Mantle-equilibrated Orthopyroxene Eclogite Pods from the Basal Gneisses in the Selje District, Western Norway |
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| Authors: | LAPPIN M A; SMITH D C |
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| Institution: | 1 Department of Geology & Mineralogy, Marischal College Broad Street, Aberdeen AB9 IAS, Scotland.
2 Geological Survey of Norway P.O. Box 3006, 7001 Trondheim, Norway |
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| Abstract: | ‘Country-rock eclogite’ pods occur enclosed withtectonic contacts within heterogeneous amphibolite-facies gneissesin the Basal Gneiss Region of Western Norway. Sixty-nine newmicroprobe analyses for garnets, clino-and orthopyroxenes, olivine,clinoamphiboles, biotite and carbonates from a number of orthopyroxeneeclogite pods in the Selje District are presented. The firstfour minerals are primary whilst the others, of which the amphibolesare described in some detail, formed during a subsequent butstill early stage in eclogite history. The primary minerals have a wider range of compositions thanorthopyroxene eclogites from other geological environments;jadeite-rich clinopyroxene and unusually grossular-poor garnetsare described from this environment for the first time. Sidero-magnesiteoccurs in apparent equilibrium with primary eclogite minerals.The early amphiboles have apparently grown at the expense of,but nevertheless in equilibrium with, primary minerals throughreactions involving OH, K+, Na+, and possibly Mg-bearing fluids.Magnesio-cummingtonite intergrown with actinolite is recognizedas an early phase in one eclogite pod. The early amphibolescan be distinguished from the symplectitic amphiboles by thelower Allv, Alvl, Ti and alkali contents and Fe/Fe + Mg ratiosand higher Si content of the former minerals. The symplectiticamphiboles form, together with plagioclase, during the stilllater amphibolitization of the eclogites.  Fe/Mg distribution coefficients are affected by the Na contentsof clinopyroxenes and probably also by the Fe/Mg contents ofthe bulk assemblages. The former is due largely to increasingacmite content in jadeite-rich clinopyroxenes whilst the latteris tentatively attributed to lower closure temperatures of Fe-richassemblages. The Ca content in garnet is significantly relatedto both of these Na and Fe/Mg factors. Nevertheless a rangeof different distribution coefficients, including the Ca/Ca+ Mg ratio in coexisting pyroxenes, suggests a very limitedrange of temperatures of equilibration, the best estimate ofwhich is 700–850 °C. Pressures of equilibration are more difficult to assess. Onemodel, based upon the assumption of the stable occurrence ofamphiboles together with primary minerals and upon the minimumpressures necessary to transform a range of rock types to eclogite,suggests pressures of 15–28 kb at 700–850°C.A second model, based upon the Al2O3 content of primary orthopyroxeneand upon the association of sidero-magnesite with pyroxenes,suggests higher pressures (30–45 kb) over the same temperaturerange. Amphiboles are not stable under these conditions andare considered to form during a subsequent lower pressure (15–28kb) event when the low Al2O3 orthopyroxenes and sidero-magnesitesurvive metastably during an essentially isothermal history. One eclogite pod contains minerals with coarse exsolution lamellae:orthopyroxene exsolving garnet and clinopyroxene exsolving orthopyroxene.These imply high T-P processes, roughly estimated at 1200–1370°C, 30–40 kb, and hence suggest eclogite generationby igneous fractionation processes. Four T-P regimes (A, B, C, D) of mineral equilibration are recognizedin the history of the Selje district orthopyroxene eclogites,between their prior origin, presumably in the upper mantle,and their present surface exposure. This initial eclogite fractionation(regime A) occurred in an olivine-poor rather than olivine-richupper mantle environment, followed by cooling, exsolution, recrystallizationand re-equilibration (regime B) in a Precambrian tectonic environment.Subsequent history involved mineral reaction, metasomatism,and probably chemical redistribution through the medium of amphibole-formingfluids (regime C) and finally Caledonian tectonic transportinto poly-metamorphic continental basement where their survivalis thought to be due to a low activity of water. Marginal symplectiticamphibolitization (regime D), due to localized fluxing of metamorphicfluids, was the last significant petrological event prior touplift and exposure. The processes of tectonic transport aretentatively considered to represent deep level obduction processesrelated to continent/continent collision. |
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