NJG82-2-03
2002
Granulite-facies contact metamorphism around the Hakefjorden Norite-Anorthosite Complex, SW Sweden
82
2
pp. 29-44
82-92394-02-8

Granulite-facies contact metamorphic mineral assemblages developed in the aureole of the ca. 916 Ma Hakefjorden norite-anorthosite complex, SW Sweden. Outside the immediate contact, country rocks consisting of Mesoproteroroic meta-greywackes preserve a pre-intrusive, regional amphibolite-facies mineral assemblage of quartz, plagioclase, K-feldspar, biotite, muscovite, and scarce garnet. Several reactions at the immediate contact produced high-temperature assemblages comprising sillimanite, cordierite, orthopyroxene, high-temperature garnet rims, corundum, hercynitic low-Zn spinel, K-feldspar, and melt. Considerable changes in the composition of biotite and plagioclase also occurred. The pre-existing amphibolite-facies assemblage was recrystallised, partially consumed, or partially molten. Garnet porphyroblasts show preserved prograde growth wning with strongly increasing pyrope content in the outermost rims. In other instances, rims are resorbed, contributing to the formation knotlike, Si02-deficient intergrowths of cordierite + spinel + sillimanite +K-feldspar + plagioclase ± corundum. Textural and compositional heterogeneity indicates that equilibrium conditions were attained only on a local mm-size scale. Some of the garnet appears to preserve a roning record of two pre-intrusive metamorphic growth episodes. Geothermobarometry of assemblages in local textural equilibrium records low-P, ultra-high-T granulitic peak contact metamorphic conditions of 890-10l5°C at pressure below 6 kbar.

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