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Peter Robinson, Geological Survey of Norway (Norway)
Nathan R. Daczko, Macquarie University (Australia)
Thomas E. Krogh, University of Toronto (Canada)
Kurt Hollocher, Union College (United States)
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Averøy eclogite is a large layered eclogite in the coastal Western Gneiss Region. Others in the group occur with coarse garnet-biotite+/-sillimanite gneiss and marble, suggesting correlation with Blhø Nappe, Upper Allochthon. This eclogite, with major and trace elements suggestive of alkaline basalt, may be metamorphosed mafic volcanics, distinct from those at margins of Mesoproterozoic gabbros intruding granitoid basement. A U-Pb age on metamorphic zircon determines eclogite recrystallization 415+/-1 Ma. A 395.3 +/-1 Ma U-Pb age of cross-cutting pegmatite suggests a 20 m.y. return of the eclogite to relatively near-surface conditions. Large metamorphic zircon tips give 411-410 Ma, likely correlated with growth of coarse secondary high-T hornblende and garnet, and possibly with the structure described here.
Typical Averøy eclogite has patches of coarse primary omphacite Jd38Ts5Wo25En25Fs7, surrounded by coarse "graphic" pyx-plag symplectite Jd11Ts11Wo35En35Fs8 + An18-19. Lower- P finer-grained patches of symplectite Jd8Ts6Wo41En37Fs8 + An32 overprint the coarse texture. Other minerals are garnet Pyr39Alm42Grs18Sps1 cores to Pyr37Alm47Grs15Sps1 rims, hornblende, rutile and ilmenite. A rare quartz layer 12mm thick contains isolated garnets 0.7mm thick flattened parallel to foliation, surrounded by a "moat" of gray-white birefringent blades 0.1mm wide, and a thin "necklace" of highly birefringent pyroxene. Initial petrography suggested the moat/necklace might be cordierite + orthopyroxene breakdown of Mg-garnet, or retrograde anorthite on Ca-garnet. EMP analyses showed zoned plagioclase moats An26 near garnet to An12 near necklaces, and garnets with Pyr42Alm42Grs15Sps1 cores and Pyr34Alm49Grs16Sps1 rims. Necklace pyroxenes are Jd9Ts1Wo41En36Fs12 and Ts1Wo0En60Fs38. All garnets show rim increase in Mn (slight = resorption) and Fe (stronger, cooling-fractionation w. pyx), while Ca changes little in quartz, but decreases in eclogite.
Initial ideas on the structure involved breakdown of a single mineral within quartz: 1) Hornblende + quartz to a prograde pseudomorph gar + plag + two pyx, but not explaining mineral arrangement. 2) Breakdown of Na- majorite-bearing (VIIINaVISi) garnet, known synthetically, though at P greater than likely in the Norwegian UHP terrane, where plagioclase would form at the quartz contact. A more plausible idea is partial chemical communication between quartz layer and eclogite. Simple ~balanced reactions reflecting different garnet behavior and product assemblages are, in eclogite, 2 Omph + 3.21 Grs18 + 0.95 Qz = 1.43 Ca-pyx + 3.02 Gr15 + 0.95 An20, in qz, 2 Omp +0.2 Grs15 + 1 Qz = 1.08 Ca-pyx + 0.32 Opx + 1 An20. In both, Al for An in plagioclase is from garnet, but with different chemography and coefficients. If correct, this implies diffusion for at least 1-2 cm during re-equilibration, and exhumation at constant or increasing T, which could explain poor preservation of HP-UHP features in these eclogites.
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