Igneous and metamorphic petrology of the eclogitic Seljeneset Meta-anorthosite and related jotunites, Western Gneiss Region, Norway

Abstract

The Seljeneset Meta-anorthosite consists of deformed and metamorphosed anorthosite and mafic rocks, is situated in the high-grade Western Gneiss Region (WGR) of Norway, and is adjacent to the 1.52 Ga Flatraket quartz monzonite (mangerite). The WGR was metamorphosed to eclogite fades during Caledonian tectonism 414 ± 16 Ma ago. The Meta-anorthosite contains 46–57 wt.% SiO 2 and displays chondrite-normalized REE patterns typical of anorthosite, indicating that metamorphism did not significantly affect REE abundances. Norms indicate that igneous anorthosite consisted of ≥ 90% modal plagioclase, minor mafic silicates, and ~ 1% Fe-Ti oxide. Mafic rocks (43–48 wt.% SiO 2) are rich in total Fe 2O 3 (13–20%), TiO 2 (3–4%), and P 2O 5 (1–3%), poor in Cr (< 4 ppm) and Ni (~ 30 ppm), and are thus classified as jotunites, which are characteristic of Proterozoic anorthosite-mangerite complexes. Jotunite originally consisted of > 50% modal andesine, ∼ 25% mafic silicates, ∼ 10% Fe-Ti oxide, and up to 7% apatite. REE patterns of jotunite are highly variable, which reflects variable igneous abundances and modification by metasomatism. Filter pressing of residual liquid trapped between cumulus plagioclase may have been important in the evolution of anorthosite and jotunite. Anorthosite contains ultramafic xenoliths (high MgO, Cr and Ni and depleted in LREE), which suggests that some mantle-derived peridotite was present in the Baltic crust by mid-Proterozoic time; this may extend the crustal history of Norwegian garnet peridotites by over one billion years. Seljeneset rocks display evidence of retrogression from eclogite through amphibolite facies. Meta-anorthosite consists of ovoidal clots of margarite, quartz, plagioclase, and relict kyanite set in a matrix of zoisite, clinozoisite, plagioclase, quartz, hornblende, and biotite. Phase equilibria analysis of sequential assemblages shows that meta-anorthosite records a retrograde P-T path from ≥ 15–17 kbar to 4.3–8.2 kbar at ~ 450–600 °C. Mafic rocks vary from moderately well-preserved eclogite, through varieties containing symplectite, to dynamically recrystallized amphibolite. In one sample, the rare Na-Ca amphibole magnesio-alumino-taramite appears to be part of the eclogite assemblage. Calculations indicate eclogite formation at P ≥ 16–18 kbar at ~ 750 °C. Some mafic rocks contain veins and segregations of biotite, muscovite, pyrite, allanite, and zircon, that, considered together with geochemical data, indicate metasomatic introduction of K, Rb, Ba, Ga, S and rare-earth and other high-field-strength elements by channelized fluids during retrogression. The Seljeneset Meta-anorthosite, associated jotunites, and adjacent Flatraket quartz monzonite likely constitute a single complex, here named the Nordpollen Plutonic Complex. This complex was emplaced 1.52 Ga ago, metamorphosed to eclogite facies during Silurian continent-continent collision, largely retrograded to amphibolite facies during uplift of the orogen, and locally metasomatised during retrogression.