Myrmekite and muscovite developed by retrograde metamorphism at Broken Hill, New South Wales

Abstract

SUMMARY. Retrograde metamorphism of gneisses and pegmatites leads in part to the destruction of feldspar and its replacement by late-stage lobate myrmekite and muscovite. Reactions promoted by retrogression suggest a range in volume of quartz production that may supplement that developed by exsolution and lead to deviations from the strict proportionality relationship suggested by previous workers. There is no need, however, to propose that quartz in myrmekite originates by constriction of pre-existing quartz within exsolved albite.RECENT papers by Phillips and Ransom (I97O) and Shelley (197o, have drawn attention to the co-existence of muscovite and myrmekite in certain metamorphic rocks. Concomitant with this association in some gneisses is the apparent paucity of potash feldspar. This stands in contrast to many other occurrences, particularly in igneous rocks, where rim and intergranular myrmekite, usually without muscovite but commonly with secondary albite, are associated with well-developed perthite (e.g. Phillips, I964).Preliminary petrographic investigations of the Potosi gneiss at Broken Hill revealed the development of a lobate myrmekite associated with fine-grained muscovite and a corresponding low potash feldspar content (Phillips and Ransom, x97o, P. 729). It was suggested that such an association was connected with retrograde metamorphism affecting some of the Broken Hill gneisses (Hobbs et al., 2968, p. 296; Vernon, 1969; Vernon and Ransom, 1971). Additional work has shown that a similar myrmekitemuscovite association occurs in some deformed pegmatites of the Broken Hill region. This paper presents descriptions of the myrmekite-muscovite intergrowths occurring in both the pegmatites and the gneisses and discusses their mode of origin.Myrmekite and muscovite in the pegmatites. Coarse-grained pegmatites occur in many of the high-grade metamorphic rocks of the Broken Hill area (Vernon, 1969, PP. 33-4). Where intersected by retrograde schist-zones they have been extensively deformed and partly recrystallized. Deformed pegmatites have been examined in detail in the area 9

Myrmekites of exsolution and replacement origins—a discussion

“…Under (1), there is no conflict. It is clear that Ashworth (1972) observed a different reaction from that described by Phillips et al (1972).…”

“…I am certainly willing to compromise by allowing roles for other factors. Even though excess silica apparently remained in the myrmekite volume when generated by the non-exsolution reactions whose postulation is necessitated by muscovite participation in the rocks described by Phillips et al (1972), the question why myrmekite formed at all would still seem amenable to treatment in terms of constraints essentially corresponding to those invoked by Ashworth.…”

“…While noting the pos-ŝ ibility of quartz proportionalities less than the ideal, I did not admit the possibility that values greater than ideal could theoretically arise by similar reactions. Phillips et al (1972) clearly correct this omission and tentatively record an actual occurrence. It now seems that alternative mechanisms could not be distinguished by deviant quartz pror portionality as I had suggested (Ashworth, 1972, p. 46).…”

“…As I have stressed elsewhere (Phillips, 1964(Phillips, ,1972 many of the common forms of myrmekite in granites are very similar in their spatial relationships to such albite and it seems logical to suppose that myrmekite has a similar origin to secondary albite. (I would also include here the quartz-bearing plagioclase found in some perthites.)…”

“…If Dr Ashworth tentatively accepts this possibility we may find some grounds for a compromise. The three equations listed above do not indicate where the quartz derived from the reactions is actually situated and I can envisage that it may go into three distinct positions within a pseudomorph: (i) discrete quartz grains, (ii) quartz intergrown with muscovite as is suggested by Ashworth, and (iii) 'excess' or supplementary quartz associated with plagioclase in a myrmekitic intergrowth as suggested by Phillips et al (1972). The reasons for the quartz occurring at any one of these positions, rather than another, are not apparent to me at present, but it appears that the overall reaction is promoted by retrograde metamorphism, and that ultimately a fine-grained aggregate of quartz, muscovite and feldspar is produced.…”