Mechanism of Mineral Reactions Inferred from Textures of Impure Dolomitic Marbles from East Greenland

Mineral assemblages and chemical compositions of minerals found in impure dolomitic marbles embedded in gneisses and migmatites of the E. Greenland Caledonian fold belt (Scoresby Sund) suggest that the marbles were metamorphosed near 630 °C at 5 kb pressure. The analysis of complex textural and mineralogical relations among minerals such as dolomite, calcite, forsterite, pargasite, chlorite, spinel, diopside and phlogopite led to the conclusion that the major mineralogical features of the rocks were probably caused by sodium metasomatism at constant temperature and pressure. The effect of the inferred sodium metasomatism may be summarized by three schematic reactions all involving modal changes of excess dolomite, calcite, forsterite, chlorite and spinel: (a) nucleation and growth of pargasite, (b) resorption of phlogopite, and (c) growth of pargasite from phlogopit

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

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 T at high P (>7 kb). Partial melting. (ii) A maximum T of ∼830 °C attained at ∼10 kb. (iii) Almost isothermal decompression, reaching 750 °C at ∼5 kb, under conditions of low μH2O. (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 extraordinarity strong zoning in the garnet