Magma Immiscibility in the Shonkin Sag Laccolith Highwood Mountains Montana

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Magma immiscibility in the Shonkin Sag laccolith Highwood Mountains Montana

Carolyn Lorraine Edmond The University of Montana

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Mansfield Library University of Montana Date: QFC 1 7 1980

MAGMA IMMISCIBILITY

IN THE SHONKIN SAG LACCOLITH,

HIGHWOOD MOUNTAINS, MONTANA

Carolyn Lorraine Edmond

B.A., Colorado College, 1976

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

Master of Science

UNIVERSITY OF MONTANA

1980

Approved by:

Chairman, Board of Examiners

Dear^ Graduate Sch

- ! lo~ )jo Date UMI Number: EP37974

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ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 ABSTRACT

Edmond, Carolyn L o rra in e , M .S., S p rin g , 1980 Geology

Magma Im m iscibility in the Shonkin Sag Laccolith, Highwood Moun tains, Montana

Director: Dr. David Alt

Although the Shonkin Sag laccolith has long been considered a classic example of differentiation in place through crystal settling, recent field work, pétrographie studies, and chemical analyses provide evidence for the gravitational separation of tv;o immiscible magmas with different densities. The laccolith, con sisting generally of augite and potassium feldspar, contains a layer of syenite resting directly upon a darker and heavier shonkinite. The fla t horizontal contact suggests a gravity-controlled mechanism. However, evidence for crystal settling such as cumu late textures, rhythmic or cryptic layering, and size sorting of grains is absent. Unusual syenite segregations that are rounded or spherical and seem to coalesce in some places suggest immis cible separation as do chemical trends incompatible with a model based solely upon fractional crystallization. The "incompatible" elements, which tend to concentrate in felsic rocks, are either not enriched in either rock (U, Th) or are higher in the shonkinite (Li, Be). These trends can be attributed to liquid immisci b ility , which also explains the sharp contacts, lack of inter mediate rock types, and the unusual rock textures. Globules of syenite are totally enclosed within shonkinite. These represent immiscible syenite magma that was trapped within shonkinite as the laccolith cooled. Earlier syenite globules were able to separate completely from the mafic magma. These coalesced and rose to the top of the magma chamber to form a thick layer of uniform syenite. Acknowledgments

This work would have been impossible without the help of several people.

I would like to thank Dave A lt, who suggested the idea and pro vided continual encouragement. Don Hyndman helped me gain the back ground knowledge necessary for this study, and helped edit the manu script. John Scott kindly reviewed the manuscript, and Ian Lange allowed me to use chemical data from samples he collected. Larry Shore spent many hours helping me retrieve the data. George Kendrick pro vided an excellent sounding board during hours of discussion, specula tion, and frustration. I would like to express my appreciation to all these people, and to the fine folks in Geraldine, Montana who made my stay there extremely pleasant. Special thanks to John, Mary, and

W illia m Tanner, and to Chuck and Lois Tonne f o r t h e ir e x tra o rd in a ry hospitality. Last, but most important, I would like to thank my parents for their unfailing moral (and sometimes financial) support.

I l l TABLE OF CONTENTS

Page

ABSTRACT...... ü

ACKNOWLEDGMENTS...... ü i

LIST OF FIGURES...... v i l

CHAPTER

I. INTRODUCTION...... 1

Regional Setting ...... 1

Description ...... 1

Other Laccoliths in the Highwood Area ...... 4

Statement of Problem ...... 5

I I . PREVIOUS WORK...... 8

I I I . FIELD RELATIONS...... 16

Chilled Zone ...... 16

Lower Shonkinite ...... 17

Pegm atite ...... 18

S ye n ite ...... 21

Upper Shonkinite ...... 22

IV. PETROGRAPHY...... 28

Chilled Zone ...... 28

S h o n k in ite ...... 30

Textural Variations Within the Lower Shonkinite . . . 34

Upper Shonkinite ...... 37

S y e n ite ...... 37

iv TABLE OF CONTENTS (C ontinued)

Page

Pegm atite ...... 41

Felsic Dike!et ...... 44

B iotite-rich Segregation ...... 45

Aegerine Syenite ...... 45

V. GEOCHEMISTRY...... 47

R e s u lts ...... 47

Discussion of Results ...... 48

V I. DIFFERENTIATION OF THE SHONKIN SAG LACCOLITH...... 56

Assimilation ...... 56

V olatile Movement ...... 56

Thermogravitational Diffusion ...... 57

Fractional Crystallization ...... 58

Flowage D ifferentiation ...... 58

Filter-press Action ...... 58

Crystal Settling ...... 59

Differentiation through Liquid Im m iscibility ...... 63

Evidence for Crystal Settling in the Shonkin Sag Lacco l i t h ...... 69

Evidence for Magma Im m iscibility in the Shonkin Sag L a c c o lith ...... 70

V II. PETROGENESIS OF THE LACCOLITH...... 73

V III. DISCUSSION...... 77

IX. SUMMARY AND CONCLUSIONS...... 82 TABLE OF CONTENTS (Continued)

Page

REFERENCES CITED...... 85

Appendix I. Modes of Representative Samples ...... 91

Appendix I I . Chemical Analyses ...... 94

VI LIST OF FIGURES

Figure Page

1. Location map - Shonkin Sag laccolith in relation to Alkalic province ...... 2

2. Cross-section through laccolith ...... 3

3. Index map - laccoliths of the Highwood area ...... 4

4. Rock densities in Shonkin Sag laccolith ...... 18

5. Layering in the Shonkin Sag laccolith ...... 19

6. Contact relations between pegmatite and syenite ...... 21

7. Pipe-like syenite segregation in lower Shonkinite. . . . 24

8. Coarse-grained pipe containing syenite segregation . . . 23

9. Syenite segregation in lower shonkinite ...... 24

10. Syenite segregation in lower shonkinite ...... 24

11. Thin sheet of syenite in lower shonkinite ...... 26

12. Angular chunk of syenite surrounded by pegmatite .... 27

13. Thin section of chilled zone ...... 29

14. Thin section of lower shonkinite ...... 32

15. Intergrowths of zeolites and sanidine ...... 33

16. Thin section of lower shonkinite ...... 35

17. Sample from "hybrid" zone ...... 26

18. "Spotted rock" adjacent to syenite segregation ...... 36

19. Thin section of "spotted rock" ...... 38

20. Cuspate biotite crystals, sample #30 ...... 38

21. Sample from syenite-upper shonkinite contact ...... 36

VI ^ LIST OF FIGURES (C ontinued)

F igure Page

22. Thin section of syenite ...... 40

23. Augite-sanidine intergrowth in pegmatite ...... 42

24. Magnetite-biotite intergrowth in pegmatite ...... 43

25. Thin section of the felsic dike!et within the pegmatite la y e r...... 45

26 - 37. Diagrams showing some major and trace elements plotted aginast height in the laccolith

26. Sodium ...... 50

27. Potassium ...... 50

28. Magnesium ...... 51

29* C a lc iu m ...... 51

30. Thorium ...... 52

31. Uranium ...... 52

32. Beryllium ...... 53

33. Lithium ...... 53

34. Lanthanum...... 54

35. Cesium...... 54

36. Europium ...... 55

37. Titanium ...... 55

v i n CHAPTER I

INTRODUCTION

Regional Setting

The Shonkin Sag laccolith is one of several differentiated alka line intrusions in the foothills of the Highwood Mountains of north- central Montana. The Highwoods are one of the classic locales for potassic rocks, and are part of the central Montana alkalic province originally defined by Pirsson (1905) and later described by Larsen

(1940). Other subprovinces include the Castle, Judith, L ittle Belt,

Bearpaw, and L ittle Rocky Mountains and the Sweetgrass H ills (Fig. 1).

These Tertiary igneous centers contain both intrusive and extrusive rocks w ith chem ical, m in e ra lo g ic a l, and te x tu ra l s im ila r it ie s . Rocks within the subprovinces vary in composition from calc-alkaline to alka line and from mafic to felsic, but in general, the province is charac terized by mafic alkaline rocks.

Description

The most distinguishing feature of the Shonkin Sag laccolith is its compositional layering (Fig. 2). A horizontal band of syenite lies between two layers of shonkinite which are enclosed within a dark, fine-grained chilled border, shonkinitic in composition. The shonki nite is composed of augite, sanidine and pseudoleucite, and differs from the syenite only by having a larger proportion of mafic minerals. Both 116 112 ' 108 JL UNlTtOi kTATiS

P. o o Sweetorass Hills

Bearpaw Mtns G (y Little Rocky Mtns Of#*# roll* Hinhwood Mtns JK#«W»n S«9 lo««l(||» Adel fit - Judith Judit Mtns

Mocassin Mtns Little Belt Mtns * H#l#m# § Castle Mtns

O Crazy Mtns # >wt#« 46*

* DiUtns#

f - • - ______MONTANA \ 1 YslSowstpfi* 1 WVOMtNO Norienai | 2 % Dark t / IDAHO

50 km

Figure 1. Location Map- Shonkin Sag laccolith in relation to Central Montana Alkalic Province, (modified from Larsen,1940) Eagle Sandstone — y q

Gradational contact'^ / n'’';:* /;v ,

Aegerine syenite intrudes the laccolith in thin s ills , generally between the syenite fyS no n k.%% and upper shonkinite layers

Irregular, wavy contact

c: p © c; rn a r * v s Extremely sharp, horizontal contact T'*' n i — { Mottled or "hybrid" zone/^Pj^ rn

= V c \ i i /*> . '/X' '

to C «— ♦I— A3 q; c •«— •f— s_g

This contact is actually grade- chl i j g H tional and is marked by an upward"^ Z ^ ' : - ' ^ - i'C ' '. \ \ A;. ; increase in grain size .. Eagle Sandstone

Figure 2, Cross-section through Shonkin Sag laccolith the syenite and shonkinite have a massive texture but are separated by a very coarse-grained unit, called "transition rock" by Hurl but (1939) and pegmatite by Barksdale (1937).

Other Laccoliths in the Highwood Area

Except for the pegmatite layer, the lithology of the other lacco liths (Fig. 3) in the area is virtually identical. Four have a layer of syenite between a thick layer of lower shonkinite and a thinner layer

r 'MCNTÀGUE 2 /LAO.OLITH ~ =

\LOST LAKE Î JtACCOUTH y CER.ALCINS V_

antelope,— ^ _ LAfCCLi \ Geratdire

ninrsèa.

'lA.l Sqü3re_S0lleï

C&WBOV CREEK LACCOLiTH.kt*: . Exposed areas laccohths ---- projected b o u n d a rie s POUND t iu r r e UVCCCHTrt i laccolit»'

M.i2S

P i i ; l e £ 3 . —It.d c x irittp o j ihe h ccciifhÿ o} iftc Uighvcood area

(from Hurl but, 1939) of upper shonkinite. Three others have syenite above shonkinite but have their upper contacts eroded (Hurlbut, 1939). Near the Bearpaw

Mountains, about 110 km northeast of the Shonkin Sag, the Boxelder laccolith is also made up of syenite overlying shonkinite (Pecora,

1941).

The Shonkin Sag laccolith is considered the key to differentiation in all these laccoliths (Osborne and Roberts, 1931). It retains its sedimentary cover in some places, but is well exposed where cut by the

Shonkin Sag, an abandoned glacial channel of the Missouri River. Its relatively complete preservation combined with easy access, has made it the subject of many studies in the past (Weed and Pirsson, 1895, 1901;

Osborne and Roberts, 1931; Larsen and others, 1935; Hurl but, 1935;

Barksdale, 1937; Hurl but, 1939; Barksdale, 1952; Nash and Wilkinson,

1970, 1971).

Statement of Problem

The Shonkin Sag was accepted as one of the prime examples of d if ferentiation in place through crystal settling - but only after two alternate hypotheses were rejected. The idea of separate injections of syenite and shonkinite magmas was abandoned because of overwhelming evidence in favor of differentiation in place. The idea of differentia tion in place through magma im m iscibility was suggested by early workers

(Weed and Pirsson, 1895; Daly, 1912), and then abruptly dropped as a working hypothesis simply because the idea went out of style, not be cause of adverse evidence found in the rocks. Magma im m iscibility was proposed early in the development of ig neous petrology to explain juxtaposition of contrasting rocks without intermediate types. The process was widely applied until early experi mental work by G reig (1927) showed th a t although some systems d id show fields of im m iscibility, they were only stable at extremely high tem peratures (nearly 1700^C). Bowen (1928) set forth criteria for the recognition of rocks separated by immiscible magmas and stressed that no evidence of im m iscibility had ever been cited in naturally occurring rocks. As a final blow, he demonstrated that crystallization differen tiation could also produce discontinuous compositional variations in igneous rocks. Silicate liquid im m iscibility was no longer considered a viable petrologic process. For several decades, no mention was made of it. It is no surprise then, that the differentiation of the Shonkin

Sag laccolith was attributed to crystal settling - no other acceptable gravity - controlled mechanism remained.

With Roedder's (1951) discovery of a second, low-temperature immis c i b i l i t y f i e l d , in te r e s t in the process began to re v iv e . I t was not until examples of quenched immiscible silicate liquids were reported in natural rocks, however, that the idea resumed its place among the accep ted mechanisms of differentiation. Both lunar and terrestrial basalts showing textures indicative of liquid im m iscibility have been described in the literature (Roedder and Weiblen, 1970a, 1970b, 1972; De, 1974;

Philpotts, 1978). More recently, liquid im m iscibility has been proposed to explain the differentiation of larger and deeper-seated bodies of rock (Philpotts, 1976; Wiebe, 1979; Lelek, 1979; Eby, 1979). A more detailed history of the experimental work relating to liquid immiscibi lity can be found in Lelek (1979) and Roedder (1979).

Although much experimental work remains to be done, some strong contrasts are apparent between rocks differentiated by crystal set tling and those differentiated by liquid im m iscibility. With these recent developments in mind, it seems an appropriate time to take ano ther look at the cause of the layering in the Shonkin Sag laccolith. CHAPTER I I

PREVIOUS WORK

Descriptions of the Shonkin Sag Laccolith appeared in early papers by Weed and Pirsson (1895, 1901). They pointed out the sim ilarities betv/een the Shonkin Sag laccolith. Round Butte, and Square Butte and suggested that all three were produced by the same process. They dis missed the possibility of separate'injectionsof shonkinitic and syenitic magma and concluded that the laccoliths had differentiated in place from a single magma.

In their earlier paper, which dealt prim arily with Square Butte,

Weed and Pirsson (1895) proposed several different mechanisms as being responsible for the differentiation. They suggested that as the magma cooled, it became unstable and sp lit into two liquids. This process, perhaps modified by diffusion, convection, and crystallization, created layers of rocks of contrasting composition.

In 1901, when they published the firs t detailed description of the

Shonkin Sag laccolith, any speculation concerning the differentiation process was omitted. A few years later, Pirsson (1905) interpreted the layering of the laccolith as the result of differentiation through a combination of convection and crystallization.

In this model, magma cools at the top and sides of the laccolith, becomes denser, and s in k s . C irc u la tio n s e t up by convection sweeps the heavy crystals along the bottom, where they accumulate. The sides and

8 floor cool firs t and the edges of this shonkinitic "crust" rise toward the top and spread over it, making it thinner on top than elsewhere.

Residual syenitic magma, depleted in mafic constituents, gradually accumulates toward the center, where it solidifies.

Pirsson had described a coarse-grained unit above the syenite and sim ilar to the one beneath it. He considered this pegmatite to be a shell completely surrounding the syenite and attributed it to the con centration of water vapor along the inner walls of the solidified shon kinite. His explanation for the diverse rock types within the lacco lith assumes gradational contacts between a ll the units.

The next mention of the Shonkin Sag laccolith was by Daly (1912) in his Geology of the North American Cordillera at the Fortyninth

Parallel. He considered fractional crystallization an inadequate ex planation for the juxtaposition of syenite and shonkinite in the Shon kin Sag and Square Butte laccoliths. He thought it more like ly that they had differentiated in place by gravitational separation of two im m is cib le magmas.

In their interpretation of the Shonkin Sag laccolith, Osborne and

Roberts (1931) accepted Bowen's conclusion (1928) that liquid immisci b ility does not operate under geologically reasonable conditions. This le ft them with fractional crystallization as the only process available

They modified Pirsson*s (1905) model by rejecting the idea of convec tion throughout the entire laccolith.

Their calculations indicated that as it cools, a saturated magma in contact with its solid phases becomes lighter rather than heavier. 10

That relationship would allow convection to occur at the bottom, but not at the top, making it impossible to enrich the upper shonkinite in mafic minerals. It would retain the same composition as the original magma o r become more f e ls ic .

By minimizing the role of convection, Osborne and Roberts drew solely upon crystal settling to explain the layering. Augite and o li vine crystals form and settle in the lower shonkinite, leaving the residual melt to crystallize as the syenite. They believed the large crystals in the pegmatite formed while settling slowly through the magma ahead o f the downforming syenite.

Osborne and Roberts never studied the laccolith in the fie ld , but examined thin sections borrowed from Yale and the University of Iowa.

They assumed that the most mafic rocks came from the bottom o f the lower shonkinite, an assumption later discounted by both Barksdale (1937) and

Hurl but (1939).

Assimilation was appealed to by Reynolds (1937) to explain the existence of the syenite layer. Never having seen the laccolith in the fie ld , she suggested that it was a layer of the Eagle sandstone swal lowed up by the magma. Since the contact metamorphism around the lacco lith is so slight, and since the assimilation of such a siliceous rock would not yield a syenite anyway, the idea was never considered serious ly by any other workers.

Thirty-seven years after Weed and Pirsson's firs t description, two groups of geologists, one from Harvard and another from Yale, decided to do the firs t detailed study of the laccolith. The two groups 11 d u p lic a te d much o f t h e ir work d u rin g the same summer. A fte r making geologic maps and topographical maps, determining rock densities for vertical sections through the laccolith, and studying the rocks in thin section, the two sets of data agreed very well. The conclusions, how ever, were diam etrically opposed.

In itia lly , the Harvard group (Larsen, Hurl but, Burgess, Griggs, and Buie, 1935) proposed that the layering of the laccolith was not the result of differentiation in place, but of separate successive intru sions. Later that year. Hurl but (1935) changed his mind and tentatively concluded that the layering was due to differentiation in place through crystal settling.

Barksdale (1937) emphasized the cross-cutting relationships found at some places within the laccolith, and supported the idea of three separate injections of magma. In his model, the firs t intrusion was shonkinitic in composition, and formed a laccolith about 55 m thick.

While sampling the laccolith from bottom to top, Barksdale had found that the lower shonkinite became darker and more dense toward the top, the reverse of what one would expect from crystal settling. He conclu ded that the magma became more mafic as the firs t intrusion progressed - probably the result of some kind of gravity differentiation in a magma reservoir beneath the laccolith.

Before the shonkinitic magma solidified completely, the second, syenitic magma intruded. Moving through the same conduit, it encoun tered the upper chilled layer of shonkinite and spread laterally to form the main body of syenite. The magma mixed incompletely with the 12

shonkinite, forming the “hybrid zone" at the top of the lower shonki

nite and the blotchy or mottled zone at the base of the upper shonki

nite. At the same time, small dikes of syenite were injected into the

overlying shonkinite and out toward the edges of the laccolith.

The t h ir d in tr u s io n , the pe g m a tite , was emplaced along th e zone o f

weakness between the lower shonkinite, now so lid ifie d , and the syenite,

s till partly molten. A minor injection forming the felsic dikelet

within the pegmatite was the final event in the formation of the lacco

l i t h .

When Hurlbut (1939) published his final conclusions, he had to ta lly

abandoned the idea of separate injections. After looking at other lac

coliths in the area, and seeing that many contained syenite between two

layers of shonkinite, he realized that a great deal of coincidence would

be required to produce the same sequence of rock types in each laccolith.

In the Shonkin Sag laccolith, he found that the average mafic mineral

content of the syenite plus the shonkinite was approximately equal to

that of the chilled margin. He concluded that the laccolith had d if

ferentiated in place through crystal settling and floating. His model

differs from that of Osborne and Roberts in that he was forced to

explain the upward-increasing density in both shonkinite layers.

Hurlbut agreed with Osborne and Roberts that the chilled phase was the original magma and that immediately below it cooling would

increase viscosity too quickly to allow for crystal settling. He pos

tulated that each successively lower layer would lose more and more of its heavy minerals as it approached the hotter, less viscous center 13

of the laccolith. At some point, most of the heavy minerals would have

separated out, and the magma would consist of the residual felsic melt, with perhaps a small amount of floated leucite. This compositional

boundary would become the upper shonkinite-syenite contact.

As the heavy m in era ls s e ttle d o u t o f the la y e r th a t was to become

the syenite, they concentrated below. As at the top, the chilled rock

in contact with the sediments cooled too quickly to allow for any crys

tal settling. Increasing viscosity allowed each successively higher

layer to "catch" more of the heavy minerals raining down from above.

At the top of this zone a very dense layer of closely packed heavy

minerals formed.

By this time, settling of heavy minerals within the layer was very

slow. Increasing viscosity prevented complete separation of the heavy

minerals, and above the dense layer a gradational zone formed. Only a

few feet thick, it was a mixture of the felsic residual liquid and mafic

crystals that stopped before settling any farther. This gradational

layer at the top of the lower shonkinite is the same as Barksdale's

"hybrid zone", which he attributed to the mixing of two magmas.

Before solidification of the syenite, a late surge of magma caused some fracturing of the shonkinite near the periphery of the lacco

lith . These fractures were fille d with residual syenitic liquid.

As the laccolith crystallized from the top and bottom inward, volatiles concentrated in the liquid at the center. This magma, lying between the solid bodies of syenite and lower shonkinite, was well

insulated and cooled very slowly to form the pegmatite. 14

A fte r H url b u t* s work on the Shonkin Sag la c c o lit h , i t became widely accepted and very well known as a classic example of differen tiation in place by crystal settling. Several years later, Barksdale

(1952) reevaluated the evidence and revised his original hypothesis of three separate injections. He concluded that the laccolith had indeed differentiated in place from a single intrusion and that the cross-cutting relationships emphasized in his earlier report could be the result of "auto-intrusion" as late surges of magma squeezed resi dual fluids into fractures.

Barksdale speculated upon the origin of the pegmatite and con sidered three possibilities: 1) intrusion of a late differentiate from below; 2) recrystallization of the syenite due to the high volatile content in the center of the laccolith; and 3) crystallization of a residual differentiate moved into its present position by auto-intrusion

The third possibility, which he favored, best explains the eccen tric position of the pegmatite layer, which is thickest not in the center of the laccolith, but near its southern edge. Structure con tours (Barksdale, 1952, Fig. la and lb) show that the laccolith has been warped since the layering developed. In Barksdale's model, the increase in internal pressure due to volatile concentration allowed the pegmatite to move into areas over anticlinal noses. Subsequent failure of the system resulted in a sudden drop in pressure and ex tremely rapid crystallization. 15

With Barksdale's change in position, the controversy came to an end and the verdict was unanimous. Assimilation and separate injec tions had been rejected for several good reasons, and only one possi b ility remained - layering in the Shonkin Sag laccolith must have developed by differentiation in place through crystal settling. Later work assumed this interpretation to be correct and focused on more detailed aspects of the petrology and mineralogy of the laccolith

(Nash and Wilkinson, 1970, 1971). CHAPTER I I I

FIELD RELATIONS

The laccolith, more résistent to erosion than the sandstone it intrudes, forms a broad dome eroded on the south side by a glacial channel of the Missouri River that carved the Shonkin Sag. A c liff 30m high and nearly 2 km long dominates the valley, exposing dark shonkinite resting horizontally upon undisturbed Eagle sandstone. Prominent columnar jointing has scattered blocks of shonkinite over the grassy slope below the c liff. At the east end of the c liff, a small valley cuts into the interior of the dome, exposing the most complete vertical section through the laccolith.

The contact with the underlying sedimentary rock is sharp and does not disturb bedding. The buff-colored sandstone is light blue for a distance of less than 1 m below the contact. The altered rock retains its sedimentary layering, but has partially recrystallized, and has a b rittle , flin ty character.

C h ille d Zone

Igneous rock immediately above the sandstone has a dark, fine grained matrix containing phenocrysts of augite (1-4 mm) and pseudoleucite (1-2 mm). Pseudoleucite grains look spherical in hand specimen and give the rock its distinctive spotted appearance. The chilled rock at the contact has been given various names, including pseudoleucite

16 17 basalt porphyry (Pirsson, 1905), pseudoleucite augite vogesite (Barks dale, 1937), and mafic phono!ite (Hurlbut, 1939).

Lower Shonkinite

The chilled zone grades upward into a massive, dark, medium-grained rock, the lower shonkinite. Through a vertical distance of about 5 m, the pseudoleucite crystals become smaller and fin a lly disappear, and the augltes become larger. Augite crystals (2-7 mm long) in a matrix of potassium feldspar dominate the shonkinite.

A traverse up the canyon, vertically through the laccolith, shows the shonkinite becoming progressively darker. Near the bottom, it con tains less than 50% mafic minerals, near the top mafic minerals make up more than 65% of the rock. This transition is gradual and can best be seen graphically in plots of the density measurements made by Barks dale (1937) and Hurlbut (1939) (Fig. 4).

At the top of the lower shonkinite is a layer about 1 m thick that is lighter both in color and in density than the main layer of shonki nite. It contains much more feldspar, and much less pyroxene, than the rest of the shonkinite layer and has been called "hybrid syenite- shonkinite" (Barksdale, 1937); "the gradational zone", (Hurlbut, 1939); and "gradational syenite" (Barksdale, 1952). The contact between this rock and the more typical lower shonkinite is more gradational than any other contact within the laccolith. Its upper contact, however, is quite sharp (Fig. 5). 18

UPPER ! ! i SHONKINITE j r — j ---- SYENITE lOWER SHONKI NITE 1— 1---- 1 I-----1- »• f 1 r •L. &

VI 1— i - 1 h - h - 1 .«t SYENITE *" i 1 1 " V r " T - . trr -* : • i 1 I OWE R » r ! 1 Î4.1 * j shonkinite ^ ^ ! i i V : ' i i *• H t ’■ ' I 1 ! !••' 1 b S Æ t'-o' '

SYENITE : .v iii t v ;u î'i ■ • i LOWER I I ; i— SHONKINITE D*‘- - - i- i '• ‘ ! 1 « ! r. |r,l ' ! 1 1 J 1 ; 1 ------k i--V ' J L

*.!**• — îy w '9 - s b%se o f la tc o lD * 9 9 Hv 1 * HufibyV* Origif**' tl% S«

Figure 4. Rock densities in the Shonkin Sag laccolith (from Hurlbut, 1939).

Pegmatite

Lying above the gradational zone is the most distinctive rock in

the laccolith, the pegmatite. From a maximum thickness of 5 m in the

center, it thins and fina lly pinches out toward the edge of the lacco

lith . Augite intergrown with sanidine forms large radiating star

shaped clusters. The augite crystals, some of which are replaced by 19

“O m CD

GO in o z

Figure 5- Layering in the Shonkin Sag laccolith. Note the extremely sharp contact between syenite (in the form of pegmatite) and shonkinite. The "hybrid" zone is directly beneath this contact. The chalky while rock in the background is syenite. 20

b1 o tite , are up to 15 cm long. Grain size within the pegmatite is

quite e rra tic, however, and coarse- and medium-grained patches show

no consistent distribution. The augite-sanidine intergrowth is also

present on a smaller scale, in areas 1-2 cm across. Called "finger

print intergrowth" by Barksdale (1937), and "graphic intergrowth" by

Hurlbut (1939), these patches are scattered throughout the pegmatite

la y e r.

Another distinctive feature of the pegmatite is the felsic dike-

let that wanders throughout the layer. The fine-grained dikelet is

usually about 15 cm wide and composed prim arily of zeolites and potas

sium feldspar. In some places, small green needle-like crystals of

aegerine are visible. Except for two places in the north drainage

where it intrudes the syenite for a short distance, the dikelet is

confined to the coarse-grained layer. The dikelet is persistent in the

center of the laccolith, and pinches out toward the edges of the peg-

m atitic layer.

Dark, bio tite -rich segregations exist within the pegmatite. These

masses are irregular, small, and have gradational contacts. They re

semble shonkinite in mineral composition (Hurlbut, 1939, p. 1070), and

in at least one place contain a lens of highly zeolitized microsyenite

(Barksdale, 1937).

The pegmatite weathers back into the laccolith, forming a bench and commonly obscuring its lower contact. The contact between the peg matite and the overlying syenite is generally in the form of a slightly overhanging c liff. 21

The pegmatite-syem'te contact is much less regular than the other

contacts within the laccolith (Fig. 6). Although it is horizontal in

places, it is more commonly wavy. Offshoots of the pegmatite extend

nearly 1 m into the syenite in places. Small patches (5-15 cm across)

of pegmatite within syenite are common near the contact. In several

places, both rounded and angular chunks of syenite are to ta lly enclosed

in pegmatite. In each case, the contact is quite abrupt.

Syen i te

^ Pegmatite / —/

Figure 6. Contact relations between pegmatite and syenite. Tanner Canyon.

S yenite

The syenite is a light-colored, medium-grained rock with biotite and augite crystals scattered evenly throughout a matrix of potassium fe ld s p a r. I t is q u ite homogeneous throughout i t s 15 m th ic k n e s s . 22

The syenite weathers to a chalky white color, and the contrast with the dark gray shonkinite is visible from several kilometers away.

Upper Shonkinite

Above the syenite lies another layer of shonkinite. This upper shonkinite ranges in thickness from 8-12 m near the center of the laccolith to about 3 m near the edge. It has been eroded from most of the laccolith, but is exposed in isolated outcrops at the head of

Tanner Canyon. The contact between the syenite and upper shonkinite is quite sharp in most places but in others a mottled zone is present with stringers of syenite cutting the shonkinite and irregular patches of shonkinite within the syenite.

In some places, a coarse-grained aegerine syenite lies between the syenite and upper shonkinite, but in most cases it intrudes the upper shonkinite in small stringers and horizontal sheets ranging in thick ness from a few cm to 1 m.

The upper shonkinite has the same mineral composition and texture as the lower shonkinite, but is lighter in color and contains fewer mafic minerals. It shows the same pattern of upward-increasing density,

The upper shonkinite grades vertically into a 3 m-thick fine-grained chilled border identical to that at the base of the laccolith.

Although the layers within the laccolith look generally homogen eous, several anomalous areas within the lower shonkinite cannot be ignored. Though not previously described, they may yield important clues to the differentiation process. 23

Several different types of syenitic segregations occur in the

lower shonkinite. Most common are cylindrical pipe-like bodies (Fig.

7). They are vertical or near-vertical and many, but not a ll, are

coarser-grained than the surrounding shonkinite. Most are about 4 cm

across and about 25 cm long. They contain large amounts of sanidine

and zeolites, and a sprinkling of mafic minerals. The pipes exist near

the base of the laccolith, but are most common near the top of the

lower shonkinite. In some places, they are numerous and only 15-25 cm

a p a rt. Most are f a i r l y homogeneous throughout t h e ir le n g th , but a few

contain segregations of more felsic material at the top or directly

above the pipe (Fig. 8). None were found in the upper shonkinite layer.

S yenite

coarse-grained pipe in lower shonkinite

Figure 8. Coarse-grained pipe containing syenite segregation. 24

Figure 7. Pipe-like syenite segregation in the lower shonkinite» west side of laccolith, near top of lower shonkinite, but below the most mafic layer.

Figures 9 and 10. Syenite segregations near top of lower shonkinite, west side of laccolith. 25

A second type of syenite segregation concentrates near the top of the lower shonkinite. Small spherical or irregular bodies about 2 cm across are common on one outcrop surface about 3 m below the pegmatite- lower shonkinite contact (Figs. 9 and 10). Rare or absent in other parts of the lower shonkinite, here they are abundant and spaced only about 10-20 cm apart. Contacts with the surrounding shonkinite are sharp, and the mineral composition is identical.

The third type of syenite segregation is least common. Thin sheets of syenite, 15 cm thick exist in the lower shonkinite (Fig. 11).

They are horizontal, have a fla t lower surface, and a fla t or gently undulating upper surface.

The lower shonkinite also contains several kinds of xenoliths, generally within the lower 5-10 m. Some are dark and fine-grained and have been recrystallized, although most show only slight contact ef fects. Xenoliths of Eagle sandstone s till show bedding and inclusions of the chilled rock, fresh and unaltered, are easily recognized. The fragments of chilled rock are surrounded by a narrow white selvedge.

The syenite is the most uniform of all the layers, but it also contains inhomogeneities. Near the contact with the underlying pegma tite , the syenite contains irregular pockets of extremely coarse grained material sim ilar in composition to the enclosing rock but sharply separated from it (Fig. 6). The pegmatite pockets in the sye nite are of various sizes, most are less than 10 x 20 cm. The pegma tite contains inclusions of syenite which differ from other inclusions in being angular (Fig. 12). 26

î sample #30

Figure 11. Thin sheet of syenite in lower shonkinite, north side of Tanner Canyon.

sample# 42

Figure 17. Sample from "hybrid" or gradational zone at top of lower shonkinite. 27

i / Pegmatite

\ ÂA/

,-/« l^lOcm / V"# / X I ^

Figure 12. Angular chunk of syenite surrounded by pegmatite, west side of laccolith.

The c h ille d zone and upper s h o n k in ite show no ir r e g u la r it ie s , either because none exist, or because outcrop is so lim ited. CHAPTER IV

PETROGRAPHY

All the rocks within the Shonkin Sag laccolith have essentially

the same minera logy-pyroxene in a matrix of potassium feldspar. Varia

tions between rock types are in the proportions of lig h t to dark min

erals, as well as some variation in accessory minerals (Appendix I).

The pyroxene is more calcium-rich than augite and is technically

a sali te. The feldspar has a low axial angle (0-10®), and is therefore

called sanidine. Biotite, olivine, apatite, and titanomagnetite are

common accessory minerals. Sanidine usually contains intergrowths of

z eolite minerals, and some carbonate and serpentine occur as alteration

pro du cts.

C h ille d Zone

The rock a t the c h ille d border has been given seve ral names in c lu ding pseudoleucite basalt, pseudoleucite-augite vogesite, pseudoleucite basalt porphyry, and mafic phonolite. Regardless, it is chemically equivalent to the adjacent shonkinite. It differs in mineral composi tion in that it contains pseudoleucite rather than sanidine. Euhedral phenocrysts of pseudoleucite, augite, and olivine are set in a dark, fine-grained matrix (Fig. 13). Small hexagonal apatite needles are scattered throughout the rock and magnetite and serpentine mineral are present as alteration products of olivine.

28 29

0> 01 o r—c: -<-»t/> «J *a LU c 2rrm

The contact with the underlying sandstone is very sharp, but slightly irregular over a distance of a fev/ centimeters. The groundmass at the contact is brownish and isotropic for a distance of about

1 mm. Bedding within the sandstone is s till visible, but some recrys tallization has taken place. Some of the phenocrysts rest directly on the sandstone floor. Most are whole, but some broken fragments are present. Tiny microlites of both augite and pseudoleucite exist in the groundmass between phenocrysts. There are very few crystals of intermediate size.

Although the pseudoleucite crystals are now an intergrowth of sanidine and zeolites, those near the contact retain the form of the 30 original leucite crystals. Most are about .5 mm across. The largest are 1.5 mm and the tiny crystals in the groundmass are less than .2 mm.

Augite in the chilled zone is commonly found in clumps of three or more crystals. A few are poikill tic , with "wormy-looking" inclu sions of biotite. Most show oscillatory zoning. Those near the con tact are generally 2-3 mm across, and some are up to 7 mm long. The small augites in the groundmass are less than .1 mm across.

The olivine crystals are also euhedral, but most look somewhat corroded. Where they touch the matrix, they are rimmed by brown bio tite . A few contain augite, but more commonly they are enclosed in augite. Most of the olivine crystals are ,5-1 mm, but some are as large as 2.5 mm. The smallest crystals identifiable as olivine are less than .1 mm across. Some of the olivine grains have been completely replaced by a serpentine mineral plus magnetite, others show alteration only along fractures.

Some carbonate is present in the groundmass and within the pseudo leucite, Zeolite minerals occur only within the pseudoleucite crystals.

Biotite occurs only as an alteration product of olivine. Most rims olivine, but in a few cases, the olivine has been almost completely replaced. Small euhedral apatite crystals are common both within the groundmass and as inclusions in augite.

S h o n k in ite

The shonkinite is a medium- to coarse-grained rock with a massive texture. Euhedral augite, biotite, and olivine are scattered through out a matrix of sanidine and zeolites. Both the upper and lower 31

shonkinite layers become darker and more dense upwards, corresponding

to an increase in the proportion of mafic minerals. The lower shonki

nite varies from 6S% dark minerals near the top to 45% near the base.

A sample from the lowest part of the upper shonkinite contains only

40% dark minerals.

Large euhedral augite crystals dominate the shonkinite. The

larger crystals are 4x7 mm, but most are 1x2 mm or .5x.7 mm. The

larger crystals show distinct oscillatory zoning which is absent or

less pronounced in the smaller crystals. Some of the augites are

rimmed with brown b io tite , others have narrow green veinlets or rims

of aegerine-augite. Aegerine-auglte is also present in a few, very

small crystals separate from the augite.

B iotite may be either green or brown and several crystals have

irregular areas of both colors. Strongly pleochroic and commonly

p o ikill tic , it may contain apatite, magnetite, and in some cases,

augite. Brown biotite rims olivine where it is next to the matrix, and

rims some of the augite crystals as well. Green biotite rims brown

biotite in places, but the optical orientation is perpendicular to

that of the earlier crystal and forms tiny spires protruding outward,

called "battlement structure" by Osborne and Roberts (Fig. 14).

Olivine is present in small euhedral equidimensional crystals

.3x.3 mm. Grains in contact with augite or apatite are relatively

unaltered, but those in contact with the felsic matrix are rimmed by

biotite. Fractures in the olivine show alteration to a serpentine mineral plus magnetite, and many grains have been completely replaced. r i

Figure 14. Thin section of Tower shonkinite. Note "battlement” struc ture of biotite overgrowths. Augites commonly show oscil latory zoning. Sample Z130, near top of layer (31 m above lower sandstone contact). (Au = augite, Ae = aegerine, B = biotite, 01 = olivine, matrix is sanidine intergrown with z e o lite s ) .

Olivine increases from less than 1% near the base to more than S% near the to p .

The matrix of the shonkinite is made up of complex intergrowths of sanidine and zeolites. In some places, sanidine (clear in plane light) is euhedral against fuzzy-looking patches of zeolites, but more commonly it contains tiny zeolitic areas in either "graphic" or

Figure 15. Graphie (a , b) and "fingerprint" (c) intergrowths of zeolite minerals (stippled) and s a n id in e .

.25mm

"fingerprint" intergrowths (Fig. 15). The graphic areas have tr i angular, rectangular, or cuneiform inclusions of zeolites, while the

"fingerprint" areas have irregular inclusions arranged in a swirling or radiating pattern. Small areas of sanidine free of zeolites exist but patches of zeolites without sanidine are rare. A few grains of unaltered nepheline were reported by Osborne and Roberts (1931) but 34 none were evident in these slides. The total percentage of felsic minerals decreases from 50% near the base of the lower shonkinite to

32% near the top.

Apatite, magnetite and a few grains of sphene are present as small euhedral crystals enclosed in all the other minerals. A serpentine mineral occurs as an alteration product of olivine.

Textural Variations Within the Lower Shonkinite. Though the typi cal shonkinite has a medium- to coarse-grained granitic texture, varia tions are common. A slide from the lower part of the lower shonkinite

(Fig. 16, #106) shows a slightly different texture, clumps and strings of mafic minerals connected by small flakes of biotite.

A sample from the "hybrid zone" or "gradational zone" (#42) d ir ectly below the pegmatite-lower shonkinite contact shows a peculiar mottled texture (Fig. 17, p. 26), small irregular light-colored areas surrounding by dark areas. The ligh t areas are predominantly feldspar, but contain tiny augite crystals. The dark areas consist of closely packed mafic crystal, primarily augite. In thin section, the light areas look like syenite. Millimeters away, the rock looks like a mafic shonkinite. It was this inconsistency that led Barksdale to call it a

"hybrid" rock.

An even more unusual texture was found in a rock from the central part of the lower shonkinite (#37). This rock was adjacent to one of the irregular syenite segregations described earlier. It shows tiny spherical felsic areas surrounded by mafic areas (Fig. 18). Contacts 35

A u -4

F igure 16. Thin section of lower shonkinite. This rock has an unu sually high biotite content. Sample #106, 7 m above lower sandstone contact. (Au = augite, Ae = aeaerine, B = biotite).

between the two are extremely sharp. In hand specimen, the white

areas give it a distinctive spotted appearance. In thin section, the

spherical areas are less well-defined, but s till evident (Fig. 19).

They are composed o f sanidine and zeolites and are surrounded by mafic crystals, most commonly by biotite. Some of the biotite crystals have a strange cuspate shape. 36

sample #37 n 1 2

Figure 18. “Spotted rock", sample #37, found adjacent to an irregular syenite segregation in Tanner Canyon.

sample #19

Figure 21. Syenite- upper shonkinite contact, head of Tanner Canyon. 37

A sim ilar texture appears in another sample adjacent to a syenite segregation (Fig. 20, #30). Embayed biotite crystals are commonly cus pate, and once again they surround felsic areas. Near the syenite segregation, a thin sheet in this case, some of the light-colored blobs approach each other and some seem to merge (Fig. 11). Directly below the syenite layer, the closely packed augite crystals form a mafic layer that is darker than the typical shonkinite.

Upper Shonkinite. The upper shonkinite is sim ilar to the lower shonkinite except that it is less mafic. It typically has the same massive texture, but at the contact with the syenite, it becomes spot ted, sim ilar to sample #37 (Fig. 21, p. 36). Spherical felsic areas are in sharp contrast to the surrounding mafic matrix. As in sample

#30, the fe lsic areas seem to merge where they touch one another.

These unusual textures have not been previously described in the

Shonkin Sag laccolith, but they are common, and may shed some lig h t on the origin of the rock types present.

S yenite

A light-colored, medium-grained equigranular rock with a massive texture, the syenite has essentially the same mineral composition as the shonkinite. It differs in that the felsic minerals are more abun dant than the m a fic m in e ra ls , and i t is a more homogeneous body, show ing none of the vertical variations in composition that are so obvious in the shonkinite layers. 38

Sample if37

b io t it e

o liv in e

i a u g ite

m a trix , sa n id in e plus z e o lite s

2 mm Figure 19. "Spotted rock" in thin section

Sample ë30

1 mm

Figure 20. Thin section of lower shonkinite showing cuspate biotite crystals in a matrix of sanidine and zeolites. 39

The augite crystals In the syenite differ from those in the shon

kinite. They are embayed and corroded and lack the oscillatory zoning

characteristic of the augite in the chilled zone and shonkinite layers.

Many more show rims of brown biotite, and several of the smaller grains

are almost completely replaced. Most of the augite crystals are be

tween 1 X 2 mm and 1 x 3 mm. Commonly p o ik ilitic , they may contain

apatite, magnetite, or in a few cases, olivine. Aegerine-augite is

present as rims on augite and in a few small grains scattered through

out the rock.

Strongly pleochroic brown biotite exists both in reaction rims

around olivine and augite and as small euhedral flakes about 0.1 mm

across. Overgrowths of green biotite are commonly oriented perpendi

cular to the earlier crystal and show the "battlement structure" de

scribed above in the lower shonkinite. Some biotite is p o ikilitic, with inclusions of magnetite and apatite most common, and olivine and

felsic minerals in a few cases. Like the augite, the biotite crystals

are corroded and embayed by th e groundmass. Some lo n g , th in b io t it e

crystals (7 mm) are optically continuous, but physically discontinuous

(Fig. 22). The fragments are now separated by several patches of sanidine and zeolite minerals.

A few small grains of olivine are present. Without exception, they are either enclosed in augite or rimmed by brown biotite.

The syenite contains the same kinds of intergrowth of sanidine and zeolites described in the shonkinite. Large areas of sanidine and zeolites also occur separately here, in many cases with sharp 40

» L

Figure 22. Thin section of syenite. The long segmented biotite crystal is optically continuous. Note "battlement" structure of biotite overgrowths. (Au = augite, Ae = aegerine, B = biotite, Ap = apatite). euhedral outlines. According to Barksdale (1937), natrolite is the most common zeolite, but substantial amounts of s tilb ite are present as well as minor amounts of chabasite and anal cite. A few grains of albite are associate with sanidine in places.

Magnetite crystals are scattered throughout the rock as modified octahedra averaging 0.5 x 0.5 mm. Apatite is quite common, especially as inclusions in augite, slightly less common in biotite and sanidine.

Sphene is slightly more common in the syenite than in the shonkinite.

Barksdale (1937) reported small amounts of tita n ite , melanitic garnet, and a mineral tentatively identified as grossularite. 41

Pegm atite

The coarse-grained nature of this unit makes it d iffic u lt to study

in thin section. Some interesting features appear, however, that are

not obvious when looking at a hand specimen of the rock.

The most distinctive feature of the pegmatite Is large radiating

clusters of pyroxene and feldspar. Thin sections show that each ray

of a star-shaped cluster is a single crystal, optically continuous,

but physicallyunconnected. The fragments are separated by randomly oriented areas of zeolitized sanidine. This texture is sim ilar to that

found in the syenite where long thin biotite crystals are separated by

f el sic minerals.

The pegmatite also shows the "fingerprint intergrowth" of augite and sanidine described by Osborne and Roberts (1931). These oval pat ches of pyroxene and feldspar exist at the center of the star-shaped clusters, as well as separate from them. They weather out of the peg matite c liff in small rounded pieces about the size of a marble. Thin sections reveal that the augite grains are single skeletal crystals of

large size (Barksdale, 1952). Some of the intergrowth areas have only one augite crystal; others are made up of two or three. Many have a larger piece of augite at the center of the intergrowth, with smaller fragments radiating outward from this nucleus (Fig. 23).

Like those in the syenite, the augites in the pegmatite are em bayed and corroded, commonly showing reaction rims o f brown b iotite.

Augite outside the intergrowth areas is greener and more pleochroic, presumably reflecting a higher aegerine content (Osborne and Roberts, 42

A 'A * v y .

3mn

Figure 23, *'Fingerprint" intergrowth of augite and sanidine in pegma tite layer. These commonly contain a larger augite crystal at the center of each intergrowth.

1931). Some aegerine*augite occurs as rims on augite or penetrating along fractures. A few separate grains are scattered irregularly throughout the rock.

Brown b io t it e commonly rim s a u g ite . Some o f these rim s show the perpendicular overgrowths of green biotite. Biotite exists as equidimensional flakes up to 2 cm across.

Olivine is rare in the pegmatite. The few grains that can be found are surrounded by augite.

As in the other layers, the felsic groundmass is predominantly sanidine. Some of the sanidine crystals look relatively fresh, while others contain abundant irregular inclusions of zeolites. Zeolites 43 are also present in irregularly-shaped patches in te rstitia l to euhedral sanidine crystals.

Small amounts of orthoclase, nepheline, and sodalite have been

reported (Hurlbut, 1939) but most have been thoroughly zeolitized.

According to Osborne and Roberts (1931), the zeolite minerals in the

pegmatite are natrolite, stilb ite , and hydronephelinite. Barksdale

(1952) identified thomsonite as well. Hurlbut (1937) reported chabasite

Apatite, magnetite, and sphene are the common accessory minerals.

The crystals are slightly larger in this unit than elsewhere. The mag

netite crystals are commonly quite p o ik ilitic and may appear in a com

plex intergrowth with biotite and/or augite (Fig. 24). Apatite is pre

sent in long thin needles usually associated with the felsic minerals.

. 25mm

Figure 24. Magnetite-biotite intergrowth in pegmatite layer. 44

Some carbonate is present as small irregular inclusions in sanidine.

Osborne and Roberts (1931) reported finding one grain of tita n ite and one grain of chalcopyrite.

Within the pegmatite layer, two other rock types are present.

Most conspicuous is the narrow felsic dikelet that wanders throughout the layer. Less obvious, and less regular in its appearance, is a dark, biotite-rich segregation.

Felsic Dikelet. This fine-grained, light-colored rock, syenitic

in composition, is predominantly feldspar and zeolites. A sprinkling of dark minerals are unevenly distributed throughout the rock. Euhe dral to subhedral sanidine crystals about 5 mm long are set in a matrix of sanidine plus zeolites. Large patches of zeolite minerals exist, and many of the sanidine crystals are zeolitized. The zeolite patches, along with the mafic minerals, occupy the angular spaces between the randomly oriented feldspar laths (Fig. 25). The prominent zeolite minerals are natrolite, stilb ite , and chabasite, but large parts of these areas are made up of an intermediate mixture (Barksdale, 1937).

Some sections contain a brown amphibole that is probably barkevikite. A few grains of riebeckite have been reported (Barksdale,

1937), Biotite replaces the brown amphibole in places and also occurs as small grains scattered through the rock. Most of the biotite is extremely pleochroic and many grains are quite corroded. Magnetite and apatite are present as accessory minerals. Apatite is less common in this rock than the other rock types in the laccolith. Barksdale 45

1 m«A

Figure 25. Thin section of the fe ls ic dikelet within the pegmatite layer. (S = sanidine, Z = zeolites, Ae = aegerine, B = biotite),

(1937) reported a colorless garnet and a few grains of titanite as v/el 1.

Biotite-rich Segregation. This rock contains biotite as tabular euhedral crystals and as rims altering from b io tite . Except fo r the absence of olivine, this rock resembles a shonkinite. The sanidine, zeolites, and accessory minerals are the same as those in the rest of the laccolith.

Aegerine Syenite

This rock is intruded in thin s ills into the upper shonkinite.

Coarse-grained, i t resembles the pegmatite in hand specimen. Large feldspar crystals, commonly 2 cm across, p o ik ilitic a lly enclose long augite crystals and blades of b io tite . 46

In thin section, it is evident that most of the augite has been either totally or partially altered to aegerine. Aegerine is also pre sent in bundles of tiny radiating needle-like crystals.

Biotite crystals are brown and highly pleochroic. Most are about

1 x 2 mm. A few have the perpendicular green overgrowths found else where in the laccolith.

The large sanidine crystals have been extremely zeolitized. Zeo lite minerals also occur in large patches in te rstitia l to sanidine.

Accessory minerals are the same as in the other rocks - tiny magnetite and apatite crystals are abundant. CHAPTER V

GEOCHEMISTRY

Samples collected in a vertical section of the laccolith were analyzed for 31 major and trace elements (Ian Lange, unpublished data).

The results are plotted according to height above the base of the laccolith (Figs. 26-37).

R esults

Elements that tend to be concentrated in the shonkinite include

Cu, Hi, Ca, Co, Fe, Mg, Nb, Sm, Sc, Be, Li, Eu- La, and Rb. Some ele ments (Be, Cu) shov/ a wide range of values and are listed here because they are higher on the average than the syenite. Others, such as Eu, are only slightly higher in the shonkinite, but by a fa irly consistent amount.

Some elements (Ca, Mg, Co, Ni, and Li) show a general increase upward through the lower shonkinite. Rubidium, however, decreases through this interval.

Elements that tend to be concentrated in the syenite include A1,

K, Na, Ba, Ce, Ti (slightly) and Mn. Other elements do not concen trate in either the shonkinite or the syenite. This group includes U,

Th, Pb, V, Cr, Cs, Dy, Hf, and Sr.

47 48

Discussion of Results

ise of Ca and Mg in the lower shonkinite reflects the increase in the proportion of mafic minerals. Iron, on the other hand, f a il s to show such a tre n d . Co and Ni do show t h is tre n d , p ro bably because they substitute for Fe in the mafic minerals.

The behavior of Li, Be, and tv/o of the rare-earth elements (La and Eu) seems unusual. They are elements that do not f it readily into the structures of the early-forming minerals and are generally concen trated in felsic differentiates. In the Shonkin Sag laccolith, how ever, they are more concentrated in the shonkinite than in the syenite

The behavior of Rb is also surprising, because it normally substitutes for potassium and concentrates in felsic rocks.

Along with the upward increase of Ca and Mg in the lower shonki nite, a corresponding decrease in Na and K is apparent. This can be explained for by the decrease in felsic minerals (feldspar and zeo lites) in that interval. A1 and Ba show the same pattern.

The behavior of the REEs is surprising in that they normally be have identically. In this case. La, Sm, and Eu are more abundant in the shonkinite. Ce is higher (on the average) in the syenite, and Dy is not enriched in either. They probably behaved very sim ilarly, however, since the enrichment of La, Sm, and Eu in the shonkinite is not pronounced and Ce shows no general enrichm ent, b u t has a few anom alous values in the syenite. The differences in behavior among the rare earths are probably very small. 49

The pegm atite seems to be more d iffe r e n tia te d than the s h o n k in ite and syenite, as evidenced by its chemical trends. It is richer in U,

Th, Be, Na, and L i and im poverished in Ca, Fe, Mg, Co, Cr and N i. The felsic dikelet within the pegmatite is even more differentiated than the pegmatite.

Potassium shows a decrease from the syenite to the pegmatite, while Na continues to increase. This behavior of K results in an unu sually high Na/K ratio in the latest differentiates. La is like K in that it also decreases from the sytenite to the pegmatite.

Most of the chemical trends can be attributed to changing mineral proportions, but others cannot. The behavior of Rb and Mn seems par ticularly anomalous, but this may be due to the fact that the x-ray spectrographs were not matrix-corrected. Other chemical trends w ill be considered in relation to the differentiation mechanisms discussed below. 50

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:c/ 733 1450 215S 2611 34*7 4153 4319 5*j 15 C193 6337 Titanium {ppi.i) CHAPTER VI

DIFFERENTIATION OF THE SHONKIN SAG LACCOLITH

Several differentiation processes have been proposed to explain the close juxtaposition of mafic and felsic rocks. Among the possible explanations for the layering in the Shonkin Sag laccolith are assimil ation, volatile movement, thermogravitational diffusion, fractional crystallization (flowage differentiation^ filte r pressing, or crystal settling), and magma im m iscibility.

Assimilation

Assimilation of a large chunk of country rock has been proposed

(Reynolds, 1937) to explain the existence of the syenite layer sand wiched between two layers of shonkinite. This seems unlikely for two reasons. F irst, the narrow zone of contact metamorphism around the laccolith indicates that, by the time of intrusion, the magma was too cool to melt the surrounding rock. Sandstone xenoliths are only slightly recrystallized and s till show remnant bedding. Second, assimilation of the country rock, the Eagle sandstone, would yield a rock high in silica, not a syenite.

V o la tile Movement

A magma with a high volatile content crystallizing under shallow conditions mioht differentiate by gaseous transfer (Fenner, 1926; Bowen,

56 57

1928). In this model» gas bubbles moving up through the magma concen trate volatiles and other pneumatolytic constituents such as Ne» Fe,

Mn, T i, P, Rb, La, Ce, Y, Zr, Nb, and U (Hyndman, in preparation). A sim ilar process, volatile streaming, moves the volatiles and related elements to areas of lower pressure without the development of a gas phase (Kennedy, 1955; Hamilton, 1965).

Since l i t t l e experim ental work e x is ts , d if f e r e n t ia t io n by movement of volatiles is hard to recognize. While the process might create

rather abrupt chemical changes, these would probably be more gradual than those produced by liquid im m iscibility. Furthermore, elemental partitioning between the shonkinite and syenite is not consistent with this model (Chapter IV).

Thermogravitational Diffusion

Compositional zoning in magma chambers may also be produced by thermogravitational diffusion (Hildreth, 1979). L ittle is known of this process except that it is probably driven by convection and oper ates independent of crystal-liquid equilibria and prior to any crystal settling. Lack of experimental work makes diffusion a d iffic u lt pro cess to recognize. According to Hildreth (1979), diffusion creates chemical gradients distinct from those attributable to liquid immis c ib ility . Major-element changes are small, and a ll compositional parameters vary continuously, rather than abruptly. Diffusion also tends to enrich the felsic fraction in beryllium, niobium, thorium, heavy rare earth elements, and especially in uranium and lithium .

These trends are opposite those seen in the Shonkin Sag laccolith. 58

Fractional Crystallization

Fractional crystallization includes any mechanical process by which early-formed crystals are immediately isolated from the system, preventing them from equilibrating with the liquid (Bowen, 1928).

Successive residual liquids become enriched in silica and alkalies.

Separation of early-formed mafic crystals and the more felsic residual liquid can be accomplished in several ways, including flowage differen tiation, filter-press action, and crystal settling (Hyndman, 1972, p. 77-78).

Flowage Differentiation. This process concentrates solid crystals within a rapid flow region, away from the walls of moving fluid.

Usually described in dikes, it may only operate in intrusions less than about 100 m across (Bhattachari and Smith, 1964). In any case, the result is an intrusion more mafic toward the center, the opposite of the shonkinite-syenite relationship in the laccolith.

Filter-press Action. This process takes place during later stages of crystallization, when residual liquid is squeezed out of a crystal mush. Forced out of the interstices, the liquid floats to the top of the magma chamber (Bowen, 1919). In the resulting rock, the mafic crystals should be tig h tly packed and commonly crushed or fractured.

The Shonkin Sag la c c o lit h lie s in u n distu rbe d sedim ents and shows no internal textures that suggest such intense deformation. Filter-press action could not have played a major role in its differentiation. 59

Assimilation, volatile movement, and diffusion are inadequate to explain the field relations and chemical trends within the laccolith.

There is no evidence for fractional crystallization by flowage differ entiation or filter-press action. Magma im m iscibility and fractional crystallization by crystal settling have the best potential to explain the differentiation of syenite from shonkinite. These two mechanisms are described more fu lly , and the expected results of each process are compared to the evidence found in the rocks.

Crystal Settling

In this process, early-formed crystals of heavy minerals sink slowly through the magma chamber and accumulate near the base (Bowen,

1928). Given sufficient time, a zoned magma chamber develops, with accumulated mafic crystals overlain by the displaced residual liquid.

After cooling and crystallization of the magma, the final result is a light-colored, low-density rock lying above a dark-colored, high- density rock.

Layered gabbroic intrusions are the clearest examples of frac tional crystallization of basalt. Crystal settling has been well docu mented in the Skaergaard, Bushveld, and S tillw ater complexes, among others (Wager and Brown, 1968). Layers rich in olivine, iron oxide, or pyroxene illustra te the effectiveness of the process in these large intrusions. Felsic differentiates occur near the top of these com plexes. 60

The mafic layers generally display cumulate textures, sim ilar to

what would be expected after crystallization of residual liquid sur

rounding a loose pile of crystals. According to Wager (1963), these

layers accumulate from slow, regular convection currents. Such layers

may be tens to hundreds of meters thick and extend for tens of kilo

m eters.

Other layers, up to 1 m thick and traceable for a few hundred

meters, show sorting by grain size or density. They are attributed to

deposition from local rapid density or turbidity currents. They have

a well-defined base and consist of a lower part rich in heavy minerals

which passes upwards into a rock rich in less dense, light-colored

feldspar. This type of layering has been called gravity stratification

(Buddlngton, 1936) or igneous lamination (Wager and Brown, 1968) and is

strong visual evidence for bottom accumulation of the material. The

resular repetition of gravity-stratified layers separated by layers of

"average" rock is called rhythmic layering and is a characteristic of

many layered intrusions (Wager and Brown, 1968).

Large bodies of rock assumed to have formed by crystal settling

show another kind of vertical variation, called cryptic layering.

This usually consists of an upward change in the plagioclase from cal

cic to more sodic and in the olivines and pyroxenes from Mg-rich to

Fe-rich compositions (Wager and Deer, 1939).

The tremendous difference in size between huge layered intrusive complexes and a small alkalic laccolith makes direct comparisons d if

ficu lt. A smaller body might not have the long cooling time and the 61 extensive convection currents required to form some of the features mentioned. Some features, such as cumulate textures or a layer of heavy minerals near the base, should be present even in small bodies where crystal settling has occurred.

Lava flows and thin intrusive sheets rarely form under conditions that are conducive to crystal settling (Wager and Brown, 1968). Some of the thicker basic s ills do show some evidence of crystal accumula tion, however. The 350 m-thick Palisades s ill, for example, contains an olivine-rich layer near its base (Walker, 1940). Rhythmic layering and igneous lamination are rare or absent in these smaller intrusions, and even though some crystals do accumulate, crystal settling usually plays a minor role in their differentiation, and is generally confined to the early stages of crystallization (Wager and Brown, 1968).

In addition to textural and field evidence, the operation of fractional crystallization, whether by crystal settling or another mechanism, results in distinctive chemical trends. Based on Gold schmidt's rules of substitution according to ionic radius and charge

(1937), several generalizations can be made:

1) Cations with large radii and low charge tend to sub stitute for K, and are concentrated in the felsic rocks. This group, the large-ion lithophile elements, includes Rb, Cs, Ba, and Pb.

2) Li substitutes for Mg in late-forming Mg minerals, which concentrates it in the felsic rocks.

3) Small, highly charged cations are unable to substi tute for the major ions. This group, the so-called "incompatible elements" includes Be and Nb. They tend to concentrate in late-stage residual solutions and may reach high concentrations in pegmatites. 62

4) Large, highly charged cations such as U and Th are also unable to fit into the structures of early- formed m inerals. They also concentrate in fe ls ic rocks, along with the lanthanide, or rare-earth elements (REE).

5) Many of the medium-sized transition elements sub stitute for Fe and Mg. This group includes Cr, N1, and Co, and tends to be enriched in ultramafic rocks. Others (Mn, V, T i) reach th e ir maximum abundance in gabbros and basalts.

These patterns are common to a diverse group of rocks. Elemental trends in the Skaergaard complex (Wager and M itchell, 1940, 1951) are sim ilar to other rock series assumed to have formed by fractional crystallization (Ringwood, 1955). Both calc-alkaline (Nockolds and

Allen, 1953) and alkaline (Nockolds and Alien, 1954) series show the same general trends. Mafic and ultramafic rocks are enriched in Mg,

Ca, Fe, Cr, N i, V, and Co re la tiv e to th e ir fe ls ic d iffe re n tia te s . The fe ls ic rocks are enriched in Na, K, Rb, Ba, Z r, L i, Cu, REE, Be, U, and Th.

In summary, the evidence for fractional crystallization is of several types:

Field evidence - layers of heavy minerals near the base of the igneous body; rhythmic layering; sorting by crystal size or density.

Pétrographie evidence - cumulate textures, especially within the dense layer of heavy minerals.

Chemical evidence - cryptic layering; characteristic variation trends of major and trace elements. 63

Differentiation through Liquid Immiscibility

In th is process, an i n i t i a l l y homogeneous magma becomes unstable amd splits into two melts, one relatively iron-rich, one relatively silica-rich. First, small droplets of the felsic melt form within the mafic melt. Eventually, some of them coalesce. Since the melts are of different densities, they begin to separate. The rate of separation is a direct function of the size and density difference of the dis persed globules and an inverse function of the viscosity of the mafic melt (Roedder, 1979). If the felsic globules rise far enough to pre clude further equilibration with the mafic melt, fractionation w ill occur, and each magma can then develop independently.

Magma im m iscibility is usually explained on the basis of struc tural models of the liquid state. A silicate liquid is assumed to consist primarily of silica tetrahedra. The sharing of oxygen anions between tv/o tetrahedra re s u lts in the linkage o f te tra h e d ra . This polymerization results in chain-like or branching structures forming a three-dimensional network sim ilar to that in a crystal (Hess, 1971).

A l*3 commonly substitues fo r Si+4, and and Na'*' balance the charge d e f ic it .

There are two fundamentally different positions occupied by ions in a silicate melt. The "network formers" (most commonly Si) form the centers of oxygen tetrahedra possessing varying degrees of linkage.

The "network modifiers" or "modifying oxides" do not enter the network, but exist in holes between the tetrahedra. Network modifiers include

L i, Rb, Ma, Cs, Ca, Sr, Ba, Pb, Mn, and REE (Ringwood, 1955b). 64

Oxygen within the melt can occur as either free oxygen (0^“ ), bridging oxygen (QO), or non-bridging oxygen (Ô"). Non-bridging oxygens are needed to coordinate cations which cannot en ter te tra h e d ra l s ite s or are not needed to charge balance Al+3 in fourfold coordination.

This introduces regions of local order within the melt on a submicroscopic scale.

Regions dominated by non-bridging oxygens are mixed with regions dominated by bridging oxygens. To reduce the area of interface between these domains, the submicroscopic regions of local order are collected in to two d is tin c t phases (Hess, 1977). The re s u lt o f th is process is the formation of a pair of immiscible liquids with distinctly different internal structures. Since the entropy and enthalpy terms of the free energy change of this mixing process can presumably be nearly equal and opposite, the occurrence, extent, and temperature of im m iscibility can be extremely sensitive to composition (Roedder, 1979).

The best textural evidence of the existence of one liquid as an immiscible separation from another is the occurrence of glassy globules of one composition in a glassy rock of another composition (Bowen, 1928).

Such evidence was firs t found in lunar basalts (Roedder and Weibien,

1970a, 1970b; Wei bien and Roedder, 1970a, 1973). Two glasses, one iron- rich and the other silica-rich, show various textures that are easily explained by liquid im m iscibility. They are embedded in one another, but separated by a sharp meniscus. Some of these basalts show globules deformed against and coalescing with each other. After recognition of late-stage immiscibility in lunar rocks, sim ilar textures were described 65 in terrestrial basalts (De, 1974; Roedder, 1978).

Although this type of evidence is the most convincing, it can only be preserved in quenched magmas. Crystallization w ill either obscure or eliminate evidence of this type, but nucléation and crys tallization features can s till provide important evidence.

Nucléation may occur at the margins of an immiscible globule, not only because o f the c ry s ta ls in the surrounding mush, but because any interface can aid nucléation. It is also possible, however, that crystal growth from the rim inward would be too slow and nucléation might occur throughout the whole mass firs t (Roedder, 1979).

Another line of evidence used to suggest im m iscibility is the pre sence of ocelli, small spherical felsic segregations within a more mafic rock. They are typically surrounded by tangential crystals in a pattern reminiscent of crystals surrounding a bubble in an industrial flotation process. Their interpretation is more tentative than that o f the quenched magmas, however, and a lte rn a te explanations o f th e ir genesis have been suggested (Carmichael, Turner, and Verhoogen, 1974, p. 66).

If crystallization has taken place, the mineralogy of both rock types should be essentially the same. Since the two liquids must be in eqilibrium with one another, they must also be in equilibrium with any a d d itio n a l phase th a t forms (Bowen, 1928).

Globules of one rock type enclosed in another are not uncommon.

The terms orbicular, ocellar, spherulitic, va rio litic, and globular a ll describe such textures. While many of the rocks showing these 66 textures do not demonstrate the equilibrium required for immiscible liq u id s , others do.

Field evidence for im m iscibility consists of several features described by Roedder (1979), including the following:

1) globules distorted by flow, or at the point of contact with another globule.

2) coalescence of globules, most probable in those parts expected to solidify last. The end product would be the accumulation of larger tabular bodies of light melt against what was then the top of the magma chamber.

3) gas vesicles in the center or at the top of globules. These are due to the lower crystallization temperature of the felsic melt. After the mafic host is a rigid mass, the felsic melt crsytallizes and shrinks, pro ducing a bubble which may later be fille d with carbonate or quartz, forming an amygdule,

4) systematic size variation of globules. Globules may become larger away from the contact since the longer cooling time allows more opportunity for coalescence.

Along with pétrographie and field evidence, chemical trends can be used to distinguish the products of im m iscibility from thsoe of frac tional crystallization. The partitioning of major and trace elements between immiscible melts has been determined experimentally for a wide range o f syn th e tic systems (Hess and o th e rs, 1975; Massion and Koster van Groos, 1973; Naslund, 1976, 1977; Visser and Koster van Groos,

1976a, 1976b, 1979; and Watson, 1975, 1976, among others).

Although the distribution of major elements during im m iscibility yields a granitic or syenitic alkali-aluminosilicate melt (Irvine,

1976) resembling that from crystal fractionation, there are important differences in the partitioning of some minor elements. Watson (1975, 67

1976) calculated partition coefficients for Cs, Ba, Sr, Ca, Mg, La,

Sm, Lu, Mn, Ti, Cr, Ta, Zr, and P in the system K20- A l203-Fe0-S i0 2 -

All the elements except Cs were enriched in the basic melt, with P showing the strongest enrichment. Some of these elements show enrich ment opposite to that expected from fractional crystallization, which concentrates P, Ti, and the rare earth elements in the more felsic differentiates.

Watson's experimental work verified most of the predictions made by Hess and Rutherford (1974). Based on a s tru c tu ra l model o f s i l i cate melts, they had concluded:

1) cations able to replace 51*4 melts (e.g. Al*^) and cations required for local charge balance in the copolymerization of AlOg with SiO^ tetrahedra (K*, Na*) would be enriched in the more acidic of the two liquids;

2) cations of high field strength (charge/radius) would favor the basic melt if copolymerization with Si 02 were not possible.

As in fractional crystallization, Ca and Mg are concentrated in the basic melt, where they presumably substitute for Fe. In Watson’s experiments, however, the ratio Fe/Mg is the same in both liquids, in contrast to the Fe-enrichment common in the felsic differentiates of bodies produced by crystal settling (e.g. the Skaergaard intrusion).

The rare-earth elements are depleted in the felsic melt, probably because they are too large to replace Si*4 -fn the manner o f A l*3

(Watson, 1976). Although no data are available concerning partitioning o f U and Th, they might be expected to behave s im ila r ly due to th e ir large size. This pattern is the opposite of that expected for 68 contrasting magmas resulting from fractional crystallization processes.

Roedder and Weiblen (1971) suggested that fractionation of elements between immiscible silicate liquids could be an Important process in the evolution of the lunar crust. Noting the enrichment of incompatible elements in the more basic of the two coexisting liquids, they pointed out that such a pattern, if found in rocks, might be evidence for the occurrence of liquid immiscibility.

Experimental work relating to liquid immiscibility is s till in its infancy, however, and its interpretation is made d iffic u lt by inade quate controls of variables such as total FeO, pq^» and H 2 O (Roedder,

1979). Small changes in composition can in it ia t e o r e lim in a te immis c ib ility and vary the partition coefficients. While extrapolations from simple experimental systems to complex magmatic compositions may not prove quantitatively correct, unusual trace element distribution patterns are potentially useful for identifying instances of magma unmixing (Watson, 1976).

In summary, the evidence fo r d iffe re n tia tio n by liq u id im m isci b ility is of three types:

pétrographie evidence - the most convincing pétrographie evidence is droplets of one glass embedded in another, but this kind of evidence is eliminated by crystalliza tion. The presence of ocelli surrounded by tangential crystals is also evidence of liquid imm iscibility.

field evidence - segregations of a felsic rock in a mafic rock. The field relationships should show that both were liquidât the same time. Their mineralogy must show e q u ilib riu m between the tv/o magmas. F e lsic globules may be present in various stages of coalescence. Given sufficient time, they can form a layer near the top of the magma chamber. They should increase in size toward the center o f the body and may o r may not contain gas bubbles. 69

Chemical evidence - the more mafico f the two rock types should have an unusual trace-element d istribu tion . Enrichment in the rare-earth elements and other incom patible elements is inconsistent with a fractional crys tallization model but consistent with liquid immiscibility.

Evidence for Crystal Settling in the Shonkin Sag Laccolith

Once the evidence for differentiation in place seemed conclusive, crystal settling was immediately invoked as the explanation for the layering in the Shonkin Sag laccolith. A tabular body of light rock re s tin g d ir e c tly on a la ye r o f darker and more dense rock does indeed suggest some sort of gravitational separation. That this stratifica tion was accomplished by the settling of heavy minerals at the bottom and accumulation of residual liquid at the top is much less obvious.

The rocks themselves show no features that point toward a crystal s e ttlin g mechanism. Cumulate te x tu re s , rhythmic la y e rin g and c ry p tic layering are all conspicuously absent.

Even if the laccolith were simply too small to develop some of these features, others should certainly be present. The absence of cumulate textures is particularly suspicious. Even in the most dense layer, the mafic crystals show no evidence of having settled into their present position (Fig. 14).

Also d ifficu lt to explain is the position of this dense layer, which occurs near the top of the lower shonkinite. Though only a few basaltic s ills show any evidence of crystal settling, those that do contain layers of accumulated crystals (the Palisades s ill, for example) always have such layers near t h e ir base (Viager and Brown, 1968). 70

The absence of intermediate rock types is also surprising. In a magma chamber w ith c ry s ta ls ra in in g down from above and residual liquid being displaced upward, a sharp boundary between rock types would not be expected. A gradual change from mafic to felsic composi tio n s seems more lik e ly .

In addition to the lack of field and pétrographie support for crystal settling, some of the chemical trends in the laccolith are

in co n s iste n t w ith a fra c tio n a l c ry s ta lliz a tio n model. Elements such as U, Th, REE, Li, and Be are strongly concentrated in residual melts derived from fractional crystallization. In the Shonkin Sag laccolith, however, these elements either are not significantly concentrated in either rock (U, Th) or are concentrated in the shonkinite rather than the syenite (Li, Be, REE) (Figs. 26-37).

Field relationships, rock textures, and chemical trends all fail to support the idea that the Shonkin Sag laccolith differentiated by crystal settling. Another mechanism, involving the gravitational separation of two immiscibile magmas, is considered below.

Evidence for Magma Im m iscibility in the Shonkin Sag Laccolith

A pair of immiscible liquids of different densities, given suffi cient time to separte, should result in a layer of light rock resting directly on a layer of denser rock. The contact between the two rock types should be horizontal and sharp rather than gradational. The mineral content of the two rocks should be the same, though the pro portions should be different. Areas where separation was incomplete 71

might show globules of the felsic rock trapped in the more mafic host.

Chemical partitioning should be compatible with that experimentally

derived for pairs of immiscible liquids.

A careful examination of the Shonkin Sag laccolith reveals that

it meets these criteria. Shonkinite and syenite are both composed of

augite and orthoclase, with minor amounts of biotite and olivine.

Syenite (in the form of the pegmatite layer) rests directly upon shon

kinite and the contact is extremely sharp and horizontal. Segrega

tions of syenite within shonkinite are common, and chemical trends are

con sisten t w ith a liq u id im m is c ib ility mechanism.

The syenite segregations are sim ilar to ocelli found in other alkaline rocks, and interpreted in some cases as immiscible felsic

globules (Philpotts, 1978). Those in the Shonkin Sag laccolith lack the tangential crystals characteristic of ocelli, and for this reason w ill be referred to as "blobs" or "segregations". The syenite blobs are most common in a zone near the top o f the lower sh o n kin ite , ju s t below the "hybrid" or "gradational" zone. Their contacts with the surrounding shonkinite are extremely sharp, and the mineral content is id e n tic a l.

Near some of these syenite segregations, the surrounding rock has a peculiar texture (Figs. 11 and 18). Small round felsic bodies sur rounded by mafic crystals give the rock a spotted appearance. Some seem to be caught in the act of coalescing. On close inspection, tiny mafic crystals can be seen within the felsic blobs. Thus their mineral content is the same as the larger blobs and the same as the main syenite la y e r. 72

These syenite segregations of various sizes provide fie ld evidence of the operation of liquid immiscibility. The larger segregations are higher in the laccolith than the smaller blobs, suggesting that they had more opportunity to coalesce.

Chemical trends also support a liq u id im m is c ib ility mechanism.

The mafic shonkinite is enriched in lithium , beryllium, and rare-earth elements relative to the shonkinite. This pattern is opposite to that resulting from fractional crystallization, but is compatible with fra c tio n a tio n patterns between two im m iscible liq u id s (Watson, 1976).

Chemical trends within the Shonkin Sag laccolith, along with the rock textures and field relationships, support a differentiation mechanism involving the gravitational separation of two immiscible liquids. CHAPTER VII

PETROGENESIS OF THE LACCOLITH

The previous chapter suggests that liquid im m iscibility can best explain the field relationships, rock textures, and chemical trends within the Shonkin Sag laccolith. The following model attempts to explain the sequence o f events th a t re su lte d in the rocks seen there today.

F irst, a single homogeneous magma was injected into the Creta ceous Eagle sandstone as a thick s ill (Griggs, 1939). The magma, shonkin itic in composition, already contained crystals of augite, biotite, olivine, and leucite. As it intruded, the magma absorbed water from the sandstone, a well-known aquifer in the area. Intrusion continued and the sediments gave way, arching upward as the s i l l developed in to a laccolith.

Where the hot magma came in contact with the cooler sediments, a chilled border formed. This border totally enveloped the laccolith, making it essentially a closed system. Inward from the chilled border, at both the top and bottom of the laccolith, a massive-textured shon kinite began to crystallize.

At some point, either by cooling, crystallization, or both, the magma reached an im m iscibility field. No longer able to exist stably as a single magma, it split into two fractions. Tiny felsic globules

73 74 formed, coalesced, and rose to the top of the magma chamber, forming a thick layer of fa irly homogeneous syenitic magma. A sharp interface separated syenitic magma from underlying shonkinitic magma. The two magmas continued to crystallize and felsic globules continued to sepa rate from the shonkinitic magma.

The shonkinite cooled, became increasingly viscous, and began to develop a rigid framework. Within the increasingly s tiff shonkinitic layer were local imperfections. Rifts in the framework due to contrac tion created areas of lower pressure where the syenite magma coalesced and escaped upward. These pipe-like bodies (Fig. 7) eventually crys tallized, along with the felsic globules that were trapped before reaching the syenite layer (Figs. 9 and 10). Thin sheets of syenite formed where local movement created horizontal breaks in the nearly solid shonkinite. Felsic globules from beneath the fracture coalesced in the narrow space provided, leaving the la y e r d ir e c tly beneath them darker and more mafic (Fig. 11).

The shonkinite, having a higher melting temperature, solidified before the syenite had totally crystallized. Thus solid shonkinite was in contact with syenite magma. The higher volatile content of the syenite magma and the increasing pressure of the entire system resulted in the syenite magma corroding the early-formed mafic crystals, leaving them embayed and eliminating the oscillatory zoning characteristic of the augite in oth e r la ye rs. The fe ls ic magma was o v e rla in by downward- crystal 1 izing syenite. As the syenite crystallized, volatiles and

"incompatible elements" such as U, Th, Be, and Li concentrated in the 75 residual liquid. Angular chunks of syenite broke o ff the roof and settled into the fluid. Small fractures and cavities within the syenite allowed volatiles to migrate and collect within them, later forming coarse-grained areas within the main syenite layer.

When the laccolith was almost solidified, tectonic forces began to gently bend it. At some point, the system failed and pressure was released, rapidly chilling the residual liquid. The high volatile content inhibited nucléation and the resulting crystals were large.

Augite and potassium feldspar were intimately intergrowh (Fig. 23) as were magnetite and biotite (Fig. 24).

This description of the genesis of the pegmatite layer is from

Barksdale (1952) who pointed out that the pegmatite layer is off- center and that structure contours show that the entire laccolith has been slightly warped. He also concluded that the large star-shaped augite-sanidine clusters must have cooled quickly since there is no evidence of any crystal settling in the pegmatite layer. Large, heavy mafic crystals would have been extremely susceptible to crystal set tling in a thin, watery pegmatitic fluid.

The origin of the narrow felsic dike!et in the pegmatite layer is not clear. Its chemical composition suggests that it differen tiated from the pegmatite (Figs. 27-40), and its fine-grained aplitic texture indicates a low water content, but the exact relationship be tween the d ik e le t and the pegmatite is uncertain. The irre g u la r biotite-rich segregations are also anomalous. Shonkinitic in composi tion, they could represent chunks of the upper shonkinite that broke 76 o ff and settled through the syenite magma. If so, their gradational

boundaries with the surrounding pegmatite suggests that they were partly

resorbed by the volatile-rich fluid.

The origin of the coarse-grained aegerine syenite that intrudes

the upper part of the syenite is also enigmatic. Like the felsic dikelet, its chemical composition suggests that it is a differentiate

from the syenite magma. Where this differentiation took place is un certain, but it may have moved to its present position when the lacco lith was being gently folded, or as a late surge of magma moved up from below, displacing parts of the laccolith that were s till molten.

Hurlbut (1939) suggested that a late surge of shonkinitic magma en tered the laccolith after differentiation had developed the syenite magma and caused the syenite magma to inject the already solidified shonkinite in some places. In one place, the late surge forced some of the syenitic magma to the extreme western edge of the laccolith, where it cooled to form a fla t dike about half a meter thick (Barks dale, 1952). Such movements could presumably account for the injec tion of the aegerine syenite as well.

After the laccolith crystallized, hot fluids flushed through the system. They may have been magmatic flu id s l e f t over a fte r c ry s ta l lization, or ground waters continuing to move through the Eagle sand stone. Whatever their source, they permeated the intrusion and caused extensive zeolitization of many parts of the laccolith. CHAPTER V III

DISCUSSION

While magma im m iscibility best explains the differentiation in place of syenite from shonkinite, several questions remain. The exis tence of the "hybrid" or gradational zone and the upward-increasing density in both shonkinite layers must be explained. These features are not evidence for or against liquid im m iscibility, but are compatible with the process.

The "h yb rid " zone lie s below the pegmatite and grades downward into the most mafic part of the lower shonkinite. The rock is mottled, with syenitic areas separated from shonkinitic areas (Fig. 17). Cor respondingly , its chemical composition is intermediate between syenite and shonkinite.

Possible explanations for the existence of the "hybrid" zone in c lu d e :

(1) Incomplete mixing of shonkinite and syenite magmas (proposed

by Barksdale, 1937).

I t seems u n lik e ly th a t such a process would re s u lt in such an even

distribution of shonkinite and syenite patches. If the two magmas

were miscible, the resulting intermediate rock would not have such

a mottled texture. If they were immiscible, but were somehov/

"stirred" together, the result should be streaks and swirls of

one rock in another rather than evenly-spaced patches of syenite

77 78 in shonkinite. In either case, magma mixing is inadequate to explain the existence of the "hybrid" zone.

(2) Partial re-equilibration of shonkinite and syenite.

If the system le ft the immiscibility field, whether by cooling or cooling plus crystallization, the rocks, s till retaining a great deal of heat, might exchange constituents across their boundary.

As elements diffused across the contact, the pegmatite layer would become somewhat more mafic than the syenite, and the upper parts of the shonkinite would become more felsic than the under ly in g sho nkinite . While th is mechanism might account fo r the bulk composition of the "hybrid" zone as well as its gradational con tact with the shonkinite below, it fails to explain the peculiar mottled texture.

(3) Felsic globules rising from the underlying mafic layer are

trapped in the "hybrid" zone before reaching the syenite layer,

This mechanism is the only one that explains the mottled texture of the rock as well as the gradational contact between the "hybrid" rock and the underlying mafic shonkinite. The mafic layer was that part of the shonkinite that cooled last, giving the felsic material more time to coalesce and rise to join the syenite layer. Early- formed globules rose to the shonkinite-syenite interface, leaving behind them a dense mafic liq u id th a t would c r y s ta lliz e a large proportion of augite. Later-formed globules, trying to escape an increasingly viscous shonkinite, were trapped before they escaped. 79

The mafic melt crystallized around them, and the resulting texture

shows irregular and rounded syenitic areas separated by shonkinitic

areas (Fig. 17).

The upward-increasing density in both shonkinite layers remains to be explained. Since it is obvious that differentiation in the lacco

lith v/as gravity-controlled, this observation seems surprising at firs t.

Hurl but (1939) finally attributed it to crystal settling by having mafic crystals fa ll from the base of the upper shonkinite, sink through the syenite magma, and settle at the top of the lower shonkinite layer.

This process would require that the crystals fe ll through a syenite

liquid not yet containing any mafic crystals. In a liquid immiscibility model, however, the syenite magma would probably contain crystals by this time- those that adhered to the felsic globules as they rose.

And since the augite c ry s ta ls o f the syenite are embayed, unzoned, and easily distinguished from those in the shonkinite layer, it seems un likely that crystals from the upper shonkinite could settle to the lower shonkinite while those in the syenite remained in place.

If the upper shonkinite did not lose mafic crystals, then perhaps it gained felsic material. This possibility seems more compatible with the magma im m iscibility model described above.

The upper shonkinite crystallized downward from the chilled zone at the roof and was a fa irly solid network by the time immiscibility set in. The mafic crystals had formed, and in the upper parts, the felsic liquid had cooled enough to crystallize as well. Farther down. 80 the upper shonkinite approached the syenite liquid that formed a layer flo a tin g a t the top o f the magma chamber. Here, the shonkinite was s till in a rather loose framework, and the felsic melt was s till trying to rise as far as possible. If some of the syenite magma forced its way into this network, displacing some of the less felsic residual liquid from the shonkinite, the eventual result would be a gradual downward increase in felsic minerals in the upper shonkinite. It could also account for some of the peculiar textures at this contact

(Fig. 21) and the splotchy or mottled appearance of the contact at the head of Tanner Canyon (Barksdale, 1937). As the syenite tried to rise, some of the lowest and structurally weakest parts of the lower shon kinite broke off from the main part of the roof to be replaced by syenite magma. This relationship, well exposed at the head of Tanner

Canyon, led Barksdale (1937) to call for separate injections of shon kinite and syenite. While that no longer seems necessary, it does seem obvious that syenite liquid was "intruding", though in a fairly passive manner, the base of the upper shonkinite, which was probably just a partly cohesive crystal mush at the time. It seems likely, then, that this process might also add felsic material to the base of this layer, and result in an upward increase in the proportion of mafic m inerals.

The upward increase of mafic minerals in the lower shonkinite is due to a different process. Here, felsic material was not added to the base, but was removed from the top. F ir s t, i t must be pointed out th a t the most mafic part of the upper shonkinite (the top) is of about the 81

same composition as the most felsic part of the lower shonkinite (the

base). This makes sense since both these areas are just slightly

inward from the chilled border and crystallized before any differentia

tio n had taken place. With increasing distance above the base o f the

laccolith, the lower shonkinite becomes darker and heavier due to the

increase in mafic minerals. Hurl but (1939) attributed this to the

addition of mafic minerals to the top of the layer. It could just as

well be produced by the removal of felsic material from the central,

hotter part of the laccolith.

The base of the lower shonkinite crystallized simultaneously

with the upper shonkinite and before the magma sp lit into two fractions.

Even after the magma reached the im m iscibility field, parts of the

shonkinite probably crystallized before the felsic globules had time

to coalesce and escape. Closer to the center, where the laccolith

remained hot longer, separation was complete, leaving the upper part

of the lower shonkinite more mafic than any other part of the lacco

lith , Crystallization eliminated some of the evidence for immiscible

separation, but preserved the unusual textures and syenite segregations within shonkinite. CHAPTER IX

SUMMARY AND CONCLUSIONS

The Shonkin Sag laccolith, with syenite overlying shonkinite, appears to have differentiated in place from a single magma whose com position was approximately that of the chilled border facies. The juxtaposition of a low density rock lying above a heavier, more mafic rock and separated by a horizontal contact suggests that the differen tia tio n mechanism was g ra v ity -c o n tro lle d .

The absence of rhythmic and cryptic layering and cumulate textures suggests that crystal settling was not responsible for the differentia tion of syenite from shonkinite. Chemical trends from trace-element and rare-earth element analyses are incompatible with a fractional crystal lization model. Liquid im m iscibility best explains the chemical trends, the rock textures and the field relationships. Immiscible syenite glo bules separated from the more mafic shonkinite magma, coalesced, and rose to the top of the magma chamber, to form a thick, homogeneous layer of syenite. Late-forming syenite globules were trapped within the shon k in ite as i t cooled and c ry s ta lliz e d around them. These are preserved as syenite segregations enclosed in the lower shonkinite.

The separation of two immiscible magmas accounts for the unusual trace-element pattern of the lower shonkinite which is enriched in incompatible elements such as lithium and beryllium and rare-earth elements relative to the syenite. It also accounts for the extremely

82 83

Sharp horizontal contact between syenite and shonkinite, and the lack o f intermediate rock types in the lacco lith .

Intrusions that have differentiated in place to produce felsic rocks overlying mafic rocks are not uncommon, especially in alkaline terranes. Daly (1914) listed more than seventy such intrusions, mostly s ills and laccoliths, from all over the world. Most were clearly gravity-controlled, and he suggested that many were the result of the gravitative separation of a pair of immiscible liquids. Though a wide range of rock sompositions are included, alkaline rocks are represented in a much higher proportion than their crustal abundance.

In recent years, experimental laboratory evidence of im m iscibility has been found within geologically reasonable compositions and temper atures in a wide range of silicate systems. Roedder (1979) lists over forty examples of recent literature in which liquid immiscibility is proposed as the differentiation mechanism in natural rocks. Here, too, a wide range of rock compositions are present, but alkaline systems are especially well represented.

Mafic and intermediate alkalic intrusions containing various kinds of syenite segregations are world-wide in occurrence (Tyrrell, 1928;

G illuly, 1927; Jahns, 1938; Pecora, 1941; Yagi, 1953; Wilkinson, 1958;

W ilshire, 1967; Phillpotts and Hodgson, 1968; Ferguson and Currie,

1971; Carman and others, 1975). Liquid im m iscibility has been proposed in several o f the more recent studies (P h ilp o tts and Hodgson, 1968;

Ferguson and Currie, 1971; Carman and others, 1975). 84

The Shonkin Sag laccolith has many of the characteristics of other bodies assumed to have differentiated by liquid immiscibility. It is a fa irly shallow alkaline intrusion with high K^O/total alkalies, and differentiated in a low-pressure environment. Its sim ilarity to the other laccoliths in the Highwood area indicates that these too must have d iffe re n tia te d by liq u id im m is c ib ility . This mechanism has been proposed for the Square Butte laccolith, just 10 km south of Shonkin

Sag, where large swirled streaks of syenite and shonkinite clearly indicate coexisting magmas (Kendrick, 1980).

Whether these types of intrusions are more susceptible to liquid im m iscibility or whether they are merely more effective at preserving evidence o f the process is not ye t cle a r. As Roedder (1979) noted, the process is such th a t preservation o f unambiguous evidence can be expected to be relatively exceptional, even if the process were common.

The operation of subcrustal im m iscibility to produce separate bodies of contrasting composition is d ifficu lt to prove. Studies of near-surface rocks containing both fractions combined with additional experimental work w ill be required before the petrologic significance of the process can be evaluated. 85

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APPENDIX I

Modes o f rock samples from Shonkin Sag la c c o lith

Chilled Zone

54% groundmass 22% pseudo!eucite 17% augite 6% olivine 1% apatite <1% carbonate, magnetite, serpentine

Syenite

50% sanidine 34% zeolites 15% augite 7% biotite 3% magnetite 1% apatite <1% olivine, aegerine, carbonate

Aegerine Syenite

40% zeolites 30% sanidine 10% bio tite 9% augite 8% aegerine 2% magnetite 1% a p a tite

Pel sic Pikelet in Pegmatite Layer

60% zeolites 30% sanidine 5% aegerine plus acmite 2% biotite 2% magnetite 1% barkevikite 92

B iotite-rich Segregation in Pegmatite Layer (Hurl bu t, 1939}

55% feldspar plus zeolites 21% augite 21% biotite 3% magnetite <1% sphene, apatite

Upper Shonkinite

30% augite 30% sanidine 25% zeolites 10% b iotite 3% magnetite 1% aegerine-augite 1% apatite <1% olivine

Lower Shonkinite

#106 7 m above base o f la c c o lith 50% felsic minerals (approx. 40% sanidine, 10% zeolites) 34% augite 10% biotite 2% magnetite 2% aegerine-augite 1% apatite 1% o liv in e

#116 48% felsic minerals 17 m above base (approx. 28% sanidine, 20% z e o lite s ) 40% augite 8% biotite 2% magnetite 1% apatite 1% o liv in e 93

#121 22 m above base 45% felsic minerals (approx. 25% sandine, 20% zeolites) 42% augite 8% biotite 3% magnetite 1% apatite 1% o liv in e

#131 32 m above base 45% augite 32% felsic minerals (approx. 18% sanidine, 14% zeolites) 15% b iotite 5% olivine 2% magnetite 1% apatite 1% aegerine-augite 94

APPENDIX I I

Chemical Analyses

S ix ty rock samples were chem ically analyzed by the Los Alamos

Scientific Laboratory. Values are tabulated on the following pages.

I. Analytical method - Neutron Activation Analysis

Accuracy - at concentration values one order of magnitude above

lower detection lim it, errors are generally <10%.

Note - Because of elemental interference, the detection lim its

for those elements determined by NAA w ill shift as a func

tion of the composition of the sample. Idealized values

are shown.

Minimum detection lim its (ppm): A1 (200); Ba (300); Ca (4000);

Ce (10): CO (2); Cr (20); Cs (2); Dy (2); Eu (.8); Fe (2000); Hf (1);

K (2000); La (6); Lu (.3); Mg (3000); Mn (10); Na (150); Rb (30);

Sc (.1); Sn (.5); Sr (300); Th (.8); Ti (200); V (5).

II. Analytical method - X-Ray Fluorescence

Accuracy - standard deviation approximately 10% at 100 ppm level

and 20% a t 20 ppm le v e l.

Minimum detection lim its (ppm): Nb (20); Ni (15); Pb (5).

III. Analytical method - Arc-source emission spectrography

Accuracy - Precision is approximately 50% at lower detection lim it

and 25% a t one order o f magnitude above i t .

Minimum detection (ppm): Be (1); Li (1). 95

IV. A n a ly tic a l method - Delayed-neutron detection ( fo r uranium on ly)

Accuracy - above the 1 ppm le v e l, uranium values have a one-

sigma e rro r o f less than 4%.

Minimum detection lim it is 1 ppb (part per billion).

Sample Location

All samples were collected in Tanner Canyon, on the southwest side of the laccolith.

Traverse 1 - from lower chilled zone up through syenite.

Sample Number Rock Type

190-193 c h ille d zone

194-219 lov/er shonkinite

218 "h yb rid zone"

220-222 pegmatite

223 F elsic d ik e le t

224-231 syenite

Traverse 2 - from upper shonkinite down to the top of the lower

shonkinite.

Sample Number Rock Type

232-236 upper shonkinite

237-244 syenite

245-246 felsic dikelet

247-249 pegmatite

250 lower shonkinite 96

Chemical Analysas (weight parts per m illion)

Sample Number U Cu Nb Ni Pb Be Li A1 6a Ca Ce

190 4.31 1220 20 5:0 27 50 300 583900 44750 56430 149 191 4.50 9200 210 410 13 60 340 567000 49570 62510 149 192 3.00 1180 20 580 20 50 310 471900 46460 66780 130 193 2.95 1090 20 550 29 50 350 604700 53330 53150 127 194 3.03 1150 20 670 23 60 220 582400 45230 60240 144 195 3.93 1250 20 720 30 70 104 615000 39810 55240 141 196 3.21 1040 20 610 17 50 320 585700 48320 57280 126 197 3.68 1070 20 680 IS 50 260 602700 41130 58100 122 198 4.14 1240 20 500 18 60 220 586900 40350 57080 147 199 4.03 1070 390 670 32 50 250 625400 43070 49910 139 200 4.53 1290 20 730 32 60 250 636200 43460 55.300 153 201 3.85 9900 330 370 16 50 240 5972C0 39020 55730 145 202 3.63 1390 290 640 24 60 290 640200 37520 56300 172 203 3.59 1030 20 760 18 40 290 595600 373cO 625.30 131 204 3. 72 8900 20 490 14 40 260 614700 38230 6184C 129 205 4.19 1150 240 830 22 50 260 603300 39140 56630 127 206 3.47 1050 440 590 16 50 370 5822GO 36270 63140 143 207 3.81 3100 200 620 17 40 290 641300 38590 59380 150 208 3.75 1160 340 320 22 40 320 602800 35410 . 63090 150 209 3.35 8700 260 410 90 30 340 570700 32220 66730 132 210 3.33 9000 280 510 10 40 400 525600 29240 69970 188 211 3.16 6300 510 620 11 40 270 544200 30640 67830 144 212 2.88 8400 210 820 22 40 310 535600 25160 76100 247 213 2.75 1050 20 580 22 40 360 527600 30040 71500 110 214 2.83 1090 20 '510 21 30 340 507300 22850 73790 160 215 2.74 8500 340 720 15 50 390 485200 24050 77960 115 216 3.2! 9000 20 850 12 40 360 502900 25120 73960 135 217 3.34 1010 350 700 21 40 350 484900 24130 74250 111 218 3.67 6100 20 910 '15 30 700 537200 25510 65640 177 219 2.97 1160 20 230 25 50 220 788500 51410 43130 144 220 9.16 3590 350 15 57 70 100 832C00 26220 29870 211 221 5.25 2070 530 16 25 60 340 675900 68430 51800 154 222 4.48 2480 320 15 49 60 280 739100 67530 • 42760 174 223 13.16 1140 300 15 73 80 800 103200 14670 10890 240 224 3,40 1130 430 15 30 30 230 861600 50350 35730 121 225 3.99 1330 20 160 37 40 160 834400 47820 34500 114 226 3.01 12&0 20 15 28 30 130 860700 44510 33680 113 227 3.66 1420 20 150 32 40 300 7.33100 95250 50120 136 228 2.70 8900 240 15 28 40 210 905900 51940 31170 940 229 3.81 1050 20 210 18 50 130 841300 48760 33840 760 230 3.93 1130 20 180 16 50 210 983100 53980 37070 131 231^__ 2.82 8ÛG0 20 15 24 40 140 890400 57620 33470 960 232 3.12 1020 20 230 25 40 240 842300 53580 42300 990 233 3.01 1230 20 410 22 50 240 640330 38440 59530 890 234 3.06 9300 300 640 30 40 250 674100 35260 52040 143 235 3.14 1210 20 670 15 30 250 617400 37490 58330 115 236 3.68 1210 20 200 31 40 200 7831-0 43860 42780 119 237 3.39 1140 20 340 26 50 200 781400 49860 39900 950 238 3.04 1:90 20 15 25 50 160 818300 46930 35420 115 239 3.91 1180 20 15 29 50 180 843600 48930 35620 119 240 3.00 1210 20 15 23 50 180 875700 55680 36280 116 241 3.42 1210 20 15 30 50 170 830100 46850 39110 970 242 3.37 1030 20 15 27 50 210 873100 51210 32150 120 243 3.25 1220 670 15 21 40 180 842500 49970 38020 113 244 2.63 1030 20 15 16 40 160 673700 51150 31920 102 245 11.35 6100 250 15 55 60 230 103600 17080 93660 132 246 11.80 4100 290 15 74 70 800 933600 13550 2736 177 247 5.14 1720 210 15 21 80 180 771900 10090 36600 140 248 5.79 1010 470 15 36 40 300 852400 70620 33180 173 249 3.26 1220 350 170 25 40 310 783500 62180 49070 109 250 3.64 1260 270 15 34 40 290 743400 47800 40740 134 97

Chemical Analyses (weight parts per m illion)

Sample Number Co Cr Cs Oy Eu Fe Hf K La Lu Mg

190 27.0 174 3.9 30 2.4 44270 5.0 33220 810 0.2 744600 191 27,8 169 1.6 40 2.2 42270 4.8 35930 840 0.2 787200 192 28.0 213 2.7 40 2.1 40570 4.4 33830 760 0,2 763200 193 24.1 129 4.0 30 2.0 35530 3.4 39940 670 0.2 651200 194 28.2 236 5.2 40 2.3 42950 4.4 44330 780 0.2 776000 195 27.2 203 3.4 40 2.1 39880 4.6 36980 860 0.4 752200 196 24.9 223 3.2 30 1.4 39160 4.0 39610 780 0.5 739000 197 20.7 169 2.4 30 1.7 32070 3.2 35390 620 0.4 677000 198 27.1 235 4.1 30 2.7 43750 4.7 41470 860 0.7 751400 199 24.1 202 2.3 30 2.7 43750 4.7 41470 860 0.7 751400 200 24.4 195 3.4 40 1.7 40070 5.0 39930 940 0.3 677400 201 22.1 172 2.4 40 2.1 32840 3.5 43460 830 0.2 669300 202 25.5 198 4.2 40 2.7 42060 4.8 39030 104 0.5 627600 203 25.5 219 3.2 40 2.3 33110 4.3 39090 800 0.2 695000 204 25.9 204 4.0 40 2.2 39040 4.4 43200 900 0.5 741600 205 20.5 165 2.2 40 2.1 32450 4.2 35340 840 0.2 630500 206 25.6 224 3.5 30 2.5 40200 4.3 42090 910 0.4 716400 207 25.5 193 3.6 40 2.2 38060 4.5 42590 900 0.3 684900 208 26.4 193 3.4 40 2.3 39850 4.0 37030 970 0..2 772900 209 22,3 136 2.8 40 2.2 32270 2.8 36510 750 0.2 714800 210 31.9 269 2.4 40 2.9 44160 4.1 36250 940 0.2 911900 211 29.6 260 2.8 50 2.4 40960 3.4 35160 890 0.2 083500 212 29.6 233 3.1 40 2.4 39290 3.3 30070 840 0.3 910500 213 24.1 193 2.5 40 1.9 31310 3.1 295C0 630 0.2 833700 214 33.1 251 2.4 40 2.9 44610 3,6 29360 800 0.2 925300 215 32.0 240 1.8 30 2.2 41110 3.5 25960 700 0.2 103400 216 32.6 211 2.0 40 ' 2, 3 42430 3.6 28510 840 0.3 106100 217 28.4 175 3.2 40 2.0 33650 3.0 24350 730 0.3 937100 218 35.8 132 12.4 40 2.7 47570 3.6 36280 940 0.2 935200 219 16.4 330 3.1 40 2.1 30990 4.1 49960 870 0.2 352000 220 12.6 80 1.5 40 2.2 39190 7.5 41460 131 0.4 129500 221 16.5 10 3.3 40 2.2 37140 4.3 35700 920 0.3 311000 222 17.1 13 3.9 30 2.7 42190 5.8 44560 119 0.4 306500 223 5.8 60 2.5 50 1.4 18170 11.3 39990 150 0.4 65120 224 13.6 180 3.4 30 1. 1 28030 3.5 54750 820 0.2 284000 225 11.4 210 3.1 40 1.6 21240 3.4 56890 730 0.2 255200 226 14.1 11 3.7 30 .2.1 32690 4.0 55100 660 0.2 273500 227 17.1 280 3.5 40 2.2 35410 4.0 49190 830 0.1 380900 228 13.9 230 3.9 30 1.7 29690 3.6 57730 580 0.4 271800 229 11.9 180 3.3 20 1.5 24300 4.2 54070 510 0-2 288200 230 16.1 280 3.8 30 2.2 34270 4.4 54640 690 0.2 279800 231____ 14.4 250 4.9 30 1.7 30120 3.4 60780 540 0.1 306300 232 16.3 460 4.3 30 1.8 32480 4.4 55030 640 0.2 372700 233 19.3 151 2.5 30 1.6 30040 3.0 38830 590 0.2 654800 234 24.3 196 3.2 20 2.2 39650 3.8 42410 740 0.2 572600 235 24.6 209 2.7 40 1.8 37680 3.5 38440 730 0.1 701500 236 16.9 710 4.4 30 .8 33950 3.8 52270 720 0.3 393900 237 14.4 520 3.6 30 1.7 26790 3.3 47190 560 0.2 390100 238 12.9 450 3.2 30 2.1 32620 4.1 43670 660 0.3 347700 239 14.5 310 4.0 30 1.8 29570 3.8 52620 650 0.3 297700 240 13.4 240 4.9 30 1.9 30640 3.7 53290 690 0.3 267800 241 11.5 180 3.8 30 1.5 24010 3.1 54580 470 0.1 333900 242 14.5 230 4.5 30 1.8 30030 3.9 55780 660 0.4 284700 243 14.7 300 4.2 30 1.6 30490 3.7 49530 600 0.2 335200 244 13.6 170 3.9 30 1.8 31100 4. 1 54580 650 0.4 290400 245 5.5 70 1.2 30 1.2 13810 7.2 45280 930 0.3 67370 246 3.5 80 0.9 40 1.5 16560 10.1 49160 133 0.4 71200 247 15.3 90 2.2 30 2.3 40140 4.8 43780 830 0.2 233100 248 15.4 90 2.3 50 2.2 31650 5.2 58210 109 0.5 230200 249 15.7 11 1.6 30 1.8 30500 3.5 41040 650 0.5 398200 250 18.1 540 1.9 40 2.2 37470 4.5 40780 830 0.4 395900 98

Chemical Analyses (weight parts per m illion)

S a m p le Number Mn Ma Rb Sc Sm Sr Th Ti V

190 1080 19680 106 21.3 9.4 1477 18.2 3605 1520 191 1117 18020 750 22.0 10.0 1785 17.5 4398 1720 192 1088 18170 990 24.7 9.4 1585 13.9 4402 1500 193 1127 16770 770 14.7 7.8 1459 13.2 4464 1470 194 1073 13820 830 23.0 9.0 2611 15.3 3983 1610 195 1979 19140 830 18.7 9.8 1793 16-8 3998 1510 196 1082 19500 930 21.0 9.9 1722 13.2 3709 1500 197 1060 21410 750 16.1 8.2 1577 11.9 3366 1550 198 1093 19400 890 22.1 11.9 1496 18.2 2995 1510 199 1045 20630 850 18.5 10.2 1653 16.5 3862 1580 200 1090 20170 860 18.0 12.3 1997 17.6 3701 1540 201 1120 18490 770 13.7 10.2 1644 14.9 3467 1600 202 9970 20070 113 18.2 9.9 1673 15.8 2517 1450 203 1024 18480 820 20.7 10.8 1544 15.0 3355 1570 204 1085 17720 970 20.9 10.3 1539 14.6 3341 1500 205 1032 17270 70C 15.1 9.6 1485 13.4 3990 1610 206 1008 16480 910 23.2 7.7 1418 13.5 3695 1510 207 1061 17940 750 19.0 10.4 2028 16.7 3874 1580 208 1039 17060 870 21.4 10.9 1721 14.5 2845 1540 209 1020 15750 780 18.0 9.7 1715 11.6 2895 1420 210 1096 14530 41 27.0 10.0 1165 12.5 3107 1580 211 1081 14940 570 23.0 11.7 1550 13.4 3259 1580 212 1071 15380 650 27.5 11.6 1333 13.5 3082 1360 213 1028 13870 510 22.8 6.9 1046 11.4 3266 1240 214 1157 14100 500 29.2 8.5 1097 12.7 2555 1340 215 1167 14830 620 30.0 9.6 1425 11.3 2644 1360 216 1204 15040 600 26.5 11.4 2033 13.0 2490 1400 217 1215 12630 24 20.7 , 9.8 1272 11.9 2678 1390 218 1198 12960 39 22.6 14.1 1856 15.7 2615 1470 219 8450 21450 102 11.4 9.6 1983 17.7 3879 1610 220 1100 35750 690 5.5 12.9 1929 35.5 5374 1890 221 . 1236 19480 700 10.8 10.6 2544 17,4 6848 2670 222 1034 21170 105 11.8 9.5 1815 20.2 4735 2510 223 7910 52600 710 1.0 13.4 458 65.2 107 4800 224 7730 22970 126 8.6 3.7 1929 14.6 3481 1440 225 7500 23210 103 6.7 7.1 1334 14.3 3729 1220 226 7450 26300 137 9.3 7.1 1498 13.8 3021 1410 227 8640 16110 980 11.4 10.9 2379 15.3 5046 1940 228 7140 26870 116 8.5 6.8 1660 12.1 3218 1430 229 7600 24090 820 7.3 5.6 1274 12.6 4386 1340 230 8060 20530 143 10.5 6.1 1706 16.2 3996 1600 231 7370 19720 1 ^ _7._1 1500 12.8 4390 1420 232 8170 21680 ~ ill ” l l ; 3 7.4 1503 13. 7 4115 1520 233 9880 16590 630 15.8 7,2 1787 12.0 3352 1510 234 9640 18430 102 20.2 7,3 1282 13.3 3671 1460 235 1014 16680 610 20.3 9.2 1703 12.6 3139 1530 236 8660 19710 115 11.7 8.9 1531 13.8 3866 1490 237 8370 19970 950 9.0 6.5 1491 10.5 3122 1460 238 7490 25840 128 9.9 7.4 1512 15.1 2842 1390 239 7550 23080 105 8.7 7.8 1727 16.0 3489 1430 240 7690 25320 133 8.9 8.7 1684 14.9 4066 1480 241 7510 21960 102 7.4 6.4 1761 12.8 3414 1340 242 7460 24620 121 8.9 7.6 1854 15.3 3535 1450 243 7790 25300 136 9.1 7.5 1447 14.6 4222 1510 244 7690 31360 120 9.0 7.5 1546 13.2 3646 1560 245 7060 48760 680 0.6 7.8 1046 39.8 114 2800 246 6650 49310 640 0.1 10.3 486 59.9 114 120 247 1049 26950 670 7.8 9.7 3040 18.3 5709 2250 248 8440 26000 1C6 5.9 11.5 2175 23.8 4937 1640 249 8710 24370 790 13.1 7.9 1-41 11.7 5497 1980 250 8400 26110 116 13,5 8.2 1658 15.0 3811 1680