Spatial Distribution and Origin of Magnetite in an Intrusive Igneous Mass

DEWEY D. SANDERSON Department of Geology, Marshall University, Huntington, West Virginia 25701

ABSTRACT monzonite. The former is thought to have been emplaced first. Data are derived from 48 collection sites distributed fairly uni- A quantitative technique was developed to aid the determination formly over the stock (Fig. 1). of the spatial distribution of magnetite 1:0 the constituent minerals in an igneous rock. The results show the magnetite to be pref- METHOD USED erentially associated with the ferromagnesian silicates; they further To establish the neighborhood around a particular mineral, lend support that the magnetite was formed by precipitation, which in this study is magnetite, a point count is carried out on a oxidation reaction, and alteration. The amount of magnetite regular grid over the area of the thin section. Data are recorded formed by each process can be estimated and was found to be in when a magnetite grain is encountered on a grid point. The miner- decreasing order, as listed above. The technique allows intergranular relations to be noted and quantilied. This permits a more justified speculation as to the crystallization history of an igneous pluton. Key words: igneous petrology, grain contacts, statistics.

INTRODUCTION In observations of rocks in thin section, relations are commonly noted that are more intimate than would be expected if the distribution of the minerals was purely random. Observations on a suite of thin sections from the Melrose stock, an Early Cretaceous intrusive mass in east-central Nevada, appear to show a preferential distribution or association of magnetite with the ferromagnesian silicates. A method was developed to allow a quantitative determination of the spatial distribution of the constituent minerals around the magnetite grains. If a preferred association is suggested, or even if it is not, a better insight can be gained into the crystallization history of the magnetite in particular and of the rock in general. Though the method developed has been used to study the association of an accessory mineral with respect to the constituent minerals of the host rock, it could also be used to study other mineral grain assemblages in various types of rocks. Quantitative investiga- tions on intergranular relations have been reported by Rogers and Bogy (1958), Mahan and Rogers (1968), Flinn (1969), and Kretz (1969). In the rocks of the Melrose stock, it is suspected that a part of the magnetite associated with the ferromagnesian minerals may be genetically related to these iron silicates. If the results of the association study suggest this to be statistically valid, then appropriate reactions can be postulated for the observed relations. This permits suggestions as to relative changes of the oxygen fugacity in the magma chamber with respect to time and space. The oxidation state of iron is sensitive to variations in :he oxygen fugacity, and, hence, depends on the minerals in which the iron will ultimately reside. The Melrose stock is located in the Dolly Varden Mountains within the Basin and Range province. A radiometric date of 125 m.y. (Armstrong, 1963) places the time of emplacement and crystallization as Early Cretaceous. The 30-sq-km exposure of the Figure 1. Generalized geology of Melrose stock, Elko County, Nevada, with loca- stock can be delineated into two phases, a monzonite and a quartz tions of collection sites (modified after Snow, 1963).

Geological Society of America Bulletin, v. 85, p. 1183 -1188, 4 figs., July 1974

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als on either side of a grid point are indicated by the graticule line Let N be the number of magnetite grains encountered in travers- of the ocular and are recorded. For an accessory mineral such as ing a thin section. Then 2N will be the total number of point magnetite, a large number of: grid points are obviously necessary. In counts, as each magnetite grain yields two counts, one count for this study, 150 to 200 magnetite grain contact relations were re- each mineral that flanks opposite sides of the magnetite grain. corded per slide. Table 1 outlines the framework of the model used in this study. Figure 2 schematically illustrates a photomicrograph having For sake of discussion, three minerals (X, Y, and Z) are associated three constituent minerals, X, Y, and Z, plus the mineral W, for with W. The first column shows the various combinations of X, which the neighborhood is to be established. Minerals X, Y, and Z Y, and Z minerals on the opposite side of a W grain. The proba- could be three rock-forming minerals, and W, an accessory mineral bility for each combinatian is the product of their respective modal such as magnetite. For purposes of illustration, the three W grains volumes. The expected point counts for N grains of W for each of are located at grid points. This results in a point tally of 2, 3, and 1 the constituent minerals of X, Y, and Z are given in the three right- for minerals X, Y, and Z, respectively. One tally is given to each hand columns of Table 1. Mineral X, for example, receives counts mineral at the point where the graticule line exits a W grain. for the (X, X), (X, Y), and (Y, X) combinations.

PROBABILITY MODEL OF ASSOCIATION COEFFICIENTS FOR THREE CONSTITUENT MINERALS

Combinations Probability Point counts for minerais X Y Z

(X, X) P 2 (P p )n X X (X, Y), (Y, X) 2 P 2 (P x x p )s (X. Z), (Z, X) 2 P 2 (F, P 2(p. • P,)N (Y, Y) 2 P P Iff

Subtotal 2P II 2«y Total 1.0

The following expressions are the basic assumptions in the de- velopment of the association parameters. P, = M, P,xPi = M,xM,.

Figure 2. Photomicrograph schematic of data collection procedure for association P and M are, respectively, the probabilities and modal volumes for coefficients of minerals X, Y, and Z with respect to W. Graticule of ocular shown on the i and;' mineral species in the rock. If there are a total of k con- each W grain. Counts made for points indicated by arrows. stituent minerals in the rock, then

ASSOCIATION MODEL P, + P2 + . . . +Pk = 1.0 M, + M + . . . + M = 1.0 . A probability model was chosen, and three parameters were de- 2 k rived from the expected distribution of mineral grain assemblages, The subtotal of point counts for each mineral is ST, for N grains of assuming a random distribution. Parametric values are calculated W. from the recorded tallies and mineral modes. Values of the param- ST, = 2 (P, x P,) N + 2 (P, x P ) N + . . . + 2 (P, x P ) N eters, which deviate from the expected norms, are interpreted as 2 k indicating a nonrandom distribution of the magnetite grains rel- which reduces to ative to the constituent minerals. ST, = 2P,N . Intergranular relations can be modeled a number of ways. The model of this study is relatively simple. More complicated and The total point count (7") is merely the compilation of the subtotals elaborate statistical models can be used in an effort to yield a closer (ST,), which upon factoring and substitution yields correspondence to the study of rocks. However, it is believed that T = 2P,N + 2P N + . . . + 2P N = 2N . meaningful results and information can be derived from the follow- 2 k ing model. The association (A,) of W, with respect to mineral i, is defined as Two assumptions are made in the application of this model to an A = ST,IT . igneous rock. (1) If the mineral grains are randomly distributed, the f probability of finding a given mineral at a point on a thin section is Substituting the expressions of ST, and T into i:he above equation equal to that mineral's mode; that is, its volume is represented by gives its area in thin section. (2) If a thin section has a great number of A, = 2P,NI2N = P, . grains, the probability of observing any combination of two minerals in contact with the magnetite is an independent event (equal to As the probability and modal volume are equal, the association of the product of the two respective probabilities). W to a given mineral is expected to equal its modal volume. The The contact of a mineral with magnetite may be either a grain- association can be normalized by its modal volume M, to give the to-grain or inclusion contact. It appears from petrographic obser- association coefficient (AC,): vations that magnetite is as likely to be intragranular as inter- AC, = A,IM, = 1.0 . granular. Assuming it may be primarily intergranular, there will then be an influence due to the size and shape of the constituent For a random arrangement of mineral material around the W mineral grains. Grains of minerals with a large ratio of surface area grains, the expected value of the association coefficient for a given to modal volume will preferentially receive a greater proportion of mineral is 1.0. counts. A check of these factors does not appreciably alter the re- Table 2 shows the association point counts, modal fractions, and sults of this study. association calculation results of a typical monzonite thin section.

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TABLE 2. EXAMPLE OF ASSOCIATION DATA FROM the strength of crystallization is likely to be related to crystal- MONZONITE THIN SECTION (VN-4E) lography direction. Association Mode Association Association If magnetite is introduced into a fractured rock, certain minerals count m (X) coefficient may have a greater tendency to fracture than other minerals, and, Plagioclase 43 38.8 15.8 0.41 hence, these would have a greater capacity to host the magnetite. K-feldspar 75 41.2 27.6 0.67 An inherited association might result from two minerals (A and B) Quartz 3 7.0 1.1 0.16 that were originally associated; at a later time, mineral C replaced Hornblende 137 10.2 50.4 4.92 B. This would leave A in association with C. One mineral can act as Biotite 14 1.8 5.1 2.84 a host for the nucleation of another mineral, and, hence, this would Total 272 99.0 100.0 display preferential association. In some cases, this may be considered a genetic association, in others it may be considered a The greater the departure of the AC values from 1.0, the chances nongenetic association. are smaller that the distribution is random. A chi-square test can be Genetic associations can be considered as both formative and de- used to test the significance of a measured set of AC values from structive. A formative association, in this study, will be referred to their expected values of 1.0. Using the AC values for the test yields as an association of magnetite to another mineral as a result of an a more conservative estimate of the significance than using the ex- incongruent reaction. The candidate minerals for this relation are pected and observed point count data. the ferromagnesian silicates. Under suitable oxidizing and P-T The numerical differences in percent between the association conditions, the incongruent melting of a ferromagnesian can pro- parameter and the modal volume of the mineral are termed the ex- vide enough Fe2+ and Fe3+, which are necessary for magnetite cess association (£A,j. This is the magnetite fraction associated (Czamanske and Wones, 1970). Magnetite that is formed in this with another mineral above that is expected from a simple random manner will be referred to as an oxidation reaction. Another form- distribution. This expression is negative if the modal volume ex- ative association results from exsolution, a phenomenon that oc- ceeds the association, and it is termed a "negative excess associa- casionally produces magnetite in ferromagnesian silicates. In such a tion." The summation of the excess associations necessarily has to case, the association would be readily noted, and the association be zero. study would not provide new insight. Destructive associations indicate minerals formed subsequent to GEOLOGIC INFLUENCES ON the normal crystallization sequence, namely deuteric and hydro- MAGNETITE ASSOCIATION COEFFICIENT thermal alterations. Magnetite released during chloritization and The association coefficient can give valuable information which serpentinization is an example of destructive association. concerns intergranular and intragranular relations of minerals. Preferential associations may not be maintained throughout the Some of the possibilities that can explain excess associations of course of crystallization. Mechanisms that tend to destroy associa- magnetite to the constituent minerals will be considered. The as- tions are (1) differential movement in the crystallizing magma and sociation parameter and coefficient were developed on a theoretical (2) chemical reactions that remove one of two minerals originally basis without reference to geologic processes that might affect their associated. values. The relations implied by the association calculation do not If it is assumed that magnetite forms by direct precipitation indicate the origin of the association. Consequently, it is then the (thus, it has no genetic association to the other minerals) and also task of the investigator to evaluate the data, along with petrologic that it forms early during petrogenesis, then it should show no pref- and petrographic observations, in order to draw meaningful con- erential spatial distribution to the constituent minerals of the rock. clusions about the rock genesis. The grains would then be randomly scattered and would be ex- The following discussion will be concerned primarily with as- pected to yield association coefficients of 1.0, or nearly so. Dis- sociations that might arise in an intrusive igneous rock. The de- tributions of these values from 1.0 would suggest the influence of scription will be concerned with associations that are termed gene- other factors, as discussed above. tic and nongenetic. The former includes associations of the opaque minerals with other minerals that have a common chemical com- RESULTS ponent. Nongenetic associations arise from circumstances that pre- The association coefficient was determined from 54 thin sections vail in the magmatic environment where there is no chemical corre- for each of the constituent minerals — plagioclase, K-feldspar, spondence. These shall be considered first. quartz, biotite, and hornblende. Thirty-four of the sections are The formation of an igneous rock is not, in most cases, the mix- from the quartz-monzonite phase and twenty are from the monzo- ing of preformed crystals. Rather, the formation is a long-termed nite phase of the Melrose stock. The modal compositions and their event in which various minerals form in response to the physical- ranges are shown in Table 3. The magnetite of the stock has a chemical conditions of the system. As a result, minerals of different stoichiometric composition. The results of the AC determinations species will be crystallized at different times and, in some cases, are compiled in Figures 3 and 4. subsequently resorbed. Some minerals may have the bulk of their The first striking feature of the histograms is the notable varia- modal volume formed before the crystallization of another mineral. tion of the AC values for the constituent minerals within a rock For instance, the early crystallization of mineral A and the late arri- unit. A chi-square test on the deviation of the AC values from 1.0 val of B precludes the possibility of having B included in A. This TABLE 3. MODAL COMPOSITIONS would limit the association of B with respect to A. On the other OF MELROSE STOCK* hand, if A and B are the last minerals to crystallize, then there is a high probability for a strong association, because both will be Quartz monzonite Monzonite Minimum Mean Maximum Minimum Mean Maximum forced to occupy the remaining interstitial volume. If A and B formed early, they would be expected to show a weak association. Plagioclase 27 38 48 14 36 52 An association might arise that can be termed an imposed as- K-feldspar 15 26 39 9 41 15 sociation. This association results when the viscosity of the melt is Quartz 18 24 37 0 4 9 Hornblende 2 5 8 6 12 17 sufficiently high enough to cause the inclusion of one mineral Biotite 1 6 10 1 5 21 within another mineral, because they are unable to repel each other Magnetite 0 1 2 1 2 3 as one or both grow. Various minerals may differ in their power of crystallization to push away adjacent magnetite grains. In addition, Values are in percent.

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i 2 0L i 2 0 1 4 n . n m i nAn

ASSOCIATION COEFFICIENT ASSOCIATION COEFFICIENT Figure 3. Frequency of occurrence of association coefficients of constituent miner- Figure 4. Frequency of occurrence of association coefficient! of constituent minerals plagioclase (P), K-feldspar (K), cuartz (Q), biotite (B), and hornblende (H), with als plagioclase (P), K-feldspar (1<), quartz (Q), biotite (B), and hornblende (H), with respect to magnetite for the quartz-monzonite phase. respect to magnetite for monzonite phase.

shows that the quartz-monzonite distribution is significantly differ- would imply that there was tight clustering of magnetite grains, ent from an expected random distribution; however, the monzonite rather than a more uniform distribution. distribution is not. The idea that the magnetite and ferromagnesian minerals formed In the quartz monzonite, the feldspars and quartz have less contemporaneously is of special interest. The relation of ferro- magnetite association than the ferromagnesian minerals. Plagio- magnesian silicates and iron-titanium oxides has been studied by clase has only one-third of the expected magnetite association. The Carmichael (1963, 1967) and Carmichael and Nicholls (1967). quartz and K-feldspar have very nearly the expected associations of Their main interest is the effect of ferromagnesian compositions on magnetite. The distribution of AC values for hornblende shows no the time of precipitation of the magnetite and vice versa. overlap with the iron-free silicates. The spread of hornblende AC Czamanske and Wones : 1970) considered the possibility of magne- values is the greatest of the constituent minerals. The biotite dis- tite formation as a product of ferromagnesian m inerals that break plays a reasonably compact distribution that has twice the expected down to iron-free silicates, under oxidizing conditions. The role of proportion of magnetite. oxygen in the formation of iron-titanium oxides, which directly There are three histograms in the monzonite group that show precipitate from the melt, is well known (Buddington and Lindsley, noteworthy contrast to the quartz monzonite. There is an average 1964). of twice the amount of magnetite associated with the monzonite The AC results and pétrographie observations suggest that a por- plagioclase as there is with the quartz-monzonite plagioclase. A tion of the magnetite formed during the incongruent reactions of change, noted in the association coefficient of the K-feldspar, is a the ferromagnesian silicates. Instead of having the end product an slight excess in the quartz monzonite and a slight deficiency in the iron-free silicate, which would imply highly oxidizing conditions, it monzonite. The average association coefficient of hornblende in the is suggested that the reactions may be more moderate: quartz monzonite is roughly twice that observed in the monzonite, 1. Orthopyroxene + 0 —» clinopyroxene 4 magnetite. and the association coefficient of quartz and biotite is approxi- 2 2. Clinopyroxene + 02 -» hornblende + magnetite. mately equal in the two rock types. 3. Hornblende +• 02 -» biotite + magnetite.

DISCUSSION All three reactions require an oxidizing atmosphere. The lack of a strong association of magnetite with the late- Evidence of the second reaction is seen in some of the monzonite formed felsic minerals, K-feldspar and quartz, indicates that the thin sections. Reaction rims of hornblende around clinopyroxene magnetite did not crystallize late. The ferromagnesian minerals show larger amounts of magnetite in the hornblende than in the formed early during paragenesis, as they are not interstitial and are pyroxene. Evidence of the third reaction is present, but it is less commonly subhedral to euhedral. Direct precipitation of the abundant than the second. There are hornblende pseudomorphs magnetite with the ferromagnesian minerals would not be expected after pyroxene with appreciable amounts of magnetite that could to display a strong association between the two minerals. This be the result of oxidation reaction. The Fe would be available from point is supported by the fact that plagioclase, which also forms the solid solution of (Mg2+, Fe2+, Al3+) in the pyroxenes and from relatively early, shows no strong association with magnetite. There- (Mg2+, Fe2+, Al3+, Fe'lr) in the hornblende. A portion of the Fe2+ fore, the strong association of the magnetite with the ferro- from the silicates has to be oxidized to Fe3+ in order to create mag-

magnesian minerals is believed to represent some kind of genetic netite. It is noted that H20 is needed for both the reactions of association. clinopyroxene to horn alende and also is needed to produce the ox- The proximity of the magnetite and ferromagnesian silicates to idizing atmosphere. Czamanske and Wones (1970) did note a de- each other, as indicated by the association coefficient, shows that crease in the Fe/(Fe + Mg) ratio in amphiboles and biotites in their iron is concentrated into microzones. It is a problem at times to de- work, though they did not attribute this to a ferromagnesian min- termine the paragenesis of the ferromagnesian silicates and mag- eral reaction. Unfortunately, no chemical analyses are available netite. The inclusion of one mineral within another does not prove from this investigation to determine the change in the Fe/(Fe + Mg) that the inclusion came first, as supported by the phenomenon of ratios from augite to hornblende to biotite. exsolution. Exsolution, ir_ the normal sense, has been recorded fcr The suggestion that magnetite formed by oxidation of the ortho- the ferromagnesian-magrietite assemblage, but the textural rela- pyroxenes is speculative, as there is no evidence of orthopyroxenes. tions observed in this study do not indicate exsolution. The ques- The reaction of augite to hornblende is seen in the monzonite, as tions then posed are: did the magnetite grains, which in many cases well as the conversion of hornblende to biotite, though this latter are included, serve as centers for ferrosilicate growth by being reaction appears to be subordinate. These reactions could well ex- sources of Fe; did the magnetite and ferromagnesians form plain the high association of the magnetite to the ferromagnesians. contemporaneously; or did the magnetite form from the ferro- The greater amount of magnetite that is found with the horn- magnesians? Ferromagnesian mineral grains that host several mag- blende than with the biotite suggests that more magnetite formed netite grains (a common occurrence in this suite of rocks) are not when the hornblende crystallized than when the biotite formed. likely to have formed by the dissociation of magnetite grains. This Magnetite, once created, would be stable and remain in its relative

position in the rock, if there was little differential movement in the alteration. In the unaltered rock, the largest portion or approxi- magma. If the greatest portion of the biotite formed by incongruent mately two-thirds of the magnetite is considered to be from direct reaction of hornblende under oxidizing conditions, a higher associ- precipitation. This leaves the remaining one-third to be genetically ation of magnetite to biotite would be expected as a result of in- associated with the ferromagnesian minerals. heriting magnetite when the biotite was constituted. Because the AC value of hornblende is three times that of biotite, it is reason- SUMMARY AND CONCLUSIONS able to conclude that much of the biotite formed by direct precipi- In a broad sense, the histograms represent an encoding of the tation as contrasted to oxidation reaction. This is in lieu of the pos- intrusive's cooling history. Comparison of the histograms of differ- sibility that the magnetite was resorbed to form the biotite, a reac- ent rock types, or even within a unit, can help identify dissimilar tion not supported by petrographic observations. cooling histories that might be too subtle to detect qualitatively. A number of thin sections show significant hydrothermal altera- With the aid of observations made during the petrographic study, it tion of the hornblende, which resulted: n the release of magnetite. It is believed that a reasonably clear picture can be drawn of the crys- appears that sites with AC values >6.0 (the quartz monzonite) rep- tallization sequence of the two primary rock types within the Mel- resent areas of late-stage alteration of the hornblende. Biotite does rose stock. not appear to have been as susceptible to alteration as the horn- The quartz monzonite will be considered first. Figure 3 shows blende, and its AC histogram supports i:his observation. that the magnetite has only minor association with the plagioclase. The petrographic observations and AC results support the idea This arises from the early crystallization of plagioclase. However, that magnetite has formed by three processes: (1) direct precipita- there are some inclusions of magnetite within the plagioclase, indi- tion from the melt, (2) oxidation reactions involving the ferro- cating that a portion of the plagioclase formed after precipitation magnesian minerals, and (3) hydrothermal or late-stage alteration of the magnetite. The K-feldspar, excluding the few phenocrysts, of the ferromagnesian minerals. Furthermore, it is believed that was constituted late in the quartz monzonite's history, for it is these are the three most important factors that have influenced the primarily interstitial and consequently it includes much magnetite. AC values. The AC values for a given thin section reveal a com- The numerical results support this point, because the association posite effect, and it is not possible to quantitatively resolve the con- coefficients are >1.0. Surprisingly, the quartz appears to have tribution of each factor. If there are only two factors influencing the formed before most of the K-feldspar: quartz is usually AC values, then it is possible to estimate the contribution of each. equidimensional and not relegated to an interstitial position. It is The amount of magnetite formed by each process can be esti- suggested, in view of the AC values for quartz and textural obser- mated with the aid of microscopic observations and the association vations of the thin sections, that the quartz and magnetite were parameter. Direct precipitation is expected to yield an association crystallizing concurrently, but the magnetite exhausted itself before coefficient of 1.0, which means that the fraction of magnetite as- the quartz. The supply of iron for magnetite was depleted before sociated with a given mineral numerically equals the modal volume the supply of free silica was depleted for quartz. of that mineral. Therefore, the fraction of magnetite formed by di- From the evaluation of the excess magnetite association of horn- rect precipitation can be estimated by summing the results for each blende, it appears that a portion of the magnetite formation can be of the constituent minerals. Because their AC values are >1.0 (ex- correlated with the commencement and cessation of the period of ceeding their modal volumes), the fraction of magnetite associated hornblende formation. Textural relations indicate that the horn- with the ferromagnesian minerals is attributed to oxidation and blende formed early in the crystallization sequence. Biotite, for the hydrothermal alteration. The average biotite AC value of each rock most part, formed subsequent to the hornblende. type is ~2.0. As biotite does not show appreciable hydrothermal The association coefficients and textural relations suggest a alteration, the excess association of magnetite to biotite is inter- slightly different paragenesis in the monzonite. The plagioclase in preted as caused by an oxidation reaction. Hornblende shows a the monzonite shows a closer spatial association with the magnetite bimodal frequency distribution that is be Sieved to represent the as- than it shows in the quartz monzonite. This is considered to result sociation arising from an oxidation reaction and alteration of the from the magnetite crystallizing contemporaneously with the hornblende late in the intrusive's history. The high AC value group- plagioclase to a greater degree than it did in the quartz monzonite. ing is from thin sections that display abundant alteration, and the A slight deficiency in the association of magnetite with K-feldspar low value grouping is from sections showing small amounts of al- could have resulted from the K-feldspar precipitating at a higher teration. Average values of 5.0 and 7.0 lor the quartz monzonite temperature. The relative amount of magnetite associated with the and 3.0 and 5.0 for the monzonite can be estimated from the histo- hornblende is less than the amount in the quartz monzonite, al- grams of Figures 3 and 4. The lower value for each rock type is though in absolute volume, the opposite is the case. The precipita- used to calculate the volume of magnetite formed by the oxidation tion of biotite and quartz with magnetite is believed to be nearly the reaction, and the higher values are used to determine the magnetite same as in the quartz monzonite. formed by alteration. Results of the excess magnetite associated The two lithologic units of the Melrose stock have probably had with the ferromagnesian minerals are shown in Table 4. similar cooling histories, because they do not show a sharp field The results reveal that, in both rock types, about one-fourth of contact, and both display similar mineral assemblages. Composi- the magnetite can be attributed to oxidation reactions during the tionally, the monzonite is slightly more basic than the quartz mon- course of crystallization. In areas of the stock where there has been zonite. It is postulated that the main difference between the two hydrothermal alteration, about one-tenth of the volume of magne- phases was in their oxygen fugacity content, which is believed to tite in the quartz monzonite is believed to have formed by late-stage have been greater in the quartz monzonite—evidence shown by the greater association of magnetite to hornblende. This can be ex-

TABLE 4. PERCENTAGE OF MAGNETITE plained reasonably by a higher concentration of HaO in the quartz TO TOTAL MAGNETITE monzonite. The overall smaller range of AC averages in the mon-

Rock type Mode of origin zonite suggests that it had a shorter crystallization history than did (%)* the quartz monzonite. Direct Oxidation Alteration crystallization reaction Field relations indicate the monzonite phase to have been emplaced first, followed by the quartz monzonite. The monzonite as Quartz monzonite 65 25 10 an initial phase might be expected to have a shorter crystallization Monzonite 45 30 25 •history. The quartz monzonite, because it is a late phase, might be * To nearest 5*. expected to have a greater hydrous component that would enhance

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the oxidizing effects. As pointed out by Buddington and Lindsley Carmichael, I.S.E., and Nicholls, J., 1967, Iron-titanium oxides and oxygen (1964), composition, temperature, and pressure also help to deter- fugacities in volcanic rocks: [our. Geophys. Research, v. 72, p. mine the relative oxidizing-reaucing condition in a cooling magma. 4665-4687. Czamanske, G. K., and Wonts, D. R., 1970, Amphiboles as indicators of oxidations during magmitic differentiation: Geol. Soc. America, Abs. ACKNOWLEDGMENTS with Programs for 1970 (Ann. Mtg.), v. 2, no. 7, p. 531. Flinn, D., 1969, Grain contacts in crystalline rocks: Lithos, v. 3, p. I would like to express my appreciation to W. J. Hinze and T. A. 361-370. Vogel for their discussions and comments on the early drafts of this Kretz, R., 1969, On the spatial distribution of crystals in rocks: Lithos, v. 2, paper and to the Department of Geology, Michigan State Univer- p. 33-66. sity, for the use of its laboratory facilities. Viahan, S. M., and Rogers, J. J.W., 1968, A study of grain contacts in some high-grade metamorphu: rocks: Am. Mineralogist, v. 54, p. 323-327. REFERENCES CITED Rogers, J.J.W., and Bogy, D. B., 1958. A study of grain contacts in granitic Armstrong, R. L., 1963, Geochronology and geology of the eastern Great rocks: Science, v. 127, p. 470-471. Basin [Ph.D. thesis]: New Haven, Yale Univ. Snow, G. G., 1963, Mineralogy and geology of the Do'.ly Varden Moun- Buddington, A. F., and Lindsley, D. H., 1964, Iron-titanium oxide minerals tains, Elko County, Nevada [Ph.D. thesis]: Salt Laki; City, Univ. Utah, and synthetic equivalents: Jour. Petrology, v. 5, p. 310-357. 153 p. Carmichael, I.S.E., 1963, The occurrence of magnesian pyroxenes and magnetite in porphyritic acid glasses: Mineralog. Mag., v. 33, p. 394-403. 1967, The iron-titanium oxides of salic volcanic rocks and their associated ferromagnesian silicates: Contr. Mineralogy and Petrology, v. MANUSCRIPT RECEIVED BY THE SOCIETY MAY 15,1973 14, p. 36-64. REVISED MANUSCRIPT RECEIVED FEBRUARY 19, 1974

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