Two different crystallization trends of pyroxene in a tholeiitic dolerite,Semi, northern Japan |
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| Authors: | Minoru Yamakawa |
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| Institution: | (1) Geological Institute, University of Tokyo, Tokyo, Japan |
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| Abstract: | Compositional and textural relations of coexisting augite and pigeonite in a tholeiitic dolerite in Semi, northern Japan have been analysed with the electron probe microanalyser. Two different crystallization trends of augite have been recognized. In the first case, augite varies in composition from Ca37 Mg41 Fe22 to Ca35 Mg32 Fe33 with nearly constant Ca/Ca +Mg+Fe ratio, whereas in the second case, augite varies from Ca36Mg40Fe24 to Ca28Mg35Fe37 with a considerable decrease of Ca/Ca+Mg+Fe ratio. The compositional trend of augite in the first case may be explained by cotectic crystallization of augite and pigeonite, and that in the second case may be explained by metastable crystallization of subcalcic augite due to undercooling of magma. Such metastable crystallization may have resulted in local heterogeneity of magma. |
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