Formation of Eclogites and the Coronas in Anorthosites, Bergen Arcs, Norway

The anorthosite arc of the Bergen area contains a thick series of gabbroic to anorthositic rocks, with garnet-pyroxene-plagioclase mineralogy. These rocks are infolded with amphibolite-facies Cambro-Silurian supracrustal rocks, deformed during the Caledonian orogeny. Eclogites occur as thick layers and as crosscutting dikes in the anorthosites. Olivine pods, apparently formed from disrupted layers, have reacted with plagioclase during cooling, passing through the generalized reactions: 1. olivine + plagioclase → aluminous pyroxenes + spinel, and 2. aluminous pyroxenes + spinel + plagioclase → low-Al pyroxenes + garnet. The resultant corona structures have orthopyroxene cores, surrounded successively by shells of low-Al clinopyroxene and garnet (+ inclusions of aluminous clinopyroxene and spinel). Microprobe analyses show the aluminous clinopyroxene to be enriched in Tschermak’s silicate (Ts) and low in jadeite (Jd), whereas the low-Al clinopyroxene has Jd/Ts > ½. Slow uplift has forced simultaneous exsolution of jadeite from the low-Al clinopyroxene and of grossular from garnet, leading to a strong zoning of the garnet shell and eventual appearance of a plagioclase shell between the clinopyroxene and the garnet. The eclogites found as conformable layers are quartz free and the pyroxenes are low in Na, but with high Jd/Ts ratios; the eclogites originally may have been cumulate olivine-pyroxene-plagioclase layers. The crosscutting eclogites are quartz bearing and the pyroxenes are omphacites or chloromelanites, typical of low-T eclogites. The mineralogy of the coronas and eclogites suggests that the anorthositic rocks originally crystallized from a magma at P < 8 kb, and were buried to 10 to 12 kb during cooling. The great difference in metamorphic grade between the anorthosites and the Cambro-Silurian rocks rules out interpretation of the anorthositic rocks as either Caledonian intrusives or as a late Precambian sequence conformably underlying the Cambro-Silurian rocks. Decompression breakdown of garnet and Tschermakitic pyroxene is common in the anorthosites of the Caledonian Jotun nappes, but is absent in the Bergen anorthosites. This difference suggests there was no rapid vertical movement in the P-T history of Bergen anorthosites, and thus that they are tectonically distinct from the anorthosites of the Jotun nappes.