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Exsolution structures in calcic pyroxenes from the Bjerkreim-Sokndal lopolith, SW Norway

Published online by Cambridge University Press:  05 July 2018

F. J. M. Rietmeijer  and
P. E. Champness
F. J. M. Rietmeijer
Affiliation:
Department of Petrology, Instituut voor Aardwetenschappen, Budapestlaan 4, 3508 TA Utrecht, The Netherlands
P. E. Champness
Affiliation:
Department of Geology, University of Manchester, Manchester M13 9PL
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Abstract

Iron-rich (Fs:En ∼0.8) calcic pyroxenes that have been subjected to granulite-facies metamorphism contain up to seven generations of exsolution lamellae. They can be grouped into four stages. In stage 1 pigeonite exsolved parallel to ‘001’ and ‘100’ (where ‘hkl’ signifies ∼ (hkl)) and mostly inverted later to orthopyroxene. During stage 2 orthopyroxene exsolved parallel to (100), while during stage 3 orthopyroxene was quickly followed by metastable ‘001’ pigeonite. The stage 3 precipitates clearly grew and thickened together for some time. During stage 4 a ‘100’ pigeonite was exsolved. The stage 3 and 4 precipitates show evidence of reheating, dissolution and later, renewed growth. Sometimes orthopyroxenes of stage 3 have crossed a ‘001’ pigeonite lamella and caused it to invert by a shear mechanism.

Chemical analysis shows no rotation of the tie lines between Ca-rich and Ca-poor phases, in contrast to previous studies of Skaergaard and Bushveld pyroxenes. The geothermometers of Wood and Banno (1973) and Wells (1977) indicate solidus temperatures of about 850°C and 900°C respectively, but the geothermometers were found to be unsuitable for subsolidus conditions. We estimate the pressure to have been about 9 kbar during solidification. Estimates of nucleation temperatures obtained from the orientations of the exsolved lamellae (Robinson et al., 1977) were 850–700°C for stage 1, and 600–400°C for stage 3. We believe this geothermometer to be unreliable for the low temperatures involved in stage 4.

Type
Research Article
Information
Mineralogical Magazine , Volume 45 , Issue 337 , 1982 , pp. 11 - 24
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1982

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