TY - JOUR
T1 - Evolution of coronas in Norwegian anorthosites
T2 - re-evaluation based on crystal-chemistry and microstructures
AU - Griffin, W. L.
AU - Meillini, M.
AU - Oberti, R.
AU - Rossi, G.
PY - 1985/12
Y1 - 1985/12
N2 - Coarse grained garnet-pyroxene coronas in anorthositic gabbros of the Bergen and Sognefjord areas preserve relics of two stages of reaction during cooling from magmatic T: (1) ol + plag→cpx I+ opxI + spn; (2) cpxI + opxI + spn + plag→cpxII + opxII + gnt. Geothermobarometry on the products of reaction (1), using Fe3+ cpx and AlIV opx values derived by X-ray structure refinement (XRef), yields T=900-950° C, P=10-11 kb. Even higher T, probably at lower P, is inferred for reaction (2); this is consistent with TEM observations of pigeonite lamellae in cpxI. Transmission electron microscopy (TEM) observations show abundant submicroscopic amphibole lamellae in cpxII, and XRef analyses of coexisting massive amphiboles suggest incomplete occupancy of the H site; both observations suggest low but finite {Mathematical expression} during granulite-facies metamorphism. TEM observations also show that later (retrograde) hydration, particularly of orthopyroxenes, is more extensive than suggested by petrographic studies. Late amphiboles have fully occupied H sites, indicating higher {Mathematical expression}. The integration of XRef and TEM studies with more conventional petrographic and electron microprobe (EMP) data, as in this study, gives a powerful tool for the study of metamorphic reactions.
AB - Coarse grained garnet-pyroxene coronas in anorthositic gabbros of the Bergen and Sognefjord areas preserve relics of two stages of reaction during cooling from magmatic T: (1) ol + plag→cpx I+ opxI + spn; (2) cpxI + opxI + spn + plag→cpxII + opxII + gnt. Geothermobarometry on the products of reaction (1), using Fe3+ cpx and AlIV opx values derived by X-ray structure refinement (XRef), yields T=900-950° C, P=10-11 kb. Even higher T, probably at lower P, is inferred for reaction (2); this is consistent with TEM observations of pigeonite lamellae in cpxI. Transmission electron microscopy (TEM) observations show abundant submicroscopic amphibole lamellae in cpxII, and XRef analyses of coexisting massive amphiboles suggest incomplete occupancy of the H site; both observations suggest low but finite {Mathematical expression} during granulite-facies metamorphism. TEM observations also show that later (retrograde) hydration, particularly of orthopyroxenes, is more extensive than suggested by petrographic studies. Late amphiboles have fully occupied H sites, indicating higher {Mathematical expression}. The integration of XRef and TEM studies with more conventional petrographic and electron microprobe (EMP) data, as in this study, gives a powerful tool for the study of metamorphic reactions.
UR - http://www.scopus.com/inward/record.url?scp=0022183345&partnerID=8YFLogxK
U2 - 10.1007/BF00374689
DO - 10.1007/BF00374689
M3 - Article
AN - SCOPUS:0022183345
SN - 0010-7999
VL - 91
SP - 330
EP - 339
JO - Contributions to Mineralogy and Petrology
JF - Contributions to Mineralogy and Petrology
IS - 4
ER -