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Fe-enrichment in tholeiitic pyroxenes: complex two-pyroxene assemblages in Mesozoic dolerites, southern Tasmania

Published online by Cambridge University Press:  01 May 2009

R. P. Hall ,
D. J. Hughes  and
L. Joyner
R. P. Hall
Affiliation:
Department of Geology, Portsmouth Polytechnic, Portsmouth PO1 3QL, U.K.
D. J. Hughes
Affiliation:
Department of Geology, Portsmouth Polytechnic, Portsmouth PO1 3QL, U.K.
L. Joyner
Affiliation:
Department of Geology, Portsmouth Polytechnic, Portsmouth PO1 3QL, U.K.
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Abstract

The pyroxene assemblages of four samples of ferrodolerite from the Mesozoic Mount Wellington sill of southern Tasmania are described. The pyroxenes of each sample define an Fe-enrichment trend equivalent to almost half of a Skaergaard-type pyroxene evolution trend. The calcic pyroxenes range from augite to ferroaugite (Ca30–40Mg40–17Fe15–45) and late-stage hedenbergite (Ca45 Mg5Fe50), and the pigeonites from intermediate to ferriferous types (Ca13Mg55–15Fe32–70). XMg numbers (Mg/(Mg + Fe)) of the calcic pyroxenes vary from 0.8 to 0.1, and of the calcium-poor pyroxenes (pigeonites) from 0.62 to 0.17. The chemical variation in the pyroxenes within individual samples is far greater than between the different samples. One sample contains an extremely wide range of pyroxenes which include some of the most ferriferous pigeonites ever recorded (Ca14Mg15 Fe71). Ti and Al contents are also highly varied, Ti:Al ionic ratios ranging from 1:15 to 1:2 with decreasing XMg. The augites have higher Ti and Al contents than pigeonites with corresponding XMg. The hedenbergites have erratic but generally low Ti contents, due to their late precipitation together with ulvospinel. The presence of such highly variable pyroxenes within individual samples and in some cases as single, complex grains makes the recognition of coexisting pyroxene pairs in such dolerites very difficult, but graphical pyroxene thermometers suggest a crystallization temperature range of 1100 to 850 °C.

The range of pyroxene compositions in such dolerites is strongly influenced by the oxygen fugacity (fO2) of the liquid from which they derive. The primary Fe-Ti oxide phase in these Tasmanian dolerites is ulvospinel (now oxidized to ilmenite ‘oxidation-exsolution’ lamellae in titanomagnetite hosts) clearly demonstrating the originally low fO2 of the parental tholeiitic magma of the Mount Wellington sill.

Type
Articles
Information
Geological Magazine , Volume 125 , Issue 6 , November 1988 , pp. 573 - 582
Copyright
Copyright © Cambridge University Press 1988

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