Geology and Petrology of the Logan Intrusives of the Hungry Jack Lake Quadrangle, Cook County, Minnesota

Geology and petrology of the Logan intrusives of the Hungry Jack Lake Quadrangle, Cook County, Minnesota

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Authors Mathez, Edmond Albigese, 1946-

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/557306 GEOLOGY AND PETROLOGY OF THE LOGAN INTRUSIVES

OF THE HUNGRY JACK LAKE QUADRANGLE

COOK COUNTY, MINNESOTA

Edmond Albigese Mathez

A Thesis Submitted to the Faculty of the

DEPARTMENT OF GEOSCIENCES

In Partial Fulfillment of the Requirements For the Degree of

MASTER OF SCIENCE WITH A MAJOR IN GEOLOGY

In the Graduate College

THE UNIVERSITY OF ARIZONA

19 7 1 STATEMENT BY AUTHOR

This thesis has been submitted in partial fulfillment of re quirements for an advanced degree at The University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judg ment the proposed use of the material is in the interests of scholar ship. In all other instances , however, permission must be obtained from the author.

SIGNED:

APPROVAL BY THESIS DIRECTOR

This thesis h s b^fen approved on the date shown below:

i t / 9 7 / BERT E. NORDLIE Date Associate Professor of Geosciences ACKNOWLEDGMENTS

This writer is indebted to Dr. Bert Nordlie, his thesis advisor, for his inspiration and valuable suggestions during preparation of this manuscript; to Drs . Evans Mayo and Paul Damon for their review of this study; to Dr. Paul Weiblen of the University of Minnesota for his patient help with the electron microprobe; and to Dr. Glen Morey of the Minnesota Geological Survey for our helpful discussions during field work.

Grateful thanks is expressed to J. Huss, R. Mathez, J. Howard, and Brutus for their assistance in the field.

Financial support provided by the Minnesota Geological Survey and by The University of Arizona is gratefully acknowledged.

iii TABLE OF CONTENTS

. z ' - Page

LIST OF ILLUSTRATIONS „ . . „ ...... vi

LIST OF TABLES ...... ix

ABSTRACT...... x

INTRODUCTION ...... 1

Purpose and Scope of Study ...... 1 Methods of Study ...... 3 Previous Work . . , ...... 3 Extent and Definition of the Logan Intrusives ...... 4 Physiography ...... 5 Field Conditions and Accessibility ...... 7 .

REGIONAL GEOLOGY ...... 8

GEOLOGY OF THE HUNGRY JACK LAKE QUADRANGLE ...... 11

Rock Types ...... 11 Rove Formation ...... „ 11 Logan Intrusives ...... , . . , ...... 14 Duluth Complex ...... 15 Structural Relations of the Logan and Country Rock . . . . . 16 Dominant Structural Patterns...... 17 Subordinate Structural Patterns...... 22 Anomalous Intrusive Forms ...... 25 Structural Elements ...... 29 Basal Contact of the Duluth Complex . , . „ ...... 31 Field Relations ...... 31 The Contact Unit...... ; ...... 33 Pleistocene Geology ...... /..... 37

LOGAN DIABASES...... 40

Determinative andField Methods...... 40 Petrography ...... ^ . 41 Chilled Margins ...... 41 Diabase ...... 43 Porphyry Zones ...... , 48 Anorthosite Inclusions ...... 52 Granophyre ...... 54

iv V

TABLE OF CONTENTS—Continued

Page

Mineralogy ...... «...... 57 Pyroxene ...... 57 Plagioclase ...... 61 Opaque Oxides ...... - 62 Quartz and Potassium Feldspar...... 65 Alteration Products ...... 6 6 Chem istry...... 69

PETROLOGY ...... 71

The Porphyry Zones...... 71 G rout- 8 chwartz Hypothesis ...... 71 Calculation of Densities ...... 72 Discussion of Density Data . , ...... '. . 75 Nature of the Magma at the Time of Emplacement . . . . 80 Observations Pertinent to Petrogenesis ...... 80 Evaluation of the. Flotation Hypothesis ...... 81 Other Mechanisms ...... 83 Proposed Mechanism ...... 8 6 Crystallization ...... 87 Iron-titanium Oxides ...... 87 Poikilitic Pyroxene in Plagioclase ...... 8 8 Paragenesis ...... 89 Alkali Enrichment ...... 92 W ater Content of the Rose Lake L i q u i d ...... 94

SUMMARY AND CONCLUSIONS...... 96

APPENDIX: MODES OF ROSE LAKE DIABASES ...... 99

REFERENCES...... 106 LIST OF ILLUSTRATIONS

Figure Page

1.„ Index and Generalized Geologic Map of Cook County, Minnesota ...... „ „ .. . 2

2. Well-developed Saw-tooth Topography of the Rove-Logan Belt „ ...... 6

3. Linear East-west Lakes of the Rove-Logan Belt ..... 6

4. Index Map of a Portion of the Hungry Jack Lake Quadrangle Showing Lake and Section Locations . . . 12 .

5. Bedrock Geology of the Hungry Jack Lake Quadrangle, Cook County, Minnesota . . . . . in pocket

6 . Thin-bedded Graywackes of the Rove Formation Cropping Out below a Massive Diabase Sill...... 13

7 . Massive Graywacke Unit of the Rove Formation ..... 13

8 . Three Interpretations Allowed from Similar Outcrop Patterns ...... 18

9. The Flat Roof of a Sill ...... 20

10. Perspective Diagram and Sketch of the Present Land Surface at Rose Lake . . . , ...... 23

11.' Perspective Diagram and Sketch of the Present Land Surface of an Area between Clearwater and Daniels Lakes ...... 24

12. Correlated Cross Sections of Part of the Hungry Jack Lake Quadrangle ...... in pocket

13. The Daniels Lake Structure ...... 28

14. Contorted Rove Strata near the Roof of a Sill ...... 28

15. Contour Diagram of 126 Joints and Shear Zones Plotted on a Lower Hemisphere Schmidt N et ...... 32

16. A Sharp Contact between Anorthositic Gabbro of the Duluth Complex and a Gneissic Hornfels after Rove Rocks ...... 34 • v ii

LIST OF ILLUSTRATIONS—Continued

Figure Page

17. A Partially Rotated Concretion in Metamorphosed Rove . . 34

18. Photomicrograph Showing Two Generations of Plagioclase Characteristic of the Contact Unit . . . 36

19 . Map of the Hungry Jack Lake Quadrangle Showing the Distribution of Glacial Materials (shaded) . . . . 38

20. A Cross Section of the Wampus Lake Esker at the East End of Bearskin L a k e ...... 39

21. Photomicrograph of the Chilled Zone Illustrating Its Radial Texture ...... 42

22. Photomicrograph of Plagioclase Poikilitically Enclosing Augite (high relief) ...... 42

23. Schematic Textural Stratigraphy of the Rose Lake Sills . . . . „ ...... 45

24. Modal Variations of the Diabase of Sill "A" at Rose Lake ...... 46

25. Modal Variations of the Diabase of Sill "B" at Rose Lake ...... 47

26. Photomicrograph of Granophyrically Inter grown Quartz and Orthoclase Surrounding a Euhedral Plagioclase ...... 49

27. The Sharp Contact of the Diabase Porphyry Zone and Diabase above It, Sill "A" ...... 51

28. Anorthosite Inclusion (white) Embedded in the Chilled Roof of the Moss Lake Intrusion ...... 53

29 . Photomicrograph Illustrating the Typical Flower-petal Texture of Granophyre ...... 55

30. An Angular Block of Fine-grained Diabase Included in Granophyre...... 56

31. Pyroxene Compositions of Rose Lake Sill "A" as Determined with the Electron Microprobe...... 58

3.2. Compositional Variation of Plagioclase and Pyroxene through Sill "A" as Determined with the Electron Microprobe ...... 59 v iii

LIST OF ILLUSTRATIONS—Continued

Figure Page

33 . Photomicrograph of the Undulatory of Hourglass Extinction Typical of Many Pyroxenes from the Rose Lake Sills . . . . . „ ...... 60

34. Traverses across Zoned Outer Segments of Plagio- clase Phenocrysts from the Porphyry Zones ..... 63

35. Photomicrographs of Skeletal Iron-titanium Oxide Minerals . . . . . „ ...... 64

36. Photomicrograph of Circumferential Sericite Zone , (light) Surrounded by a Recrystallized Armor, of Sodic Plagioclase (at extinction) of a Plagioclase Phenocryst of the Porphyry Zone of Sill "A" ..... 67

37. Photomicrograph of an Alteration Patch ...... 67,

38. A Comparison of Densities of Various Mafic Liquids and Plagioclase in the Range AngQ-gQ at Elevated Temperatures ...... 73

39. Calculated Densities of Plagioclases at Various Temperatures ...... „ . . . „ 76

40. Phase Relations of the System CaAl 2 Si2 0 g- Mg2Si04-Fe3 0 4 - 8 1 6 2 in the Plane 50 Weight PercentCaAl2Si20g at PO 2 = 10""0 .7 ^ m ...... 90

41. An Irregular Surface of Liquid Compositions Saturated with Anorthite and 6 ne or More Other Crystalline Phases Projected onto a Plane . . . 91 LIST OF TABLES

Table Page

1. Precambrian Stratigraphy and Chronology of Rocks of Cook County, Minnesota, and Nearby Regions . . 10

2. Chemistry of the Rose Lake Diabase and Some Diabases of Northeastern Minnesota and Neighboring Ontario ...... 70

3. Chemical Compositions of the Rocks of Figure 38 .... 74

4. Estimated Water Content of the Rose Lake Liquid, in Weight Percent ...... 95

ix ABSTRACT

Detailed mapping in the Hungry Jack Lake quadrangle shows that Keweenawan Logan diabases are generally.concordant bodies; ir regular intrusive forms and thickening, thinning, or pinching out of sills along strike and along dip complicate this simple pattern.

The primary mineralogy of the diabase includes plagioclase

(An4 5 _5 Q) in a subophitic to diabasic relation with augite. Skeletal iron-titanium oxides and interstitial granophyre are common.

Two thin sills having 3-foot plagioclase porphyry zones near their roofs were studied in detail; differentiation mechanisms responsible for such zones are analyzed. Calculated cooling and s ettling rates show that gravitational separation of plagioclase can account for many por phyry zones. Whether plagioclase floats or sinks is mainly dependent on the water content of the magma since their densities are similar.

The mechanism is dependent on the relative times of appearance of the solid phases in the magma.

Neither the calculated density relationship between plagio clase and melt nor texture of the porphyry zones of these sills supports flotation. Flow differentiation and multiple intrusion are also ruled out.

A new mechanism proposes that the highest viscosity liquid enters the widening conduit last; since sills thicken by uplift of their roofs, the phenocryst-rich, more viscous melt is emplaced above phenocryst-free melt.

x INTRODUCTION

. The Logan Intrusives are a series of Keweenawan diabases and. related rocks injected into the Animikie sedimentary rocks of the Rove

Formation. In Minnesota, both rock types crop out in a narrow east- west belt located between a huge lopolith, the Duluth Complex, and the international border. They extend northward and eastward to Thun der Bay, Ontario. The Hungry Jack Lake quadrangle is in Cook County, lies within this east-west belt, and includes the international border in the north and Duluth Complex in the south (fig. 1) .

Purpose and Scope of Study

This study was undertaken to detail the structural relations of the Logan Intrusives and their host rock and to describe their intrusive form. A second objective was the investigation of the petrology of the

Logan diabases , with particular emphasis placed on the study of the diabase porphyry zones often characteristic of individual intrusions.

At the suggestion of the Minnesota Geological Survey , the

Hungry Jack Lake quadrangle was chosen for field study because the structural relations of its rocks were thought to be more complex and less well understood than they are in other areas. In addition, the quadrangle immediately to the west, having been recently mapped, of fers some geologic control; and, unlike other areas where the Logan crops out, the Hungry Jack Lake quadrangle is easily accessible. 2

CANADA NIPIGON

THUNDER

MINNESOTA

i X MICHIGAN WISCONSIN

CANADA ^SaganagaZ/^ Gunflint Iron-Formation Rove Formation and /L o g a n Intrusives

Knife Lake f r Hungry Jack LakeX , ~^=. = X X ~ - T - - )

VARAIS

Scale in miles .1 -.;zr— 10 20 30

Figure 1. Index and Generalized Geologic Map of Cook County, M innesota

The index map (above) shows the axis of the Lake Superior syncline (Halls, 1966) . The location of the Hungry Jack Lake quadrangle is indicated on the geologic map (below) (Goldich et a l . , 1961). Methods of Study

Field work was carried out during the summer months of 1969 and August 1970. Traverses were made at 1 ,000-foot intervals across strike and contacts were traced where possible and practical. Diabases cropping out over the entire quadrangle were sampled, and some were subjected to petrographic analyses . In addition, two thin sills, in which the porphyry zones exist, were chosen for microprobe and de tailed petrographic analyses; samples were collected at 5 -foot intervals through these sills . The methodology is described in greater detail in subsequent pages.

Previous Work

The Logan Intrusives have been studied at their classic locality.

Pigeon Point ( which Juts into Lake Superior at the Minnesota-Ontario border), by Bayley (1893), Daly (1917), Grout (1928), and Grout and

Schwartz (1933). The last paper reports on the entire Rove-Logan belt in Minnesota. An older report, that of Lawson (1893), deals with Logan rocks where they crop out near Lake Superior in Minnesota and Ontario.

The diabases and their acidic differentiates in Ontario are the subjects of a study by Blackadar (1956), who presents valuable chemical data.

Goldich et al. (1961) give radiometric dates for rocks from the Precam- b'rian terrane of Minnesota and neighboring Ontario, including one sup posedly representative of the Logan. Morey's (1969) study of the Rove

Formation and that of Grout, Sharp, and Schwartz (1959) of the general geology of Cook County, Minnesota, include the area of this paper.

The Duluth Complex in the Hungry Jack Lake quadrangle has been mapped by Nathan (1967, 1969). The geology of the neighboring regions of On

tario is shown on the maps of Tanton (1931) and Pye and Fenwick (1963 ,

1965).

Extent and Definition of the Logan Intrusives

The intrusive character of the diabases was first noted by

Lawson in 1893 (Grout et a l., 1959), who proposed the term "Logan

Sills , " in honor of the pioneering Canadian geologist, Sir William E.

Logan. It has been pointed out (Grout et al, 1959, p. 50), however, that the term "Logan Intrusives" is more appropriate since the intrusions are commonly in the form of dikes or irregular bodies as well as sills.

As originally defined by Lawson (1893), the name was restricted to diabases arid related rocks that intrude the Rove Formation. This is an arbitrary definition, however, because host rocks in the vicinity also include the Gunflint Iron-Formation and various Keweenawan volcanic rocks, although their distribution is limited. Nevertheless , the Logan

Intrusives are distinguishable on a geographic basis, although perhaps not genetically, from other Keweenawan diabases of the region. They include the diabases of the Beaver Bay Complex; those of the thick and well-studied End!on, Northland, and Lester River sills near Duluth; the numerous and small diabase bodies intrusive into the North Shore Vol canic Group; and the diabases that are found in. the vicinity and north of Nipigon, Ontario.

From a more general viewpoint, all the Lake Superior diabases fall within a Keweenawan quartz diabase province first defined by

Phemister (1937) . The province includes not only the periphery of Lake 5

Superior but all of southern Ontario.between the Quebec and Manitoba borders, a distance of some 700 miles.

Physiography

Since the western part of the Rove-Logan. belt is mainly under lain by gently south-dipping sills intruded into less resistant country rock, saw-tooth topography is well developed there (fig. 2). The east- west strike of the rocks is reflected by linear east-west ridges and in tervening lakes (fig. 3). In the more easterly regions of the belt, near

Lake Superior, dikes as. well as sills are important and confuse topo graphic trends . In Canada, particularly near the city of Thunder Bay, relatively flat regional dips (Pye and Fenwick, 1965) result in large diabase-capped, mesa-like structures . One such structure, Mt. McKay, rises 1,000 feet above the city.

Within the Hungry Jack Lake quadrangle, the highest ridges are approximately 2,000 feet above sea level, making this area among the highest in Minnesota. Elevations of most of the lakes are on the order of 1,700 feet, although the level of Rose Lake is 1,520 feet. Be cause of its rugged relief, this region is one of the most scenic in

M innesota.

The contact of the Duluth Complex with Rove-Logan rocks is found in the quadrangle and marks the boundary of two distinctly dif ferent physiographic provinces . In contrast to the saw-tooth topography to the north, the Complex underlies a plateau-like region with little re lief, except for occasional knobs and ridges marking the granophyres of the Complex. A poorly developed topographic grain is characterized by 6

Figure 2. Well-developed Saw-tooth Topography of the Rove- Logan Belt

The view is east-northeast from the south shore of Rose Lake (sec. 22, T. 65 N ., R. 1 W.) .

Figure 3 . Linear East-west Lakes of the Rove-Logan Belt

The view is due east; in the lower right is Wampus Lake (sec. 2, T. 64 N., R. 1 W.) . rather indistinct east-west hills which reflect the strike of various gab-

bro units and also a well-developed joint set. Since the "plateau" is

near the level of the diabase ridges but above areas underlain by the

Rover Formation, the gabbro contact is often marked by a north-facing

slope.

Field Conditions and Accessibility

Unlike the more isolated quadrangles to the east, even the

furthest reaches of the Hungry Jack Lake quadrangle are accessible

within several hours by canoe, the most practical means of transporta

tion. Several abandoned railroad grades, relics of former logging opera

tions, improve accessibility.

Depending on one's point of view, the main obstacles to field

work are dense brush, heavy June rains, and insects. The first, in par

ticular, hampers the location of outcrops, since it is not uncommon to

,become entangled in a deep forest where lateral visibility in any direc

tion may be restricted to some tens of feet. REGIONAL GEOLOGY

The oldest rocks of Cook County are the Ely Greenstones,. metamorphosed basic lavas of the early Precambrian (Goldich et a l.,

1961). They were intruded during Laurentian time by an elliptical batho- lith, the Saganaga Granite (Grout et a l., 1959), located in the far northest corner of the county. These rocks are unconformably overlain by the fine to coarse elastics of the Knife Lake Group.

The hiatus separating the early Precambrian from younger rocks represents the Algoman orogeny, which has been assigned a 2.5 b.y. age (Goldich et a l., 1961). The orogeny coincides with the formation of voluminous bodies of granitic rocks which underlie large areas west and north of Cook County; the Vermilion and Giants Range Granites are examples . However, in the county itself the only result was deforma tion and metamorphism of Knife Lake and older ro ck s.

The middle Precambrian Animikie Group unconformably overlies older rocks . It is comprised, in Cook County and Ontario, of the thin basal Pokegama Quartzite, the Gunflint Iron-Formation (radiometrically dated at 1.9 b.y. by.Hurley et a l., 1962) and the argillites and graywackes of the Rove Formation. Although geographically separated from them by the younger Duluth Complex, they are lithologic equivalents of the Pokegama, Biwabik, and Virginia Formations, respectively, of the

M esabi Range (Grout et al . , 1959).

Deposition of Animikean sediments ceased with the Penokean orogeny which, although more intense further south, resulted in a small angular unconformity between Animikean and Keweenawan rocks in Cook

County and neighboring Canada (Morey, 1969) . Early Keweenawan sedi

mentary rocks are restricted to a thin sequence of coarse elastics of the

Puckwunge Formation (or Sibley Series in Canada).

Middle Keweenawan igneous rocks include, from oldest to youngest, (1) the North Shore Volcanic Group, a thick sequence com posed dominantly of basalt with subordinate amounts of more acidic materials and correlative with the Portage Lake lavas of the Keweenaw

Peninsula, Michigan; (2) numerous hypabyssal diabasic bodies, which include the Logan Intrusives; and (3) the Duluth Complex, a multiple intrusion of a wide variety of gabbroic and related rocks (Taylor, 1964).

Although field relations in other areas suggest that ages of the diabases may overlap those of the volcanics and Duluth Complex to some extent

(Halls, 1966), in northern Cook County numerous sills and dikes cut lavas of the North Shore Volcanic Group, and the Duluth Complex meta morphosed and exhibits intrusive relations to both. In any event, both radiometric data (Silver and Green, 1963; Faure, Chaudhuri, and Fenton,

1969; Goldich et a l., 1961, 1966) and paleomagnetic data (Beck, 1970;

DuBois, 1962) indicate that ages of all Keweenawan igneous rocks clus ter about 1.1 b .y . When one considers (1) the extent of Phemister's

(1937) Precambrian diabase province and the probability of its contin uance beneath Paleozoic cover of the central states, (2) that the Duluth

Complex is 150 miles long, reaches a maximum width of 30 miles and may extend a considerable distance beneath Lake Superior, (3) that the volume of the Keweenawan lavas "must exceed 100,000 cubic miles"

(White, 1960), and (4) that the southward extension of the mid-continental 10 gravity high to Kansas suggests Keweenawan rocks extend that far south

also (Halls, 1966) , it becomes apparent that the region is one of the

major continental igneous provinces of the world and was of major tec

tonic importance 1.1 b .y . ago.

The reader is referred to the generalized geologic map of Cook

County (fig. 1) . Table 1 summarizes the Precambrian stratigraphy and

chronology. Excepting that portion represented by minor Pleistocene

materials , the geologic record of the last billion years is nonexistent

in Cook County.

Table 1. Precambrian Stratigraphy and Chronology of Rocks of Cook County, Minnesota, and Nearby Regions. Reproduced from Morey, 1969, p. 7.

Era Period-System Major Sequence Formation Intrusive Rocks

(1.1 b .y .) Duluth Complex Logan Intrus ives North Shore undivided Volcanic Grp Late Keweenawan Precambrian -discontinuity ? - Puckwunge - unconformity ----- (1.7 b .y.) Rove M iddle Huronian (?) Animikie Grp Precambrian Gunflint Pokegama ------unconformity (2.5 b .y.) "Algoman" Granites Timiskamian Knife Lake Grp undivided Early ------unconformity Precambrian Saganaga Granite Ontarian Keewatin Grp Ely Green stone GEOLOGY OF THE HUNGRY JACK LAKE QUADRANGLE

Precambrian rocks which crop out in the quadrangle and struc tural relations among them are described below. The reader should refer to the index map (fig. 4) for section numbers and lake names and to the geologic map accompanying this report (fig. 5, in pocket).

Rock Types

Rove Formation

According to Morey's (1969) study the 3 , 200-foot thick section of Rove includes, in stratigraphic order, 400 feet of basal argillite, 70-

100 feet of interbedded argillites and graywackes, and 2 ,700 feet of thin-bedded graywackes . Portions of each unit exist in the Hungry Jack

Lake quadrangle (Morey, oral communication). However, since both the argillites and most of the graywackes are dark gray to black, are ex tremely thin bedded and friable, and have nearly identical textures, they are almost impossible to distinguish in the field. Their general and most common character can be seen in figure 6 . The only immediate-^ ly distinguishable unit is a massive graywacke (fig. 7). Because of the poor and infrequent exposures of Rove throughout the quadrangle, the unit is traceable only along the south shore of Daniels Lake in secs .

35 and 36 .

Intraformational conglomerates, quartzites, and carbonates have been reported (Morey, 1969), along with carbonate concretions

■ 50 29 2# 27 26 30

DUNCAN LAKE FIG .11 o

l a k e

T63N T65N T64N T 64N

HUNGRY JACK LAKE \UPUS

LAKE

POPLAR LAKE

SCALE

Figure 4. Index Map of a Portion of the Hungry Jack Lake Quadrangle Showing Lake and Section Locations 13

Figure 6 . Thin-bedded Graywackes of the Rove Formation Crop ping Out below a Massive Diabase Sill

NW1/4NE1/4 sec. 27, T. 65 N. , R. 1 W.

Figure 7. Massive Graywacke Unit of the Rove Formation

Tree in upper right is about 12 inches in diameter. SW1/4 NW1/4 sec. 36, T. 65 N., R. 1 W. 14 and various primary s edimentary structures .. Within the Hungry Jack Lake

quadrangle ripple marks and small cross-beds can often be seen. The

presence of what apparently is a metamorphpsed quartzite lens has been

noted in sec. 6 , T. 64 N., R. 1 W ., at the gabbro contact, and.carbon

ate concretions have been found at the base of the high cliff on the east

end of Hungry Jack Lake and at the gabbro contact several.hundred feet west of the road to Clearwater Lake in sec. 10,T.64N.,R. 1 W.

Logan Intrusives

The relatively simple primary mineralogy of the diabases in cludes plagioclase (ave. = An^g) in a diabasic to subophitic relation with augite. These comprise, respectively, 40 to 50 and 30 to 45 per cent of the unaltered rock. The remainder includes magnetite and ilmenite, interstitial granophyric quartz and orthoclase and minor accessories.

Olivine has only rarely been noted in hand specimen. Commonly, in tense deuteric alteration severely complicates this simple picture.

On a weathered surface, kaolinitized (Grout and Schwartz,

1933) plagioclases are. white, and a rock which is only moderately weathered may assume a splotchy, brownish appearance. More intense weathering produces an irregular surface since the feldspars weather more quickly than mafic constituents. A long-exposed surface usually appears spheroidal and breaks down to a coarse, gravelly, brown soil.

The most frequently occurring rock is true diabase with the texture and mineralogy described above. Near the roofs of thicker sills this grades into more intermediate rock with conspicuous amounts of red feldspar and, probably, quartz. Other varieties include ( 1) an extremely 15

fine grained zone at the chilled margins of the intrusion; ( 2) a chilled

zone that is slightly porphyritic, the phenocrysts being plagioclase;

(3) a diabase porphyry zone having abundant phenocrysts of plagioclase

but which is unrelated to the porphyritic chilled zones on the basis of

occurrence; (4) anorthosites, which occur as megacrysts or xenoliths

in some sills; and (5) granophyre, which is always later than, although

perhaps genetically related to, the diabase. The latter has only a

limited distribution, cropping out south of Ruby and Rudy Lakes and

occasionally as dikelets near the gabbro contact.

Porphyritic chilled zones seem to be restricted to particular

sills . Thus, they may be of potential use in the correlation of sills

along strike where outcrop is lacking or structural complications exist.

Only rarely do any of the Logan rocks exhibit lineation or

foliation, and when foliation is seen it is always faint and parallels the general attitude of the sill. Foliation in diabase porphyry zones has

been reported elsewhere (Grout and Schwartz, 1933), but it has not been

noted in the Hungry Jack Lake quadrangle despite the typical brick-shape

of plagioclase phenocrysts.

A later section treats the petrology and petrography of the

Logan Intrusives in detail. For a description of the intrusives where they crop out in other areas, the reader is referred to Grout and Schwartz

(1933).

Duluth Complex

Rocks of the Duluth Complex underlie approximately the south

ern third of the quadrangle. The basal contact can be found near the 16

south.shore of East Bearskin Lake and short distances; north of the Gun-

flint Trail further westward. The basal contact of the gabbro with

country rocks is discussed briefly in a later section.

It has been the experience of this writer that the rocks of the

Duluth Complex are extremely variable, and vast areas of the Complex are covered by glacial materials or thick forest growths. Much of the southern portion of the Hungry Jack Lake quadrangle is covered by swamp

Or glacial drift. The rocks are mostly gabbroic; that they are hetero geneous is shown by Nathan (1969), who mapped this area. The gabbros typically contain plagioclase and clinopyroxene; the main variants are iron-titanium oxides , olivines , or other mafic constituents . Subordinate amounts of granophyre are also present. . .

Structural Relations of the Logan and Country Rock

Other workers have contended (Grout and Schwartz, 1933) that the topography of northern Cook County is a rather accurate reflection of even subtle differences in bedrock geology. Pleistocene glacial erosion or deposition is of only local importance in this respect. The contention is supported by this worker's experience in the Hungry Jack

Lake quadrangle.

Here, and indeed this is true in vast areas of the Canadian

Shield, outcrop is locally abundant, but large areas are covered by thin veneers of forest or swamp-derived soils or glacial debris and may fur nish only isolated exposures of bedrock. In the construction of the geologic map accompanying this report (fig. 5), regions in which obser-. vational data of bedrock are lacking required considerable inference -v - /: ' ' z 17 first from topographic patterns apparent in the field and on aerial photo graphs and second from structural patterns established elsewhere. It is pertient, then, to devote sOme discussion to the latter.

Regions of igneous terrane present special complications be cause the forms of igneous intrusions are not necessarily bound by geometrical constraints applied successfully, for example, to folded regions. Carey (1958) emphasizes that simple outcrop patterns of ig neous terranes may support a number of interpretations (fig. 8). The

Hungry Jack Lake quadrangle is suspect in this regard because certain areas, particularly those east and south of Duncan Lake, do not reflect the well-developed east-west topographic trends characteristic of neighboring regions .

The sill-like nature of most of the Logan Intrusives has been demonstrated by the detailed mapping of Morey (1969) in the South Lake quadrangle immediately to the west of the Hungry Jack Lake quadrangle and by the reconnaissance mapping of the entire region by Grout and his associates. This is also,the case in the Hungry Jack Lake quadrangle.

In addition, thickening, thinning, and termination of the sills is a fre quent occurrence both along strike as well as up and down dip. Obser vations justifying these conclusion and the interpretations of areas ex hibiting anomalous structural patterns follow.

Dominant Structural. Patterns

Typically, the pattern found traversing across strike is dia grammed in cross section in fig. 8. The gentle, south-facing slopes correspond to dip slopes, and diabase-country rock contacts are 18

(a)

(b)

Figure 8. Three Interpretations Allowed from Similar Outcrop Patterns

The country rock is indicated by a dashed pattern, (a) Shows the typical cross section of sills in the Hungry Lake quadrangle. Where two sills are found in the same ridge, the north slope (right) acquires a steplike topography. After Carey (1958) . . "■ ' . ' . : . ' ■■■"• ■ . 19 commonly exposed on their lower half. Progressively deeper levels of

the sill are exposed further northward and crop out in a north-facing

cliff or ledge at the ridge crest. Only rarely is Rove strata exposed at

the base of the ledge since talus usually covers the remainder of the

slope. Small diabase sills or other intrusives are, no doubt, buried

beneath the talus and not mapped. It has been noted, however, that

in at least two localities (near the south shore of Daniels Lake in sec.

25 and of Rose Lake in sec. 22) sills thicker than some tens of feet

generally result in a topographic expression; that is, the north^facing

slopes assume a steplike pattern.

The following observations show that the diabase intrusions

. are generally concordant;

1. Usually, the upper contacts of Sills exposed on dip slopes

are either perfectly flat or slightly irregular (fig. 9). Chilled

diabase, usually with a thin veneer of hornfels, crops out on

these surfaces.

2. Contact exposures of the sill roofs maintain approximately

constant elevations, occasionally for long distances . For

example, a contact is exposed for over a mile in intermittent,

wave-washed outcrops along the north shore of Daniels Lake

(secs . 25 and 26).

3. Where the Rove is exposed at the base of Sills, its bedding is

closely parallel to the supposed attitude of the overlying sill

as inferred from topography and the upper contacts.

4. Certain localities afford exception three-dimensional views of

the sill form. These include the west shore of Rose Lake in 20

Figure 9. The Flat Roof of a Sill

The surface is capped by a thin veneer of hornfels . Sec. 22, T. 65 R. 1 W. 21 sec. 22, the south shore of Flour Lake in sec. 1, and the

small northern peninsula of sec. 21 into Rose Lake.

Strike and dip measurements on the Rove are generally within

20 degrees of east-west and range from 5 to 20 degrees south, respec tively . Dips near the Duluth Complex contact are ordinarily on the order of 40 degrees .

The broader ridges and lakes of the northern part of the quad rangle reflect the greater thickness of the sills underlying them. For example, a thickness of about 1,000 feet is likely for the sill separat ing Rose, and Daniels Lakes, In contrast, thin sills crop out in the southeast portipn of the map area. These result in small, closely spaced ridges which are not apparent on the 1:24,000 topographic base map. The area immediately south of Aspen Lake is characteristic; here, the main evidence for the presence of several thin sills is a number of small ledges or flat outcrops of chilled diabase found along traverses across strike. On the small ridge of the southwest side of a small beaver pond between Ruby and East Bearskin Lakes (sec. 7, T. 64 N . ,

R. 1 E.) four individual sills crop out. Three of these make up a 40- foot ledge, the south side of which is capped by a thin belt, of Rove with a sill above it. The uppermost sill can be followed as a single unit westward to Aspen Lake. The three lower sills (the thinnest is 8 feet thick) collectively comprise a steep, north-facing slope which be comes the ridge south of Rudy Lake (sec. 12). Whether or not the sills prevail as individual units westward along strike is not known. Their small size requires that they be shown as a single unit on the map. Subordinate Structural Patterns

A subordinate pattern of thickening, thinning, and termination of sills is established by occurrences in numerous localities. Several examples are described.

Exceptionally good exposures near the west shore of Rose Lake lead to the interpretation shown in fig. 10. The base of the southern most sill is more or less continuously exposed on the large ridge in the foreground of the diagram. Bedding measurements in the Rove beneath the sill suggest an irregular attitude. This Rove belt can be traced westward to the small beaver pond in the southwest comer of the dia- . gram. It apparently wraps around the base of the knob just north of the pond and crops out on its north face. Followed westward, the Rove ter minates about 1,200 feet into sec. 21. However, it should be noted that in this valley the Rove belt is no longer floored by the same sill.

The south-facing slope above sill "A" corresponds to that sill's upper contact. But in the small valley between sills "A" and "B" Rove rocks are absent except near Rose Lake. Such a situation is accomplished if

(1) the two sills are separated by a wedge of Rove which, in part, ter minates down-dlp at approximately the present position of the valley, and (2) sill "B" wedges out up-dip, as shown by the dotted lines illus trating its hypothetical extension above the present erosional surface.

A somewhat analogous interpretation is accorded to the area between Daniels and Clearwater Lakes (fig. 11). The sills in the fore ground make up the ridge on the south side of Daniels Lake and pinch out westward along strike. This situation is emphasized by the fact Figure 10. Perspective Diagram and Sketch of the Present Land Surface at Rose Lake Reference to figure 4 shows its location. The block is 4,000 feet on a side. The surface distribution of the Rove is shown by dashed lines; the dotted lines show the hypothetical extension of diabase above the present land surface; and the dashed pattern of the cross sections is the country rock. Vi

Figure 11. Perspective Diagram and Sketch of the Present Land Surface of an Area between Clearwater and Daniels Lakes

Reference to figure 4 shows its location. The block is 4,000 feet deep and 3,000 feet wide. The dashed pattern of the cross sections is the country rock.

to that their strike is at an acute angle with that of the sill underlying the

ridge in the background.

. In another case, the basal contact of the sill underlying the

broad peninsula in the west end of Bearskin Lake is exposed for a short distance near the,north tip of the peninsula „ Its upper contact approxi

mately coincides with the southern shore of the peninsula; in all prob

ability the sill dies out eastward beneath the lake. The pattern is

similar at the east end of Hungry Jack Lake, where the Rove strata

interfinger with thin sills at the lake shore but taper out eastward.

The necessity for establishing the above, patterns is in their application to areas such as the southeast quarter of the map. Here, numerous thin sills complicate the geology and cover limits direct ob

servation. The reader is referred to the set of cross sections con

structed at 1,000-foot intervals through this area (fig. 12, in pocket).

Anomalous Intrusive Forms

Deviations from a strictly concordant form are accurately re flected by anomalous topographic patterns and are common in the Hungry

Jack Lake quadrangle. The most easily documented case is that found on the diabase ridge northeast of Moss Lake (sec. 33), here referred to as the Moss Lake structure. At the far eastern end of Moss Lake, the southern contact of diabase and Rove appears irregular in several small outcrops near the level of the lake. The south slope of the ridge here is characterized by a series of large ledges arranged in steplike fashion and composed of fine-grained, chilled diabase and hornfels .

One hundred thirty feet above the lake there are large, flat outcrops in 26 which the roof of the intrusion is exposed „ Attitudes of hornfelses cap ping these outcrops are characterized by gentle south to south-

southwest dips. To the northwest, the ledges resolve themselves into a 30-foot cliff, again of predominantly chilled diabase or varieties known to be near the contact. Further northwestward, at the Moss-

Duncan Lake portage, erosion and a fault result in a cut that provides excellent exposure of the interior of the intrusion. Chilled diabase in a 20^foot vertical cliff marks its southern wall. On the north side the contact rises vertically 3 feet above a cover of talus and then assumes a concordant position with the gently dipping Rove strata. The contact can be traced 150 feet westward, maintaining its concordance. These observations leave little doubt that in the Moss Lake structure diabase transgresses Rove strata in a steplike fashion. :

Massive ridges northeast of the Moss Lake structure, between

Duncan Lake and Bearskin and Daniels Lakes, imply other anomalous intrusive forms . Although strategic areas along their flanks are covered one may speculate that the more southerly knob (the Duncan Lake struc ture) is a semi-concordant pod of diabase which, inferred from exposures on its south side, transgresses the country rock. In addition, gentle south dips of Rove strata exposed near the base of the cliff of the northwest corner of sec. 34 suggest it has a relatively flat form. These exposures can be followed for only a hundred feet or so and appear to terminate against diabase along strike, suggesting that either the Rove here is part of an irregular floor of the intrusion or it is a xenolith of large proportions. At the present erosional level a thin belt of Rove may separate this mass from that to the north (the D aniels Lake structure) - : ' - 27 this is suggested by the presence, of several small outcrops of hornfels in the valley between them.

In the Daniels Lake structure, relatively fine grained diabase constitutes the cliffs of its east and west sides (fig. 13), suggesting contacts were at one time nearby . Many diabases cropping out on its top contain more than the usual amounts of red feldspar, a characteristic feature of diabases near the top of intrusions. Thus, the present topo graphic form is indicative of the approximate shape of the original in trusion, which had walls conforming approximately to the cliffs or slopes that presently bound it on the east and west sides and a roof slightly above the present erosional surface.

Intrusives commonly deviate from strictly planar and concor dant forms, on smaller scales. A good example of this exists at the northwest shore of Clearwater Lake. Here, a small lip in a south- dipping sill is marked by a 15-foot, south-facing cliff of chilled dia base at the shore of the lake and gentle northward dips in the hornfels that crops out above it, to the north. The resulting syncline is only a surficial structure, reflecting the shape of the intrusion.

At present, one can only speculate on the causes of variations in the intrusive forms described above. Possibly, they are due to some regional structural pattern predating emplacement of Logan rocks. More specifically, the walls of the Daniels Lake structure may reflect trends of a pre-Logan fracture pattern. There are, indeed, subparallel linea ments to be found in the pre-Keweenawan terrain of Ontario; however, study of the Logan over a wider region will be necessary in order to test this hypothesis. 28

Figure 13 . The Daniels Lake Structure

The view is north-northwest. The west end of Daniels Lake is in the lower right. Note the recent east-west-trending scarp, a result of a young fault, north of Daniels Lake.

Figure 14. Contorted Rove Strata near the Roof of a Sill

The spruce (left) is about 4 inches in diameter. At the shore of Daniels Lake, SE1/4 sec. 26, T. 65 N. , R. 1 W. Structural Elements

Folds. Localized areas of steeply dipping Rove strata exist in the Hungry Jack Lake quadrangle and have been reported in the South

Lake quadrangle (Morey, oral communication); they may indicate folding.

Dips of nearly 70° S.. have been measured on the south shore of Aspen

Lake (in sec. 12, just west of the Aspen Lake-East Bearskin Lake port age) , but in nearby outcrops the bedding is again gentle. On the old railroad grade near the eastern section line of sec. 3, T. 64 N, R. 1 W. the Rove dips steeply in one exposure, while 20 feet away, and near a sill, the dip is low angle. The genesis of these structures, in particu lar, whether or not they are apophyses of intrusive events, is not known. In any case, they are uncommon and will not be discussed further.

A more common feature also noted elsewhere is the severe and irregular contortions of hornfelsic Rove strata found directly above the sill south of Spen Lake in sec. 10 and in several outcrops along the north shore of Daniels Lake (fig. 14). This "crumpling" is interpreted to be caused by the intrusive event itself (Morey, oral communication).

Observations supporting this interpretation are as follows: (1) fold axes and axial planes are randomly oriented except that fold axes sel dom plunge more than 30 degrees; (2) folds of various types (open, symmetrical, asymmetrical, isoclinal, and recumbent) may be present in one outcrop, and there appears to be no single predominant type; (3) one fold can change in both orientation and type along its fold axis; and (4) the folds exist on all scales ranging from inches to several feet. 30

Faults. Eighteen faults, mostly small, have been found in the

quadrangle; many of these are questionable. The difficulty in their

recognition arises from the fact that they can normally be seen only

where slickensides are well developed of where contacts are offset.

Since exposures of actual contacts are relatively rare, it is probable

that many more faults exist.

Fault planes most often strike east-west and dip at steep

angles, generally north. They are parallel to a prominent set of joints

and shear zones.

The presence of major faults rests on several lines of evidence

' x which, taken individually, may not be conclusive. For example, the

northwest-southeast fault near the west side of the quadrangle is in

dicated by breaks in topography on the north .shores of Moss and Leo

Lakes and on the ridges south of these lakes and by a small spring on the slope north of Moss Lake. Also, it is the southern extension of a

fault hypothesized by Morey (written communication) for the South Lake

quadrangle, but this writer was unaware of Morey's fault when the pres

ent one was first postulated. This particular fault also appears on the

Canadian maps (Pye and Fenwick, 1965) and is part of a prominent set

of lineaments, many of which exhibit a peculiar curved trend. It is thought that this set predates the emplacement of the Duluth Complex

(Morey, oral communication). However, on the map accompanying this

report (fig. 5 ) it is hypothetically extended through the gabbro because,

although covered at the critical juncture, the contact is displaced in the necessary manner. 31

East-west faults on the north and south sides of Daniels Lake are marked by scarps, the remarkable persistence and linearity of which are sufficient evidence for their presence (fig. 13). Also, their sharp ness suggests they are relatively young phenomena, perhaps related to vertical movements following the retreat of the Rainy Ice Lobe.

The fault through Duncan Lake is inferred on the basis of a probable offset of a contact at the Moss Lake-Duncan Lake portage and a highly probable offset of a contact near the stairway portage between

Duncan and Rose Lakes. Again, there are parallel fault sets in the im mediate vicinity of Ontario (Pye and Fenwick, 1965) .

Joints. Figure 15 shows attitudes of various joint sets and shear zones . (Shear zones are used here as deeply weathered scars along which movement cannot be demonstrated.) The most prominent set strikes east-west and is nearly vertical. Other subordinate sets strike north-south, northeast-southwest, and northwest-southeast. A prom inent sheeting, characteristic of practically all diabase sheets world wide, is also present here. It most likely owes its origin to contraction of a cooling magma. Grout and Schwartz (1933) reported the presence of poorly developed columnar joints. They are not prominent in the Hungry

Jack Lake quadrangle but can be seen on some of the larger palisades.

Basal Contact of the Duluth Complex

Field Relations

Dips of country rock of about 60 degrees near the base of the

Duluth Complex suggest the contact has a similar attitude. Locally, however, hornfels near the contact exhibits nearly horizontal bedding; □ ill ess n 0 - 3 % 3- 6 % 6 - 9 % 9 - 12% > 12%

Figure 15. Contour Diagram of 126 Joints and Shear Zones Plotted on a Lower Hemisphere Schmidt Net ' 33 for example, in sec. 6, T. 64 N, R. 1 W. Precise location of the con

tact is not possible over much of its. length because bedrock exposures

near it are generally isolated, and numerous hornfelsic xenoliths are

present within the gabbro several hundred lateral feet from its base.

On the map accompanying this report, the position of the contact is.the

most northerly extent of Complex rocks. A rare exposure of a sharp con

tact between gabbro and gneissic hornfels (after Rove) is shown in fig.

16. Here, partial mobilization of Rove strata occurred, as witnessed by

the "pressure shadows" around a partially rotated carbonate concretion

(fig .1 7 ) about 20 feet from the gabbro.

An investigation of the metamorphic effect of the Duluth Com

plex is beyond the scope of the present study. The reader is referred

to Grout (1933) for a more comprehensive view.

The Contact Unit

At numerous localities the contact is marked by a fine-grained,

diabasic to sugary textured rock, here referred to as the contact unit.

Exceptional exposures of this are present in secs i 5 and 6, T. 64 N .,

R. 1 W. Observations pertinent to its genesis are as follows:

1. At the above locations. Rove strata in contact with this unit

have been metamorphosed to a dense, powdery textured horn

fels . ■■■ ■■■

2. A rock which microscopically resembles the contact unit

(fig. 18) of secs. 5 and 6 (see below) shows intrusive rela

tions to Rove hornfels at the location of fig. 17

3 . Four thin sections of rock samples taken near the contact have

been classified as the contact unit. All have in common two 34

Figure 16. A Sharp Contact between Anorthositic Gabbro of the Duluth Complex and a Gneissic Hornfels after Rove Rocks

About 70 feet west of the powerline, NE1/4SW1/4 sec. 10, T. 64 N. , R. 1 W.

r '

% , .

Figure 17. A Partially Rotated Concretion in Metamorphosed Rove

Note the "pressure shadow" around it. About 20 feet north of the contact of figure 16. , 35 distinct generations of plagioclase and pyroxene. Strong, nor

mally zoned sodic labradorite laths are embayed by small,

discrete anhedra of albite and clinopyroxene (fig. 18); over

growths have been observed on many of the pyroxenes. In

other respects these samples differ. There is a continual tran

sition from a medium-grained, gabbroic rock having fresh, Mg-

' rich olivines, clinopyroxene, ophitic hypersthene, and biotite

to a fine-grained rock characterized by an abundance of hydrous

minerals (hornblendes and biotites) replacing pyroxene.

There is little doubt that the rocks contain an igneous assem blage reflecting severe disequilibrium conditions. The question is whether or not the unit is metamorphosed Logan diabase or a chilled phase of the Duluth Complex. Because of the field relations discussed above and the fact that metamorphic effects of Logan rocks on the Rove are subtle, the latter alternative is favored and interpreted as follows .

The intrusion of the Duluth Complex was accompanied by the formation of a hybrid zone at.the contact, represented by the contact unit, formed by contamination of a chilled and partially congealed gabbro with mainly alkali and volatile components from the country rock. The medium- grained gabbro in contact with meta-Rove in figure 16 would be explained if chilled gabbro had been displaced to higher levels of the magma cham ber while the gabbro in figure 16 was emplaced against the already hot country rock.

A more definite res olution of this problem will require more work over a wider area. On the map (fig. 5) the contact unit has been included with Duluth Complex rocks . 36

Figure 18. Photomicrograph Showing Two Generations of Plagioclase Characteristic of the Contact Unit Pleistocene Geology

Locally abundant Pleistocene glacial drift covers portions of northern Cook County and has been attributed to the Rainy Lobe, one of two ice lobes that invaded Cook County during Wisconsin times (Sharp,

1953). Throughout much of the region, however, the only evidence of glaciation is the existence of a variety of scattered foreign boulders.

Figure 19 shows the distribution of glacial materials.

Drift is restricted mainly to glaciofluvial deposits „ Two major esker "systems" are developed in the Hungry Jack Lake quadrangle..

These are the Wampus Lake Esker (fig. 20), which can be followed from the east end of Clearwater Lake to the west end of Aspen Lake, and what is here referred to as the Leo Lake Esker, which dams Leo Lake and is best developed between it and Hungry Jack Lake. The former and a portion of the latter esker appear on the map of Sharp in Grout et al.

(1959, plate 2). This writer prefers to extend the Leo Lake Esker further northward rather than limit it to the area of Leo and Hungry Jack Lakes, as shown by Sharp, because of the glaciofluvial drift found on the north and south shores of Moss and Duncan Lakes and of the bathymetry of these lakes„ A third, smaller esker can be followed for several thousand feet along, the north shore of East Bearskin Lake, where it forms the small peninsula of sec„ 12.

Knob and kettle drift has been reported by Grout et al. (1959, p. 66-67) in a half-dozen locations in the quadrangle and has been con firmed by this study. Its existence has been noted in additional loca tions (fig. 19). The reader is referred to Sharp (1953) or Grout et al.

(1959) for further details on Pleistocene deposits. RIW RIE

20 22

22 23 24

30 29 28 27 26 25 30

DUNCAN

LAKE DANIELS LAKE

32 34 35 36

"as? zx/rf BEARSKIN LAKE 1 65 N T65N T64N T 64 N

HUNGRY JACK LAKE

LAKE

RUDY RUBY LAKE LAKE POPLAR LAKE

SCALE

2000 4 0 0 0 6 0 0 0 0 0 0 0 F E E T

Figure 19. Map of the Hungry Jack Lake Quadrangle Showing the Distribution of Glacial Materials (shaded). 39

Figure 20. A Cross Section of the Wampus Lake Esker at the East End of Bearskin Lake LOGAN DIABASES

The two sills cropping out on the west shore of Rose Lake were

chosen for detailed petrographic analyses and are.here referred to as

the Rose Lake sills. Structural relations in this area have been pre

viously discussed and are illustrated in figure 10. It is evident that

they are but tongues of a much thicker sill to the west and also that

they become one and the same mass down dip. Sills "A" and "B" refer

to the northerly and southerly ones , respectively. Their thicknesses ,

at the sampling sites on the shore of Rose Lake, are 72 + 2 (sill "A")

and 83 + 2 feet (sill "B").

These sills were chosen for study because (1) the excellent

exposure in this region allows good control, and (2) it was thought that

detailed examination of thin (rapidly consolidated) sills might supply

. useful constraints for some petrologically important hypotheses. Of

prime interest to this report is the origin of diabase porphyry zones;

these exist in the Rose Lake sills.

A number of thin sections of other Logan rocks of the quadrangle

were examined in order to obtain information about rocks not found at

Rose Lake. Their descriptions are included in the following discussion.

Determinative and Field Methods

A Jacob's staff was used in collecting samples at 5-foot inter

vals through each sill. At least one and usually two thin sections of

each sample were microscopically examined. Modal analyses were

made and used to calculate chemical compositions. Plagidclase and

40 X : : . 41 pyroxene compositions for sill "A" and several plagioclases from sill "B"

were determined using the electron microprobe at the University of Min

nesota under the guidance of Dr. P. W. Weiblen. Calcium was deter

mined for plagioclases using standards of 0, 20, 40, 60, 80, and 100

mole percent anorthite; potassium was not determined in every sample

since it generally proved to be present in negligible amounts. In clino-

pyroxene, major cation abundances (Ca, Fe, Mg) were determined using

standards of Wo47EnnFs42, Wo47En2lFs32, Wo4sEn34Fs21, and

Wo 5 ()En4 7 Fs3 . More detailed compositional information on the latter three is given by Smith (1966).

Petrography

Chilled Margins ■_

The marginal chilled phases typically are felty aggregates of thin needles of opaque oxides with shreds of plagioclase and brownish

mafic constituents. The latter are probably pyroxene and alteration prod

ucts . These are commonly arranged in needlelike masses radiating from

common centers, imparting a distinctly radial microtexture to the rock

(fig „ 21). Individual radiating clumps are haphazardly distributed and

show varying degrees of development. An apparently similar texture has

been described in Hawaiian basalts (Richter and Moore, 1966), as "radi

ating star-burst clusters of (plagioclase) laths, each lath mantled by con

centrations of small clinopyroxene crystals . . ." In addition, areas of

subparallel, minute needles of iron titanium oxides are present and sug

gest skeletal remnants of former grains . Sulfides are only a minor part 42

Figure 21. Photomicrograph of the Chilled Zone Illustrating Its Radial Texture

Figure 22. Photomicrograph of Plagioclase Poikilitically En closing Augite (high relief)

Note the euhedral nature of the opaques . . 43 of the opaques. Tiny voids within this felty mass are often filled with clear quartz or potassium feldspar and tiny shreds of biotite„

Plagioclase phenocrysts are diagnostic of many chilled zones , including those at Rose Lake. They are up to 6 mm in length, usually are calcic andesine, and in some cases show moderate to strong normal zonation. It was suggested earlier that they may aid in the correlation of individual sills along strike since they appear to be restricted to particular sills. Commonly, the phenocrysts poikilitically enclose small inclusions of the groundmass constituents. The volume of plagio clase phenocrysts in the Rose Lake sills is no more than several percent of the total; in other sills, they may comprise 10 to 15 percent. Parallel alignment of plagioclase phenocrysts has not been noted in the Hungry

Jack Lake •quadrangle.

D iabase

Excepting the diabase porphyry zones, there is a continual textural evolution,expressed mainly by a gradual increase in grain size, from the felty chilled margins to the interior portions of the sills. Here, diabasic to subophitic textures are well developed. This textural transi tion takes an unusual path at Rose Lake, however, with the development of a poikilitic relationship between plagioclase and pyroxene in which subhedral to anhedral laths and plates of plagioclase poikilitically en close small (usually less than 0.3 mm) blebs of augite (fig. 22) . At least in part, the individual augite blebs are optically continuous, and their distribution suggests a radial pattern. As opposed to their occa sionally highly altered state elsewhere, pyroxenes completely enclosed :v '; 44 by plagioclase are fresh. Commonly, subhedral pyroxenes suggest crystallization before plagioclase; however, poorly developed diabasic relations usually coexist with the poikilitic texture.

. In sill "A" , the above textural relations are faintly seen 12 feet above the basal contact and become highly developed nearer the contact. They can be found within approximately 15 feet of the upper contact. In sill "B", they are developed within 20 feet of the lower and

30 feet of the upper contact. In addition, within 5 feet of the sill mar gins it is not uncommon to find augite assuming a fish-skeleton type appearance, with elongation along the c-axes . The shape of pyroxenes in other regions of the sills are controlled by shapes of interstices be tween plagioclase laths. The textural variation through the Rose Lake sills is shown schematically in figure 23 . It is evident that near the intrusive margins crystallization of the two phases proceeded simul taneously. However, there is no universal development of these anoma lous textures in diabases from other localities in the Hungry Jack Lake quadrangle.

Approximate average grain sizes increase from 0.6 and 0.4 mm for plagioclase and augite, respectively, within 3 feet of the sill mar gins, to 1.8 and 0.9 mm in their interiors. Skeletal magnetite-ilmenites range from 0.3 to 1.0 mm within the sill; near the margins they are not well developed and occur as elongate skeleton (up to 1 mm) or as tiny needles, whose average length is about 0.3 mm.

Modal compositions of samples from the Rose Lake sills are given on figures 24 and 25 and in the appendix. They reveal the rela tively homogeneous mineralogy of the sills with respect to plagioclase Sills THICKNESS IN FEET 10 20 30 40 50 70 80 60 iue 3 Shmtc etrl tairpy fte oe Lake Rose the of Stratigraphy Textural Schematic 23. Figure J: * y • • y * \ M* IL "B" SILL • • • • • • • • • IL "A"SILL IBS PORPHYRY DIABASE CHILLED ZONE ZONE CHILLED OKLTC DIABASE POIKILITIC DIABASE 45 Lake acsretn ? ad pdt. lgols icue sericite. includes Plagioclase epidote. and , (?) talc-serpentine POSITION IN SILL (feet from roof) 70 30 40 20 50 60 10 Alteration products of pyroxene include uralite, biotite, chlorite, chlorite, biotite, uralite, include pyroxene of products Alteration iue 4 Mdl aitos fte ibs o il A a Rose at "A" Sill of Diabase the of Variations Modal 24. Figure ‘OPAQUE URZ AND QUARTZ 10 K-FELDSPAR OXIDES IL A ' "A" SILL

20 30 OUE */• VOLUME 40 50 PLAGIOCLASE 60 46 70 47

PLAGIOCLASE

20 PYROXENE (incl. alteration)

QUARTZ AND K-FELDSPAR 30

o 2 40 E o pV ) 2%

OPAQUE " OXIDES

SILL "B 70

10 2 0 30 40 50 60

VOLUME •/. Figure 25. Modal Variations of the Diabase of Sill "B" at Rose Lake Alteration products of pyroxene include uralite, biotite, chlorite, talc-serpentine (?) , and epidote. Plagioclase includes sericite. : ' ' ' . . : 48 and pyroxene. Modal variations in sill "B" are believed to be due main ly to severe alteration and therefore are not a result of primary mag mat ic p ro c e sse s.

Quartz and orthoclase increase slightly toward the tops of the sills. In some cases, they are granophyrically intergrown and border euhedra of plagioclase (fig. 26); elsewhere, sodic and altered plagioclase grain borders are involved in the intergrowth, resulting in a myrmekite.

Opaque oxides are ubiquitous as euhedral crystals and skeletal grains „ Alteration products, accessory zircons and apatites, and minute quantities of secondary sulfides are also present.

Modal compositions of the Rose Lake sills compare closely with those reported for Logan rocks in Ontario (Blackadar, 1956) and for diabases of the Endion sill near Duluth, Minnesota (Ernst, 1960).

Porphyry Zones

Diabase porphyry from Pigeon Point has been discussed by

Grout and Schwartz (1933) and has been reported at numerous localities throughout the Logan belt. In the Hungry Jack Lake quadrangle, in addi tion to its presence at Rose Lake, diabase porphyry has been found in outcrops on the slope northwest of Duncan Lake (secs. 28, 29, and 32) and west of Partridge Lake (sec. 30). Portions of the Moss Lake intru sion have numerous phenocrysts, but they are not so numerous as to warrant the term "porphyry."

Porphyry zones of sills "A" and "B" are found 8-11 and 23-26 feet below their roofs (fig. 23). Although Grout and Schwartz (1933) 0.5 MM I------1

Figure 26. Photomicrograph of Granophyrically Intergrown Quartz and Orthoclase Surrounding a Euhedral Plagioclase ■■■7 . 50 suggest it to be the rule, more recent field observations (N„ Jones,. un published report, Minnesota Geological Survey) indicate.that the por phyry zones are not necessarily confined to the near-roof regions of sills „ In sill "A" , a contact.with the porphyry zone below and diabase above is well exposed. Figure 27 illustrates the sharp transition. Other contacts at Rose Lake are also abrupt, although they are not directly observable. Grout and Schwartz (1933) make no mention of sharp con tacts, and one observed by N. Jones (unpublished report, Minnesota

Geological Survey) is "gradational yet abrupt."

Dimensions of plagioclase phenocrysts average 1.4 x 0.7 cm and comprise 36 + 3 volume percent of the rock. Their edges are common ly altered, as described in a later section. Alteration of the groundmass is .characteristically severe, often masking the primary relations among its minerals. indeed, this may be an additional diagnostic character istic. Also, the coexistence of both diabasic and poikilitic relations between groundmass pyroxenes and plagioclases can be observed. Pri mary inclusions, in particular those of pyroxene or opaque oxides, are not present in the phenocrysts. Interstitial quartz and feldspar (both potassium and sodium rich) are prominent members of the groundmass.

The porphyry zones within the Rose Lake sills may not be com pletely typical of those of other sills because (1) other porphyry zones may be thicker, especially in thicker sills; (2) plagioclase phenocrysts up to 15 cm long have been reported; and (3) other porphyry zones may have significant amounts of red feldspar, suggestive of a more inter mediate composition. Whether these feldspars are actually potash rich or simply contain hematite (Poldervaart and Gilkey, 1954) is not known. 51

Figure 27. The Sharp Contact of the Diabase Porphyry Zone and Diabase above It, Sill "A"

Note random orientation of plagioclase phenocrysts . • 52 In any event, the abundances of acidic components in the Rose Lake sills are not sufficiently great to lighten the color significantly.

Despite their elongate shape, there is only a vague suggestion of parallel alignment of phenocrysts in the Rose Lake sills; and this is at the immediate contact of the porphyry zones with diabase. Faint foliation in porphyry zones has been reported elsewhere in the Logan, however. In all cases, it is parallel to the attitude of the diabase sheet.

Anorthosite Inclusions

Anorthositic inclusions are common in the Logan and, in fact, in the Duluth Complex and other diabases of northeastern Minnesota.

Their other occurrences are comprehensively discu'ssed by Grout and

Schwartz (1939). Within the Hungry Jack Lake quadrangle, both xeno- lithic inclusions and megacrysts are present in diabase of the Moss

Lake intrusion and between Bearskin and Hungry Jack Lakes .. The largest are at least 3 feet in diameter, the smallest on the order of inches .

One of the first interpretations (Grout and Schwartz, 1933) argued that the inclusions originated in the magma in which they are found by the collection of early formed crystals which then floated to the roofs of the sills. In a subsequent revision, they state that the in clusions "are obviously foreign, picked up by the magma on the way to its present position where it solidified to form diabase." Observations supporting this latter interpretation are (1) diabase shows intrusive re lationships with the inclusions and (2) one of the inclusions found near

Moss Lake is embedded in the chilled zone of the roof of the intrusion

(fig. 28). It is also pertinent that the inclusions mineralogically 53

Figure 28. Anorthosite Inclusion (white) Embedded in the Chilled Roof of the Moss Lake Intrusion

NE1/4SW1/4 sec. 33, T. 65 N. , R. 1 W. resemble the diabase in which they are enclosed in terms of mineral phases present and that the anorthosite plagioclase has a composition of An 5 9 + 5 , which is similar to that of phenocryst cores from the por phyry zones. These facts suggest that the inclusions may be cognate.

Granophyre

Small dikelettes and dikes of granophyre are common within the

Duluth Complex (Weiblen, oral communication) and along its basal con tact. In the Logan Intrusive s , near the roofs of thick sills, are usually developed what is best termed a granophyre-rich diabase. This is evi dently a result of differentiation. Granophyre proper crops out south of

Rudy and Ruby Lake in the northeast and northwest corners of secs. 12 and 7, respectively, where it is the dominant member of the small sill underlying that area.

Microscopically, the texture is dominated by sodic plagioclase surrounded by granophyric quartz and orthoclase (fig. 29) . This texture is characteristic of granophyres and resembles a flower-petal or "cauli flower structure" (Walker, 1969). Minor mafic constituents, which com prise less than 10 volume percent of the rock, are completely altered to ch lorite.

Contact relations with diabase indicate that the granophyre is later (fig. 30). Whether or not they are genetically related is not known.

Grout and Schwartz (1933) have noted the occurrence of granophyre sills within the Logan region; Blackadar (1956) has concluded that granophyres associated with Logan rocks in Ontario are due in part to assimi lation of sedimentary or other acidic materials by the mafic magma. 55

Figure 29. Photomicrograph Illustrating the Typical Flower- petal Texture of Granophyre Figure 30. An Angular Block of Fine-grained Diabase Included in Granophyre

NW1/4NW1/4 sec. 7, T. 64 N., R. 1 E. 57 Mineralogy

Pyroxene

Aug it e is the single pyroxene phase of the Rose Lake sills. A , possible exception is that some augites of coarser diabases reveal minute exsolution lamellae; they are parallel to (100) and therefore indi cate an orthorhombic form (Poldervaart and Hess , 1951).

Compositional data obtained from the electron microprobe are given in figures 31 and 32 and in the appendix . Averages for all analy ses of individual slides from sill "A" (fig. 31) represent at least two points on four to eight different grains. Undoubtedly, some composi tional scatter is due to inaccuracy of the determinative methods , which may be caused in part by elemental substitution in X and Y sites.

Titanium is suspect in this regard. This writer subjectively judges the compositional error to be + 4 mole percent of any one pyroxene end- member composition.

The data emphasize the following points; (1) Rose Lake augites cluster about those represented by a portion of the Skaergaard trend

(fig. 31); (2) pyroxenes from different parts of the sill show no clear compositional trends (fig. 32); (3) within any one sample there is con siderable compositional heterogeneity (fig. 32), which results in the. scatter on the contour diagram (fig. 31); and (4) compositional inhomo- . geneities are also large within individual grains. Figure 33 shows the resulting undulatory or hourglass extinction. Customarily, points chosen for analysis were located near the centers and edges of indi vidual grains; these commonly differed by as much as 10 mole percent TV

6 0 10

3 #

Figure 31. Pyroxene Compositions of Rose Lake Sill "A" as Determined with the Electron Microprobe

The contour diagram is of 111 points with an interval of 3 percent of all points in 0.1 percent of the CaSiOg-MgSiOg-FeSiOg equilateral triangle. The Skaergaard trend for calcic pyroxenes (Brown, 1967) is shown for comparison. Inset shows average compositions of pyroxenes from various positions in the sill, expressed in feet below the roof. ~ Figure 32. Compositional Variation of Plagioclase and Pyroxene through Sill "A" as Deter mined with the Electron Microprobe

(a) Averages of all plagioclase analyses; (b) maximum (x) and minimum (o) plagioclase determinations; (c) total Fe:Mg ratio for pyroxenes. The line expresses the range from next-to- highest to next-to-lowest determinations. Points are median values. (P) denotes compositions of phenocrysts. cn co I— — — I

Figure 33. Photomicrograph of the Undulatory or Hourglass Extinction Typical of Many Pyroxenes from the Rose Lake Sills ■ : . ' 6i or more for any one pyroxene end-member composition. Such disequilib rium must result from slow Ionic diffusion rates in these rapidly cooling s ills .

Plagioclase

Plagioclase compositions in the range of sodic labradorite to andesine are characteristic. The electron microprobe data presented in figure 32 and in the appendix show the variation through sill "A" of average, maximum, and minimum values from individual samples (i.e., locations); limited data from sill "B" are also included in the appendix.

The error is judged to be +2 mole percent An. Maximum and minimum values are thought to be more significant than averages because (1) although an effort was made to analyze geometrical centers of each plagioclase grain, the point actually analyzed was dictated more by the condition of the grain surface; (2) if different plagioclases started growth at various times, compositional differences in their initial points of nucleation might result; and (3) symmetrical growth around nucleation points need not have taken place, owing to the presence of nearby c ry sta ls.

Pertinent conclusions from the data are as follows; (1) compo sitions of the center portions of grains show a remarkable consistency through the entire sill (An^g); (2) normal zoning generally amounts to an increase in 4 to 8 mole percent An, based upon average values, but becomes much more severe in the upper portion of the sill; (3) cores of plagioclase phenocrysts of the porphyry zones are richer in the anorthite molecule than are any other plagioclase (averaging Angg); (4) phenocrysts borders have a composition similar ..to that of plagioclase from the center and base of the sill rather than from neighboring regions near the top; in sill "B" groundmass plagioclase from the porphyry zone is similar to that from neighboring samples; and (5) plagioclase of both the upper and lower chilled zones of sill "A" is soda-rich (Ans 0 - 4 0 ) and has un usually high (2-3 mole percent Or) potassium content.

Oscillatory zoning is present in most phenocrysts from the porphyry zones (fig. 34) . It has also been observed occasionally in plagioclase from other parts of the sill. Its coexistence with normally zoned plagioclase in this rapidly consolidated sill supports the thesis that zoning is a result of cation concentration in the liquid of the im mediate vicinity of the growing grain; thus, in these sills zoning de pended upon rates of ionic diffusion and growth of plagioclase (Harloff,

1927; Bottinga, Kudo, and Weill, 1966). Albite and, to a lesser extent, carls bad twins are common; twinning according to other laws is rare.

Opaque Oxides

The presence of magnetite-ilmenite skeletons (fig. 35) is most striking in the Rose Lake sills. In addition, euhedral crystals and needles are present but less common. Both forms reflect a cubic habit.

Compositional microprobe determinations of two skeletal ribs show them to be nearly pure ilmenite (Fe:Ti::47:53); no compositional data are available for other opaque phases . Goethite and other iron-rich al teration products probably occur between the opaque ribs .

TexturaLrelations between opaque oxides and other phases are complex and may be summarized as follows: (1) euhedral crystals and 63

55-1

53-

51 -

Z 4 9 -1 < 55i

53-

51-

49-1 i O 200 400 600 800 1000

MICRONS

Figure 34. Traverses across Zoned Outer Segments of Plagio- clase Phenocrysts from the Porphyry Zones

Sill "A", bottom; sill "B", top. 64

Figure 35. Photomicrographs of Skeletal Iron-titanium Oxide Minerals

The skeletons are within altered plagioclase, which forms the fillings between the ribs . 65 skeletons reflecting euhedral outlines are enclosed by either plagioclase or pyroxene; (2) either euhedra, needles , or skeletons may crosscut completely optically continuous grains of plagioclase or pyroxene; (3) these latter minerals can, in addition, be found between the ribs of skeletons and are often continuous, with the grains in which the skeletons reside.

The well-developed cubic forms of skeletons leave no doubt that the initial oxide to crystallize was cubic itself; the rather large abundance of hexagonal ilmenite indicates that it was titanium-rich.

Therefore, an ulvospinel is likely to have been the initial phase

(Buddington and Lindsley, 1964). The opaque oxides are discussed in greater detail below.

Quartz and Potassium Feldspar

Most often, quartz and a potassium feldspar phase are in late- stage, granophyric intergrowths. Quartz can also occur as discrete, tiny grains in the ground mass with alteration minerals and in this case is certainly an alteration product. For example, silica is liberated in sericitic alteration of plagioclase and also in the biotite to chlorite reaction.

When quartz and potassium feldspar are intergrown with sodic plagioclase, the plagioclase is usually cloudy. It is the sodic, altered borders of earlier plagioclase laths which take part in the intergrowth.

Granophyric diabase has been explained by magma fractionation, and the reader is referred to Ernst (1960) for a more detailed account. ' " • ■ ' 66 Alteration Products

Alteration is predominantly of the propylitic type and moderate to severe in almost all of the samples examined. This introduces some uncertainty in the accuracy of modal percentages .

Sericitic alteration of plagioclase is ubiquitous, and in most instances the centers and calcic portions of plagioclase grains have been preferably attacked over their more sodic borders . In contrast , plagioclase phenocrysts of the diabase porphyry zones generally exhibit a sericitic border surrounded by a thin armor of cloudy feldspar, either albite or oligoclas e . Figure 36 illustrates a circumferential sericite zone surrounded by the recrystallized armor, which is optically discontinuous with the rest of the phenocryst. It is suggested that the sodium liber ated in the plagioclase ^sericite reaction has migrated outward to form the sodic armor, again, probably a deuteric process .

Sodic plagioclase intergrown with quartz and potassium feld spar is interpreted to have resulted from a late-stage magmatic or deu teric reorganization of alkalis and silica. Tiny needles of accessory apatites are ubiquitous in the quartz.

Uralitic coronas around augites are present in most instances; completely fresh pyroxenes are rare, and the augites are either bleached or cloudy. As alteration proceeded, biotite formed after uralite; this is suggested by their close association. The presence of minute accessory zircons is suggested by pleochroic haloes seen in many biotites .

Olivine has not been observed in any of the nearly 70 diabase thin sections examined; only on rare occasion has its questionable oc currence been noted in hand specimen. Several modes of Logan rocks 67

Figure 36. Photomicrograph of Circumferential Sericite Zone (light) Surrounded by a Recrystallized Armor of Sodic Plagioclase (at extinction) of a Plagioclase Phenocryst of the Porphyry Zone of Sill "A"

Figure 37. Photomicrograph of an Alteration Patch

The needles are of actinolite enclosed in a fine aggregate of talc-serpentine (?) . Minute grains of epidote and magnetite are also present. , . 68 reported by Blackedar (1956) include olivine, as do two (of approximately

40 reported) in the study of Ernst (1960) . The latter work states " . . . olivine was apparently an essential constituent of most specimens, but it is now largely replaced by intergrowths of chlorite, talc, iddingsite, magnetite-ilmenite, and serpentine."

Particularly characteristic of most diabases of the Hungry Jack

Lake quadrangle are discrete, rounded patches of alteration minerals

(fig. 3 7). The patches are usualy about 0.5 to 1 mm in diameter but may be as large as 4-5 mm. The association seen in practically all cases is of chlorite and talc-serpentine (?) with lesser amounts of magnetite-ilmenite and epidote as essential constituents. Actinolite may also be present as tiny needles. The interpretation of Ernst (1960) could likewise be applied to Logan rocks.; however, it is not favored here because of textural relations, which suggest that pyroxene is the main source mineral of the alteration (along with minor amounts of plagioclase) and because of the lack of even trace quantities of olivine.

Since the patches are discrete, that is, in no way connected with frac tures or veins, it is likely that they represent volatile components pres ent during the late magmatic stage.

Other alteration products include carbonate and talc. Magne tite is altered to goethite; and rocks exposed to the surface for some time contain abundant hematite. Rutile (after ilmenite) is not uncommon, and secondary sulfides have been noted in several samples. The den teric fluid must have been mainly a hydrous phase rich in alkali consti tuents. Lesser amounts of COg and HgS also characterized it. - 69

Chemistry

Because of its bearing on following discussions, a composite chemical composition has been calculated (table 2) from modal data of sill "A" (represented by approximately 8,000 points) and from known plagioclase and pyroxene compositions. This method of calculation is thought to provide precise results (Friedman, 1960), especially because of the large number of points counted. In addition, the 14 samples taken at regular intervals through the 70-foot thick sill effectively eliminate sampling problems inherent in a limited number of chemical analyses „

The factor limiting the accuracy is the presence of abundant alteration products which are difficult to determine optically „ No attempt has been made to characterize chemically diabase from different parts of sill "A" or to calculate the composition of sill "B", in which alteration intro duces doubt into the accuracy of modal compositions .

A comparison of compositions of Rose Lake and other diabases of the region emphasizes their chemical similarity (table 2) . A more complete compilation of analyses of Keweenawan diabases is given by

Ruotsala and Tufford (1965).

Interesting aspects of these analyses are their low silica and high iron contents . Average abundances in weight percent of SiOg, FeO, arid Fe 2 0 g of all dolerites are 50.6, 8.9, and 2.4 percent, respectively

(Mason, 1967). Although there is a greater chemical variation in Lake

Superior diabases than suggested by the analyses of table 2, they are more or less typical of the region. 70 Table 2 „ Chemistry of the Rose Lake Diabase and of Some Diabases of Northeastern Minnesota and Neighboring Ontario

1 2 3 4 . 5 6 7

S i0 2 47.22 50.04 49.88 48.69 47.31 48.99 47.25

T i0 2 2.79 3.76 1.19 3.55 0.78 1.93 2.89

AI2 O3 14.23 11.70 18.55 14.08 17.76 17.88 15.00

Fe2 0 3 5.61 2.28 2.06 3.42 1.25 3.99 2.64

FeO 9.86 13.51 8.37 12.06 9.08 8 .0 1 11.09

MnO ------0.15 0.09 0 .2 0 ------0 .2 1 MgO 7.21 4.20 5.77 4.70 8.89 4.33 6.52

CaO 8.32 7.16 9.70 6 . 8 8 8.64 7.86 8.40 NagO 2.57 3.47 2.59 2.92 1.39 2.90 2.52 k 2o 0.75 1.03 0 . 6 8 1.05 1.99 2.13 0.81 h2 o+ 0.94 1.28 1.04 , 1.82 1.80 1.40 1.63 h2 o - —---- 0.07 ------0.57 0 .36 0 .59 0.35 P2°5 tr 0.47 ----- 0.29 0 .2 2 0.95 0.56 co 2 ------0.25 ------0.16 0.33 —---- 99.50 99.60* 1 0 0 . 20* 100.23 99.63 101.29 99.87

*More elements analyzed than reported. 1. Rose Lake> calculated from all modal analyses of sill "A" . 2. Chilled diabase, sec. 25, T. 65. N ., R. 2 W . (Grout and Schwartz, 1933, table 8 , no. 1). 3. Composite of five samples of gabbro from the Pigeon Point sill (Grout and Schwartz, 1933, table 8 , no. 2). 4. Composite of.30 samples of Mt. McKay sill, upper Ontario (Blackadar, 1956, table 8 , no. 8 ). ■ . . 5. Composite of Keshkabuon Island sill, Ontario, four samples (Blackadar, 1956, table 8 , no. 5).

6 . Composite of Loon Lake sill, Ontario,seven samples (Blackadar, 1956, table 8 , no. 1.0). 7. Ophitic diabase, Endion sill (Schwartz and Sandburg, 1940, table 1 , no. 1). PETROLOGY

It is evident from the above discussion that the stratigraphy of the Rose Lake sills can be distinguished on two bases, a textural one and a compositional one. Rapid cooling, owing to sill thinness , must have resulted in a departure from equilibrium. This is expressed by zoned plagioclases and heterogeneous pyroxenes, for example, and the short duration over which various differentiation mechanisms were ap parently allowed to proceed, Yet, the stratigraphy bears witness to the activity of these mechanisms during, and perhaps after, the intrusive event. The following discussion is an inquiry, into the. nature of dif ferentiation mechanisms , particularly with respect to petrogenesis of the porphyry zone,and crystallization of the sill.

The Porphyry Zones

Grout-Schwartz Hypothesis

The porphyry zone of the Pigeon Point sill, according to the original hypothesis of Grout and Schwartz (1933), developed in response to flotation of early-formed plagioclase phenocrysts which precipitated after intrusion. The hypothesis is based on field relations, namely that porphyry zones are found usually near the roofs of sills, and on the den sity difference between plagioclase and solid diabase at room tempera ture. More recent field data and information on the physical properties of feldspars and, in particular, mafic melts require a reevaluation of the problem.

. 71 ■ . ■ ■ : 72 There has been no general agreement on whether or not plagioclase would float or sink in mafic melts. Cumulus plagioclase and anorthositic zones suggest sinking in many mafic bodies, for example, in the Skaergaard (Wager and Brown, 1967) and some Karroo (Walker and

Poldervaart, 1949) and Tasmanian (Edwards, 1942) dolorites. On the other hand, flotation is. suggested as the dominant motion from field relations elsewhere, as, for example, in some Greenland rocks (Bridge water and Harry, 1968). Hess (I960), in a classic investigation of the

Stillwater complex, suggested that since the density contrast between plagioclase and the magma is so small, plagioclase is likely to remain suspended.

Calculation of Densities

Recent data on partial molar volumes of oxide components of rocks and their mutual effect in combination allow reliable calculations of densities of liquid silicate systems of known composition (Bottinga and Weill, 1970). Using their method, densities of Logan liquids have been calculated. The results are shown in figure 38, along with experi mentally determined melt densities . Corresponding chemical composi tions are given in table 3 .

Figure 38 also shows a range for plagioclase densities corres ponding to feldspar compositions from Angg to Angg. These have been calculated as follows . Combining density data for high plagioclases at room temperature (Barth, 1969, fig. 4.7) with data on the volume ex pansion of four plagioclases given in the "Handbook of Physical Con stants" (Clark, 1966), densities for the four plagioclases were calcu lated to 1000oCi The resulting density-temperature curves were E 2.60 cn

250

1400 1300 1200 1100

■ w „ o, 0 „ ,u ,. o, v,„-= M...O « u . a .- — - th*

W water. Dashed lines areexpenmen 74 Table 3 . Chemical,Compositions of Rocks of Figure 38

1 2 3 4

S i0 2 47.22 49.88 48.24 50.71

T i0 2 2.79 1.19 0.82 1.70

A12 C3 14.23 18.55 18.12 14.48

Fe2°3 5.61 2.06 1.25 4.89 FeO 9.86. 8.37 6.70 9.07

MnO —--- 0.09 ---- 0 .2 2

MgO 7.21 5.77 9.41 4.86

CaO 8.32 9.70 11.84 8.83

Na2C 2.57 2.59 2.56 3.16 k 2o 0.75 0 .6 8 0.26 0.77 h 2o 0.94 1.04 0 .6 6 1.04 ---- P20 5 tr 0.16 0.36

1 „ Rose Lake diabase, sill "A" .

2„ Gabbro, Pigeon Point (no. 3, table-2), not all elements reported (Grout and Schwartz, 1933, table 8 , no. 2).

3 . Van Haven diabase (Dane, 1941, table 3 , no. 1).

4. Columbia River basalt (Sottinga and Weill, 1970, table 7) . . . , ' . 75 extrapolated to 1200oC. From these and a similar curve constructed from data for plagioclase of the composition AngyAbggOri (Stewart et a l.,

1966), curves relating density to plagioclase composition were con structed at various temperatures (fig. 39).

Discussion of Density Data

The essential problem of density relationships is emphasized by their comparison in figure 38. It is obvious that small variations in any of these values can drastically alter the behavior of plagioclase in melts. Factors on which these relations are most dependent are (1) com position of the melt, particularly with respect to water, or other light volatiles: ( 2) load pressure, on which the effect of water is severely dependent; (3) oxidation state of iron; and (4) initial feldspar composi tion. . Water in the melt. The effect of water has been evaluated using the equation of its partial molar volume in the Ab-H^O system as given by Burnham and Davis (1969)*,

Vh20 = 24.7+7.0x10-3 (T -900) - 1 . 5P + 8 .4 x lO " 2? 2 - l„ 4 x 10~ 3 P3 , where is the partial molar volume of water (cm 3 x mole- !), T is temperature (°C), and P is pressure (kb). The equation emphasizes the dependence of molar volume on high pressures .

*A recent revision of this equation (Burnham and D avis, 1971) is given as;

VH2 O = 10.98+0.962T + 0.1005T2 - 0 .00199T3 - P(0 .53 + 0.203 IT + 0.00158T2) + p2 (0.078 + 0 .0 1 144T)-P3(0.00396), where VH2 O is in cm2 mole-1, P is in Kb, and T is in 1 0 2 degrees C. Although the partial molar volume is more temperature dependent in this equation, it does not differ significantly from those calculated from the former equation under physical conditions of concern here. 400 600 900 "C 1000°C

1200 0C

Of - : ; ' 77

A pressure of 0.5 kb has been assumed in the density calcula tions and has been estimated as follows . The primary mineralogy of the diabase is a low-pressure assemblage since amphibole is not present.

(The latter appears as a primary mineral in the hydrous olivine tholiite of Yoder and Tilley, 1962, fig. 27, at approximately 1.5 kb.) Secondly, if it is assumed that the upper contact of the Rove Formation is at the approximate position of the present contact of the Duluth Complex (since volcanics crop out above the Duluth Complex to the south in figure 1), then the sills at Rose Lake were probably intruded beneath a cover Of

1,000 feet of Rove rocks. The thickness of volcanics covering this area, in middle Keweenawan time is not known; it is likely to be minimal, how ever, since they thin rapidly away from the axis of the Lake Superior syncline. Therefore., 0 .5 kb may represent a minimum load pressure, and 1 kb is probably a maximum.

Since the volume function of the above equation is not signifi cantly dependent on pressures of less than a kilobar or so, pressure ' effects are not really germane to the present discussion. It should be noted, however, that the density of a hydrous magma in a deep chamber is less dependent on water than it is at shallow depths, where water may decrease its density by several percent. Accordihgly, it is more likely for plagioclase of a given composition to float or remain sus pended in a deep magma than if the magma were intruded to shallower depths. The effect of elevated pressures on the partial molar volume of other components is negligible (Bottinga and Weill, 1970).

A comparison of curves 3 and 4 in figure 38 stresses the impor tance of water content to the density of liquid silicate systems. Curve 5 is calculated from an analysis of gabbrd from the Pigeon Point sill, neg lecting water, whereas curve 4 is a recalculation including its original

1.04 weight percent water. Curve 5 is the calculated density of sill

"A" with 0.94 weight percent water, the value calculated from modes.

However, the amount of water in a magma at near liquidus temperatures is difficult to estimate, since it may not be accurately reflected by composition. A recent estimate (Moore, 1970) indicates that oceanic tholeiites, Hawaiian tholeiites, and alkali-rich basalts from ocean regions average about 0.25, 0.5, and 0.9 weight percent water. Recal culated densities of Rose Lake diabase using the lowest figure (0.25 percent) are significantly higher than densities of labradoritic plagioclase at corresponding temperatures . However, the moderate to severe deuteric alteration of Rose Lake rocks and independent estimates of the water content of the original magma, based on density requirements and water solubilities (see below), suggest 1 .0 percent water as being a more realistic value.

Oxidation State of Iron. The oxidation state of iron at high magmatic temperatures is dependent on oxygen pressures (PQg), which may be indirectly controlled by water pressure (Hamilton and Anderson,

1967) . High Pji20 tends to cause high Pq 2 if Hg is allowed to escape from the magma. As discussed in greater detail below, early precipita tion of an Fe-Ti spinel phase in the Rose Lake diabases suggests ini tially high Pq2 .

Density calculations are severely dependent on the oxidation state of iron because densities of FeO and FegOg are 5.6 and 3.1 gm/cc, respectively, at 1400°C (Bottinga and Weill, 1970). The curves of figure 39 are based on the FeO/FegOg of the original chemical analyses .

Since most F@ 2 ^ 3 *s contributed by primary iron-titanium oxide minerals,

it is thought that FeO/FegOg of the analysis given for Rose Lake diabase closely approximates that of the original melt. Thus, calculated densi ties of Rose Lake liquids are correct with.respect to the oxidation variable.

Feldspar Composition. The average composition of plagioclase phenocrysts from the porphyry zone of sill "A" is about An 5 4 . This den sity is slightly higher than the diabase at corresponding temperatures.,

Densities of plagioclases range from 2.51 to 2.71 gm/cc for albite and anorthite, respectively, at 1200°C (fig. 39). It is clear that the effect of fractional crystallization of a mafic magma in which progressively more sodic feldspars form may be to change completely the relative densities of the plagioclase and liquid phases. Thus, slopes of density- temperature Curves of normally zoned plagioclases are flatter than those of plagioclases of constant compositions and.even may assume negative values. With lower temperatures, melt and plagioclase phenocryst den sities should approach each other (assuming the density-temperature curve of the former maintains a constant slope). This was probably not an important factor in sill "A" because the compositional zonation of the phenocrysts is small, ranging from An^g in their cores to An^g at their edges. However, in Skaergaard or Stillwater magmas, where the range in feldspar compositions is much greater, their changing densities may have had an important influence on differentiation. Nature of the Magma at the Time of Emplacement

The nearly simultaneous appearance of plagioclase and pyroxene under physical conditions likely to prevail in many shallow natural mafic systems follows from the melting curves of Yoder and Tilley (1962, fig.

27-30). In addition, data of Weiblen and Wright (Shaw, 1969) show that pyroxene and plagioclase began precipitating at approximately the same temperature in some Hawaiian tholeiites. Richter and Moore (1966) have shown that the crust of the Kilauea Iki lava lake, Hawaii, is character ized at its base by a zone several inches thick where 50 percent of the crystallization takes place. The change in crystallinity here occurs over a temperature interval of about 5°C. Walker (1957) concluded that aphitic to diabasic textures develop because plagioclase generally nuc leates more rapidly than pyroxene, although both phases may appear simultaneously. Consideration of the above and the fact that the chilled zones of the Rose Lake sills contain occasional plagioclase phenocrysts, and the more calcic nature of phenocrysts of the porphyry zones leave little doubt that plagioclase phenocrysts precipitated before and under different physiochemical conditions than the remainder of the sill, that is, at some time prior to intrusion.

Observations Pertinent to Petrogenesis

The list below summarizes observations which a petrogenetic model must explain:

1. Phenocrysts—Plagioclase phenocryst compositions range from

Antjg in their cores to An^g at their edges . Phenocryst borders

have compositions similar to groundmass plagioclase, which is not strongly zoned in sill "A" but more so in sill "B" (fig „

32, appendix).

2 o Contact relationships—Contacts of porphyry zones and diabase

are sharp (fig. 27). They are usually found near sill roofs, al

though there may be exceptions .

3. Texture--Elongate feldspar phenocrysts are randomly oriented

at Rose Lake; the ground mass is characterized by pyroxenes

poikilitically enclosed by plagioclase. Phenocrysts make up

36 + 3 volume percent of rock, not being closely packed (fig. 27).

4. Composition—Porphyry zones in both Rose Lake sills share

slight alkali and silica enrichment with the diabase immediately

above and below them.

Evaluation of the Flotation Hypothesis

It is evident from the above discussion that the density relation ship between plagioclase and the Rose Lake liquid does not favor the flotation mechanism, of Grout and Schwartz (1933). However, the rela tionship does not argue conclusively against it, since small changes in variables, such as the water content of the magma and oxidation state of iron,allow melt densities greater than plagioclase densities . Assuming this to be the case, a crude calculation can be made of the density con trast required to produce the porphyry zone.

Thermal Requirements. Shaw (1969, fig. 2) has shown that the . viscosities of tholeiitic melts from Hawaii increase from about 10^ to

1C)5 poise in a temperature interval of approximately 40°C. It is assumed that the Rose Lake magma was intruded at 1200°C and that it had cooled 82 to 1150°C before its viscosity became sufficiently high to prohibit fur ther movement of crystals. Since the porphyry zone is 8 feet (2.4 m) below the roof of sill "A" , the upper 8 feet of the sill must have cooled to 1150°C before notable flotation occurred. The time required for this can be calculated according to the methods of Jaeger (1968). He relates various values of n= T /ro to x / 2 (kt) , where T0 is the temperature of the magma at the time of intrusion, T is the temperature at some time, t (sec), after intrusion of some point, x (cm), distance from the con tact, and k is the thermal diffusivity (taken as 0.01 cm^/sec) . In the simplified model for the Rose Lake magma, n= 0.955. The corresponding value of x/2(kt) 1/2 is -1.2 (Jaeger, 1968, table I). If x= 240 cm (the distance of the porphyry zone from the sill's upper contact), t is 10? sec. Therefore, approximately one -third of a year is the time allowed for flotation. "

Flotation Rates . Flotation rates can be calculated using

Stokes' Law for settling of spheres ,

v - . 2gr2Ad v an where V is the settling rate (cm/sec), Ad the density contrast between the particle and fluid (gm/cc), g the acceleration due to gravity, r the radius of the particle (cm), and the coefficient of viscosity (poise) .

Typical values of Yj for basaltic liquid at 1200°C are on the order of

300 poises (Kani, 1934; Minakami, 1951; Shaw, 1969). The average radius of plagioclase phenocrysts is 0.25 cm. From an initial position . in the center of the sill, plagioclase phenocrysts would have to migrate a distance of 8 .6 meters to reach the upper part of the porphyry zone. 83

To move this distance in one^tbird of a year, flotation velocities on the. order of 26 m/yr are required. From Stokes' Law, this corresponds to a density contrast on the order of 1.8 x 1 0 ""^ gm/ccl If the coefficient of viscosity were 3000 poises, the required density contrast would in crease by one order of magnitude.

An important conclusion to be drawn from this is that even in thin, rapidly consolidated sills , gravitational stratification is.likely to occur if a solid phase is present. The mechanism is probably mainly controlled by the viscosity of the melt and by the time of appearance of a particular solid phase relative to that of other solid phases. This is emphasized by inspection of Stokes' Law, which shows that settling velocities are directly dependent on Ad but also on the square of the particle radius.

One observation which apparently cannot be explained by the flotation mechanism is the'distinctly noncumulus texture of porphyry zones „ Plagioclase phenocrysts are not closely packed nor do they exhibit a preferred orientation (fig. 27) . Both should be expected of plagioclase crystals floating in a fluid melt. Also, flotation cannot account for differences in the position of the porphyry zones of sills "A" and "B" . For these reasons , and also because the density relationships previously calculated do not favor flotation, other differentiation mecha nisms are considered.

Other Mechanisms

Multiple Intrusion. Frankel (1967) has defined multiple.intru sion as resulting "from two or more surges of the same magma injected at time intervals sufficient to allow for some cooling and partial to complete crystallization of each preceding surge before the injection of the next portion of the magma." It is important that the distinction be tween multiple and composite intrusions be understood. The latter originates from two or more different magmas, and rather striking simi larities between diabase porphyries and their hosts argue against com posite intrusion. The multiple intrusion mechanism presently enjoys general popularity. For example, it has recently been applied to the

Palisade Sill (K. R. Walker, 1969) and the Sierra Ancha complex of

Arizona (Nehru and Prinz, 1970).

If diabase porphyry zones represent multiple intrusion, narrow limits can be placed on the time difference between the first and second magmatic pulses . The lack of interior chilled zones indicates that the first intrusion would still have been hot at the time of the second, that is, hotter than 300 °C (Jaeger, 1958). Furthermore, equivalent enrich ment of silica and alkalis in both the porphyry and neighboring diabase suggests that the second pulse would have to have been intruded while the first was still at low magmatic temperatures, that is, while some of the earlier intrusion was still liquid (corresponding to a temperature of about 1000°C). On the other hand, since groundmass plagioclase of the porphyry zone does not exhibit strong normal zonation whereas plagioclase of the surrounding diabase does , plagioclase of the first intrusions must have been solidified by the time of the second. This probably corresponds to a temperature of 1100oC.

It seems intuitively difficult, however, to explain the facts that porphyry zones are usually found near sill roofs, have generally tabular and regular forms, and, at least at Rose Lake, apparently maintain their lateral continuity for some distance„ Application of this mechanism to the Rose Lake sills, therefore, is not favored.

Flow Differentiation. Axial migration of particles in a suspen sion is the expected result in Poiseuille flow (Segre and Silbergerg,

1962). Karnis, Goldsmith, and Mason (1963) have shown that variously shaped particles migrate during flow toward an equilibrium position that is at a distance of one-half the radius of the conduit from its wall.

This corresponds to a position of about 20 feet below the roof of the Rose

Lake sills. The mechanism has been given geologic application

(Bhattacharji and Smith, 1964; Bhattacharji, 1967) and successfully ap plied to natural systems (Simkin, 1967). It is of interest that flow dif ferentiation accurately predicts the observed position of the porphyry zone of sill "B" . Because of this and the fact that axial migration of a suspension is a physical necessity, given sufficient time for migration and Poiseuille flow, the flow differentiation mechanism becomes an at tractive alternative.

Poiseuille's Law requires laminar flow. If it is assumed that

(1) preferred orientation of plagioclase laths is necessitated by laminar flow and ( 2) with nonlaminar flow, migration of a suspension toward a common axis does not occur, then the random orientation of plagioclase phenocrysts in the porphyry zone argues against flow differentiation.

It is difficult to evaluate these assumptions. However, nonlaminar flow in mafic sills, such as those at Rose Lake,is not unreasonable. The factors which determine whether or not a fluid flowing through a conduit is turbulent or laminar are embodied in the Reynolds number, Nr . For a tabular conduit of infinite lateral extent, Nr = 2pva/n, where p is the 86 density of the fluid, v its flow velocity, n its coefficient of viscosity, and a the thickness of the conduit. For laminar flow, values of Nf be low about 2000 are required. Sill "A" is 2200 cm thick; if p is taken as 2.6 gm/cc and n as 300 poises, the values of v > 53 cm/sec yield values of Nr > 2000. Such intrusion rates are reasonable (Shaw, 1965).

Proposed Mechanism

A mechanism is suggested which emphasizes the order in which liquids enter the conduit as an important factor in differentiation.

Intrusion by the well-known fluid wedge mechanism can be a rapid event (Shaw, 1965); and if so, then the viscosity of the magma should play an important role in the way in which fluid enters the widen ing conduit. That is, the least viscous liquid should be more sensitive to pressure gradients over a ^hort time duration and should enter the region of least pressure (the developing conduit) first. Since the effect of suspended solids is to increase viscosity of a liquid (Roscoe, 1952), liquids rich in phenocrysts should enter the conduit at some later time.

It is logical to assume that a nearly horizontal intruding wedge thickens, by uplift of its roof. Therefore,as later phenocryst-rich liquids enter a conduit, they should be emplaced above preceding liquids, assuming the sheet is not of infinite extent.

This process may be regarded as closely akin to both multiple intrusion and flow differentiation. Indeed, with variables, such as the amounts and types of suspensions in parent chambers , degree of crys tallization in the conduit, viscosity, and so on, one might expect a continuous gradation between the two mechanisms. ■ 87

This mechanism is favored to account for the porphyry zones at

Rose Lake because it is not, for the moment, as objectionable as the others. It is recognized that continued study should provide further limitations..

Crystallization

Iron-titanium Oxides

Textural relations involving the opaque oxides with other min erals are important to paragenetic interpretations „ These have been previously summarized on page 62 „ Their euhedral nature and inclusion within plagioclase and pyroxene indicate that the spinel phase crystal lized at rather high magmatic temperatures . Two alternative hypotheses are present „

1. Early and rapid crystallization of an ulvospinel was necessi

tated by high initial Pq 2 of the magma; it was immediately

oxidized to form ilmenite and magnetite-rich pairs (Buddington

and Linds ley, 1964). This was followed by resorption of the

latter phase and its replacement by pyroxene and plagioclase

at slightly lower magmatic temperatures„ Magnetite resorption

requires a decrease in PQg, which also has the effect of en

larging the stability fields of olivine and pyroxene at the ex

pense of magnetite (Boeder and Osborn, 1966). Since the liquid

is already in equilibrium with magnetite and pyroxene, a slight

decrease in Pg^ will place it entirely within the pyroxene field,

where magnetite is unstable. This hypothesis may suffer from the fact that it requires

spinel crystallization, exsolution, and resorption at high tem

peratures and over a narrow temperature and time interval,

that is, after the formation of a crystal mush (because there

is no gravitational sorting of the opaque phase) and before

complete crystallization of plagioclase and pyroxene.

2 „ A second alternative is favored here because it is more simple

and allows simultaneous growth of ulvospinel, plagioclase,

and pyroxene. In this case, ulvospinel grew where it could in

a more or less interfingering, three-dimensional fabric with

plagioclase and pyroxene. The ribbed nature of the opaques

developed because they grow most easily parallel to major

crystallographic planes and during this lateral growth enclosed

minor amounts of plagioclase and pyroxene. Exsolution of mag

netite and ilmenite-rich phases, in this case, is a distinctly

later, subsolidus event and bears no relation to silicate fill

ings between ribs „ More data on the composition of the opaque

phases are needed to estimate the temperature of exsolution and

to evaluate this alternative.

Poikilitic Pyroxene in Plagioclase

The point has already been made that crystallization of most plagioclase and pyroxene, to which ulvospinel should now be added, proceeded simultaneously. The poikilitic relationship between plagio clase and pyroxene supports this thesis . The close proximity of the poikilitic zones to intrusive margins suggests that their formation may in part be a function of rates of cooling. Rapid cooling may result in supercooling of the magma with respect to feldspar and cause its more rapid crystallization (Weiblen, oral communication) . The presence of pyroxene, in addition to the opaque oxide, as inclusions within plagioclase indicates that this mineral had also begun to nucleate. Thus, the times of appearance of these phases could not have been very dif ferent, but plagioclase growth was much more rapid.

Paragenesis

As discussed previously, the presence of plagioclase phenocrysts of the porphyry zone and in chilled diabase indicates that it was apparently the single solid phase in some deep chamber prior to intru sion. In addition, the presence of small amounts of iron-titanium oxides during intrusion is suggested by occasional groups of parallel needles of opaques in the chilled margins. However, their abundance must have been limited since no large-scale settling has been observed.

After intrusion, the formation of most plagioclase, pyroxene, and mag netite occurred nearly simultaneously . This sequence is illustrated in figure 40, which shows the phase relations in the system SiOg-MggSiO^-

FegO 4 -CaAl2Si2 Og at 50 weight percent anorthite and PQg = 10“® atm

(air). From the Rose Lake liquid, X, plagioclase is the first phase to separate. It is soon joined by magnetite, and the liquid moves down the phase boundary to the invariant point. A, where pyroxene also begins crystallizing. However, figure 41 shows the important consequences of crystallization at lower, more realistic values of P 0 2 ' the invariant point migrates toward the iron apex, and the fields of stability of plagio clase and pyroxene expand at the expense of magnetite. Thus, in order to reach the invariant point, the liquid travels a shorter path mainly 90

SILICA

ANORTHITE

PYROXENE

O liv in e / MAGNETITE

Figure 40. Phase Relations of the System CaAlgSigOg-IV^SiO^- Pe20^-Si02 in the Plane 50 Weight Percent CaAlgSigOg at pQg = 10~0.7 atm

X is the composition of the Rose Lake liquid. The course of liquid with crystallization is shown by the small arrow (Roeder and Osborn, 1966) . 91

/ Pn •IO'aro!m (oir) S'L Pn •IO',aN AN AN 'HEM pr

OL OL AN

(b)

AN SH ♦ AN

, AN

60 MAG ♦ AN.

Figure 41. An Irregular Surface of Liquid Compositions Satu rated with Anorthite and One or More Other Crystalline Phases Projected onto a Plane.

The dashed lines are contours of anorthite in the liquid. These diagrams have been termed "anorthite-saturation planes" (Roeder and Osborn, 1966) . Note that as pQ 2 decreases, the size of the olivine, pyroxene, and anorthite fields increase at the expense of that of mag netite . '>-> . 92

through the anorthite field; and the pyroxene phase joins plagioclase

and magnetite sooner. This agrees with petrographic interpretation.

Alkali Enrichment

The classical viewpoint (Walker, 1940; Edwards, 1942; Turner

and Verhoogen, 1960) maintains that alkali enrichment in the upper re

gions of sills is accomplished by fractional crystallization, settling of

early-formed solid phases, and upward displacement of residual liquids.

Nearly simultaneous crystallization of pyroxene and plagioclase severe ly limits this process, however, since it does not allow large-scale

settling of pyroxene (Hamilton, 1965). Therefore, mechanisms involving

upward migration of residual liquids (for example, Hotz, 1953; Walker and Poldervaart, 1949). or differential upward migration of alkali and volatile components in the melt (for example, Tomkeieff, 1937; Broderick,

1935; Kennedy, 1955; Hamilton, 1965; Richter and Moore, 1966) have

gained support. The latter authors observed alkali enrichment in the liquid immediately below the crust of the lava lake of Kilauea Iki,

Hawaii. They relate the enrichment to a hypothesis advanced by Kennedy

(1955), who reasoned that water in a melt, either in solution or as a gas, attempts to achieve a uniform chemical potential and migrates to areas

of least confining pressure and temperature (i.e., upwards). Since

solubilities of alkalis are high in water, these, too, are concentrated upw ards.

At Rose Lake, differentiation by crystal settling is untenable; textural relations do not permit it, and the sills exhibit modal homo geneity throughout. Consideration of this and the compositional and modal stratigraphy of sill "A" leads to the following conclusions; The constant compositions of plagioclase cores suggest that crystallization of this mineral commenced in all places under approximately equivalent chemical conditions; that is, notice able amounts of alkali constituents migrated upward either in solution or as a separate gaseous phase only after plagioclase had begun nucleation and growth.

The lack of strong zonation of groundmass plagioclase of the porphyry zone is fortuitous and had no relationship to the presence of plagioclase phenocrysts „ The absence of zonation is explained by the upper 12 feet of the sill having crystallized before any fractionation process became active.

The more alkalic nature of plagioclase within 5 feet of both the floor and roof of the sill may be partly the result of inward migration of alkalis from the wall rock. Simple chilling and fractionation within the interior of the sill does not explain either their strong zonation or slight potassic enrichment.

Volatile constituents, potassium, and silica remained mobile and chemically active for a longer time and are thus more even ly distributed throughout the upper part of the sill.

Compositional data of pyroxenes do not clearly define any frac tionation trend. Severe disequilibrium with respect to major pyroxene cations is on the scale of individual grains and re sulted from slow rates of cation diffusion within liquid and solid phases in the rapidly consolidated body. ' ' 94

Water Content of the Rose Lake Liquid

Since the density of mafic melts at low pressures is mainly dependent on water, the water content of a magma of a known density

can be calculated assuming the oxidation; state of iron is known. For the Rose Lake liquid, this latter condition is met (see page 78). Any hypothesis except multiple intrusion requires that the density contrast between the Rose Lake liquid and phenocrysts of the porphyry zone be

small in order for plagioclase to remain suspended. Therefore, the plagioclase and melt densities can be taken as equivalent at a given temperature. Since the slopes of density-temperature curves for plagio clase and the liquid are similar, little error is introduced by choosing an arbitrary temperature. At 0.5 kb and 1200°C, a plagioclase of the composition Ang^, the. average phenocryst composition, has a density of 2.635 gm/cc (fig. 39). The Rose Lake liquid under similar conditions and with a similar density contains 0.83 weight percent water.

The water content of a magma can be estimated independently if the amount of solidification necessary to saturate the remaining melt with water is known. It is assumed that saturation was necessary before alkali migration began. This is especially likely if a gaseous transfer mechanism were mainly responsible for fractionation because saturation is required to form a gas phase. As noted previously, the upper 12 feet of sill "A" solidified before alkali fractionation and, therefore, before saturation. Furthermore, since pyroxenes from about the lower 12 feet are not altered (appendix), these regions must also have crystallized before saturation released water for alteration. Accordingly, 12+12 = 24 feet or one-third of the 72-foot thick sill had crystallized before the 95 remaining melt became saturated. The amount of water present in the original magma must have been two-thirds of that required to saturate it.

The solubility of water in the Columbia River basalt at 1100°C is 1.8 percent.at 0.5 kb and 3.0 percent at 1 kb (Hamilton, Burnham, and Osborn, 1964) . The solubility of water in an albite liquid decreases with increasing temperature (Burnham and Jahns, 1962), so the above solubility figures must be considered a maximum for melts at 1200°C.

The maximum water content of the original Rose Lake liquid must have been 2/3 (1.8) = 1.2 weight percent at 0.5 kb and 2/3 (3.0) = 2.0 weight percent at 1 kb. Thus, two independent methods yield closely similar estimates. Table 4 summarizes these results.

Table 4. Estimated Water Content of the Rose Lake Liquid, in Weight Percent

Permissible Amount in Magma Estimate from Pressure W ater According to Estimate from Chemical (bars) Solubility Solubilities Density Data Analysis

500 1.8 1.2 0 .8 0.94 o CO 1000 3.0 2.0 0.94 SUMMARY AND CONCLUSIONS

Detailed mapping of the Hungry Jack Lake quadrangle has shown that the Logan diabases were generally intruded concordantly into the

Rove host rock. This simple pattern is complicated, however, by a sub ordinate structural pattern of thickening, thinning , or pinching out of the diabase up and down dip and along strike (fig. 10 and 11) and by local deviation of these bodies from a strictly tabular form.

Diabase of the sills cropping out at Rose Lake is characterized byplagioclase (An 43)in a diabasic to subophitic relationship with augite.

Together, they make up 75 to 85 percent of the unaltered rock. Skeletal magnetite-ilmenites reflecting euhedral outlines and granophyric quartz and orthoclase make up most of the remainder of the rock. The sills are essentially modally homogeneous with the exception of their plagio clase porphyry zones. Their textural stratigraphy is characterized by poikilitic pyroxene in plagioclase adjacent to the chilled margins, diabasic interiors, and porphyry zones near their roofs (fig. 23).

The compositional variation of pyroxene and plagioclase in sill "A" (figs. 31 and 32) is as follows . Cores of plagioclase maintain constant compositions, whereas their borders become much more sodic in the upper parts of the sill. That ground mass plagioclase of the por phyry zone does not exhibit such sodium enrichment is thought to be

-fortuitous and due to the fact that this zone crystallized before signifi cant upward migration of alkalies. Plagioclase near the borders of the sill exhibits slight alkali enrichment, a result of inward migration of

96 these elements from the host rock. Pyroxene compositions are homo geneous throughout the sill but markedly heterogeneous within any one grain. The disequilibrium is explained by slow cation diffusion in these rapidly consolidated rocks. : - 1 . '■ : V ' The calculated density of the Rose Lake.liquid with 1 weight percent water is slightly lower than that of plagioclase phenocrysts of the porphyry zone at corresponding temperatures. This and the lack of cumulate texture of the porphyry argue against the flotation hypothesis of Grout and Schwartz (1933) for the origin of these zones . However, small variations in the oxidation state of iron and water content of the magma can completely change plagioclase-melt density relationships, and the density argument is inconclusive. Results of calculations of cooling and settling rates show that gravitational stratification is a possibility, even in thin sills ( < 100 feet thick), if a solid phase is present.

Other mechanisms are considered. Multiple intrusion does not seem probable because the porphyry zones are usually found near sill roofs, are tabular, and show apparent lateral continuity for some dis tance in the Hungry Jack Lake quadrangle. Flow differentiation, al though it predicts the observed cross section of sill "B" , does not ex plain the random orientation of phenocrysts in the porphyry zone. The bias of this study favors a new mechanism whereby phenocrysts sus-^ pended in a deep chamber enter the developing conduit later than the phenocryst-free, more fluid melt. Since the sill probably thickens by uplift of its roof, the phenocryst-rich liquid is emplaced above earlier liquid. 7. ; - " 98 At the time of intrusion, the Rose Lake liquid contained occa

sional plagioclase phenocrysts and possibly an opaque oxide. After in trusion, plagioclase, pyroxene, and magnetite crystallization proceeded

nearly simultaneously. Small quantities of alkali, silica, and volatile

components migrated upward as a gas phase or in solution in the liquid, and the chemically active volatiles deuterically altered the primary p h a se s.

The water content of the original magma has been estimated by three independent means (table 4) . The amount of water in hydrous min erals of the diabase is 0.94 weight percent. Assuming that the density of the magma and plagioclase were the same, the water content would be 0.8 percent. If the sill crystallized at 0.5 kb, calculations using saturation levels yield a maximum value of 1.2 percent.

Finally, the importance of the study of small bodies for the purpose of evaluating igneous processes should, be emphasized. Not only can they be studied in greater detail, but also they offer many more constraints to igneous hypotheses than do studies of larger bodies. APPENDIX

MODES OF ROSE LAKE DIABASES

99 100

Sill "A" (Sample number corresponds to feet below roof; standard deviation at 95% confidence interval to nearest percent, from curves of Van Der Plas and Tobi, 1965; tr, less than 0.5% ; ----- , not present)

Sample Number 5 10 15 20 25

Plagioclase (incl. s ericite) 44.6 4 55.9 4 . 47.3 4 47.4 4 48.1 4 Pyroxene (incl. uralite) 26.0 4 20.6 4 24.5 4 . 23.4 4 24.6 3

Opaque oxides 9.1 2 4.7 2 6.6 2 6 .0 2 7.3 2

Quartz. & orthoclase 8.4 2 6.4 2 7.3 2 5.2 2 4.5 1

Biotite & chlorite 6 .2 2 2.6 1 7.1 2 7.5 2 9.6 2

Serpentine (?) & talc 4.1 1 7.8 2 3.2 1 6 . 6 2 3.7 1

Epiddte 1.6 2.0 3.1 1 2.7 1 1.6 Hematite tr 0.9

Carbonate ----- tr ——— tr 0.6 Apatite tr tr tr tr . tr Rutile & iron-rich alteration products tr tr tr tr tr Zircon tr ----- tr tr -----

Sulfides —— —---' ——— 1.2 ----- % An in plagioclase a 42 2 ,48 2 42 2 46 2 43 2 b54 2 Pyroxene composition c 34-38 -28 39-34 -27 37-35-28 37-39-24 33-39 -28 Total points counted 570 490 840 540 650

a. Average values; error is subjective judgment of this writer; probe determination unless stated otherwise . b. Average composition of phenocrysts . C. Average compositions in mole percent Wo-En-Fs; probe determinations. 101

Sill "A"—Continued (Sample number corresponds to feet below roof; standard deviation at 95% confidence interval to nearest percent, from curves of Van Der Plas and Tobi, 1965; tr, less than 0.5% ; ----- , not present)

Sample Number 30 35 40 46 50

Plagioclase (incl. sericite) 46.6 3 50.8 4 47.7 4 52.0 4 49.2 3 Pyroxene (incl. uralite) 23.7 3 26.7 4 23.5 4 24.3 4 24.0 3

Opaque oxides 8.3 2 6.4 2 4.1 2 4.3 2 6.7 2

Quartz & orthoclase 4.9 1 3.9 2 4.5 2 2.2 1 2.1 1

Biotite & chlorite 8.0 2 5.5 2 12.9 3 6.7 2 9.3 2

Serpentine (?) & talc 5.0 1 5 .2 2 4.2 2 8.8 2 7 .2 2

Epidote 3.5 1 0 .8 1.2 1.1 0.5

Hematite tr •----- 0.9 tr 1.0 Carbonate tr ----- ”™— — ----- —--- Apatite tr tr tr tr tr Rutile & iron-rich alteration products tr tr tr tr tr Zircon tr tr tr tr tr

Sulfides ----- 0.7 1.0 0 .6 — —— % An in plagioclase a 48 2 b 54 4 49 2 48 2 48 2 Pyroxene composition c 36-39 -25 34-40 -26 36-35 -28 Total points counted 820 500 500 500 860

a. Average values; error is subjective judgment of this writer; probe determination unless stated otherwise. b. Optical determinations, maximum % An. c. Average compositions in mole percent Wo-En-Fs; probe determinations. 102

Sample Number 55 60 65 70

Plagioclase (incl. sericite) 49.1 3 49.5 4 47.0 3 48.5 3 Pyroxene (incl. uralite) 25.3 3 28.9 4 33.2 3 a 3 7 .8 3

Opaque oxides 7.8 2 7.9 2 6.9 2 8.6 2

Quartz & orthoclase 2.6 1 5.0 2 5.2 1 4.1 1

Biotite & chlorite 9.1 2 5.7 2 5.0 1

Serpentine (?) & talc 4.1 1 1.9 0 .8

Epidote 1.4 1.1 1.8 1.0

Hematite 0.6 tr ------—

Carbonate ------Apatite tr tr tr Rut lie & iron-rich alteration products tr tr tr Zircon tr tr tr Sulfides tr tr tr % An in plagioclases “ c 55 4 47 2 c 53 4 42 2 Pyroxene composition d 38-38 -24 Total points counted 900 650 840 830

a. Includes biotite, chlorite, serpentine, and talc. b. Average values; error is subjective judgment of this writer; probe determination unless stated otherwise. c. Optical determinations , maximum % An. d . Average compositions in mole percent Wo-En-Fs; probe determinations. 103

Sill "B" (Sample number corresponds to feet below roof; standard deviation at 95% confidence interval to nearest percent, from curves of Van Der Plas and Tobi, 1965; tr, less than 0.5% ; -----, not present)

Sample Number 9 14 19 25 30

Plagioclase (incl „ sericite) 44.7 4 42.1 4 44.3 4 56.5 4 44.9 4 Pyroxene (incl. uralite) 31.2 4 28.8 3 25.4 4 18.8 3 29.1 3

Opaque oxides 7.2 2 10.0 2 7.4 2 6 .0 2 6 .8 2

Quartz & orthoclase 11.8 2 10.7 2 9.3 2 7.1 2 9.3 2

Biotite & chlorite 2.2 1 4 .4 1 7.0 2 5.4 1 . 6 .2 2

Serpentine (?) & talc 0.9 0.6 3.4 1 4.7 1 tr

Epidote 1.1 2 .0 3.2 1 0 .8 3.2 1

Hematite — — — ------—------

Carbonate L 0.9 0.9 tr 0.5 0.5 Apatite tr tr tr tr tr Rutile & iron-rich alteration products tr tr tr tr tr Zircon ----- tr ' tr tr tr Sulfide tr 0.5 tr tr tr % An in plagioclase a 51 4 51 4 55 4 2 bSs7 2 Pyroxene composition c Total points counted 560 790 590 750 760

a. Average values; error is subjective judgment of this writer; probe determination unless stated otherwise. b. Average composition of phenocrysts . c„ Average compositions in mole percent Wo-En-Fs; probe determinations. 104

' Sill "B"—Continued (Sample number corresponds to feet below roof; standard deviation at 95% confidence interval to nearest percent, from curves of Van Der Plas and Tobi, 1965; tr, less than 0.5% ; -----, not present)

Sample Number 35 40 45 50 55

Plagioclase (incl„ sericite) 49.5 4 44.3 4 48.6 4 46.9 4 . 46.7 4 Pyroxene (incl. uralite) 31.9 3 23.2 3 27.2 3 17.6 3 24.0 3 Opaque oxides 7.7 2 6.4 2 6.2 2 5.1 2 6.4 2 Quartz & orthoclase 9.6 2 9.1 2 9.3 2 7.4 2 4.5 1 Biotite & chlorite tr 1.1 1.8 8.0 2 4.8 1 Serpentine (?) & talc tr 8.2 2 4.6 1 10.8 2 11.6 2 Epidote 1.3 1.8 0.8 1.1 2.0 Hematite ----- 2.5 0.5 tr tr Carbonate ----- tr tr tr ----- Apatite tr tr tr tr tr Rutile & iron-rich alteration products tr tr tr tr tr Zircon —— ---— tr tr tr Sulfide ----- 3 .4 1 1.0 3.1 1 ——— % An in plagioclase a 48 2 Pyroxene composition ” Total points counted 760 730 770 650 730

a „ Average values; error is subjective judgment of this writer; probe determination unless stated otherwise„

b„ Average compositions in mole percent Wo-En-Fs; probe determinations. 105

Sill "B"—Continued

Sample Number 60 70 75 80

Plagioclase (incl. sericite) 47.5 4 46.9 4 45.3 4 40.0 4 Pyroxene (incl. uralite) 22.8 3 24.6 3 33.9 4 37.4 4 Opaque oxides 6.9 2 6.6 2 7.9 2 11.5 2 Quartz & orthoclase 4.5 1 2.9 1 4.8 1 5.2 1 Biotite & chlorite 7.0 2 7.7 2 3.1 1 5.4 1 Serpentine (?) & talc 9.6 2 : 9.3 2 3.2 1 ---- - Epidote 1.0 2.0 1.3 0.5 Hematite tr tr 0.5 ' —--- Carbonate ---! — tr ----- tr Apatite tr tr tr tr Rutile & iron-rich alteration products 0.7 tr tr tr Zircon tr tr tr tr Sulfide tr —--- tr tr % An in plagioclase a . 46 2 Pyroxene composition b Total points counted 730 750 620 740

a „ Average values; error is subjective judgment of this writer; probe determination unless stated otherwise.

b. Average compositions in mole percent Wo-En-Fs; probe determinations. REFERENCES

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HUNGRY JACK LAKE

1000 1 4 0 0 0 W

A v

HUNGRY JACK LAKE / * 4 1000 12000W

ASPEN HUNGRY JACK LAKE LAKE

1000 - 10000W

1000 9 0 0 0 W

ASPEN SOUTH LAKE NORTH

1000 - 8 0 0 0 W

1000 7 0 0 0 W / / // 1 ! I II I I ASPEN LAKE \ \ L i / / _ / I

1000 6 0 0 0 W

1000 \ 5 0 0 0 W \ e. b e a r sk in \ LAKE I FLOUR LAKE

1000- 4 0 0 0 W

FLOUR \ \ \ l i'E. BEARSKIN j \// / / LAKE

1000 - 2 0 0 0 W

l! ll l! BEARSKIN I FLOUR \ LAKE LAKE

1000 "

FEET ABOVE SEA LEVEL FEET WEST OF FOURTH PRINCIPAL MERIDIAN VERTICAL AND HORIZONTAL SCALE

0 5 0 0 1000 ______2 0 0 0 fe e t

ill DULUTH COMPLEX LOGAN INTRUSIVES ROVE FORMATION

FIG.12 CORRELATED CROSS SECTIONS OF PART OF THE HUNGRY JACK LAKE QUADRANGLE

MATHEZ. M.S. THESIS, GEOLOGY, 1971 90*30 90*22 30" 4 8 * 0 6 4 5 4 8 * 0 6 4 5 EXPLANATION LU > Z oc LU < O z Glaciofluvial sands and gravels of cn P LU CO the Rainy Ice Lobe, esker axes shown I- 0 1 019 LU < 3

ngaer A1013 n .1 0 2 5 O1016 r /y y' —— -——------z < I DULUTH COMPLEX cr. ^ K — ...... z m LU Predominently gabbroic rocks, undifferentiated 5 LU < u £ LU LU CL X CL

* « # «• * • # , • • • • * I I LU _J cr LU '/ O a . Q CL 3 LOGAN INTRUSIVES Sills and dikes; Id, diabase, diabase porphyry, intermediate differentiates; Ig, granophyre, later than diabases V V — 1700

Daniels b° v < LU LU cr ^ CL _i oo I |d Q s Q v.VSNt < IV u : m ! ^ ^zi LU ROVE FORMATION CL CL Argillites and graywackes ...... ■ - -....j■••<■ ; c:%: u _ 7 „ ~ — ::zcza Zz ; y s u T E !ZR--.;j -o k . N A I„yD N A L ; ' /«o0 _

14 Bearskin Xenoliths or small bodies of country rock within Z ' ' _ Boqeiiho^-i7oo— igneous masses T65N f ------T65N T64N v:.v; ------T64N ------(jf—" \ ja,..,,:;.; • :- Zci- •? «o V ^ T ^ * , | 9 C

z . Approximate area of outcrop s - o y f . * 6 Id

M C ■ F f e Id rTl' FloJf^Lake'^*- Campground !,i > CONTACT :\° 8M -/soo- Approximate locations indicated by long dashes,

6M rsts indefinite and concealed ones by short dashes; ^ 0 - _ — - — ^>-d—---._- queried ones are doubtful •«■■ ,UE S :';: /? o a d Lakef d "3% WU Pewee 1 ...... ------D ™ x VS^mt dx$> 9 < -rn^rf^r- FAULT /Sff° " 4 - . '*'** Make Approximate locations indicated by dashes; U Lace Lake A B M 1 9 1 0 and D identify upthrown and downthrown sides, o | ►yX XZxl Nrx.'^< respectively • • ■ — ■ "— id iast oearsriii ^ ‘ j Campground^ ^ ^ / . ''' fie» '° iz Gravet Fil /. 1 5 © Inclined Horizontal Strike and dip of bedding -6 M 1 8 8 8 f867 — j Q Siva 12 Bow Lake Hooker Lake' 1 6 Strike and dip of igneous contact 1 5 Quiver Lake i : t S 4 Vo Fag Lake - 17 Strike and dip of foliation Caribou Lake

Glenn Lake ~1 , 48*01 10 .1 . Axes of major eskers 90*30" RIW RIE 90*22*30 Base from U. S. Geological Survey SCALE 1:24 0 0 0 Geology by E. A. Mathez, 1 9 6 9 -7 0 1/2 Topographic map, 1959 0 I MILE \ KILOMETER

CONTOUR INTERVAL 20 FEET 3 DATUM IS MEAN SEA LEVEL MN MINN. GN FIG. 5 BEDROCK GEOLOGY OF THE HUNGRY JACK LAKE QUADRANGLE, COOK COUNTY, MINNESOTA QUADRANGLE LOCATION

MATHEZ, M.S.THESIS. GEOLOGY.1971