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∶Mg∶Fe 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.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
Atkins, F. B.: Pyroxenes of the Bushveld Intrusion, South Africa. J. Petrol. 10, 229–249 (1969)
Bence, A. E., Albee, A. L.: Empirical correction factors for the electron microanalysis of silicates and oxides. J. Geol. 76, 382–403 (1968)
Bence, A. E., Papike, J. J., Lindsley, D. H.: Crystallization histories of clinopyroxenes in two porphyritic rocks from Oceanus Porcellarum. Proc. 2nd Lunar Science Conference 1, 559–574 (1971)
Bence, A. E., Papike, J. J., Prewitt, C. T.: Apollo 12 clinopyroxenes: Chemical trend. Earth Planet. Sci. Lett. 8, 393–399 (1970)
Benson, W. N.: The basic igneous rocks of eastern Otago and their tectonic environment. Part IV. The mid-Tertiary basalts, tholeiites and dolerites of north-eastern Otago. Section A: Distribution and geological occurrence. Trans. Roy. Soc. New Zealand 73, 116–138 (1943)
Benson, W. N.: The basic igneous rocks of eastern Otago and their tectonic environment. Part IV. The mid-Tertiary basalts, tholeiites and dolerites of north-eastern Otago. Section B: Petrology, with special reference to the crystallization of pyroxene. Trans. Roy. Soc. New Zealand 74, 71–123 (1944)
Benson, W. N.: The basic igneous rocks of eastern Otago and their tectonic environment. Part IV. The mid-Tertiary basalts, tholeiites and dolerites of north-eastern Otago. Section C: Exogenous and endogenous metamorphism associated with the tholeiites and dolerites of Moeraki. Trans. Roy. Soc. New Zealand 75, 288–318 (1945)
Boyd, F. R., Smith, D.: Compositional zoning in pyroxenes from lunar rock 12021, Oceanus Procellarum. J. Petrol. 12, 439–464 (1971)
Brown, G. M.: Pyroxenes from the early and middle stages of fractionation of the Skaergaard intrusion, east Greenland. Mineral. Mag. 31, 511–543 (1957)
Carmichael, I. S. E., Nicholls, J., Smith, A. L.: Silica activity in igneous rocks. Am. Mineralogist 55, 246–263 (1970)
Clark, S. P., Schairer, J. F., De Neuville, J.: Phase relations in the system CaMgSi2O6-CaAl2SiO6-SiO2 at low and high pressure. Carnegie Inst. Wash. Yearbook 61, 132–135 (1962)
Coombs, D. S., Dickey, J. S.: The early Tertiary petrographic province of north-east Otago: Waiareka and Deborah Volcanic Formations. N.Z. Dept. Scientific and Industrial Research, Information Series 51, 38–54 (1965)
Deer, W. A., Howie, R. A., Zussman, J.: Rock-forming minerals, vol. 2, Chain silicates, 379 p. London: Longmans 1963
Hollister, L. S., Gancarz, A. J.: Compositional sector-zoning in clinopyroxene from the Narce area, Italy. Am. Mineralogist 56, 969–979 (1971)
Hollister, L. S., Trzcienki, W. E. Jr., Hargraves, R. B., Kullick, C. G.: Petrogenic significance of pyroxenes in two Apollo 12 samples. Proc. 2nd Lunar Science Conference 1, 529–558 (1971)
Jaeger, J. C.: Cooling and solidification of igneous rocks in Basalts: The Poldervaart treatise on rocks of basaltic composition (Hess, H. H., Poldervaart, A., eds.), vol. 2, p. 503–536. New York: Interscience Publishers 1968
Kushiro, I.: Si-Al relations of clinopyroxenes from igneous rocks. Am. J. Sci. 258, 548–554 (1960)
Kushiro, I.: Clinopyroxene solid solutions at high pressures. Carnegie Inst. Wash. Yearbook 64, 112–117 (1965)
Kushiro, I.: Clinopyroxene solid solutions formed by reactions between diopside and plagioclase at high pressures. Min. Soc. Am., Special Paper 2, 179–191 (1969)
Kushiro, I., Nakamura, Y.: Petrology of some lunar crystalline rocks. Proc. Apollo 11 Lunar Science Conference 1, 607–626 (1970)
Le Bas, M. J.: The role of aluminum in igneous clinopyroxenes with relation to their parentage. Am. J. Sci. 260, 267–288 (1962)
Lindsley, D. H., Emslie, R. F.: Effect of pressure on the boundary curve in the system diopside-albite-anorthite. Carnegie Inst. Wash. Yearbook 66, 479–480 (1968)
Nakamura, Y.: Origin of sector-zoning of igneous clinopyroxenes. Am. Mineralogist (In press)
Nakamura, Y., Coombs, D. S., McBirney, A. R.: Petrology of the Tawhiroko tholeiitic dolerite at Moeraki, North-eastern Otago, New Zealand. (In preparation)
Nakamura, Y., Kushiro, I.: Compositional relations of coexisting orthopyroxene, pigeonite and augite in a tholeiitic andesite from Hakone volcano. Contr. Mineral. and Petrol. 26, 265–275 (1970a)
Nakamura, Y., Kushiro, I.: Equilibrium relations of hypersthene, pigeonite and augite in crystallizing magmas: Microprobe study of a pigeonite andesite from Weiselberg, Germany. Am. Mineralogist 55, 1999–2015 (1970b)
Osborn, E. F., Tait, D. B.: The system diopside-forsterite-anorthite. Am. J. Sci., Bowen Volume, 413–433 (1952)
Prince, A. T.: Liquidus relationships on 10% MgO plane of the system lime-magnesia-alumina-silica. J. Am. Ceram. Soc. 37, 402–408 (1954)
Schairer, J. F., Yoder, H. S., Jr.: The nature of residual liquids from crystallization with data on the system nepheline-diopside-silica. Am. J. Sci. 258 A, 273–283 (1960)
Warren, B. E., Bragg, W. L.: The structure of diopside CaMg(SiO3)2. Z. Krist. 69, 168–193 (1928)
Weill, D. F., McCallum, I. S., Bottinga, Y., Drake, M. J., McKay, G. A.: Mineralogy and petrology of some Apollo 11 igneous rocks. Proc. Apollo 11 Lunar Science Conference 1, 937–955 (1970)
Williams, R. J.: Reaction constants in the system Fe-MgO-SiO2-O2; Intensive parameters in the Skaergaard intrusion, east Greenland. Am. J. Sci. 271, 132–146 (1971)
Author information
Authors and Affiliations
Rights and permissions
About this article
Cite this article
Nakamura, Y., Coombs, D.S. Clinopyroxenes in the tawhiroko tholeiitic dolerite at Moeraki, north-eastern Otago, New Zealand. Contr. Mineral. and Petrol. 42, 213–228 (1973). https://doi.org/10.1007/BF00371586
Received:
Issue Date:
DOI: https://doi.org/10.1007/BF00371586