of a protracted metamorphic evolution.Reaction textures are common and include: (i) pseudomorphs (e.g. of sillimanite after kyanite); (ii) relatively coarse-grained monomineralic reaction rims (e.g. of cordierite between sapphirine and quartz); (iii) fine-grained symplectitic coronas (e.g. of orthopyroxene + sapphirine round garnet); (iv) inclusions, in garnet cores, of minerals (e.g. staurolite) not found elsewhere in the rocks.Detailed microprobe study has revealed large chemical variations within each phase. Different textural types of each phase have different compositions, and strong zoning is preserved in garnet (Mg/(Mg + Fe) from 0-30 to 0-61) and coarse sapphirine. Inclusion populations in garnet correlate with host composition.The textural and chemical features are interpreted in terms of successive equilibrium assemblages and reactions. Metamorphic conditions operative at each stage in the evolution are calculated using published geothermometers and geobarometers as well as thermodynamically calibrated MAS and FASH equilibria. The results are used to construct a P-T-time path for the sapphirine-granulites, which can be summarized as follows:(i) Increasing Tat high P (>7 kb). Partial melting.(ii) A maximum 7"of -830 °C attained at -10 kb.(iii) Almost isothermal decompression, reaching 750 °Cat -5 kb, under conditions of low /i H ,o-(iv) Further cooling, and decompression. Localized hydration. Rocks exposed. The P-T-time path is interpreted as the product of a single metamorphic cycle (the tertiary 'Lepontine' event) and is extrapolated to the Gruf Complex as a whole. When combined with published geochronological data, the results indicate an average uplift rate in excess of 2 mm/yr for the Gruf Complex between 38 and 30 Ma ago.An in situ partial melting origin for the sapphirine-granulites is favoured. Extraction of an iron-rich granitic liquid from a normal pelitic palaeosome could generate a refractory residue with the required Mg, Al-rich composition. The change in bulk solid composition during partial melting is thought to account for the extraordinarily strong zoning in the garnets.

Tectonometamorphic history of the Gruf complex (Central Alps): exhumation of a granulite–migmatite complex with the Bergell pluton

“…Most of the previous interpretations considered the Gruf complex as a migmatitic unit that at least locally also suffered granulite-facies metamorphism during Alpine metamorphism (Droop and Bucher-Nurminen 1984;Gebauer 2003, Schmitz et al 2009). Structurally, the Gruf complex was regarded as either the eastern continuation of the Adula nappe (Schmid et al 1996a, b;Davidson et al 1996;Frey and Ferreiro Mählmann 1999;Berger et al 2005) or an equivalent of the Bellinzona-Dascio zone (Wenk 1973;Wenk and Cornelius 1977;Milnes and Pfiffner 1980).…”

“…Granulites and charnockites formed under ultra-high temperature conditions of T = 920-940°C and P = 8.5-9.5 kbar (Galli et al 2011). Droop and Bucher-Nurminen (1984), and Schmitz et al (2009) suggested an Alpine age for the granulites, whereas Huber (1999), Burri et al (2005) and Galli et al (2011) envisaged a pre-Alpine age. Subsequently, the charnockites have been dated at 282-260 Ma (U-Pb on zircon) placing granulite facies conditions in the Permian (Galli et al 2012).…”

“…The existing geological maps cover only small parts of this complex (north-eastern part by Wenk and Cornelius 1977; southernmost part by Moticska 1970); for most of the complex detailed maps or other field information are not available. The few available petrological (Bucher-Nurminen and Droop 1983;Droop and Bucher-Nurminen 1984;Diethelm 1989) and geochronological (Gulson 1973;Schmitz et al 2009) investigations were mostly performed using fallen blocks; hence there is no documentation regarding the tectonic or geological context of the samples. Since detailed mapping, a prerequisite for developing a coherent structural scheme of the Gruf complex, is missing, there is no consensus on the tectonic position of the Gruf complex within the Penninic nappes yet.…”

Melt Reintegration Modelling: Testing against a Subsolidus Reference Assemblage

“…However, later evidence of subtle partial melting was found [62], suggesting that the orthopyroxene-bearing assemblages formed under suprasolidus conditions. Although some studies have recorded mineral assemblages with prograde subsolidus orthopyroxene e.g., [63,64], these situations generally involve infiltration of a low H 2 O activity fluid (aH 2 O), such as CO 2 -rich fluids or highly concentrated brines, and occur in rocks that have experienced granulite facies temperatures. An example of this is the formation of fine-grained orthopyroxene-andalusite symplectites under low aH 2 O conditions during retrogression of pelitic granulites, described by Ballèvre, et al [65].…”

Interprétation tectonométamorphique du nord de la province du Supérieur, Québec, Canada /

“…Fine-grained symplectitic intergrowths that have been locally developed are generally interpreted to have formed during retrograde breakdown of high-grade phases (Droop & Bucher-Nurminem 1984, Ballèvre et al 1997, Nijland et al 1998 between 890 -1160 ±50°C. We estimate the P-T conditions of the Sil -Spl -Crd -Spr -Qtz assemblage studied here between 900 -950 ±50°C and 9 -10 ±1 kbar.…”