Air abrasion experiments in UPb dating of zircon

Chemical Geology (Isotope Geoscience Section), 58 (1986) 195-215 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

195

AIR-ABRASION EXPERIMENTS IN U-Pb DATING OF ZIRCON S.S. GOLDICH’>’ 1 Department ’ U.S. Geological

and LYNN

B. FISCHER’

of Geology, Colorado School of Mines, Golden, CO 80401 (U.S.A.) Survey, Box 25046, MS 963, Federal Center, Denver, CO 80225 (U.S.A.)

(Received

October

X3,1985;

accepted

for publication

November

18, 1985)

Abstract Goldich, S.S. and Fischer, L.B., 1986. (Isot. Geosci. Sect.), 58: 195-215.

Air-abrasion

experiments

in U-Pb

dating

of zircon.

Chem. Geol.

Air abrasion of zircon grains can remove metamict material that has lost radiogenic Pb and zircon overgrowths that were added during younger events and thereby improve the precision of the age measurements and permit closer estimates of the original age. Age discordance that resulted from a single disturbance of the U-Pb isotopic decay systems, as had been demonstrated by T.E. Krogh, can be considerably reduced, and, under favorable conditions, the ages brought into concordancy. Two or more events complicate the U-Pb systematics, but a series of abrasion experiments can be helpful in deciphering the geologic history and in arriving at a useful interpretation of the probable times of origin and disturbances. In east-central Minnesota, U.S.A., Penokean tonalite gneiss is dated at 1869 f 5 Ma, and sheared granite gneiss is shown to have been a high-level granite intrusion at 1982 * 5 Ma in the McGrath Gneiss precursor. Tonalite gneiss and a mafic granodiorite in the Rainy Lake area, Ontario, Canada, are dated at 2736 * 16 and 2682 + 4 Ma, respectively. The tonalitic phase of the Morton Gneiss, southwestern Minnesota, is dated at 3662 f 42 Ma.

1. Introduction We present U-Pb analytical data and age determinations on zircon from igneous and metamorphie rocks from four localities where previous geochronological studies did not yield ages of precision adequate to resolve some basic geologic problems. An air-abrasion technique was used to reduce the age discordance of the zircon. Basically, the technique is similar to that of Krogh (1982) but with some differences both in technique and in objectives. An air-driven diamond-studded abrasion mill was used to remove metamict zircon that had undergone Pb loss, thereby reducing the age discordance and giving a probable age of crystal0168-9622/86/$03.50

o 1986 Elsevier Science Publishers

lization. In rocks that have had a complex metamorphic history, the abrasion technique is effective in removing not only the metamict portions, but also the secondary overgrowths. The changes in the U-Pb systematics revealed by the abrasion experiments can be used to identify some of the geologic processes. Much of the present-day practice in U-Pb age measurements on zircon was developed by L.T. Silver, and a detailed paper (Silver and Deutsch, 1963) covers many aspects, such as, utilizing a number of size fractions of zircon concentrated from a single large block, morphology, zoning, inclusions, acidstripping experiments, annealing, and so forth. From this work a number of generalizations have developed: (1) the largest zircons B.V.

196

commonly have the lowest U; (2) the U content increases with decreasing grain size; and (3) metamictization and magnetic SUSCePtibility increase with U and Th contents with decreasing grain size. and, therefore, The morphology and growth StrUCtUreS of zircon have been studied for many years. The various growth features mentioned in the brief descriptions in the Appendix have all been described and illustrated, as for example, by Poldervaart and Eckelmann (1955), Silver and Deutsch (1963), Leggo et al. (1971), Kiippel and Griinenfelder (1971), Pidgeon and Aftalion (1972), and Bickford et al. (1981). The problem of inherited zircon, cores or xenocrysts, also was recognized many years ago, for example, by Tyler et al. (1940) and Stern et al. (1966). It is well known that the U-Pb and z3zTh*08Pb isotopic systems in titanite are relatively easily disturbed by metamorphic processes. Tilton and Griinenfelder (1968) noted that the ages determined on titanite generally were concordant or nearly so, whereas the U-Pb ages on zircon generally were discordant. They noted also that titanite commonly dates the time of metamorphism rather than of crystallization of the rock. Hanson et al. (1971) showed that titanite and zircon behaved differently in the contact zone between the Giants Range Granite (- 2650 Ma) and the Duluth Gabbro (- 1100 Ma) in northeastern Minnesota, U.S.A. They related the age discordance in the titanite directly to the llOO-Ma event, but the discordance in the zircon was more complicated. Some of the age discordance in the zircon was developed after the 1100-Ma event. The age discordance in zircon commonly is related to radiation damage with the development of metamict structures. The larger the content of U and Th, the greater the radiation damage and the greater the probability of increasing discordance. Compared to zircons, titanites generally have much lower concentrations of U and Th, but this alone will not explain the differences in age discordance. Chemical composition

and crystal structure are controlling factors. High-U minerals, as for example uraninite, commonly have concordant ages. Radiation damages in these minerals, and apparently also in titan&, do not persist but is annealed on a short-time basis. In zircon, and also in other silicates, such as a&mite, radiation damage develops new mineral phases and a microcapillary porosity which permits entry of water. Because of the large surface area and heat generated by the radioactive decay, chemical reactions between the solid and aqueous phases are accelerated. Zircon, but not titanite, therefore, usually shows an age discordance that developed late in the history of the rock commonly unrelated to a known thermal event - the dilatancy model of Goldich and Mudrey (1972). The literature on U-Th-Pb dating is extensive. The review of Gebauer and Griinenfelder (1979), with a reference list of papers to 1978, provides a useful introduction. 2. Methods Chemical analyses (Table I) represent large splits of the field samples taken during the crushing of the field samples and prior to the mechanical grinding. The grinding and final preparation of these samples was accomplished by methods used in conventional silicate analysis. The whole-rock chemical analyses are considered to be more representative than the modal analyses (Appendix). Zircon was concentrated by commonly used methods. Samples of - 60 kg were crushed and ground with intermittent sieving to reduce breakage of the larger zircon crystals. Iron filings and most of the magnetite were removed with a hand magnet. In the absence of a suitable Wilfley table, sieved fractions were elutriated in a rising water column to remove the light minerals, principally feldspar and quartz. The concentrates were further separated with bromoform and methylene iodide. The final mineral separations were made with a

197 TABLE

I

Chemical analyses of rocks from east-central Valley, southwestern Minnesota Area Sample No. SiO, (wt.%) AhO, TiO, FM, Fe0 MnO MN CaO Na,O K,O p,o, H,O+ H,Oco2 TE* Total

Rb (wm)

Sr Ba Cr Ni V Y Zr Nb

East-central KA551

Minnesota KA515

72.7 14.1 0.18 0.71 2.11 0.08 1.02 3.75 3.85 1.33 0.08 0.51 0.04 0.01 0.13 100.6,

74.9 11.6 0.21 1.02 1.91 0.05 0.15 0.57 3.52 4.90 0.08 0.33 0.07 0.05 0.14 99.5,

24 447 430
Minnesota, Rainy

Rainy Lake, Ontario, Lake, Ontario

RL41-66 58.0 14.3 0.71 2.16 5.02 0.11 5.99 5.96 3.61 2.43 0.35 1.27 0.04 0.11 0.38 100.4,

162 13.3 88 106 527 147

62.5 1,193 1,165 291 159 109 17 140 7

KA844 71.5 14.7 0.27 0.67 1.36 0.04 0.68 2.78 5.00 1.80 0.08 0.36 0.07 0.01 0.12 99.4, 62 458 390 10 8 25 7 90 -

and Minnesota

River

Minnesota

River Valley

KA835

KA857

67.4 17.0 0.47 0.48 1.80 0.03 0.81 3.90 6.02 1.06 0.09 0.22 0.06 0.00 0.20 99.5, 50 974 350 10 13 29 7 270

75.0 13.2 0.18 0.35 0.95 0.02 0.27 0.80 3.00 5.55 0.04 0.34 0.07 0.02 0.15 99.9, 229 97 805 25 146 13

*Trace elements calculated as appropriate oxides. Major and minor elements; Cr, Ni and V by N.H. Suhr, Pennsylvania State University by atomic absorption spectrophotometry and emission spectrography. FeO, H,O+ and CO,, conventional methods; Rb and Sr, isotope dilution; Ba, Y, Zr and Nb, X-ray fluorescence.

Frantz@** magnetic separator and by handpicking. Each fraction was cleaned by handpicking under a binocular microscope and examined with a petrographic microscope in high-index liquid. Zonal variations in interference colors readily reveal rhythmic variations in U content that have been depicted in radiographs or fission-track maps (Lud*Use of a brand name is for descriptive only and does not constitute endorsement U.S. Geological Survey.

purposes by the

wig and Stuckless, 1978). Zonal growth patterns may show numerous delicate layers or thicker zones (sector growth), or a combination of these features. Zonal growth patterns that are euhedral generally are considered to be magmatic, and uniform interference colors across the zones are suggestive of small time differences. Overgrowths commonly show marked differences between core and shell. The cores may be much darker, hematite stained, highly metamict, or corroded. Cracks in the shell may be unrelated to those in the core. Such

198

features alert, the observer of the possibility of a significant difference in the age of the core and the shell. Crystallographers have been making mineral spheres for many years (Bond, 1951), and R.D. Hamilton of the Colorado School of Mines called our attention to a commercially available gas-driven diamond-studded mill*. Very small samples of small zircon crystals behaved in an unpredictable manner in the abrasion mill, and to avoid introducing inert material that later would have to be removed, we used splits of lo-20 mg. If large amounts of material were available, preliminary magnetic separations were made; otherwise the final sample for dissolution and analysis was cleaned, largely by handpicking, after abrasion. Zircon samples were abraded from 15 min. to 1 hr. or more using clean, dry compressed air at l-2 psi (6.895-13.79 kPa). .The mill is effective in removing altered or metamict zircon. The abraded grains were recovered and weighed to obtain an estimate of the amount of material removed or lost by abrasion. Analyzed samples are listed in the tables in order of decreasing grain size. Abraded samples are distinguished by the analysis numbers in brackets, both in the tables and in the figures. In general, the larger size fractions were abraded because they present fewer problems of handling and clean up, but some smaller sizes were used if adequate material was not available, or the U content was unfavorable. The zircons were leached in warm 1 N HN03, 1 N HCl, and triple-distilled water for 20 min. each to remove surficial nonradiogenic Pb. After final rinsing in pentadistilled water, the samples were loaded into Teflon@’ digestion capsules with ultrapure concentrated HF and HNO+ The digestion assembly (Krogh, 1973) was heated at - 205°C for 2 days. Pb was extracted from the sample solution using a bromideform anionexchange resin column (modified *McCrone

Research Associates

Ltd., London.

from Krogh, 1973) and analyzed on a 30cm-radius 909sector mass spectrometer using the single-filament silica-gel technique. U and Th were extracted on a nitrate-form anionexchange resin (Tatsumoto, 1966) and analyzed by mass spectrometer using a triple-filament assembly. Analytical precision of isotope ratios (2~) are + 0.1% for Pb ratios except zoaPb/ *04Pb ratios which may have an uncertainty of as much as 3% (ratios > 1000). Corrections for Pb fractionation were determined from analysis of the U.S. National Bureau of Standards sample SRM-981. U, Th and Pb concentration data are accurate to within + 1%. Regressions of the data were made with the least-squares program of Ludwig (1980) and plotted on conventional concordia diagrams (Wetherill, 1956). All the radiometric ages, including those quoted from the literature, have been calculated with the decay constants and atomic ratios recommended by the Subcommission on Geochronology of the I.U.G.S. (Steiger and Jiiger, 1977). 3. U-Pb zircon

dating

and age determination

3.1. East-ten tral Minnesota,

of

U.S. A.

3.1.1. Tonalite gneiss of Bradbury Brook The mediumto coarse-grained biotitetonalite gneiss south of Onamia, Minnesota (Fig. 1) was included in the Hillman Gneissoid Biotite Tonalite of Woyski (1949) who assigned the rock to the “Late Algoman”. Goldich et al. (1961), on the basis of the K-Ar mineral age determinations, showed that the Hillman Tonalite and associated igneous rocks in east-central Minnesota are Penokean (- 1800 Ma) rather than Algoman (- 2700 Ma). K-Ar and Rb-Sr mineral ages and whole-rock studies by Goldich et al. (1961), Peterman (1966), and others are summarized in Keighin et al. (1972). The range, 1500-1780 Ma, reflects metamorphic events principally post-Penokean

199

1

Frances

u Int’l. UNITED

KA551

- TONALITE

GNEISS

*j

Ontario Falls

\---

STATES

Fig. 2. Concordia plot for zircon gneiss of Bradbury Brook (Table tion 2 abraded. U in ppm in italics.

u)

( ‘---A__-____

Minnesota Iowa 1

1 ;skl

/I

I m’

Fig. 1. Map showing approximate locations of sample areas: I = tonalite gneiss of Bradbury Brook; 2 = granite gneiss of Bremen Creek; 3 = granodiorite and tonalite gneiss, Rainy Lake, Ontario, 4 = tonalitic phase of the Morton Gneiss.

faulting and shearing. The problem was the failure of the earlier age determinations to date specific Penokean rocks and events precisely. Sample KA551, tonalite gneiss, was collected from the low-lying weathered boulder-y outcrops along the South Fork of Bradbury Brook. The precise location and an approximate mode are included in the Appendix; the chemical analysis in Table I; and the U-Pb analyses of three size fractions and one abraded fraction in Table II. The zircon fractions (Fig. 2) show a moderate age discordance that in terms of apparent Pb loss ranges from 18% for the coarsest, which also has the largest U con-

II).

from [3]

tonalite is frac-

tent, to 16% for fraction 4. Fraction 2 was abraded with a small reduction in the U content, but the age discordance was appreciably reduced from 18% (2) to 10% ([3] ). A regression of the U-Pb data gives an original age of 1869 f 5 Ma with a lower intercept of 480 f 37 Ma. The three unabraded size fractions give upper and lower intercepts of 1875 + 8 and 502 + 38 Ma, respectively, within analytical error. A principal benefit of the abrasion experiment, therefore, lies in the support it provides for interpretation of the age discordance as single stage. A relatively homogeneous zircon population was formed at 1869 Ma and lost Pb at - 480 Ma. The events that affected the Rb-Sr and K-Ar isotopic systems apparently had little effect on the U-Pb systems in the zircon. A mineralrock Rb-Sr isochron age of 1690 f 75 Ma from this locality, for example, is 180 m.y. younger than the zircon age. The 1869 + 5 Ma age for the tonalite gneiss of Bradbury Brook fits well with the ages reported by Van Schmus (1980) for Penokean intrusives in Wisconsin, U.S.A. 3.1.2.

Granite

gneiss

of

Bremen

Creek

The McGrath Gneiss (Woyski, 1949) at the type locality is an augen gneiss of

200 TABLE

II

TJ, Th-Pb analytical data and apparent ages for zircon from tonalite gneiss (KA551) bury Brook and granite gneiss (KA515) of Bremen Creek, east-central Minnesota Analysis No.

Fraction analyzed

Sample weight (mg)

Concentration u Th

(ppm) Pb

Common

of Brad-

‘06Pb/a04Pb

Pb @I

KA551: 1

+150 mesh

6.30

2

-150,

6.79

[31

+250 LM

No. 2 abraded - 57% removed -250 LM

4

357.6

0.68

7,387

819.1

246.0

0.61

7,581

5.38

758.3

247.1

0.80

5,463

6.98

858.0

262.4

0.40

11,051

143.0

0.49

4,665

155.2

0.70

5,930

128.5

1.55

1,850

146.6

0.85

4,757

1,466

KA515: 1

121 3 [41

-60,

+lOO

No. 1 abradsd - 30% removed -100, +200

30.0 8.26 15.0

No. 3 abraded - 45% removed

6.19

375.9

216.2

404.2 343.7 365.6

198.4

See Appendix for fraction descriptions. ‘06Pb/‘04Pb corrected for fractionation. corrections, KA551 - 1 : 15.49 : 15.27 : 35.12, KA515 - 1 : 15.22 : 15.21 least magnetic.

quartz monzonite composition. The large augen are deformed microcline crystals. In the Denham area (Fig. l), the Archean McGrath Gneiss is in fault contact with the Proterozoic Thomson Formation (Keighin et al., 1972). North of the east-west fault, the argillite and graywacke of the Thomson are recrystallized phyllite and biotite-quartzfeldspar gneiss. On the south, shearing has converted the McGrath Gneiss to a roughly banded cataclasite in which relicts of the large pink microcline crystals generally can be found. Locally, the recrystallized Thomson Formation is intermingled with sheared and granulated McGrath Gneiss. South of Denham in the vicinity of Bremen Creek, a sheared and recrystallized red granitic gneiss trends N65”W and is vertical in attitude. Relict microcline grains, such as

Common Pb : 34.86. LM =

those found in the shear zone north of Denham are lacking, but fluorite and tourmaline are present in both rocks. Rb-Sr mineral and whole-rock ages that range from 1500 to 1800 Ma suggested that the two shear zones are temporally related, and it appeared likely that the gneiss at Bremen Creek might be sheared and recrystallized McGrath Gneiss. A bulk-chemical analysis (Table I) of the rock at Bremen Creek does not differentiate it from the McGrath Gneiss (Goldich et al., 1961, p.117), and two size fractions of zircon analyzed by T.W. Stem, U.S. Geological Survey, gave 207Pb/Z06Pb ages approaching 2000 Ma, but the ‘OaPb/ 232Th ages are much younger (Table II, KA515, 1 and 3). In view of the field interpretation of the shear zones, Stem’s analyses suggested that the zircon from the rock at

201

Pb composition

(at.%)

lo4Pb

‘06Pb

=O’Pb

0.01026

85.504

9.3356

Atomic --Pb

5.1496

0.00924

83.012

9.3956

7.5830

0.01203

82.533

9.4702

7.9846

0.00611

82.727

9.3445

7.9220

0.00750

77.904

9.5347

12.553

0.01067

77.367

9.5127

13.110

0.02366

77.898

9.6891

12.389

0.01291

76.814

9.4998

13.673

Z06W23*U

ratios

(and

ages

*07W*35U

in Ma) 10’Pb/z06Pb

0.24206

3.5895

0.10755

(1,397) 0.28941 (1,639) 0.31200 (1,750) 0.29382

(1,547) 4.4563 4.8514 (1,794) 4.5355

(1,758) 0.11168 (1,827) 0.11277 (1,844) 0.11196

(1,661)

(1,738)

(1,832)

0.34381 (1,905) 0.34457 (1,909) 0.33684 (1,871) 0.35706 (1,968)

5.7409 (1,938) 5.7540 (1,940) 5.5881 (1,914) 5.9781 (1,973)

0.12110 (1,972) 0.12111 (1,972) 0.12032 (1,961) 0.12143 (1,977)

(1,723)

Bremen Creek had been recrystallized and reset to fit along a mixing line from - 1800 Ma to 2700 Ma. New unpublished geochemical data, particularly the trace elements, however, differentiated the granitic gneiss of Bremen Creek from the McGrath Gneiss, and abrasion experiments were undertaken to resolve the age of the granitic gneiss. The U-Pb analyses of the abraded zircons (Table II, [2] and [4]) present somewhat complicated results. In both tests, abrasion increased the U contents in the same proportion, but the calculated ages of abraded fraction [2] are about the same as those for the original fraction. In contrast, the ages for abraded sample [4] are essentially concordant (Fig. 3). A regression of the four samples gives model-l (Ludwig, 1980) ages of 1982 f 5 and 491 + 200 Ma (Fig. 3).

20’Pb/231Th

0.09143 (1,768)

0.08423

(1,634)

0.365' K&515 - GRANITE GdEISS OF BRENJ3 CREEK

0.355, 0.355, 0.345. 0.345.

INTERCEPTS

AT

1982 f 5 and 491 r 200 Ma

4.8

5.0

5.2

5.4

5.6

5.8

6.0

6.2

Fig. 3. Concordia plot for zircon from granite gneiss of Bremen Creek (Table II). [2] is fraction 1 abraded; [4] is fraction 3 abraded. U in ppm in italics.

202

There is appreciable scatter in the data points, and a model-2 regression gives upper and lower intercepts of 1981 f 24 and 456 + 600 Ma, respectively. Some of the scatter may result from laboratory bias; the original fraction analyses were made in Washington, D.C., in 1976 with borax fusions and the abraded samples were analyzed in Denver, Colorado, in 1982. The U-Pb age confirms the indications of the trace-element analyses that the gneiss of Bremen Creek is a pre-Penokean high-level granite pluton that was emplaced in the quartz monzonite precursor of the McGrath Gneiss. Both rocks were involved in shearing and recrystallization, and the possibility of disturbance of the U--Pb systems in the zircon of the Bremen Creek rock by Penokean events cannot be dismissed. Abraded sample KA515. [4] , however, has essentially concordant ages with a 207Pb/206Pb age of 1977 Ma that is within analytical error of the model-l age of 1982 + 5 Ma which we accept as a reasonable value. 3.2. Rainy

Lake, Ontario,

Canada

Earlier geochronologic studies of the Precambrian of northern Minnesota and adjacent parts of Ontario were tied to the rock succession developed by Lawson (1913) in the Rainy Lake area, Ontario. K-Ar and Rb-Sr mineral ages (Goldich et al., 1961) failed to resolve the time of the Laurentian and Algoman orogenies of Lawson, but suggested that the time interval was relatively short. Hart and Davis (1969) reported whole-rock Rb-Sr isochron and zircon U-Pb ages for Lawson’s major rock units with the exception of the late- or postkinematic Algoman Granite. The latter were studied by Peterman et al. (1972). In both studies the U-Pb ages were found to be considerably older than the corresponding Rb-Sr ages. Hart and Davis (1969) concluded that the events, including the two orogenies, probably occurred within a time interval of 60 m.y.; Peterman et al. (1972) suggested 50 m.y.

Lawson’s terminology, which is used here simply to define the problem, was based on his recognition of two periods of folding accompanied by granitic intrusive activity. The older Laurentian granite invaded the Keewatin (Ely Greenstone in Minnesota) and was pre-Seine Group (Knife Lake Group in Conglomerates in the Seine Minnesota). and in the Knife Lake locally are basal and of great thickness (Gruner, 1941). The metasedimentary and intercalated metavolcanic rocks of both the Seine and the Knife Lake unconformably overlie the folded Keewatin and Laurentian rocks. Subsequently they were folded and intruded by Algoman granitic plutons, which, as emphasized by Lawson (1913), are massive and essentially undeformed. Hart and Davis (1969) interpreted their U-Pb data using Tilton’s (1960) model of Pb loss by continuous diffusion with a modification in which D/a2 is a function of time (Wasserburg, 1963). They included zircon and titanite from localities in Ontario outside of the Rainy Lake area and obtained an age of 2710 + 30 Ma. Goldich and Mudrey (1972), however, argued that the U-Pb zircon data from the Rainy Lake area fit a straight line much better than a continuous diffusion curve. A regression of 10 zircon analyses of Hart and Davis (1969) from Rainy Lake only gave upper and lower intercepts of 2650 _+ 40 and 80 + 150 Ma, respectively. Peterman et al. (1972) regressed data for 11 samples of zircon and one titanite and obtaihed upper and lower intercepts on concordia of 2670 and 90 Ma, respectively. Thirteen whole-rock samples of Algoman Granite, however, gave a Rb-Sr isochron age of 2540 + 90 Ma; Ri = 0.7015 f 9. Similar Rb-Sr isochron ages from 2543 to 2584 Ma have been reported by Birk and McNutt (1977) and Birk (1979) for granitic plutons at Burditt Lake, Flora Lake, Taylor Lake, and the Lake Despair area, - 60 km northwest of Rice Bay. We report here U-Pb analyses of zircon and titanite from leucotonalite (trondhjemite) gneiss and from a mafic granodiorite.

203

These rocks are Laurentian and Algoman, respectively, in Lawson’s usage. For ease of discussion, the younger rock is addressed first. 3.2.1. Mafic granodiorite of the Otter-tail Lake stock Sample RL$I-66 (Peterman et al., 1972) represents a mafic granodiorite border phase of the Ottertail Lake stock (Harris, 1974) that was mapped as the Redgut Bay Granite by Lawson (1913). The chemical analysis (Goldich and Peterman, 1980) in Table I has been augmented with trace-element ,determinations from Shirey and Hanson (1984). An unusual aspect of the U-Pb analytical data is the surprisingly large age discordance in view of the low U content (Table III). The larger zircon fractions (U, 117 ppm) have age discordance corresponding to Pb loss of 37%; the smallest size (No. 4, Table III) with 167 ppm of U shows an apparent Pb loss of 40%. Fraction 1 did not afford sufficient material for abrasion, and a split of 14.3 mg from fraction 2 was used. The rounded grains (5.80 mg) were further processed by removing a magnetic fraction and by handpicking. The analyzed fraction that weighed 2.83 mg is a substantial amount (20%) of the split. The zircon population has two components. A high-U fraction was largely eliminated by abrasion and by handpicking of the dark-gray to black rounded grains leaving - 20% of low-U colorless to light-pink abraded zircon [3]. The age discordance was dramatically reduced, and a regression of the four analyses yields intercepts of 2682 f 4 and 88 + 18 Ma, respectively. Abrasion also greatly reduced the percentage of common Pb as has been found by Krogh (1982). Common Pb in the zircon fractions was high, 16--180/C of the total Pb (Table III); abrasion removed 80% of the common Pb. The present work confirms earlier conclusions: (1) The U-Pb ages are much older (- 150 m.y.) than the Rb-Sr isochron ages (Peterman et al., 1972).

(2) The age discordance is the result of relatively recent Pb loss (Goldich and Mudrey, 1972). In addition, the U-Pb analyses strongly support a single-stage age disturbance of a population free of inherited or xenocrystic zircon, which is in line with the conclusion of Shirey and Hanson (1984) that the mafic granodiorite, which they call monzodiorite, is a sanukitoid derived by “direct km”.

melting

of

the

mantle

at

depths

of

<

50

3.2.2. Tonalite gneiss North of Rice Bay, and north of the Quetico fault, there are large outcrops of gray fine-grained thinly banded biotite-quartzplagioclase gneiss and migmatite intruded by gray to red massive quartz monzonite that is similar in appearance and composition to the Algoman Granite of Lawson (1913). The relationships of the gneiss and younger granitic rocks is well exposed along Provincial Highway 812 northeast of Rice Bay. Sample KA844 (Appendix) represents the gneiss. The chemical analysis (Table I) is similar to an analysis of (Grout, 1938, p.494): “gray gneiss, Bay of Rainy

and

Fig. 4. diorite, fraction

to

probably Lake”

an analysis

Laurentian,

Upper

of gneiss from

Concordia plot for zircon from Rainy Lake, Ontario (Table 2 abraded. U in ppm in italics.

mafic III).

Rice

Crowe

grano[3] is

204 TABLE

III

IJ, Th-IJ analytical data and apparent ages for zircon and titanite 66) and tonalite gneiss (KA844) from Rainy Lake, Ontario

from mafic granodiorite Common Pb (%)

‘osPh/zO’Pb

Fraction analyzed

Sample weight (me)

Concentration u Th

1

+lOO mesh

17.5

115.9

57.13

16.2

233

2

-100,

15.7

117.9

58.76

15.6

239

Analysis No.

(ppm) Pb

(RL41-

RL41-66:

+150

No. 2 abraded - 60% removed -250

19.0

166.8

1

+120

11.3

395.4

173.6

1.33

3,339

2

-120

8.22

368.2

169.7

0.80

5,520

No. 2 abraded - 50% removed No, 2 abraded - 80% removed -100, +200

4.17

307.2

152.0

0.65

5,524

1.96

352.3

193.7

6.26

686

5.74

398.3

185.5

0.49

7,978

7.04

353.8

174.2

0.54

7,001

4.18

381.3

175.3

0.32

9,328

7.07

478

131 4

2.83

64.03

66.66

39.46 82.22

4.59

740 203

18.0

KA844 :

[31 [41 5

161 7 8

No. 5 abraded - 33% removed -200 LM titanite +120

See Appendix for fraction corrections, RL41-66-l

15.5

53.50

46.81

descriptions. 206Pb/Z04Pb corrected : 13.77 : 14.75 : 33,47,KA844-1

Island that Harris (1974, p.34) regarded as one of the oldest units in the Rainy Lake area. All these analyses are similar in major, minor and trace elements to analyses of the Northern Light Gneiss (Goldich et al., 1972). The analytical data and apparent ages for four zircon size fractions, three abraded samples, and one titanite are given ip Table III, and a concordia plot (see Fig. 5) clearly displays some fundamental differences between the zircon of the gneiss and that of the mafic granodiorite: (1) The age discordance of the zircon

35.28 for fractionation. : 13.64 : 14.69

Common : 33.37.

Pb

in the tonalite gneiss is much less than in the granodiorite, although the U content is roughly three times greater (Figs. 4 and 5). (2) The largest size fraction of the zircon in the tonalite gneiss shows the greatest age discordance, corresponding to Pb loss of - 24%. The apparent Pb loss in the other fractions is 18-20s. (3) There is much more scatter in the data points for the tonalite gneiss, and the calculated errors are much larger. A regression of the data for the seven zircon analyses gives upper and lower intercepts of 2734 f. 26 and 420 + 273 Ma, respectively. Exclusion

205

Pb composition (at.%) 20aPb losPb 'O'Pb

zosPb

0.2609

61.082

14.244

24.414

0.2506

60.705

14.080

24.964

0.0738

65.027

12.781

22.118

0.2890

58.937

14.193

26.581

0.0215

80.318

14.955

4.7054

0.0128

81.020

15.257

3.7103

0.0104

80.866

15.201

3.9227

0.1010

76.578

15.578

7.7436

0.00786

81.222

15.080

3.6905

0.00864

80.791

15.081

4.1187

0.00519

81.373

15.100

3.5211

0.1142

65.376

13.317

21.193

Atomic ratios(and agesin Ma) 206pb,238U 207,,,'""~ '"7Pb/Z06Pb

0.32896 (1,833) 0.33121 (1,844) 0.45793 (2,430) 0.31424 (1,762)

8.2017 (2,254) 8.2858 (2,263) 11.533 (2,567) 7.8293 (2,212)

0.18083 (2,660) 0.18144 (2,666) 0.18266 (2,677) 0.18070 (2,659)

0.40855 (2,208) 0.43309 (2,320) 0.46431 (2,459) 0.48054 (2,530) 0.43938 (2,348) 0.46165 C&447) 0.43466 (2,327) 0.48853 (2,564)

10.305 (2,463) 11.130 (2,534) 11.934 (2,599) 12.418 (2,636) 11.176 (2,538) 11.799 (2,589) 11.075 (2,529) 12.286 (2,626)

0.18294 (2,680) 0.18639 (2,710) 0.18641 (2,711) 0.18742 (2,720) 0.18448 (2,693) 0.18537 (2,702) 0.18479 (2,696) 0.18239 (2,675)

of analysis No. 2 yields 2736 f: 16 and 468 + 161 Ma, a minimum age for the tonalite. (4) The titanite (8) plots apart from the zircon points and clearly indicates a disturbance in the isotopic decay systems. (5) The lower intercept of the chord for the tonalite gneiss has an older apparent age than the granodiorite (Fig. 6). If near-surface dilatancy effected the Pb loss from zircon in the granodiorite at - 90 Ma, it should have caused similar Pb loss in the zircon of the tonalite gneiss. This did happen, but the age discordance in the zircon in the tonalite gneiss was caused

108Pb/232Th

0.13070 (2,483)

0.14720 (2,776)

by two events; whereas, the age discordance in the granodiorite zircon, within the precision of our measurements, was produced by a single event. Fractions of a zircon population that was formed at one time and subsequently was affected by only one event, regardless of size, crystal habit, U content, and extent of radiation damage, will define a line within analytical error. Deviations of data points from the line indicate a more complex history or inherited Pb (e.g., xenocrystic zircon). Relatively small amounts of Pb were lost from the zircon in the tonalite at the

206

0.54

KA844

TONALITE

GNEISS

0.46}

0.341

/. 6

1 mp&;"10

6

14

Fig. 5. Concordia plot for zircon and titanite (8) from tonal& gneiss, Rainy Lake, Ontario (Table III). [3 ] and [4] are abraded portions of fraction 2; [6 ] is fraction 5 abraded. U in ppm in italics. Analyses Nos. 2 and 8 were excluded from the regression.

0.52 -

RAINY LAKE, ONTARIO

INTERCEPTS AT A - 2682 2662 f 4 and 88 * 18 Ma B - 2736 * 16 and 468 f 161Ma 3

5

I 7

,

I

,

I

I

I 13

.

Fig. 6. Composite concordia plot for zircon from (+) mafic granodiorite (Fig. 4); zircon and titanite from (X ) tonalite gneiss (Fig. 5); titanite (29T, Tilton and Griinenfelder, 1968) and zircon (292, Hart and Davis, 1969) frdm (0) porphyritic syenite. Rocky Islet Bay, Rainy Lake.

time of emplacement of the granodiorite compared to the large losses that occurred at - 90 Ma. The 207Pb/206Pb ages of the zircon fractions (Fig. 5) fall in a regular sequence from 2720 to 2680 Ma, except for analysis No. 2 which plots below the line. In this regard, analysis No. 2 is aber-

9

rant, and, therefore it was excluded from the age regression as was the titanite (KA844.8); Analysis No. 2 is an unabraded fraction, and the analyzed split may not have been representative. The analyzed samples from both the tonalite gneiss (KA844) and the granodiorite (RL41-66) are shown in Fig. 6. In addition to the samples from the granodiorite (RL4166) and the tonalite gneiss (KA844), two points have been added to Fig. 6; 29T is a concordant titanite (Tilton and Griinenfelder, 1968), and 292 is zircon (Hart and Davis, 1969) from Lawson’s (1913) unit 1 I, porphyritic syenite in Rocky Islet Bay west of Rice Bay. This locality is 22 km W of RL41-66 and 18 km SW of KA844. The zircon (292, Fig. 6) plots on abraded sample RL41-66 [3]; the titanite (29T), with a 207Pb/206Pb age of 2690 Ma, plots near the 2682-88-Ma chord of the mafic granodiorite. The titanite (8, Fig. 6) from tonalite gneiss (KA844) also plots along the line. All the analyses fall within a narrow wedge which can be constructed by connecting points A and B on concordia (Fig. 6) with a lower intercept at 88 Ma. The alignment of the zircon data points for KA844 results from an initial displacement along the chord A-B and a second displacement caused by Pb loss at - 88 Ma. The intercept of 469 f: 161 Ma, then, has no geological significance. The age discordance of the titanite (KA844.8) is a problem. It might be explained as a two-stage discordance resulting from resetting at 2680 Ma, and dilatancy loss of Pb at - 90 Ma. [This interpretation is not supported by the 20sPbf232Th age of 2776 Ma (Table III), which argues against Pb loss] ; and the concordant ages of titanite from the porphyritic syenite (29T, Fig. 6) of Rocky Islet Bay with discordant U-Pb ages for zircon from the same rock (292, Fig. 6). It is possible that a local event (e.g., small younger intrusives) at the KA844 locality affected the titan& but not the zircon. The position of t&mite on the 2682-88-Ma chord, then, is fortuitous. This speculative

explanation needs field and laboratory checking. A second interesting problem that requires further study is the large amount of common Pb in the zircon from the granodiorite. Abrasion, as noted earlier, reduced the common Pb by 80% by eliminating a reddishbrown metamict phase. In contrast, the zircon fractions of KA844 have small amounts of common Pb that range from 0.33% to 1.3% of the total Pb. The abraded fractions [3] and [6] also have relatively low common Pb. The time of entry of the common Pb controls the Pb corrections that are made in the age calculations, and generally it is assumed that the common Pb was incorporated at the time of formation of the zircon. The large amounts of common Pb in RL41-66 zircon, however, suggest that a good part of this Pb may have been rock Pb that diffused into the metamitt zircon within relatively recent geologic time. In summary, the ages of 2736 f 16 and 2682 + 4 Ma that we relate to the Laurentian and Algoman intrusives of Lawson (1913) do not resolve the problems of the geology of the Rainy Lake area. Harris (1974) did not use Lawson’s terminology, but arranged his map units in chronologic order. He made good use of the radiometric age determinations available at the time, but his work showed that the lithic units and their relationships are much more complicated than previously mapped. For example, Lawson’s unit 11, porphyritic syenite in Rocky Islet Bay is Harris’ porphyritic quartz monzonite, but Harris recognized that a variety of rock types were included in Lawson’s unit II. He also recognized that the rocks of Rocky Islet Bay are an intrusive complex; he mapped faults for the first time. More recently, Poulsen et al. (1980) demonstrated that the Archean rocks in the southern part of the Rice Bay dome are overturned. Detailed geochronologic studies are needed in conjunction with field studies similar to the recent work in the Favourable Lake area by Corfu and

Ayres (1984) to resolve the problems. We note, however, that results similar to those reported here have been recorded by Nunes and Thurston (1980), Davis et al. (1981), and Ermanovics and Wanless (1983).

3.3. Minnesota 3.3.1.

River Valley,

Tonalite

U.S.A.

gneiss

The light-gray tonalitic phase of the Morton Gneiss is relatively well exposed in the Delhi area of the Minnesota River Valley. This is locality 5 of Goldich et al. (1980b), Wooden et al. (1980), and Goldich and Wooden (1980), and the reader is referred to these and to papers by Bauer (1980), Doe and Delevaux (1980), Goldich et al. (1980a), and Nielsen and Weiblen (1980) for a review of the geology and the literature pertaining to the Precambrian rocks of southwestern Minnesota. The tonalite gneiss at locality 5 is cut by numerous straight and relatively undeformed dikes of aplite and pegmatite and small irregular masses of adamellite-2 of Goldich et al. (1980b). The 3000-Ma pegmatite and closely ‘associated adamellite-1, which are well developed in the Morton vicinity, were not found at locality 5. For this reason locality 5 was selected for detailed study, and five size fractions of zircon from KA6 73 (Goldich and Wooden, 1980) provided solid support for the two-stage age discordance model postulated by Goldich et al. (1970). In this hypothesis it was suggested that a primary age discordance resulted from a high-grade metamorphic and magmatic event, w 2600 Ma, with a secondary discordance caused by dilatancy Pb loss in Cretaceous time, * 100 Ma. The present work was undertaken to test the two-stage age discordance model and to obtain a more precise age for the tonalitic phase of the Morton Gneiss . In addition to sample KA6 73 (Goldich and Wooden, 1980), two samples of tonalite gneiss (KA846 and KA835) and a sample

208 TABLE

IV

U, Th-Pb analytical River Valley Analysis No.

data and apparent

Fraction analyzed

ages for zircon

Sample weight (md

and titanite

Concentration u Th

from the Delhi area, Minnesota

(ppm) Pb

Common Pb (%I

206Pb/=0’Pb

KA673: I

121

+80 mesh

2.0

195.4

148.9

1.36

2,985

No. 1 abraded - 58% removed

7.61

218.9

173.6

0.73

5,002

229.2

163.3

0.39

8,544

KA846:

3

+60

141 151

11.0

No. 3 abraded - 30% removed

7.66

291.8

219.0

0.37

9,670

No. 3, - 50%

3.77

257.0

196.0

0.50

6,572

3.25

244.3

188.7

1.82

1,879

4.36

229.7

178.4

0.58

5,997

5.36

211.4

163.6

0.73

4,703

0.61

592.3

427.8

4.13

894

7.30

551.0

331.4

2.16

1,943

removed

No. 3, - 50%

I61

removed

[71

No. 3, - 55% removed

No. 3, - 55%

[81

removed KA835:

10

+80 abraded - 70% removed +lOO light brown

11

+lOO dark brown

13.9

787.8

456.6

2.31

1,832

12

-100

11.1

547.3

297.1

1.66

2,467

13

titanite

53.1

105.7

18.38

202

15.65

257

191

72.75

83.45

-80, +120 KA836:

14

-80, +120

7.32

See Appendix for fraction descriptions. ,rections, Nos. l-12 - 1 : 11.94 : 13.69

1259

707.6

206Pb/20’Pb corrected for fractionation. : 31.88, Nos. 13, 14 - 1 : 13.86 : 14.78

from a large late- or postkinematic dike of granite aplite that traverses the regional foliation of the tonalite gneiss at locality 5 were collected and processed. A chemical analysis of KA835 (Table I) falls within the range of the leucocratic tonalite gneiss in

Common : 33.57,

Pb cor-

the Morton area (Wooden et al., 1980, p.60). The variations in chemical composition of the analyzed samples are readily explained by crystallization-fractionation. Few socalled batholiths are a single intrusion, and the tonalite gneiss at locality 5 consists of a

209

Pb composition (at.%) 20aPb '06Pb =O'Pb

'--Pb

0.0242

72.35

19.80

7.83

0.01256

70.624

20.357

9.0066

0.00668

74.156

20.299

5.5388

0.00626

73.422

20.242

6.3288

0.00849

72.660

20.484

6.8467

0.03120

72.153

20.583

7.2336

0.00993

72.848

20.769

6.3734

0.01253

72.948

20.834

6.2060

0.07148

70.389

19.069

0.03733

72.641

18.532

8.7889

0.04005

73.476

18.472

8.0127

0.02880

73.498

18.717

7.7560

0.2982

60.370

14.797

0.2506

64.510

14.223

10.472

24.535

21.017

Atomic ratios(and agesinMa) 2OSpb,'38U '"'Pb/""U 10'Pb/106Pb

0.6293 (3,150) 0.64935 (3,226)

23.44 (3,245) 25.643 (3,333)

(3,398)

0.61341 (3,084) 0.63959 (3,187) 0.64310 (3,201) 0.64431 (3,206) 0.65631 (3,253) 0.65444 (3,246)

23.071 (3,230) 24.234 (3,278) 24.890 (3,304) 24.941 (3,306) 25.670 (3,334) 25.610 (3,332)

0.27278 (3,322) 0.27480 (3,334) 0.28070 (3,367) 0.28074 (3,367) 0.28367 (3,383) 0.28381 (3,384)

0.58352 (2,963) 0.50463 (2,634) 0.49174 (2,578) 0.46140 (2,446) 0.51667 (2,685)

20.932 (3,135) 17.368

CVQ6)

0.26016 (3,248) 0.24962 (3,182) 0.24553 (3,156) 0.25046 (3,188) 0.18562 (2,704)

9.4597 (2,384)

0.17233 (2,580)

0.39813 (2,160)

(2,955)

16.647 (2,915) 15.934 (2,873) 13.224

number of intrusions. The available field and chemical data all are in accord with a small, if measurable, time difference between the types of leucocratic tonalite gneiss in the Morton area. The gray tonalitic gneisses contain clasts of amphibolite, some

208Pb/Z3ZTh

0.2702 (3,305) 0.28641

0.18712 (3,467)

of which are remnants of folded sills or dikes, but some may very well be xenoliths derived from the preexisting terrane that was intruded by the tonalite. The tonalite and amphibolite were first folded prior to emplacement of pegmatite and adamellite,

210

at 3000 Ma. A chemical analysis of the granite aplite (KA587, Wooden et al., 1980, p.70) is included in Table I. The analytical data and U-Pb ages are given in Table IV. Sample KA673.1 and six abraded zircon samples define a chord, similar to that shown in Fig. 7, with upper and lower intercepts on concordia of 3654 + 320 and. 2657 f 439 Ma, respectively. The large errors reflect the clustering of the data points near the midpoint of the line and the long extrapolations to concordia. At least two mechanisms were involved in the development of the age discordance: (1) Pb loss during highgrade (amphibolite facies) metamorphism; and (2) addition of U in overgrowths on original zircon from fluids related to the granitic intrusives. It appears likely that an appreciable time span (- 50-100 m.y.) separated the loss of large amounts of radiogenic Pb generated in the tonalite zircon in the interval from 3.6 to 2.7 Ga, and the addition of U in overgrowths on recrystallized zircon at - 2.6 Ga. The metamict structure of the zircon permitted entry of water derived from interstitial openings, fluid inclusions, and nearby biotite. The aqueous phase

0.41 10

I 14

ia

207pb,&” 22 3o

34

38

Fig. 7. Concordia plot for zircons from the aonalitic phase of the Morton Gneiss, all of which show marked age discordance related to the 2.6-2.7Ga event, and titanite (13), which is essentially concordant, giving line A. Using 2600 Ma as the time of disturbance gives line B (see text), data in Table IV.

facilitated recrystallization during which it was expelled removing some of the radiogenic Pb. Titanite (Table IV, KA835.13) has essentially concordant U-Pb ages which differ by less than l%, with a zo7Pb/206Pb age of 2704 Ma. The ‘O*Pb/ 232Th age is 3467 Ma, and we cannot account for this old value because the titanite was formed or recrystallized during the high-grade metamorphism. A regression of the zircon and titanite data yields intercepts of 3662 + 42 and 2669 + 52 Ma, respectively, on concordia (Fig. 7, line A). Abraded samples [5] and [6] are accidental replicates. The U contents differ by - 5%, but the ages (Table IV) are similar. Analyses [ 71 and [8] represent an effort to test the reproducibility of the abrasion technique and as a check on the difference between KA673. [2] and KA846. [ 71 (Fig. 7). The second analysis [8] differs from [71 by - 8% in U content, but the ages are similar and well within analytical error. We attribute the clustering of the data points (Fig. 7) primarily to the partial loss of Pb and recrystallization of the metamict phase during the high-grade metamorphism. The seven zircon samples on the 3.62.6-Ga chord (Fig. 7) have low U contents, ranging from 195 to 292 ppm and averaging 235 ppm. In a general way there is a positive correlation between U content and ‘degree of discordance, but some exceptions are obvious. Sample KA846.3 (Fig. 8), with 229 ppm U, falls well within the range of samples on the chord but plots well below. The size fractions of KA835 have large amounts of U and pronounced age discordance. Sample KA835.1 I with 788 ppm U plots between Nos. 10 and II, both of which have - 550 ppm U. The abraded fraction [9] plots well above the three normal size fractions but failed to reach the 3.6-2.6-Ga chord. Zircon from the large postkinematic aplite (Table IV, KA836.14) has sharply discordant U-Pb ages (Fig. 8) with a 207Pb/ ‘OaPb age of 2580 Ma, A whole-rock Rb- Sr

211 TOWLITIC PHASE, MJRTCN GWISS 0.70 -

INTERCEPTSAT A 3662 l 42 and 2669 + 52 Ma B - 3363 + 59 and 1130 + 310 Ma C - 2580 Ma (207/206 age )

Fig. 8. Concordia plot for zircon and titanite from the tonalitic phase of the Morton Gneiss and zircon from granite aplite, line A is from Fig. 7. Line B is a regression for unabraded zircon fractions. Line C is the *07Pb/*“6Pb age of zircon (14) from the aplite.

isochron-age for six samples of aplite from the Morton area is 2590 f 40 Ma, Ri = 0.7036 f 20. Sixteen samples of aplite and adamellite-2 gave 2590 + 40 Ma, Ri = 0.7024 f 11 (Goldich and Wooden, 1980, p.87). If 2600 Ma is taken as the time of metamorphism and titanite KA835.13 is omitted from the regression, the upper and lower intercepts on concordia are 3624 + 46 and 2600 + 68 Ma, respectively (Fig. 7, line B). If we examine the U-Pb analyses of unabraded zircon, the contribution of the abrasion experiments becomes evident. The four smaller sizes of KA673 give upper and lower intercepts of 3522 + 313 and 1739 f 657 Ma. Goldich and Wooden (1980, p.89) regressed these samples together with five older analyses and obtained upper and lower intercepts of 3487 f 123 and 1732 i 258 Ma, respectively. Three size fractions and an abraded zircon from KA835 give upper and lower intercepts of 3277 * 205 and 736 f 1058 Ma, respectively. A regression of nine samples of unabraded zircon from KA673, KA846 and KA835 gives upper and lower intercepts of 3363 + 59 and 1130 + 310 Ma, respectively (Fig. 8). The age of the tonalitic phase of the Morton Gneiss, on the basis of all the un-

abraded zircon analyses from locality 5, would be 3365 + 60 Ma. It might also be said, that the close approach of the data points to concordia supports this age. This conclusion is erroneous; the data points lie near concordia, but the ages are sharply discordant. The abraded samples all lie on line A, and none, on line B (Fig. 8). Three abraded samples cluster at a point on line A corresponding to a 207Pb/206Pb age of 3390 Ma. Our explanation is that rock fluids entered the metamict zircon dissolving some of the radiogenic Pb, and a large part of this Pb was lost during the highgrade protracted metamorphism. 4. Concluding

remarks

We view our present study as exploratory, but the abrasion technique is a valuable addition to geochronology. Improvements in field sampling, processing, and analytical procedures will greatly increase its usefulness, not only in obtaining precise age measurements, but also in providing information on geologic processes that disturb the radioactive decay systems. Work in progress will expand two aspects of the present study. Overgrowths and xenocrystic cores are prominent in the migmatitic Morton Gneiss northwest and southeast of the Delhi area where the granitic components of 3000 and 2600 Ma overwhelmed the older tonalite gneiss-amphibolite component. The common Pb problem, which was discussed briefly in Section 3.2.2. on the mafic granodiorite of Rainy Lake, also is evident in titanite (KA-835.13) and in zircon from the aplite (KA-836.14, Table IV). The titanite contains 15.34 ppm of common Pb (18.4% of the total Pb), but the U-Pb ages are nearly concordant, and a large component of common Pb is rock Pb incorporated at the time of recrystallization of the titanite. The zircon from the granite aplite also has a high common Pb of 110.7 ppm (15.6% of the total Pb), but like the zircon from

212

the mafic granodiorite (RL41-66) is metamict with marked age discordance. It should be noted that the mafic granodiorite and the granite aplite are very different rock types, and that this difference is reflected in the U contents of the zircon that is nearly 10 times greater in the zircon from the aplite than in the zircon from the granodiorite. It seems likely that a large component of the common Pb in the zircon was added from groundwater in very recent time. Entry of surface water introduces a new factor not to be confused with the rock water of the dilatancy model. Large amounts of rock Pb and other cations may be added to the metamict zircon by chemical diffusion, with radiogenic Pb and other ions moving out. The calculated U-Pb ages for zircon from RL41-66 and KA836 are not strongly affected by assumptions made concerning the isotopic composition of the common Pb. We have found similarly high common Pb in zircon from other localities, and the mode of incorporation of the common Pb is problematical. The with aqueous phase, and its interaction the metamict phase of zircon, is fundamental in controlling age discordance. Acknowledgments This investigation was supported by the National Science Foundation Grant No. EAR77-23345 to the Colorado School of Mines, with supplementary funds from the Colorado School of Mines, and logistical support from the Branch of Isotope Geology, U.S. Geological Survey, and the Minnesota Geological Survey. We are indebted to L.M. Kwak, Z.E. Peterman, J.L. Wooden, and particularly to J.S. Stacey and T.W. Stern for their interest and assistance. Various drafts of the manuscript were read by R.D. Hamilton, H.R. Northrop, J.N. Rosholt, Jr., T.W. Stern, D.M. Unruh and J.L. Wooden. A number of graduate students, Colorado School of Mines, assisted in the field and laboratory work, and we thank Linda G. Martin and James R. Shannon.

Appendix East-central

Minnesota,

U.S.A.

KA551. Tonalite gneiss of Bradbury Brook. NW l/4 SW l/4 sec. 30, T. 41 N, R. 26 W, Mille Lacs County, lat. 46”00’05”N, long. 93”40’3O”W, 8 km S and 1.6 km W of Onamia. Gray coarse-grained gneiss: andesine 60%, quartz 28%, biotite 7%, Kfeldspar 2%, hornblende 2%; titanite, magnetite, apatite, zircon; muscovite, epidote, chlorite, pyrite, hematite. U-Pb analyses: 1, light-brown, euhedral, and dipyramidal forms, zonal growth prismatic (L.B.F.); 2, pink to light-brown crystals, least magnetic (L.B.F.); [3], No. 2 abraded (L.B.F.); 4, lightbrown crystals, least magnetic (L.B.F.). KA515. Granite gneiss of Bremen Creek. Midpoint west line sec. 20, T. 44 N; R. 21 W, Pine County, lat. 46”16’45”N, long. 93”02’W, 8 km S and 6.4 km W of Denham. Red medium granoblastic gneiss: microcline 38%, oligoclase 18%, quartz 37%, biotite 5%, hornblende; fluorite, titanite, apatite, zircon, tourmaline, opaque; epidote, chlorite, calcite. U-Pb analyses: 1, clear crystals and fragments, prismatic and dipyramidal forms, small opaque inclusions, zircon crystallites (T.W.S.); [2], No. 1 abraded (L.B.F.); 3, mostly clear crystals, minor light-brown (T.W.S.); [4], No. 3 abraded (L.B.F.).

Rainy Lake, Ontario,

Canada

RL4 l-66. Mafic granodiorite (quartz monzodiorite) border zone of Ottertail Lake stock, roadcut on Highway 11, 9.5 km E of Bear Passage bridge, lat. 48”43’40”N, long. 92”52’2O”W, Map 2443, Kenora-Fort Frances, Ontario Geological Survey. Dark-gray, medium-grained: andesine 35%, K-feldspar II%, quartz lo%, hornblende 26%, biotite 8%; magnetite, titanite, apatite, zircon; epidote 7%, chlorite 2%, calcite. U-Pb analyses: 1, pink to brown crystals and fragments; small opaque inclusions, uniform high interference colors (L.B.F.); 2, minor pink slender crystals, major-brown stubby with complex terminations and inclusions: small opaque, small-rounded with high relief, small rounded pleochroic, green to rose (L.B.F.); [3], No. 2 abraded (L.M.K.); 4, light-brown crystals, uniform high interference colors (L.B.F.). KA844. Tonalite gneiss, cut on Highway 812, 19.5 km N of intersection with Highway 11, lat. 48”51’35”N, long. 93”41’1O”W, light-gray banded: oligoclase 61%, K-feldspar 5%, quartz 20%, biotite 7%; magnetite, titanite, apatite, zircon; epidote 4%, sericite, calcite, pyrite. U-Pb analyses: 1, mostly brown simple crystals, minor smaller clear to pink

213

crystals (J.L.W.); 2, light-brown slender crystals with zoning that appears to be magmatic, interference colors are similar and not interrupted from zone to zone (J.L.W.); [3] and [4] are No. 2 abraded (L.B.F.); 5, mostly brown with delicate euhedral zoning, commonly with alternating high and low interference colors, minor clear crystals without zoning and with uniform interference colors (L.B.F.); [6], No. 5 abraded (L.B.F.); 7, slender, light-brown crystals, zoned with euhedral sectors, some fine zones with variations in interference colors (L.B.F.); 8, titanite, amber to brown crystals and fragments (L.B.F.). Minnesota Locality 1980, Renville

River

Valley,

U.S.A.

5. Northeast of Delhi (Goldich p.92), SE car. sec. 33, T. 114 County, lat. 44” 37’46”N, long.

and Wooden, N, R. 36 W, 95” 10’49”W.

KA673. Approximately 8 m W of large aplite dike. Gray medium-grained gneiss: oligoclase 79%, microcline 3%, quartz 5%, biotite 9%, hornblende 2%, titanite 1%; apatite, opaque, zircon, allanite; sericite (Goldich et al., 1980b, chlorite, epidote, p.49). U-Pb analyses: 1, clear to pink crystals (J.L.W.); [2], No. 1 abraded (L.B.F.). KA846. Lower outcrop, Gray coarse-grained gneiss, pink crystals and fragments; have thin overgrowths(?) abraded, sectioned mounts ference colors (L.B.F.).

275 m WSW of U-Pb analyses: sectioned epoxy (L.B.F.); [4--81, show uniform

KA673. 3, clear mounts NO. 3 inter-

medium-grained gneiss from KA835. Light-gray ledge east of aplite dike. Oligoclase 45%, quartz 35%, K-feldspar 3%, hornblende 8%, biotite 4%, titanite 2%; allanite, apatite, zircon; opaque 3%, epidote, chlorite, sericite, hematite. U-Pb analyses: [9], abraded (L.B.F.); 10, light brown +lOO mesh crystals with complex terminations, zoned with outer zones that appear to be overgrowths that commonly are clear compared to brown to red inner zone. Cracks in overgrowths commonly are independent of cracks in inner zone (L.B.F.), 11, dark-brown, +lOO mesh crystals similar to No. 10 except for darker color (L.B.F.); 12, lightto dark-brown crystals, similar to above (L.B.F.); 13, titanite (L.B.F.). KA836. Granite aplite. Dike striking N30” W, dipping 62”NE, cutting across the structure of the tonalitic at NW end and 3.8 m SE end, gneiss; - 5.5 m wide 60 m long; weak secondary foliation. Microcline 40%, oligoclase 20%, quartz 32%, biotite 5%, magnetite 2%; allanite, titanite, apatite, zircon; chlorite,

epidote, brown,

sericite, complexly

Analysts: Stern; Wooden.

L.B.F. L.M.K.

hematite. U-Pb analyses: 14, zoned crystals, magmatic (L.B.F.).

=

= Lynn Loretta

B. Fischer; M. Kwak,

T.W.S. J.L.W.

= T.W. = J.L.

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