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1978

Petrography and chemistry of the East Fork dike swarm Ravalli Co. Montana

Ruth Hall Badley The University of Montana

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Recommended Citation Badley, Ruth Hall, "Petrography and chemistry of the East Fork dike swarm Ravalli Co. Montana" (1978). Graduate Student Theses, Dissertations, & Professional Papers. 7726. https://scholarworks.umt.edu/etd/7726

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DIKE SWARM, RAVALLI CO., MONTANA

by

Ruth H. Badley

Presented in partial fulfillm ent of the requirements for the degree of

Master of Science

UNIVERSITY OF MONTANA

1978

Approved by

Chairman, Board of/Examiners

Deaif, Graduate Schoolÿÿ^

//- / - y j Date UMI Number; EP38527

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ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 481 0 6- 1346 ABSTRACT

Badley, Ruth H. , M.S., Spring, 1977 Geology

Petrography and Chemistry of the East Fork Dike Swarm, Ravalli Co., Montana

Director: Dr. Donald Hyndman

The East Fork dike swarm consists of several d iffe re n t types of porphyritic felsic dikes. It outcrops along the East Fork of the Bitterroot River in an area approximately sixteen by four kilo­ meters in outline and is thought to represent a small portion of the Idaho Porphyry Belt which outcrops in te rm itte n tly from Boise, Idaho to Helena, Montana. The Idaho Porphyry Belt consists of a series of calc-alkaline volcanic centers, the largest of which is located near C hallis, Idaho. The East Fork Swarm is thought to represent the northeasternmost lim it of volcanic activity of a smaller volcanic center located in the West Fork of the Bitterroot River Valley. Six major dike types were distinguished using both pétrographie and chemical characteristics of the dikes. The most easily recognizable of these, the Mink Creek rhyodacite porphyry and the Mink Creek porphyritic andésite, are characterized by large (up to six centimeters) euhedral megacrysts of sanidine. All the dikes examined contain plagioclase feldspar which is commonly sericitize d and shows o scilla to ry zoning and synneusis structure. Quartz occurs as rounded phenocrysts in a ll dikes except the Meadow Creek porphyritic dacite. B iotite and hornblende have been deuterically altered to chlorite plus rutile, sphene, epidote and/or calcite in nearly a ll dikes observed. The East Fork dikes are chemically sim ilar and range in compo­ sitio n from andésite to rhyodacite. Compositions were determined using x-ray fluorescence techniques. The average composition of the East Fork dikes is rhyodacite, while the most frequently occurring rock type is dacite. Rocks names were determined with a cla ssifica tio n designed by Church (1975).

1 ^ ACKNOWLEDGMENTS

I wish to give special thanks to Dr. Donald Hyndman, at whose suggestion this study was begun, and with whose encouragement

i t was completed. I also wish to thank Dr. Jack Wehrenberg fo r his

help, especially with analytical problems. Dr. Rudy Gideon provided

valuable assistance with s ta tis tic a l problems. Drs. Gray Thompson

and David Fountain furnished helpful discussion and comments.

Gary Morrison of the USFS provided much needed maps and aerial

photographs. A very special thanks to Ken Gordon whose patience

and skill with the x-ray machine were invaluable. Finally, I wish

to thank Linda Wackwitz for helping keep me sane, and Giles Walker

and Gene Nelson for providing incentive and encouragement.

111 TABLE OF CONTENTS

Page

ABSTRACT i i

ACKNOWLEDGMENTS ...... i i i

LIST OF TABLES ...... v

LIST OF FIGURES...... vi

CHAPTER

I. INTRODUCTION ...... 1

Location of the Study A r e a...... 2

Regional Geology ...... 2

II. THE IDAHO PORPHYRY B E L T...... 5

I I I . PETROLOGY...... 11

General Textures and Sequence of C rystallization . 12

Common Dike Types of the East Fork Swarm...... 22

Potassium Feldspar Megacrysts ...... 32

A lte r a tio n ...... 36

IV. WHOLE ROCK CHEMISTRY...... 41

V. CONCLUSIONS...... 49

REFERENCES...... 52

APPENDIX ...... 54

1 V LIST OF TABLES

Table Page

1. Common Characteristics of the six most representative East Fork dikes...... 29

2. Whole rock analyses for East Fork d i k e s ...... 43

3. Comparison of XRF data with outside a n a ly s is ...... 54 LIST OF FIGURES

Figure Page

1. Location Map of the East Fork study area ...... 3

2. The Idaho Porphyry B e l t ...... 6

3A. Hypothetical model for intrusion of the East Fork 3B. Dikes along a preexisting structural weakness: plan view and cross s e c t i o n ...... 9

4. Stages in development of synneusis aggregates .... 14

5. Synneusis of plagioclase in the East Fork rhyodacite porphyry ...... 18

6 . Rounded and deeply embayed quartz in the East Fork rhyodacite porphyry ...... 18

7. Pressure versus weight percent2 O H at 750°C for synthetic granite ...... 19

8 . Rhyolite tetrahedron showing East Fork dikes ...... 21

9. Biotite phenocryst completely altered to chlorite . . 38

10. Triaxial plot showing fields of variation of most common volcanic rocks ...... 45

1 1 . SiÜ2 variation diagrams for East Fork dikes ...... 47

VI CHAPTER I

INTRODUCTION

A distinct northeastward-trending series of Tertiary dikes, the Idaho Porphyry Belt, may be observed on the Idaho and Montana state geologic maps. These dikes have been noted in an area which ranges from northeast of Boise, Idaho to north of Butte, Montana.

A portion of this dike swarm outcrops in the East Fork of the

Bitterroot River valley in southwestern Montana. The East Fork dikes are primarily dacites to rhyodacites in composition, whereas the

Idaho Porphyry Belt as a whole contains, in addition, rocks of basaltic composition, thus completing a compositional range from basalt to rhyolite. The East Fork dikes have a predominantly porphyrytic texture and contain phenocrysts of quartz, plagioclase, K-feldspar, b io tite and hornblende.

The present study was undertaken to describe and characterize the dikes located in the East Fork of the Bitterroot River valley

(hereafter referred to as the East Fork dikes or swarm). The dikes were examined and described using pétrographie and chemical techniques.

Whole rock chemical analyses were made of twelve major outcrops, using x-ray fluorescence. Two subsidiary problems appeared after the project was begun: 1) the origin of large K-feldspar megacrysts occurring in the Mink Creek rhyodacite porphyry and the Mink Creek porphyritic

1 andésite; and 2) placement of the dikes w ithin the Idaho Porphyry

Belt and their relationship to nearby volcanic centers.

Location of the Study Area

The study area is located eighty miles (134 km) south of Missoula,

Montana (Fig. 1) and ten miles (16.6 km) northeast of Sula, Montana along the East Fork of the B itte rro o t River. Major access to the area is provided by U.S. Highway 93, unpaved Ravalli County and U.S. Forest

Service roads. The East Fork area has been extensively logged, and abandoned logging roads provide access as well as exposure. The study area is bounded by L ittle Mink Creek on the western margin and by

Martin and McCart Creek roads on the eastern margin. Dikes tend to be concentrated at the western end of the area along Mink Creek Road and the East Fork of the Bitterroot River.

Regional Geology

The East Fork of the Bitterroot River is located within the eastern margin of the Idaho batholith and at the southern end of the Sapphire

Tectonic block. I t is in the northeast portion of the Idaho Porphyry

Belt. The dikes are one of the more resistant rock types in the area and form a large portion of outcropping rock types. Other rock units in the area include a biotite-hornblende quartz diorite phase of the

Idaho batholith and preCambrian metamorphic rocks consisting of medium grade c a lc -s i1icate gneisses and p e litic schists. Volcanic rocks were Montana

• MlssouI a Idaho

Butte

Oregon )

Wyoming

r V L ' ^

ANACONC ' r ]7'" '

Figure 1. Location map of the East Fork Study Area, outlined with solid line. Typed letters designate type locations fo r East Fork Dikes. observed only as flo a t but are prominent on some ridge tops.

There are seven major types of dikes in the East Fork Swarm.

Composite dikes are common, and contain up to five different types of dike. A particular dike type may be repeated several times in a composite dike; however, no d is tin c t patterns of intrusion appeared.

The East Fork dikes rarely show chilled margins and nearly all contacts with other dikes and the country rock are vertical. Sample locations were noted on USFS 1:24000 planimetric road maps.

Dikes sim ilar to those studied in the East Fork area have been reported both east and west of the study area (Fig. 1). Flood (1974),

Wiswall (1976), and Rebal (verbal communication, 1976) have noted porphyritic dikes similar to the East Fork dikes in the vicinity of

Fishtrap Creek in the Anaconda P intlar Wilderness Area. West of the study area, Berg (1973) has reported porphyritic dikes in the West

Fork of the Bitterroot River Valley. The relationship of some dikes as feeders to the overlying volcanics is apparent in this area

(Hyndman, verbal communication, 1976). CHAPTER II

THE IDAHO PORPHYRY BELT

Early Tertiary dikes have been found in a broad northeast- trending zone throughout south-central Idaho and southwestern Montana

(Fig. 2). These dikes, which vary in composition from basalt to rh yo lite , have been grouped together and called the Idaho Porphyry

Belt, or less commonly, the Anaconda Range Dike Swarm (Badgely, 1965).

Previous workers in the Idaho Porphyry Belt include Ross (1934),

Anderson (1947) and Olson (1968). The "type area" fo r the Idaho

Porphyry Belt is located in the Boise Basin (Anderson, 1947) and many of the dikes belonging to the Belt are found in areas containing economic concentrations of precious metals. The relationship between mineralization and intrusion of the dikes is unclear and the dikes are not always found with a corresponding area of mineralization.

No specific boundaries have been set for the Idaho Porphyry Belt.

Boise, Idaho is commonly accepted as the southwestern lim it, while

Butte, Montana is considered to represent the northernmost lim it.

Other authors (Olson, 1968) have extended the northern lim it of the

Idaho Porphyry Belt (IPB) to the Canadian border. With this extension, the IPB then would include most of the Central Montana Pétrographie

Province (Larsen, 1940) (Fig. 2) which is composed of rocks of different chemical composition (primarily strongly alkaline igneous

5 g

Central Montana

• '\//Petrographic .Prçyincé

Guider Batfiol H h . /

Idaho athol

Anaconda Range ^ Dike Swarm (Badgely, 1965)

Tertiary Dikes (Olson, 1968)

Tertiary Plutons 0 (Olson. 1968)

I ^ Jurassic-Cre- 4^ taceous Plutons ‘ (Olson, 1968)

Figure 2. The Idaho Porphyry Belt (after Olson, 1968 and Badgely, 1965) rocks). It can only be included in the IPB if the controlling structure is a pre-existing basement zone of weakness which has had no effect on chemical composition. The more restrictive limits as de­ fined by Anderson (1947) and Badgely (1965) w ill be used in this paper.

A pre-existing zone of structural weakness w ithin the basement has been called upon by recent workers (Olson, 1968, and Hyndman, Badley, and Rebal, 1977) to account for the emplacement and length (approximately

400 km) of the Idaho Porphyry Belt. A locally important process in the formation of the IPB is the intrusion of dikes along fractures that resulted in doming related to Tertiary plutons (Olson, 1968, p. 121).

The East Fork dikes have been included in the Porphyry Belt because of their position within the zone of weakness.

To assume that the entire length of the Idaho Porphyry Belt is under­ lain by a single source of magma is unreasonable. I t appears more likely that it represents a series of separate but contemporaneous centers of volcanism and intrusion which are similar in composition.

These centers of volcanism may be a result of fracturing of the crust which permits rise of basaltic magmas from the mantle. Their heat in turn generates localized centers of volcanism which are andesitic to rh y o litic in composition. Continued heating and fracturing would allow extrusion of mantle derived basalts (D. Fountain, verbal communication,

1977). Olson (1968)and Reid (1963) have noted a series of basaltic dikes which appear to be later than the rh y o litic dikes. Reid (1963, p. 15) has placed diabase dikes in the Sawtooth Wilderness as feeders 8 to the Columbia River basalts. The tectonic history of the Idaho

Porphyry Belt is unknown and beyond the scope of this work. However, the sim plified model presented above appears reasonable in lig h t of available information.

Centers of volcanism found on the Idaho Porphyry Belt include the

Challis Volcanics, an unnamed area in the West Fork of the Bitterroot

River Valley and the Lowland Creek Volcanics. The rocks found in these areas are predominantly dacite to rhyolite in composition. In the

West Fork area, dikes can be seen as feeders to the overlying volcanics

Volcanic rocks in the East Fork area appear to occur as a thin veneer over the Idaho batholith, whereas in the West Fork area, a thick p ile of volcanics is apparent. No extensive outcrops or flows were observed

I t is proposed here that the dikes found in the East Fork area are not the source of the overlying veneer of volcanics, but that this veneer represents more extensive flows which extend northeast from the West

Fork. The West Fork area represents the source for the magma center

included in the East Fork Swarm. The dikes have been intruded along

the zone of weakness, decreasing in number with increasing distance

from the West Fork Center (Fig. 3a). The dikes were probably intruded

la te ra lly from the magma chamber, rather than from upward from i t

(Fig. 3b).

The absence of extensive flows in the East Fork area can be ex­ plained in two ways. First, if the center of volcanism is not located below the East Fork dikes but to the west in the West Fork area, only West Fork Area East fork Area

A

B

Magma Chamber

Volcanic Flows Di kes VoIcano

Figure 3A & B Hypothetical model for intrusion of the East Fork Dikes along a preexisting structural weakness (IPB) A. Plan view B. Cross section Note: Drawings not to scale. 10 a small portion of the material extruded from the West Fork area would ever reach the East Fork area as flows or ash fa lls . Second, removal of the volcanic cover by erosion may have been greater in the East

Fork area while the West Fork area was le ft re la tiv e ly untouched. At this time evidence for either hypothesis is in s u ffic ie n t to form a conclusion.

The f ir s t hypothesis mentioned in the preceding paragraph is preferred fo r the following reasons. Dikes in the East Fork area are more numerous in the southwest portion of the study area (Fig. 3a).

Very few dikes were observed northeast of Meadow Creek. I f the East

Fork represents a separate center, then both flows and dikes would be expected to be more widespread. No dikes in the East Fork area were observed feeding directly into flows. However, this does not exclude the existence of such a relationship.

Since the West Fork volcanic center may be located along a zone of basement weakness, i t appears lik e ly that volcanic a c tiv ity would not be restricted to a small area (e.g. a neck or plug) and that small dikes could form away from the volcanic center. Dikes intruded in this manner would probably decrease in number away from the volcanic center, and would form a situation sim ilar to that found in the East Fork area

(Fig. 3a). The dikes then intruded la te ra lly from the magma chamber, rather than upward from i t . CHAPTER I I I

PETROLOGY

Dikes found in the East Fork area generally occur as composite dikes. No more than five d iffe re n t types were found in a composite outcrop, though the same type of dike could be found repeated through the outcrops. Contacts between the deeply weathered Idaho batholith and the dikes were not observed, though dike walls were generally ve rtica l. Contacts between d iffe rin g types of dike are vertical or nearly vertical.

Flow structures were observed only in one thin section and i t was not possible to place the structures in relationship to the dike walls. On a large scale, preferred orientation of K-feldspar mega­ crysts was observed in dikes at two locations along Mink Creek Road.

Flow structures on an outcrop scale were seen at one lo c a lity on Mink

Creek Road, where portions of a Mink Creek porphyritic andesite-type dike containing an enriched amount of plagioclase phenocrysts appear to have been broken apart and surrounded by a less plagioclase-rich magma.

Contacts are diffuse, and there is no other change in the character of the dike, indicating probable crystal settling and subsequent turbulence

These flow structures were found in only two types of dike and in only four outcrops, making them uncommon features in the East Fork dikes.

11 12

Xenocrysts and xenoliths are also uncommon in the East Fork Swarm.

Inclusions of one dike within another were found at only one locality along Mink Creek Road. Here, rounded inclusions of the Mink Creek porphyritic andésite were found in an outcrop of the Mink Creek

rhyodacite porphyry. The two dikes are juxtaposed in outcrop, the Mink

Creek rhyodacite porphyry apparently having been intruded along one side of the already crystallized Mink Creek porphyritic andésite. The Meadow

Creek andésite contains inclusions of the Idaho batholith, as well as xenocrysts of quartz. Xenocrysts of quartz were observed in only a few thin sections, but were distinguished from indigenous quartz by the presence of undulose extinction across sutured grain boundaries within an aggregate of several crystals. One such xenocryst also showed a slightly corroded outline, indicating that it was partially resorbed.

Although there is some textural and mineralogical variation in the East Fork dikes, many of the dikes share common features. The most prominent of these, rounded, embayed quartz, synneusis structure, and oscillatory zoning, are described in detail in a later section. Other features such as the degree of alteration and type of groundmass, as well as those previously mentioned, have been compiled in Table 1 fo r comparison. Descriptions of characteristics peculiar to a certain type of dike w ill be described in a b rie f section on each major dike group.

General Textures and Sequence of C rystallization

Although the East Fork dikes are variable in both appearance and texture, three textural features are shared by most porphyritic dikes: 13 synneusis structures, o scilla to ry zoning, and rounded, embayed grains of quartz. Synneusis structures involve several minerals, most notably quartz, plagioclase, and K-feldspar. They provide information about sequence of crystallization and turbulence within the magma, as well as evidence for magmatic origin of the rock (Vance, 1969). Rounded embayed quartz is thought to result from magmatic corrosion resulting from changes of pressure within the magma (Whitney, 1975). The repeated release of volatiles or diffusion-supersaturation of anorthite adjacent to a plagioclase crystal have been called upon to account for oscillatory zoning in this mineral. Brief summaries of these processes w ill be pre­ sented here in order to relate them to the East Fork dikes.

Synneusis structures form as crystals d rift together and attach themselves to one another (Figure 4A-C). Although the term synneusis has not been widely adopted, synneusis structures are widespread in igneous rocks. Synneusis structures are indicative of magmatic origin for rocks containing them, requiring a medium fluid enough to allow unrestricted movement of phenocrysts. They are thought to have originated through episodic magmatic turbulence early in the crystallization of the magma. They include glomeroporphyritic structures found in volcanic rocks. There are two main c rite ria fo r recognizing synneusis structures:

1 ) two or more relatively large crystals were involved, and 2 ) a prominent crystal face in one crystal is parallel to a prominent crystal face in the other (Vance, 1969, p. 8 ). 14

A B C

Figure 4 A-C. Stages in development of synneusis aggregates. A. Two isolated crystals B. Drifting together and union. C. Post synneusis overgrowth (Vance, 1969, p. 9).

The most recognizable feature in synneusis is m is fit of the broad faces between two joined crystals (Figure 4C). Re-entrant angles and polygonal outlines are common, though with extended crysta lliza tio n the re-entrant angles are prefe re n tia lly f ille d in. Synneusis twins are distinguished from growth twins by the presence of more than one in ­ dividual, misfit, and irregularities of the composition plane. Synneu­ sis is distinguished from epitaxial growth by the presence of two in­ dependent, comparatively large crystals at the beginning of mutual growth,

Epitaxial growth requires the nucléation of a smaller crystal on a larger one (Vance, 1969, p. 8-13).

Synneusis is recognized in minerals such as plagioclase, quartz,

K-feldspar, hornblende, and pyroxene in either volcanic or plutonic rocks. These minerals are often observed in glomeroporphyritic (randomly oriented) structure in volcanic or hypabyssal rocks. It is easiest to recognize synneusis structures in zoned minerals such as plagioclase 15 and much more d if f ic u lt in unzoned minerals such as quartz and K- feldspar. Plagioclase is most commonly seen in parallelism or near parallelism , of the 010 face of one crystal with respect to the010 face of the other. This is called parallel synneusis. Most common twin laws for plagioclase have been observed as synneusis twins. How­ ever, none of these laws were observed in the East Fork dikes. Parallel synneusis was observed in a ll major dikes. Vance (1969, p. 18) states that synneusis of quartz is most readily recognized as glomeroporphyritic clusters in quartz porphyries. Quartz in the East Fork dikes is commonly seen as single grains, but synneusis clusters were observed in the Mink

Creek Rhyodacite Porphyry and the East Fork Porphyritic Rhydodacite.

The most d if f ic u lt mineral to recognize in synneusis relation is

K-feldspar; primarily because of the lack of zoning. Two examples of

K-feldspar in synneusis relation were found in the Mink Creek porphyri­ tic andésite.

K-feldspar megacrysts also offer an excellent example of synneusis between d iffe re n t minerals. Growing K-feldspar megacrysts often enclose plagioclase, quartz, and biotite, as well as minor accessory minerals such as sphene and zircon. Plagioclase found w ithin the East Fork megacrysts is probably a combination of both synneusis and epitaxial growth. Grain boundaries are often in d is tin c t and o scilla to ry zoning occurs only in wel1-developed crystals. Quartz in the East Fork mega­ crysts is not euhedral as one might expect of an epitaxially-grown c ry s ta l, but is rounded, resembling quartz found as phenocrysts. This suggests that i t was corroded p rio r to its inclusion in the megacryst. 16

Oscillatory zoning is currently thought to result from diffusion- supersaturation of anorthite adjacent to a plagioclase crystal (Sibley, et al, 1976). During crystallization, anorthite molecules diffuse to an area of lower concentration (i.e., a plagioclase crystal which has just crystallized a zone of anorthite) until the area bordering the crystal becomes supersaturated with anorthite molecules and another zone is precipitated. In this manner, small, uniform oscillations in anorthite content can form dozens of zones within a single plagioclase c ry s ta l. Among other mechanisms proposed to form o s c illa to ry zoning, most notable and widespread is the repeated release of volatiles, and subsequent formation of zones. Vance (1962, p. 750) has pointed out that a body of magma has no regulatory system to determine a rhythmic release of volatiles. This process would be feasible in a subvolcanic environ­ ment where proximity to the surface might allow such a process to occur, but oscillatory zoning is also widespread in plutonic rocks.

Plagioclase occurring as phenocrysts in the East Fork dikes commonly shows oscillatory zoning. The phenocrysts show up to thirty to forty oscillations and the zoning may be deflected around inclusions (Fig. 5).

Deflections such as these have been described in a paper by Blackerby

(1968). He has designated this effect as convolute zoning and proposed that it results from obstruction of the growing crystal by a semi porous object (in his case a bleb of glass). Convolute zoning is common in dikes such as the East Fork rhyodacite porphyry and appears to have formed 17 when growth of oscillatory zones was blocked by inclusions of other minerals such as biotite.

The third common textural feature of the East Fork dikes is rounded, embayed quartz (Fig. 6 ). This characteristic is especially well de­ veloped in the Mink Creek rhyodacite porphyry and the East Fork dacite porphyry. Quartz occurs as one to six millimeter, clear gray grains with a rounded o u tlin e . Synneusis clusters are common. Locally rounded quartz phenocrysts display poorly-developed borders of quartz and a lk a li feldspar intergrown w ith the groundmass. Rounded, embayed quartz is found in early all the East Fork dikes and does not show undulose e x tin c tio n . Quartz phenocrysts with euhedral hexagonal out­ lines are exceedingly rare.

The formation of rounded, embayed quartz phenocrysts is generally attributed to magmatic corrosion. This is a very common feature of shallow intrusive rocks such as quartz porphyries. Corrosion of early- formed quartz phenocrysts is thought to occur when a decrease in pressure causes the melt to cross the phase boundary marking the disappearance of quartz (Fig. 7). This process commonly occurs in rocks such as rhyolites and rhyodacites with a high SiÛ 2 content. The synthetic granite on which Figure 7 is based contains 73.98 percent SiOg which forms approximately 26 percent quartz. The disappearance of quartz is augmented by the increased solubility of SiÛ2 in quartz-saturated melts at lower pressures (Whitney, 1975, p. 28). The cooling history of any 18

Figure 5. Synneusis of plagioclase in the East Fork rhyodacite porphyry. Note oscillatory zoning which has been defected around a small phenocryst of b io tite . Crossed nichols, length o f plagioclase approximately 1 mm

a m

Figure 6 . Rounded and deeply embayed quartz in the East Fork rhyodacite porphyry. Crossed nichols, quartz crystals approximately 3 mm across. 19

10

Af

8

6 Af

(Kb) 4

2

0 2 4 6 8 10 12 14

Wt. % HgO

Figure 7 Pressure versus weight percent2O Hat 750^C for synthetic granite (after Whitney, 1975) 20 porphyritic rock requires at least one rapid change in temperature and pressure which is manifested by the change in grain size from phenocrysts to groundmass. Although l i t t l e is known about the length o f time re ­ quired for corrosion of early-formed quartz, it is conceivable that corrosion could have occurred during intrusion and drop in pressure and before final crystallization (Hyndman, verbal communication, 1977).

The sequence of crystallization in any igneous rock is controlled by several factors. These include composition as well as temperature and pressure. The order of crystallization is controlled primarily by the order in which the minerals contained in the melt reach saturation

(Hyndman, 1972, p. 76). Compositions of five East Fork dikes have been plotted on a rh y o lite tetrahedron a fte r Carmichael (1974, p. 229).

Although all the dikes crystallize either plagioclase or alkali feldspar as the firs t phase (Fig. 8 ), alkali feldspar is found as phenocrysts in only two of the five dikes, all of which contain K-feldspar in the groundmass. The remainder of the dikes contain phenocrysts of quartz and plagioclase.

The highly poikilitic phenocrysts of K-feldspar found in the East

Fork dikes could provide a clue to the sequence of crystallization, as they were the firs t phase to form phenocrysts in two dikes. Unfortunately, the minerals included are not distributed in zones in the megacrysts

(i.e., plagioclase, quartz and biotite are found mixed together), making a definite conclusion about the order of crystallization difficult. 21

SiQ

so

30,

20 3 0 to 70 ao

Wgykf percent

Figure 8 . Rhyolite tetrahedron showing East Fork dikes plotted as circled points. Note: point q is part of the diagram. (After Carmichael, 1974). 22

Another factor which must be considered is the antipathy of some minerals toward synneusis with others. Plagioclase and quartz show a marked antipathy toward synneusis with one another (Vance, 1968,

p. 23). This is visible in the East Fork dikes, where only one example of plagioclase-quartz synneusis was observed. In the past, such a lack of synneusis would have indicated late-stage crystallization of quartz and plagioclase. Chemical composition and known crystallization trends of quartz and plagioclase in porphyritic rocks indicate that this is

not the case (Vance, 1969, p. 23).

In summary,it appears, on the basis of chemical composition and euhedral outline, that either plagioclase or K-feldspar was the firs t phase to crystallize in the East Fork dikes. Biotite is found as in­ clusions in quartz, plagioclase and K-feldspar, which may be an indication of preferential synneusis as well as early crystallization. Chemical compositions suggest that quartz began crystallizing early in the history of most dikes, rather than later as indicated by the lack of synneusis between quartz and plagioclase. On the whole, the order ofcrystalliza­ tio n o f the East Fork dikes is not complicated and appears to follow normal crystallization trends for their given rock types.

Common Dike Types of the East Fork Swarm

Mink Creek rhyodacite porphyry. The Mink Creek rhyodacite porphyry is one of the most conspicuous rock types in the East Fork swarm and is characterized by large (up to 5 cm in length) euhedral megacrysts of 23

K-feldspar which have been identified as sanidine. It occurs within composite dikes and is always found with the Mink Creek porphyry andésite. The dikes commonly form prominent outcropts up to ten meters high and eight to ten meters wide. The length of the dikes is difficult to determine, as they do not outcrop continuously and are best traced by float and exposures in road cuts.

The Mink Creek rhyodacite porphyry is light green-gray on both fresh and weathered surfaces, though the latter is sometimes streaked reddish brown by iron oxides. Approximately 50 percent of the rock is very fine­ grained groundmass, composed o f quartz, and K-feldspar in nearly equal amounts. The remainder o f the rock is composed of phenocrysts o f quartz,

K-feldspar, plagioclase and biotite. Phenocrysts of chalky white plagioclase from 5 mm to 15 mm in length compose thirty percent of the rock. Quartz occurs as rounded, embayed phenocrysts, one to four mm long as well as very small (less than 0 .2 mm) anhedral crystals in the ground­ mass. Both b io tite and hornblende have been altered to dark green chlorite, which generally occurs as pseudomorphs after the original minerals. A few grains of unaltered biotite are commonly found in the cores of K-feldspar megacrysts. These K-feldspar megacrysts range in length from 0.5 cm to 6.5 cm and in width from 0.5 cm to 4.5 cm. The crystals are commonly euhedral and may be untwinned or twinned according to the Carlsbad law. In cross section the crystals appear zoned. The outer portion of the crystal is pink to b u ff whereas the inner portion is gray. This zoning is though to be a result of 24 alteration and is discussed in that section. Accessory minerals in the Mink Creek rhyodacite porphyry include a p a tite , sphene, zircon, and allanite. Allanite occurs as small, euhedral, zoned crystals with simple twins. It is strongly pleochroic with x' = beige and z* = brown. Apatite and zircon are also euhedral to subhedral whereas sphene is generally anhedral. All the accessory minerals may be found as an inclusion within a large phenocryst or as a constituent of the groundmass.

In th in section, the Mink Creek rhyodacite porphyry appears pervasively altered. The feldspars are moderately to strongly altered to sericite and clay minerals. The mafic minerals, biotite and hornblende, have been altered to c h lo rite plus r u tile , sphene, epidote, or calcite. The green-gray color of the Mink Creek rhyodacite porphyry is probably due to the presence of chlorite.

Mink Creek p o rp h yritic andésite. The Mink Creek p o rp h yritic andésite is one of the more widespread of the East Fork dikes. I t is is found w ith the Mink Creek rhyodacite porphyry as well as several other types of dikes. The size of the dikes in the Mink Creek porphyritic andésite is more variable than most dike types, ranging from 0.5 to nine meters in width. The Mink Creek porphyritic andésite is also found in varying stages of crystallization. The dikes with finer grain size and fewer phenocrysts are generally smaller. Definite flow structures were found in one outcrop, and probable flow structures were seen in two other outcrops. 25

The Mink Creek p o rp h y ritic andésite is dark green-gray and

contains approximately thirty percent phenocrysts consisting of

quartz, plagioclase, K-feldspar and altered mafic minerals, and

seventy percent aphanitic to fine-grained groundmass. The groundmass

is unusual in that it is one of only two dike types which contains

abundant hornblende or other mafic minerals. The groundmass contains

approximately fifteen to twenty percent euhedral hornblende, with minor

biotite, with plagioclase, quartz, and K-feldspar comprising the re­

mainder. Quartz and plagioclase phenocrysts occur in much the same

manner as the Mink Creek rhyodacite porphyry. However, the K-feldspar

megacrysts tend to smaller (0.5 - 4cm), and in rapidly chilled dikes

they are subhedral in outline. Zircon, sphene and apatite are the most

common accessory minerals.

East Fork rhyodacite porphyry. The East Fork rhyodacite porphyry

is more restricted in outcrop than either the Mink Creek rhyodacite

porphyry or porphyritic andésite. It is found in a very large outcrop

along the East Fork of the B itte rro o t River as well as in a smaller

outcrop near the head of Mink Creek Road. The East Fork dike was one

of the only dikes which outcropped for any distance (0.4 km). The dike

is not variable in either texture or mineralogy along its length. A

fa u lt contact or shear zone appears to separate the East Fork rhyo­ dacite porphyry and a less quartz-rich dike which occurs as wedges along

the southwestern and northeastern ends of the dike. 26

The East Fork rhyodacite porphyry is very similar to the Mink

Creek rhyodacite porphyry, with one exception: K-feldspar occurs only in the groundmass of the East Fork rhyodacite porphyry. It is a strongly porphyritic rock, with forty percent groundmass and sixty percent phenocrysts. The dike is light green-gray in color, due to the presence of chlorite, both in the groundmass and as phenocrysts. The groundmass is composed of approximately equal parts of quartz and

K-feldspar, with up to five percent chlorite after biotite. The pheno­ crysts consist of strongly embayed and rounded quartz, plagioclase phenocrysts with strong oscillatory and convolute zoning, and mafic minerals such as biotite and hornblende which have been altered to c h lo rite plus sphene, ^ i d o t e , and c a lc ite . Accessory minerals include allanite, which occurs in the same manner as that found in the Mink Creek rhyodacite porphyry.

Meadow Creek p o rp h yritic d a c ite . The Meadow Creek p o rp h yritic dacite is the most representative rock type of the northeast portion of the study area. It outcrops along Meadow Creek and along Springer

Creek Road. Outcrops are generally large, from seven to fifteen meters in width and five to seven meters in height. The Meadow Creek por­ phyritic dacite is commonly found as single dikes, rather than com­ posite dikes.

The Meadow Creek porphyritic dacite is different from the three previously mentioned dike types in that it contains very little quartz in the groundmass which consists almost entirely of K-feldspar. 27

Small (1-2 mm) rounded phenocrysts of quartz make up one to two per­ cent of the rock. It is commonly buff to medium-gray in color.

Phenocrysts other than quartz in the Meadow Creek porphyritic dacite include plagioclase and altered mafic minerals, originally biotite and hornblende. Plagioclase is strongly altered to sericite and clays.

Zoning has generally been obscured by this alteration and is rarely discernable. Fresh biotite or hornblende is rare in this dike type.

Accessory minerals consist of apatite and zircon, with most sphene occurring as a secondary alteration product.

One outcrop of the Meadow Creek porphyritic dacite was drilled and cored by the U.S. Forest Service in order to determine the extent of the dike at depth. It is approximately sixty feet in width; the depth of the dike is unknown. D r ill core showed the unexposed margin of the dike to have a one- to two-foot c h ille d margin. The Idaho batholith into which the dike was intruded was altered, with abundant epidote and chlorite present at the contact.

Martin Creek porphyritic rhyodacite. The Martin Creek porphyritic rhyodacite is very similar to the Meadow Creek porphyritic dacite and names for the two dikes were determined chemically. The Martin

Creek porphyritic rhyodacite contains a little more quartz. The Martin

Creek porphyritic rhyodacite is found along Martin Creek Road, which is the northeastern boundary of the study area. I t is re s tric te d to a single dike in outcrop and shows a slightly chilled margin. The dike is approximately ten to fifteen meters wide and 0 . 2 km long. 28

The Martin Creek porphyritic rhyodacite is medium gray in color, and does not appear as green as the majority of the East Fork dikes.

I t contains b io tite in varying stages of a lte ra tio n ; however, most biotite is fresh and unaltered. In contrast, hornblende is completely altered to a euhedral rim of chlorite and epidote and a core of calcite.

Both plagioclase and K-feldspar are altered, with plagioclase alteration more severe than K-feldspar. The fine-grained groundmass of the Martin

Creek porphyritic rhyodacite is predominantly K-feldspar and contains only a few percent quartz.

Meadow Creek andésite. The Meadow Creek andésite is the only nearly nonporphyritic dike which outcrops repeatedly in the East Fork area.

It contains only a few (up to two) percent phenocrysts, the bulk of which are quartz and chlorite. It is the darkest dike observed in the East Fork area, being nearly black. It generally occurs as small dikes, one to three meters in width. It is found either as a composite dike, or as single dikes cutting the Idaho batholith directly. It is one of the only dike types which contain xenocrysts of quartz which are sub-rounded in outline and consist of an aggregate of grains with

sutured boundaries. The Meadow Creek andésite is found most commonly

in the northeast half of the study area.

The Meadow Creek p o rp h y ritic andésite is dark green gray to greenish black. It consists primarily of plagioclase and strongly altered mafic minerals. K-feldspar was identified with sodium cobaltinitrite stain. Alteration of the Meadow Creek andésite is so Table 1. Characteristics of East Fork Dikes

Percent Percent Pheno- Ground- crysts mass Quartz K-feldspar Plaqioclase Mafics Alteration Accessories

Mink Creek 50% 50* approx. 28% 50% rhyodacite equal parts 23% in 35% in ground­ 30% 2% po ohyrv interlockinq groundmass mass 30% euhedral in both ground­ Plagioclase: allanite (MCRPl quari: and 5% rounded 5% euhedral phenocrysts, mass and rare moderate to K-spar, 1 - 2 and deeply megacrysts of white to cream phenocrysts. severe, twinning apatite chlorite af­ embayed sanidine in color. Biotite commonly obscured. K-spar. ter biotite phenocrysts Osci1latory altered to moderate in zircon (phenos). (oscil) zoning chlorite and phenocrysts, and parallel sphene. Epidote severe in synneusis with chlorite. groundmass. Mafics: common. severe

Mink Creek 30% 70% 7% 20% 53% 20% porphyritic 30% pi agio- 5% in ground- 19% ground­ 30% groundmass 15% euhedral Plagioclase: zircon andesi te class mass mass anhedral, in te r­ green-brown moderate to (MCPA) 20% K-spar 2% rounded 1% subhedral locking grains hornblende severe, 15% horn­ and deeply megacrysts 23% phenos; prisms in twinning ob­ blende and embayed of sanidine. euhedral, white groundmass. scure. K-felds- b iotite phenos. to cream crystals, 5% sub-euhedral par: moderate 5% quartz oscil. Zoning biotite and in pheno­ quartz and and parallel hornblende crysts, severe feldspars synneusis common. phenos altered in groundmass. with inter­ to chlorite Mafics: biotite:

locking severe. Horn­ ro grains. blende; weak. Table 1. (Continued)

Percent Percent Pheno­ Ground­ crysts mass Quartz K-feldspar Plagioclase Mafics Alteration Accessories

East Fork 50% 40% approx. 30% 30% 30% 10% rhyodacite equal parts 22': in 30% In 30% phenocrysts 8% b io tite Plagioclase allanite porp^- vry interlocki nc rroundmass groundmass as white to phenos altered weak, zoning (EFRP) quartz and 7' rounded. with quartz. cream colored to chlorite present. apatite K-spar, up Gceply em­ xls. Oscil. and ru tile K-feldspar- to 5% chlor­ bayed zoning and and/or sphene. severe. zircon ite after phenos. parallel Tr. epidote Mafics; bioti te. synneusi s 2% horn­ hornblende common. blende a l­ moderately tered to severe. chlorite. Biotite - severe.

Meadow Creek 35% 65% 1-2% 63% 30% 5% porphyri tic almost all i ndividual in groundmass 30% phenos as 5% bio tite Plagioclase: apatite dacite K-spar, up grains in anhedral in­ cream-colored, altered to severe; zoning (MEPD) to 2% groundmass terlocki ng subhedral chlorite and twinning zircon quartz. grains grains. and sphene. obscured. Biotite: severe. K-feldspar; severe.

w o Table 1. (Continued)

Percent Percent Pheno­ Ground­ crysts mass Quartz K-feldspar Plagioclase Mafics Alteration Accessories

Martin Creek 40% 60% 7% 55% 23% 15% porphyritic almost all ruunded, in groundmass 23% phenos as 10% biotite, Plaqi oclase: apatite rhyodacite K-spar in embayed as anhedral cream colored, mostly fresh severe. K- { m ? } lock!ng oheno- interlocking subhedral xls. and dark brown . feldspar: zircon grains. crysts and grains. Parallel syn­ 5% hornblende, weak to anhedral neusis common. completely a l­ moderate. grains in tered re­ Biotite: ground­ placed by weak. mass . CaCOj, Hornblende: chlorite and severe epidote.

Meadow Creek 0-2% 98-100% 2% 15% 58% 25% andésite approx. 25% rounded anhedral, subhedral laths, 20% biotite Plagioclase: apatite (MEA) chlorite phenos. severely a l­ intergrown with completely moderate. after and magne­ tered grains chlorite and altered to Mafics: chlorite tite . in groundmass. K-spar. chlorite. severe, to approx. 73% 5% magnetite. chlorite plus sphene plagioclase fine-grained after and K-feld­ CaCOg. chlorite spar 32 severe that the primary mafic mineral was unidentifiable and exists now as chlorite and epidote. One thin-section contained spherulitic structures composed entirely of chlorite.

Potassium Feldspar Megacrysts

K-feldspar megacrysts are a phenomenon which has intrigued petrologists for many years. They are found in both metamorphic and igneous rocks and are generally restricted to the more fe ls ic varieties of these rocks. Metamorphic K-feldspar megacrysts are commonly found in gneisses. Igneous rocks bearing megacrysts vary considerably in texture. Medium-grained granites (cla ssifica tio n of Streckeisen, 1967 in Hyndman, 1972) are the most common host rocks. However, in the

East Fork area, megacrysts occur in hypabyssal porphyritic dikes.

There are two types of dikes in the East Fork swarm which contain megacrysts: these are represented by the Mink Creek rhyodacite porphyry and the Mink Creek porphyritic andésite. They are commonly found together in composite dikes. Both dikes contain rounded, embayed quartz, plagioclase phenocrysts which have been sericitized to varying degrees, and chloritized biotite in a groundmass of quartz and altered

K-feldspar in the Mink Creek rhyodacite porphyry, and hornblende, quartz and altered feldspar in the Mink Creek porphyritic andésite.

The Mink Creek rhyodacite porphyry is light green-gray in color and strongly porphyritic. The groundmass consists of approximately equal parts of quartz and K-feldspar and makes up about 50 percent of 33 the rock. Plagioclase phenocrysts are clumped together in parallel synneusis relation. Where not severely altered, they may show faint oscillatory zoning. Biotite has been pervasively altered to chlorite, sphene and minor epidote. Phenocrysts of quartz are rounded and embayed

K-feldspar megacrysts are euhedral in outline, buff to pinkish-gray in color, one to six centimeters in length, and highly poikilitic with in­ clusions of chlorite, biotite, quartz and plagioclase. They occur as single crystals, some with Carlsbad twins.

The Mink Creek prophyritic andésite is dark green-gray in color and strongly porphyritic. It is distinguished from the Mink Creek rhyo­ dacite porphyry on the basis of color, amount of quartz, hornblende and K-feldspar megacrysts. Quartz and K-feldspar megacrysts in the

Mink Creek porphyritic andésite are less abundant, whereas hornblende is more abundant. Megacrysts are generally subhedral, buff to pinkish gray, and smaller (up to three centimeters in length) than in the Mink

Creek rhyodacite porphyry. Carlsbad twins are less common. A slig h t preferred orientation of megacrysts was noted in two dikes.

K-feldspar in the megacrysts was id e n tifie d as sanidine using x-ray diffraction techniques. The distinction between sanidine and orthoclase was based on the 131 peak, which was expected to show some separation if orthoclase were present. Qualitative x-ray diffraction indicates approximately 80 percent Or molecule (Wehrenberg, verbal communication, 1977). Optical properties of the K-feldspar (2V =

15-20°, negative optic sign and negative relief) confirm the x-ray 34 id e n tific a tio n . Nearly a ll megacrysts are euhedral and 130 faces are well developed. As a general rule, euhedral crystal outlines are thought to result from early crystallization (Carmichael, et al, 1974).

The euhedral outlines and 130 faces of the megacrysts indicate that they began crystallization early and proceeded to crystallize over a prolonged period of time (Wehrenberg, verbal communication, 1977).

Subhedral megacrysts are less prevalent and are generally restricted to the Mink Creek prophyritic andésite. Three explanations exist for th e ir less perfect outline. F irs t, changing pressure w ithin the magma as it crystallized could result in partial resorption or corrosion of the megacrysts by the melt (Whitney, 1975, p. 28). Partial resorption of quartz is widespread in the East Fork dikes, however, a d irect correlation between quartz and K-feldspar cannot be made. Second, both biotite and plagioclase show extensive deuteric alteration and the subrounded outlines of the megacrysts could result from alteration.

However, this process appears to be unlikely, as widespread evidence of deuteric alteration is present in the Mink Creek rhyodacite porphyry and its megacrysts are euhedral. F inally, since the size of perfection of the crystals may be in part dependent on time, the subhedral megacrysts could result from shorter periods of cooling and crystallization.

The megacrysts are always p o ik ilitic and contain inclusions of biotite, sphene, plagioclase, quartz, apatite, allanite, zircon, epidote and chlorite. Epidote and chlorite are present as alteration products of biotite and hornblende. Biotite and/or chlorite are aligned in 35

concentric zones parallel to [0 1 0 ], [ 1 1 0 ], [ 0 0 1 ], and [ 2 0 1 ], giving the megacrysts a zoned appearance. The megacrysts are also zoned in color; the cores of the crystals are gray and the margins are pink to buff. This apparent zoning is thought to be a product of deuteric alteration or weathering, the pinkish color resulting from the oxidation of iron.

K-feldspar megacrysts are thought to be formed in two ways: 1) c ry s ta lliz a tio n from a magma, or 2 ) growth by potassium metasomatism.

Workers supporting either method have cited the same evidence, primarily the alignment of both plagioclase and mafic minerals in zones parallel to the crystal outlines. Vance (1969, p. 21) has suggested that some metamorphic megacrysts may be r e lic t phenocrysts from a porphyritic granite. Metasomatic megacrysts are commonly found in gneisses and other rocks which have undergone regional metamorphism and deformation.

Magmatic megacrysts are generally found in g ranitic rocks which show no evidence of recrystallization or deformation. Determination of the origin of K-feldspar megacrysts should take into consideration the nature of the host rocks as well as chemical, structural, and crystallo- graphic criteria.

The East Fork megacrysts show the zoning of b io tite and plagioclase that is characteristic of many K-feldspar megacrysts. Consequently, other criteria must be used to help determine their origin. Two examples of sanidine in synneusis relation were found at lo c a lity EF-2b.

The crystals are not twinned according to any rational twin law for 36

K-feldspar. The sample consists of two Carlsbad twins; twin B growing with its [001] face parallel to the [010] face of twin A. Their juxtaposition is believed to have occurred when the two crystals came into contact during c ry s ta lliz a tio n . Vance [1969, p. 7 ] states that the presence of synneusis structures provides evidence for unconfined motion w ithin a liquid phase and "therefore unequivocal textural evidence for magmatic origin".

Thus, the East Fork megacrysts appear to have crystallized d ire c tly from a magma. Both optical and x-ray d iffra c tio n evidence indicates that the K-feldspar crystallized as sanidine which is also indicative of a volcanic origin fo r the megacrysts. Euhedral megacrysts tend to form when crys ta lliz a tio n is prolonged, whereas subhedral megacrysts probably result from shorter periods of cry s ta lliz a tio n . The megacrysts are prominent and dikes containing them are easily recognized and thus characteri zed.

A1teration

Deuteric alteration is classica lly defined [AGI Glossary of Geology,

197 2, p. 191; Sargent, 1918, p. 19] as the alteration of an igneous body by its own late stage hydrous fluids. It seems quite likely that in some igneous bodies, especially small ones such as dikes, that not all of the altering fluids are magmatic, but rather a mixture of magmatic flu id s and circulating groundwater [D. Hyndman, verbal communi­ cation, 1977]. Rocks which have undergone alteration as they cooled 37 commonly show one or more of the following characteristics in thin section: 1 ) mafic minerals such as b io tite , hornblende or pyroxene have been altered to chlorite or less frequently, actinolite,

2) plagioclase has been altered to sericite, 3) rims of K-feldspar grains or veinlets have been altered to a lb ite (Hyndman, 1972, p. 84).

Deuteric alteration can be distinguished from surface weathering by the presence or ch lo rite and se ric ite rather than iron oxides and clay minerals such as smectite (Thompson, verbal communication, 1977).

C hlorite in the East Fork area is the most common alteration product of b io tite and hornblende which can be seen in varying stages of alteration. It characteristically occurs as a pseudormorph after biotite, forming light green, pseudohexagonal plates. The outline of hornblende is rarely preserved and ch lo rite occurs as a mass of un­ oriented crystals. The former generally occurs as phenocrysts, but i t may also make up part of the groundmass. Rutile and sphene are fre ­ quently found growing along the cleavage (perpendicular to the C axis) of chlorite (Fig. 9). Rutile was identified by its acicular habit and its local occurrence with sphene. Sphene was recognized by its high positive relief and rare wedge-shaped crystals. Epidote was found with chlorite and sphene. It showed high positive relief and pale yellow pleochroism. Chlorite in the East Fork dikes varies in color from pale blue-green to pale green and is commonly strongly pleochroic with x' - light green and z' = dark green. The composition of chlorite 38

Figure 9. Biotite phenocryst completely altered to chlorite Dark needle-like mineral is r u tile ; note wedge- shaped sphene crystal in upper le ft corner and scattered clear grains of apatite. Plane light, grain diameter approximately 1 mm.

is generally related to the composition of the original mafic mineral and stronger pleochroism is due to a higher iron content (Deer, Howie and Zussman, 1966, p. 237-9). Under crossed nichols, the East Fork chlorite exhibits the anomalous deep blue interference colors charac­ te ris tic of c h lo rite . The presence of ch lo rite , both as phenocrysts and disseminated through the groundmass, accounts for the gray-green color of many East Fork dikes. 39

Sericite can best be defined as a fine-grained, clear, white

mica which has a composition equivalent to that of muscovite

I^*^2A l4 (SigAl2 0 2 0 )( 0 H,F)^]. However, the composition of sericite may be

more complex and variable, where the altered rock does not contain or

has not had a large amount of potassium introduced. In a rock containing

normal amounts of sodium and potassium, such as the East Fork or Mink

Creek rhyodacite, the mineral which appears to be fine-grained sericite

may, in fact, include other white micas such as paragonite (Na-rich white mica) or margarite (Ca-rich white mica). The term sericite is

used in descriptions of the East Fork rocks with the understanding that

the white micas observed in East Fork alteration may be compositionally more complex than the term s e ric ite implies.

Plagioclase occurs both as phenocrysts and a component of the groundmass. Most of the plagioclase has been altered to variable amounts

of sericite and clays. Commonly, alteration is so strong that twinning and zoning in the plagioclase have been obscured. Sericite occurs as very small, feathery grains which show first-order interference colors.

In severely altered dikes, it is coarse-grained and shows bird's-eye

extinction more characteristic of muscovite. Sericitization may occur as an overprint, leaving recognizable features such as twinning and

zoning; or i t may be severe around the edge of the c ry s ta l, obscuring

twinning and zoning, leaving an unaltered core in which these features are recognizable. 40

K-feldspar occurring in the groundmass as well as in megacrysts also has been altered to sericite and clays. It was recognized in the groundmass with the aid of a sodium-cobaltinitrite stain, since al­ teration of the groundmass was so severe that id e n tifica tio n of K- feldspar using optical properties was very difficult. K-feldspar in the groundmass was often observed with a cross-hatched pattern of a l­ teration, the origin of which is unknown. A lteration of the megacrysts is evident in handspecimen from the change of color from the edge (buff) of the crystal to the center (gray). The inner boundary of this zone is diffuse and rounded and varies in thickness from one crystal to another. The altered zone is composed of severely-altered feldspar

(mostly clay and s e ricite ) which surrounds small patches of unaltered

K-feldspar.

Thin sections of the East Fork dikes reveal that they have been deuterically altered and show a slight overprint of surface weathering.

The alteration of biotite and hornblende to chlorite and minor epidote is pervasive. Unaltered biotite is uncommon. Plagioclase and

K-feldspar have been altered to sericite and clays. The degree of seritization is variable and tends to be stronger in dikes without quartz phenocrysts. CHAPTER IV

WHOLE ROCK CHEMISTRY

Twelve whole rock samples were analyzed fo r eight major elements using x-ray fluorescence and atomic absorption techniques in order to determine chemical composition. Quantitative analyses for Si0 2 ,

AI2O3 , Fe^Og, CaO, MgO, K^O and Ti 02 were made using x-ray fluorescence techniques described by Benoit (1972, appendix I I ) . NagO was de­ termined by Skyline Labs, Inc. which used atomic absorption procedures.

Two factors were included in the selection of a dike for analysis:

1 ) frequency and size of outcrops, and 2 ) position within the dike swarm, so that chemical trends, i f present, would be recognizable.

Dikes such as the Meadow Creek andésite were included because they represent a very different type of dike, though only a few outcrops were located.

Raw x-ray fluorescence data were reduced with the aid of a calculator program as the University of Montana computer program was not operable in January, 1977. Percentages of unknown oxides were determined with Monroe programmable calculator and a program which calculated three values: 1 ) m, the slope of a line; 2 ) b, the y-intercept of this line, and 3) r, the correlation coefficient. Data was fed into the program with "x" values equal to the average counts per ten seconds of a standard fo r a given oxide and "y" values equal

41 42

to the known concentration for that standard. Unknown concentrations

were generated from the following formula:

y = m X + b

Four U.S. Geological Survey Standards were counted before and after

each run of four unknowns in order to monitor machine d r if t and to

provide more data for the regression line. A ll elements with the

exception of magnesium were counted for 5,000 counts or more.

Table 2 shows the results of the XRF analysis and those of

Skyline Labs combined. Totals of all rocks were then plotted on the

cla ssifica tio n diagram designed by Church. Because of an apparent drop in SiOg of four to six weight percent, four samples were sent to

Skyline Labs for analysis. Results of the Skyline analyses and the original XRF data are shown in Appendix I. No values for precision in

either type of analysis were determined.

Rock names for the East Fork dikes were determined using a cla ssi­

fic a tio n developed by Church (1975, Fig. 10). The cla ssifica tio n is

based on an orthogonal plot of NagO+KgO versus FeO + Fe^O^ +

(CaO + MgO) versus A l2 0 3 /S i0 2 , in weight percent. It is easy to use

because only addition and division of the major oxides are required to

plot the data. Compounding of analytical error through data manipu­

la tio n is also at a minimum. The cla ssifica tio n was developed by

p lo ttin g 1,422 analyses of volcanic rocks (165 rhyolites, 112 dacites. Table Whole Roc|^ Analj;^es for _

1 1 1 1.2 USGS Standards^ -12b Ml- 16c MC-7 MC-12 MC-19 EF-3c2 EF- 3c4 _S^- 5 _ EF-2d 12- 3 G-2 GSP.J AGV-1 BCR-f

J.16 72. 16 69.18 60.43 66.91 65.58 66. 54 69. 1 69, 3 6A4 66. 4 69. 11 67.38 59.00 54.50 S.07 15. 71 15.62 15.37 15.86 15.86 16. 11 15. 13 15. 46 15.00 14. 18 15. 40 15.25 15.75 13.61 ■ .70 3. 13 3.94 6.19 3.95 3.89 3. 90 3. 38 3. 14 4.06 8. 82 2. 65 4.33 6.76 13.40 3.01 0 98 1,17 4.14 1.26 1.78 1. 61 1. 16 1. 94 0.46 1. 52 0. 76 0.96 1.53 3.46 AO.qO :.02 1. 10 2.01 4.89 2.07 2.86 2. 67 1. 69 2. 15 3.22 2. 89 1. 94 2.02 4.90 6.92 C a O 3.40 3 74 3.93 2.36 3.73 3.08 3 07 3. 37 2 89 3.26 3. 60 4. 51 5.53 2.89 1.70 MfnO 70 4. 2 J.60 4 30 4.60 3.40 4.00 4.20 4 40 4. 90 4 20 4.00 3. 50 4. 07 2.80 4.26 3.27 ISz 0 '2 O . 38 0.53 0 32 0.56 0.62 0.45 0.40 0 39 0 38 0 39 0.45 0. 45 0. 50 0.66 1.04 2.20

al 102 36 '100728" Toi ."44 ■■ l o i i o r"9 1 7 0 0 98.I T 97.65 98.69 99 11 99.47 97.85 101. 3~6“ 98 94 98.93 96 T f " T9.“06"'

“1 10 30 5 10 35 5 50 10 40 5 11. 7 33.3 59.7 18.4

2 2 2 2 2 2 2 2 2 2 0. 36 0.90 2.3 1.1 n 20 50 25 30 15 50 65 15 115 15 31. 2 51.3 35. * 17.6 p ST, 60 75 60 70 80 75 75 55 80 50 85. 0 98.0 84.0 12.0 n

1 Analysis redone by Skyline Labs, Inc., new values for all oxides except Fe203 are plotted. ? Saiiple contaminated by Fe when ground to pulp in disc-grinder. 3 Values shown after Flanagan, 1973. te: A. All Na20 values determined by Skyline Labs, Inc. B. No lim its of precision were determined for XRF analyses; none were supplied for work done by Skyline Labs, Inc. 44

364 andésites, 155 trachytes, 444 basalts). One hundred eighty-five analyses were eliminated from the contouring procedure on the basis cf the ir jrrjS'jcl ard obscure na^es. The array of points was contoured, and the category was defined by two-thirds of the total points for that rock name (Church, 1975, p. 260). The East Fork dikes and the

U.S.G.S. Standards used during analysis are shown in Figure 10. Only two samples, the Meadow Creek andésite and the Mink Creek porphyritic andésite fa ll outside the dacite boundary. The majority of the analyzed rocks are dacites. Four samples, as well as the U.S.G.S. standard

G-2, fall in the rhyodacite field, as defined by the overlap between the rhyolite and dacite fields.

Although only three major chemical rock types were defined using

Church's (1975) c la ssifica tio n , the mineralogies of these rhyodacites, dacites and andésites show some internal variation. The rhyodacites are the least variable rock type. They generally have phenocrysts of quartz, plagioclase, and biotite (or chlorite) plus or minus

K-feldspar megacrysts in a matrix of quartz and K-feldspar. The dacites contain phenocrysts of plagioclase and biotite (plus chlorite) with or without quartz in a groundmass of both quartz and K-feldspar or

K-feldspar alone. The two andésites show the most variation in mineralogy. The Meadow Creek andésite contains a few small phenocrysts of quartz and no phenocrysts of plagioclase in a groundmass of plagioclase and altered mafic minerals whereas the Mink Creek porphyritic NaP f t \ 0

^o ■

TVachyte PKonolt+e East Fork Oike. V)&S Sturt4ard

IZ

.000 • ISO io o ISO .350 .400

A1

.pi* Figure 10. Triaxial plot showing fields of variation of most common volcanic un rocks; contours are inclusive of two-thirds of total points counted for each rock type. (After Church, 1975). Rock names for the East Fork dikes were chosen using this classification. 46 andésite contains phenocrysts of quartz, plagioclase, hornblende, and

K-feldspar in a groundmass of the same minerals. Differences in chemical composition as well as changes in temperature and pressure are reflected by variation in mineralogy and texture.

Plotted on SiOg variation diagrams (Marker diagrams), analyses of the East Fork dikes vary about a positively or negatively sloping stra igh t line which was visually determined for a ll elements (Figure 11).

Rocks which show straight line variation have been interpreted as co-magmatic; that is , they are derived from the same source. The

Meadow Creek andésite isthe most mafic of the East Fork dikes, with

60.4 percent SiOg and 6.96 percent Fe^Og, There is an absolute difference of four percent SiÛ2 , between the Meadow Creek andésite and the next most silicic dike on the variation diagram, a break twice as large as any other between the East Fork dikes.This break may or may not be statistically significant. Thin sections reveal a nearly equi- granular texture, which is unusual for rocks in the area. The Meadow

Creek andésite may in fact, represent a slightly different type of dike or perhaps a later stage of intrusion.

Hyndman (1972, p. 8 8 ) has pointed out several hazards associated with the use of Marker diagrams. Two of these apply d ire ctly to the

East Fork diagrams. F irs t, enough analyses must be used in order to determine a s ta tis tic a lly accurate curve. Admittedly, eleven analyses may not determine a curve typical of the East Fork rocks. Therefore, 47

17 16 oi5 o

13 7

i f 5 »c 4 3

% 4 o 3 0) ■o 2 X o 1 r> +j s; ^ cn 2 OJ K*

5 - o cCVJ 4 03

o X *

2 ' C\J o K K K X X * *

60 61 62 63 64 65 66 67 68 69 70 71 72

Weight percent Si02

Figure 11. SiOp Variation Diagrams for East Fork Dikes 48

any conclusions made from these diagrams must be made with this in mind. Second, Bowen (1928, iji Hyndman, 1972) states that the d if ­ ferentiation curve fo r glassy or fine-grained rocks may be va lid , but that porphyritic or coarse-grained rocks may have formed in part by crystal accumulation. To what extent, if any, the porphyritic nature of the East Fork dikes has been determined by crystal accumu­ la tio n , is unknown. Evidence of crystal settling was observed in only one location for one type of dike. The role of crystal settling in chemistry and formation of the remainder of the East Fork dikes remains uncertain.

The East Fork Dikes, with the exception of the Meadow Creek andésite, are a group of chemically consistent porphyritic dikes.

The average composition is 67 percent SiOg, a rhyodacite. Ten samples were analyzed fo r base metals (copper, molybdenum, lead, and zinc) and results, shown in Table 2, indicate that the dikes contain average concentrations of base metals. No unusual chemical signature appears to exist fo r the East Fork dikes. They do not contain abnormal amounts of any other major oxides or base metals. CHAPTER V

CONCLUSIONS

The East Fork dikes were found to be a group of chemically similar, but texturally diverse, rocks. The compositions of the

East Fork dikes vary from andésite to rhyodacite, with a mean com­ position of rhyodacite and a mode consisting of dacite. When plotted on Marker variation diagrams, most oxides vary about a visually estimated straight line, indicating that they represent the same period of intrusion. I t is probable that the gap between the Meadow Creek andésite (60.45 percent SiO^) and the rest of the analyses indicates a time interval between periods of intrusion. The Meadow Creek andésite, which is thought to be later, is the only non-porphyritic dike type in the East Fork area, and appears to cut the e a rlie r, more fe ls ic porphyritic dikes. The first phase to begin crystallization in most dikes is either plagioclase or alkali feldspar (sanidine). Some of the dikes plot near the thermal trough dividing the crysta lliza tio n of quartz from feldspar, indicating that quartz in most cases was pre­ cipitated with feldspar.

The mineralogy of the dikes is also sim ilar; the major components of the dikes are quartz, plagioclase, biotite, K-feldspar, and in some cases hornblende. The mafic minerals have been pervasively altered to chlorite plus rutile, sphene, epidote, or calcite. The

49 50

a lte ra tio n minerals are commonly found as pseudormorphs a fte r b io tite

and hornblende. Both plagioclase and K-feldspar show alteration to

sericite and clays, though the degree of alteration is strongly

variable. K-feldspar is found prim arily in the groundmass, though

two types of dike, the Mink Creek rhyodacite porphyry and the Mink

Creek porphyritic andésite contain large (up to six centimeters in

length) euhedral megacrysts of sanidine.

Nearly all of the East Fork dikes are porphyrytic, with pheno­

crysts of quartz, plagioclase, K-feldspar, biotite, and/or hornblende.

The amount of phenocrysts may vary within a dike type, though typically

this variation is only a few percent. More commonly, the amount of

phenocrysts varies from dike type to dike type, from 30 to 60 percent.

The East Fork dikes commonly share three textural features:

rounded and embayed quartz, synneusis structures, and oscilla to ry

zoning in plagioclase. Rounded, embayed quartz is currently thought

to form as a result of a drop in pressure, which renders quartz an unstable phase, and resorption takes place. Quartz phenocrysts in the

East Fork dikes are commonly rounded and less frequently deeply embayed,

Synneusis structures form when two or more well-developed crystals float together in a melt and are not broken apart by later turbulence.

They are best observed in plagioclase, where oscillatory or normal zoning is disrupted and the individual crystals may be readily dis­ tinguished. Synneusis structures are abundant in the plagioclase found 51 in the East Fork dikes. Synneusis structures may also be observed in groups of rounded quartz crystals and rarely, between two mega­ crysts of sanidine. O scillatory zoning is also abundant in the East

Fork dikes. It is currently thought to result from diffusion and supersaturation of anorthite adjacent to a growing crystal, though other hypotheses include the precipitation of a zone of anorthite as a result of the release of volatiles.

The East Fork swarm is thought to represent the northeasternmost expression of the West Fork volcanic center. Dikes are more abundant in the southwestern portion of the study area and may possibly denote the northeast lim it of volcanic activity in the West Fork area. The

East Fork area may also represent a separate volcanic center, though with lim ited exposure and outcrop of volcanic rocks in the area this seems unlikely. REFERENCES

AGI Glossary of Geology, 1972. American Geological In s titu te , Washington, D.C., 805 p.

Anderson, A.L., 1947. Geology and ore deposits of the Boise Basin, Idaho. U.S.G.S. Bulletin 944-C, 319 p.

Badgely, P.C., 1965. Structural and tectonic principles. Harper & Row, New York, 495 p.

Benoit, W.R., 1972. Vertical zoning and differentiation in granitic rocks — central F lin t Creek Range, Montana. Unpub. M.S. thesis. University of Montana, 53 p.

Berg, R.B., 1973. Geology of southernmost Ravalli County, Montana, Northwest Geology, vol. 2, p. 1-5.

Blackerby, B.A., 1968. Convolute zoning of plagioclase phenocrysts in Miocene volcanics from the Western Santa Monica Mtns., C a lif., American Mineralogist, vol. 53, p. 954-962.

Carmichael, I.S .E ., Turner, F.J., and Verhoogen, 0 ., 1974. Igneous Petrology, McGraw-Hill Book Co., 739 p.

Church, B.N., 1975. Quantitative cla ssifica tio n and chemical comparison of common volcanic rocks, G.S.A. Bull., v. 8 6 , p. 257-263.

Deer, W.A., Howie, R.A., and Zussman, J ., 1966. An introduction to the Rock-Forming Minerals, Longman Group Ltd., London, p. 237-9.

Flanagan, F.J., 1972. Values for international geochemical reference samples, Geochimica & Cosmochimica Acta, vol. 37, p. 1189-1200.

Flood, R.E., 1974. Structural geology of the upper Fistrap Creek Area, Central Anaconda Range, Montana: Unpubl. Master's thesis. University of Montana, 71 p.

Hyndman, D.W., 1972. Petrology of Igneous & Metamorphic Rocks, McGraw-Hill Book Co., 533 p.

, Badley, R., and Rebal, D., 1977. Northeast-trending early dike swarm in central Idaho and western Montana, Geol. Soc. Amer. Abstracts with programs, vol. 9, no. 6 , p. 734.

52 53

Larsen, E.S., 1940. The pétrographie province of Central Montana, Geol. Soc. Amer. B u ll., v. 51, pp. 887-948.

Olson, H.J., 1968. The geology and tectonics of the Idaho porphyry belt from the Boise Basin to the Casto Quadrangle: Ph.D. dissertation, Univ. of Arizona, 154 p.

Reid, R.R. , 1963. Reconnaissance geology of the Sawtooth Range, Idaho Bur. of Mines & Geol., Pamphlet 129, 37 p.

Ross, C.P., 1934. Geology and ore deposits of the Casto Quadrangle, U.S. Geol. Survey Bull. 854, 135 p.

Sargent, H.C., 1918. iü Tyrel1, G.W., 1929, The Principles of Petrology, E. P. Dutton & Co., Inc., N.Y., 349 p.

Sildey, D.F., Vogel, T.A., Walker, B.M., and Byerly, G. 1976. The Origin of o scilla to ry zoning in plagioclase: A diffusion and growth controlled model. Amer. Jour. Sci., vol. 276, p. 275- 284.

Vance, J.A., 1962. Zoning in igneous plagioclase: Normal and oscillatory zoning. Amer. Jour. Sci., vol. 260, p. 746-760.

. , 1969. On Synneusis, Contributions to Mineralogy and Petrology, vol. 24, p. 7-29.

Whitney, J.A., 1975. The effects of pressure, temperature, and q on phase assemblage in four synthetic rock compositions, ^ The Journal of Geology, vol. 83, no. 1, p. 1-31.

Wiswall, C. G il, 1976. Structural Styles of the Southern boundary of the Sapphire Tectonic Block, Anaconda-Pintlar Wilderness Area, Montana: Unpubl. M.A. Thesis, Univ. of Montana, 62 p. APPENDIX I

Table 3. Conparlson of XRF data with outside analyses

GROUP 1 GROUP 2 GROUP 3 STANDARDS

2a Nil-12b Ml-16c MC-7 MC-12 MC--19 EF-3c2 EF-■3c2j MA-■1 SC-•5 EF-■2d 12- -3 1 G--2 GSP-1 AGV-1 BCR-1

SiOg 62. 59 64.26 71.39 68.35 60 41 67 05 65 68 66 67 62 81 63 96 61 52 59 34 j 69 11 67.38 59.00 54.50 A12Ü3 16 03 15.13 15.80 15.70 15. 21 15 74 15 74 16 01 15 17 15 53 15 03 14 14 15 40 15.25 15.75 13.61 Pe203 à 79 6.57 3.40 4.77 : 6 96 4 68 4 63 4 64 1 4 14 3 89 4 83 9 70 2 65 4.33 6.76 13.40 XgO 1 39 3.09 1.04 1.23 j 4 20 1 32 1 84 1 67 1 1 59 0 51 2 01 1 22 0 76 0.96 1.53 3.46 CaO 2 21 3.91 1.07 1.95 4 82 2 04 2 81 2 63 1 67 2 12 3 17 2 86 1 94 2.02 4.90 6.92 F'20 4 07 3.54 3.88 4.07 1 2 53 3 88 3 24 3 23! 3 51 3 04 3 40 3 74 4 51 5.53 2.89 1.70 TiOg 0 38 0.54 0.32 0.48 j 0 62 0 45 0 40 0 39 I 0 37 0 39 0 44 0 45 0 50 0.66 1.04 2.20

Total 98.46 97.04 96.90 96.55 94 .75 95.16 94.34 95 24 89 26 89 44 90 4 91 45 94. 87 96.13 91.87 95.79

XRF data without corrections from outside laboratory. Samples are grouped as they were run. Note the drop of 4 to 6 percent SiO^ in Group 3, Sample 12-3 was contaminated during grinding. Ml-12a MA-1 SC-5 EF-2d 12-3

SiÛ2 68.4 69.1 69.3 67.4 66.4

AlgOj 15.3 14.9 15.7 15.3 13.9 tn Fe203 3.4 3.1 3.1 3.9 8.1 MgO 1.3 1.5 0.46 1.9 1.2 CaO 2.3 1.7 2.2 3.2 2.9 KgO 4.2 3.7 3.4 3.6 3.8 Ti02 0.40 0.37 0.38 0.42 0.44 NagO 4.2 4.9 4.2 4.0 3.5 Note: No analyses to determine Whole rockanalyses from Skyline Labs, Inc. precision were made.