Mantle-equilibrated Orthopyroxene Eclogite Pods from the Basal Gneisses in the Selje District, Western Norway

‘Country-rock eclogite’ pods occur enclosed with tectonic contacts within heterogeneous amphibolite-facies gneisses in the Basal Gneiss Region of Western Norway. Sixty-nine new microprobe analyses for garnets, clino-and orthopyroxenes, olivine, clinoamphiboles, biotite and carbonates from a number of orthopyroxene eclogite pods in the Selje District are presented. The first four minerals are primary whilst the others, of which the amphiboles are described in some detail, formed during a subsequent but still early stage in eclogite history.

The primary minerals have a wider range of compositions than orthopyroxene eclogites from other geological environments; jadeite-rich clinopyroxene and unusually grossular-poor garnets are described from this environment for the first time. Sidero-magnesite occurs in apparent equilibrium with primary eclogite minerals. The early amphiboles have apparently grown at the expense of, but nevertheless in equilibrium with, primary minerals through reactions involving OH, K⁺, Na⁺, and possibly Mg-bearing fluids. Magnesio-cummingtonite intergrown with actinolite is recognized as an early phase in one eclogite pod. The early amphiboles can be distinguished from the symplectitic amphiboles by the lower Al^lv, Al^vl, Ti and alkali contents and Fe/Fe + Mg ratios and higher Si content of the former minerals. The symplectitic amphiboles form, together with plagioclase, during the still later amphibolitization of the eclogites.

ΣFe/Mg distribution coefficients are affected by the Na contents of clinopyroxenes and probably also by the ΣFe/Mg contents of the bulk assemblages. The former is due largely to increasing acmite content in jadeite-rich clinopyroxenes whilst the latter is tentatively attributed to lower closure temperatures of Fe-rich assemblages. The Ca content in garnet is significantly related to both of these Na and ΣFe/Mg factors. Nevertheless a range of different distribution coefficients, including the Ca/Ca + Mg ratio in coexisting pyroxenes, suggests a very limited range of temperatures of equilibration, the best estimate of which is 700–850 °C.

Pressures of equilibration are more difficult to assess. One model, based upon the assumption of the stable occurrence of amphiboles together with primary minerals and upon the minimum pressures 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 orthopyroxene and upon the association of sidero-magnesite with pyroxenes, suggests higher pressures (30–45 kb) over the same temperature range. Amphiboles are not stable under these conditions and are considered to form during a subsequent lower pressure (15–28 kb) event when the low Al₂O₃ orthopyroxenes and sidero-magnesite survive 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 generation by igneous fractionation processes.

Four T-P regimes (A, B, C, D) of mineral equilibration are recognized in 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-rich upper mantle environment, followed by cooling, exsolution, recrystallization and re-equilibration (regime B) in a Precambrian tectonic environment. Subsequent history involved mineral reaction, metasomatism, and probably chemical redistribution through the medium of amphibole-forming fluids (regime C) and finally Caledonian tectonic transport into poly-metamorphic continental basement where their survival is thought to be due to a low activity of water. Marginal symplectitic amphibolitization (regime D), due to localized fluxing of metamorphic fluids, was the last significant petrological event prior to uplift and exposure. The processes of tectonic transport are tentatively considered to represent deep level obduction processes related to continent/continent collision.