Convergent metamorphism of eclogites and dolerites,Kristiansund area,Norway |
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| Authors: | WL Griffin A Raheim |
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| Institution: | 1. Mineralogisk-Geologisk Museum, Oslo, Norway;2. Department of Geology and Geophysics, A.N.U., Canberra, Australia |
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| Abstract: | Amphibolite-facies gneisses of the Frei group include pelitic migmatites, mica schists, quartzites, marbles, augen-gneisses and ecologites. Field relations indicate that the eclogites are supracrustal rocks metamorphosed in situ. Kyanit- Kspar-quartz that the assemblages in the gneisses indicate PH2O <Pload.Late-tectonic dolerite sills in the gneisses show corona-forming reactions leading to omphacite-garnet-plagioclase associations. Pyroxene-garnet pairs formed by these reactions yield equilibration temperatures near 700°C, though the experimentally determined equilibrium positions of the reactions cover a wide range of T and P. The dolerites were probably intruded between 4.5 and 9 Kb and cooled rapidly; the corona-forming reactions were initially overstepped and proceeded metastably at the T and P of the surrounding eclogite terrane.During retrogression of eclogites to granulite-facies assemblages (cpx + gnt + lag + qtz), omphacite loses Na and Al as plagioclase (An10–20) exsolves in perthite-like textures, and garnet loses Ca as it is resorbed. Similar reactions are seen between pyroxene and garnet in the coronites and eclogitic metadolerites. These reactions are ascribed to uplift during cooling of the terrane. |
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