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

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

ABSTRACT The Sand Springs model states that the symplectic intergrowth of and , namely myrmekite, is the chief marker of A nonreplacive, nonexsolution model of myrmekite growth is aqueous-phase saturation, and that crystallization style prior to based on textural relationships in the Sand Springs porphyritic growth of myrmekite contrasts sharply with that occurring after. , west-central Nevada. A sequence of crystallization is Myrmekite in the Sand Springs rock is well developed, and its rela- divided into (1) a preaqueous-phase saturation stage, characterized tionship to other silicate phases is exceptionally clear. The origin of by major growth of (zoned), quartz, and K- myrmekite depends upon growth in late magmatic fluids at and (phenocrysts), and (2) a postaqueous-phase saturation stage somewhat beyond the time of aqueous-phase saturation. This is characterized by myrmekite, final euhedral growth of plagioclase sharply at variance with replacement and exsolution models to and quartz, and final growth of K-feldspar phenocrysts and most which myrmekite is usually referred. K-feldspar of the matrix, including some crystals with adularia- Myrmekite is especially useful in this new context because it is habit characteristics. Myrmekite results from micropressure widespread in granitic rocks and is easily identified in thin section. quenching during the separation of an aqueous phase as crystalli- According to the proposed model, the presence of myrmekite sig- zation progresses. The occurrence of myrmekite as lobate units on nifies at least a partial magmatic history for the rock in which it plagioclase, extending into K-feldspar, results from precipitation of occurs. For example, if myrmekite occurs in an otherwise oligoclase (the basic ingredient of myrmekite) as local continua- metamorphic-looking granitic , then, by the model, it is likely tions of plagioclase growth from a melt that simultaneously expels that the gneiss is a magmatic rock that has been modified by either an aqueous-rich fluid enriched in K-feldspar component. Late a synmagmatic or postmagmatic deformational event. In another K-feldspar crystallizes from the aqueous-rich fluid, filling in around context, the abundance of myrmekite in aplite-pegmatite systems the myrmekite. Quartz in myrmekite represents the inability of indicates that fluids were extracted from a parent system late, but silica to diffuse from the quenched melt and occurs as vermicules just prior to the myrmekite stage. The deuteric and hydrothermal chiefly in accord with the principles of binary eutectic crystalliza- stages to be expected beyond the myrmekite stage in a crystallizing tion. granitic must sooner or later be characterized by replace- The Sand Springs myrmekite model is tested by evaluating its oc- ment and alteration phenomena. This occurs in the Sand Springs currences in aplite-pegmatite systems, in granitic , and in pluton, but it can be expected to be more intense in tectonic situa- the hydrothermal secondary K-feldspar environment. Myrmekite tions conducive to concentration of very late fluids, to the point commonly occurs in all but the hydrothermal environment, which where major reworking processes acting on earlier fabrics can be is postmyrmekite, and a fundamentally magmatic origin can be expected to produce a final rock quite unlike its silicate-melt—stage reasoned for the other rock types if the tectonic environment during predecessor. Without knowledge of this previous magmatic history, crystallization is also considered. as deduced from the presence of myrmekite, interpretation of the rock's origin would be erroneous or incomplete. INTRODUCTION SAND SPRINGS PORPHYRITIC GRANODIORITE: This paper deals with the magmatic-hydrothermal boundary RECORD OF TRANSITION FROM SILICATE MELT problem (Burnham, 1967) from a textural point of view, and is TO AQUEOUS-RICH FLUID therefore concerned with distinction between textures resulting from crystallization in silicate melts and those deriving from The Sand Springs pluton is a stock-size, Sierran-type body, pre- "deuteric" and "hydrothermal" phenomena. The study develops a viously studied by Beal and others (1964). It consists of porphyritic crystallization model of the Sand Springs porphyritic granodiorite granodiorite to the east and nonporphyritic, more mafic granodiorite of west-central Nevada. In the early stages, crystallization of sili- to the west (Fig. 1). Typical concentration of K-feldspar pheno- cate melt undersaturated in aqueous phase proceeds in the quinary crysts in the porphyritic granodiorite is shown in Figure 2, A; how- granitic system, to and then along the cotectic line with quartz and ever, locally there is a clustering of phenocrysts (Fig. 2, B). Aplite- two-feldspar phases crystallizing simultaneously. The system then pegmatite dikes, a few centimetres to about 6 m thick are distribu- changes dramatically, behaving more like a hydrothermal system ted mainly in a belt running along the western crest of the range than a melt system, as aqueous-phase saturation occurs. In the Sand (Fig. 1), roughly within a gradational contact zone between the two Spring pluton, this change is marked by a visible textural- granodioritic phases (J. H. Schilling, 1977, personal commun.) The mineralogical record, providing an unusual opportunity to identify dikes range from pure aplite, to pure pegmatite, (Fig. 2, C), to pure a presaturation magmatic fabric from postsaturation phenomena. quartz veins, although zoned dikes with aplite margins and a peg-

Geological Society of America Bulletin, Part I, v. 90, p. 1047-1062, 10 figs., 3 tables, November 1979, Doc. no. 91107.

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ir silt, sand, gravel ^ _O I |QTv volcanic rocks intrusive breccia and andesite, M l t and rhyolite dikes (shown g diagrammatically) S Ü ' Sand Springs pluton: is eastern phase; porphyritic s granodiorite western phase; mafic grano- both units cut by aplite- pegmatite dikes (shown diagrammatically) of essentially same age

metasedi mentary- !{a metavolcanic rocks

high-angle fault

thrust fault

5 km J

Figure 1. Location and generalized geologic map of Sand Springs Range, Nevada. Modified from Schilling (1964).

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_ f ' s « * J W XáJü < matite core are common. Within the aplite-pegmatite dike belt, there is a marked tendency for northwest orientation parallel to a major joint system, and since the ages of the dikes are probably very close to that of the host pluton (Schilling, 1964), at least some of the jointing could have occurred during the late stages of mag- matic consolidation.

Petrography of Porphyritic Granodiorite

The combined phenocryst-matrix mode of a typical sample (Ta- -S ble 1) is a granodiorite (Streckeisen, 1976), and the average plagio- X clase composition is An2i (Table 3). K-feldspar phenocrysts are typ- ically 2 to 2.5 cm, somewhat elongate parallel to crystallographies; a few crystals are as large as 6 cm. The "matrix" grain size is uni- f: ? . - 'V form, with crystals chiefly 0.5 to 2 mm maximum length. Bulk-rock .1 cm, chemistry (Table 1) is taken from Weyler and Volborth (1964) and P. Beaulieu (1977, Nevada Bureau of Mines and Geology, unpub. analysis). Figure 3 shows typical textural relationships in an idealized sec- tion from the core of a K-feldspar phenocryst extending into the t matrix of the rock. The following textures and minerals charac- terize zone 1 (Fig. 3), which is the core region of the phenocryst. » • j. Vf V. K-feldspar is white to pale buff, slightly perthitic, with probable or- * * i - thoclase structural characteristics (Table 2A). 2Vex is about 64°, the -i composition in optically homogeneous parts is Or87, and Ba con- tent is relatively high (Tables 2B, 2C). A single Carlsbad twin is V.. common, but cross-hatched twinning is virtually absent except, I rarely, in very local areas adjacent to plagioclase inclusions. A faint ?.. r * j * •íí zoning in the K-feldspar is characteristic and is defined by both ex- tinction angle differences in otherwise optically homogeneous crys- * , r1 Í1 VJ - > V tals and by minor exsolved ablite. Plagioclase occurs as oriented M ,

TABLE 1. CHEMICAL ANALYSIS, NORMS, AND MODE OF t . , i «< .t .. ' SAND SPRINGS PORPHYRITIC GRANDIORITE CIPW norm (molecular) Mode-modified norm

Chemical analysis

Si02 68.78 qz 24.8 qz 24.8 AI2O3 14.96 or 18.5 K-spar 15.9 Fe203 2.35 ab 37.4 ab 40.4 Na20 4.42 an 11.7 an 10.7 K2O 3.12 en 1.6 biot 4.5 CaO 2.61 fs 0.6 horn 0.2 r- '-W;-^' < MgO 0.66 mt 1.8 mt 1.7 rWi. Ti02 0.32 il 0.6 ap 0.4 MnO 0.05 ap 0.4 sph 0.7 P2O5 0.18 en 1.6 plag = An2o.9 Total 97.45 plag = An2:

• •'TR 'V-'- Mode Matrix K-feldspar Rock Q + A + P : 100 (90.3%) phenos (9.7%) Quartz 24.1 0.1 24.2 25.5 Plagioclase 50.7 1.8 52.5 55.5 > , v • - - . K-feldspar 10.5 7.6 18.1 19.0 1 • i A- P J* » f ' ¡t 'f W' i (T * Biotite (chlorite) 3.4 Tr. 3.5 Hornblende 0.05 Tr. 0.1 Figure 2. Mesoscopic characteristics of Sand Springs porphy- Opaque 0.58 Tr. 0.6 ritic granodiorite. A: Porphyritic granodiorite with typical Sphene + apatite K-feldspar phenocryst distribution. B: Clustered phenocrysts with + zircon 0.86 Tr. 0.9 color distinction between white core (lighter) and pink rim (darker) Note: Chemical analysis from Weyler and Volborth (1964, p. 210), ex- K-feldspar more apparent than in A. C: Pegmatite dike in cept Na20, MgO, and P2O5, from Beaulieu (unpub.); FeO taken as Fe203. porphyritic granodiorite. Graphic K-feldspar-quartz (darker) and Norms assume Fe203: FeO = 1:2; mode-modified norm from Nelson and Hibbard (1976). oligoclase-quartz (lighter) intergrowths.

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inclusions in the K-feldspar phenocrysts and modally makes up as K-feldspar host (Table 3). Plagioclase-plagioclase synneusis (Vance, much as 20% of the phenocryst (Table 1). Plagioclase crystals 1969), rarely with unifying overgrowth, is rather common. Zone 1 range from perfectly euhedral forms, with or without what are is further characterized by inclusions of sphene, hornblende, bio- probably albitic overgrowths, to very irregular shapes, locally with tite, apatite, zircon, and opaques; the first three are commonly blebby quartz associations, with or without albitic rims. The oriented parallel, along with plagioclase, relative to growth surfaces plagioclase has oscillatory zoning with an overall normal trend. of the K-feldspar host. Very rare, rounded or irregular blebs of Although the rim compositions of plagioclase are clearly not as quartz occur in the core region of phenocrysts. albitic (exclusive of the albitic overgrowths mentioned above) as Zone 2 comprises the marginal shell of K-feldspar phenocrysts those of large matrix plagioclase crystals, there is no significant dif- (Fig. 3). These K-feldspar rims have a distinct salmon-pink color, ference in their rim composition from inner to outer core of the contrasting sharply with the white cores (Fig. 2, B). Rim K-feldspar

ZONE 3 ZONE 2 ZONE 1 Figure 3. Textural characteristics from core to rim of K-feldspar phenocryst and of matrix of Sand Springs porphyritic granodiorite. Zone 1 contains white K-feldspar and preferentially oriented plagioclase inclusions. Zone 2 has major myrmekite, pink K-feldspar (includ- ing adularia-habit crystals), and euhedral quartz. Zone 3 is matrix, consisting of fully developed plagioclase and quartz and containing some pink K-feldspar and myrmekite.

TABLE 2A. STRUCTURAL DATA FOR K-FELDSPAR IN SAND SPRINGS PLUTON

K-feldspar 201 060 204 131-131 Suggested structural state

Phenocryst core 21.087 41.765 50.753 Weak broadening Orthoclase Phenocryst rim 21.046 41.791 50.634 Some broadening Intermediate Finer grained pegmatite 20.994 (est.) 41.792 50.671 Double peak Intermediate microcline Coarse-grained pegmatite 20.979 (est.) 41.839 50.649 Broad Intermediate microcline Note: X-ray using three-peak method of Wright (1968).

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differs in several other respects: (1) lack of visible zoning, (2) even Biotite of zone 2 is larger and more abundant than in zone 1; com- less perthitic , (3) more areas of cross-hatched twinning, (4) monly, it is partly chloritized and may contain epidote in cleavage 2Va values similar but somewhat larger than those of the interior planes. Hornblende, sphene, apatite, zircon, and opaques also (Table 2B), (5) an apparent structural state of intermediate micro- occur in zone 2. cline rather than orthoclase (Table 2A), (6) the presence of a few Zone 3 is the matrix of the porphyritic granodiorite, and the dis- small well-formed salmon-pink K-feldspar crystals, which therefore tinction between it and zone 2 is largely geometric. K-feldspar tends lie in K-feldspar of the same color, (7) a composition slightly en- to occur intergranularly relative to plagioclase, but otherwise it is riched in the orthoclase component (Table 2C), and (8) a sig- identical to that in zone 2. Well-zoned plagioclase has rims and an nificantly lower Ba content than the core (Table 2C). The small average composition more albitic than plagioclase included in K-feldspar crystals are absent from the inner part of the pheno- K-feldspar phenocrysts. Myrmekite is common at K- crysts (zone 1). These crystals are somewhat elongate parallel to c feldspar—plagioclase contacts. Some plagioclase cores are altered to and are characterized by {111}, { 001}, {101}, and { 010}, forms (Fig. sericitic white mica, epidote (or calcite), and albite. 4, A), similar to Kalb's (1924) Zellertal crystals of "adularía habit" (see also, Smith, 1974, p. 256). Plagioclase of Zone 2 is essentially Interpretation of Porphyritic Granodiorite Texture restricted to those parts where large matrix crystals face the pheno- crysts and therefore lie partly in zone 2 K-feldspar. The smaller, A series of sketches (Fig. 6) show the progressive construction of more calcic plagioclase inclusions that have pronounced crystallo- the porphyritic granodiorite texture. The objective is to demon- graphy orientation relative to the host K-feldspar are generally ab- strate how growth of myrmekite fits naturally into a sequence of sent in zone 2. "Overgrowths" of well-developed myrmekite (Fig. crystallization of a water-bearing granitic magma. As discussed be- 4, B) on large plagioclase crystals are characteristic of this zone, low, a bulk magma composition in the ( and therefore myrmekite occurs at the contact between plagioclase of Steckeisen, 1976) and granodiorite range is ideal for the growth and K-feldspar where it is so commonly observed in other rocks. of myrmekite. A porphyritic rock in this range, particularly one The myrmekite occurs most commonly as extensions of the plagio- with a coarse-grained matrix, such as the Sand Springs rock, is clase, locally finishing growth of the plagioclase with euhedral sur- especially well-suited to show the sequence of crystallization, inas- faces, as would be expected of nonmyrmekitic plagioclase (Fig. 5, much as crystallization probably occurred at low undercoolings E). However, "wartlike" myrmekite, with or without euhedral sur- (Swanson, 1977), and that crystallization has been uncomplicated faces, also occurs attached to plagioclase (Fig. 5, A). Rarely, but by mechanical factors likely to be important if there had been rapid very significantly, some myrmekite is in very irregular contact with and turbulent intrusion of partly crystallized magma. K-feldspar, and some quartz vermicules are isolated in the Stage A (Fig. 6) is nucleation and initial growth (most calcic) of K-feldspar (Figs. 4, D; 5, B). Quartz of zone 2 occurs as (1) small plagioclase, as would be expected in a cooling magma of euhedral crystals isolated in K-feldspar (Fig. 3), and (2) large grains granodioritic composition. Stage B begins with the nucleation of a of the matrix, euhedral against K-feldspar of zone 2 (Figs. 3; 4, C). few K-feldspar crystals, indicating that the melt composition has moved to the boundary hypersurface between the plagioclase and TABLE 2B. OPTICAL DATA FOR K-FELDSPAR IN K-feldspar fields, beyond which the following occurs: (1) simul- SAND SPRINGS PLUTON taneous crystallization of plagioclase and K-feldspar, (2) repetitive synneusis of the small plagioclase crystals onto the growing sur- No. of No. of 2V„ faces of K-feldspar, with continuous incorporation of the plagio- crystals measurements range average clase as K-feldspar grows (see, for example, Frasl, 1954; Maucher, Phenocryst core 6 12 58-73 64 1943; Schermerhorn, 1956; Hibbard, 1965), and (3) continuous Phenocryst rim zoning of plagioclase, prior to complete incorporation, along a and matrix 10 22 59-80 66 normal trend with oscillations, resulting in a lower zoning range (least sodic rims) of plagioclase the earlier it is included in growing K-feldspar; that is, the plagioclase is protected from further

TABLE 2C. MICROPROBE DATA FOR K-FELDSPAR IN SAND SPRINGS PLUTON TABLE 3. PLAGIOCLASE COMPOSITION IN SAND SPRINGS PORPHYRITIC GRANODIORITE Rim Core Crystals in Crystals in Matrix All crystals innermost outermost core crystals in rock Si02 64.10 61.60 core of of K-feldspar AI2O3 18.30 18.48 K-feldspar phenocryst Na20 0.90 0.48 phenocryst K2O 15.12 14.73 CaO 0.05 0.01 BaO 0.27 1.99 Highest value 32 34 25 (core) Total 98.74 97.67 Mean of highs 28 29 23 Norm (wt %) Lowest value 11 16 11 ab 7.6 4.1 (rim) or 89.4 87.0 Mean of lows 20 20 13 an 0.2 0.05 Weighted mean 24 24 20 21 cn 0.1 4.9 for all crystals Total 97.9 96.1 Note: A-normal method using data of Tobi and Kroll (1975). Values are Note: Analysts: R. Drumheller and R. Wittkopp. percent by weight anorthoclase.

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growth. A number of points need further discussion. Since some of next to the growing crystals. Relatively high Ba content of this ear- the hornblende, biotite, zircon, apatite, opaque oxides, and sphene lier K-feldspar relative to that of marginal K-feldspar in stage F is included in the innermost core of K-feldspar phenocrysts, they (Table 2) is consistent with Kerrick's (1969) conclusion that the must have at least begun crystallization in stage A. Plagioclase of core region of phenocrysts in the Cathedral Peak prophyritic quartz similar porphyritic rocks has been found to have progressively monzonite of Yosemite crystallized earlier and probably at higher more sodic rims the farther out from the K-feldspar core they occur temperatures than marginal K-feldspar. Some "embayments" as (Hibbard, 1965). Because the data (Table 3) in this case do not in- defined by zoning (Fig. 3) in the very core of some K-feldspar dicate such a progressive change, it is suggested that exsolution of phenocrysts might be interpreted as further evidence of resorption albite from the K-feldspar host has resulted in abnormally sodic during the early stages of plagioclase and K-feldspar crystallization. rims of plagioclase in the innermost core of the K-feldspar pheno- The occurrence of a few quartz blebs in early K-feldspar, some cryst, and the subtle difference in plagioclase rim composition to be closely associated with included plagioclase, might prompt a con- expected from core to rim of the phenocrysts has been obscured. clusion that quartz began to crystallize very early, but alternatively Some of the plagioclase inclusions are very poorly formed; perhaps it may be that there was local entrapment of melt, due to ir- some turbulence required for K-feldspar—plagioclase synneusis is regularities in plagioclase or K-feldspar growth surfaces, that could also represented by partially resorbed (and therefore irregular) later precipitate small amounts of quartz along with a little albitic plagioclase crystals as the entire system is intruded to regions of plagioclase and K-feldspar. A similar model was discussed by lower confining pressure during stages A and B. The question arises Schermerhorn (1956, p. 335—336). as to the possible genetic relation of albitic rims to ragged plagio- Stage C (Fig. 6, C) is the beginning of major quartz crystalliza- clase crystals. In this case, albitic rims seem to be more related to tion. Thus the trivariant K-feldspar-quartz-sodic plagioclase exsolution processes than to decalcification (compare Scher- boundary hypersurface (commonly referred to as the "cotectic merhorn, 1956, p. 346), since both irregular and euhedral plagio- line") in the isobaric quinary (Ab-An-0r-Q-H20) granitic system clase crystals commonly have what appear to be overgrowths of has been reached. Since by now, K-feldspar crystals are large and albite, rather than albite as a product of replacement. are nearly their final phenocryst size, few, if any, quartz crystals K-feldspar growth during stage B is characterized by some weak can, by synneusis, be included in them, and therefore quartz is rele- zoning, probably reflecting minor fluctuations in melt composition gated largely to the coarse-grained matrix of the rock. Plagioclase

t

B D Figure 4. Relationships in zone 2 (Fig. 3). A: "Adularia-habit" -feldspar crystal (center, in extinction) in marginal K-feldspar of phenocryst. B: Myrmekite as extension of plagioclase crystal, facing ^-feldspar. C: Euhedral quartz (two large crystals) of matrix facing K-feldspar of zone 2 (Fig. 3). Oriented plagioclase in K-feldspar of: ne 1 (upper right). D: Partly replaced myrmekite with quartz ver- micules isolated in replacing K-feldspar.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/11/1047/3444251/i0016-7606-90-11-1047.pdf by guest on 01 October 2021 Figure 5. Myrmekite characteristics in Sand Springs porphyritic granodiorite (A through F) and granite of Gillis Range, Nevada (G.). A: Euhedral myrmekite unit (center) at- tached to larger plagioclase crystal (both in extinction). B: Myrmekite unit partly euhedral (lower right), partly replaced by K-feldspar, with isolation of some quartz vermicules (left side of myrmekite "crystal"). C: Coarse and fine myrmekite. Thinner quartz vermicules in outer (later) more sodic oligo- clase zone. D: Marked contrast between coarse and very fine (rim) myrmekite. E: Myrmekite as simple continuation of euhedral plagioclase crystallization. Note lack of many quartz vermicules in plagioclase zone (left) equivalent to zone with their abundance (center and right). F: Myrmekite "vein" be- tween two differently oriented K-feldspar crystals. G: Myrmekite "vein" in single K-feldspar crystal. Note displaced S 0.5 mm Carlsbad twin along plane containing myrmekite. Quartz Xi*, vermicules not readily visible.

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crystals not included in K-feldspar phenocrysts continue to grow forced to crystallize along with ojigoclase as an intergrowth. during this stage and progressively zone to more and more sodic "Wartlike" growth of myrmekite units might also be controlled by rim compositions (Table 3). K-feldspar phenocrysts continue to the quenches, namely by way of rapid growth at localized sites grow, but since the growth surface of phenocrysts is relatively rather than by distributive crystallization requiring considerable enormous, little change in crystal size occurs at this time. It is pro- diffusion of growth materials. Myrmekite, so interpreted, grows di- posed that the remaining magma at this stage is still unsaturated rectly from a late magmatic fluid, and therefore by a completely with respect to aqueous phase. nonreplacive mechanism. Stages D, E, and F (Fig. 6) represent a major and highly sig- Stage F (Fig. 6) is a postsaturation aqueous-rich environment in nificant change of conditions, during which the remaining fluids of which essentially one final phase must complete crystallization, the magma are consumed. It is, first of all, the stage of aqueous- namely the pink K-feldspar of the marginal part of phenocrysts, phase saturation in the remaining melt (stage D). From this point that in the matrix, and the "adularia-habit" crystals. The very latest on, crystallization of remaining plagioclase, K-feldspar, and quartz fluids in stage F are dominantly aqueous. This fluid is very reactive is from a fluid in which the aqueous phase plays a major role, and and very locally replaces myrmekite, leaving quartz vermicules iso- in which the style of crystal growth is quite unlike that of earlier lated in final K-feldspar (Figs. 4, D; 5, B; 6, F). Thus, the fluid that crystallization from the unsaturated magma. What is the evidence has just previously precipitated myrmekite has now reversed the suggesting that an aqueous fluid (or at least a melt near saturation) process and is taking oligoclase back into the aqueous-rich phase is present during crystallization in these stages? Freedom of growth and precipitating K-feldspar. The events at this time are controlled and euhedral form characterize such conditions in part, and atten- more by the activities of aqueous fluids containing dissolved alkalis, tion is directed to two occurrences of final quartz growth; namely rather than silicate melt equilibrium phenomena. The replacement the new euhedral crystals (Fig. 3) in fluid that will later crystallize of oligoclase is due to the instability of the anorthite component, as marginal pink K-feldspar, and the euhedral termination of which is readily converted to epidote (calcite), which relocates quartz crystals of stage D (Fig. 6, D) facing this same late fluid be- elsewhere, not uncommonly in chloritized biotite. The albite com- tween phenocrysts and matrix (Fig. 4, C). Further evidence of crys- ponent of oligoclase, which also must be removed, perhaps goes tallization in this aqueous active environment is represented by a into the very latest K-feldspar or relocates as intergranular "sec- few, but highly significant, "adularia-habit" K-feldspar crystals ondary" albite. Chloritization of biotite and sericitization of that occur in the marginal phenocryst zone along with the small plagioclase cores are also tied to the latest effects of these euhedral quartz crystals. It is not suggested that these are adularia "deuteric" or "hydrothermal" fluids. crystals, but it is thought that an aqueous-rich fluid environment has favored growth of a few new K-feldspar crystals with TEST OF THE GENERALITY OF adularia-like morphology. SAND SPRINGS MYRMEKITE MODEL Stage E (Fig. 6) is marked, very importantly, by the growth of myrmekite. It is suggested that the separation of aqueous phase The proposition that myrmekite is a product of direct crystalli- from melt results in two important phenomena. First, it produces zation of late magmatic fluids in the Sand Springs porphyritic an aqueous-rich fluid into which K-feldspar components are prefer- granodiorite is at variance with various metasomatic and exsolu- entially partitioned, ending earlier coprecipitation with quartz and tion replacive models popular from the time of Becke (1908), sodic plagioclase and preventing crystallization of K-feldspar as an Schwantke (1909), Spencer (1945), Drescher-Kaden (1948), and, additional constituent of myrmekite. Second, as the aqueous phase more recently, Shelley (1964), Phillips and others (1972), and many separates from melt, domains of overpressure develop, which when others. Summaries of myrmekite research (Smith, 1974); Phillips, relieved by microtectonic events, result in micropressure quenches 1974) include a few references to models similar to the direct (pri- freezing remaining melt (in spite of an overall lowering of the liq- mary) one proposed herein. Among the latter, Spencer's (1938) uidus by saturation of the melt in aqueous phase; Fenn, 1977). model is notable, and it is comparable except for the concept of two Since K-feldspar component at this point is partitioning into the coexisting but immiscible silicate melts. The following series of aqueous-rich phase itself, remaining melt to be quenched must have questions concern well-known characteristics of myrmekite and are the composition of sodic plagioclase (oligoclase) and quartz — es- answered in the light of the Sand Springs model. sentially the composition of myrmekite. The quenching environ- Why does myrmekite occur in the contact region between ment might account for the presence of quartz vermicules in oligo- plagioclase and K-feldspar? There are few exceptions to this rule in clase, because some quartz component may not have time to diffuse the Sand Springs example, only rare cases of "vein" myrmekite to major centers of earlier quartz crystallization and is effectively lying between two K-feldspar crystals (described later) or where

•

Figure 6. Schematic crystallization history of Sand Springs porphyritic quartz monzonite (granite). A: Beginning of plagioclase crystal- lization (most calcic). B: Early growth of K-feldspar. Attachment and incorporation of oriented plagioclase, continued growth of less calcic plagioclase as zones on earlier plagioclase and probably as newly nucleated crystals. C: Early growth of quartz, simultaneous with further crystallization of more sodic plagioclase and K-feldspar phenocryst enlargement. Melt not yet saturated in aqueous phase. D: Euhedral quartz growth facing and isolated in remaining melt. Continued growth of matrix plagioclase. Melt just saturated in aqueous phase. E: Pressure quench and major growth of myrmekite on existing plagioclase crystals. F: Postmyrmekite growth of K-feldspar as (1) continuation of phenocryst, (2) matrix fill-in, and (3) a few adularia-habit crystals. Latest fluid is very aqueous rich and locally replaces earlier plagioclase and myrmekite. Chloritization of biotite, production of minor epidote, and local sericitization of plagioclase cores occurs.

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there are "clusters" of myrmekite units, not clearly related to any toward K-feldspar. Usage is not strictly descriptive and has, unfor- major feldspar phase. The location of myrmekite in most cases is tunately, led to the conclusion that the myrmekite must have re- simply a function of where its components originate. The quartz placed K-feldspar. Proportionally little consideration has been and oligoclase of myrmekite crystallize from a melt that is expelling given to those occurrences of myrmekite that are (1) simply con- an aqueous-rich fluid enriched in K-feldspar component, a fluid tinuation of growth of earlier plagioclase cyrstals (Fig. 5, E), which that will subsequently precipitate relatively pure K-feldspar after would not even be noticed if the quartz vermicules were absent, (2) and around the myrmekite units. Thus, where myrmekite crystal- euhedrally zoned like any other plagioclase crystals (Fig. 5, C), in- lizes, so does the K-feldspar. The amount of myrmekite (size and/or dicating growth advancement as euhedral crystals, and (3) number of units) that crystallizes from the late fluid is controlled by "wartlike" in overall aspect, but whose final growth surfaces are what oligoclase and quartz components remain in this melt at the euhedral (Fig. 5, A and B). The conclusion is inescapable that many myrmekite stage. The question then becomes one of the attachment myrmekite growth units have no replacive geometry at all, requir- of myrmekite to the plagioclase side of the intergranular relation. ing a reassessment of all myrmekite. The convex character of Earlier plagioclase usually is present in contact with late myrmekite in the context of the Sand Springs myrmekite model can K-feldspar—rich fluids, and continued growth of oligoclase (along be taken as analogous to crystals facing miarolitic cavities (Hib- with its quartz vermicules) as a rim zone on this plagioclase would bard, 1977), although the rounded forms of some myrmekite units be favored over homogeneous nucleation of new oligoclase indicate that growth was not into such a low-viscosity vapor phase. (myrmekite) crystals that would eventually be enclosed in final What is the meaning of more than one "layer" of myrmekite? crystallization of K-feldspar. Furthermore, epitaxial growth of There are two types of "second-generation" relationships: (1) those oligoclase on older plagioclase would be favored, on structural in which a myrmekite layer or unit against K-feldspar is "finer grounds alone, over epitaxial (or nontaxial) growth on previously grained" (that is, the quartz vermicules are smaller and more crystallized K-feldspar (that is, the surface of partly crystallized closely spaced), contrasting with a "coarser grained" myrmekite K-feldspar phenocrysts), since the K-feldspar and plagioclase struc- toward the plagioclase host (Fig. 5, C and D), the difference being tures are quite different (see discussion in Smith, 1974, p. 505). If related to an increase in quenching and/or compositional changes this were not true, plagioclase mantling K-feldspar in granitic rocks (note the more sodic zone containing the smaller vermicules in Fig. would be far more common than it is. 5, C), and (2) those in which there is a second period of myrmekite Why does oligoclase crystallize (as a component of myrmekite), growth superposed nontaxially on another, which by the Sand whereas the K-feldspar component remains in the aqueous-rich Springs model would be related to a second pressure quench during fluid? The idea of partitioning of these components in two fluid the myrmekite stage, perhaps controlled by events external to the phases, such as is proposed in the Sand Springs model at the time of immediate site of myrmekite growth. myrmekite growth, is not a new one: "Although separation of Why does quartz, as a component of myrmekite, crystallize along constitutents between the two fluids never is complete or even with oligoclase? It should be pointed out that even the replacement nearly so, a strong tendency nonetheless exists for preferential de- models would have these two phases crystallizing simultaneously, velopment of common potash-bearing minerals via the aqueous and there are only a few references to mechanisms involving quartz phase and soda-bearing minerals via the melt during most stages in replacement of plagioclase to form the myrmekite. The relationship which both kinds of minerals are being formed" (Jahns and Burn- of quartz to oligoclase in myrmekite approximates a binary eutectic ham, 1969, p. 856). To my knowledge, there has not yet been an system. Prior to the myrmekite stage in the Sand Springs model, in-depth explanation for this partitioning. Nevertheless, one related major quartz crystallization occurred, indicating that the quartz factor in the partitioning seems to be clear: the importance of the boundary field certainly was reached. Oligoclase and quartz would anorthite component in preferentially "tying up" the albite compo- be expected to crystallize simultaneously from melt if there is con- nent (to form oligoclase) that might otherwise go into K-feldspar, comitant partitioning of K-feldspar component into the separating or perhaps remain in solution beyond the myrmekite stage. aqueous-rich fluid phase. The characteristically "wormy" rods Why is oligoclase far more common as a component of myrme- (vermicules) of quartz in myrmekite are a question in themselves. kite than albite? In the crystallization of intermediate granitic com- There is a definite tendency for these rods to lie perpendicular to positions, plagioclase will usually zone to oligoclase (not albite) growth direction, including fan-shaped geometry in many lobate rims, and if other conditions are favorable, myrmekite with oligo- forms. Striking analogy can be drawn between myrmekite and clase should develop. In some , especially alkali granites, two-phase crystallization in metal alloy systems, where freezing where the anorthite content of the system is exceedingly low, albite velocities and growth kinetics control crystal morphology, spacing would be expected to affiliate with alkali feldspar if hypersolvus, of rod structures, and whether one phase will be continuous or dis- rather than form albite myrmekite units, and this could be referred continuous (Chadwick, 1963; Tiller, 1968; Flemings, 1973). The to as the "albite restriction." Subsolvus albite granites are rare, but main points are (1) in a quenched system where myrmekite is postu- those that become subsolvus as crystallization proceeds and lated to form, the rate of crystallization of quartz is much greater aqueous-phase enrichment occurs, should crystallize myrmekite than diffusion rates that otherwise would allow silica to diffuse to with an albite host, if quartz is available and other myrmekite- major centers of quartz growth already begun at an earlier stage, producing conditions are satisfied. Such myrmekite has been ob- and (2) rod eutectic structures are favored over granular served (Fig. 7, B) but the plagioclase of this myrmekite is about oligoclase-quartz mosaics by essentially the same theoretical factors as applied to metal alloys (Tiller, 1968). AnI0, not An0_5. In systems with only modest amounts of anorthite component, oligoclase myrmekite might well associate with a Why does myrmekite form instead of granophyric or graphic nonmyrmekitic intergranular albite. quartz-plagioclase intergrowths? Blebby and coarse vermicular Do the shapes of myrmekite units indicate anything about their quartz in plagioclase is commonly referred to as a granophyric tex- origin? Terms such as warlike, parasitic, lobate, bulbous, and prot- ture, and its distinction from myrmekite is partly one of grain size. rusive are used to describe the shape of myrmekite units convex Perhaps granophyric intergrowths of this kind represent less intense

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quenching of a system of appropriate composition, not dependent than escaping and concomitantly quenching what remains. Accord- on the presence of an aqueous phase. Smith (1974, p. 592—593) ingly, in a system capable of crystallizing myrmekite or granophyric concluded that granophyric K-feldspar-quartz is the result of plagioclase-quartz texture, graphic texture would occur instead, if "rapid simultaneous crystallization of alkali feldspar and quartz," the system were relatively closed to escape of aqueous phase. and presumably he would apply the same to plagioclase-quartz Another aspect of the quartz-oligoclase relationship in myrme- granophyric textures. Graphic relation between plagioclase and kite is that of the relative volume relations between the two com- quartz also is possible, and the question is, What are the factors ponents as a function of An content of the plagioclase. Some sig- promoting the "angular" relations of the phases in this case over nificance has been given to this, originally by Becke (1908), and the "wormy" intergrowth characterizing myrmekite and grano- among more recent work by Ashworth (1972), in which an increase phyric textures? Smith (1974, p. 607—608), from a survey of the in quartz is said to be proportional to higher An content of the host literature on graphic K-feldspar-quartz, concluded that the texture plagioclase. Ashworth's conclusions would seem not to apply results from simultaneous crystallization under vapor-rich condi- where myrmekite can be shown to grade laterally along a plagio- tions depending on epitaxial kinetic factors. It is tentatively clase compositional zone, from parts rich in quartz vermicules to suggested here that growth of sodic plagioclase in a relatively parts with practically none at all (Fig. 5, E). Where the "propor- closed, aqueous-rich system will form graphic texture if excess tionality of quartz" seems to be a reality, it may alternatively be silica is available. The cuneiform relationships can then be ascribed interpreted in accord with the Sand Springs model as the result of to freedom of growth in the system that is progressively becoming predictable fluid equilibrium compositions existing at the myrme- richer in aqueous phase. Such an environment would be possible in kite stage for any given bulk magma composition. postmyrmekite time, after aqueous-phase saturation, and in those A proposition set forth by Shelley (1964) relates to a common structural situations where the aqueous phase is contained, rather observation that abundant myrmekite seems to be most common in

A

K-feld crystal 1

Figure 7. Characteristics of "vein" myrmekite occurring between two differently oriented K-feldspar crystals. A: Myrmekite units (pi 2) are dominantly epitaxial with one K-feldspar crystal (crystal 2), but a few myrmekite units (pi 1) have paradoxical relationship shown more clearly in B. Sand Springs porphyritic granodiorite. B: Weakly myrmekitic albite units having paradoxical relations to respective hosts with which they are epitaxial, except part (left) in which albite 1 myrmekite units are poorly developed and paradoxical relationship is lost. Baveno, Italy.

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rocks with cataclastic textures. Shelley maintained that recrystalliz- must be present in the "vein" plane prior to the very latest growth ing quartz, inherited from a cataclastic event, is incoporated in of the host K-féldspar crystal or crystals. In the case of two differ- myrmekite as quartz vermicules. In many situations a prior or syn- ently oriented K-feldspar crystals, the fluid is either (1) introduced myrmekite cataclastic event can be clearly ruled out, so the real along the intergranular or (2) becomes intergranular between two question becomes one not of the origin of myrmekite itself, but of advancing growth fronts of K-feldspar. In the first case, a fluid with the origin of abundant myrmekite in some cataclastic rocks. Many myrmekite potential has been relocated in the crystallizing system augen gneisses and fine-grained two-mica granitic rocks fall into as the result of some (perhaps very local) tectonic event, and the this category. If tectonic events are occurring at the myrmekite second case can be explained by crystal motion in the magma, jux- stage in a crystallizing magma (and they may occur prior and after, taposing two K-feldspar crystals to the exclusion of early plagio- as well), the micropressure quenches, required in the Sand Springs clase crystals. An intergranular example, with myrmekite units ex- model, may be enhanced and crystallization of myrmekite facili- tending mainly into one K-feldspar grain is shown in Figures 5, F tated. and 7, A. A double row of myrmekite units (in part) is shown in One of the most compelling arguments that replacement plays a Figure 7, B. A particularly interesting relationship may occur that major role in myrmekite growth are the "veins" of myrmekite that has myrmekite epitaxially related to both K-feldspar individuals, occur (1) between two differently oriented K-feldspar crystals (see, either as continuous or alternating myrmekite units along the for example, Phillips, 1964) and (2) within a single, optically con- "vein," but appearing on the "wrong" side of the "vein" relative to tinuous K-feldspar crystal (Phillips and others, 1972). In either case its epitaxial host (Fig. 7, B). An idealized group of sketches (Fig. 8) there may be a single, an alternately occurring, or a double set of explains these relationships in the light of thin-sectioning direc- myrmekite units along the "vein." In the first case, lobate myrme- tions. Figure 9 shows the relations to be expected with a myrmekite kite units base on one K-feldspar surface and extend out into the "vein" in a single K-feldspar crystal. In this case, a rupture must opposing K-feldspar. In the second, myrmekite units alternately occur prior to final consolidation of the magma system (such as base on both K-feldspar surfaces along the course of the "vein." In would be expected in a moving crystal-liquid mush), in which a late the third case, there are two rows of myrmekite units, basing fluid with myrmekite potential penetrates and crystallizes myrme- against each other along a roughly defined medial plane, resulting kite and a final filling-in K-feldspar, all of which effectively reheals in lobate forms extending out into each K-feldspar individual. The the original rupture ( Fig. 5, G). In this case, the paradoxical epi- impression is that replacement began along an intergranular or taxial relations do not occur, because there is only one host indi- fracture surface and progressed into K-feldspar in one or both di- vidual. rections. An alternative to this interpretation, consistent with the The final and perhaps most difficult test of the general applica- Sand Springs myrmekite model, is as follows. In either the two or bility of the Sand Springs myrmekite model is sharply echoed by single K-feldspar crystal situation described above, a fluid with Phillips (1974, p. 186), commenting on previous suggestions of di- myrmekite-precipitating potential (that is, a melt with K-feldspar, rect crystallization of myrmekite: "Further, it is difficult to explain oligoclase, and quartz components, saturated with aqueous phase) all the myrmekite found in metamorphic rocks as the product of

Figure 8. Diagrammatic ex- planation of lobate myrmekite units based along a medial plane between two differently oriented K-feldspar crystals (myrmekite "vein"). Nonreplacive origin re- quires a stage of premyrmekite and postmyrmekite growth of K-feldspar of both individuals. Four thin-section planes are shown with respect to two ideal- ized myrmekite units, epitaxial with respective K-feldspar hosts. Paradoxical relation occurs in section 2-2' (C).

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simultaneous crystallization from a melt." Three options are (1) cator of the presence and nature of rock-forming fluids. Accord- there is more than one origin for myrmekite, (2) many of the ingly, interpretations can be framed as to whether myrmekite forms metamorphic rocks have had a significant magmatic history, or (3) before, during, or after cataclasis and recrystallization of the gneiss, simplectic intergrowths of quartz and plagioclase in some or corresponding to, respectively (1) later of a metamorphic rocks are not myrmekite. I prefer to reject the first myrmekite-bearing magmatic granitic rock, (2) simultaneous possibility, because I believe it is unlikely that two "identical" magmatic crystallization and deformation of an intruding crystal geologic features have more than one origin. The third situation re- mush, and (3) partial melting or magmatic hybridization of an quires an explanation for myrmekite-like textures occurring in otherwise nonmagmatic system. The gneissose Kinsman Quartz rocks that could not have had even a partial magmatic history (see Monzonite of Billings's (1956) New Hampshire Plutonic Series has Shelley, 1973). Bleb and, rarely, semivermicular quartz can occur in a spectacular development of myrmekite (John B. Lyons, 1968, plagioclase (see, for example, Fig. 10), particularly in thermal personal commun.) and offers an ideal opportunity to test these metamorphic environments where sieved textures are common. In possibilities. The plutons are sheetlike masses characterized by a

these cases, the question is why the semimyrmekitic texture is fa- distinct St foliation (Nielson and others, 1976) in which K-feldspar vored over a granular polygonal mosaic relationship so common in megacrysts commonly lie with long-axis alignment. The rock is dynamothermal metamorphic rocks. The question must remain strongly cataclastic and varies from rather coarse-grained gneiss unanswered for the moment. Going to the second possibility, most with moderately well-formed K-feldspar megacrysts to very fine- occurrences of real myrmekite in metamorphic rocks can be ex- grained, cataclastic and granoblastic gneiss with distinctly rounded plained by the fact that they had a significant magmatic history, in megacrysts. Nielson and others, (1976) concluded on structural that they are either (1) magmatic rocks that crystallized in a grounds that the Kinsman represents magmatic injection into a dynamic environment and acquired metamorphic characteristics, tectonically active environment. The fundamentally magmatic be- (2) magmatic rocks that have been subsequently metamorphosed, ginning of the rock can be visualized by sifting out the metamor- (3) partially melted rocks, or (4) hybrids. In the first two cases, evi- phic features, effectively leaving textures remarkably similar to the dence for deformed and partially recrystallized myrmekite should Sand Springs porphyritic granodiorite. For example, the size of exist, as it does in many granitic gneisses. megacrysts, the relatively homogeneous distribution of megacrysts in the pluton, the occurrence of at least some oriented plagioclase MEANING OF MYRMEKITE IN ROCKS OF inclusions in the megacrysts, and some euhedral tendency of matrix CONTRASTING GEOLOGIC EVOLUTION quartz facing these megacrysts suggest that the megacrysts are in fact relict phenocrysts. If so, myrmekite should and does occur typ- Myrmekite is very common in granitic rocks through a wide ically between the rock matrix and the phenocrysts, corresponding range of composition and texture. It is especially abundant in to zone 2 (Fig. 3) in the Sand Springs rock. Metamorphism of the quartz monzonitic (granite of Streckeisen, 1976) and granodioritic Kinsman has resulted in (1) reshaping of the phenocrysts, (2) de- rocks, but it also occurs in quartz dioritic and dioritic systems formation of plagioclase crystals of the matrix, (3) comminution where fractionation produces a late fluid or fluids containing and, locally, recrystallization of the matrix and phenocrysts, (4) K-feldspar, oligoclase, quartz, and aqueous phase. Systems general development of mortar texture, and (5) probable assistance deficient in anorthite component (and perhaps also in aqueous in the development of cross-hatched twinning of the K-feldspar. phase), such as hypersolvus alkali granites, are devoid of myrme- The question is, When did this cataclastic metamorphism take kite. place? Was it (1) after crystallization of the Kinsman magma or (2) Myrmekite is as abundant in most granitic gneisses as it is in their during crystallization, either during emplacement of a crystal-liquid directionless counterparts. This offers the opportunity to approach mush, or after emplacement as crystallization occurred in a still- gneiss petrogenesis from a textural point of view, in that, by the active tectonic environment? With myrmekite as the marker, dis- proposed Sand Springs myrmekite model, myrmekite is an indi- tinctly postmagmatic regional metamorphism should result in

Figure 9. Diagrammatic explanation of nonreplacive origin of myrmekite "vein" in single, optically continuous K-feldspar crystal. Rupture of K-feldspar crystal is required just prior to or during myrmekite stage, followed by additional growth of K-feldspar after myrmekite (compare Fig. 5, G).

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major deformation and recrystallization of myrmekite. In contrast, therefore beyond the myrmekite stage. Thus, the timing of a some deformation and recrystallization during the late stages of tectonic event (such as fracturing) is critical. In the absence of magmatic consolidation should, and in fact has, resulted in some tectonics the aqueous-rich fluid would precipitate K-feldspar inter- strained myrmekite units and locally in recrystallization and reor- granularly in the host pluton, and the final aqueous component ganization of myrmekite into granoblastic quartz and oligoclase, would dissipate from the system. Therefore, myrmekite should not leaving much myrmekite completely or relatively unaffected. A occur with secondary K-feldspar veins, because by the time semisolid system in motion would tend to disrupt physical parti- "deuteric" or "hydrothermal" K-feldspar crystallizes, myrmekite tioning of remaining fluid, and final growth of grains might reflect has already formed elsewhere. Secondary K-feldspar from a irregular geometry in this sense more than crystal habit; a rock with number of porphyry copper deposits have been examined in this something less than clearly magmatic textures would result. Fur- light, and myrmekite is notably absent. It is suggested that any thermore, crystal deformation might also lead to local solid-state augmentation of the late aqueous-rich, magmatically derived fluid recrystallization within crystals (or between crystals devoid of by meteoric water entering the system would simply accentuate the much intergranular fluid), even though fluids are generally present replacement and redistribution phenomena possible without it, but intergranularly. What could be more favorable for recrystallization Taylor (1974) considered K-feldspar alteration in the porphyry than a mineral just below its solidus undergoing deformation? The copper system to be isotopically within the field of primary mag- later the deformation lasts relative to crystallization of fluids, the matic waters, and the problem may not even exist. more deformed and recrystallized the rock will be. It is suggested The aplite-pegmatite model of Jahns and Burnham (1969) is re- that late-stage penetrative events in the Kinsman, as contrasted markably parallel to the model for the development of myrmekite with relative quiescence in the Sand Springs system, have resulted in in the Sand Springs porphyritic granodiorite, and the experimental a marked textural distinction of a superficial but not a fundamental work indirectly reflects on the myrmekite model itself. The aplite nature. Other well-known gneisses, such as the Shuswap of south- and pegmatitic fractions, by their model, are derived from a single ern British Columbia, the Colville gneiss of north-central Washing- silicate melt system containing a substantial aqueous phase. The ton State (Waters and Krauskopf, 1941; Snook, 1965), and the melt, compositionally in the vicinity of the granite eutectic or min- gneissose finer grained granites of New England (Chayes, 1952), all imum becomes saturated and then supersaturated with respect to contain myrmekite. The presence of myrmekite in these rocks the aqueous phase as crystallization of quartz, K-feldspar, and suggests that they too have evolved in large part through the crys- sodic plagioclase proceeds. If there is locally a rapid loss (or tallization of magma, albeit in a variety of tectonic environments. readjustment) of the aqueous phase (presumably related to fractur- The Sand Springs myrmekite model identifies a distinct break be- ing of the system), rapid crystallization (pressure quench) occurs, tween preaqueous-phase and postaqueous-phase saturation freezing aplite. If the departing aqueous-rich phase is at least par- K-feldspar crystallizing in granitic systems, and this might corre- tially retained in the system next to the aplite, there is a slow and spond to the distinction between the so-called primary and second- free growth of pegmatitic crystals. Thus, both aplite and pegmatite ary K-feldspar associated with porphyry copper mineralization. have been generated and their close spacial relation neatly ex- "Secondary K-feldspar alteration" is used to describe K-feldspar plained. Correlation of these events with those yielding myrmekite that has not had a primary magmatic origin, or, translated to the in the Sand Springs pluton is as follows. The Sand Springs Sand Springs model, that would be K-feldspar crystallized after porphyritic granodiorite fractionally crystallizes to a stage just aqueous-phase saturation. An occurrence of secondary K-feldspar prior to myrmekite growth. The melt at this time is enriched in veins in granitic plutons may represent mechanical release of sodic plagioclase, K-feldspar, and quartz components, and is nearly aqueous-rich fluids (carrying K-feldspar components) at a time saturated in an aqueous phase — precisely the condition of a sys- somewhat beyond aqueous-phase saturation of the system and tem about to yield aplite and pegmatite. Continued crystallization with oversaturation in the aqueous phase, followed by a pressure quench yielding myrmekite, is an exact replica of the aplite quench, except that myrmekite is produced instead of aplite. The equivalent of K-feldspar-rich pegmatite is the postmyrmekite fluids that yield the pink intermediate microcline in the Sand Springs system. Tex- tural-structural differences between the final product, an aplite- pegmatite dike on the one hand and the K-feldspar-myrme- kite—quartz of zone 2 (Fig. 3) on the other are not as conflicting as they at first may seem. If the conditions of myrmekite growth are so similar to those existing during aplite crystallization, ap- lite should contain abundant myrmekite. A group of aplites have been investigated in this context, and except for a few with no K-feldspar and some with plagioclase

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attention is directed to the possibility that a tectonic event, such can myrmekite-like symplectic intergrowths occurring in com- as shearing or jointing, occurring just prior to myrmekite growth pletely nonmagmatic environments be properly evaluated, and only in the Sand Springs pluton, would have permitted a melt of then can myrmekite as a quench texture in water-bearing granitic essentially aplite-pegmatite composition to move as small-body magmatic systems be confirmed. intrusions along these structural weaknesses. Since myrmekite had not yet crystallized, it would be expected to occur in the aplite ACKNOWLEDGMENTS as an integral part of its fabric. Some volume relations are quite re- vealing. The pink K-feldspar in the Sand Springs rock was artifi- This paper would not have been possible without stimulating 2 cially colored on a 500-cm slabbed surface, and was point counted and inspiring contact with J. B. Lyons, P. Misch, and J. A. Vance. as 6.2% (volume). This includes myrmekite, some quartz, and The support of the Nevada Bureau of Mines and Geology in prepa- major postmyrmekite K-feldspar, that is zone 2 (Fig. 3), and is ration of thin sections and figures is gratefully acknowledged. equivalent to an aplite-pegmatite dike 6.2 cm thick cutting an out- 2 crop of 1 m , or to an aplite-pegmatite body derivable from a mass REFERENCES CITED of Sand Springs porphyritic granodiorite the size of Mount Rushmore (South Dakota), that would be five times as large as the Ashworth, J. F., 1972, Myrmekites of exsolution and replacement origins: famous Etta pegmatite lying southeast of Mount Rushmore (Page Geological Magazine, v. 109, p. 45-62. and others, 1953). It is interesting, in this respect, that the Sand Beal, L., and others, 1964, Geology of the Sand Springs Range, in Final re- port, Project SHOAL, geological, geophysical, and hydrological in- Springs pluton does have a belt of aplite-pegmatite dikes (Fig. 1), vestigations of the Sand Springs Range, Fairview Valley and Fourmile and they contain K-feldspar with color and structural characteris- Flat, Churchill County, Nevada: Nevada Bureau of Mines, Nevada tics very similar to the K-feldspar in zone 2 of the main pluton rock Mining Analytical Laboratory, Desert Research Institute, University of (Fig. 2, C; Table 2). Nevada, p. 24-35. Becke, F., 1908, Uber Myrmekite: Mineralogische und Petrographische Mitteilungen, v. 27, p. 377-390. SAND SPRINGS MYRMEKITE MODEL AND Billings, M. P., 1956, The geology of New Hampshire. Pt. 2, Bedrock geol- VALIDITY OF TEXTURAL INTERPRETATION ogy: New Hampshire State Planning and Development Commission, 203 p. Burnham, C. 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K., 1948, Die Feldspat-Quartz-Reaktionsgefüge der favoring a replacive origin; however, there is little justification for Granite und Gneise: Berlin-Göttingen-Heidelberg, Springer-Verlag, concluding that myrmekite units have replaced K-feldspar on the 259 p. Fenn, P. M., 1977, The nucleation and growth of alkali from hy- basis of "embayment geometry" when considered in the total tex- drous melts: Canadian Mineralogist, v. 15, p. 135—161. tural picture of the rock in which this occurs. The nonreplacive Flemings, M. C., 1973, Solidification processing: New York, McGraw-Hill origin of myrmekite-in the Sand Springs model has been made on Book Co., 364 p. the basis of a reasonable sequence of textural relationships de- Frasl, G., 1954, Anzeichen schmelzeflüssigin und hochtemperiertem veloped during the entire period of crystallization of the magma Wachstum: An den grosen Kalifeldspaten einiger Porphyrgranite, Porphyrgranitgneise und Augengneise Österreichs: Geologische Bun- and not just on the myrmekite stage alone. desanstalt Austria Jahrbuch, v. 97, p. 71-132. The nonreplacive origin of myrmekite as proposed releases it Hibbard, M. 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Kalb, G., 1924, Die Kristalltracht des kalifeldspates in minerogenetischer melt stage. The very latest fluids in a crystallizing granitic system Betrachtung: Contributions to Mineralogy Abstract A, p. 449—460. are capable of replacing plagioclase with K-feldspar, as described in Kerrick, D. M., 1969, K-feldspar megacrysts from a porphyritic quartz the Sand Springs rock, and the replacement yields irregular or rag- monzonite, central Sierra Nevada, California: American Mineralogist, ged plagioclase and myrmekite units in contact with what looks v. 54, p. 839-848. Maucher, A., 1943, Uber geregelte plagioclaseneinschlusse in Orthoklas like an invasion of K-feldspar. These features catch the eye, tending (Sanidin): Zeitschrift für Kristallographie und Mineralogie, v. 105, p. to result in overemphasis on replacement processes in a rock that is 82-90. actually dominated by nonreplacive textures. Nelson, Carl, and Hibbard, M. 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