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.

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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. The Microprobe facility, yielding 1310 ± 30 ppm Bateman and Chappell, 1979; in Yosem- concept that the four large megacrystic plutons Zr (n = 61). Zr concentrations for two titan- ite National Park); the Mono Creek Granite are interconnected at depth and the whole was ite rims are below the limit of quantifi cation, (~380 km2; Bateman, 1992; renamed from the hot and molten concurrently might be termed and one is at the limit of quantifi cation, from quartz monzonite of Mono Recesses; Lock- the “giant tank” model. host-granitoid sample LPL-7. These rims were

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crystal exposed on a fractured or glacially planed surface. The easiest repeatable measure of megacryst size is the apparent length of all crystals above a specifi c size on a suitable fl at surface. Mega- crysts exposed on artifi cially cut and polished surfaces totaling 2.01 m2 in area were examined on two monoliths from the granodiorite of Topaz Lake. (These blocks can be observed on the grounds of the U.S. Geological Survey in Menlo Park, California.) Of 638 measured and tabulated K-feldspar crystals >1 cm in length (Fig. 3), the largest crystal is 9.8 cm long, the median length is nearly 2 cm, and 5% of the crystals are >4 cm. Their abundance (~300/m2) is typical for mega- crystic Sierran plutons. The largest crystal found on glaciated outcrops of the Cathedral Peak Granodiorite on the shore of Cathedral Lake is 7.4 cm in length. Hirt (1989) and Moore (1981) both reported Whitney Granodiorite megacrysts to 8 cm in length. The maximum size of all crystals would be somewhat larger because the length of the exposed part of any crystal is gener- ally shorter than the true length. To avoid this exposure factor, separate K-feld- spar megacrysts were collected where weath- ered out of moraine deposits. From a moraine near Cathedral Lake, the largest Cathedral Peak Granodiorite crystal of 30 collected is 7.2 cm in length, and from a moraine near Lone Pine Lake in a collection of about the same size, the larg- est Whitney Granodiorite megacryst is 6.0 cm in length. The largest of a suite of ~12 weathered-out phenocrysts from the Golden Bear dike is 5.1 cm in length. These separate crystals are useful in determining the geometric center for sectioning. The available data suggest that while mega- crysts are abundant at 3–4 cm in length, a small proportion attain 5–7 cm, but only rarely do the largest ones approach 10 cm. No megacrysts were found that exceed 10 cm in length in the Sierra Nevada, but rare ones no doubt exist. The largest megacrysts we have examined are from the Triassic quartz monzonite megapor- phyry of Twentynine Palms south of the Sierra Figure 2. K-feldspar megacrysts. Top: granodiorite of Topaz Lake. Middle left: Carlsbad- Nevada in the Pinto Mountains. Hopson (1996, twinned megacryst from Golden Bear dike weathered out of fi ne-grained groundmass. Bottom p. 28) described megacrysts from the Joshua left: Cathedral Peak Granodiorite megacryst showing internal zoning by alignment of red- Mountains exposure of the megaporphyry up stained plagioclase inclusions. Bottom right: megacryst and host of granodiorite of Topaz Lake to 16 cm in length. Brett Cox collected one with plagioclase stained red with amaranth. Scale bar is 4 cm long and applies to all samples. K-feldspar Carlsbad twin megacryst weathered out from its matrix in this unit that had a length of 15.3 cm, a density of 2.57 g/cm3, and a weight analyzed in triplicate to verify their low Zr K-FELDSPAR MEGACRYSTS of 1.1 kg. Others have reported megacrysts else- concentrations. where up to 18–20 cm in size (Vernon, 1986). X-ray maps of K, Na, Si, Ca, and Ba were Most of the K-feldspar megacrysts are made over features of interest in megacrysts microcline with perthite structure and Carls- TEXTURAL AND COMPOSITIONAL in areas of ~1 cm2 using the JEOL instrument bad twinning. Precise quantitative infor- ZONING with a 20 nA focused beam, a 10 µm point mation on their size distribution is limited spacing, and a dwell time of 120 µs requiring because they are embedded in the granitic Zoning is ubiquitous in the K-feldspar mega- ~1.5 days per map. matrix with commonly only a part of each crysts (Fig. 2). Many of the features defi ning

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the zones are repeated in all of the four granitic sions. In some plutons outside the Sierra Nevada The zones defi ned by Ba concentration range intrusions examined, although considerable batholith, mineral inclusions are preferentially from 0.05 to several millimeters in thickness variation is found between megacrysts in the concentrated in certain megacryst growth sec- (Figs. 5 and 7), and are, of course, affected in same outcrop and between megacrysts from dif- tors (Vaniman, 1978), but this feature has not thickness by the crystallographic face that they ferent plutons. been found in the examined Sierran localities. parallel. Backscattered electron beam images Dark inclusions of biotite, hornblende, titan- The mineral inclusions generally show a show that zonal layers enriched in Ba are almost ite, and iron oxide are arranged in internal zones marked alignment of their long axes parallel to invariably asymmetrical. They begin sharply parallel to the outer margins of megacrysts, and the zones in which they occur (Figs. 5, 6, and and end more gradually (assuming they grow also commonly near the outer margin (Fig. 2). 7). This is especially so for plagioclase, horn- outward from core to rim in the crystal; Figs. 9, In some settings, particularly on weathered sur- blende, and titanite. The more equidimensional 10, 11, and 12). Prominent zone boundaries faces, inclusions of small plagioclase crystals habit of biotite, quartz, and iron oxide inclusions show an abrupt increase in BaO content of ~1 are visible aligned along zone boundaries. They does not lend itself to such alignment. wt%, with the BaO content commonly dou- can be seen to advantage if the megacryst is sec- Scanning electron microscope and electron bling from 1% to 2% (representative results tioned, etched with HF, and stained for plagio- microprobe backscattered electron images reveal in Table 1). The abrupt increase in Ba content clase with amaranth solution (Fig. 2). coarse oscillatory zoning in all megacrysts. The occurs within ~0.02 mm whereas it returns to an Microscopically, small mineral inclusions in zoning is highlighted by the variation in Ba ambient value in 0.1–0.2 mm (Fig. 9), and thus the K-feldspar megacrysts are seen to be abun- concentration (Figs. 5 and 6) because Ba read- produces a sawtooth concentration profi le. dant in some zones and rare or missing from ily replaces K in the feldspar lattice (in coupled While most of the zone boundaries refl ect others (Fig. 4). Plagioclase is the most abundant substitution of Al for Si) and ranges from 0.5 growth of the megacryst, some indicate inclusion mineral. The other minerals, in com- to 3.5 wt% BaO. The high atomic weight of Ba resorption, solution, or erosion of the outer mon order of abundance, are hornblende, biotite, and its ready acceptance in the feldspar makes it edge of the megacryst before renewed growth quartz, titanite, iron oxide, apatite, and zircon. ideal to illustrate zoning in the megacrysts. and crystallization (Figs. 10 and 13). Such The inclusion crystals (commonly euhedral to All examined megacrysts from the six stud- subhedral) are concentrated in layers along zone ied intrusions are oscillatory zoned. Gener- boundaries. Quartz is the only inclusion that ally 10–16 discrete zones are present in the rarely shows crystal form; it generally forms megacrysts (Fig. 8). The smaller K-feldspar rounded blebs, or elongate stringers. The inclu- phenocrysts in older marginal plutons (such sions are usually sorted by size so that along one as the Paradise Granodiorite, which surrounds zonal layer the plagioclase crystals may average the Whitney Granodiorite; Moore, 1981), have ~1 mm in length whereas in other layers they fewer, less-well-defi ned zones, and interstitial may average 0.1–0.5 mm (Figs. 5 and 6). Com- K-feldspar in the host granitoids generally lacks monly zones up to 0.5 mm wide between the oscillatory zoning observable by the methods inclusion layers have virtually no mineral inclu- employed in this study.

Figure 4. Photomicrograph of Whitney Gra- nodiorite megacryst (CL-11) under crossed polars showing zonally arranged inclusions, principally plagioclase. The microscopic arrangement of inclusions indicates zones more numerous than are apparent mega- Figure 3. Cumulative percent of K-feldspar phenocrysts longer than 1 cm measured on two scopically. A biotite inclusion is near the core large samples of the granodiorite of Topaz Lake. A total of 638 such crystals are exposed on of the crystal. Short dimension of image is fl at cut and polished surfaces with an area of 2.01 m2. 22 mm.

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resorption boundaries are particularly abrupt or channel cut in the zone older than the one parallel to one another and to 001. The perthite and may be wavy and channelized. It is not containing it. veins are often at a high angle to zone bound- uncommon for one or more of the inclusion Under the microscope the perthitic nature of aries. In addition, irregular masses of patch crystals to appear displaced from the orderly the K-feldspar megacrysts is apparent (Figs. 7, perthite are common. alignment for its particular zone of simi- 10, and 11). Veins of perthitic albite formed In some megacrysts zonal boundaries (appar- lar crystals. On close inspection the errant by exsolution upon slow cooling of K-feldspar ently previous crystal faces) are visible as thin crystal proves to be lodged in a depression attain widths of 1–40 µm that are commonly straight dusty lines a few microns thick that parallel other zonal boundaries. Under highest power magnifi cation, some of the lines are seen to be the locus of tiny minerals generally of the same kind as those appearing as larger inclu- sions. Other lines prove to be narrow veins of perthitic albite that has apparently migrated to form a seam coincident with a previous crystal face (Figs. 7 and 11). Without electron beam images showing Ba abundance, the details of oscillatory zoning are not readily apparent. Under the petrographic microscope varying abundances of perthitic albite from one zone to the next may highlight some zones as subtle differences in extinction angle when viewed in polarized light (Fig. 12). These varying abundances of perthite appar- ently refl ect differences in the sodium content of the original K-feldspar zone before exsolu- tion of albite. Aside from the four K-feldspar megacryst– bearing large intrusions that have been exam- ined that range from 380 to 1030 km2 in area, K-feldspar megacrysts have been examined in two much smaller fi ne-grained intrusions. The Johnson Granite Porphyry intricately intrudes Figure 5. False color mosaic of microprobe electron backscatter images of small parts of the south-central part of the Cathedral Peak Whitney K-feldspar megacryst showing major Ba-rich zone boundaries (blue) along which Granodiorite (Bateman, 1992). The Golden plagioclase inclusions (red, yellow) as well as biotite, hornblende, and titanite (dark purple) Bear dike, apparently fed by the Whitney Gra- are concentrated. Bottom image shows inner zone that curves around crystal corner on nodiorite (yet beginning 0.5 km north of the left, and dark purple 0.2 mm rhombic titanite crystals in top right. Top image shows the exposed Whitney pluton), extends 15 km north- long axes of elongate plagioclase inclusions that are preferentially aligned parallel to a zone east across the Sierra crest and range front to boundary. Megacryst rim and growth direction is to top. Note how Ba-rich bands start the eastern foothills (Moore, 1963, 1981). The abruptly and fade gradually. dike ranges from 5 to 30 m in thickness and contains zoned K-feldspar megacrysts similar to those in the main megacrystic masses. The 83 Ma dike has been correlated with another set of dikes to the southeast in the Coso Moun- tains, and the offset of the two suggests a 65 km post-dike, right-lateral displacement on faults along the alignment of Owens Valley; the com- bined length of the two dike sets exceeds 50 km (Kylander-Clark et al., 2005). These granite porphyry intrusions are clearly magmatic in origin. They have sharp chilled mar- gins and euhedral Carlsbad-twinned, K-feldspar phenocrysts (Fig. 3) set in a fi ne-grained ground- mass. Scanning electron microscope images show BaO-enriched oscillatory zones similar to Figure 6. Mosaic of electron backscatter images of half of a megacryst in Whitney Grano- those of the main porphyritic plutons (Fig. 13). diorite showing subtle BaO oscillatory zoning. Dark plagioclase inclusions are concentrated The abundant inclusions in the K-feldspar are and aligned along zone boundaries. Inclusions of biotite, hornblende, and accessory miner- the same mineral species as those previously als (white) are concentrated near outer margin. Irregular outermost rind of the megacryst described and some zonal boundaries show dis- indicates its continued enlargement during growth of groundmass minerals. tinct evidence of resorption. Elongate inclusions

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Figure 7. Three electron-microprobe scans, each of the same 1 cm2 area of a K-feldspar megacryst from the Cathedral Peak Granodiorite. CP—backscatter electron image with plagioclase inclusions (dark gray) and biotite, hornblende, and accessory mineral inclusions (white). Ba—barium concentration map showing seven barium-enriched zones (white) defi ning upper right corner of crystal. Na—sodium con- centration map showing chiefl y blocky plagioclase inclusions and parallel perthitic albite stringers inclined upward to right. A few albite stringers parallel zone boundaries (previous crystal faces) near center.

survived with their delicate oscillatory zoning during dike intrusion and quenching of the fi ne- grained groundmass. Ba oscillatory zoning in K-feldspar mega- crysts has been identifi ed at other plutonic localities (Vernon, 1986). K-feldspar mega- crysts in the Shap granite in northern England show particularly fi ne oscillatory zones, defi ned by Ba concentration, that have been attributed to igneous crystallization (Cox et al., 1996). Ba oscillatory zoning also occurs in K- feldspar megacrysts in volcanic settings. Multiple Ba-defi ned sawtooth oscillatory zones are present in sanidine phenocrysts from the Fish Canyon Tuff that forms a vast ash-fl ow sheet in the San Juan Volcanic Field, Colorado (Lipman et al., 1997; Bachmann et al., 2002). We have observed about a dozen such zones in K-feldspar megacrysts from the South Mountain volcanic dacite dome in the San Juan volcanic fi eld, Colorado (Steven and Ratte, 1960). The outstanding electron backscatter images of Ba oscillatory zoning in sanidine megacrysts from the Taapaca rhyodacite, Chile (Wegner et al., Figure 8. Number of oscillatory zones as defi ned by BaO concentration in K-feldspar 2005), were an inspiration for our efforts with megacrysts from several megacrysts of the Cathedral Peak Granodiorite (open squares) electron microscopy. and Whitney Granodiorite (solid circles). Generally 10–16 zones are present that are fairly evenly spaced from edge to core of the megacrysts. ORIGIN OF ZONING

The K-feldspar megacrysts show oscillatory are preferentially aligned parallel to zone bound- rated from that earlier intrusion, but megacrysts zoning patterns that record the size and shape aries within the megacrysts (Fig. 13). Some from the Golden Bear dike lack such xenolithic of previous boundaries of the crystals during megacrysts in the Johnson Granite porphyry material. We conclude that the Golden Bear dike growth. The zones are characterized by mul- are partly encased by medium-grained granitic was fed from the Whitney pluton when a part of tiple, fi ne, asymmetric, Ba-enriched layers, rock similar to the Cathedral Peak granodiorite. it was largely molten, yet contained suspended channelized and resorbed zone boundaries, and Those megacrysts may be xenocrysts incorpo- previously zoned megacrysts. The megacrysts zonally concentrated inclusions that are sorted

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by size and aligned by shape parallel to zone boundaries. The similarity of these patterns in the four studied plutons, as well as in the dikes Na 0 2 related to these plutons, supports the notion that BaO the crystals grew surrounded by melt. These fea- tures are most compatible with crystal growth in a melt (Kerrick, 1969; Long and Luth, 1986; Vernon, 1986; Cox et al., 1996), rather than with subsolidus crystallization (Dickson and Sabine, 1967; Johnson et al., 2006a, 2006b) engulfi ng and replacing other mineral grains. The K-feldspar zoning is apparently produced by an abrupt increase in the Ba concentration of

Na20 the nourishing melt, coupled in some instances BaO by partial solution of K-feldspar. This increase is then followed by a more gradual growth of the K-feldspar during which the Ba concentration is depleted back to its previous concentration. This cycle is likely caused by replenishment of the cooling intrusion by repeated injections of fresh, hot, relatively Ba-rich granitic magma, followed

Na20 BaO

Na20 BaO

Na 0 2 Figure 10. Two major Ba-enriched (lighter BaO colored) zone boundaries (A-A and B-B) in a Cathedral Peak K-feldspar megacryst. Growth direction (and crystal rim) is to the right. These prominent irregular boundar- ies apparently signal recharge of the magma chamber with fresh, hotter Ba-rich granitic melt that has partly dissolved the mega-

Figure 9. Electron microprobe BaO and Na2O concentration profi les across BaO-enriched cryst, producing an irregular and channel- zones in each of fi ve megacrysts from several intrusive masses. Growth direction of the ized boundary. Plagioclase inclusions (dark) crystal (toward the rim) is to the right. Arrow shows beginning of zone boundary as defi ned are aligned parallel to zone boundaries, but by BaO content. BaO buildup takes place in 0.015–0.025 mm and returns to the ambient some are nestled in depressions eroded and/

level in ~0.2 mm. Na2O content shows little relationship to BaO, but refl ects the presence of or dissolved in the older Ba-poor zone. Back- perthitic albite stringers. scattered electron image is 4.5 mm wide.

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by a period when crystallizing K-feldspar In some cases megacrysts were apparently close FeTi oxides. All of the examined K-feldspar quickly extracted Ba from the melt, causing its to the region where a large and hot batch of new megacrysts contain zonally arranged idiomor- concentration to return to a lower level. Replen- magma entered the chamber. This entry heated phic titanite inclusions, and idiomorphic titanite ishment may have been accompanied by cycling and shifted the local melt composition so that it is also a widespread accessory mineral in the of megacrysts between crystal-rich mushy, and was temporarily undersaturated in K-feldspar surrounding host granitoids. We use electron melt-rich portions of the magma bodies. With components, causing partial dissolution and resul- microprobe analyses of Zr in the titanite grains replenishment, the higher concentration of Ba in tant irregular wavy zone boundaries. to estimate the temperatures of megacryst the new magma relative to the host melt, caused Introduction of each batch of fresh magma growth and to investigate the thermal histories the growing K-feldspars to rapidly incorporate into the chamber, presumably through its lower of the intrusions. Ba into their crystal lattices because of the par- portions, would have reheated the magma, off- Concentrations of Zr, Nb, Ce, and Y were ticularly strong affi nity of Ba for K-feldspar. set its conductive cooling, and prolonged the life measured in the cores and rims of numerous This accounts for the sharp beginning of Ba of the chamber as a viable reservoir of granitic titanite grains (generally 0.1–0.2 mm in size) enrichment at each zone boundary. Continued magma. Intermittent recharge would also have along traverses across megacrysts, and in the growth of the new zone then depleted the liq- accelerated stirring and mixing of the magma contiguous host granitoid for selected samples uid in Ba, leading to the progressive decrease in bodies, thereby lofting settling crystals back up (see Table 2 for representative results). Anhe- Ba concentration moving outward through each into the interior of the chamber. It is clear that dral granules of titanite included in locally chlo- sawtooth oscillatory zone. all of the minerals of the fi nal granodiorite rock ritized biotite were not analyzed. Zr-in-titanite were periodically suspended in the melt during temperature (T) is a weak function of pressure K-feldspar megacrystic growth because they are (~+12 °C/100 MPa) and also of the activities (a)

all found as inclusions in the megacrysts. of SiO2 and TiO2, and is given by (Hayden et al., 2007): T (°C) = [7708 + 960P(GPa)]/[10.52

MEGACRYST GROWTH – log10(aTiO2) – log10(aSiO2) – log10(ppm Zr, TEMPERATURES titanite)] – 273. Temperatures presented here assume crys- The concentration of Zr in titanite depends tallization at 250 MPa, consistent with diverse primarily on temperature in zircon-saturated estimates of intrusion depth in the central part of magmas (Hayden et al., 2007). Since Zr dif- the Sierra Nevada batholith (Ague and Brimhall fuses slowly, Zr-in-titanite geothermometry 1988; Sisson et al., 1996). Quartz inclusions

can preserve details of thermal histories that are show that the aSiO2 was 1 when the megacrysts

lost by mineral phases that continue to react at grew. The aTiO2 is more diffi cult to assess, but subsolidus temperatures, such as feldspars and in practice is not a serious limitation. Estimates

Figure 11. False color electron backscatter image of Cathedral Peak K-feldspar mega- cryst showing multiple curving Ba-rich zone boundaries (darker blue) near a previous lower right crystal corner. Growth direction (megacryst rim) is to the right. Plagioclase (red, yellow, green) occurs as inclusions and as small parallel perthitic albite stringers. These stringers aligned upward to the right generally cut zone boundaries. However, a Figure 12. Three images of a zone boundary in a megacryst from the Whitney Granodiorite, perthitic albite stringer starting on lower all of the same area at the same scale with growth direction to the right. (Left) False color left cuts other perthite stringers and is par- backscatter electron image of Ba-enriched zone boundary (dark blue). Plagioclase inclu- allel to zone boundaries. It has apparently sions are blocky red and yellow crystals ~0.2–0.5 mm in size. Perthite is represented by invaded a seam along a previous crystal face. nearly horizontal red and yellow stringers. (Middle) Optical microscope image with plane This stringer will be visible under an optical light showing faint lineation induced by perthite, with no hint of zone boundary except for microscope and will hint as to the extent of alignment of plagioclase inclusions. (Right) Optical microscope image with crossed polars fi ne zoning. Image is 4 mm wide. showing faint indication of zoning by subtle changes in character of perthite.

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TABLE 1. ELECTRON-MICROPROBE ANALYSES (WT%) OF MEGACRYST K-FELDSPAR TRAVERSING ACROSS A REPRESENTATIVE SAWTOOTH BA OSCILLATION, MOUNT WHITNEY GRANODIORITE SAMPLE LPL-11

Position SiO2 Al2O3 FeO* MgO CaO Na2O K2O SrO BaO Total 1 (inner) 64.7 18.8 0.08 0.01 0.02 2.03 13.5 0.08 0.85 100.0 2 64.8 18.7 0.09 0.01 0.04 2.86 12.4 0.07 0.87 99.8 3 64.8 18.8 0.10 0.00 0.02 2.18 13.5 0.03 0.74 100.2 4 64.6 18.6 0.10 0.00 0.01 1.35 14.4 0.14 0.76 99.9 5 64.6 18.5 0.11 0.00 0.03 1.52 14.4 0.12 0.72 99.9 6 64.7 18.7 0.07 0.01 0.00 1.62 14.2 0.09 0.78 100.2 7 65.4 18.7 0.09 0.00 0.06 3.07 12.0 0.02 0.63 99.9 8 64.3 18.6 0.10 0.01 0.03 2.36 13.0 0.12 0.66 99.2 9 65.7 18.6 0.07 0.00 0.47 3.02 11.4 0.06 0.58 99.8 10 64.8 18.4 0.09 0.01 0.00 2.47 12.8 0.10 0.61 99.3 11 64.8 18.8 0.09 0.01 0.00 1.88 13.9 0.06 0.67 100.3 12 65.2 18.9 0.11 0.00 0.05 3.64 11.4 0.03 0.64 100.0 13 65.0 18.9 0.09 0.00 0.05 3.62 11.4 0.06 0.88 100.1 14 64.1 18.8 0.07 0.00 0.01 1.81 13.7 0.09 1.00 99.6 15 64.3 18.6 0.11 0.00 0.02 1.58 14.0 0.08 0.99 99.6 16 64.1 18.7 0.08 0.01 0.01 1.71 13.7 0.08 0.95 99.4 17 64.6 18.5 0.09 0.00 0.02 1.99 13.3 0.06 0.95 99.5 18 64.9 19.0 0.10 0.01 0.01 2.66 12.4 0.04 0.99 100.1 19 63.9 19.1 0.09 0.00 0.12 3.18 11.3 0.06 1.63 99.3 20 63.4 19.2 0.09 0.02 0.14 2.80 12.0 0.08 2.18 99.8 21 63.4 19.3 0.09 0.01 0.07 2.49 12.4 0.10 2.16 99.9 22 63.9 19.2 0.07 0.01 0.04 2.53 12.4 0.05 2.02 100.2 23 63.8 18.9 0.06 0.02 0.08 2.72 12.1 0.02 2.00 99.7 24 64.3 19.1 0.08 0.01 0.15 3.41 11.0 0.00 1.78 99.9 25 64.0 18.9 0.11 0.01 0.11 3.05 11.5 0.07 1.63 99.4 26 63.3 19.1 0.11 0.01 0.01 1.64 13.5 0.08 1.93 99.7 27 64.8 19.1 0.11 0.01 0.02 2.13 12.9 0.08 1.68 100.8 28 64.7 19.1 0.10 0.02 0.07 2.68 12.4 0.08 1.67 100.8 29 64.4 19.2 0.08 0.01 0.05 2.68 12.1 0.04 1.57 100.1 30 64.0 18.7 0.09 0.01 0.01 2.09 13.1 0.04 1.54 99.6 31 64.1 18.9 0.08 0.02 0.01 2.58 12.6 0.03 1.38 99.8 32 63.7 18.8 0.09 0.03 0.03 2.41 12.9 0.03 1.32 99.3 33 64.6 18.9 0.07 0.02 0.08 3.01 12.0 0.05 1.19 99.9 34 64.8 18.8 0.09 0.01 0.10 3.90 10.7 0.00 0.95 99.4 35 64.8 18.9 0.06 0.02 0.05 2.77 12.3 0.06 0.96 99.9 36 64.6 19.0 0.08 0.01 0.07 2.65 12.5 0.03 0.90 99.9 37 64.4 18.5 0.10 0.02 0.05 2.21 13.1 0.10 0.73 99.2 38 63.7 18.4 0.11 0.01 0.03 1.38 14.3 0.04 0.73 98.7 39 64.0 18.3 0.07 0.01 0.05 1.20 14.8 0.08 0.62 99.2 40 (outer) 64.5 18.6 0.09 0.00 0.04 1.19 14.7 0.04 0.62 99.8

of aTiO2 in titanite-bearing dacites and rhyolites small that we calculate temperatures with aTiO2 (± pyroxene, amphibole) (Hildreth, 1981), and are in the range 0.7–0.95 (Hayden et al., 2007). set to 1. The Zr-in-titanite temperature results toward the low end of two-feldspar and FeTi-

Near 750 °C, an aTiO2 of 0.7 would lower are plotted against distance across their host oxide temperatures of titanite-bearing dacites estimated temperatures by 20 °C relative to an megacryst (Fig. 14; negative values are distance (Nakada, 1991; de Silva et al., 1994; Bachmann

aTiO2 of 1. Two titanite-bearing dacites that into the host granitoid), and as curves of relative et al., 2002; Maughan et al., 2002). (2) Titanite were used to develop the geothermometer, and frequency (Fig. 15). grains in the granitoid host rocks have growth that have compositions and mineralogy similar The analyses give the following results. (1) temperatures that overlap those of titanite grains to megacrystic Sierran granitoids (Lund and The megacryst-hosted titanite grains all yield included in the megacrysts, but also extend to

Fish Canyon Tuffs), have weighted-mean aTiO2 igneous temperatures, mainly in the range lower temperatures—in some cases below soli- of 0.8 (Hayden and Watson, 2007), which would 735–760 °C, similar to FeTi-oxide pre-eruption dus values. (3) With the possible exception of lower estimated temperatures by 13 °C relative temperatures of rhyolites with phenocrysts the Golden Bear dike sample, titanite growth

to aTiO2 of 1. These differences are suffi ciently of quartz, sanidine, plagioclase, and biotite temperatures do not defi ne systematic heating or

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cooling gradients from megacryst cores to rims. temperatures can be substantially cooler (to with subsolidus growth temperatures within the In the Golden Bear dike sample some titanite >100 °C) than those of respective cores. megacrysts is evidence that the intrusions never grains near the edges of the studied megacryst The limited range and igneous values of cooled below the solidus during megacryst record higher growth temperatures than those growth temperatures for megacryst-hosted growth. Combined with the observation of ubiq- in the megacryst interior, possibly revealing titanite grains support the interpretation that the uitous sawtooth Ba zoning in the megacrysts, a gradual heating event prior to dike emplace- megacrysts formed as igneous sanidine phe- the narrow igneous temperature range recorded ment. (4) For the grains included in megacrysts, nocrysts, and that intrusion temperatures varied by included titanite grains is consistent with most titanite rims record growth temperatures by only small amounts while the megacrysts episodic recharge of the magma bodies by gra- close to or only slightly cooler than those of grew. Titanite growth temperatures from the nitic liquids that sustained the intrusions above titanite cores (51 of 66 core-rim pairs have rim host granitoids extend to much lower values, their solidus temperatures and replenished the temperatures less than, to no more than fi ve but such solidus and subsolidus temperatures Ba concentrations in the melt. degrees above, associated cores). (5) For grains are absent for titanite grains included within Each idiomorphic megacryst is overgrown by in the surrounding granitoid, titanite rim growth the K-feldspar megacrysts. Absence of titanite a rind, ~0.5 mm thick, of amoeboid K-feldspar that extends interstitially between the plagio- clase, quartz, and other minerals of the host rocks (Fig. 6). Oscillatory sawtooth Ba zoning is absent from these overgrowth rinds and in the anhedral interstitial K-feldspar of the host rocks. Barium concentrations are also gener- ally lower in the interstitial K-feldspar than in the oscillatory-zoned megacrysts (Kerrick, 1969). The cessation of sawtooth Ba zoning and the decrease in K-feldspar Ba concentrations, accompanied by the appearance of titanite with near-solidus and subsolidus temperatures, indi- cate that the megacrysts ended their idiomorphic growth when they became isolated from replen- ishing hot granitic liquids. Idiomorphic growth may have ceased when the megacrysts settled into and were captured by solidifying mushy portions of the intrusions, or replenishment events may have stopped for that magma body, allowing it to fi nally cool and solidify.

HISTORY OF CRYSTALLIZATION

Megacryst-bearing granitoids have an aver- age composition near the granodiorite-granite boundary in the classifi cation of Streckeisen (1973). Their average modal abundances are ~45 vol% plagioclase, 30 quartz, and 25 K-feldspar (normalized to 100%, Bateman, 1992, p. 32). Experimental studies and textural relations indi- cate that the order of initial crystallization of the salic minerals in the Cathedral Peak Granodior- ite began with plagioclase, followed by quartz, and then by K-feldspar (Bateman, 1992). With cooling, crystallization drove the melt composi- tion to the plagioclase-quartz-sanidine cotectic so that all three minerals crystallized together, driving the residual melt composition toward the ternary minimum of true granite composi- Figure 13. Mosaic of electron backscatter images of part of a megacryst from the Golden tion where complete solidifi cation took place. Bear dike showing BaO oscillatory zoning nearly identical to that seen in pluton-hosted Therefore, during much of the solidifi cation of megacrysts. Crystal rim and direction of zone growth is to the top. The seven zone boundar- these megacrystic rocks, the three major phases, ies (numbers on left) begin abruptly with higher concentration of BaO (lighter shade) and plagioclase, quartz, and K-feldspar, crystallized fade gradually to lower concentration (darker shade). Boundaries 6 and 7 (toward top edge) simultaneously, as demonstrated by the inclu- show evidence of resorption with crystal inclusions in pits. Dark inclusions are plagioclase sion in the megacrysts of the other two miner- and light are biotite, hornblende, and accessory minerals. Small white 0.1–0.2 mm rhombic als (as well as the mafi c and accessory miner- titanite crystals occur in upper part of the image. Width of mosaic is 6 mm. als). During this period of simultaneous major

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mineral crystallization, the system behaved in TABLE 2. TITANITE TRACE ELEMENT CONCENTRATIONS AND ZR-IN-TITANITE a near-eutectic fashion where release of latent TEMPERATURES FOR MOUNT WHITNEY GRANODIORITE SAMPLE LPL-7 MEASURED BY ELECTRON MICROPROBE heat buffered the temperature over wide ranges of crystallinity. Grain Position Ce Y Nb Zr T number (mm) (ppm) (ppm) (ppm) (ppm) (oC) Cycling of megacrysts into recharged magmas In K-feldspar megacryst would have required that the magma bodies were 1 core 9.06 10760 1300 1540 780 769 relatively fl uid, at least episodically, seemingly 1 rim 9.05 7730 1540 800 430 735 inconsistent with K-feldspar as the last of the 2 core 7.41 6910 1310 630 570 751 major minerals to commence crystallizing. Evi- 2 rim 7.40 6340 950 750 460 738 dence that the magmas were not highly crystal- 3 core 6.91 7260 1130 1000 450 737 line when the megacrysts grew includes the small 3 rim 6.91 7100 1130 800 460 738 sizes of the plagioclase, quartz, hornblende, and 4 core 6.73 7150 1580 610 610 754 biotite grains included in the megacrysts. The 4 rim 6.73 6610 1150 720 510 744 abundance of fi ne-grained felsic matrix in the 5 core 5.46 6770 1000 920 410 733 megacrystic Golden Bear dike also demonstrates 5 rim 5.45 7850 1190 1030 580 752 the melt-rich character of the megacryst-bearing 6 core 3.93 8460 1350 1090 600 753 magma at the time of quenching. Possibly the 6 rim 3.93 6790 1120 740 440 736 megacrystic intrusions lost appreciable rhyolitic 7 core 3.68 6300 920 780 550 748 7 rim 3.67 6420 830 730 440 736 melt to eruptions as they solidifi ed, so their bulk 8 core 3.65 9240 1160 1320 580 752 compositions are more mafi c than those of the 8 rim 3.65 6420 930 690 470 740 magmas they crystallized from. If so, sanidine 9 core 3.45 6050 1030 730 560 750 megacrysts may have grown at more melt-rich 9 rim 3.44 6540 1070 850 530 747 conditions than would be inferred from phase 10 core 2.21 8760 1250 1290 540 747 equilibrium considerations of the (melt depleted) 10 rim 2.21 6740 980 600 550 748 granodioritic rocks. 11 core 1.50 6370 950 870 480 741 Temperatures close to 740 °C for the rims of 11 rim 1.50 6400 940 700 470 739 megacryst-hosted titanites are a shared feature of 12 core 1.48 6770 960 780 440 736 all the studied intrusions and support the inter- 12 rim 1.48 7850 920 1080 470 740 pretation that sustained near-isothermal condi- 13 core 1.46 6060 940 680 580 751 tions were critical for the K-feldpar phenocrysts 13 rim 1.45 6810 980 780 550 748 14 core 1.31 7360 980 1100 440 736 to grow to megacrystic dimensions. Large gra- 14 rim 1.30 7560 1000 1090 480 741 nitic magma reservoirs that are well mixed and 15 core 0.74 6450 880 850 480 741 semicontinuously replenished by new additions 15 rim 0.73 9090 970 1340 490 742 of felsic magma may be optimal for growth of 16 core 0.59 7100 910 650 570 751 megacrysts, consistent with the megacrystic 16 rim 0.58 6570 940 620 590 753 intrusions in the Sierra Nevada being among the 17 core 0.30 9290 1120 1360 500 743 largest in the batholith (Fig. 1) with protracted 17 rim 0.30 6800 970 810 440 736 intrusive histories (Coleman et al., 2004). Many In amoeboid K-feldspar rind surrounding megacryst large granitic intrusions in the Sierra Nevada 18 core –0.74 6990 850 950 460 738 batholith and elsewhere lack K-feldspar mega- 18 rim –0.75 6390 820 730 430 735 crysts, and instead are either equigranular or 19 core –0.83 6480 1010 640 610 754 have inconspicuous to small (1–2 cm) K-feld- 19 rim –0.83 6510 920 750 590 753 spar phenocrysts. These intrusions may have In host granodiorite cooled through the K-feldspar crystallization 20 core –3.35 5780 500 1570 390 730 interval faster than the megacrystic bodies due to 20 rim –3.36 1150 340 370 60 636 less frequent recharge events, or they may have 21 core –3.72 7020 1030 820 400 731 attained high crystallinities before saturating 21 rim –3.73 670 230 130 <60 <636 with K-feldspar due to more mafi c bulk compo- 22 core –4.70 6530 910 850 410 733 sitions (e.g., Mount Givens Granodiorite; Bate- 22 rim –4.71 550 230 120 <60 <636 man and Nokleberg, 1978). Some megacrystic 23 core –6.30 3480 350 790 250 706 plutons do not have unusually large exposure 23 rim –6.31 2890 300 570 260 707 areas (e.g., the granodiorite of White Moun- 24 core –9.12 4190 460 710 250 706 24 rim –9.13 6670 870 790 460 738 tain; du Bray and Dellinger [1981]; the “quartz 25 core –9.57 5000 430 1200 310 717 monzonite” [granite] of Papoose Flat; Nelson 25 rim –9.58 6850 840 960 400 731 et al. [1978]), seemingly inconsistent with pro- Note: Position is measured perpendicular to one edge of megacryst, positive tracted near- isothermal growth of megacrysts in values are in the megacryst, negative values are in host granitoid. T—temperature. a large, multiply replenished magma reservoir. However, these smaller megacrystic intrusions may be apophyses above larger plutons, or the megacrysts may have grown in larger magma

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reservoirs at greater depths and were later car- silicate melts generally decrease in the sequence necessary for nucleation of new crystals. These ried to the site of emplacement. Na ≥ K > Ca > Al >> Si (Hofmann, 1980), so arguments account for why crystal sizes in true Why do the K-feldspar megacrysts commonly the rate-limiting process for growth of feldspars granites are routinely K-feldspar ≥ plagioclase become 10–100 times larger than plagioclase and quartz is attainment of the appropriate pro- > quartz. In less evolved granitoid magmas this crystals, even though the rock contains many more portions of Si:Al on the crystal-melt interface. ordering of growth and nucleation rates due to discrete plagioclase than K-feldspar crystals? This Granodiorite-to-granite series liquids have Si/ relative “diffusive melt-similarity” is manifest fact has long been a puzzle, but most workers Al ratios in the range 4–5 (molar). Sanidines do by the common poikilitic habit of late intersti- attribute it to low nucleation rates and high growth not depart appreciably from the alkali-feldspar tial K-feldspar that is practically unknown for rates for K-feldspar crystals under certain condi- join and therefore have Si/Al ratios very close to late quartz. In contrast, growth from an aqueous tions in melts of granitic composition (Vernon, 3. Since nearly all plagioclase grown in natural metasomatic fl uid provides no simple mecha- 1986; Cox et al., 1996; Paterson et al., 2005) as subalkaline magmas have signifi cant anorthite nistic explanation for why the K-feldspars attain verifi ed by experimental studies (Swanson, 1977; components, their Si/Al ratios are appreciably megacrystic dimensions.

Day and Fenn, 1982; Whitney, 1975). <3 (An30 2.1, An50 1.7). Igneous quartz contains In addition to the evidence from the zoning We propose that K-feldspars can grow to only trace Al and thus has a nearly infi nite Si/ of the megacrysts recounted above, fi eld evi- megacrystic dimensions, whereas other granite- Al ratio. Viewed in this light, K-feldspar is most dence indicates that the large K-feldspar crystals forming minerals do not, because K-feldspar similar in composition to the coexisting gra- grew as phenocrysts in a melt. Telling evidence is most similar in composition to the granitic nitic melt and therefore has the least diffusive is the concentration of megacrysts in swarms, liquid in terms of the major components that barrier to growth, followed by plagioclase, fol- their presence in dikes, the orientation of their diffuse slowly in silicate melts. For a crystal lowed by quartz. Those minerals with greater long axes parallel to fl ow sorting and dike walls to grow, essential components must diffuse diffusive barriers to growth are also expected (Vernon, 1986; Bateman, 1992; Paterson et al., through the melt to the crystal-melt interface, to have higher nucleation rates because larger 2005), and their association in ladder dikes and and excess or excluded components must dif- volumes of melt distant from established crys- schlieren with swarms of mafi c enclaves. Some fuse away from the interface. Diffusivities in tals can achieve the high supersaturation levels orbicular granitic rocks (as within the mapped

Figure 14. Zr-in-titanite temperatures (Hayden et al., 2007) plotted versus position across K-feldspar megacrysts (diamonds), or in host gran- itoid (circles, negative distance values). Filled symbols are temperatures from titanite cores, open symbols are from titanite rims. A, B, C, and D: Mount Whitney Intrusive Suite samples. C, D, and F show edge-to-edge traverses across megacrysts without adhering host granitoid.

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Recharge brought fresh hot magma into the hosted titanite grains all yield igneous tempera- chamber, which renewed or accelerated convec- tures, mainly in the range 735–760 °C. The tion and mobilized existing crystals on the fl oor growth temperatures of titanite grains in the gra- and walls. It enriched the melt in Ba, much of nodiorite hosts marginal to the megacrysts over- which had previously been depleted because of lap, but are commonly cooler than those of titan- its strong affi nity to the growing K-feldspar phe- ite grains in the megacrysts. The limited range nocrysts. The re-entrained small earlier crystals and igneous values of growth temperatures for then adhered to the surface of the suspended, megacryst-hosted titanite grains support the growing K-feldspar crystals and formed layers interpretation that the megacrysts formed as of crystal inclusions. At a particularly vigorous igneous sanidine phenocrysts, and that intru- and hot recharge event, some material was dis- sion temperatures varied by only small amounts solved from K-feldspar crystals before crystal while the megacrysts grew. Subsolidus tempera- growth began anew. tures are only recorded by titanite grains in the The repeated recharge of a large magma cham- host, indicating that megacrysts ceased growing ber over a period measured in millions of years before the magmas cooled below the solidus. provides a mechanism to maintain it in a hot and The oscillatory zoning and temperature data Figure 15. Probability density functions partially molten state, a condition that would not demonstrate that the megacrysts are igneous (from Isoplot; Ludwig, 2003) of Zr-in-titan- be possible if the intrusion were emplaced at a phenocrysts that grew in melt. The individual ite temperatures for titanite grains included single event as envisaged in simplifi ed thermal Ba-enhanced zones are believed to signal dis- in K-feldspar megacrysts (black curve) and models (Glazner et al., 2004). This extension of crete surges of new, hotter felsic melt injected in contiguous host granitoids (gray curve), the life of a large magma chamber by repeated from beneath the magma chamber, and main- derived for samples W25, LPL7, and SP2E. injections of hot melt could maintain a relatively tained the partially molten or mushy chamber Note that low-temperature shoulder for stable composition and temperature for a long for an extended period until the next recharge host-granitoid titanites is absent for titanites time, an environment in which crystallizing surge. Each recharge of magma would have included in megacrysts. K-feldspar would be stable. Apparently such stirred up crystals on the fl oor and walls of the an environment is required to generate these chamber, and induced convective currents that extraordinarily large crystals. lofted the settling megacrysts back up into the orbicular pipe on the west ridge of Mount Young chamber, where suspended small crystals would and along the Kern-Kaweah River 1 km west CONCLUSIONS stick onto their outer faces, producing the zonal of Rockslide Lake in the Whitney Granodior- arrangements of inclusions. Because of the ite; Moore, 1981) contain nuclei of K-feldspar Megacrysts of K-feldspar from all four of the extended period of maintenance of the magma megacrysts, indicating that they were available granitic plutons containing the largest mega- chamber, this process continued long enough to as free phenocrysts to be coated with comb lay- crysts (4–10 cm in length) in the Sierra Nevada provide the ideal environment for the growth of ers (Moore and Lockwood, 1973). These fea- composite batholith, and from megacryst- these extraordinarily large megaphenocrysts. tures all point to a magmatic origin. bearing dikes associated with these plutons, all Radiometric dating of granitic rocks in the show sawtooth oscillatory zoning marked by ACKNOWLEDGMENTS central and southern Sierra Nevada indicates varying concentration of BaO. Generally each of Mike Diggles aided in the fi eld with photogra- that the axis of plutonism migrated eastward about a dozen rather evenly spaced zones in the phy and collecting; Allen Glazner championed fresh at ~2.7 km/m.y. from 120 to 85 Ma. This rate megacrysts begins rimward with a sharp increase investigations of Sierra problems; Leslie Hayden gen- was independently measured for two traverses in Ba concentration over a distance of 10–25 µm erously provided her titanite geothermometer prior to 100 km apart (Chen and Moore, 1982). This rate and declines over a much greater distance. Some publication and also advised us on its use; William defi nes the eastward movement of active gran- of the more pronounced zones begin with resorp- Hirt supplied samples of the Whitney granodiorite; Forrest Hopson made available information on the ite making in the upper mantle and lower crust, tion and channeling of the underlying zone. Twentynine Palms locality and Brett Cox produced which fed the batholith. Hence beneath a pluton Parallel to the BaO delineated zones are samples from it; Keith Howard provided information 14 km wide, such as the Whitney granodior- zonal arrangements of small mineral inclusions, on the K-feldspar porphyritic plutons in the Mojave ite, activity would continue for 5 m.y. Beneath principally plagioclase, but also biotite, quartz, Desert; Peter Lipman supplied samples from the South Mountain dacite dome on the margin of the the 24-km-wide Whitney sequence of plutons, hornblende, titanite, opaques, and other acces- Platoro Caldera, Colorado; Jacob Lowenstern helped including the Sugarloaf, Lone Pine, Paradise, sory minerals. These inclusions are concentrated with microphotography; Muriel Myers tabulated and Whitney Granodiorites, the period of subja- along zone boundaries, sorted by size along the crystal size information; Robert Oscarson facilitated cent activity would be 9 m.y. During these peri- boundaries with their long axes preferentially electron microscopic and electron probe investiga- ods the growing intrusions are underlain by sites aligned parallel to the boundaries. These fea- tions; Karen Sundback helped with backcountry col- lecting; and Gerhard Wörner supplied information on of magmatic storage, which keep them hot, and tures indicate that the K-feldspar megacrysts backscatter electron images. Charles Bacon, Michael subject them repeatedly to intrusive recharge grew while suspended in melt, allowing the Clynne, Peter Lipman, Sheila Seaman, and Ronald of fresh magma. Each of the 10 or more zones inclusion minerals periodically to attach them- Vernon reviewed the manuscript and all offered valu- within a single Whitney megacryst apparently selves to all the faces of the growing crystals. able advice. We are grateful for this help. records a recharge event of fresh hot granitic The temperature of growth of titanite inclu- REFERENCES CITED magma into the chamber. Such recharge could sions in the K-feldspar megacrysts has been have extended over more than a million years, estimated by use of a zirconium-in-titanite geo- Ague, J.J., and Brimhall, G.H., 1988, Magmatic arc asymme- with individual recharge events spaced more thermometer. Microprobe analyses of zirconium try and distribution of anomalous plutonic belts in the than 0.1 m.y. apart. in titanite inclusions indicate that the megacryst- batholiths of California: Effects of assimilation, crustal

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