High-Potassium Intrusive Rocks of the Crandall Ring-Dike Complex, Absaroka Mountains, Wyoming

High-potassium intrusive rocks of the Crandall ring-dike complex, Absaroka Mountains, Wyoming

A. M. KUDO Department of Geology, University of New Mexico, Albuquerque, New Mexico 87131 DAVID E. BROXTON ESS-1, Mail Stop D-462, Los Alamos National Laboratory, Los Alamos, New Mexico 87545

ABSTRACT

The Crandall ring-dike complex has a gabbro-diorite core intruded by a shoshonitic ring dike. Both have been cut by granular and porphyritic quartz monzonite dikes. The most abundant rock is medium-grained diorite. In contrast, the ring-dike shoshonite has large phenocrysts of plagioclase and augite in a microcrystalline groundmass of biotite, Siinidine, plagioclase, amphibole, magnetite-ilmenite, and variable amounts of glass. Texturally, the plagioclase and pyroxene appear out of equilibrium with the surrounding matrix in both the ring dike and the intrusive core rocks. The gabbro, diorite, and porphyritic quartz monzonite are characterized by plagioclases with bimodal core and rim compositions, indicating that one group is possibly xenocrystic. Trends on variation diagrams for both minor and trace elements cannot fa« traced from the diorite to the quartz monzonite. On variation diagrams, the shoshonite rocks have the widest scatter of data points, which can be explained by the addition of plagioclase and augite phenocrysts. Geochemical modeling is successful in demonstrat- ing that the gabbro can be formed as a cumulate from the diorite by fractional crystallisation but is unsuccessful in relating the diorite and the quartz monzonite by this process. The strongly positive Eu anomaly in the gabbro supports the cumulate origin; the diorite and shoshonite have chemical and textural signatures which strongly indicate contamination by plagioclase and pyroxene accumulation. Even the porphyritic quartz monzonite has experienced plagioclase accumulation. The nature of the uncontaminated magma is uncertain. Chemi- cally, the diorite is a plutonic equivalent of shoshonite, and support is given to the hypothesis that shoshonites are formed by contamination with plagioclase and pyroxene (Prostka, 1973).

INTRODUCTION

The Absaroka volcanic province of northwestern Wyoming crops out over 23,000 krn^ and consists mainly of calc-alkalic Eocene andesite flows and breccias (Smedes and Prostka, 1972). The rocks range in com- l> j I Map Area position from basalts to rhyolites, and it was here that Iddings (1899a) first " IDAHO .< • Cody described and namijd the potassium-rich mafic lavas as absarokite, shoshonite, and banakite from exposures in the northern Absaroka Mountains. Chadwick (1970) postulated two subparallel belts of eruptive WYOMING centers spanning the length of the province. The belts trend northwest, and the rocks of the eastern belt are generally more potassic than those of the Figure 1. Location and geologic map of the Crandall ring-dike western belt. complex, Absaroka Mountains, Wyoming. Symbols are as follows: Our paper is a petrogenetic study of intrusive rocks from the eastern dikes and sills are represented by dark, heavy lines; g = gabbro, d = belt (Fig. 1). The majority of the rocks analyzed come from the dissected diorite, qm = quartz; monzonite, s = shoshonite, Tv = volcanic and Crandall "volcano" (on Hurricane Mesa) of Iddings (1899b). The Cran- volcaniclastic rocks of the Absaroka Supergroup.

Additional material for this article, Tables A and B, may be secured free of charge by requesting Supplementary Data 85-19 from the GSA Documents Secret!iry.

Geological Society of America Bulletin, v. 96, p. 522-528, 11 figs., 1 table, April 1985.

522

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dall volcano has a core of medium-grained diorite with minor gabbro and quartz monzonite surrounded by a ring dike composed of porphyritic aphanite, the composition of which is similar to shoshonites described by Iddings (1899a) or to high-K andesite. Krushensky (1960) performed a detailed pétrographie study of the intrusive rocks, and Pierce and others (1973) published a geologic map of the quadrangle which contains the Crandall pluton. We will discuss the genesis of these rocks using mineral chemistry and major- and trace-element geochemistry of the whole rocks. We hope to show similarities between the shoshonites and the dominant rock type in the intrusion, the diorites. Evidence for some contamination of the Crandall magmas by pyroxene and plagioclase will be presented.

FIELD RELATIONSHIPS

Two plutons, a ring dike, and swarms of late-stage dikes have intrusive contacts with the Eocene Wapiti Formation and the Trout Peak trachyandesite (Nelson and Pierce, 1968). The largest intrusion is the Crandall pluton and ring dike on the south-central part of the mesa Figure 2. Photomicrograph of diorite with glomerocrystic aggre- (Fig. 1). The Crandall ring-dike complex is primarily diorite but also gate of plagioclase arranged subparalleUy with jagged contacts and contains rocks ranging from gabbro to quartz monzonite. In the core of the corroded rims. These are surrounded by potash feldspar (black) con- complex, quartz monzonite is found as dikes cutting the diorite, but the taining poikilitic inclusions of plagioclase and pyroxene. Crossed ni- contact between the gabbro and the diorite appears gradational. In some chols. Long dimension = 3.5 mm. areas of the core, the quartz monzonite occurs in large outcrops continuous over tens of metres, suggesting stock-like bodies rather than dikes. The central diorite is almost completely enclosed by a younger, circular ring dike composed of a porphyritic, K-rich andesite or shoshonite. The ring dike, in turn, has been intruded by dikes and dikelets of quartz monzonite, especially in the southwestern exposures of the complex. Some of these dikes project into the stock-like bodies of the quartz monzonite in the core. No quartz monzonite dike was observed outside of the ring dike. These quartz monzonite dikes appear lithologically similar to the quartz monzonite of the core and appear to emanate from there, therefore, the sequence of the intrusion is most likely gabbro-diorite, ring dike, and lastly, quartz monzonite. The Crandall ring-dike complex has a width of 3.2 km, and the interior stock measures ~2.5 km in diameter. The thickness of the ring dike varies from <0.5 km to >1.0 km. It is interesting that, although the volumes of the ring dike and the core are approximately equal, the ring- dike rocks are porphyritic with an aphanitic matrix, and the core rocks are medium grained. A zircon fission-track age of 40.2 ±1.7 m.y. has been obtained for a quartz monzonite sample from the core of the intrusion (dated by C. Naeser, 1980, written commun.). Figure 3. Photomicrograph of augite aggregate in ring-dike rock. PETROGRAPHY Long dimension = 3.5 mm. Crossed nichols.

Medium-grained granular gabbros, diorites, and quartz monzonites subparallel orientation (Fig. 2). In contact with the interstitial potash contain plagioclase, two pyroxenes, minor biotite, and magnetite-ilmenite feldspar, the rims are corroded (Fig. 2). Subophitic intergrowth of plagio- (Table A).1 There are minor variations in texture, and minor olivine clase and augite is not uncommon. Plagioclase, being the largest, appears (mostly altered) occurs in the gabbro. Some of the quartz monzonites are to be the earliest crystallized. This is unusual for the gabbros if they are porphyritic with a phaneritic groundmass, but others are fine grained and equivalent to absarokites because plagioclase phenocrysts are absent in the aphyric. Some of the diorites have an intergranular texture. The most latter. The aphanitic rocks from the ring dike have phenocrysts of plagio- striking texture, however, in almost all rock types, but especially in the clase and augite set in a hypocrystalline to holocrystalline groundmass. diorite, is represented by early formed, large subhedral to euhedral plagio- The augites occur in aggregates with interlocking grain contacts (Fig. 3) clase laths with jagged rims and stubby pyroxene prisms. These are sur- and in single grains with rounded resorbed outlines. These features have rounded by later interstitial potash feldspar (sanidine?) and/or quartz been observed in shoshonite flows by Prostka (1973) who inferred a poikilitically enclosing the smaller plagioclase and pyroxene grains. Many hybrid origin for these rocks, the plagioclase and pyroxenes being xeno- of the plagioclases occur in monomineralic aggregates in which the indi- crystic or xenolithic. vidual anhedral grains have irregular wavy contacts between them and a MINERAL CHEMISTRY

'Tables A and B are in the GSA Data Repository. Request Supplementary All mineral compositions have been determined on two electron Data 85-19 for free copies. microprobes (ARL-EMX and the JEOL Superprobe 733) which are fully

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automated. At least 15 individual grains in each microprobe section have realize that sectioning of rocks will not always expose actual cores for been analyzed for both core and rim compositions. Several step scans microprobing, but at least 25 individual cores were measured for sach across large, zoned plagioclases also were done. Six element analyses for section. The gap occurs for rim compositions as well, so that the bimodal plagioclases, nine for pyroxenes and olivines, and eight for the Fe-Ti distribution appears real. In a sample of a sparsely porphyritic quartz oxides have been corrected by Bence-Albee. monzonite with <1% plagioclase phenocrysts, a bimodal distribution is lacking, however. No calcic plagioclase composition was detected. In this

Feldspar sample also, large interstitial sanidine (Or6i to Orgl) has a more Na-rich composition than the smaller interstitial grains which have Or82 to C)r91. The compositions of plagioclase in these medium-grained rocks are Sanidine compositions are Or80_74 in diorite and Org^ in quartz mon- plotted in Figure 4. The most An-rich composition observed for all rocks is zonite. Two micropeithites in diorite had plagioclase-sanidine paiis of about An7g; the most calcic are from cores, although some rim composi- Ab59An4!-Abi90rg[ and Ab63An37-Ab|7Or83. One pair from the quartz

tions of An57 have been obtained. The most sodic core composition ob- monzonite was AbsgAn^-AbigOrg!. These give similar temperatures served occurs at about An]g. Reversed zoning is common. Significantly, (580 to 600 °C at 1 kb pressure) using the two-feldspar geothermoir eter the most calcic core compositions for the gabbro are in the same range as (Stormer, 1975). The plagioclase compositions in these intrusive rocks the core compositions in the porphyritic quartz monzonite. In fact, there is compare well with those measured by Nicholls and Carmichael (1969) for a bimodal distribution of the feldspar core (and rim) compositions in these shoshonites in the Abiiarokas. The major differences are slight; however,

samples; one distribution occurs between about An45 to An^, and another the volcanic rocks are more enriched in Or and An. occurs between An^j to An^. The more sodic compositions appear to be In the ring-dike samples, the plagioclase compositions are generally from the smaller grains poikilitically enclosed in the potash feldspars. We more An-rich than are those of the plutonic rocks and have a maximum of

An70. Normal zoning is common. Rare core compositions in the Ar^j^o range have been found, but such sodic compositions are most common in

phenocryst rims and in the groundmass. Reverse zoning from An45 to

An49 was detected in the feldspars in one ring-dike sample. In another

sample, both rim and groundmass had a sanidine of Or4j Ab5sAn4.

Pyroxene

Pyroxene compositions are restricted throughout the gabbro to quartz monzonite series (Fig. 5). Clinopyroxene compositions change from

En47FsgWo47 in the gabbro to En^Fs^Wo^ in the most silicic rock. These compositions are similar to those in shoshonites (Nicholls and Car- michael, 1969). The restricted range in pyroxene compositions over a wide silica range has been observed in calc-alkalic rocks (Ewart and others, 1977; Cameron and others, 1980) and has been attributed to high oxygen fugacity (Cameron and others, 1980). In the gabbro, exsolution of pyroxene is common, especially for the Ab Ab Ab Ab Ab hypersthene. Pyroxene end-member compositions, however, cannot be measured with the smallest beam on the microprobe. Compositions in the \ miscibility gap of the pyroxene quadrilateral are obtained instead (Fig. 5). Ab/ \Or 11.z In the ring-dike rocks, pyroxene compositions range from En^FsgWo^» to En42Fs16W042, which is almost identical to the entire range from gabbro L \ to quartz monzonite. Two-pyroxene temperatures (Wood and Banno, L A 1973) are similar for the diorites and the quartz monzonites, both averag- 5 ing -925 °C. The Mg-rich pairs always yield a lower temperature L/ Ab AO r (-20 °C) than do the Fe-rich pairs.

Figure 4. Feldiipar compositions for representative rock types Opaque Minerals from the Crandall ring-dike complex. Open circles are core compositions of large grains; solid circles are rim and small-grain composition. Both magnetite and ilmenite occur as separate grains; in single grains, 1 = gabbro; 2 = dioriite; 3 = porphyritic quartz monzonite; 4 = aphyric they occur as exsolution lamellae. The mole fraction of ulvospinel in r.he quartz monzonite; 5 = ring dike. magnetite varies from 0.019 to 0.137 and that of hematite in the ilmerite

Figure 5. Pyroxene compositions for representative rocks from the Crandall ring-dike complex. Dashed outline represents compositions of clinopyroxene phenocrysts from the ring-dike rocks.

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1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 . X AI2O3 Figure 6. Variation diagram of -X ° P V OD _ various oxides plotted against FeO* 00 lie O (total Fe as FeO)/MgO. Crosses = 5b SÌO2 6'0 6b gabbro; open triangles = diorite; open

MgO - squares = quartz monzonite; open cir- "x cles = ring dike. X A - - JK * / OFF : 1 1 1 1 Figure 7. Silica variation diagrams -X CaO for A1203, MgO, and CaO. Symbols _ X same as in previous figures. Addition of plagioclase (p) and augite (c) phenocrysts is indicated by arrows. Length - - - of arrows has no significance. 0

OQ _ 1 1 55 610 1 1 1 1 1 1 S1O2

Jakes and Smith (1970). The porphyritic quartz monzonite plots in the calc-alkalic field, but the sparsely phyric and granular quartz monzonite types are alkali-calcic. Here, also, the ring-dike-rock trend diverges from the trend of the diorite, gabbro, and the granular quartz monzonite. Two of ^FeCf/MgO2 the samples plot with the diorite; the remaining four plot in the calc-alkalic field. Being potassium rich, these rocks have been compared by Iddings from 0.027 to 0.119. Geothermometry is useless because of the exsolution (1899a) to his absarokite-shoshonite-banakite suite. Indeed, most of the and the highly variable composition. diorite may be considered a plutonic equivalent of the shoshonite; however, the gabbro, with its abundant plagioclase and high alumina content, MAJOR-ELEMENT VARIATIONS is more akin to high-alumina basalt and most unlike absarokites associated with shoshonites (Gest and McBirney, 1979; Nicholls and Carmichael, Examination of the whole-rock chemistries (done by J. Husler at 1969). Petrographically, in absarokites, plagioclase is absent as pheno- UNM using a combination of standard wet chemical and atomic absorp- crysts, but plagioclase in the gabbro is interpreted as having been present tion spectroscopy) shows three distinct groups corresponding to gabbro, before the pyroxene and olivine. This would explain the high alumina diorite, and quartz monzonite with large gaps in silica between them content of the gabbros in this study. (Table B).2 On variation diagrams using FeO* (total Fe as FeO)/MgO as the abscissa, these groups are clearly distinguished (Fig. 6). Being enriched TRACE-ELEMENT VARIATION AND Sr ISOTOPY in CaO and AI2O3 and depleted in K20, the gabbro is clearly separated from diorite. On the CaO, K20, Ti02, and P2Os curves, breaks occur in Sr, Ba, Th, Sc, Cr, and some REE have been obtained for most of the the trends from the diorite to the quartz monzonite. Only two of the rocks. All of these trace elements were determined by INAA (at Los ring-dike samples plot with the diorite, whereas the remaining samples, Alamos National Laboratory), except for Sr, which was determined by especially for Al203, MgO, and CaO, consistently deviate from the atomic absorption spectrophotometry. Some of the heavy rare-earth ele- "trend" of the main intrusive rocks. Of the ring-dike rocks, the samples ments were analyzed and found to be as much as ten times chondritic with the higher modal proportions of augite relative to plagioclase pheno- values, but because the sensitivity and precision of the analyses of these crysts (Table A) have higher CaO and MgO and lower A1203 than does elements were not of the same quality as they were for the light rare-earth the diorite. On the other hand, samples with a higher proportion of plagio- elements, these have not been considered. It has been suggested, from the clase to augite phenocrysts are richer in A1203 and are depleted in K20. major-element analyses, that diorite may be compared to shoshonite. Ac- On silica variation diagrams for A1203, MgO, and CaO (Fig. 7), the cording to Pearce (1982), who used trace elements to characterize lavas ring-dike rocks lie on trends which are at a high angle to the trend between from convergent plate boundaries, calc-alkalic basalt and shoshonite are gabbro to diorite to quartz monzonite. Addition of clinopyroxene and enriched relative to MORB in Sr, K, Rb, Ba, Th, Ce, P, and Sm and plagioclase (which occur as phenocrysts in the ring-dike rocks) to an depleted in Ta, Nb, Zr, Hf, Ti, Y, and Yb. Using our trace elements, it is "average diorite" appears to explain this divergent trend. apparent that the Absaroka diorite has characteristics very similar to those When the analyses are plotted in diagrams using the logarithms of the of the shoshonite from New Hebrides (Fig. 9). With the exception of K

calc-alkalic ratio (Ca0/Na20 + K20) versus Si02 (from Brown, 1982), and Sm, which are more depleted than in the rock from New Hebrides, similarities to a very mature arc suite, such as New Guinea, become even the gabbro has similarities. apparent (Fig. 8). These rock suites are correlated with thick continental Many plots of one trace element versus silica or FeO*/MgO or crust, are alkali-calcic, and are associated with high-K shoshonite rocks of covariation of pairs of trace elements have been made. All these need not be shown because all support the major-element division of the rock types 2In GSA Data Repository. See footnote 1. into distinct groups with consistent breaks in trends from the diorite to the

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100

Figure 8. Plot of the logarithm of

the calc-alkalic ratio (Ca0/Na20 + K20) versus Si02 from Brown (1982). Symbols identical to those in Figure 4. Dashed line represents New Guinea arc rocks. The field between the two lines represents the "normal" calc- alkalic rocks according to Brown (1982).

60 Si02fVo) Figure 9. Selected trace elements for the Crandall ring-dike complex normalized to MORB (mid-ocean ridge basalts). This diagram is adapted from Pearce (1982), who attempted to characterize rocks from various destructive plate margins. Solid circles with lines drawn between them is the high-K shoshonite from New Hebrides; the other symbols are as before. 0.1 SmTi ScCr quartz monzonite and in the scatter of the shoshonite from the ring dike. A few examples are shown in Figure 10. Chondrite-noimalized REE plots are shown in Figure 11. All sam- -4.0 ples are LREE enriched, La being 70 to >400 times more abundant than in chondrite. Diorite is characterized by the LREE enrichment in which chondrite-normali2.ed La ratios are between 110 and 140 and by slightly positive Eu anomalies (Fig. 11). Gabbro has the lowest LREE enrichment. La ratios are below 80, and there is a strongly positive Eu anomaly (Fig. 11). The most silicic rocks have significantly lower Eu values than do any of the other rocks. The chondrite-normalized La ratio of one of the U ppm 2.0 samples is —170, although another sample has a La ratio >400. These do FeOVMgO not have a Eu anomaly. The ring-dike samples again show a large range: their chondrite-noimalized La ratios lie between 80 and 220. The shosho-

nite samples with l;he lowest K20 values have the lowest La ratio below 94 and a slightly positive Eu anomaly. The two K-rich samples have high La ratios above 170, and one of these has a slightly positive Eu anomaly. A 100 relationship between K20 and the existence of a Eu anomaly is suggested and, indeed, a statistically significant correlation is obtained between the

K20 content and the Eu/Sm ratios. Higher K20 contents are correlated with lower Eu/Sm ratios. By courtesy of D. G. Brookins of the University of New Mexico, Sr-isotope ratios were obtained on five samples using the antiquated mass Sc ppm Sc ppm spectrometer in the department. One gabbro yielded a 87Sr/86 Sr ratio of 0.7042 ± 0.0002 (1 standard deviation); three diorites yielded ratios of Figure 10. Variation diagram and covariation plots of selected 0.7041, 0.7049, and 0.7067; and one ring-dike sample yielded a ratio of trace elements. Symbols identical to those in previous figures. 0.7077. diorite, are composed of medium-grained quartz monzonite or porphyritic DISCUSSION quartz monzonite with a phaneritic groundmass. This anomalous associa- tion of rock textures could be explained if it is assumed that the gabbro- The Crandal). ring-dike complex, in spite of its small size, has a diorite and the quartz monzonite were injected as crystal-rich mushes, the diversity of rock types that raises a number of questions. One of these crystals already being medium grained, reflecting the thermal conditions of questions concerns the difference in texture between the core rocks and the a deeper environment. In addition, the ring-dike may have been vented to ring dike. The ring-dike rocks have phenocrysts which are as large as the the surface, allowing for the escape of volatiles and the resultant chilling. largest ones in the diorites, but they have a groundmass which is micro- A model for simple fractional crystallization of a single parent crystalline (some samples have glass that suggests a fast quench). The magma was tested initially. In view of the chemical trends in the variation volumes of the two bodies are not too different. The gabbro-diorite was diagrams, it seems unlikely, however, that the different groups cm be injected first, but the margins of this body are not chilled into an aphanite. related to one another by fractional crystallization. Indeed, geochemical Small dikes less than a metre in width, which cut the ring dikes and the modeling has been unsuccessful in supporting the generation of the quartz

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-I—I—I—I I I Diorite Gabbro Quartz Monzonite \

100

50 \ \ \ \ M w .c -y o 10

-I I I—I—i-—I ' ' ' • I I I 1 1 L- La Ce SmEu LaCe SmEu Dy

Figure 11. Chondrite-normalized rare-earth-element plot for the various rock types of the Crandall ring-dike complex; a, diorite/gabbro; b, quartz monzonite; c, ring dike. Dashed line in "a" is for the average gabbro, and the shaded field is for the diorite samples. The two lines in "b" are for the porphyritic quartz monzonite (long dash) and the aphyric quartz monzonite (short dash). The shaded area in "c" has the four low-potash ring-dike samples. The remaining two are plotted separately.

monzonite from the diorite or the gabbro, but it was possible to demon- The diorite is a chemically and petrographically distinct and homo- strate that the gabbro can be formed as a cumulate from the diorite. A geneous group. The silica content is restricted to a range of slightly >2% good fit is obtained in modeling of the REE when 67% plagioclase, 16% and all samples appear to be grouped tightly on chondrite-normalized augite, 10% hypersthene, 3% biotite, and 4% opaque are accumulated REE plots with La ratios between 110 and 140. For this reason, it is most (Table 1). This matches the mode of the gabbro (Table A). Differences in likely that the diorite had a common origin from a single source rock. The Fs (fraction of melt remaining) of <0.100 are considered acceptable, and similarity of the diorite to shoshonite has been noted. Pearce (1982) and the model is considered significant (Cameron and others, 1980). The effect Gill (1974) suggested that shoshonite may result from partial melting of a of alteration products (sericite and biotite) and minor amounts of potash, hydrous mantle previously enriched in the required elements. The abun- feldspar, and apatite were not tested, but certainly these phases would dance of the REE and Th-U supports derivation from an enriched source.

contribute K20 and P205 to the bulk composition of the gabbro. The Mg The low Mg number of these magmas, however, argues against derivation number of the gabbro is similar to that of the diorite, but this should be from normal mantle material which has Mg-rich olivine. What is more, expected, according to the cumulate model, as the gabbro is formed by the slightly positive Eu anomaly in the diorite is difficult to obtain from accumulation of the mafic phases from the diorite. One problem is the partial melts from most mantle material without accumulation of some absence, in our samples, of the silicic liquid that formed during the forma- plagioclase either as a contaminant or as a cumulate. Another possibility is tion of the cumulate. The composition of such a liquid has not been found that the diorite is derived from continental material. Brown (1982) corre- and must be assumed to have been eroded or overlooked or unexposed. lated crustal thickness with the alkalic-to-calcic nature of the arc magmas. Iddings (1899b) reported an analysis of a granitic rock with >71 percent The more alkali-calcic andesites are correlated with thicker continental silica, but it is uncertain whether this represents the lost liquid or not. The crusts. The diorite is highly alkali-calcic. If this correlation is true, the occurrence of a cumulate rock in an unlayered pluton is not unusual, chance for some involvement of continental material either as a source or inasmuch as examples of zoned granitic plutons exhibiting cumulate as a contaminant is very gTeat. The variable 87Sr/86Sr ratios from 0.7041 nature are common (Pitcher, 1979). They are believed to have been to 0.7067 suggest that some contamination may have occurred. Certainly intruded as a crystal mush after cumulate-type crystallization at depth. the bimodal distribution in the plagioclase compositions and the normal zoning with the odd reverse zoning support some disequilibrium process in which assimilation was incomplete. Clearly, the calcic plagioclase must be

TABLE 1. EXAMPLE OF FRACTIONAL CRYSTALLIZATION MODEL the contaminant. The texture and the slightly positive Eu anomaly suggest some accumulation of plagioclase laths to the diorite. The minor occur- Diorite Gabbro Bulk distribution coefficient* Calculated F rence of subophitic intergrowth of plagioclase and pyroxene and the (D) (fraction of melt remaining) glomerocrystic aggregates of augite with interlocking grain boundaries suggest possible xenolithic origin as well. The restricted range in pyroxene La 34 ppm 23 ppm 0.2256 0.2422 Ce 65 36 0.1991 0.2788 compositions may result from the addition of xenocrystic pyroxene. The Sm 5.3 3.0 0.2190 0.2965 early-formed crystals are distinctly different from the interstitial late phase; Eu 1.9 1.6 0.3575 0.2636 the early phases include calcic plagioclase, pyroxenes, and possibly minor La Ce Sm Eu Plagiodase 0.27 0.19 0.12 0.4 olivine, but the late phases are sanidine, sodic plagioclase, biotite, and Augite 0.20 0.30 0.6 0.5 quartz. These two assemblages do not appear to represent equilibrium. The Orthopyroxene 0.02 0.03 0.07 0.07 Biotite 1.0 0.9 0.6 0.5 former assemblage could possibly represent a gabbroic "magma" and the Magnetite 0.03 0.03 0.03 0.03 latter a granitic magma; these two magmas underwent mixing to form the

Noie: in this model, the motor equivalent of 67 volume percent plagioclase, 16% augite, 10% hypersthene, 4% biotite, and diorite. It is difficult to accept this, however, especially since the gabbroic 3% opaque are removed by fractional crystallization from diorite (8-7-8-6) to form cumulate gabbro (A-20). Compare with mode in Table 1. assemblage crystallized first, before the interstitial granitic material. In 'Distribution coefficients from compilation by Börnhorst (1980). view of the arguments presented by McBirney (1980), the presence of

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phenocrysts does not support magma mixing; also, non-linear trends occur CONCLUSIONS for the diorites in variation diagrams (Fig. 6), and the trend does not line up with the gabbxo or quartz monzonite. The original uncontaminated The Crandall diorite and quartz monzonite have been formed by magma may be quartz monzonite, as suggested by the late interstitial different degrees of contamination by plagioclase and two pyroxene s. The phases. Prior to intrusion, this magma was contaminated with xenocrystic parent magma for both may have been different, but a quartz-saturated plagioclase and pyroxenes from a crustal gabbro to form the diorite; assim- magma with primary biotite and sanidine is suggested, the diorite having ilation was incomplete. Subsequently, this contaminated magma of diorite been formed, intruded, and solidified before the quartz monzonite. Both composition underwent fractional crystallization to form the cumulate may have been injected as crystal mushes. The diorite magma also has gabbro. Due to the fact that the gabbro is compositionally equivalent to a differentiated to form a cumulate gabbro. A shoshonite ring dik; was high-alumina basalt, and the diorite is compositionally equivalent to a injected after the diorite and before the quartz monzonite and was shoshonite magma, differentiation of shoshonite at crustal levels produces quenched to form a porphyritic-aphanitic rock. The shoshonite also has cumulate high-alumina gabbro rather than absarokite. been contaminated but by plagioclase and augite only; its parent magma The compositional gap» between the diorite and quartz monzonite appears to be comparable to the diorite in major-element composition. and the geochemical modeling indicate that fractional crystallization can- Both on a major- ami trace-element basis, the diorite is a plutonic equiva- not relate the two. The highly enriched La, the lack of a Eu anomaly, and lent of shoshonite. If shoshonite magma, with its abundant plagioclase the overall chemistry suggest a source rock highly enriched in LIL ele- phenocrysts, were to fractionally crystallize, the solid cumulate formed ments if the quartz monzonite was formed by partial melting. The porphy- should have a high-alumina composition unlike that of the basic ritic quartz monzonite has bimodal plagioclase compositions and two absarokites. pyroxenes with disequilibrium textures, so that some contamination of this sample is suggested. It appears as if plagioclase of composition similar to ACKNOWLEDGMENTS that found in diorite may have been added in minor amounts to the quartz monzonite, but the pyroxenes, being more Fe-rich, must have undergone This work was supported in part by the University of New Mexico some equilibration. The lack of bimodal plagioclase compositions in the Research Allocations Committee. Aaron C. Waters and Crayton J. Yapp sparsely phyric and granular quartz monzonite may support the pristine kindly reviewed the initial draft of the manuscript. The thoughtful and nature of these types of rocks, but the presence of < 5% amphibole suggests incisive comments by D. S. Barker, D. H. Eggler, L. A. Fernandez, and the possible addition of minor pyroxene which has been subsequently D. H. Stout were most helpful and greatly appreciated. altered. Recently, based on their studies of the absarokite-shoshonite suite from the Absaroka range, Meen (1983) and Gest and McBirney (1979) REFERENCES CITED

demonstrated that absarokite and shoshonite show little contamination Bornhorst, T. J., 1980, Major- an J trace-element geochemistry and mineralogy of upper Eocene to Quaternary volcanic effects. Meen (1983) demonstrated that absarokite may differentiate to rocks of the Mogollon-Datil volcanic field, southwestern New Mexico [Ph.D. dissert.]: Albuquerque, New Mexico, University of New Mexico, 1,108 p. shoshonite if there is removal of early crystallized pyroxenes and olivines. Brown, G. C., 1982, Calc-alkaline intrusive rocks: Their diversity, evolution, and relation to volcanic arcs, in Thorpe, R. S„ ed„ Andesites: New York, John Wiley & Sons, p. 437-4«!. The suppression of the crystallization of plagioclase is most critical to this Cameron, M., Bagby, W. C., and Omeron, K. L., 1980, Pedogenesis of voluminous mid-Tertiary ignimbrites of the Sierra process. In the case of these intrusive rocks, however, some differences are Madre Occidental, Chihuahua, Mexico: Contributions to Mineralogy and Petrology, v. 74, p. 271-284. Chadwick, R. A., 1970, Belts of i;ruptive centers in the Absaroka-Gallatin volcanic province, Wyoming and Montana: apparent. The primary magma which was injected into Hurricane Mesa Geological Society of America Bulletin, v. 81, p. 267-274. Ewart, A., Brothers, R. N., and Mateen, A., 1977, An outline of the geology and geochemistry and tht possible was a diorite with shoshonitic affinities; however, this magma probably petrogenetic evolution of the volcanic rocks of the Tonga-Kermadec-New Zealand island arc: Journal of suffered some crustal contamination and incomplete assimilation similar to Volcanology and Geotherrnal Research, v. 2, p. 205-250. Gest, D. E., and McBirney, A. R.. 1979, Genetic relations of shoshonitic and absarokitic magmas, Absaroka Mountains, the process suggested by Prostka (1973). This dioritic magma, with its Wyoming: Journal of Volcanology and Geotherrnal Research, v. 6, p. 85-104. Gill, J. B., 1974, Role of underthrust oceanic crust in the genesis of a Fijian calc-alkaline suite: Contributions to phenocrysts of large plagioclase and two pyroxenes, accumulated to form Mineralogy and Petrology, v. 43, p. 29-45. the high-alumina gabbro. Analogously, shoshonite, with its abundant plagi- Iddings, J. P., 1899a, Absarokite-shoshonite-banakite series, in Geology of Yellowstone National Park: U.S. Ceological Survey Monograph 32, pal 2, p. 326-355. oclase phenocrysts, should form a cumulate rock of more basic composi- 1899b, The dissected volrano of Crandall basin, Wyoming, in Geology of Yellowstone National Park: U.S. Geological Survey Monograph 32, p. 215-268. tion with a high-alumina composition, not a rock of absarokite Jakes, P., and Smith, J. E., 1970, High potassium calc-alkaline rocks from Cape Nelson, Eastern Papua: Contributions to composition. Rods equivalent to banakite have not been found in the Mineralogy and Petrology, v. 28, p. 259-271. Krushensky, R. D., 1960, Geology of the volcanic features of the Hurricane Mesa area. Park County, Wyomi ig [Ph.D. Crandall pluton; however, a silicic quartz monzonite magma was gener- dissert]: Columbus, Ohio, Ohio State University, 217 p. McBirney, A. R., 1980, Mixing ;ind unmixing of magmas: Journal of Volcanology and Geotherrnal Research, v. 7, ated subsequent to the diorite, and some of this magma may have suffered p. 357-371. xenocrystic contamination as well. Meen, J. K., 1983, Isotopic composition of some Laramide volcanics, Absaroka Mountains, Montana: Carnegie Institu- tion of Washington Yearbook 82, p. 481-486. Nelson, W. H., and Pierce, W. G., 1968, Wapiti Formation and Trout Peak trachyandesite, northwestern Wyoming; U.S. Of all the rook types in the Crandall complex, the ring-dike rocks Geological Survey Bulletin 1254-H, p. 1-11. display the widest scatter of data points. They have a porphyritic-aphanitic Nicholls, J., and Carmichael, I.SE., 1969, A commentary on the absarokite^hoshonite-banakite series of Wyoming, U.S.A.: Schweizerische Mineralogische und Petrographische Mittelungen, v. 49, p. 47-64. texture, and so the large grains of plagioclase do not show the bimodal Pearce, J. A., 1982, Trace element characteristics of lavas from destructive plate boundaries, in Thorpe, P.. S., ed., Andesites: New York, Joh l Wiley & Sons, p. 525-548. distribution; however, the rims and groundmass compositions are more Pierce, W. G„ Nelson, W. H„ and Prostka, H. J., 1973, Geologic map of the Pilot Peak quadrangle. Part County, sodic than are the phenocryst cores. The textural features which point to Wyoming: U.S. Geological Survey Miscellaneous Investigations Map 1-816. Pitcher, W. S., 1979, The nature, accent and emplacement of granite magmas: Geologic Society of London Journal, possible xenocrystic or xenolithic origin for the plagioclase and augite v. 136, p. 627-662. Prostka, H. J., 1973, Hybrid origin of the absarokite-shoshonite-banakite series, Absarokite volcanic field, Wyoming: phenocrysts, as well as the chemistry, support a hybrid origin for the Geological Society of America Bulletin, v. 84, p. 697-702. ring-dike shoshonite. This is supported by the one 87Sr/86Sr ratio of Smedes, H. W., and Prostka, H. J., 1972, Stratigraphic framework of the Absarokite Volcanic Supergroup in Ye lowstone National Park region: U.S. Geological Survey Professional Paper 792-C, 33 p. 0.7077. A contaminant slightly different from that of the diorite and quartz Stornier, J. C., Jr., 1975, A practical two-feldspar geothermometer American Mineralogist, v. 60, p. 667-674. Wood, B. J., and Banno, S., 1973, Gamet-orthopyroxene and orthopyroxene-dinopyroxene relationships in simple and monzonite is suggested because of the presence of trace olivine and the complex systems: Contributions to Mineralogy and Petrology, v. 42, p. 109-124. lack of hypersthens in the shoshonite. The least contaminated shoshonite which has major-element composition similar to that of the diorite has a different REE plot. This is the result of its high La and Ce content, compared to that of the diorite. A slightly different source rock is therefore MANUSCRIPT RECEIVED BY THE SOCIETY JULY 13, 1983 REVISED MANUSCRIPT RECEIVED AUGUST 29, 1984 suggested. MANUSCRIPT ACCEPTED SEPTEMBEII 19, 1984 Printed i:i U.S.A.

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