Myrmekite as a marker between preaqueous and postaqueous phase saturation in granitic systems: Discussion and reply Discussion

EVAN R. PHILLIPS* Department of Geology, University of W. llongoug, P.O. Box 1144, Wollongoiig, N.S.W. 2S00, Australia

In his paper on myrmekite, Hibbard (1979) supported a simul- servations on in K- as summarized bv Vlehnert taneous or direct crystallization hypothesis (Phillips, 1974, p. 186) (1968, p. 192) who believed that "idiomorphic quartz" inclusions that relates the quartz and in myrmekite to an approxi- in K-feldspar megacrysts are early-formed high quartz which crys- mate binary . He believes that myrmekite, the final tallized before the K-feldspar. Further, Vlehnert (1968, p. 189) euhedral growth of and quartz, and the final growth of suggested that the xenornorphic outer part of some K-feldspar K-feldspar on phenocrysts and in the matrix develop during a megacrysts — a volume not unlike Hibbard's zone 2 — is derived postaqueous-phase saturation stage of progressive crystallization of by metasomatic introduction of K-feldspar rather than by late-stage magmatic . The inter-relationship of textures is illustrated crystallization from a fluid. in his Figure .3, and his interpretation is outlined in his Figure 6. Of The onus of proof is on Hibbard to show that the K-feldspar particular interest is zone 1 of Figure 3, where some plagioclase in- components at stage E will partition into the water-rich phase, clusions in a K-feldspar phenocrvst have partial rims but no thereby promoting the direct crystallization of only quartz and myrmekite rims. His hypothesis implies that myrmekite cannot plagioclase. His proposed hiatus in the growth of K-feldspar may occur in zone I because the crystallization sequence does not reach be more complex than necessary. In addition, Hibbard's suggested the critical stage for myrmekite development until a water- equivalence between aplite-pegmatite dikes and a K-feldspar — saturated fluid accumulates in what becomes zone 1 of the myrmekite-quartz association within the main pluton is rather K-feldspar phenocryst. tenuous. Phillips and Carr (1973, Fig. 3) have illustrated a plagioclase in- Hibbard has implied that metamorphic rocks which contain clusion which is surrounded by myrmekite (see also Voll, I 960, Fig. myrmekite must have once been granites. Let me note briefly one 7). Such myrmekite rims on plagioclase inclusions occur within region where this notion cannot apply. Despite disagreements K-feldspar phenocrysts near their cores and throughout the mega- about the geology of Broken Hill, N.S.W., there is no doubt that crysts. Certainly they occur in the volume designated by Hibbard as penologists working in this terrane believe that the sillimanite zone 1. This texture indicates that myrmekite does not form during (Stanton, 1976, p. B226) are simplv metamorphic rocks the crystallization sequence of a as outlined by Hibbard — derived from argillaceous sediments. They have no history of gran- unless it is postulated that binary eutectic-type crystallization of ite descendancy, yet they hold significant amounts of myrmekite. quartz and oligoclase occurs frequently through the progress of Likewise, the so-called "granitic gneisses" and Potosi at Bro- crystallization, and this is untenable. Nor are conditions suitable ken Hill contain abundant myrmekite, but it is doubtful if they ever for a series of pressure quenches as suggested by Hibbard for sec- crystallized originally as granites — although they once may have ond generation relationships. Equally untenable is the notion that been rhyolitic or dacitic rocks (Stanton, 1976, p. B228). liquid remains as a sheath around plagioclase inclusions held It is unacceptable to say, as Hibbard has, that symplectites of within a solid K-feldspar megacryst until conditions are right for quartz and plagioclase in some metamorphic rocks are not one late-stage eutectic-type crystallization event. myrmekite without detailing their textil ral features. If the texture The texture illustrated in Figure 2 of Phillips and Carr (1973) fulfills the requirements outlined by the majority of petrologists raises further doubts about Hibbard's model. Here a K-feldspar (Phillips, 1974, p. 185,', then the intergrovvth should be called porphyroblast occurring in a mafic enclave of a porphvritic myrmekite. Further, Hibbard's implication of the relationship be- adamellite has a partial rim of myrmekite formed on plagioclase tween granophyric intergrowths and myrmekite is unfortunate. which is part of the enclave "matrix." Hibbard's model cannot There is no doubt that myrmekite and granophyric intergrowths apply in this example because the K-feldspar porphyroblast and its are distinctive textures and have different modes of origin (Barker, "matrix" undoubtedly developed in the solid state, and there is no 1970). place a "remaining melt" (see Hibbard's Fig. 6D) could be con- In his discussion of "vein" myrmekite (his Figs. 7, 8, 9), Hibbard tained. did not consider the association between mats of fine muscovite Hibbard has noted that rare blebs of quartz occur in the cores of and myrmekite lobes (Phillips, 1980, Fig. 1G). The presence of K-feldspar phenocrysts, but he believes that quartz crystallization is mica in such intergrowths is closely related to retrograde held until a late stage when plagioclase, K-feldspar, and quartz and the destruction of Or. It probably involves form together. He proposed that crystallization of K-feldspar as outlined in reactions listed by Phillips (1973). The phenocrysts caused local entrapment of melt which later precipi- suggestion of Hibbard (lus Fig. 9) that the production of myrmekite tated as quartz inclusions. This suggestion is at variance with ob- in veins requires the rupture of an original K-feldspar, the filling of the crack with fluid wlv.ch precipitates myrmekite, and then the * Deceased May 11, 1980. additional growth of K-feldspar, dictates an unnecessary and im- The article discussed was published in the Bulletin, Part I, v. 90, probable series of events. p. 1047-1062. It is clear that retrograde metamorphism (including cataclasis) is

Geological Society of America Bulletin, Part 1, v. 91, p. 672-674, 1 fig., November li 30, Doc. no. 01107.

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an essentially solid-state phenomenon independent of magmatic REFERENCES CITED granites. Often a direct correlation exists between the amount of myrmekite and muscovite formed and the degree to which ret- Barker, D. S., 1970, Compositions of , myrmekite, and graphic : Geological Society of America Bulletin, v. 81, p. 3339-3350. rogression has advanced. This commonly leads to the almost total Hibbard, M. J., 1979, iMyrmckite as a marker between preaqueous and replacement of K-feldspar (Phillips, 1980, Fig. 1H) by abundant postaqueous phase saturation in granitic systems: Geological Society myrmekite. In such rocks, myrmekite occurs in amounts that must of America Bulletin, Part 1, v. 90, p. 1047-1062. exclude its production as a minor eutectic-like intergrowth formed Mehnert, K. R., 1968, Migmatites and the origin of granitic rocks: Amster- at a stage of magma consolidation where fluids are nearly dam, Elsevier, 393 p. Phillips, E. R., 1973, Myrmekites of exsolution and replacement origins — exhausted and the volume of potential myrmekite limited. A discussion: Geological Magazine, v. 1 10, p. 74—77. Opinions other than those of Hibbard on matters such as the 1974, Myrmekite — One hundred years later: Lithos, v. 7, p. 181- significance of the involvement of secondary albite and myrmekite, 194. the proportionality relationship between the volume of quartz in 1980, On polygenetic myrmekite: Geological Magazine, v. 117, p. 29-36. myrmekite and the An content of the associated plagioclase, the Phillips, E. R., and Carr, G. R., 1973, Myrmekite associated with alkali convex form of some myrmekite, the replacement of plagioclase in feldspar megacrysts in felsic rocks from New South Wales: Lithos, myrmekite by K-feldspar, and the relationship between systems v. 6, p. 245-260. deficient in An and the absence of myrmekite may be obtained, for Stanton, R. L., 1976, Petrochemical studies of the ore environment at Bro- example, from Mehnerr (1968, p. 200) and Phillips (1974, 1980). ken Hill, New South Wales: 4 — Environmental synthesis: Institution of Mining and Metallurgy Transactions, Section B, v. 85, p. B221- Lastly, the propostion that myrmekite is polygenetic has been de- B233. tailed recently by Phillips (1980). Voll, G., 1960, New work on petrofabrics: Liverpool and Manchester Geological Journal, v. 2, p. 503-567. ACKNOWLEDGMENTS

1 am grateful to Dr. R. A. Facer, Mrs. H. Bunyan, Mr. T. Thew, MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 25, 1980 and Mr. and Mrs. B. Watson for their assistance with this note. MANUSCRIPT ACCEPTED APRIL 22, 1980

Reply

M. J. HIBBARD Department of Geological Sciences, Mackay School of Mines, University of Nevada, Reno, Nevada 89557

Evan R. Phillips has raised several thoughtful questions regard- melt in skeletal or dendritic crystals, or portions of crystals, can be ing the late-magmatic model of myrmekite crystallization (Hib- expected, and the textures shown in Figure 1 are presented as bard, 1979), and 1 welcome the opportunity to respond to them. modest representatives. The plagioclase in Figure 1A is partly in- Phillips has observed the occurrence of myrmekitic plagioclase cluded in a K-feldspar phenocryst. Local entrapment of melt has included within the core region of some K-feldspar phenocrvsts. He occurred between the plagioclase and the K-feldspar host, later is quite correct that if this myrmekite crystallized early, along with crystallizing into a micrographic K-feldspar-quartz, plagioclase, or before the core K-feldspar, there can be no relation of myrmekite and quartz assemblage identical to the quenched matrix of the growth to aqueous phase saturation in a crystallizing granitic porphyry. The plagioclase in Figure IB has finished crystallization magmatic system initially well undersaturated in aqueous phase in a sheath of fluid in which sodic plagioclase component finished (assuming no drastic change in total pressure). Reference was made out the plagioclase with several crystal faces, the K-feldspar com- (Hibbard, 1979, p. 1050) to the occurrence of blebby quartz as- ponent added to the K-feldspar host tending to form in-facing crys- sociated with plagioclase included in the core region (zone I) of tal faces, and the quartz component filled in the intermediate space. K-feldspar phenocrvsts. That blebby quartz is actually locally ver- Locally, there was development of micrographic K-feld- micular within the plagioclase. There is no doubt then that spar-quartz. Similar relegation of final fluids has occurred in myrmekite may occur in the core region, although, at least in Figure 1C, but there has also been local myrmekitic intergrowth of examples familiar to this writer, its development is exceedingly vermicular quartz in the plagioclase. Thus, if other factors favoring minor compared to myrmekite occurring in the rim zone (zone 2). myrmekite growth are satisfied, myrmekite may crystallize in these Of course, this is true if the section taken through the phenocrvst is local entrapment regions within the core of the K-feldspar at the really through its core and not a slice near the rim itself, in which same time as major myrmekite develops at the margins of the case well-developed myrmekite would appear in the "core" of the phenocrysts, as heat is lost from the system as a whole. phenocryst. Reference was also made (p. 1052) to the possibility Phillips also suggests that myrmekite associated with the "solid that albitic rims on the small plagioclase crystals included in the state" crystallization of K-feldspar in mafic inclusions contained in K-feldspar core may represent trapped melt. This interpretation has some granitic plutons precludes crystallization of myrmekite in taken on renewed significance as the result of an investigation by anything resembling silicate melt. In my experience, most, if not all, the writer which indicates that the growth of K-feldspar and plagi- of these occurrences of morphologically well-developed K-feldspar oclase is significantly cellular in character under certain circum- megacrysts occurring in wallrock and in inclusions have been mis- stances, even in plutonic environments. Trapped or semi-trapped interpreted for a very long time and lie under a cloud of dogma. As

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Figure 1. Plagioclase included in K-feldspar phenocrysts with evidence for intergranular crystallization of trapped fluids. A: Mi- crographic K-feldspar-quartz, sodic plagioclase, and quartz of granite porphyry matrix also occurs between partly included plagi- oclase crystal (partly sericitized) and K-feldspar host. Gillis Range, Nevada. B: Partial quartz shell around plagioclase, locally with micrographie K-feldspar-quartz. Massif Central, France. C: Partial quartz shell with local vermicular quartz in the plagioclase (myrmekite). Taratap granite, southern Australia. 4

an alternative, I propose that the K-feldspar (commonly also plagi- oclase and quartz) megacrysts occurring in fine-grained, mafic-rich, rounded, dioritic, or granodioritic inclusions of manv granites and arc in fact phenocrysts, not porphyroblasts. They are 1 /2mm the phenocrysts of partly crystallized least-mafic granitic that have been mechanically mixed with more mafic magma (typi- cally diabasic). Such a hybrid system is eventually disrupted by additional surges of the more felsic system, incorporating the largely, but not completely, crystallized porphyritic mixture as in- clusions in which late crystallizing myrmekite may occur. There are many field examples fitting this model, but those of Pabsr (1928) and Thomas and Smith (1 932) are particularly vivid. K-feldspar phenocrysts and associated myrmekite may also occur in, for example, pelitic schists occurring as xenoliths in granites or as wall- rock. Mechanical injection of crystal-melt systems is indicated, and I am familiar with one example where the schists probably bowed open along foliation planes, much as a deck of cards does with lat- eral shortening, permitting introduction of the K-feldspar mega- crysts and attendant melt. Recompression of the expanded system tended to expel some of the melt, leaving the K-feldspar pheno- crysts (with some myrmekite) seemingly isolated in the schist, mimicking porphyroblasts.

I would suggest tha: the traditional interpretation of many granitic gneisses also needs revision. The case for cataclasis and partial recrvstallization of a partly crystallized granitic magmatic 1 /8mm system has been preliminarily presented (Hibbard, 1979, p. 1059-1060). The example discussed is a myrmekite-bearing, two-mica, two-feldspar granite which locally contains: (1) sillima- B nite (also garnet and cordierite), (2) unusual concentrations or clus- ters of myrmekite units, and (3) muscovite (sericite), occurring in part along the medial plane between opposing rows of myrmekite units. These types of features are taken by Phillips to indicate metamorphic, not magmatic processes, and the presence of myrmekite indicates to him a nonmagmaric origin of the myrme- kite. I proposed (Hibbard, 1979) that a magmatic system, perhaps more than 80°/) crystallized, involved in major tectonic deforma- tion will yield both magmatic and metamorphic features, and that myrmekite, if present, will be a normal component of the former (see also p. 1057-105 8 in Hibbard, 1979).

REFERENCES CITED

Hibbard, M. J., 1979, Myrmekite as a marker between preaqueous and postaqueous phase saturation in granitic systems: Geological Society of America Bulletin, v. 90, p. 1047-1062.' Pabst, A., 1928, Observations on inclusions in the granitic rocks of the Sierra Nevada: University of California Department of Geological Sci- ences Bulletin, v. 17, p. 325-386. Thomas, H. H., and Smith, W. C., 1932, Xenoliths of igneous origin in the Tregastel-Ploumanac'h Granite, Cotes-du-Nord, France: Geological Society of London Quarterly Journal, v. 88, p. 274—295.

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