Geochemistry of the melt and country rocks of the Lake St. Martin impact structure, Manitoba, Canada

Geochimica et Cosmochimica Ada Vol. 54, pp. 2093-21 I I Copyright 0 1990 Pergamon Press plc. Printed in U.S.A.

0016-7037/W/$3.00 + .oO

Geochemistry of the melt and country rocks of the Lake St. Martin

impact structure, Manitoba, Canada

W. U. REIMOLD,“*‘~ J. M. BARR, ‘.* R. A. F. GRIEVE,~ and R. J. DURRHEIM~

‘Schonland Research Centre, University of the Witwatersrand, P 0 Wits 2050, Johannesburg, RSA *Bernard Price Institute of Geophysical Research, University of the Witwatersrand, Johannesburg, RSA

3Economic Geology Research Unit, University of the Witwatersrand, Johannesburg, RSA** 4Geophysics Division, Geological Survey of Canada, 1 Observatory Crescent, Ottawa, Canada, KIA 0Y3

‘Department of Geophysics, University of the Witwatersrand, Johannesburg, RSA

(Received September 28, 1989; accepted in revised form March 12, 1990)

Abstract--Impact melt and country rocks from the 23 km diameter Lake St. Martin structure in Manitoba were analysed for major and trace element abundances and for Rb-Sr isotopic compositions. Our aim was to better understand the formation of the melt rocks in this probable impact structure, to search for traces of the meteoritic projectile, to date the melt sheet in order to improve the chronological record of impact events on the Earth’s surface, and to constrain the target, from which the melt rocks could have formed.

Major element analyses are consistent with the observation by SIMONDS and MCGEE ( 1979) that many of the melt rocks from Lake St. Martin are chemically homogenized, to a degree similar to that of melt bodies from other impact structures. However, some samples are of slightly different composition. This can be explained by post-impact hydrothermal alteration (particularly near the present erosion surface), chemical exchange between melt-bearing breccia and basement rock near the bottom of the melt sheet, or limited mixing of different target lithologies. So-called “pseudotachylites” from the central uplift of the structure are chemically diverse and do not always represent the composition of their host rock. We conclude that such samples are not pseudotachylite but probably represent injected impact breccia.

REEs and other lithophile elements are well homogenized in the melt in comparison with the country rock variation. There is a slight enrichment of the siderophile elements Co and Au and of Cr in the melt rocks, but further analyses are required to verify whether this is caused by a small contribution from the meteoritic projectile. The melt rocks are also homogenized isotopically, when compared with the spread of isotopic ratios determined for the country rocks. The melt rock isotopic composition, however, can only be achieved by mixing of various granitoid country rock types with another component of high 87Rb/86Sr ratio. Furthermore, the melt rocks are isotopically reequilibrated: whole rock Rb-Sr isotope data yield an age of 2 12.4 + 43 Ma (Isr = .7 132 T 3) at 2a level. Whole rock and mineral separate data yield an age of T = 2 19 * 32 Ma (Isr = .7 13 1 + 3). This age, consistent with previously reported K-Ar ages of 200 + 25 and 250 + 25 Ma, is thought to be the best approximation to the age of the melt rocks and thus of the impact event. However, the 2a error of 32 Ma is too high to allow this age to become a significant addition to the data base of well-constrained impact ages for analysis of periodicity in the cratering record. An “age” of T = 2785.3 + 157 Ma (2~) for Isr = .6997 T 11 was obtained for the basement rocks of the Lake St. Martin structure, but the true meaning of this “age” is still unclear. While the age obtained is comparable to other Superior Province data, it appears that the Lake St. Martin country rock data define a mixing line.

The new chemical and isotopic results are consistent with an impact origin for the Lake St. Martin structure. Harmonic least-squares mixing calculations suggest that the melt rocks were formed from ca. 80% monzonitic and 15% granitic basement plus a small addition of ca. 6% carbonate rock. This is in good agreement with the results by SIMONDS and MCGEE (1979) and favors their suggestion that carbonate- rich target rocks, even when they form the upper portion of the target stratigraphy, do not significantly take part in impact melt formation but are largely removed in a CO*-rich vapor phase.

INTRODUCTION central area of the structure have been provided by MCCABE

THE LAKE ST. MARTIN structure is located at 98”32W/ 51”47w about 230 km NNW of Winnipeg in Manitoba, Canada (inset, Fig. 1). Detailed descriptions of the local geology, structure, and, in particular, of shock metamorphism in basement lithologies and of melt breccia occurrences in the

and BANNATYNE (1970) and SIMOND~ and MCGEE (1979). From structural evidence, shock metamorphism ranging from stage 0 to 4 (STOFFLER, 197 I), and the occurrence of various types of elastic and melt breccias within the crater, an origin by meteorite impact for the Lake St. Martin structure has been widely accepted. However, CURRIE (1970) concluded that the structure had an endogenic origin.

The structure lies near the eastern edge of the Paleozoic

* Present address: Department of Geochemistry, University of cover overlying Superior Province granitic gneisses of the Cape Town, Rondebosch, 7700, RSA. Canadian Shield. Conventional K-Ar ages of 250 + 25 and

** Present address. 200 f 25 Ma are cited by MCCABE and BANNATYNE ( 1970)

2093

2094 W. U. Reimold et al.

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IMPACT LITHOLOGIES

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FIG. 1. Location of (inset), surface geology, drill hole, and surface sampling localities in the Lake St. Martin crater structure, Manitoba, Canada. Geology after MCCABE and BANNATYNE f 1970). Schematic, composite cross section through the structure as derived from drilling information. Relative positions of some drill holes are shown for reference, No vertical exaggeration. Outer circle represents inferred original position of crater rim. inner circle represents outer edge of annular, melt rock-bearing trough. Note very poor surface exposure; white areas are covered by varying thicknesses of glacial drift.

for two melt rock specimens and, as SIMONDS and MCGEE (1979) point out, such an age is consistent with the observation that the youngest rocks of precratering age are De- vonian and that the crater was filled with Jurassic redbeds and evaporites.

A drilling programme within the structure was undertaken during the late Sixties. The results are critically evaluated by MCCABE and BANNATYNE ( 1970). Figure 1 presents a sketch of the local geography and of the limited surface exposures in the structure, and also shows a geological cross section, as

Geechemical study of the Lake St. Martin impact 2095

derived from drilling information. The Lake St. Martin structure is a structurally modified complex crater of ca. 23 km in diameter. A central uplift consisting of Superior Prov- ince gneiss and also containing some impact melt rock is transected by numerous veins of pseudotachylite carrying intensely shocked clasts (SIMONDS and MCGEE, 1979). The zone surrounding the central uplift consists of various layers, some consisting of impact melt rock, others of polymict

breccia, that are described in detail in the cited literature. Strongest shock metamorphism of country rocks was en- countered in drill core or surface specimens from the central

uplift. Despite the fact that a number of major element analyses

for melt and country rocks from the Lake St. Martin structure have been published, we carried out further detailed analysis with the following main objectives:

a) Multi-trace element analyses are needed to better char- acterize the melt rocks and provide further information on their degree of homogeneity and the nature and proportions of potential parent rock components. In addition, we hoped that traces of the meteoritic projectile could be detected in the siderophile element record of Lake St. Martin melt rocks.

b) The medium grain-size of some melt rock specimens per- mits mineral separation for isotopic dating of the age of the cratering (i.e., melt rock-forming) event. The Rb-Sr method has been previously successfully applied to the dating of terrestrial and lunar impact melt rocks (e.g., JAHN et al., 1978; REIMOLD et al., 1981; and others). In addition, few studies have to-date considered isotopic homogenization and equilibration during formation of terrestrial impact melt rocks (e.g., REIMOLD, 1980, 1982, and refs. therein). A better constrained age is germane to the major controversy, whether mass extinctions in the biostratigraphic record can be related to periodic increases in the rate of meteorite or comet impacts onto the Earth’s

surface.

SAMPLES AND EXPERIMENTAL PROCEDURES

Small specimens (~2-70 g) from a number of drill cores and some of the better surface exposures of both melt and country rock (Fig. 1) were used. Powder aliquots of most of these samples were analysed for major elements by XRF at the Geology Department of the Uni- versity of the Witwatersrand. Accuracy levels as obtained from standard and duplicate sample analysis are indicated in the data list of Table 3a. XRF analyses for samples 12A-78 and 2 IA-78 were performed by the Geological Survey of Canada. Trace element concentrations were determined by instrumental neutron activation analysis at the Schonland Research Centre applying the technique described by ERASMUS et al. (1977).

Whole rock specimens were prepared by careful crushing in several stages, while visible clasts were removed by handpicking. Preparation of mineral separates (i.e., pyroxene- and feldspar-enriched fractions) for Rb-Sr isotope analysis involved magnetic separation followed by handpicking. Rb and Sr concentrations and isotopic compositions were obtained by standard cation exchange/isotope dilution tech- niques and mass spectrometry at the Bernard Price Institute of Geo- physical Research. Rb and Sr total chemistry blanks analysed during the period of this work were in the order of 2 and 4 ng, respectively. Typical SRM-987 standard values from the MM 30 mass spectrometer at the BP1 Geophysics during that time were averaged to .71034 f 17 (2u).

Detail of sample mineralogy is given in Tables 1 (country rocks) and 2 (melt rocks). The country rocks represent a suite of granitic to monzodioritic gneisses varying in biotite and hornblende abundances and grain-size (fine- to coarse-grained), and of amphibolite metamorphic grade. They appear generally fresh (Fig. 2a and b). Sericit- ization of feldspar can occasionally be observed, but alteration of biotite or hornblende is extremely rare. A few specimens contain traces of carbonate (sample 10-78, - 1 ~01%). With the exception of the three specimens 18A/B/G-78 from the central uplift and drill core samples 4-382 and 4-395 (Table 1) no characteristic shock metamorphic effects and thermal deformation were observed. A few examples of shock effects (multiple sets of planar features and partial conversion to diaplectic glass) in the strongly shocked specimens are shown in Fig. 2c and d. The melt rocks are generally clast-poor (<5- 10 ~01%) and. with the exceotion of samples 6G-78. 12B-78 and 4- 387, more or less holocrystalhne (compare mesostasis contents given in Table 2). Figures 2e and f and 3a-d show examples of melt rock textures and clast deformation. Grain sizes for melt rock matrix minerals (plagioclase and pyroxene) vary from fine (SO-100 pm) to medium-grained (100-300 pm, rarely up to 500 pm). Clasts recognised are generally feldspar (mostly plagioclase) and some minor quartz. Significantly, only very few relatively pristine, unshocked and weakly shocked clasts were observed. By far the majority of clasts occur as either sieved (checkerboard, parquetted-Fig. 2e and f) crystals (e.g., FLORAN et al., 1976; SIMONDS et al., 1978; PHINNEY et al., 1978; REIMOLD, 1982; BISCHOFF, 198 1) or in the form of so-called “ghosts” (completely fused and finest-grained recrystallized grains; e.g., REI- MOLD, 1982; BISCHOFF, 198 1, Fig. 3a and b). The melt rock samples appear largely unaltered, with only minor carbonate or clay mineral alteration in the matrices. However, sample 4-387 contains several, up to 4 mm large, amygdules fdIed with chlorite, saponite, nontronite, oxides, and other unidentified secondary minerals (Figs. 2f and 3e), and mesostasis in this sample is also partially oxidised (Fig. 3d). Several ~01% of biotite and some chlorite are also present in the matrix of this sample. The mesostasis between matrix plagioclase and pyroxene consists of very line-grained to cryptocrystalline, intergrown quartz and alkali-feldspar, with accessory iron oxides.

EXPERIMENTAL RESULTS

Major Element Chemistry

XRF results and CIPW normative compositions for melt and country rocks are compiled in Tables 3a and b, respectively. Only selected samples were analysed for major element abundances to supplement the existing data of MCCABE and BANNATYNE (1970) and SIMONDS and MCGEE (1979) and to consider the compositions of more exotic specimens (e.g., 4-387) for mixing calculations.

Two ternary diagrams (Fig. 4a and b) represent the new data in terms of KzO-CaO-NazO and A1203-Fe203-MgO concentrations. Figure 4a shows a rather homogeneous composition with respect to alkali elements in the melt rock specimens, but clearly indicates a significant shift towards higher CaO contents, in comparison with the country rock data. Partial enrichment of melt rocks in CaO was also described by SIMONDS and MCGEE (1979), and was discussed by these authors as being either the result of admixture of a small carbonate contribution or of hydrothermal alteration. Some

of the country rocks have higher KzO contents than the melt rocks. In Fig. 4b (AlzOs-FezOs (total Fe)-MgO) the melt rocks

show the same variation as the few analysed country rocks. Several melt rocks appear to be slightly enriched in MgO (cf. also SIMONDS and MCGEE, 1979). These authors also report an excess in Al203 for their samples from drill core LSM4, an observation that is not confirmed by our new data. (For comparison with Figs. 4a, Sa and b, and 6 note the different scale for Fig. 4b).

2096 W. LJ. Reimold et al.

Table 1 Mineralogy of cout~try rocks (estimated vol 46; xx-accessories), rock compositions after STRECKEISEN (qz - quartz, ptag-ptagioctase).

7-78 8-78 9-78 1X-78 16B-78

m 30 30 30 20 25 __.__ Plagioclase K-feldspar Biotite Muscovite Hornblende Sphene Clinop~oxene Apatite opasues Clay minerals Carbonate

:: 15

30 35 5

25 30 5

55 40 20 10 15

10 10 5

xx xx

xx xx xx xx xx

xx xx xx xx

xx xx 5

xx

Granodioritic

phaneritic med.-grained

qz-und. extinction

xx

Granitic shocked

phaneritic, med. grained

qz-as in 8-l 8

Granitic

phaneritic, med. grained

qz-ud. extinction

plag. to sericite (very minor) Inclusion in

9-78

Graoitic

phaneritic COii%W-pined

qz-und. ext., some annealing

plag. to sericite (common)

phaoeritic med.-coarse grained

fsp-low birefr., down to near-

isotropism; hbl strongly fractured;

kinkbanding in biotite; quaitz & feldspar: multiple sets of planar features;

beginning isotropixation

very fresh pIag. to &cite (minor)

20.78 1078 4-382 1478 a-79 4-395

ouarls 20 30 3035 30 20 30 piagioclare K-feldspar Biotita h4uswvite Hanblende SPheae Clinopymxene Al&e

clay minsals Ca&OlUte

3s 20 10

1s xx

10-U 35-40

10 d

45

15-20

30 25 15 xx

30 40 d xx

40

25

d xx

1-2 xs

xx xx

xx xx xx

xx xx

xx Lx s

<I

oraaitic

xx

Grsaitic Granitic oTsniIic

msd.-coarse. grsined fine-med. grsincd fine-med. grained fine med. grained

multiple sets of planar featmw in qz and feldspar; partial

isotropization (dia- p1ectic ql/fsp glarsc+

local fusion; sligbteat amount of

saicitization

minor kreguIsr

ffscturing; bent p1ag. lamllpe;

Wp slightly se&i- tisad, biotite slightly

alwed (some. wbonate. oxides);

minor fracturing wry fresh; minor

some irre@ar fmmring; cleavage in

fsp well-developed; minor aericitiration

tlsshecked; fsp. very fresh-some

minor wtrbonate d8vclopment at contact8 fspN0; some

pnne&ng, appsfially of qz

&nilar to 18(3-78; sligbtly mom isotiopi- _

ration; some fcldsDar alte-

ration; biotii strongly OXidkd

Figure Sa and b shows CIPW normative quartz, orthoclase, and plagioclase abundances in melt and country rocks, along with the results provided by SIMONDS and MCGEE (19791, including their “ps~udotachylite” data. The melt rocks form a relatively tight cluster. Two of the three “pseudotachylite” samples, however, have different compositions (Fig. 5a). In comparison with the wide scatter of country rock compositions-of granitic, monzonitic/monzodioritic, and dioritic mineralogy (Fig. Sb)-the melt rocks appear welt homogenized. SIMONDS and MCGEE ( 1979) compared the degree of homogeneity displayed by the major element compositions

of Lake St. Martin melt rocks to those of melt bodies from other impact structures and found it to be similar.

Figure 6 (MgO + FezOs-AIzO&aO + K20 + Na,O) compares major element com~sitions for all available analyses of Lake St. Martin melt and country rocks. Besides a slight shift for most melt rocks towards higher alkali element contents, the data spread for melt and country rocks appears to be of similar magnitude. This finding is in apparent contrast to the discussion of Fig. Sb. The results of artificial norm calculations do not fully reflect the small v~abiliti~ with respect to MgO, A1203, and alkali elements.

Tab

le 2

M

iner

alog

ical

ch

arac

teri

stic

s of

som

e an

alys

ed m

elt

rock

s (e

stim

ated

vol

%,

n.d.

= n

ot d

eter

min

ed,

x =

trac

es)

----

----

--__

__

____

____

_-__

____

_

l-29

l-

135

5-78

66

-78

12B

-78

1-9

4-78

4-

387

13B

-78

66-7

8

Mat

rix

Feld

spar

65

Py

roxe

ne(a

ugiti

c)

30

5 M

esos

tasi

s C

arbo

oate

(sec

ond.

) -

Bio

tite

med

grai

ned,

op

hitic

Feld

spar

<5

Cla

st

<lm

m

opaq

ues

X

Mus

covi

te

Car

bona

te

x H

emat

ite

Hor

nble

nde

Apa

tita

60

30

5 S5

fine

-mod

. gr

aine

d op

hitic

5 m

inor

<.5m

m

X

all

clas

ta a

m

all

clas

ts a

re

all

clas

ts a

re

siev

ed o

r “g

host

” si

eved

or

“gho

st”

siev

ed o

r “g

host

” fe

ldsp

ar

feld

spar

fe

ldsp

ar

65

30

5

med

grai

ned,

op

hitic

(s

light

ly

coar

ser

than

l-2

9)

<5

n.d.

60

60

20

20

15

15

min

or

med

. gr

aine

d,

as 6

G-7

8,

ophi

tic

slig

htly

co

arse

r

10

5 1

unsh

ocke

d gr

ain

n.d.

n.

d.

X

X

X

as l

-29,

so

me

as 6

G-7

8,

min

or

carb

onat

e m

esos

tasi

s al

tera

tion

som

ewha

t al

tere

dto

gree

nish

cl

ay m

iner

al

70

15

5 5

fine

-mad

. gr

aine

d su

boph

itic

5

<2.0

mm

X

X

all

clas

ts

are

ghos

t fe

ldsp

ar;

som

e gr

eeni

sh

clay

m

iner

al;

amyg

dule

s

65

55

65

15

20

20

5 10

5

15

x

X

2

med

. gr

aioe

d fi

ne-m

ed.

grai

ned

med

. gr

aine

d (s

ub)o

phiti

c/in

ters

erta

l op

h.-s

ubop

h.

5-10

m

inor

<1.5

mm

10

<2m

m

S5

min

or

1.5m

m

X

X

X

X

x

X

X

X

X

feld

spar

al

l cl

asts

cl

uste

rs

of

clas

ts

are

siev

ed

pym

xene

in

ar

e “g

host

s”

or

“gho

st”

mat

rix,

cl

asts

fe

ldsp

ar;

ma-

ar

e si

eved

tr

ix

part

ially

fe

ldsp

ar

alte

red

(mes

o-

stas

is!)

; up

to

4mm

ant

ygdu

les

60

I5

5 10

X

med

. gr

aine

d op

hitic

5 m

inor

<2.0

mm

X

X

all

feld

spar

cl

asts

ar

e “g

host

s”


b

e

RIG. 2. (a) Texture of relatively coarse-g&red granitic country rock sample 10-78. Besides minor sericitization of plagioclase (grain in center, upper half) this rock appears reasonably fresh. Width of field of view: 3.4 mm, crossed nicols. (b) Rather fresh granite 11%78. Width: 3.4 mm, crossed nicols. (c) Several quartz grains displaying multiple sets of planar features from strongly shocked b~ment rock SC-78 from the central uplift. Width: 285 Fm, parallel nicds. (d) A ph&oclase (bent twin lameliae can still be recognised) crystal in country rock 4-395. This grain displays partial isotropization (transformation to maskelynite) at left and right as well as in center. Width: 355 pm, crossed

Geoehemical study of the Lake St. Martin impact 2099

c d

FIG. 3. (a) Sample 66-78: “ghost” clast (upper right, note many small feldspar crystals) in subophitic melt matrix. Width: 2.2 mm, crossed nicols. (b) Sample 4-387: this melt rock displays two matrix textures, rather well-crystallized subophitic areas and the fine-grained material of the upper half of this picture. It is not clear whether the fine-grained areas are due to the presence of “ghost” inclusions. Width: 3.4 mm, crossed nicols. (c) A large vesicle filled with chlorite and dark oxides in melt rock 4-387. The vesicle is surrounded by a concentration of pyroxene crystals. Width: 3.4 mm, parallel nicols. (d) In addition to the two textural extremes in sample 4-387 (Fig. 3b) this photo illustrates alteration that at&ted some areas of this rock: black areas in subophitic matrix are altered mesostasis. Width: 3.4 mm, crossed nicols.

It is inte~sting to compare “~eudo~chylite” ~m~sitions with those oftheir respective host rocks, as indicated by arrows in Fig. 6 (also compare Table 6b). While two “pseudotachylit&‘-host rock pains are only slightly different in com~~tion, the third pair does not relate compositionally. In the light of the general debate on the genesis of pseudotachylite-in tec- tonic settings, cryptoexplosion and impact structures (e.g., REIMOLD et al., 1987a,b)-and in order to improve the criteria for the distinction of pseudotachylite and impact breccias, further analysis of “p~udo~chylit~‘‘-host rock pairs is

obviously warranted. In Table 6a compositions of both so- called ‘“pseudotachylite” and melt rock from drill core LSM- 4 are compared with the two basement rock varieties all intersected within a section ea. 6 m (i.e., 18 feet) long. Whereas melt rock and “pseudotachylite” compositions match very well, both differ strongly from the local basement rock types.

SIMONDS and MCGEE (1979) argue that the limited variation of melt rock compositions could either be the result of incomplete mixing of basement rock varieties or of post-cratering chemical aheration. They favor the latter process on

polarizers. (e) Sieve structure in plagioclase clast in subophitic (upper right) melt rock 12B-78. Semicoherent portions of twin lamellae are strongly curved. Incipient melting can be seen near center of image. Width: 2.2 mm, crossed nicols. (f) A checkerboard-plagioclase clast (center) and an oval amygdule (upper rig&) surrounded by dark oxides and filled with clay minerals in melt rock 4-387. Matrix in this portion of the sample is rather coarse-grained ophitic. Width: 3.4 mm, crossed nicols.

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o1

4.2

3.8

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3.52

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2.9

2.6

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66.5

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2.8

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58.0

58

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68.5

70

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90

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16.6

13

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8 9.

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45

2.65

6.

7 .0

8 .I

2 .0

3 .0

3 .0

8 4.

3 3.

6 2.

2 1.

95

4.1

4.3

4.5

1.3

1.3

3.8

5.5

3.6

3.4

3.9

4.7

1.9

3.2

5.4

5.0

2.05

l 3

.65

.l .l

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2.6

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7.0

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2.8

64.0

.56

15.9

4.

5 .05

3.1

3.25

5.

2 2.

2 .3

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62.9

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16.7

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5 .l 1.

1 2.

15

5.5

3.1 .2

3.

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75.3

.04

13.5

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.06

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5.7 .0

6 .5

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63.3

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1 l 2

2.0

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3.3

-

TO

TA

L

99.8

6 99

.91

99.8

9 99

.86

99.8

3 99

.72

99.6

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1.03

99

.96

99.9

2 99

.87

100.

0 99

.87

99.8

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.96

100.

05

99.9

9 99

.88

- 4 c

TA

BL

E

3b

3 :

CIP

W

NO

RM

AT

IVE

MIN

ER

AL

OG

Y O

F

LA

KE

St

MA

RT

IN

ME

LT

AN

D C

OU

NT

RY

RO

CK

S

(cal

cula

ted

from

da

ta

of

Tab

le

3a1

0 b 2 W

K

E

L

T

R 0

C K

S

C

0

U

N

T-R

Y

R

0

C

K

S

r

6C-7

8 4-

78

l-9

138-

78

4-38

7 12

~-7

8 21

~-7

8 16

8-78

18

C-7

8 7.

78

10.7

8 14

.78

lSA

-78

20-7

8 4-

382

8-79

4-

385

Apa

tite

.I

Il

men

ite

.9

Ort

hoc

lase

20

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ite

32.3

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nor

thit

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net

ite

3.0

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19.1

33

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20.8

19

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35.9

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9 29

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6.9

5.8

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42

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27.7

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1.6

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17.9

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31

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3.1

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8 .v

11.8

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29

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33

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5.9

4.9

2.9

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5.6

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- 22

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6 4.

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41

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44.5

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- 9.

6 7.

6 2.

8 4.

6 1.

4 1.

7 9.

7 14

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13.1

l 14

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1.4

33.9

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30.6

51

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3.5

6.6

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0 .5

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.I5

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2.4

31.1

13

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Foo

tnot

e :

Wh

en

tota

ls

are

belo

w

100

(sam

ples

4-

387.

21

8-78

an

d 4-

382)

. th

is

is

due

to

con

trib

uti

ons

from

n

orm

ativ

e m

iner

al

prop

orti

ons

that

ar

e n

ot

list

ed

(e.g

.. eo

run

dum

l.

Geochemical study of the Lake St. Martin impact 2101

CaO NqO

b

FIG. 4. (a) Melt and country rock compositions in terms of KrO-CaO-Na20. (b) Melt and country rock compositions in terms of A120s-Fe203 (total Fe)-MgO abundances.

the basis of some mineralogical and morphologicdl observations.

A rather complete major element profile is available for the melt sheet intersected in drill core LSM-I between -3 and -2 I 1’; some detail on this core was presented by SIMONDS and MCGEE (1979; Table 2). As systematic compositional variations through melt bodies have been recorded from some other impact structures (e.g., Boltysh crater, GRIEVE et al., 1987, Fig. 6; or Cleanvater Past structure, PALME et al., 1979), we examined the major element concentrations from samples of the LSM- 1 profile in detail. The total alkali element, FezOx, and MgO concentrations do not significantly change over the range studied. However, both K20 and CaO profiles have changes along core 1 (Fig. 7). Minor changes in concentration are noted near the surface and at depth below - 18 1’ near the contact to a breccia composed of melt plus basement and

a LSM MELT ROCKS

+ this work Sim+We,l979:

carbonate, whereas the interior of the melt sheet is homogeneous. Weathering of near-surface melt probably caused the upper anomaly. The anomaly at the bottom of the melt sheet may result from either hy~othe~~ interaction between melt and underlying breccia and basement at-or shortly after-the time of impact, or to later fluid movement along that contact. This is clearly consistent with the hy- pothesis that hydrothermal activity is responsible for the compositional variations observed amongst the Lake St. Martin melt rocks.

Trace Element Chemistry

Abundances of 22 trace elements and of K and Fe were determined on 9 melt and 9 country rock samples by INAA (Table 4). Iridium con~ntmtions in all these samples are

b Qz

A

LSM Country rocks

CIPW m Simonds + McGee,1979

~2 all available melt

FIG. 5. (a) CIPW normative quartz (Qz)-orthoclase (Or)-plagioclase (PI) proportions of melt rocks and “pseudotachyhte” analysed in this study and by SIMONDS and MCGEE (1979). (b) CIPW Qz-Or-PI proportions of country rocks. For comparison the melt rock field has been indicated. Data clusters are as used for HMX mixing calculations, and some samples used for mixing modelhng are indicated. Rock classification according to STRECKEISEN.


McCabe+Bonmtvne.1970 . melt rock o “pseudotachyliW 0 country rock

I,bu!xk . melt rock o country rock

Simonds+McGee.l979

A melt rock x “pseudotachylite”

CaO A country rock

+K 0 ~“pseudotochylite”-hostrc$k pafrs + a20 N2

FIG. 6. MgO + Fez03-A1203-CaO + K,O + NazO diagram representing the complete data base for Lake St. Martin samples.

lower than our detection limit of 1 ppb. An additional analysis of a melt rock from the Lake St. Martin structure has been published by G~BEL et al. (1980), who determined an upper limit for Ir of .03 ppb. PALME (1982) concluded from this that the Lake St. Martin melt shows little evidence for enrichment of Ir, i.e., for contamination by an undifferentiated meteoritic projectile. Patterns, slopes, and average concentrations of REEs, as well as slight negative Eu anomalies, for the melt and country rocks are rather similar (Fig. 8a and b), with the exception of country rock 8-79. However, variation in REE composition of the melt rocks is a factor of 10 less than that of the country rocks. The limited variation is similar to that observed in other impact melt bodies (e.g.,

50

100

2oc

Depth (ft)

LSM-1

0CaO . K20

FIG. 7. Vertical CaO and KzO profiles in borehole LSM-1 (data from Simonds and McGee, 1979).

REIMOLD, 1982; Fig. 10). The lack of appreciable fraction- ation of impact melt rocks has been attributed to their relatively homogeneous original composition and subsequent emplacement as a rapidly cooled unit (FLORAN et al., 1978). The abundances of other lithophile elements in melt and country rocks are compared in Fig. 9. With the exception of melt rock 4-387 (exceptional SC, U, and Th contents), all other melt rocks again display rather uniform composition- intermediate to country rock compositions. Several melt rocks are depleted in Cs. In summary, both the REEs and other lithophile elements indicate a significant degree of homogeneity for the melt rocks. Sample 4-387 is different from the other melt rocks due to the vesicles and secondary phases noted in this sample. Also, it was pointed out by SIMONDS and MCGEE (1979) that the basement sampled in borehole LSM-4 is very heterogeneous, and 4-387 (as the deepest analysed melt rock and derived from the vicinity of underlying basement) could be affected by chemical alteration, as noted for the deepest melt samples of drill core LSM-1 (Fig. 7). On the other hand, mixing with a particular basement material cannot be excluded.

Siderophile element (e.g. Co, Ni, and Au) and Cr concentrations are low in both melt and country rocks. Sample 4- 387 displays exceptionally high Co and Cr values, which- hypothetically-could be regarded as evidence for a particular basement component that was not homogeneously mixed into the general melt body, but should also be viewed as due to the altered state of this sample. There may be minor en- richments of Co, Cr, and Au in the melt rocks (compare Table 5), but we hesitate to interpret these as evidence for a meteoritic (chondritic) component admixed to the melt, as the standard deviations for melt and country rock averages (Table 5) are of the order of 30-60% of average elemental concentrations.

Co/Cr and Ni/Co ratios (Fig. 10) indicate that, with the exception of sample 4-387, the melt rocks are of intermediate composition to the widely scattering country rock data. This diagram suggests at least two parent rock components labelled CA and cB (also compare Fig. 8b), but it is possible to see indications for two further components (cc-sample 18A- 78; and CD-contributing possibly to sample 4-387). This diagram does not indicate any compelling evidence for the existence of an identifiable meteoritic component.

TA

BLE

4

: IN

AA

RE

SU

LT

S FOR MELT R

OCK

S AN

D CO

UNTR

Y RO

CKS

FRO

M THE LAKE St. MARTIN CRATER STRUCTURE (all trace element data in ppm).

Ssu

plc

No.

M

ELT

RO

CKS

Cleo

eat

5-M

l-

29

128-7

8

1-1

35

bG-7

8

4-7

8

4-3

87

t-9

138-7

8

K(X

) PC

(X)

CO

N

i C

r A

u

La

Ce

Rd

Sm

R

U

Th

Yh

Lu

2.9

2.4

3.8

2.9

3.8

3.6

3.2

3.6

3.8

1.6

2

1.4

4

2.1

3

1.7

8

2.4

1

3.2

4

6.2

1

2.5

3

2.8

0

4.8

3

7.5

6.1

8

11

27

9

10

8

9

14

11

15

18

19

17

13

20

25

26

27

::

94

32

55

.021

.012 l 031

,030

.013

.034

.030

.025

.027

46

2

61

53

67

25

65

76

79

91

::

32

32

13

27

79

30

29

lb

32

31

5.3

4.5

8.0

7.0

8.0

5.1

3.8

4.9

4.8

1.2

.8

2

1.0

,8

9

1.0

8

1.2

8

1.4

5

1.1

2

1.1

4

.4

.4

.6

.5

.6

.75

.8

.7

.7

1.1

.8

1.2

1.2

1.3

.7

.2

.l

.2

l l

.l

.I7

::6

.5

.b

.I6

.18

Hf

3.2

3.1

5.1

4.4

4.9

4.2

2.4

3.8

3.9

T

a .3

.2

.4

l 4

.4

.4

.2

.4

.4

SC

4.3

4.0

5.1

4.6

7.3

8.0

18.8

6.2

6.0

A

s .4

-

‘3

- .5

1.0

1.1

-

.6

Rb

85.5

68.4

114

103

115

94

92

81

83

cs

.94

.62

.85

*VI

1.2

1

1.7

1.5

.9

1.1

B

a 654

566

895

845

908

897

717

848

864

U

Th

2.6

1.0

1.8

2.0

1.6

1.0

-

1.3

1.2

7.0

5.8

10.0

9.0

10.5

2.6

1.5

2.2

8.3

CO

UR

TR

Y

ROCK

S 9-7

8

168-7

8

8-7

9

18C

-78

7-7

8

lo-7

8

14-7

8

lBA

-78

20-7

8

4.6

2.7

3.6

1.3

3.5

6.6

5

4.8

3.0

1.9

5

2.1

4

.21

2.8

1

3.9

1

2.8

3

2.0

8

3.3

6

3.3

5.6

.7

11.8

lb

.2

6 5

22

11

8

11

2

18

29

15

9

33

23

17

13

7

28

36

39

25

123

53

,013

.OlO

.0

27

.036

.019

.025

.013

.037

.019

209

120

8.5

21

192

211

36

24

84

345

191

6

30

267

299

63

23

142

72

47

7

22

90

110

25

26

53

lb.0

14.5

.b

10.0

28.5

17.6

3.4

8.9

8.0

1.7

4

1.9

3

.3b

1.1

3.5

9

2.2

4

.78

1.7

2

2.1

2

1.2

1.3

.I

2.5

2.3

1.6

1.0

3.2

3.7

.2

2::

3.5

2.0

1:‘:

.8

.2

.3

.l

.l

.3

.I6

.27

.07

::6

13.8

11.2

.7

6.4

17.1

14.5

9.

6 4.5

4.7

1.7

2.0

l 2

.4

1.8

2.5

2.3

.4

.4

3.4

5.4

.4

9.5

15.6

4.1

2.9

9.6

6.9

.5

*

.3

- -

1.2

-

- -

154

179

128

70

159

159

209

33

88

.98

1.2

9

1.9

1

1.5

1

1.5

6

5.4

2.8

2.0

1.7

1325

709

780

241

1389

lb89

1117

320

831

.9

2.0

1.1

1.5

2.4

1.2

1.8

-

.88

23.0

17.6

1.3

1.1

10.8

24.3

4.4

2.1

12.2

Note

:

Ir in

al

l sa

mple

s rl

ppb

(det

ecti

on

lim

it

at

SR

C);

and

con

firm

ed

by

furt

her

IN

AA

com

mis

sion

ed by

O

ttaw

a G

eol.

Surv

ey.

W. U. Reimold et al.

Melt Rocks

F--T 12578 u 5-78 6-4 h-78

\ v---v MS-78 - 1-29 w 1-135

\ o--o 13578

Lake St. Martin

1

Country Rocks

o----o 7-78 M 16078 p----o 9-78 - 14-78 9--v IO-78 +---a 18G78 v---+ 8-79 l --. l%A-78 o---o 20-78

Lake St. Martin

FIG. 8. (a) REE patterns for melt rocks. Note: the Nd value for sample l- 135 is considered questionable. (b) Variation of REE abundances in Lake St. Martin country rocks.

Mass Spectremetric Results

With the exception of sample 4-387, the melt rocks are homogeneous with respect to Rb and Sr when compared with the country rock data spread (Table 7, Fig. 11). However, homogenization is clearly not perfect (compare samples 5- 78, l-29, and 1-t 35) with respect to Sr contents in particular. Signi~cant Rb and Sr homogeni~tion is often observed in impact melt bodies (REIMOLD, 1980, 1982) and must be regarded as the reason why Rb-Sr dating of whole rock impact melt samples has only been attempted a few times, and sel- dom with success. It is also obvious from Fig. 11 that simple two-component mixing, as previously suggested by &3MOLD et al. (1987a), cannot account for the Lake St. Martin melt rock composition.

Rb and Sr isotopic data on the melt rocks (Table 7) display a notable spread of *‘Rb/‘% ratios (Fig. 12a). Whole rock data are rather well aligned (note: the scatter around the regression line in Fig. 12a is largely due to the exaggerated y-scale). Together with all mineral data, they were regressed to yield an age of T = 164.5 2 2 1 Ma (I u error) for an initial ratio Isr = .71361 T 16 (all new data regressions reported here were carried out by utilizing the GEODATE YORK PRO- GRAMME, 1989). Only the whole rock data for exotic melt rock 4-387 was excluded from this calculation. REIMOLD et al. (1987a) reported an age of T = 220.5 f 18 Ma (I, = .7 13 1 T 1) based on fewer whole rock data and excluding data for one pyroxene-enriched and one feldspar-enriched separate (indicated by arrows in Fig. 12a) that were thought to reflect either the effect of alteration or of admixture of

unequilibmt~ parent rock clasts. However, ~tro~phic observations suggest that the large majority of feldspathic clasts of the checkerboard or ghost types in these samples must be considered as being at least partially equilibrated with the melt matrices (cf. REIMOLD, 1982), as they are in states of severe thermal reaction with the surrounding matrix. Thus, a definitive explanation for the scatter of these two mineral separates is not available. When the two pyroxene/feldspar data of aberrant nature are excluded from new data regression, anageofT=219+32(2u)Ma(Is,= .7131 +3)iscalculated. Regression of only the whole rock data yields a very similar age of 2 12.4 +- 43 (2,) Ma for an initial ratio Isr = .7 132 T 3. Both these ages are within the range of the 200-250 Ma K- Ar ages reported by MCCABE and BANNA~NE (1970). Therefore, we conclude that our age of T = 2 19 t 32 (2g) Ma is currently the best value for the age of the Lake St. Martin impact event.

Melt rock 4-387, shown to be chemically different from the other analysed melt rocks from Lake St. Martin, is also different with regard to its Rb-Sr isotopic composition (Figs. I2a and b). It plots close to the country rock regression line (Fig. 12b), which caused REIMOLD et al. (1987a) to suggest that 4-387 may have been formed by local melting (in situ) rather than as part of a well-mixed melt body. The possibility that the sample may have contained a large unequilibrated clast cannot be excluded. Al1 other melt rocks have higher 87RblS6Sr ratios than found on the section of the country rock regression line corresponding to melt rock “Sr/%r ratios (Fig. 12a). If 4-387 was a mixture of “normal” melt plus unequilibrated clast, it would be expected to plot, as it does,


Lake St. Martin

cl l Melt rocks

w M.R. No. 4-387

o Country rocks

Bo Rb Hf la SC As Cs ih

FIG. 9. Abundances of some lithophile elements and of As in melt and country rocks.

to the right of the country rock regression line. However, this is based on the assumption that the obtained range of isotopic compositions for melt rocks is a true representation of the melt body composition and that it does not extend to lower

87Rb/86Sr ratios. Also, REIMOLD (1982) showed that the central melt body in the Lappajarvi impact structure is chemically and isotopically well homogenized, but that this body is far from being well-equilibrated with respect to Rb and Sr iso- topes. For whatever reason, sample LSM-4-387 may have not been reequilibrated with the bulk of the melt at Lake St. Martin.

In the course of this study the possibility of analysing mineral separates from sample 4-387 was considered. However, the amount of secondary phases or altered matrix minerals (estimated at 20-25 ~01%) as well as the fine (generally <80- 100 pm) grain size of this sample prohibited effective mineral separation.

A wide variety of isotopic compositions was measured for the country rocks from the Lake St. Martin structure (Fig. 12b). GEODATE (1989) regression of these data resulted in a best-fit line corresponding to an “age” of T = 2785.3 f 157 (2~) Ma for an initial ratio Isr = .6997 7 11. This “age” lies well within the range of ages determined for Superior Province lithologies from ca. 3 100-2400 Ma (ERMANOVICS and DAVI- SON, 1976; VERHOOGEN et al., 1970). The latter authors presented a histogram of Superior Province ages that shows a strong maximum around 2200-2700 Ma ages. The initial ratio determined for this sample suite is typical for Archean to Early Proterozoic felsic rocks. However, it appears doubtful that the “age” determined for Lake St. Martin country rocks is of actual chronological value. Testing for the possibility that the best-fit line could represent a mixing-line indicated that the country rock data plot onto a mixing hyperbola in a “Sr/“Sr versus Sr concentration diagram. It is important to bear in mind that most analysed specimens were very small drill-core splits that most probably only represented single bands in the gneissic basement, rather than the regional lith- ological variation.

Mixing Calculations

In order to model the composition of the melt rocks on the basis of the known country rock types, a series of mixing calculations was carried out with the Harmonic Least-Squares (HMX) mixing calculation programme (ST~CKELMANN and REIMOLD, 1989). The compositions of the main components-granite, monzonite, and diorite-are given in Table 8a and were calculated as averages for the sample groupings indicated in Fig. 5b. Table 8a also contains the average composition of the melt rock mixture that we attempted to model. In addition to calculations with these three basement components, two calculations were performed with additional felsic components that, on the basis of Fig. 5b, are exotic with respect to the main basement types. Finally, three calculations were carried out with an average limestone com-

TABLE 5 : AVERAGE (+11 STANDARD DEVIATION) CONCENTRATIONS OF SIDEROPHILE ELEKENTS IN LAKE St. MARTIN MELT AND COUNTRY ROCKS (CALC. FROM DATA IN TABLE 4); DATA IN PPM.

CO Ni Cr Al1

Melt rocks 9.8f6.8 15.3:5.9 42.2i26.8 .025f.008 Country rocks 9.1i6.8 16.4t10.3 38.1234.7 .022+.010


CO/C1

0.4

0.3

0.2

0.1

I

CD?

k l P-387

. 128-78 ’ 66-78

. l-29

o Melt rocks 0 08-79

o Country rocks CB

1 I I , I I ,

0.5 1.0 1.5 2.0 2.5 3.0 Ni/Co

Ro. 10. Co/Cr versus Ni/Co ratios for melt and country rocks (see text for explanation). Arrows point towards possible target component compositions.

position (after PETTIJOHN, 1957) as additional eminent (the uncertainties given in Table 8a were chosen to roughly represent the actual variation among typical iimestone-de-

posits). Whether ~tenti~Iy irn~~nt components had been omitted was also tested by allowing for deviation from 100% for the total of calculated component proportions.

TABLE 6: (a) CCWARISON OF “PSEUDDTACRTLITE” (-15’). MRLT BRRCCIA t-17’) AND RASRMRNT ROCK (-23’. -33’) FROM DRILL CORE LSM-4 (DATA PRON McCARE AND BARNATTNE. 1971 AND FROli SIMONDS ARD MCGEE. 1979). Fe,O, = total Fe,

Depth (ft) 15’ 17 23 33

SO0 TiOo Al& FMb

g cao NszO KzO P.OS

TOTAL

65.2 62.99 59.5 .49 .66 .5

16.2 17.43 18.33 6.32 6.15 6.36

.05 .03 .09 1.81 2.41 3.59 2.37 2.87 5.26 2.86 2.98 3.36 4.50 4.32 2.14

.17 .12 .09

99.97 99.96 99.22

76.51 .02

12.78 .53 .08 .46 .40

1.88 6.13

98.79

* recalculated water-free composition

(bf COMPARISON OF “PSEUDOTACHYLIT&” (Samples 218, 237. 123) AND BASRMRNT ROCKS (227, 134) FROM DRILL CORE LSM-4 (DATA PROM SIMDNDS ARD McGRR, 19793. Pe,O, = total Fe.

Depth fftf 218 237 227 123 134

SiOa 70.62 53.87 74.3 61.35 58.08_ TiOa .35 .70 .13 .63 .83 Al.01 13.99 17.10 13.49 16.32 16.48 Fe.03 3.16 8.82 2.00 7.36 8.28 Ho0 .05 927 .08 .13 NgO 1.41 4.02 .18 3.23 4.10 CaO 2.27 6.03 1.67 3.76 5.01 NaaO 3.34 4.40 3.58 3.23 3.53 K.0 5.66 3.19 5.36 3.56 2.35 p.05 .07 .22 .12 .ll

TOTAL. 100.92 98.62 100.71 99.64 98.9


TABLE 7 : Rb-Sr ISOTOPE DATA FOR LAKE St. MARTIN MELT AND COUNTRY ROCKS

MBLT ROCKS

Sanple Rb (PPm)

Sr (PPml

.‘Rb/**Sr

~ ~~ 128-78

‘YOIJ Px Mag(Psp+Pxf NOMW (Psp)

1-Y 13B-78 4-78

1-29 Px FsP *12q.l

1-135 74.80 546.75 .3961 5-78 92.24 341 .b9 .7816 bG-78 75.64 407.51 .5374 4-387 76.87 677.62 .3283

72.63 317.81 .bb17 68.71 322.40 .6171 46.65 161.69 .8533 27.12 lb5 .OO .4759 20.40 177.14 .3334

72.12 461.68 .4523 .71449rb 72.63 422.47 .4977 .71454t8 77.71 419.0 .5370 .71494tb

59.37 385.19 -4462 23.77 269.14 .2772 93.58 493 * 77 .5487 94.25 509.88 .5352

.71511t8

.71532t12

.7157Otb

.71464r8

.71h85t12

.71458r8

.71439tb

.71450t15

.71465t8

.71441ilO

.71557fb

.71498fb

.71091fb

cooNTRY ROCKS

Sample Rb (PPrn)

Sr (Ppm)

l ‘Rbla%r

20-78 76.86 802.27 .2773 1 b-78 190.43 256.32 2.1648 18A-78 60.93 588.95 .2994 10-8 129.86 290.03 1.3009 8-78 89.82 305.14 .8537 9-78 106.06 323.30 .9523 7-78 103.39 477.80 .6272 168-78 128.08 306.14 1.2155 18C-78 54.54 693.72 .2275

.71157tb

.7803bf7

.71028t8

.75063f7

.73243*10

.74183t26

.72565f19

.75029tlO

.70914t12

- refers to last digit(s); Px/Psp - attempts were made to obtain fractions enriched in pyroxene or feldspar, respectively.

The results are summarized in Table 8b, where the restric- tions imposed on the various runs are also shown. The discrepancy factor calculated for each run is a value directly proportional to the degree to which the composition of the mixture could be reproduced with the chosen components- perfect reproduction results in a discrepancy factor of 1. Comparison of the discrepancy factors for Lake St. Martin calculations demonstrates that only those calculations allowing for a contribution from sediments (limestone) yielded reasonable results. In Table 8c, the original composition of the melt rock mixture and the calculated composition from run 8 are compared. For all major elements excellent agreement was achieved, with the largest difference obtained for MgO of .I5 wt%. This discrepancy is readily explained by the large uncertainty in the actual MgO/CaO (i.e., dolomite/ calcite) ratio of the Lake St. Martin target sediments. In con- clusion, the composition of the Lake St. Martin melt rocks can be well modelled with available basement rock components and an approximately 6.5% contribution from carbon-

ate sediments. The best mixing model is 93.6% felsic country rock (ca. 80% of monzonitic material plus 15% of the equiv- alent to granitic sample 4-33) and 6.5% of carbonate rock.

The final calculation (Table 8b, run 9) with tripled uncertainties assigned to the limestone component indicates: (I) the actual uncertainties do not significantly affect the deter-

mination of the absolute proportion of the limestone component required to model this mixture; (2) however, uncer-

tainties in multiple-component mixing problems are generally not to be underestimated. The HMX-method (ST~CKEL-

MANN and REIMOLD, 1989) has the advantage over other mixing programmes that uncertainties obscuring the param- eters of components and mixture enter the computation. The reason for the high discrepancy factor calculated for run 9 is

that, in this case, it is not possible to distinguish CaO con- tributions from the various felsic components, which may

be possible in the other types of mixing tests, as the limestone CaO content is obscured by an unrealistically high error.


18C Rb hwm)

14C

1oc

G(

2(

r

I-

014-78 l melt rocks

0 country rocks

I-

q o

a

I- 0

q J-70

. l

20;7’ . . .

J-135 l 4-361

> 01-29 q

WC-76

I-

200 400 600 800

Sr (ppm1

FIG. 11. Rb and Sr concentrations in melt and country rocks. Note: two-component mixing would result in a linear array of data points. Several sample nos. are given to allow easy comparison with isochron plot Fig. 12a.

SUMMARY AND DISCUSSION

Major and trace element analysis and Rb-Sr isotopic results have shown that the overall homogeneity of the Lake St. Martin melt sheet, relative to the compositional variation measured for the country rocks, compares well with those of other impact structures. In particular, the REE data and the whole rock Rb-Sr isotope data demonstrate homogenization. However, distinct limitations to this process in the Lake St. Martin case are indicated in the concentrations of MgO, Alz03, and, to a somewhat lesser extent, of alkali elements. Some variation in trace element abundances (e.g., SC, Cs, U, Th-Fig. 9 and Sr-Fig. 11) was also observed. SIMONDS and MCGEE (1979) suggested that there were differences between sample groupings from different drill cores and that unsampled basement rock types could be involved in melt generation. Based on Fig. 6, this suggestion appears unnec- essary, as the total data base does not require additional basement rock types in order to account for the spread of melt rock compositions. Some melt rocks are enriched in CaO, and to a lesser extent in K20, effects that were previously noted by SIMONDS and MCGEE (1979) and interpreted as being either the result of sediment admixture or due to hydrothermal alteration. Some evidence for MgO-enrichment of melt rock samples is also present.

The results of this study favour hydrothermal alteration to explain the limited variability of the Lake St. Martin melt rocks. In a vertical profile through the melt sheet in drill core LSM-1, both K,O and CaO concentrations vary at the im- mediate surface and at the bottom. This can be explained by surficial hydrothermal alteration and by chemical interaction

(4 (b) 760.

.n8- . . . . . . . LAKE ST. MARTIN

Lake St. Martin // Gi ” Country Rocks

$f .716. Melt Rocks

,‘<;,o;$ rack a.3 -Z ,760.

# . 03

,;j 03 .7x- ,740.

"T464.5 t21MaIlo) 15r=.71361S16 . wholerocksamp. Mwl=5.02 0 min seps.lB-78

0 min. sepsl-29 . I nlOY.oml.error

,7,0],/

I

_ , c l.l.LZrxi'

2.5

FIG. 12. (a) Rb/Sr isotopic compositions of melt rocks (Table 7). Age calculations were performed with a modified York 69 regression method (GEODATE, 1989). For comparison the regression line for analysed country rocks was added. Arrows indicate two aberrant data points (Px, Fsp) not considered in calculation of the best estimate for the age of the Lake St. Martin impact melt of T = 219 + 32 Ma (20). Note strongly exaggerated y-scale. The “pyroxene”- separates analysed probably contained varied amounts of mesostasis, which would explain their spread of isotopic compositions. Feldspar separates are plagioclase, and the i 125 and ~90 Frn fractions were sieve fractions thought to be enriched in mesostasis in comparison with the whole rock samples. Symbol sizes generally exceed analytical errors. Maximum analytical error as indicated in legend. (b) Rb/Sr isotopic compositions of basement rocks. The spread of whole rock melt rock samples is indicated for comparison (the solid square refers to location of melt rock 4-387 data).

Geochemical study of the Lake St, Martin impact 2109

TAEIIE 8a : -XTICNSQFRIERAGEMELTAND INDIVIWAL- AS USED IN HMX MIXINE CALCULATIONS (MEANAND STANDARD DEVIATIONS)

Si02 TiO,

Melt Mixture*

Granite

N=4 Ii=2

61.3k.8 69.4+1.2 .44+.02 .46+.02 13.8C.3 13.W.4 4.0t.2 3.1f.6 .06rt.O1 .03*.01 4.52.3 2.1+.2 5.02.25 1.3*.1 3.9rt.2 3.65k.35 3.2rf.l 5.2f.3 .13*.03 .1+.01

btxv.onite/ Mxmzdiorite

N=6

18C-7$1+ Diorite Limestone* *

62.5f3.0 .8+.5

15*1+1.1 5.9+1,9 .08f.03 2.6k1.3 3.0fl.O 5.02.8 2.7+.5 .3+.2

58.Ot.4 .54+.03 16.6k.2 5.8f.l .08+.01 4.35.2 4.3+.1 5.5f.25 1.92.1 .3f.01

5.19fl.O .06&.02 .81f.2 .541.2

7.E39+1.0 42.57k5.0 .05-+.01 .33*.1 .04f.01

~,srmpare Table 3a; ** after Pettijohn (1957), arbitrary unmrtainties assigned; analytxal uncea;linties according to analyst C. Day.

between the bottom of the melt sheet and underlying breccias. Further evidence for post-impact hydrothermal activity is obtained from petrographic observations: some exotic specimens, such as 4-387, contain considerable numbers of vesicles with fillings of secondary minerals or contain secondary carbonate or clay minerals in their matrices. On the basis of the well-homogenized REE abundances in melt rocks and the limited spread in isotopic composition, we tend to reject the possibility that the somewhat variable chemical composition of the melt rocks could be due to incomplete mixing. For individual samples, such as 4-387, this possibility can, however, not be completely excluded.

The so-called “pseudotachylite” samples from the central uplift have been described by SIMONDS and MCGEE (1979)

to contain abundant inclusions with shock metamorphic effects. The analyses reported by these authors show that their “p~udotachylite” samples are either chemically similar to or different from the respective host rocks. REIMOLD et al. ( 1987b) suggested closer study of pseudotachylite-like material from impact structures in order to better define criteria dis- tinguishing bonajde pseudotachylite (formed in situ) from injected impact breccias, such as the Al breccias defined by LAMBERT (1981) (cf. also SCHWARZMAN et al., 1983, on pseudotachylite and impact breccias from the Vredefort Structure and the Moon). The problem is that the term “pseudotachylite” is frequently used in a purely descriptive sense, but in general structural geology it refers to melt rocks produced by friction {genetic definition). Such friction melts have never been described to contain shock metamo~hosed clasts (with the exception of the debated Vredefort case). As the analyses from LSM4 core reported by SIMONDS and MCGEE (1979) can be interpreted to represent mixtures of local and foreign components, it is suggested here that the “pseudotachylite” from the Lake St. Martin structure is most probably impact breccia of the Al-type (LAMBERT, 198 1).

Analysis of siderophile elements in melt and country rocks from Lake St. Martin did not result in the clear identification of a contribution from the meteoritic projectile. However, a statistical comparison of average melt and country rock abundances of Co, Cr, and Au indicates a very slight enrichment of these elements in the melt rocks.

Harmonic least-squares mixing calculations show that it

is possible to produce a mixture corresponding to the composition of the Lake St. Martin melt rocks by bulk melting of the various identified basement components plus a small, about 6%, cont~bution from carbonate sediments. According to MCCABE and BANNATYNE (1970) the pre-impact stratigraphy contained an upper portion of several 100 m carbonate- rich sediments. This result is also in agreement with the con- clusion by SIMONDS and MCGEE (1979), who postulated that carbonate-rich target lithologies are not incorporated into impact melt mixtures at a significant level but are mostly removed in a CO*-rich vapor cloud. In cases where carbonate- rich lithologies form the upper part of the stratigraphy in the target area, as in the Lake St. Martin structure, this is an important observation, as one would expect that material from this section would be melted before the underlying crystalline basement. Limited admixture of a carbonate com~nent to the Lake St. Martin melt rocks is also indicated by the frequently observed enrichment in Ca, relative to the country rock compositions.

Rb-Sr isotopic analyses indicate that the melt rocks are not only chemically but also isotopically homogenized. Mix- ing of the analysed country rocks alone cannot explain the high s7Rb/86Sr ratios of the melt rocks, but a *‘Rb contribution from Sr-rich carbonate, as demanded by the HMX mixing calculation, is in keeping with these observations. A well- constrained Rb-Sr isochron age of T = 219 rt 32 Ma& = .7 13 1 7 3; 2~) was obtained for welI-equilibrated whole rock melt samples and mineral separates. This age is in agreement with the K-Ar ages of 200-250 Ma quoted by MCCABE and BANNATYNE(~~~~) andshouldthusbetaken as the currently best estimate for the age ofthe Lake St. Martin impact structure. However, because of the rather large statistical error of 32 Ma (at 2u level) this age cannot contribute significantly to the record of absolutely dated impact events, as hoped at the onset of this study. Dating with the 40Ar- 39Ar stepheating technique should be further attempted on some Lake St. Martin melt rocks.

A country rock data regression yielded an age of T = 2785.3 -t 157 (20) Ma for an initial ratio isr = .6997 ‘F: 11, an age fitting for Superior Province basement, but most probably not of true chronolo~cal value. The data indicate that the regression line could well be a mixing line.

TA

RL

E 8b

:

RB

SUL

TS O

F ll

MX

MIX

ING

CA

LC

UL

AT

ION

S (C

raa

= G

rani

te;

Non

-

Mon

zoni

te;

Dio

=

Dio

rite

-

com

pare

F

ig.

5b;

A-4

-33;

0-

4-52

; C

-4-4

4 -

afte

r Si

mm

onds

and

McG

ee 1

1979

1;

pivo

t ca

mp.

-

a m

anda

tory

co

nsti

tuen

t of

th

e m

ixtu

re

Run

C

ompo

nent

s/

RE

SUL

TS

( x

1 N

o.

Res

tric

tion

s G

rani

te

Mon

zoni

te

Dio

rite

4-

378

A

5 C

L

imes

tone

D

iser

.Pac

t.

Gra

nlM

on/D

ioilO

O%

/ G

ran

= pi

vot

cam

p,

Gra

nJU

onJD

ioJl

OO

XJ

Gra

n -

pivo

t ca

mp,

as

2;

no p

ivot

ca

mp.

; =

or

- 10

0%

as

1;

no p

ivot

ca

mp,

; =

or

- 10

0%

Gra

n/~n

JDio

lAJB

/C/

100X

JGra

n =

pivo

t ca

mp.

as

5;

no p

ivot

ca

mp.

; =

or

* 10

0%

Gra

n/l4

onJD

io/A

J0/C

J L

imes

t . J

lOO

X/n

o pi

vot

cam

p.;

= or

=

100%

as

7;

Gra

n -

pivo

t co

mp.

/100

%

ss

8;

limes

tone

un

- ce

rtai

ntie

s tr

iple

d

30.1

24.6

O

il.1

69.9

24.7

24

.0

30.1

t4.3

0t

1.1

69.9

t4.4

ot

1.0

24.6

29.5

24.2

02

1.0

68.6

24.4

ot

1.0

24.0

29.5

t4.5

ot

1.0

bB.6

t4.6

23

.5

2.02

1.3

0t1.

3 51

.4t6

.0

25.2

t2.3

02

1.2

21.4

t5.0

22

.6

Oil.

2 ot

1.3

Of.

8 7a

.9t2

.5

021.

3

02.8

28.2

k2.8

02

1.3

tl.S

t3.2

19

.5

14.8

t1.9

02

.8

0t.a

b.

4t.5

8.

9

2.52

.9

77.a

t2.5

0.

22.8

13

.2t1

.9

ot.9

O

k.8

6.42

.5

9.0

10.4

t2.a

72

.922

.7

1.32

.8

a.St

2.0

Of1

.3

Ofl

.O

6.7k

.7

42.3

Geochemicai study of the Lake St. Martin impact 2111

TABLE 8c :ixIMwuIsoNoF- AND -TED K%LT RXK (AVERAGE) ctEM!3XITI(%J FOR RUN 8 OF TABLE 8b.

&a&

61.30

Caloulat&

61.26

Difffoba-c&C)

.04

Ml-J3

13.80 4.00 .44 13.81 4.01 .44 -.Ol -.Ol .oo

.06 .06 .oo z 4.50 5.00 4.35 5.01 -.Ol .15

NaO K2 ii

3.90 3.93 -.03 3.20 3.19 .0X

p205 .I3 .13 .oo

Ackno~Iedgment.~-We gratefully acknowledge the former Director of the Sehonland Research Centre. Prof. J. P. F. Se&chop. and the Head of the Isotope Division, Dr. c. B. Smith, at the Bernard Price Institute, for their interest in this work and permission to perform the INAA and isotope analyses at their facilities. Ms. C. Day, Dept. of Geology, Univ. of the Witwatersrand, was instrumental in collecting and reducing the INAA and XRF data. Ms. D. Mthembu and Mrs. C. S. Beadle are thanked for the typing of this rnanu~~~. This paper is Geological Survey of Canada ~ont~bution No. 34189. Reviews by G. Graup, G. Ryder, and an anonymous reviewer improved the clarity and contents of this paper. Comments by C. B. Smith are &.a much appreciated.

Ed~toriai hurling: H. Palme

REFERENCES

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CURRIE K. L. ( 1970) New Canadian cryptoexplosion crater at Lake St. Martin, Manitoba. ~aiure 226,839-841.

ERASMUS C. S., FESQ H. W., KABLE E. J. D., RASMUSSEN S. E., and SELLSCHOP J. P. F. (1977) The NIMROC samples as reference materials for neutron activation analysis. J. Radioanal. Chem. 39, 323-334.

ERMANOV~CS I. F. and DAVIDSON W. L. (1976) The Pikwitonei Gmnuhtes in relation to the Northwestern Superior Province of the Canadian Shield. In The Early History of the Earth (ed. B. F. WINDLEY), pp. 33 l-347. J. Wiley & Sons.

FLORAN R. J., SIMONDS C. H., GRIEVE R. A. F., PWINNEY W. C., WARNER J. L., RHODES M. J., JAHN B.-M., and DENCE M. R. ( 1976) Petrology, structure, and origin of the Manicouagan melt sheet, Quebec, Canada. Geophys. Res. Len. 3,49-52.

FLORAN R. J., GRIEVE R. A. F., PHINNEY W. C., WARNER J. L., SIMO~VDS C. H., BUNCHARD D. P., and DENCE M. R. (1978) Manicona~n impact melt, Quebec. 1. Strati~phy, petrology, and chemistry. f. Geophys. Res. 83(B6), 2737-2759.

GEODATE YORK PRO&AMME (1989) Modified York 69 Programme bv B. E. EGLINGTON and R. E. HARMER. CSIR. Pretoria. version January 1989.

GOBEL E., RE~WOLD W. U., BADDENHAUSEN H., and PALME H. ( I980) The moiectile of the Laouaiarvi imnact crater. Z. Natur- . , .” . . _

~rschung 35a, i 97-203. GRIEVE R. A. F., RENY G., GUROV E. P., and RYABENKO V. A.

(1987) The melt rocks of the Bohysch impact crater, Ukraine, USSR. Contrib. Mineral. Petroi. 96, 56-62.

JAHN B. M., FL~RAN R. J., and SIMONI& C. H. (1978) Rb-Sr isochron age of the Manicouagan melt sheet, Quebec, Canada. J. Geophys. Res. 83,2799-2803.

LAMBERT P. ( I98 I) Breccia dikes; geological constraints on the formation of complex craters. In multi-Ring Basins (eds. P. H. SCHULTZ and R. B. MERRILL); Proc. Lunar Planet. Sci. 12A. pp. 59-78. Pergamon Press, New York.

MCCABE H. R. and BANNATYNE 8. B. (1970) Lake St. Martin cryptoexplosion crater and geology of surrounding area. Geol. Sum. Manitoba Geol. Paper, 3/70.

PALME H. (1982) Identification of projectiles of large terrestrial impact craters and some implications for the interpretation of b-rich Cre- taceous/Tertiary boundary layers. In Geological Imphcations of Impacts of Large Asteroids and Comets on the Earth (eds. L. T. SILVER and P. H. SCHULTZ); Geol. Sot. Amer. Spec. Pup. 190, pp. 223-233.

PALME H., G~SEL E., and GRIEVE R. A. F. (1979) The distribution of volatile and siderophile elements in the impact melt of East Clearwater (Quebec). Proc. Lunar Planet. Sci. Conf 10th. 2465- 2492.

PETTUOHN F. J. (1957) Sedimental) Rocks, 2nd edn. Harper and Brothers, New York.

PHINNEY W. C., SIMONDS C. H., COCHRAN A., and MCGEE P. E. (1978) West Clearwater, Quebec impact structure, Part II: Petrol- ogy. Proc. Lunar Planet. Sci. Conf 9th, 2659-2693.

REIMOLD W. U. (1980) Isotopen-, Haupt- und Spurenelement-Geo- chemie und Petrographie der Impaktschmelzen des Lappajlrvi- Kmters, Finnland. Ph.D. thesis, Westf. Wilh. Univ. Miinster, FRG.

REXMOLD W. II. (1982) The impact melt rocks of the Lappajlrvi meteorite crater, Finland: ~tro~aphy, Rb-Sr, major and trace element geochemistry. Geochim. Cosm~him. Acta 46, 1203-1225.

REIMOLD W. U., GRIEVE R. A. F., and PALME H. (1981) Rb-Sr dating of the impact melt from East Clearwater, Quebec. Contrib. Mineral. Petrol. 76, 73-K

REIMOLD W. U., TRED~UX M., BARR J. M., and GRIEVE R. A. F. ( 1986) A geochemical study of the Lake St. Martin melt and basement rocks. Meteoriti~s 21,490-492.

REIMOLD W. U., BARR J. M., GRIEVE R. A. F., and TR~DOWX M. (1987a) INAA and Rb-Sr isotope analysis of Lake St. Martin melt and country rocks. Lunar Planet. Sci. XVIII, 828-829.

REIMOLD W. U., OSKIERSKI W., and HUTH J. (1987b). The pseudotachylite from Champagnac in the Rochechouart meteorite crater, France. Proc. Lunar Planet. Sci. Conf 17th.; J. Geophys. Res. (supplement) 92(B4), E737-E748.

SCHWARZMAN E. C., MEYER C. E., and WILSHIRE H. G. (1983) P~udotachylite from the Vredefort Ring, South Africa, and the origin of some lunar breccias. Geoi. Sot. Amer. Bull. 94,926-935.

SIMONDS C. H. and MCGEE P. E. f1979) Petroiogv of imaactites from Lake St. Martin structure, Manitoba. Pr&.- Lunar’Planet. Sci. Conf IOth, 2493-25 18.

SIMONDS C. H., FLORA~V R. J., MCGEE P. E., PHINNEY W. C., and WARNER J. L. (1978) Petrogenesis of melt rocks, Manicouagan Impact Structure, Quebec. J. Geophys. Rex 83,273~2788.

ST~CKELMANN D. and REIMOLD W. U. (1989) The HMX (harmonic least-squares) mixing calculation programme. J. Math. Geol. 21(8), 853-860.

STOFFLER D. (197 1) Progressive metamorphism and classification of shocked and brecciated crystalline rocks at impact craters. J. Geophys. Res. 76,5541-5551.

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Geochemistry of the melt and country rocks of the Lake St. Martin impact structure, Manitoba, Canada

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