Mineralogy and Geothermobarometry of Magmatic Epidote-Bearing Dikes, Front Range, Colorado

Mineralogy and geothermobarometry of magmatic epidote-bearing dikes, Front Range, Colorado

RALPH L. DAWES I Department of Geological Sciences AJ-20, University of Washington, Seattle, Washington 98195 BERNARD W. EVANS

ABSTRACT Thermobarometric calculations provide em- old pressure required for magmatic epidote to pirical evidence that epidote is stable in silicic, form in silicic systems, a question of great signif- Epidote phenocrysts (PS19-24) in Lara- calc-alkaline magmas at high total pressure, icance to tectonic reconstructions, is still rela- mide-age, porphyritic dacite dikes in the high water content, and high oxygen fugacity. tively uncertain, and indeed somewhat contro- Front Range of Colorado prove that epidote The dacitic magmas originated by partial versial (Tulloch, 1986; Moench, 1986; Zen and can crystallize directly from a magma. The melting at temperatures above 800 °C. Tem- Hammarstrom, 1986; Zen, 1988a). dikes are high-K, calc-alkaline dacites and peratures (±50 °C) ranged from 800 to 880 °C The petrographic criteria by which epidote rhyodacites (Si02 = 64%-70%). Phenocryst during early phenocryst growth down to 620 of magmatic crystallization can be distinguished assemblages consist of combinations of epi- to 670 °C during groundmass crystallization. from that of subsolidus, hydrothermal, or meta- 3+ dote, plagioclase (An53_j9), biotite [Fe / Phenocrysts formed at pressures between 7.2 morphic crystallization are essentially those that 2+ petotai = Q 25. Mg/(Mg + Fe ) = 0.42-0.55], ± 1.0 and 12 ± 2.0 kbar. Oxygen fugacity was establish the role of epidote as a product of reac- quartz, aluminous amphibole (AI,ota| = within the magnetite stability field at two log tion during cooling between high-Al igneous 2.4-3.0 per formula unit), igneous garnet units (+1.0) below the hematite-magnetite hornblende and residual liquid (Zen, 1988a). (And2.6Gros13_27Spess2-i3Pyr7.26Alm5i_60), buffer. H2O fugacity calculations give values The textural observations on plutonic rocks are igneous muscovite, and sanidine. Holocrystal- ranging from 3.4 to 16 kbar, depending on thus consistent with the sequence of mineral line, aphanitic groundmasses consisting main- how annite activity in biotite is modeled. The growth reported in the 8-kbar experiments on ly of quartz, plagioclase, and alkali feldspar occurrences of types I and III epidote com- synthetic granodiorite by Naney (1983). It is the comprise 60%-75% of the dike rocks and bined with geobarometry give a minimum case, nevertheless, that many Cordilleran grano- require that epidote grew early in the crystal- pressure of epidote stability in the dike mag- diorite to tonalite plutons have undergone pro- lization sequence. mas of 8.0 ±1.0 kbar. These results agree tracted supersolidus and subsolidus deformation, with experimental evidence and with condi- Petrographic and microprobe data distin- with accompanying recrystallization, such that tions inferred for the crystallization of mag- guish three types of magmatic epidote pheno- in many cases it is no longer a simple matter to matic epidote in large, Cordilleran plutons. crysts, with different petrogenetic connota- decide on an igneous origin for epidote. More Phenocrysts in the dikes were preserved in tions implied by each type. Type I epidote convincing evidence for the magmatic crystal- conditions outside their stability limits by occurs as euhedral phenocrysts, up to 8 mm lization of epidote is provided by its occurrence rapid emplacement and quenching in shal- long, with low content of allanite component as phenocrysts in rapidly quenched liquids low-level dikes. (<0.02 total REE cations per 12.5 oxygen (Williams and Curtis, 1977; Evans and Vance, formula unit); many have allanitic cores upon 1987). In these somewhat more Si02- rich magmas, epidote crystallized directly from which they nucleated epitaxially. Type II epi- INTRODUCTION dote forms smaller (0.2-2 mm) phenocrysts the liquid, apparently without resorption of amphibole. which have a significant amount of allanite Recent experimental and field studies have component (from 0.01 to 0.30 total REE cat- shown that at elevated pressures epidote can The study of quenched systems has the advan- ions p.f.u.) in oscillatory zones not confined crystallize during the cooling of liquids of gra- tage of providing unambiguous information on to crystal cores. Types I and II epidote both nitic, granodiorite, and tonalitic composition the conditions and relationships involved in the show a continuous solid solution between ep- (Naney, 1983; Zen and Hammarstrom, 1982, earlier crystallization of magmas at depth. With idote and allanite. Type III epidote, inter- 1984a, 1984b, 1988). Given the compositional this in mind, this study presents further informa- preted as relict type I phenocrysts, occurs as complexity of natural systems, and the fact that tion on the field relations, magma compositions, skeletal, optically continuous inclusions inside controls are exerted by four independent inten- petrography, and mineralogy of epidote-pheno-

plagioclase phenocrysts. A reaction relation- sive parameters [T, P, f(H20), f(02)] as well as cryst-bearing, silicic dikes found by Evans and ship, in which epidote becomes unstable rela- bulk composition, it is not surprising that the Vance (1985,1987) in the Front Range of Colo- tive to plagioclase due to depressurization, is definitive, clarifying experiments on igneous ep- rado. The compositions of coexisting phenocryst inferred. idote remain to be done. As a result, the thresh- phases permit estimates to be made of the

Additional material for this article (Tables A-E) may be secured free of charge by requesting Supplementary Data 9120 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 103, p. 1017 -1031,11 figs., 6 tables, August 1991.

1017

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Late K-Eocene Intrusive Rocks Tas:alkall syenite stocks

105°' 7'30" 105° 30' Figure 1. Map showing bedrock geology and dike locations. Bedrock geology from Gable (1980), Gable and Madole (1976), and Pearson and Johnson (1980).

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temperatures, pressures, and oxygen and H2O crystalline up to the contacts. Platey or elongate fission track ages from dikes 5, 60, and 83 are fugacities attending the crystallization, not only phenocrysts, including epidote, commonly show 55.5 ± 5.5 (Evans and Vance, 1987), 52.6 ± 5.6 of igneous epidote but also igneous garnet and flow alignment subparallel to the dike walls, (Pearson and Johnson, 1980), and 53.0 ± 5.5 muscovite. which becomes more pronounced closer to the Ma (J. A. Vance, 1987, oral commun.), respec- margins of the dikes. No new mineral growth is tively. The 71 Ma age places the formation of GEOLOGIC SETTING AND visible in wall rocks. Contacts are knife-sharp. dike 5 among the earliest intrusive ages in the FIELD RELATIONS Other studies support a relatively shallow mineral belt and very near the initiation of Lar- level of dike emplacement. Prior to Laramide amide uplift of the Front Range (Tweto, 1975). More than 20 dikes have been identified as uplift, the Precambrian core of the Front Range The Eocene fission track ages may indicate later part of a suite of magmatic epidote-bearing and had a cover of ~ 3 km of Phanerozoic sedimen- reheating, either by syenitic stocks of early Eo- closely related dacites, all within 20 km of tary rocks (Tweto, 1975). Fluid-inclusion stud- cene age in the eastern part of the area (Fig. 1; Ward, Boulder County, in the Front Range of ies of mineral-belt ore deposits and related ages tabulated in Gable, 1984) or by a hypo- Colorado (Fig. 1). This region is at the north- hydrothermal alteration indicate that only 1-3 thetical Tertiary batholith underlying the Front eastern end of the Colorado mineral belt km of Precambrian basement have been re- Range portion of the mineral belt (Case, 1965). (Lovering and Goddard, 1950), a zone of moved from the Front Range since Late Cre- abundant small intrusions and associated miner- taceous time (Hudson and Atkinson, 1988; WHOLE-ROCK CHEMISTRY alization of uppermost Cretaceous to Tertiary Myint and others, 1988). The combined, maxi- age. The dikes intrude two main types of Pre- mum thickness of sedimentary cover plus Pre- Ten dike rocks were chosen as representative cambrian country rock: cordierite-sillimanite- cambrian basement eroded from the Front of the magmatic epidote-bearing suite for chem- biotite gneiss which locally contains garnet Range since emplacement of the dikes is there- ical analysis (Table A).1 AH analyzed dike rocks (Gable and Sims, 1969) and is thought to have fore ~4~6 km. It is thus reasonable to assume contain magmatic epidote, except for dike 58, formed during regional metamorphism at about that the magmatic epidote-bearing dikes were which is the most mafic sample analyzed. Its 1750 Ma (Hedge and others, 1976), and two- emplaced at shallow depth, where total pressure affinity to the epidote-bearing suite is inferred mica granitic rocks of the St. Vrain batholith, was 2 kbar or less, and country-rock tempera- from its spatial proximity, whole-rock chemis- which is of Silver Plume type (Anderson and tures were no more than about 250 °C (Evans try, and the presence of high-A1 amphibole and Thomas, 1985) and has an age of about 1400 and Vance, 1987). igneous garnet similar to those found in the Ma (Peterman and others, 1968). In much of the The causes and tectonic affinities of Laramide magmatic epidote-bearing dikes. study area, the granite has pervasively intruded magmatism in the Front Range are speculative. The dike rocks are calc-alkaline dacites and the metamorphic rocks. Emplacement of the dikes may have been re- rhyodacites (Irvine and Baragar, 1971; Streck- The field relations of the dike rocks, and the lated to deep-seated faulting during the initiation eisen, 1979), and they fall in the high-K dacite porphyritic-aphanitic texture of even the largest of Laramide uplift of the Front Range. The field of Ewart (1979). Si02 content of the dikes dikes, indicate that they were rapidly emplaced origin of the magmas is most likely related to ranges from 64 to 70 wt%. Values of Mg/(Mg + and quenched at hypabyssal depth in dilatant subduction of an oceanic plate at a low angle Fe) are between 0.30 and 0.40, except for dike fractures. The dikes range in size from approxi- beneath the Cordillera during Laramide time 83, which has a value of 0.22. Rare-earth abun- mately a meter wide and a few meters in length (Bird, 1984), and the magmas may have been dances are typical of intermediate calc-alkaline to a maximum width of -100 m and a maxi- preferentially channeled through pre-existing, rocks, and in particular light rare-earth abunr mum length of more than 1 km. Most of the crustal-scale faults reactivated in the Laramide dances are not unusually high in the magmatic dikes trend within 30° of east-west and dip orogeny (Tweto and Sims, 1963). epidote-bearing dikes. steeply. The outer 0.5-3 cm of the dikes generally are phenocryst-poor, show a slight color dif- AGE OF THE DIKES PETROGRAPHIC SUMMARY ference, and have a semi-conchoidal fracture. Phenocrysts within 1-2 cm of dike margins tend Biotite K-Ar and biotite-whole-rock Rb-Sr Phenocryst assemblages are summarized in to be more bent, fractured, or broken than in ages for dike 5 are concordant at 71 Ma (69.0 ± Table 1. Inclusion relationships show that, in dike interiors, but the groundmass remains holo- 2.0 and 74.0 ± 3.0 Ma, respectively). Zircon general, zircon preceded apatite, which preceded garnet and allanite, followed by amphibole, plagioclase, epidote, biotite, quartz, and alkali

TABLE 1. REPRESENTATIVE DIKES feldspar. In some dikes, epidote preceded plagioclase. Muscovite phenocrysts, present only in

Dike Ep Ps% % matrix C Phenocrysts An 5SSi02 corundum-normative dikes, largely overlapped biotite in the crystallization sequence. 1 I 20.5 68 n.d. PI, Q, Bt, Ep, Gnt 46-24 n.d. 5 I 21.0 73 0.54 PI, Q, Bt, Ep, Gnt 46-23 69.4 The groundmass in all of the dikes is apha- 6 III 20.7 70 0.25 PI, Bt, Hb, Q 52-32 64.4 15 II 23.1 75 2.73 PI, Q, Bt, Ms, Ep, Gnt 52-25 69.3 nitic, holocrystalline, and largely xenomorphic; 20 II 23.0 75 n.d. PI, Q, Bt, Ms, Ep, Gnt 50-24 n.d. it consists of plagioclase, quartz, alkali feldspar 26 11 23.9 75 1.37 PI, Q, Bt, Ep, Gnt 54-34 66.5 32 II 22.7 70 1.29 PI, Q, Bt, Hb, Gnt, Ep 52-24 68.4 and accessory apatite, magnetite, and zircon; no 41 III 19.7 65 0.00 PI, Bt, Hb, Q 50-28 64.7 54 III 23.6 70 n.d. PI, Q, Bt, Hb, Gnt 49-28 n.d. hydrous phases are present. Trace amounts of 57 III 21.9 65 1.61 PI, Q, Bt, Gnt 48-26 67.9 ilmenite were found in the groundmass of two 58 none 60 0.41 PI, Bt, Hb, Q, Gnt* 54-31 64.3 60 I 23.8 70 0.45 PI, Q, Bt. Ep 45-23 68.1 dikes (5 and 41). The groundmass in all of the 83 I 22.9 70 0.15 Pl. Q, Bt, San, Ep 42-20 69.8

Note: Ep = type of epidote (only one type occurs in each dike); Ps = Ps content in epidote [Fe-*+/(Fe3* • Al)]; % matrix in mode from 1,000-point counts for dikes

1 and 5, others from visual estimates (±5%); C - normative corundum in whole rock; % Si02 - wt% from whole-rock analysis; An = anorthite content in plagioclase; n.d. = not determined. Phenocrysts listed in order of abundance: PI = plagioclase; Q = quartz; Bt = biotite; Hb - hornblende; Ep - epidote; Gnt = igneous garnet; Ms = •Tables A through E may be secured free of charge muscovite; San - sanidine. •Dike 58 also contains accessory phenocrysts of titanite and allanite. by requesting Supplementary Data 9120 from the GSA Documents Secretary.

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Figure 2. (a) Two synnuesis-joined type I Petrographic and compositional criteria per- somewhat rounded, suggesting minor corrosion epidote phenocrysts from dike 5. The larger mit three types of magmatic epidote phenocrysts (Fig. 2). In dike 83, many of the type I epidote crystal has a slightly rounded aspect; the to be distinguished; namely, types I, II, and III phenocrysts are extensively rounded and em- smaller one has a darker allanitic core. Adja- epidote (Table B). Only one type of epidote is bayed, and they appear to have been corroded cent phenocrysts are partly chloritized biotite. present in any given dike. Because the REE con- from euhedral form during the intratelluric Crossed polars; scale bar = 1 mm. (b) Three tent may have a pronounced effect on epidote stage. In contrast, type I epidote included in the type I epidote phenocrysts in dike 1. The stability, it is worthwhile to clearly distinguish large sanidine phenocrysts in dike 83 is euhed- large phenocryst on the right is bent and between types of epidote in which differing hab- ral, showing only a small amount of corrosion. cracked where it is impinged upon by a its and occurrences are correlated with different Simple and polysynthetic twinning on (100) corner of a plagioclase phenocryst. Plane REE contents. are both common in type I magmatic epidote. light; scale bar = 1 mm. (c) Type II epidote Magmatic epidotes of types I, II, and III have Type I phenocrysts occur singly, or in rare syn- phenocryst in dike 32; small apatite crystal 19.5% to 25.2% of the pistacite (Ps) end nuesis clusters of up to five crystals. Subparallel partly intergrown along lower border. 3+ member, Ca2Fe 3Si3012(0H) (Table B). flow alignment of epidote, biotite, and elongate Crossed polars; scale bar = 0.5 mm. (d) Type Within each individual dike, microprobe spot plagioclase crystals is common. Within 1 cm of III epidote inside plagioclase, dike 57. Epi- analyses of epidote compositions vary from a contacts with wall rock, where flow alignment is dote crystal is partially resorbed, optically mean Ps value by no more than ±1.0. Secondary strongest, small biotite crystals are oriented in continuous. Zoned plagioclase crystal is part epidotes, replacing biotite and amphibole (along flow lines which wrap around larger epidote of a synnuesis cluster. Crossed polars; scale with chlorite), are distinctly more iron rich, phenocrysts. Some epidote crystals are bent and bar = 1.75 mm. (e) Muscovite phenocryst in PS29~33- cracked where they abut corners of plagioclase dike 15, with sharply bounded intergrowth of Type I Epidote. Type I epidote was de- or quartz phenocrysts, presumably due to the biotite along bottom boundary, including scribed previously by Evans and Vance (1985, force of magma flow during dike emplacement smooth, butt-end contact, interpreted as an 1987). It was not found in assemblages contain- (Fig. 2b). equilibrium texture. Plane light; scale bar = 1 ing hornblende or muscovite. The pleochroic Allanitic cores, strongly oscillatorily zoned in mm. (f) Same as (e), crossed polars, showing scheme is X,Y = colorless, Z = pale greenish REE, are visible in 25%-30% of the type I phe- kinking of muscovite phenocryst. yellow. Phenocrysts of type I epidote are pris- nocrysts as seen in thin section. Truncated oscil- matic and tend to be highly elongate in the b latory zones surrounded by zones from a direction. Most phenocrysts are between 0.5 and subsequent stage of growth are common in alla- 4 mm in length; exceptional ones are as much as nitic cores (Fig. 3a); often as many as three of 8 mm long. Outlines of type I epidote are com- these internal corrosion boundaries occur con- monly euhedral, but in many crystals, the faces centrically. Rare cores of allanite which are dikes typically has a somewhat hiatal grain-size are slightly irregular in detail and corners are rounded and virtually unzoned occur at the cen- distribution. Larger plagioclase, quartz, and alkali feldspar crystals have approximately equant, hypidiomorphic aspects and are 50-200 /urn across, and there is an even finer-grained groundmass of xenomorphic, felsic minerals 5-25 /^m in size. Although the freshest possible rocks were collected and analyzed, most show a small amount of alteration, as indicated by flecks of sericite in phenocryst and groundmass feldspars, and incipient to well-developed chlo- ritization of biotite and amphibole, accompanied by magnetite + epidote ± titanite. Titanite, and allanite not mantled by epidote, were present as phenocryst phase only in dike 58. Alteration was variable within individual dikes, which permitted the analysis of apparently unaltered examples of all phenocrysts.

DIKE MINERALOGY

Epidote. Note on Terminology. In this paper, we refer to a mineral of the epidote structure containing >0.5 REE p.f.u. as allanite; to Figure 3. Back-scatter electron photomicrographs of allanitic cores in type I epidote; all scale zoned epidote in which the zones predominantly bars = 100 microns, (a) Slightly rounded, homogeneous inner core surrounded by multi-staged contain 0.05-0.5 REE p.f.u. as allanitic epidote; oscillatory-zoned overgrowth of outer core. Outer core is enveloped by type I epidote, which and to epidote with <0.05 REE p.f.u. as epidote appears black (dike 5). (b) LREE "shadow" extending from zoned allanitic core into epitaxial (sensu stricto), or as low-REE epidote. overgrowth of type I epidote (dike 83).

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ter of oscillatorily zoned allanitic cores in epi- Analyses of types I and II epidote, including range of plagioclase compositions in a given

dote phenocrysts (Fig. 3a); most likely, these are allanitic cores, show complete solid solution rock (typically An40 ± 5). Type III phenocrysts xenocrysts from the residuum of partial melting. from zero REE to a maximum near 0.6 La+ are generally relatively large, from 2-7 mm The boundaries of allanitic cores with surround- Ce+Nd+Sm cations per 12.5 oxygens. A plot of long. ing epidote vary from euhedral to highly em- LREE cations against Ca-1 for type I epidote Secondary Epidote. Fine-grained, anhedral bayed. Some of the allanitic cores have a skeletal and allanitic epidote in dike 83 (Fig. 4a) con- epidote is a common alteration product of bio- aspect which may be due to rapid growth, as firms a nearly one-to-one substitution of LREE tite and amphibole in the dacite dikes. It tends to indicated by the concordance of internal zoning for Ca, and shows that La, Ce, Nd, and Sm grow along cleavages of the altered minerals and with exterior form, but in some allanitic cores, make up nearly all of the substitute component; is usually associated with chlorite ± magnetite. It euhedral internal zoning is truncated by the analyses of type I and type II epidote from other has a more noticeably green pleochroic color in outer boundary with epidote, indicating partial dikes extend over the same range. Previous stud- the Z direction than do primary phenocrysts; the corrosion prior to epitaxial overgrowth by ies have demonstrated the coupled substitution: pleochroic formula is X = clear to pale yellow, Y epidote. Ca2+ + (Al3+, Fe3+) = REE3+ + Fe2+ (Deer and = pale greenish yellow, Z = yellowish green. The Allanitic cores have maximum total LREE others, 1962). This substitution is predominant ferric iron content of secondary epidote is much oxide values of about 16 wt% in the centers of in the phenocrystic epidote. Mg also increases higher than that of magmatic epidote; Ps falls in the cores; more typically, the totals fluctuate be- systematically with the LREE in epidote (Fig. the range 29-33. Secondary epidote is also dis- tween 8 and 14 wt% (-0.3-0.5 LREE cations 4b), joining Fe2+ in a fairly constant proportion: tinct from magmatic epidote in having more 2+ Ti02 and less MgO (Table B). p.f.u.). Outside of allanitic cores, and in crystals Mg/(Mg + Fe ) = 0.20-0.30. Th02 is between wljich have no allanitic core (as seen in thin 0 and 0.02 wt% in type I epidote, and between Plagioclase. Euhedral plagioclase pheno- section), type I epidote is oscillatorily zoned in 0.01 and 0.10 wt% in allanitic epidote, showing crysts, 1-8 mm across, display normal zoning allanite component, but contains fewer than no systematic variation with other elements with superimposed fine-scale, euhedral oscilla- 0.02 LREE cations p.f.u. The oscillatory zones (compare Gromet and Silver, 1983). tory zoning. Synnuesis clusters of plagioclase are euhedral, fine scale, and appear sharply de- Type HI Epidote. Some of the dikes contain crystals are common. Core An values are in the fined, particularly when viewed down Y. As skeletal epidote inclusions within plagioclase range 40-55, and rim values are between 19-30 many as 50 concentric, oscillatory zones occur phenocrysts (Fig. 2d). These are interpreted to (Table 1). In dikes 1 and 5, where An content in individual epidote phenocrysts, at an approx- be relict type I epidote phenocrysts for the fol- declines at the rims, K-feldspar (Or) content imately constant spacing of 3-5 /¿m, although lowing reasons: (1) they are compositionally rises until a plateau is reached, in which the Or wider or more irregularly spaced zones also identical to type I epidote (Table B); (2) the content of the plagioclase phenocryst rims over- occur. skeletal fragments, where discontinuous, are still laps that of the groundmass plagioclase, which Type II Epidote. Type II magmatic epidote in optical continuity, even where they extend grew simultaneously with K-feldspar (Fig. 5). is distinguished from type I by its generally through adjacent, synnuesis-joined plagioclase Plagioclase rims in dikes 1 and 5 thus ap- smaller size (0.2-2 mm), less elongate habit crystals which have different crystallographic proached and apparently achieved saturation (Fig. 2c), and, more importantly, by having a orientations; and (3) oscillatory zoning in plagi- in K-feldspar component (Evans and Vance, significant, albeit variable, allanite component. oclase is interrupted in the vicinity of the type III 1987). This results in a slightly different pleochroic inclusions by a homogeneous, calcic plagioclase Quartz. Subhedral quartz phenocrysts, 1-7 scheme (X = colorless, Y = light yellowish zone which outlines a shape suggesting a once- mm in diameter, have dipyramidal, hexagonal brown, Z = light tan-green) and lower birefrin- complete euhedral epidote phenocryst. The external morphology. Quartz phenocrysts are gence. The phenocrysts range from euhedral to homogeneous plagioclase surrounding skeletal slightly rounded to highly rounded and subhedral with rounded corners and embay- epidote fragments is at the calcic end of the embayed. ments. Simple and lamellar twinning on (100) occurs. Phenocrysts composed of two symmetric twin units, with well-developed (100), (001), and (101) prism faces, in contact along a central (100) plane, are common. The allanite component in type II epidote is zoned, commonly in very fine, sharp, euhedral oscillations, less commonly in patchy or anhedral zones. La+Ce+Sm+Nd values (which account for >90% of the REE content) do not exceed 0.30 p.f.u., except inside sharply bounded cores distinctively richer in allanite component. A small number of type II phenocrysts have anhedral rims of low-REE epidote. These rims are compositionally indistinguishable from type I epidote. It is much more common to find oscillatory zones with appreciable REE content all the way to the rim in type II phenocrysts; in many crystals, the rims are slightly en- Figure 4. (a) A-site cations from normalized microprobe analyses of type I epidote and riched in LREE. Type II epidote phenocrysts in allanitic cores, showing complete solid solution from REE-free type I epidote to allanitic which rims contain significant allanite compo- epidote (dike 83). (b) LREE cations versus Mg cations from type I epidote and allanitic cores nent tend to be euhedral. (dikes 5 and 83).

12 cite dikes, logK of biotite-muscovite F/OH is

between 0.8 and 0.9, XPHLOG about 0.35). The distribution of Ti02 between muscovite and co- Figure 5. K-feldspar com- existing biotite phenocrysts in the rhyodacite C_ 8 ponent versus anorthite com- dikes matches that of muscovite cores and bio- CD CL ponent in plagioclase from tite in two-mica granite (Monier and Robert, in dike 1. Triangles: pheno- 1986), and the distribution ratio (Ti in musco- * 4 crysts; squares: groundmass. vite/Ti in biotite = 0.22-0.26) is in the range of Isotherms for K-feldspar-sat- biotite-muscovite pairs in high-grade pelitic urated plagioclase calculated schists (Guidotti and others, 1977).

from ternary feldspar solu- High Ti02 content in muscovite (>0.60 wt%) tion model of Fuhrman and suggests a magmatic origin (Miller and others, 24 28 32 36 Lindsley (1988) for 10 kbar. 1981; Speer, 1984; Zen, 1988b). The higher Ti content in the core may be attributed to higher % An temperature during core crystallization as well

as greater abundance of Ti02 in the melt before depletion by simultaneous muscovite and biotite Biotite. Biotite occurs in euhedral, hexagonal those in so-called two-mica granites interpreted growth. In sum, all of the textural and composi- plates up to 4 mm across. Slight bending and as being of magmatic origin (Best and others, tional criteria point to a primary magmatic kinking of the plates is the norm. Pleochroism is 1974; Miller and others, 1981; Lee and others, origin for the muscovite phenocrysts. Y, Z = light yellow-brown, X = dark sienna- 1981; Speer, 1984). Muscovite phenocrysts have Three other types of white mica besides the brown. Incipient replacement by chlorite, along been reported in extrusive rhyolites (Schleicher relatively large phenocrysts have been observed with secondary epidote and magnetite, is com- and Lippolt, 1981; Noble and others, 1984) in the dacite dikes, usually in more altered rocks: mon along crystal margins and cleavages. Unal- which are also generally similar to muscovite in (1) sericite in altered feldspar; (2) replacement tered biotite is homogeneous, both within the dacitic dikes, except that they contain less Fe product of altered biotite; and (3) fine-grained individual phenocrysts and within a given dike. and Mg and diverge in F content. Higher bulk- growth in slightly corroded type I epidote. On Fluorine content of biotite is between 0.60 rock Fe and Mg content and a higher oxidation textural grounds, it appears obvious that these and 0.70 wt%, CI consistently near 0.10 wt%, state could largely account for the greater Fe and have a secondary origin. They are also composi-

and Ti02 between 2.6 and 3.5 wt% (Table C); Mg in muscovite from the rhyodacite dikes. tionally distinct from phenocrystic muscovite all of these values are in a range typical of igne- Compared to muscovite from rhyolites (Noble (Table C; Fig. 7), most notably in having con- ous biotites in granitic rocks (Dodge and others, and others, 1984), the muscovite phenocrysts in sistently much lower Ti02 content. 1969). In the peraluminous, muscovite-bearing rhyodacite dikes near Ward have apparently not Garnet. Two distinct populations of garnet dikes, the AI2O3 content of biotite is higher been subjected to defluoridization. On the other were found in the dikes: igneous (phenocrystic) (17.5-18.5 wt%) than in dikes without musco- hand, the F content is not high enough to stabi- garnet (Fig. 8), which is thought to have been an vite (15.5-17 wt%), suggesting that biotite and lize the muscovite to low pressure (Schleicher early liquidus phase, and xenocrystic garnet, muscovite were in equilibrium. Mg/(Mg + and Lippolt, 1981; Pichavant and Manning, thought to have been extracted from mid-crustal Fe2+) values of biotite are between 0.42 and 1984). metamorphic rocks by the magma as it rose 0.51 in the magmatic epidote-bearing dikes, and The partitioning of F between muscovite from its source area. The rims of many xeno- about 0.55 in dike 58. phenocrysts and co-existing biotite in the Front cryst garnets are altered to igneous compositions Muscovite. Muscovite compositions are Range dacitic dikes indicates that they were in (Fig. 8). The two types of garnet occur together given in Table C. Muscovite phenocrysts typi- equilibrium, according to the "equilibrometer" in several of the dikes (dikes 1, 5, 26, and 54). cally form euhedral books up to 8 mm across of Munoz and Ludington (1977; for the rhyoda- Igneous Garnet. Euhedral garnet crystals, (Figs. 2e, 2f). Some phenocrysts are subhedral 0.1-0.7 mm in diameter and pale pink in thin to skeletal, appearing to have undergone partial section, most commonly occur as inclusions in resorption concentrated along cleavage ends. plagioclase phenocrysts. Rounded garnets occur Like biotite, the phenocrysts are slightly bent in the groundmass independently of other phe- and kinked. Examples of euhedral muscovite nocrysts, and euhedral garnet inclusions in bio- phenocrysts in sharp, butt-end contact with biotite are least common. It is an accessory phase; tite phenocrysts occur, suggesting stable co- more than three garnets in one thin section is existence of the two micas. The muscovite in the unusual. rhyodacite dikes has Ti02 between 0.57 and The phenocrystic garnet in general can be 0.87 wt%. Some of the larger, uncorroded phe- described as grossular-rich almandinic garnet. nocrysts show core to rim zoning of Ti in micro- In terms of mol%, the compositions fall within

probe linescans parallel to cleavage (Fig. 6). the range And2_5Grosi5^27Spess2_13Pyr7_26 As with type I epidote, the euhedral form, Alm51_6o (Table D; Fig. 8). Except for garnet in relatively large size juxtaposed against an apha- muscovite-bearing dikes, grossular content is nitic matrix, and lack of reaction textures with generally above 20 mol%. None of the magmatic other minerals make a convincing case that the garnets is strongly zoned, but microprobe line-

muscovite crystals originated as phenocrysts Figure 6. Ti02 zoning in muscovite, from scans of some of the largest garnets inside crystallized from a magma. Compositions of 20-point rim-rim microprobe linescan across plagioclase reveal systematic, antithetic fluctua- muscovite phenocrysts (Table C) are similar to 4.2-mm-wide phenocryst (dike 20). tions of grossular and spessartine component,

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with CaO varying by as much as 3.5 wt%. Line- 1976); and also to the least grossular-rich rims of rich but still quite distinct from the phenocryst scans also showed that in most garnets grossular garnet sheathed by plagioclase in the mag- compositions. content drops by 1-4 mol% at the rim relative to matic epidote-bearing Bushy Point tonalite of The strongest evidence that these high- the core (see Table 5 below); however, varia- southeastern Alaska, interpreted as being of pyrope, low-grossular garnets are accidental in- tions in almandine, pyrope, and spessartine con- high-pressure, igneous origin (Zen and Ham- clusions in the dikes is the fact that the narrow tents do not follow a consistent pattern. marstrom, 1984a). This strongly suggests a pink rims have compositions either indistin- This type of garnet is thought to have a mag- high-pressure origin of the garnet in the Front guishable from the phenocrystic garnets, or matic origin because (1) the garnet crystals are Range dikes, and the close similarity to garnets transitional between the xenocryst and pheno- either euhedral (inside plagioclase or biotite) or produced experimentally in dacitic melts cryst compositional fields (Fig. 8). This indicates somewhat rounded as if slightly resorbed (in the at high-water activities and high pressures that the xenocrystic garnets were not in equilib- matrix); (2) they contain randomly oriented, (>8 kbar) is almost certainly no coincidence rium with the dacitic magma in which they small, euhedral apatites and tiny, euhedral zir- (compare with Green, 1977). became included, but the rims became altered cons identical in appearance: to those included in Xenocrystic Garnet. A number of the dikes, toward equilibration with the melt, and there- other phenocryst minerals; (3) except for the zir- including two of the type I epidote-bearing fore attained compositions approaching, or iden- con and apatite crystals, they are clear and dikes, contain xenocrystic garnet. Xenocrystic tical to, the igneous garnets which crystallized inclusion-free; in particular, they contain no typ- garnets are quite variable in size, ranging from directly from the magma. ically metamorphic minerals; (4) most are en- 0.1-15 mm in diameter. They occur as slightly The xenocrystic garnets could not have come closed in igneous plagioclase displaying normal rounded to highly rounded and embayed crys- from the country-rock biotite gneisses now ex- and fine-scale oscillatory zoning. The fifth, tals surrounded by the dike matrix. Many have posed in the Front Range, because the and most important, indication of an igneous corroded rims with wisps of chlorite and asso- Mg/(Mg + Fe) values of garnets in the gneiss origin is the composition of the garnet. The ciated magnetite. Sillimanite needles occur (Gable and Sims, 1969) are only about half compositions bear no resemblance to garnets rarely in large xenocrystic garnets. The garnets those of the xenocryst garnets. The xenocrystic from country-rock biotite gneiss (Gable and are colorless in thin section except for the out- garnets are also dissimilar to garnets from lower Sims, 1969), or to garnets from a variety of ermost rims, which commonly have a pale pink crustal mafic granulite xenoliths from the State mafic granulite xenoliths, thought to be of tinge. Line kimberlites, 80-100 km north of Ward lower-crustal origin, found in kimberlite dia- The xenocrystic garnets are compositionally (Bradley and McCallum, 1984). tremes in the northern Front Range (Bradley quite distinct from the phenocrystic ones (Table Evans and Vance (1987) reported the discov- and McCallum, 1984), nor do they resemble D). They consist dominantly of pyrope and al- ery of a single xenolith from dike 6 with pelitic xenocrystic garnets within the dacite dike suite mandine, with % Mg/(Mg + Fe) values of granulite mineralogy (plagioclase-biotite-garnet- itself (Table D). 38-41 and much lower grossular content than The garnet compositions do bear a strong re- the phenocrysts. Most of the xenocryst garnets semblance (Fig. 8) to garnet crystallized at high from different dikes fall in a narrow composi- pressure from water-bearing dacitic melts in a tional range (Ando.4_2.4GroS[j^gSpessog-i 2 Alm number of experiments (Green and Ringwood, Pyr37.0 40.6Alm54.0-59.3)- Large garnet xeno- 1968, 1972; Stern and Wyllie, 1973; Green, crysts in dike 12 are somewhat more grossular-

.05

.04 to Magmatic c o -r-f c (O .03

OJ CVJ .02 Replacing Biotite" (U Q. Gro .01 Replacing Plag.- Figure 8. Igneous garnet compositions in terms of almandine- grossular-pyrope (after Zen and Hammarstrom, 1984a). Composition .00 fields (references given in text): (1) Bushy Point garnets; (2) eclogites; (3) high-P andesite, tonalite, and basalt experiments; I.G. = field of Mg/Mg+Fe igneous garnets and xenocryst garnet rims from this study; X.C. = field of xenocryst garnet core compositions. Arrow and temperatures Figure 7. Titanium versus Mg number in magmatic and secondary showing trend of experimentally produced igneous garnets taken from white mica. Stern and Wyllie (1973).

TABLE 2. GARNET-BIOTITE THERMOMETRY

Dike Pfkbar) Xalm Xpyr Xgro Xspe XFeBt XMgBl XAlviBt XTiBt XMnBl InK Temperature t°C)

FS HS GS IMA 1MB

1 9 0.543 0.138 0.213 0.074 0.367 0.353 0.090 0.058 0.010 1.3310 761 836 721 747 776 5 9 0.541 0.139 0.218 0.070 0.363 0.356 0.088 0.061 0.010 1.3395 766 850 778 749 830 15 9 0.599 0.200 0.134 0.041 0.344 0.338 0.145 0.054 0.004 1.0793 896 954 795 843 833 20 9 0.605 0.208 0.140 0.028 0.349 0.339 0.135 0.052 0.005 1.0386 922 986 824 876 871 32 9 0.516 0.109 0.210 0.126 0.476 0.352 0.101 0.055 0.005 1.2530 791 864 742 779 806 54 8 0.585 0.181 0.160 0.040 0.358 0.369 0.080 0.062 0.012 1.2034 818 884 773 774 820 58 8 0.529 0.158 0.177 0.095 0.444 0.447 0.031 0.070 0.007 1.2151 812 876 732 767 801

Note: FS = Ferry and Spear, 1978; HS - Hodges and Spear, 1982; GS = Ganguly and Saxcna, 1984; IMA, B = Indaresand Martignole, 1985, models A and B. Garnet mole fractions add up to less than 1.000 because Xand is not shown. Pressures from garnet-hornblende barometry. Abbreviations; X = mole fraction; aim = almandinc component in garnet, pyr = pyrope component, gro - grossular component, spe = spessartine component, and = andradite; other mole fractions are of cation in given mineral, mineral abbreviations as in Table 1; K = equilibrium constant.

kyanite-corundum). The garnet in this xenolith phenocrysts, oriented parallel to sanidine crystal has the same composition (Table D) as the xe- faces, are fairly common. Inclusions of plagio- nocrystic garnets in the other dacitic dikes and clase phenocrysts occur in all of the large also has pink rims with igneous composition sanidine crystals. (Evans and Vance, 1987). Kyanite has apparently not been found in any exposed Front GEOTHERMOBAROMETRY Range metamorphic rocks. It is intriguing that garnets with compositions nearly identical to 1. Temperature those in the pelitic xenolith occur in magmatic epidote-bearing dikes over a distance of 20 km Garnet-Biotite. The presence of euhedral across the crest of the Front Range. garnet and biotite inclusions inside of plagioclase Amphibole. Amphibole phenocrysts, euhed- phenocrysts, and of rare euhedra of garnet inside ral to subhedral, are generally between 0.5 and 3 biotite, indicates that garnet and biotite were in mm long. The amphibole ranges from approxi- equilibrium. Garnet-biotite Fe-Mg exchange mately equant in aspect to somewhat elongate; thermometry gives temperatures in the range the most strongly tschermakitic amphibole (in 720-880 °C (Table 2). Of the solution models dikes 32 and 45) is particularly stubby. An used, only that of Indares and Martignole (1985, overall pleochroic scheme is X = straw-yellow, Figure 9. Allv versus A1 total (= A1IV + "model B") explicitly accounts for non-ideal ef- Y = olive-green to grass-green, Z = dark blue- A1VI) in hornblende (normalized to 23 oxygen fects of Ca and Mn in garnet as well as Ti and VI green. The amphibole is optically (-), with 2 V in formula, all Fe as FeO). Enclosed fields in- octahedral Al (A1 ) in biotite; however, Ti in the range 55°-65°. clude both core and rim compositions from biotite from the dacite dikes does not vary along Amphibole in the magmatic epidote-bearing microprobe spot analyses. Analyses are the same substitution vector as that of the biotite dikes, and also dike 58, is dominantly tschermak- grouped into the following fields: (1) dikes 6, studied by Indares and Martignole (1985), and ite (Table E), in some cases transitional to fer- 41, and 54 (30 analyses); (2) dikes 32 and 45 so their model may not be strictly applicable. rotschermakite, pargasite, or tschermakitic horn- (23 analyses); (3) dike 58 (35 analyses). Nar- The lnKD values (Table 2) give temperatures in blende (Leake, 1978; Hawthorne, 1981). The row lines are best fits from Hammarstrom the range 690-780 °C according to the empiri- most distinctive feature of the amphibole chem- and Zen (1986) to data used to calibrate their cal calibration of Thompson (1976). Solution istry is the high aluminum content. Microprobe empirical hornblende barometer: lower A1 models which accurately account for the effect 3+ analyses, normalized to half-unit cell totals (23 line for plutons without magmatic epidote; of significant Ca, Mn, and Fe in garnet and Ti V1 oxygens), indicate that total A1 (A1T) is between higher A1 line for plutons with magmatic epi- and A1 in biotite have yet to be developed. 2.2 and 3.05, and tetrahedral A1 (Allv) between dote. Thick line is fit to the experimental data Nevertheless, existing models give temperatures 1.4 and 2.1; when averaged values from pheno- of Johnson and Rutherford (1989). which are more or less consistent and are proba- crysts are considered, AlT is between 2.4 and bly the best estimate of temperatures during the 3.0, and Allv between 1.75 and 2.05. Igneous growth of phenocrysts. The total uncertainty associated with any of the garnet-biotite geother- amphibole is rarely so rich in A1 (Dodge and which Fe3+ content is taken as intermediate be- mometers cannot be stated precisely, but an others, 1968; Hammarstrom, 1984; Wones and tween the values obtained by assuming 13 cat- estimate of +50 °C may be reasonable. Gilbert, 1982). Individual phenocrysts are not ions excluding Ca, Na, and K and by assuming systematically zoned from core to rim, but the 15 cations excluding Na and K (Robinson and Two Feldspars. The ternary feldspar activity rims themselves are where the highest A1 to- others, 1982) suggest that 25% to 35% of the iron model of Fuhrman and Lindsley (1988) was tals tend to be found. Variations of amphibole is ferric. used for two-feldspar thermometry and deriva- compositions in the dacite dikes, both from Sanidine. Dike 83 contains prominent, tion of activities of feldspar components. Figure core to rim and from one dike to another, are euhedral, blocky to tabular sanidine pheno- 6 indicates that plagioclase in dike 1 continued mainly due to combinations of the tschermakite crysts up to 1 cm in size with composition crystallizing as the magma cooled, and the pla- [Al2Mg_|SLi] and edenite [NaAlSL,] substitu- Cn3 Ab An oOrgQ. The rims are generally gioclase became saturated in Or component at a 3+ 5 15 5 1 tions (Thompson, 1981). Calculations of Fe in largely altered to sericite, but interiors of many temperature between 600 and 650 °C, which hornblende from stoichiometric constraints, in are unaltered. Inclusions of type I epidote was also the temperature of groundmass plagio-

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TABLE 3. TWO-FELDSPAR THERMOMETRY ration. Dike 15 has a much larger discrepancy (230 °C), which may indicate fractionation of Dike 83 (10 kbar) Zr relative to P2O5. Errors resulting from uncer-

Initial Adjusted T(°C) tainty in measured values of Zr and P2O5 are less than ±30 °C. XAbAl XKsAl XAnAI XAbAl XKsAl XAnAI aAb=aAb aAn=aAn aKs=aKs 0.166 0.823 0.011 0.171 0.818 0.011 677 682 677 Temperature Summary. Liquidus tempera- XAbPI XKsPI XAnPI XAbPI XKsPt XAnPI average: 679 0.715 0.041 0.244 0.705 0.051 0.244 tures of the dike rocks were probably between 800 and 1050 °C. Garnet-biotite temperatures Dike 32 (8 kbar) are in the range 720-880 °C. Most garnet-

Initial Adjusted biotite temperatures are below 800 °C, garnet- hornblende and hornblende-plagioclase temper- XAbAl XKsAl XAnAI XAbAl XKsAl XAnAI aAb=aAb aAn=aAn aKs=aKs 0.130 0.860 0.010 0.150 0.840 0.010 645 648 649 atures are between 740-810 °C, and simulta- KAbPl XKsPI XAnPI XAbPI XKsPI XAnPI average: 647 0.660 0.045 0.295 0.660 0.040 0.300 neous growth of sanidine and plagioclase in dike 83, which took place after type I epidote in the Dike I (8 kbar) rock had undergone incipient resorption, occurred at 650-700 °C. Therefore it is inferred Initial Adjusted that phenocryst growth in the dikes, including XAbAl XKsAl XAnAI XAbAl XKsAl XAnAI aAb=aAb aAn=aAn aKs=aKs 0.124 0.867 0.009 0.144 0.847 0.009 631 633 626 crystallization of magmatic epidote, took place XAbPI XKsPI XAnPI XAbPI XKsPI XAnPI average: 630 mainly in the interval 800-700 °C. Growth of 0.665 0.055 0.280 0.665 0.035 0.300 plagioclase rims and groundmass feldspars in Note: calculated according to ternary feldspar model of Fuhrman and Lindsley (1988); calculation procedure adjusts mole fractions within analytical uncertainly. dikes 1,5, 15, 32, and 45 took place between Abbreviations: X = mole fraction; Ab = albite component; Ks - orthoclase component; An = anorthite component; A1 = alkali feldspar, PI = plagioclase; aAb=aAb indicates temperature at which activity of albite end member is equivalent in alkali feldspar and plagioclase. 620 and 670 °C, and these temperatures are associated with the solidi of the dike magmas.

clase crystallization. Similarly, groundmass pla- within these dikes fall within the ranges of the 2. Pressure gioclase from dike 32 plots along a 650 °C empirical calibrations. Appropriate An values isotherm. A few analyses of groundmass alkali from zoned plagioclase phenocrysts were de- Plagioclase-Biotite-Muscovite-Garnet. The feldspar from dikes 1, 5, and 15 were obtained, rived as described under garnet-hornblende ba- reactions which yielded temperatures of 620-670 °C rometry (below). (Table 3) using the Fuhrman and Lindsley Zircon and Apatite Saturation. The zircon (1) pyrope + grossular + muscovite = (1988) two-feldspar thermometer program solubility formula of Watson and Harrison 3 anorthite + annite, and MTHERM3 at 8 kbar. (1983) and the apatite solubility formula of Har- (2) almandine + grossular + muscovite = Application of MTHERM3 to the sanidine rison and Watson (1984) can be used as geo- 3 anorthite + phlogopite and plagioclase phenocrysts in dike 83 gives thermometers, assuming that the dikes represent temperatures from 650 to 700 °C, using either melts which were saturated in zircon and apatite were empirically calibrated as a geobarometer plagioclase inclusions in sanidine, or the outer components, did not contain excess zircon or by Ghent and Stout (1981). The uncertainty in portions of separate plagioclase phenocrysts, apatite derived from restite, wall rocks, or crys- this method is probably at least ±1.0 kbar. Using which have similar compositions; averaged tal accumulation, and did not lose significant the equilibrium constant equations for the two compositions give a temperature of 680 °C amounts of zircon or apatite by crystal fractiona- reactions (given in correct form in Hodges and (Table 3). Calculations for these phenocryst tion. If these assumptions are valid, then the cal- Spear, 1982), and temperatures of 800-860 °C feldspars assume a pressure of 10 kbar. culated temperatures should approximate the as indicated by garnet-biotite thermometry, Garnet-Hornblende and Hornblende-Pla- liquidus temperatures (or temperatures at which pressures between 9 and 11 kbar were calcu- gioclase. The empirical garnet-hornblende Fe- the magmas formed by partial melting). lated for the dikes containing plagioclase, biotite, Mg exchange geothermometer of Graham and Calculated temperatures range from 800 to muscovite, and garnet (Table 4). Averaged min- Powell (1984) indicates temperatures of 740- 940 °C for zircon, whereas temperatures for eral compositions were used, except for plagio- 780 °C, and the empirical hornblende-plagio- apatite saturation range from 940 to 1030 °C. clase, for which an An value of 0.41, typical of clase geothermometer of Blundy and Holland Temperatures for apatite saturation are system- core compositions, was used. Using lower An (1990) yields temperatures of 740-810 °C for atically higher, by about 100 °C for most of the values, such as those of the plagioclase rims, dikes 32, 45, 54, and 58. Mineral compositions dikes, compared to temperatures for zircon satu- results in higher pressures.

TABLE 4. PLAGIOCLASE-MUSCOVITE-GARNET-BIOTITE GEOTHERMOBAROMETRY

Dike Temp (°C) XCaPl XCaGnt XFeGnt XMgGnt XFeBt XMgBt XKMu XAlviMu toK(l) lnK(2) PI (bars) P2(bars)

15 (a) 820 0.360 0.160 0.620 0.200 0.344 0.338 0.910 0.800 4.5476 1.2062 9,900 9,600 15 (b) 863 0.360 0.160 0.620 0.200 0.344 0.338 0.910 0.800 4.5476 1.2062 10,100 10,100 20 (a) 860 0.360 0.140 0.605 0.208 0.344 0.338 0.910 0.800 4.8305 1.6802 9,700 9,500 20 (b) 899 0.360 0.140 0.605 0.208 0.344 0.338 0.910 0.800 4.8305 1.6802 9,900 9,900

Note: calculated according to empirical calibration of Ghent and Stout (1981). Equilibrium 1: Mg end members; equilibrium 2: Fe end members. Temperature (a) from garnet-biotite thermometry; temperature (b) gives equivalent pressures for both equilibria. Abbreviations: X - mole fraction of cation in given mineral; mineral abbreviations as in Table 1; K = equilibrium constant; PI = pressure calculated from Mg end members, P2 " pressure calculated from Fe end members.

Anorthite-Grossular-Kyanite-Quartz. The TABLE 5. GASP GEOBAROMETRY reaction Dike Temp (°C) Xgro Xpyr Xalm Xspe aGro aAn P (kbar)

3 anorthite = grossular + 2 kyanite + quartz lc 750 0.213 0.138 0.543 0.074 0.0108 0.544 12.8 Ir 750 0.201 0.149 0.548 0.075 0.0090 0.544 12.5 5c 780 0.218 0.139 0.541 0.070 0.0116 0.551 13.4 provides an upper limit on pressure in the ab- 5r 780 0.188 0.157 0.555 0.072 0.0073 0.551 12.7 15c 820 0.134 0.200 0.599 0.041 0.0026 0.566 12.3 sence of an aluminosilicate mineral. Table 5 I5r 820 0.123 0.213 0.599 0.039 0.0020 0.566 11.6 20c 860 0.146 0.229 0.573 0.024 0.0030 0.563 13.3 shows pressures calculated according to the cali- 20r 860 0.138 0.232 0.579 0.025 0.0035 0.563 13.0 bration of Koziol and Newton (1988) with 32c 770 0.233 0.092 0.490 0.153 0.0098 0.568 13.2 32r 770 0.210 0.109 0.516 0.126 0.0136 0.568 12.7 grossular activities according to Ganguly and 54c 780 0.160 0.181 0.585 0.040 0.0044 0.582 11.8 54r 780 0.163 0.172 0.590 0.049 0.0046 0.582 11.8 Saxena (1984) and anorthite activities according 58c 760 0.177 0.158 0.529 0.095 0.0058 0.623 11.5 to Fuhrman and Lindsley (1988). Temperature 58r 760 0.169 0.154 0.526 0.105 0.0050 0.623 11.2 estimates are based on garnet-biotite thermome- Note: pressures calculated according to Koziol and Newton (1988), grossular activity from Ganguly and Saxena (1984), anorthite activity from Fuhrman and try. Typical plagioclase core compositions were Lindsley (1988). Abbreviations: c = core; r = rim; X = mole fraction of garnet end members (see Table 2 note); aGro = calculated grossular activity; aAn = calculated anorthite activity. Temperatures from garnet-biotite thermometry. used. Varying XAn by 0.1 changes the calculated pressures by less than 200 bars; raising the estimated temperature by 50 °C raises the pressures ~1 kbar. According to Hodges and McKenna Empirical Hornblende Barometry. Johnson TABLE 6A. GARNET-HORNBLENDE BAROMETRY (1987), the uncertainty in any of the methods and Rutherford (1989) presented an experimen- Dike 6 32 45 54 58 prior to the experiments of Koziol and Newton tal calibration, including reversed constraints on

(1988) is at least 2 kbar, and possibly as much as equilibrium, which confirmed a linear correla- XAn n.a. 0.38 0.38 0.42 0.49 3 kbar. Koziol and Newton (1988) claim that tion of A1t versus total pressure in igneous am- P (kbar) n.a. 8.8 8.8 7.6 7.6 their more precise calibration reduces the uncer- phibole coexisting with crystallizing dacitic,

tainty by a factor of two. rhyolitic, and tonalitic melts, as suggested by TABLE 6B. ALUMINUM-IN-HORNBLENDE BAROMETRY The entire range of GASP pressures thus cal- previous empirical calibrations (Hammarstrom culated for seven garnet-bearing dikes can be and Zen, 1986; Hollister and others, 1987). This A1 total 2.62 2.85 2.78 2.65 2.54 P(kbar) 7.6 8.3 8.2 7.7 7.3 summarized as 12 ± 2.0 kbar. These pressures experimental calibration is particularly appro- are an upper limit on conditions during early priate for geobarometry of the dacitic dikes Note: garnet-hornblende barometry from calibration of Kohn and Spear because the dacite used in the experiments (1990); assumes T = 750 °C. Aluminum-in-hornblende barometry from cali- growth of garnet and plagioclase in the dikes. bration of Johnson and Rutherford (1989). Abbreviations; XAn = mole frac- closely matches the epidote-bearing dacites in tion of anorthite in plagioclase; A1 total = total aluminum in hornblende; n.a. - Garnet-Hornblende. The empirical garnet- not applicable (igneous garnet absent). hornblende barometer of Kohn and Spear major-element composition, the experiments (1990) is calibrated over a range of mineral were conducted at 740-780 °C, and the experi- compositions suitable for the dike phenocrysts. mental amphibole is known to have coexisted Plagioclase compositions were taken from the with a large amount of melt. The tschermakite T two phases. Textures indicate that the amphi- values which predominated over the bulk of the substitution was the dominant control on A1 in boles equilibrated in the presence of plagioclase, plagioclase phenocrysts in the dikes, extending the experimental calibration (Johnson and biotite, and magnetite as well. None of the from small, generally more anorthitic cores to Rutherford, 1989) and was little affected by amphibole-bearing dikes contains K-feldspar narrow rims with distinctly lower An content. variations in Fe/Mg or f(02). Amphiboles from phenocrysts, and only dike 58 contains titanite. Hornblende inclusions occur in these interme- the epidote-bearing dikes are somewhat inhomo- Undersaturation of titanite in the melts may re- geneous, but A1 cation variation trends parallel diate zones. Oscillatory zoning in the interme- sult in lower Ti02 in amphibole, which could diate parts of plagioclase phenocrysts generally those defined by amphibole from the experimen- have a minor effect on The amphibole in tal calibration (Fig. 9). Hammarstrom and Zen have a narrow range of An content, so that av- the dacitic dikes has Ti02 content similar to that erage values are probably representative of the (1986) showed that hornblende in magmatic T of amphibole analyzed for the calibrations, and plagioclase which equilibrated with hornblende. epidote-bearing plutons typically has A1 of T IV the absence of titanite is not likely to have had a Hornblende compositions are taken from aver- about 2.0-2.6. A plot of A1 versus A1 (Fig. 9) significant effect on age rim analyses. Rims of hornblende inclusions shows that the amphibole in most of the mag- A1T. A few of the runs in the within plagioclase match rims of hornblende not matic epidote-bearing dikes follows the trend experimental calibration did not produce mantled by plagioclase. of the line fit by Hammarstrom and Zen (1986) K-feldspar (Johnson and Rutherford, 1989). to their data from hornblende in magmatic Pressures calculated from the Mg and Fe end- Those writers argued that the melt was neverthe- epidote-bearing plutons, but the A1T and A1IV member reactions (Kohn and Spear, 1990) for less close to being saturated in K-feldspar. values of the phenocryst amphibole extend to the dacitic dikes are in agreement to within less Furthermore, the experiments which failed to significantly higher values. than 500 bars. Calculated pressures (Table 6A) produce K-feldspar did not deviate significantly are 8.7 kbar for type II epidote-bearing dikes Johnson and Rutherford (1989) emphasized from the linear correlation they established be- t (32, 45), 7.6 kbar for a type III epidote-bearing that their experimentally calibrated barometer is tween Ptotal and A1 in amphibole. By analogy, dike (54), and 7.6 kbar for dike 58. Propagation based on amphibole rim compositions which are the amphibole-phyric dikes were probably also of errors in microprobe analyses and assumed thought to have equilibrated with silicate melt close to saturation in K-feldspar. Calculations of temperature (±50 °C) results in an uncertainty and plagioclase, K-feldspar, quartz, biotite, titan- K-feldspar activity based on plagioclase rim of 0.5-1.0 kbar for this geobarometer (Kohn ite, and magnetite. Euhedral inclusions of am- compositions give values of 0.85 to 1.0, except and Spear, 1990); a conservative estimate of phibole in quartz phenocrysts occur in dikes 32 for dike 58, which yields orthoclase activity ±1.0 kbar is therefore assumed. and 45, indicating the stable co-existence of the values between 0.72-0.78. Hornblende in the dacitic dikes equilibrated with igneous garnet,

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which is an aluminum-saturating phase and was 1 ing f (02) outlined above carries an uncertainty not present in the experimental calibration 800 °C 700 °C of ±1.0-1.5 log units f(02), the similarity of the (Johnson and Rutherford, 1989). Nevertheless, results does establish that dikes 1, 5, 6, 15, 20, pressures from Al-in-hornblende barometry and 41, and by inference all of the magmatic agree fairly well with pressures calculated from epidote-bearing dikes, have phenocrysts which the empirical garnet-hornblende barometer of crystallized under conditions which were unusu- Kohn and Spear (1990; see above). ^s^HM ally oxidizing for silicic igneous rocks (compare Results of the Johnson and Rutherford with Haggerty, 1976; Ewart, 1979). (1989) barometer applied to the dacitic dikes (Table 6B) indicate that amphibole in type II 4. Water Pressure epidote-bearing dikes formed at 8.2 ± 0.6 kbar, "^xNNO amphibole in type III epidote-bearing dikes Annite-Sanidine-Magnetite. If the pressure, formed at 7.6 ± 0.5 kbar, and amphibole in dike QFM^s. temperature, and f(02) conditions are known, 58 at 7.3 ± 0.5 kbar. The given uncertainties are then f(H20) can be derived from the reaction according to Johnson and Rutherford (1989). In

actuality, uncertainty may be ±1.0 kbar at best. 8.5 9.0 9.5 10.0 10.5 11.0 11.5 annite = sanidine + magnetite + H2 It is noteworthy that these results agree with 1/T (K) X 1000 pressures from garnet-hornblende barometry to and an expression for the equilibrium constant

within 500 bars. Figure 10. Oxygen fugacity. BAMM of H20 (Wones and Eugster, 1965). Wones Pressure Summary. The combined results of (biotite-annite-muscovite-magnetite-quartz) (1981), incorporating more recent experiments geobarometry show that pressures during crys- curves calculated according to model of on the breakdown of annite, suggested the tallization of preserved minerals in the dikes did Zen (1985) for dikes 15 and 20. Individual relation not exceed 12 ± 2.0 kbar. Dikes containing data point is calculated from Spencer and

muscovite and type II epidote crystallized phe- Lindsley (1981) for dikes 5, 41; bars show logf(H20) = 4819/T + 6.69 + 31ogXann

nocrysts at pressures of 10 ± 2.0 kbar. Garnet- uncertainty. The other curves shown for ref- + 0.51ogf(02) - 0.011 (P - 1)/T

hornblende and aluminum-in-hornblende ba- erence are quartz-fayalite-magnetite (QFM), + loga^,, + logamt, rometry on dikes containing type II epidote nickel-nickel oxide (NNO), and hematite- indicate a pressure of 8.2-8.8 kbar, whereas magnetite (HM). with T in Kelvins and P in bars. In order to be amphibole in dikes containing type III epidote consistent, the simple ionic activity model of

3 indicates 7.6-7.7 kbar. In dike 58, which does Wones (1972,1981), where aann = (XFe2.) , was not contain magmatic epidote, amphibole and applied to the BAMM buffer at 750 °C, result- garnet record pressures of 7.3-7.6 kbar. The ac- lated from stoichiometric constraints as 25% to ing in logf (02) =-12.1.

curacy of these pressure estimates may not be 35% of the Fe, is consistent with conditions in- For biotite from dike 20, using aor = 0.9 (de- better than ±1.0 kbar. Nevertheless, these results termediate to the nickel-bunsenite and HM rived from plagioclase rim compositions), calcu- indicate that formation of type III epidote by buffers (Blundy and Holland, 1990, p. 209). lated f(H20) is 16 kbar. According to the partial resorption of type I epidote occurred as Biotite-Almandine-Muscovite-Magnetite- fugacity coefficients in Burnham and others pressures dropped below ~8 kbar, and that Quartz. Dikes 15 and 20 contain a pheno- (1969) for pure H20, this corresponds to a magmatic epidote was stable in the dikes only at cryst assemblage suitable for applying the water pressure of 11 kbar. There are two likely pressures above 8.0 ±1.0 kbar. biotite-almandine-muscovite-magnetite-quartz sources of error in this estimate. The first is the (BAMM) f(02) buffer of Zen (1985). This uncertainty in the f (02) estimate. If a logf (02) 3. Oxygen Fugacity buffer is based on the reaction value of -12.2 instead of -12.1 is used, f(H20) drops to 13.6 kbar, with a corresponding PHj0

Iron-Titanium Oxides. Co-existing Fe-Ti ox- annite + almandine + 02 = muscovite + of 8.9 kbar. This shows how sensitive this meth- ides in the matrix of dikes 5 and 41 indicate that 2 magnetite + 3 quartz. od of determining f(H20) is to errors in the oxygen fugacity was about 2 log units below the estimation of f (02).

hematite-magnetite (HM) buffer (Evans and The total uncertainty in f (02) determined using The second main source of uncertainty in Vance, 1987), according to the diagram of the BAMM buffer is estimated by Zen (1985) constraining f(H20) is in the choice of an activ-

Spencer and Lindsley (1981, their Fig. 4). Using as no greater than 1.5 logf(02) units. ity model for annite. For instance, the totally equations from Spencer and Lindsley (1981) to The BAMM buffer curve calculated for dikes ionic model of Bohlen and others (1980) gives a solve for temperature, and then inputting that T 15 and 20 according to the above equation, minimum value for annite activity. When com- to solve for f(02), the results are T = 740 ± 80 using the activity models of Zen (1985), is lo- bined with the Wones (1981) equation (above), °C and logf(02) = -12.0 ± 1.0. cated between 2 and 2.5 log units below, and it results in f(H20) = 3.4 kbar, or PH2Q = 2.2 3+ Biotite and Hornblende. The high Fe con- approximately parallel with, the HM buffer kbar [using logf(02) = -12.1, T = 750 °C, P = 3+ total tent of biotite from dikes 1 and 5 (Fe /Fe = curve (Fig. 10). For a temperature of 740-750 10 kbar, a0r = 0.90], Thus, although there is a 0.25) is further evidence of a relatively high oxi- °C, and using mineral rim compositions, the in- great deal of uncertainty in calculated f(H20) 2+ 3+ dation state. The Fe , Fe , and Mg content of dicated logf(02) is -11.8 to -12.2. values, those which result from assuming that 3 the biotite in this dike indicates a logf (02) value Oxygen Fugacity Summary. All of the indi- aann = (XFe2-) are quite high and allow the ma of 2.0-2.5 log units below the HM buffer, as- cations are that oxygen fugacity in the dikes was possibility that Ph2o y have been nearly suming a temperature of 740-750 °C, according at about two log units below the HM buffer, equal to Ptotai- O" the other hand, if the alterna- tive annite activity model of Bohlen and others to the experimental work of Wones and Eugster with logf(02) = -12.0 at T = 740 °C (Fig. 10). 3+ (1965). The Fe content of amphibole, calcu- Although each of the three methods of measur- (1980), which gives a minimum value of aann, is

accurate, then f (H20) may have been about 3-4 strating that magmatic epidote can form early in suggests a reaction relationship in which epidote

kbar, and PH2Q a fraction of P)otai- In either case, the crystallization sequence under conditions of is consumed to form plagioclase. In appropriate was PH2O substantial. Magmatic epidote and high Ptotal and PH2o- conditions, epidote is a full-fledged magmatic t0 muscovite required a high Ph2o crystallize. Petrographic aspects of phenocrystic epidote mineral which may be involved in any number The matrix crystallization temperature of -650 such as an elongate habit need not be a feature of reactions involving other igneous phases. De- °C also implies a high H2O content. of truly magmatic epidote in phaneritic plutonic pending on intensive and compositional parame- rocks. Euhedral habit, particularly against mica, ters, epidote may enter a magmatic crystalliza- DISCUSSION simple and polysynthetic twinning, and euhed- tion sequence either early or late. Textural ral, oscillatory zoning of Fe content are not un- criteria for a magmatic origin of epidote in plu- Textural Criteria for Magmatic Epidote. common in metamorphic epidote. Highly euhed- tonic rocks should not be limited to indications The applicability of textural criteria from phe- ral, oscillatory zoning of allanite component, as that it formed from the breakdown of horn- nocrystic epidote to plutonic occurrences is lim- seen in phenocrystic epidote, has apparently not blende just above the solidus. ited by the disparity in crystallization and been reported in metamorphic epidote, how- Stability of Magmatic Epidote. In this study, cooling histories. The documented plutonic ever, even where allanitic cores and zoning of magmatic epidote has been found to be asso- epidote appears to have grown late in the crys- allanitic to REE-free epidote occurs (Sakai and ciated with high-pressure crystallization, but tallization sequence, forming from a reaction others, 1984). Such sharply defined, fine-scale, f(02) and f(H20), which were also relatively which consumed hornblende and anorthite oscillatory zoning, involving a charge-coupled high in the dacitic magmas, may be as important component in plagioclase and produced biotite reaction, would seem to be evidence of crystalli- as total pressure in controlling epidote stability. and quartz along with epidote (Zen and Ham- zation from a liquid, analogous to oscillatory Elevated oxygen fugacity favors the stability of 3+ marstrom, 1984b). The phenocrystic epidote, in zoning in plagioclase. Fe -bearing epidote. Experiments (Liou, 1973) contrast, crystallized relatively early (<25% crys- This study demonstrates a natural occurrence suggest that as f(Û2) approaches the HM buffer, tals, Table 1), and there is no evidence that it of magmatic epidote which formed independent epidote can exist stably in water-rich granitoid formed from a reaction involving amphibole. It of a reaction relationship with hornblende, and melts at pressures less than 5 kbar. Oxygen fu- appears to have crystallized directly from the magma. Melting and crystallization experiments on a trondhjemite at 15 kbar (Johnston and Wyllie, 1988) found epidote to be the liquidus mineral at high water content (> 12% H2O), and the next mineral on the liquidus after hornblende at slightly lower water contents, demon-

Figure 11. (A) Schematic drawing of reaction curves which intersect at invariant point i in Figure 1 IB. (B) Summary of pressure-temperature conditions of the magmatic epidote-bearing dike magmas. Fields with hatchured borders show preferred estimates of (1) partial melting of lower crust to form the dacitic magmas (850-950 °C, 10-12 kbar); (2) crystallization of phenocrysts, including magmatic epidote (700-850 °C, 8-10 kbar); and (3) quenching and final groundmass crystallization (620-670 °C, 2-4 kbar). Dashed lines show total extent of uncertainty. Field 2 is subdivided into 2a, in the alpha-quartz stability field; and field 2b, in the beta-quartz stability field. In field 2b, geothermobarometric estimates overlap with the oval that shows the range of estimates of the location of invariant point i (see discussion in text). Granite and tonalite water-saturated melting curves are from Wyllie (1977). The muscovite breakdown reaction, alpha-beta quartz equilibrium, and reactions intersecting at i were calculated by GE0-CALC PTX (Berman and others, 1987), using activities derived as indicated in the text.

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gacity in the dacitic magmas was between the of Berman and others (1987) using the data of pressures below the minimum for epidote stabil- nickel-bunsenite and HM buffers, but the Ps Berman (1988). The six phases define an invar- ity. Even in the quenched dike rocks, the mag- content of the magmatic epidote in most dikes is iant point (Fig. 11 A). Clinozoisite activity in matic epidote records textural evidence that it less than that of epidote grown in nickel- epidote Was calculated from equations in Bird was no longer in equilibrium with the magma as bunsenite-buffered experiments (PS25; Liou, and Helgeson (1980) which take into account quenching occurred. 1973). This is not seen as a contradiction, how- Fe-Al order in epidote; grossular and anorthite ever, because epidote in the dikes was not part activity were taken from calibrations used for CONCLUSIONS of an Fe3+-constraining assemblage. geothermobarometry, using compositions asso- Very little is known about the effect of REE ciated with type III epidote (Xgr0 = 0.27; XAn = Epidote crystallized as an early magmatic content on the stability of magmatic epidote. In 0.55-0.6). It is thought that the magmas were phase in Laramide-age, calc-alkaline dacites in type I epidote, even the most REE-rich oscilla- close to saturation in H20 and quartz during the Front Range near Ward, Boulder County, tory zones have <0.02 LREE cations p.f.u., resorption of type III epidote, suggesting activi- Colorado, recurring over an area about 10 * 20 therefore the stabilization effect of REE on type ties close to unity for these phases, but these km. The dike magmas rose rapidly from depths I epidote is probably insignificant. Assuming values, and the activity of aluminosilicate com- of 20-30 km and underwent quenching in a complete miscibility between allanite and epi- ponent in the melt (estirtiated to be between 0.5 hypabyssal environment, preserving phenocryst dote, and no hiatus between allanite growth and and 0.9), are not well constrained. Nevertheless, minerals grown at high pressures. Besides epi- epidote growth in the dacite magmas, a smooth even if activities of Water, quartz, and alumino- dote, these minerals include high-grossular al- compositional profile from allanite core to epi- silicate are varied over all permutations of values mandinic garnet, tschermakite, and muscovite. thought to be plausible for the magmas (that is, dote overgrowth would be expected. The bound- The dacite magmas formed by melting at 0.5-1), the calculated invariant point remains aries between allanitic cores and epitaxial temperatures above 800 °C, possibly as high as in the alpha-quartz stability field, at tempera- overgrowths of type I epidote, however, are 1050 °C. The oxygen fugacity during pheno- tures of 650-800 °C and pressures 3=9 kbar, generally abrupt and compositionally discontin- cryst growth was 2.0 ±1.0 log f(0 ) units resulting in some inconsistency with the pre- 2 uous. This suggests that depletion of REE in the below HM; at 750 °C, log f(0 ) = -12.1 ± 1.0. ferred estimates of conditions of partial melting, 2 magma by allanite growth resulted in instability Water content was also high: f(H20) = 3.4-16 phenocryst growth, and quenching of the dikes of the epidote until lower temperatures were kbar, for f(0 ) = -12:1 at 750 °C, depending on (Fig. 1 IB). 2 reached. Epidote then nucleated epitaxially on the annite activity model used. Phenocryst crys- pre-existing allanite crystals. The difficulty of Large uncertainties are attached to estimates tallization occurred predominantly at tempera- nucleating epidote has been remarked on by of activities of Si02, Al Si0 , and H2O compo- tures in the range 700-800 °C. Sanidine most of the experimentalists who attempted it 2 5 nents of the magma, .used to calculate the loca- phenocrysts, which occur in one dike, grew at (for example, Holdaway, 1972; Liou, 1973). It tion of the invariant point L Furthermore, there 650-700 °C. Groundmass feldspars, which is no surprise that, given the virtually identical are minimal data on the thermodynamics of formed as the dikes were quenched, crystallized structure of the two minerals, after epidote be- clinozoisite-epidote solid solutions. Therefore, at temperatures in the range 620-670 °C. Un- came stable in the magma it nucleated preferen- little confidence is placed in using the phase certainties in temperature estimates are on the tially, perhaps exclusively, on pre-existing equilibria according to GEO-CALC to limit type order of ±50 °C. allanite. III epidote crystallization and resorption to the Pressures during phenocryst growth did not Type II epidote formed from magmas which alpha-quartz field (location 2a in Fig. 1 IB), and exceed 12 ± 2.0 kbar; phenocrysts formed contained significant amounts of REE in the crystallization in the beta-quartz field, as sug- mainly in the range 8.0 ± 1.0 to 10 ± 2.0 kbar. melt (but not more REE than is typical for an gested by the hexagonal, dipyramidal form of Type III epidote shows that epidote became un- intermediate, calc-alkaline rock). As an indica- the quartz, is preferred (location 2b). stable in the dacitic magmas as they rose to pres- tor of pressure or other intensive parameters, the The cooling and depressurization path during sures less than 8.0 ± 1.0 kbar. Thus the significance of type II epidote, which has REE dike emplacement and quenching is poorly con- conditions inferred for the growth of magmatic abundances intermediate to low-REE type I ep- strained. The preservation of type I epidote phe- epidote in tonalitic to granodioritic plutons of idote and common allanite, has not been studied nocrysts and other minerals grown at high the western Cordillera are empirically con- experimentally. In the Front Range dacite dikes, pressure requires that the magmas rose quickly firmed. The epidote in the dikes, however, crys- however, it occurs only in phenocryst assem- from depths of greater than 20 km. This demon- tallized while the magmas were still >65%-75% blages which crystallized at >8 ± 1 kbar. strates that water-rich, silicic magma can rise liquid and did not form via a reaction relation- Type III epidote indicates that type I mag- abruptly to shallow (s£6 km) crustal levels from ship between amphibole and melt. matic epidote became unstable during evolution depths in the range 20-30 km. Phenocryst tex- Characteristics of epidote phenocrysts, such as of the magma from which it crystallized and was tures indicate that the garnet, quartz, muscovite, euhedral form set in an aphanitic matrix, REE resorbed, producing anorthite in the process. and epidote underwent varying amounts o£ par- zoning as described above, and involvement in The resorption is attributed to reduction of pres- tial resorption (that is, were no longer stable) flow fabrics, may not be directly applicable as sure rather than temperature because reactions prior to, or during, quenching. criteria for a magmatic origin of epidote in more which form anorthite from epidote generally It must be emphasized that despite the preser- slowly cooled, phaneritic plutonic rocks; this have a positive P-T slope. Furthermore, the re- vation of type I epidote in the magmas as they study does indicate, however, that magmatic ep- sults of geothermobarometry have indicated that were emplaced at relatively shallow depths, the idote can occur in textural varieties other than pressure was probably lower for type III textural relations integrated with thermoba- that emphasized in the recent literature. Epidote epidote-bearing assemblages, whereas tempera- rometry indicate a minimum pressure of epidote may once have been present in any magnetite- ture was, if anything, higher. stability in the magmas in the vicinity of 8 kbar. bearing dacite, rhyodacite, tonalite, granodiorite, Reactions in the system CaO-A^C^-SiC^- Furthermore, the evidence suggests that in more trondhjemite, or granite with a high water con-

H20 involving grossular, anorthite, quartz, slowly cooled granitoid plutons which crystal- tent which began crystallizing at >8 kbar, and aluminosilicate, water, and clinozoisite were lized magmatic epidote at depth, epidote would may have been fractionally crystallized from calculated with the GEO-CALC PTX program readily react out if the magma evolved through such a magma, along with allanitic cores.

Ghent, E. D., and Stout, M. Z., 1981, Geobarometry and geothermometry of Naney, M. T., 1983, Phase equilibrium of rock-forming ferromagnesian silicates ACKNOWLEDGMENTS plagioclase-biotite-gamet-muscovite assemblages: Contributions to in granitic systems: American Journal of Science, v. 283, p. 993-1033. Mineralogy and Petrology, v. 76, p. 92-97. Noble, D. C., Vogel, T. A., Peterson, P. S., Landis, G. P., Granti, N. K., and Graham, C. M., and Powell, R., 1984, A garnet-hornblende geothermometer; others, 1984, Rare-element enriched, S-type ash-flow tuffs containing This research was supported by National calibration, testing, and application to Pelona Schist, southern Califor- phenocrysts of muscovite, andalusite, and sillimanite, southeastern Peru: nia: Journal of Metamorphic Geology, v. 2, p. 13-31. Geology, v. 12, p. 35-39. Science Foundation Grant EAR 86-08838AO 1 Green, T. H., 1976, Experimental generation of cordierite or gamet-bearing Pearson, R. C., and Johnson, G., 1980, Mineral resources of the Indian Peaks to B. W. Evans and a Sigma Xi Grant-in-Aid of granitic liquids from a pelitic composition: Geology, v. 4, p. 85-88. study area, Boulder and Grand Counties, Colorado: U.S. Geological 1977, Garnet in silicic liquids and its possible use as a P-T indicator: Survey Bulletin 1463, 86 p. Research to R. L. Dawes. Dolores Gable intro- Contributions to Mineralogy and Petrology, v. 65, p. 59 67. Peterman, Z. E., Hedge, C. E., and Braddock, W. A., 1968, Age of Precambrian Green, T. H., and Ringwood, A. E., 1968, Origin of garnet phenocrysts in events in the northeastern Front Range, Colorado: Journal of Geophys- duced us to the dikes, and A. J. Irving provided calc-alkaline rocks: Contributions to Mineralogy and Petrology, v. 18, ical Research, v. 73, p. 2277-2296. expert assistance on the whole-rock ICP anal- p. 163-174. Pichavant, M., and Manning, D., 1984, Petrogenesis of tourmaline granites and 1972, Crystallization of garnet-bearing rhyodacite under high-pressure topaz granites: The contribution of experimental data: Physics of the yses. Reviews by J. M. Hammarstrom and M. T. hydrous conditions: Journal of the Geological Society of Australia, Earth and Planetary Interiors, v. 35, p. 31-50. v. 19, part 2, p. 203-212. Robinson, P., Spear, F. S., Schumacher, J. C., Laird, J., Klein, C., Evans, B. W., Naney significantly improved the manuscript. Gromet, P. L, and Silver, L. T., 1983, Rare earth element distributions among and Doolan, B. L., 1982, Phase relations of metamorphic amphiboles: minerals in a granodiorite and their petrogenetic implications: Geochim- Natural occurrences and theory, in Veblen, D., and Ribbe, P., eds., ica et Cosmochimica Acta, v. 47, p. 925-939. Amphiboles: Petrology and experimental phase relations: Reviews in Guidotti, C. V., Cheney, J. T., and Guggenheim, S., 1977, Distribution of Mineralogy, Volume 9B, Mineralogical Society of America, p. 1 211. titanium between coexisting muscovite and biotite in pelitic schists from Sakai, C., Higashino, T., and Enami, M., 1984, REE-bearing epidote from northwestern Maine: American Mineralogist, v. 62, p. 438-448. Sanbagawa pelitic schists, central Shikoku, Japan: Geochemical Jour- Haggerty, S. E., 1976, Opaque mineral oxides in terrestrial igneous rocks: nal, v. 18, p. 45-53. Mineralogical Society of America Short Course Notes 3: Oxide Schleicher, H., and Lippoit, H. J., 1981, Magmatic muscovite in felsic parts of REFERENCES CITED minerals. rhyolites from southwest Germany: Contributions to Mineralogy and Hammarstrom, J. M„ 1984, Microprobe analyses of hornblende from 5 calc- Petrology, v. 78, p. 220-224. Anderson, J. L., and Thomas, W. M., 1985, Proterozoic anorogenic two-mica alkaline intrusive complexes, with data tables for other calcic amphi- Speer, J. A., 1984, Micas in igneous rocks: Reviews in Mineralogy, Volume 13. granites: Silver Piume and St. Vrain bathoiith, Colorado: Geology, boles and BASIC computer programs for data manipulation: U.S. Micas, p. 299-356. v. 13, p. 177-180. Geological Survey Open-File Report 84-652, 98 p. Spencer, K. J., and Lindsley, D. H., 1981, A solution model for co-existing Bence, A. B., and Albee, A. L., 1968, Empirical correction factors for the Hammarstrom, J. M., and Zen, E-an, 1986, Aluminum in hornblende: An iron-titanium oxides: American Mineralogist, v. 66, p. 1189-1201. electron microanalysis of silicates and oxides: Journal of Geology, v. 76, empirical igneous geobarometer: American Mineralogist, v. 71, Stern, C. R., and Wyllie, P. J., 1973, Phase compositions through crystallization

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