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

1971; Oxford University Press; Volume: 12; Issue: 2 Linguagem: Inglês

10.1093/petrology/12.2.219

1460-2415

William L. Griffin,

Paleontology and Stratigraphy of Fossils

Coronas in anorthosites have resulted from reactions between plagioclase 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. Cpx I 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 equilibrium with Cpx II. The garnets resemble those of eclogites; the amphibole is 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-Al Cpx. Formation of amphibole followed reaction (2) in all cases. Comparisons of the mineralogy with experimental work suggest that the anorthosites crystallized below ∼ 8 kb, and that pressure increased to at least 10 kb during slow cooling. Symplectitic breakdown of garnet to orthopyroxene+plagioclase+spinel±clinopyroxene indicates a rapid drop in P at relatively high T. This decompression is probably related to the formation of the Jotun Nappes.