Igneous Phenocrystic Origin of K-Feldspar Megacrysts in Granitic Rocks from the Sierra Nevada Batholith
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Igneous phenocrystic origin of K-feldspar megacrysts in granitic rocks from the Sierra Nevada batholith James G. Moore Thomas W. Sisson U.S. Geological Survey, Menlo Park, California 94025, USA ABSTRACT the granodiorite hosts marginal to the mega- ditions (Dickson and Sabine, 1967; Johnson crysts range to lower growth temperatures, et al., 2006a, 2006b). In this study we focus Study of four K-feldspar megacrystic gra- in some instances into the subsolidus. The primarily on the texture and composition of nitic plutons and related dikes in the Sierra limited range and igneous values of growth megacrysts in four major intrusions span- Nevada composite batholith indicates that the temperatures for megacryst-hosted titanite ning 300 km of the eastern, crestal part of the megacrysts are phenocrysts that grew in con- grains support the interpretation that the range to investigate the nature and origin of tact with granitic melt. Growth to megacrys- megacrysts formed as igneous sanidine phe- the megacrysts. The genesis of the megacrysts tic sizes was due to repeated replenishment nocrysts, that intrusion temperatures var- bears on another problem of Sierran geology, of the magma bodies by fresh granitic melt ied by only small amounts while the mega- that of the nature of emplacement of the large that maintained temperatures above the soli- crysts grew, and that megacryst growth granitic intrusions that contain them. A phe- dus for extended time periods and that pro- ceased before the intrusions cooled below nocrystic origin of the megacrysts is compat- vided components necessary for K-feldspar the solidus. Individual Ba-enriched zones ible with the host pluton having formed as a growth. These intrusions cooled 89–83 Ma, were apparently formed by repeated surges spatially extensive crystallizing reservoir of are the youngest in the range, and repre- of new, hotter granitic melt that replen- magma, a so-called “big tank” (Glazner et al., sent the culminating magmatic phase of the ished these large magma chambers. Each 2004). A porphyroblastic (post solidifi cation) Sierra Nevada batholith. They are the gra- recharge of hot magma offset cooling, main- origin is necessary to account for the even nodiorite of Topaz Lake, the Cathedral Peak tained the partially molten or mushy charac- distribution of megacrysts if the host plutons Granodiorite, the Mono Creek Granite, the ter of the chamber, stirred up crystals, and are aggregates of numerous dikes and sills that Whitney Granodiorite, the Johnson Granite induced convective currents that lofted, set- solidifi ed shortly after their injection (Cole- Porphyry, and the Golden Bear Dike. tling megacrysts back up into the chamber. man et al., 2004; Glazner et al., 2004). Megacrysts in these igneous bodies attain Because of repeated reheating of the magma 4–10 cm in length. All have sawtooth oscilla- chamber and prolonged maintenance of the GEOLOGIC SETTING tory zoning marked by varying concentra- melt, this process apparently continued long tion of BaO ranging generally from 3.5 to 0.5 enough to provide the ideal environment for Many of the intrusive granitic rock masses wt%. Some of the more pronounced zones the growth of these extraordinarily large (here referred to as intrusions or plutons) mapped begin with resorption and channeling of the K-feldspar phenocrysts. in the Sierra Nevada batholith contain conspicu- underlying zone. ous K-feldspar crystals (Fig. 1). In these mapped Layers of mineral inclusions, principally Keywords: Sierra Nevada, megacryst, barium plutons (commonly called porphyritic), the feld- plagioclase, but also biotite, quartz, horn- zoning, K-feldspar. spars are described as variable in abundance and blende, titanite, and accessory minerals, are size, and the host plutons as faintly, partly, or parallel to the BaO-delineated zones, are INTRODUCTION strongly porphyritic. Commonly the K-feldspar sorted by size along the boundaries, and have crystals are 1–2 cm in length. their long axes preferentially aligned paral- Large, conspicuous crystals of potassium However, a series of large intrusions in the lel to the boundaries. These features indicate feldspar typify many granitic intrusions in the eastern Sierra Nevada is characterized by giant that the K-feldspar megacrysts grew while Sierra Nevada batholithic complex, California, K-feldspar crystals (Figs. 1 and 2). These surrounded by melt, allowing the inclusion and giant crystals attaining 4–10 cm in length megacrysts commonly attain 4 cm in length minerals to periodically attach themselves to occur in a few major intrusions. The origin of and rarely approach 10 cm. The host intru- the faces of the growing crystals. these megacrysts has long been a subject of sions are each the central and youngest mem- The temperature of growth of titanite debate and study. They have been attributed to ber within a sequence of nested intrusions that included within the K-feldspar megacrysts is early phenocrystic growth by crystallization are nonporphyritic or have comparatively small estimated by use of a Zr-in-titanite geother- from the melt phase of magma (Kerrick, 1969; phenocrysts. Generally the central megacryst- mometer. Megacryst-hosted titanite grains Vernon, 1986; Bateman, 1992; Cox et al., bearing intrusion is displaced east of the cen- all yield temperatures typical of felsic mag- 1996), or to late porphyroblastic growth from ter of the overall intrusion sequence of related mas, mainly 735–760 °C. Titanite grains in a water-rich fl uid phase under subsolidus con- plutons. The rock of these megacrystic plutons Geosphere; April 2008; v. 4; no. 2; p. 387–400; doi: 10.1130/GES00146.1; 15 fi gures; 2 tables. For permission to copy, contact [email protected] 387 © 2008 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/2/387/3338737/i1553-040X-4-2-387.pdf by guest on 27 September 2021 Moore and Sisson METHODS The nature of K-feldspar megacrysts in the granodiorite of Topaz Lake, Cathedral Peak Granodiorite, Mono Creek Granite, Whitney Granodiorite, Johnson Granite Porphyry, and the Golden Bear dike was examined in the fi eld and laboratory. Collections were made contain- ing megacrysts from outcrops, and at two locali- ties extensive collections were made of crystals partly weathered out in glacial moraines. Sam- ples were cut, stained, examined microscopi- cally, and photographed in thin section. Scan- ning electron microscope backscatter images (commonly with dozens of images assembled in mosaics) were made with a LEO 982 fi eld emission digital instrument with an accelerating voltage of 15 kV. Electron microprobe analyses were per- formed with the 5-spectrometer JEOL 8900R instrument at the U.S. Geological Survey, Menlo Park, California. Feldspars were ana- lyzed for Si, Al, Fe, Mg, Ca, Na, K, Ba, and Sr with wavelength-dispersive methods at an accelerating potential of 15 kV, a beam cur- rent of 10 nA, and a spot defocused to 2 µm. Standards were Si, Al, Na: Tiburon albite; K: orthoclase OR1A; Ca: synthetic anorthite; Fe, Mg: synthetic glass RGSC (Corning #N201); Ba: natural barite; and Sr: natural strontianite. Titanite grains were analyzed for Zr, Nb, Ce, and Y at an accelerating potential of 20 kV, a beam current of 200 nA, and a focused spot. Each titanite point was analyzed for a total of 5 min consisting of 5 cycles of counting 30 s Figure 1. Map of Sierra Nevada batholith naming the four Cretaceous intrusive masses (plu- on peak and 15 s each of high and low back- tons) that contain giant K-feldspar crystals (block pattern), and other less porphyritic plu- grounds (total of 2.5 min on peak). Standards tons (cross-hatch pattern). In addition, two late porphyry intrusions are shown: the Johnson were Zr: natural zircon; Ce: synthetic Ce- Granite Porphyry, cutting the Cathedral Peak Granodiorite, and the Golden Bear dike, phosphate; Y: synthetic Y-phosphate; and Nb: apparently fed from the Whitney Granodiorite. Modifi ed from Kistler and Fleck (1994). Nb metal. Background positions were selected to avoid interfering peaks and were suffi ciently close to the peak of interest to apply a linearly is of restricted composition, generally close to wood and Lydon, 1975), 30 km west of Bishop; interpolated background value at the peak posi- the granodiorite-granite boundary in the modal and the Whitney Granodiorite (~590 km2; du tion. Background-corrected count rates were classifi cation of Streckheisen (1973). Volcanic Bray and Moore, 1985; Moore, 1981; Moore converted to concentrations with the JEOL analogs to these granitoids would be evolved and Sisson, 1985; Stone et al., 2000), partly in proprietary version of the CITZAF reduction dacite or rhyodacite. Sequoia National Park. In addition, two late- program, using concentrations for CaO, TiO2, Samples were examined from the four stage hypabyssal intrusions were examined, and SiO2 fi xed to those of ideal titanite. These major Cretaceous megacrystic intrusions near the Johnson Granite Porphyry cutting the instrument conditions give a 20 ppm limit of the crest of the range along a span of more Cathedral Peak Granodiorite and the Golden detection for Zr, based on counting statistics, than 300 km (Fig. 1). The intrusions are the Bear dike (Moore, 1981), a 15-km-long dike and we take 60 ppm (three times the limit of granodiorite of Topaz Lake (~1030 km2; John, associated with the Whitney Granodiorite. detection) as the limit of quantifi cation. This 1983; John et al., 1994; previously the Sonora These megacrystic intrusions are the young- method was verifi ed by repeated analysis of pluton of Schweickert, 1976), well exposed on est in the range, having cooled 89–83 Ma. They a titanite working standard BLR-1 employed Sonora Pass; the Cathedral Peak Granodiorite represent the culminating magmatic event in the by the U.S. Geological Survey–Stanford Ion- of the Tuolumne Intrusive Suite (~620 km2; evolution of the Sierra Nevada batholith.