Genesis of Coronas in Anorthosites of the Upper Jotun Nappe, Indre Sogn, Norway

Coronas in anorthosites have resulted from reactions betweenplagioclase and olivine. The general types are: (I) Olivine-Opx-Cpx II-Gnt-Cpx I+Spinel. (II) Olivine-Opx-Cpx II-Hbl-Gnt with inclusions of Cpx I+Spinel. (III) Opx-Cpx II-Hbl-Gnt. The evolutionary sequence appears to be Type I II III. CpxI has low Jd/Ts, whereas Cpx II has lower Al and higher Jd/Ts.The orthopyroxenes are low in Al and appear to be in equilibriumwith Cpx II. The garnets resemble those of eclogites; the amphiboleis pargasite. A two-stage reaction sequence is suggested: (1) Oliv+Plag Al-Opx+Al-Cpx+Spinel. (2) AI-Opx+Al-Cpx+Spinel+Plag Gnt÷low-Al Opx+low-AlCpx. Formation of amphibole followed reaction (2) in all cases. Comparisons of the mineralogy with experimental work suggestthat the anorthosites crystallized below 8 kb, and that pressureincreased to at least 10 kb during slow cooling. Symplectiticbreakdown of garnet to orthopyroxene+plagioclase+spinel±clinopyroxeneindicates a rapid drop in P at relatively high T. This decompressionis probably related to the formation of the Jotun Nappes.