Clinopyroxenes in the tawhiroko tholeiitic dolerite at Moeraki,north-eastern Otago,New Zealand |
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| Authors: | Yasuo Nakamura D S Coombs |
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| Institution: | (1) Department of Geology, University of Otago, Dunedin, New Zealand;(2) Present address: Geophysical Laboratory, Carnegie Institution of Washington, 2801, Lipton St. N. W., 20008 Washington D. C., USA |
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| Abstract: | Clinopyroxenes from various lithologic units in a single tholeiitic dolerite intrusion 50 m thick were analysed with the electron microprobe. Microphenocrysts of augite in the chilled marginal rock have the least scattered Ca MgFe ratios, around Ca38Mg50Fe12, and lowest Ti/Al ratios, less than 1/4, of the augites examined; they crystallized prior to emplacement. The augites which crystallized in situ are more Fe-rich and have higher Ti/Al ratios, close to 1/2. Pigeonite started crystallizing later than augite and formed rims to augite crystals. Continuous zoning from augite to subcalcic augite as was previously reported by Benson (1944) for the same dolerite is not observed. Subcaloic ferroaugites, however, link augite and ferropigeonite in the outermost margins of pyroxenes in the later segregations. The small scale differentiation in situ gave rise to globules, veins, and pegmatoids, in which sector-zoning of augite is well developed. The {100} sectors are enriched in MgSiO3, FeSiO3, and MnSiO3 components relative to the {010} and the {110} sectors, and are accordingly depleted in other components. The protosite theory of sector-zoning formation proposed by Nakamura (1973) is further developed and the environment which favours the development of sector-zoning is discussed. The formation of augite rims around quartz-rich xenoliths and the compositional characteristics of the augites concerned are due to local high silica activities. Comments are made concerning some lunar pyroxenes in the light of the present study. |
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