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Myrmekite As a Marker Between Preaqueous and Postaqueous Phase Saturation in Granitic Systems

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Myrmekite As a Marker Between Preaqueous and Postaqueous Phase Saturation in Granitic Systems 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 quartz and oligoclase, 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. granodiorite, 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 plagioclase (zoned), quartz, and K-feldspar 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 gneiss, 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 magma 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 gneisses, 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. 1047 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 to Fallon to Frenchman Reno 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- diorite 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). 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 MYRMEKITE AS A MARKER IN GRANITIC SYSTEMS 1049 _ 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.
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