U–Pb systematics in heterogeneous zircon populations from the Precambrian basement of the Maryland Piedmont

Earth and Planetary Science Letters, 23 (1974) 238- 248 © North-Holland Publishing Company, Amsterdam - Printed in The Netherlands

L_LA

U - P b SYSTEMATICS IN HETEROGENEOUS ZIRCON POPULATIONS FROM THE PRECAMBRIAN BASEMENT OF THE MARYLAND PIEDMONT B. GRAUERT Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D. C. (USA) Received April 9, 1974 Revised version received July 1, 1974

The aim of the study was - besides the dating of metamorphic events to evaluate the effects of multi-stage crystal growth, episodic and continuous Pb loss, and U gain on the discordant age patterns found for zircon populations of the polymetamorphic Baltimore Gneiss, the Precambrian basement in the Maryland Piedmont. Eight gneiss and migmatite samples were collected at two localities in the Phoenix and Towson dome, respectively. Their zircon populations were separated into twenty-three fractions of different size and optical appearance. A low-contamination method (T.E. Krogh, 1973) was used for the U - P b analyses. Microscopy and electron-microprobe studies revealed internal heterogeneities of the zircon crystals: at least half of the grains of each population reflect more than one stage of crystal growth, with the last stage consisting of U-poor overgrowths (U: below 400 ppm, mostly below 200 ppm). Evidence for episodic U gain and overgrown material other than zircon has not been found. On a concordia diagram the "ages" obtained by upward extrapolations (1080 and 1180 m.y.) and downward extrapolations (421 and 455 m.y.) of the best-fit lines to the data points are in fair agreement with the geochronologic data found by other investigators and the probable times of metamorphic periods of Precambrian (Grenville) and early Paleozoic (Taconic) orogenies. Models of Pb loss by continuous diffusion cannot adequately explain the discordant age patterns: these are essentially the result of superposition of episodic Pb loss and zircon overgrowth during the Taconic (and Acadian?) metamorphisms. The zircon overgrowth appears to be present in all fractions, but its influence on the U - P b systematics is generally not perceptible because it is overridden by the effect of episodic Pb loss. For the fractions showing the most discordant ages, the contribution of Pb loss to the discordancy was found to be at least 85 %. From the microscopic picture and the isotopic data, it appears that the bulk of the zircon substance crystallized during one or several high-grade metamorphisms accompanied by migmatization and granitization of the rocks in the course of the Grenville orogeny. Under consideration of zircon ages of Baltimore Gneiss rocks of Pennsylvania, the results point to a complex Grenville metamorphic history in the Maryland and Pennsylvania Piedmont, that lasted from at least 1200 m.y. until about 980 m.y. The granulite-facies metamorphism in the West Chester Prong, Pennsylvania, may be 50-200 m.y. younger than the metamorphic events in the gneiss domes of the Baltimore area. Although it seems that real differences exist with respect to the Precambrian ages of major zircon-forming events between the Phoenix and the Towson dome, the apparent difference of about 100 m.y. should be interpreted with caution, because it is impossible, so far, to evaluate quantitatively the influence of possibly much older inherited zircon components.

1. Introduction

(1) H o w is o p e n - s y s t e m b e h a v i o r o f zircon for Pb and U related to pressure, t e m p e r a t u r e , chemical com-

T w o p o i n t s are o f f u n d a m e n t a l i m p o r t a n c e for the i n t e r p r e t a t i o n o f zircon U - P b ages:

p o s i t i o n , and possible s e c o n d a r y a l t e r a t i o n s o f the crystal lattice such as radiation damage [ 1 ] , h y d r a t i o n [ 2 - 4 ] , annealing [5] etc.?

* Present address: Institut fiir Kristallographie und Petrographie der Eidgen. Technischen Hochschule, Sonneggstr. 5, CH-8006 Ziirich, Switzerland.

(2) H o w far m a y a given z i r c o n p o p u l a t i o n or an individual zircon crystal be c o n s i d e r e d h o m o g e n e o u s and cogenetic? The e l u c i d a t i o n o f the m e c h a n i s m s w h i c h

U-Pb SYSTEMAT1CS IN HETEROGENEOUS ZIRCON POPULATIONS

cause U and Pb migration out of or into zircon crystals becomes more difficult or impossible if samples are composites of various zircon "phases" [6] with possibly different primary ages. A heterogeneous origin is obvious for many zircon populations found in sediments and metasediments [ 7 - 1 0 ] , but even in many magmatic rocks the optical properties of the zircon crystals reflect two or more stages of crystal growth [11-13]. Even clear, eubedral, and homogeneous-looking zircon samples of intrusive rocks have been found to contain inherited but microscopically unrecongnizable components with 207pb/ 206pb and 2°6pb/238U apparent ages that are at least several hundred million years higher than the age of the intrusion /14]. In most cases it cannot be proven directly whether the various components of a composite zircon population crystallized during a "single" geologic event within a relatively short period, or whether they were formed during two or more events separated by long time intervals. A decision becomes particularly difficult where the microscopic picture suggests that even the individual crystals are composites of older and younger zircon substance - e.g. cores with later overgrowths - or unaltered and distinctly altered components of an originally homogeneous crystal. Recently, KroNa and Davis [15] were able to break off the overgrown ends from relatively large crystals found in a paragneiss. The isotopic analyses revealed distinctly different ages: old, highly discordant cores and young, nearly concordant ends. However, because of small grain size or zonal or patchy intergrowths of the various zircon phases, a separation is generally not possible or would be too time-consuming. In addition, not much is known about the possibility of Pb transfer from one phase to another within a single zircon grain. However, as will be shown in the following, a close consideration of the isotopic data and the concentration and distribution of U occasionally allows one to decide whether a given pattern of discordant U-Pb ages is primarily caused by phase mixing or whether it is the result of U gain and/or Pb loss. The aim of this investigation was to find out whether the U - P b systematics in heterogeneous zircon populations of a polymetamorphic gneiss complex, the Precambrian basement of the Maryland Piedmont, reflects the major metamorphic events. In addition, I have attempted to clarify whether the age patterns obtained are due to Pb loss, U gain or zircon mixing. As

i

239

5

10

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Coastal Plain sedimentary rocks ~

GLENARM SERIES Wissahickon Formation

Baltimore Gabbro Comptex

~

Baltimore Gneiss incl. Gunpowder Granite (Towson Dome)

Cockeysville Marble Setters Formation

•

Sample locality

Fig.l. Simplified geologic sketch map of the Appalachian Piedmont north of Baltimore, Maryland, showing the Precambrian basement (Baltimore Gneiss) which outcrops in the cores of a series of mantled gneiss domes: Chattolanee dome (C), Phoenix dome, Texas dome (T), Towson dome, and Woodstock dome (W).

discordant U - T h - P b ages of zircon from the same basement - partly from the same outcrops - have been previously interpreted as the result of Pb loss by continuous diffusion [16, 17], this study should also test the limits in the applicability of simple continuous diffusion models.

2. Geological setting The Precambrian basement of the Maryland Piedmont, generally referred to as "Baltimore Gneiss", outcrops in a series of gneiss domes which are mantled by the younger metasediments and metavolcanics of the "Glenarm Series" [18] (Fig.l). The basement rocks are a complex assemblage of biotite gneiss, biotite-hornblende gneiss, amphibolite, migmatites, and various types of granitic gneiss. Field geologic observations [18] and previous geochronologic studies on total

240

B. GRAUERT

rocks and minerals [16, 17, 19, 20] yielded evidence for at least two metamorphic periods: (1) A strong orogeny at about 1.0-1.2 b.y. (Grenville orogeny) transformed earlier rocks to gneisses and migmatites and produced coarse-grained granitic rocks. (2) During an early Paleozoic orogeny (Taconic orogeny) the basement was reactivated and rose as domes beneath its mantle. R b - S r whole-rock analyses of large post-Glenarm pegmatite swarms which surround the gneiss domes and are interpreted as terminating the regional deformation and metamorphism yielded an age of 425 +- 20 m.y. (X87Rb = 1.39 × 10-11yr -1) [19]. (3) Temperatures remained sufficiently high for Ar loss and isotopic re-equilibration of Sr in minerals until about 290-280 m.y. [16, 19, 20]. The zircon samples were separated from various types of gneisses and migmatites exposed at two localities about 20 km apart, one in the Phoenix dome, the other in the Towson dome (Fig.l). The exact locations and the rock types are given in the Appendix. An excellent description of the rocks of the gneiss domes has been published by Hopson [18]. In the Phoenix dome the biotite gneiss (sample BAL-4), biotite-hornblende gneiss (BAL-2), and banded gneiss (BAL-3) are possible derivatives of sediments and some of them could have formed from volcanic material. According to Hopson, the neosomes of the migmatites developed in the solid state by metasomatism or exsudation rather than by injection of granitic magma. R b - S r isotopic analyses of whole-rock samples [21 ] suggest that the bulk of the aplitic neosomes (BAL-6) and the K-feldspar blastesis in the adjacent gneisses (BAL-5) are not the result of in situ segregation and that potassic material is likely to have migrated in from an outside source. At the sample locality in the Towson dome the predominant rock type is a coarse augen to flaser gneiss (BAL-11). Veins of medium-grained granite (BALd 3) have been deformed along with the augen gneiss.

3. Microscopic

properties of the zircon populations

All zircon populations studied show a heterogeneous appearance. Microscopic observations revealed that at least half of the grains reflect more than one stage of crystal growth. As a first approximation, three

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Fig.2. Zircon grains of the Baltimore Gneiss showing typical heterogeneity. The parts of the zircon grains that were formed during the various stages of crystal growth are termed here "phases". stages of zircon growth can be distinguished (Fig.2): (1) Dark, translucent, sometimes also clear cores can be observed in many of the larger crystals. They most likely represent the oldest, possibly inherited zircon component. (2) The second stage is characterized by euhedral zircon growth, evident from repeated zoning (up to ten zones). This stage is best developed in the zircon populations of the migmatitic and granitic gneisses, where it accounts generally for more than 80 % of the grain volumes. In the U-poor zircons the zoned part is clear, whereas the U-rich grains are brownish to reddish and translucent. From the microscopic picture it appears that the zircons of samples BAL-5, BAL-6, BAL-11, and BAL-13 went through a period during which the development of euhedral and repeatedly zoned crystals was possible, thus suggesting a mobile, presumably molten environment during crystallization. (3) During a third stage, clear, U-poor overgrowths modified the shape of many crystals, forming all sorts of grains from nearly euhedral individuals to extremely irregularly shaped clots. Other phases than zircon have not been identified. From the microscopic observations and the cathode luminescence patterns (Fig.3) it appears that in some cases stage three was preceded by a period of moderate zircon corrosion and resorption, apparent from irregular, sometimes concave surfaces of zircon phase two

U-Pb SYSTEMATICS IN HETEROGENEOUS ZIRCON POPULATIONS

241

Fig.3. Photographs showing cathode luminescence of polished zircon grains from the Baltimore Gneiss. The patterns reflect the relative distribution of heavy elements such as U, Th, and Pb. (light: low concentration of heavy elements, dark: high concentration of heavy elements). A. Broken prismatic zircon crystal of sample BAL-5 showing a thin rim of U-poor overgrowth on the prism and knobby outgrowth on the pyramid. The thin, light band at the right margin of the grain cannot be identified with certainty as newgrowth (reflexes?). The recent surface of fracture (lower margin) shows no overgrowth. B. Characteristic overgrowth patterns on the ends of prismatic and roundish zircon grains of sample BAL-11. C. U-poor overgrowth on a core that displays a weak, zonal pattern (BAL-5). The irregular surface of the core, cutting here and there across the internal zoning of the core (arrow), points to a stage of corrosion before the overgrowth started. D. Fractured zircon grain which was possibly healed by U-poor new-growth (arrow). U-poor overgrowths are clearly identifiable at the ends of the crystal. (see Fig.3C). The a s s u m p t i o n t h a t t h e clear o v e r g r o w t h s are p o o r in U is based: (1) o n a c o r r e l a t i o n o f t h e optical a p p e a r a n c e w i t h t h e U c o n c e n t r a t i o n s f o u n d for

t h e s e v e n t e e n z i r c o n f r a c t i o n s a n a l y z e d (clear: 3 5 0 6 0 0 p p m ; b r o w n i s h t o reddish, t r a n s l u c e n t : 5 0 0 - 2 5 0 0 ppm; milky-looking, turbid: more than 2500 ppm), and

242

B. GRAUERT

TABLE 1 U - P b analytical data •

Concentrations

1

Atomic ratios observed2

Atomic ratios calculated 3'4

Sample No.

Sieve fraction (microns)

BAL-2-1

< 40

496

89.7

7700

0.07593

0.2116

0.1638

1.669

0.07389

BAL-2-2

> 125

483

89.2

18,250

0.07508

0.1950

0.1691

1.733

0.07431

BAL-3-1

< 40

1641

198

4277

0.07381

0.03263

0.1279

1.243

0.07048

BAL-3-2

100 150

1475

197

6268

0.07491

0.03547

0.1409

1.410

0.07265

uranium rad. lead 206pb/204pb 207pb/206pb 208pb/206pb 206pb/238 U 207pb/235 U 207pb/206pb (ppm) (ppm)

BAL-3-3

> 150

2283

235

3732

0.07200

0.02988

0.1099

1.033

0.06817

BAL-3-4

> 150 m

2735

278

4303

0.07017

0.02340

0.1093

1.007

0.06684

BAL-4-1

< 40

358

65.1

33,500

0.07402

0.2245

0.1625

1.650

0.07360

BAL-4-2

> 150

562

94.9

11,600

0.07454

0.1778

0.1572

1.589

0.07331

BAL-5-1

< 40

642

47,500

0.07311

0.03792

0.1455

1.461

0.07281

BAL-5-2

40-53

866

125

89.2

43,000

0.07380

0.04038

0.1503

1.522

0.07347

BAL-5-3

53-75

890

130

48,000

0.07402

0.04257

0.1522

1.548

0.07373

BAL-5-4

75-100

973

142

27,000

0.07396

0.04393

0.1518

1.536

0.07339

BAL-5-5 BAL-5-6

100-150 > 150

1162 1342

166 189

7790 1940

0.07389 0.07901

0.04394 0.05865

0.1495 0.1471

1.486 1.454

0.07207 0.07169

BAL-5-7

> 150 m

2776

264

4614

0.06920

0.02351

0.1022

0.9318

0.06610

BAL-6-1

> 125

1818

214

10,910

0.07120

0.02203

0.1260

1.214

0.06989

BAL-6-2

> 150 m

3038

294

5010

0.06941

0.01776

0.1043

0.9573

0.06655

BAL-11-1

< 40

789

112

16,200

0.07532

0.07501

0.1443

1.482

0.07445

BAL-11-2

100-150

663

109

8560

0.07819

0.08094

0.1656

1.748

0.07654

BAL-11-3

>, 150

660

107

8300

0.07796

0.07865

0.1637

1.721

0.07626

BAL-12-1

100 150

1393

170

5850

0.07397

0.05559

0.1263

1.246

0.07154

BAL-12-2

> 150

2033

213

2285

0.07482

0.05391

0.1103

1.043

0.06857

BAL-13-1

> 125

1673

183

5795

,0.07142

0.05467

0.1139

1.083

0.06896

1 For concentration measurements all aliquots were spiked with the same combined 2°Spb/235U tracer. A 238U/235U ratio of 137.88 was used for the computations [28]• 2 The internal precision of the mass spectrometric ratio measurements expressed in two standard deviations of the mean (= two standard errors) was: 2°7pb/2°6pb: better than -+ 0.03 %; 2°spb/Z°6pb: better than ± 0.03 %; 2°6pb/2°4pb: better than ± 2 % (2°6pb/2°4pb < 10,000); better than +- 5 % (2°6pb/2°4Pb > 10,000). 3 Isotopic composition of common Pb used for the corrections: 206 pb/2O4 Pb: 18.0; 207 pb/2O4 Pb: 15.5 ; 208 pb/2O4 Pb: 37.0. 4 Analytical uncertainties assigned: 206 Pb/ 238 U: ± 0.5 %; 207 Pb/ 206 Pb: 4 0.3 % (2°6pb/2°4pb > 15,000); 2°7pb/Z35u: -+ 1.0 %; ± 0.5 % (2°6pb/2°4pb < 15.000). mMilky-looking (metamict) zircon fraction separated by handpicking.

U-Pb SYSTEMATICS IN HETEROGENEOUS ZIRCON POPULATIONS (2) on electron-microprobe studies. The U concentration of phase three was always found to be below 400 ppm, in most cases below the detection limit of about 200 ppm. On the cathode luminescence photographs (Fig.3), zircon phase three is characterized by sharp contrast against phase two. The estimated maximum proportion o f the clear, U-poor overgrowths may reach 20 % in some large grains, and 50 % in small ones. However, the volume ratio of the three zircon phases varies significantly from grain to grain. A correlation of the age discordancies with the volume ratios has, therefore, not been made. Patterns that could be interpreted as the result of U gain, as recently found for some detrital zircon populations [22], have not been observed. In addition, there are U-rich, partly or totally turbid, milky-looking grains that appear opaque under the microscope. They were found in all populations but are most abundant (up to 2 %) in the migmatites and the light layers of the banded gneisses. Contrary to the magnetic properties of similar grains in many other zircon populations, these "metamict" zircons are not sensitive or poorly sensitive to the separation in a magnetic field.

4. Discussion of the analytical results The isotopic data of the various zircon fractions separated according to size and optical appearance are listed in Table 1 and are plotted on a 2°6pb/238U versus 2°7pb/235U diagram (concordia diagram) in Fig.4. The data points lie on or fall close to two best-fit lines, P and T, which were calculated for the zircon fractions of the Phoenix and Towson dome, separately. The diagram in Fig.4 illustrates that the data pattern cannot be interpreted by means of simple models of Pb loss by continuous diffusion: the linear arrays o f data points show distinct angles to the diffusion curves proposed by Tilton [23] and Wasserburg [1]. In addition, experimental studies by Shestakov [24] revealed that the diffusion coefficient of Pb in zircon at room temperature is negligibly small and too low to explain discordant zircon ages such as plotted in Fig.4 by lowtemperature diffusion. It seems, therefore, straightforward to explain the approximately rectilinear patterns in Fig.4 either by a model of episodic Pb loss from Precambrian zircon or by a mixing model of Precam-

243

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238U

0.15

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PHOENIX DOME BAL-2 Biotite-hornbt. gneiss <3. BAL-3 Light banded gn. o BAL-4 Biotite gneiss © BAL-5 Migmatite

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Fig.4.2°6pb/238U versus 2°7pb/23Su diagram (concordia diagram) showing the data points of the various zircon fractions. T and P indicate best-fit lines to the data points of samples from the Towson dome and Phoenix dome, respectively. D(t) = D o indicates a curve for Pb loss by continuous diffusion as proposed by Tilton [23]. The diffusion curve for 1080 m.y. old U-Pb systems is approximated by its extrapolated tangents at 1080 m.y. brian zircon with newly grown or overgrown zircon of early Paleozoic age. With some exceptions that shall be discussed below, the age discordancies of the zircon fractions are correlated with the U concentrations. Similar observations have been made by Silver [25] and many others on presumably cogenetic zircon populations which were subjected to later episodic disturbances, thus supporting a model of episodic Pb loss as a function of radiation damage for the zircons of the Phoenix and Towson dome. On the other hand, from the striking heterogeneity of the zircon grains (Figs.2 and 3), a mixing model appears plausible. It is, therefore, important to know whether the presently observed relative U concentrations of the various fractions reflect a primary U distribution, or whether they are essentially the result of later zircon newgrowth. From Fig.4 alone, an answer cannot be obtained without the knowledge of the mixing ratios. In this case, however, one can directly demonstrate by means of a 206pb/238U versus 1/238U plot that the relative U distribution must be a primary one. This is shown in Fig.5 where the data points form a convex, band-like array.

244

B. G R A U E R T

,

(1180 rn.y.) t~ (1080m y)

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206Pb 238 U :

--'~ --4

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(455my) 1

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10-3 ppm-1

Fig.5. 2°6pb/238U versus ]/238U diagram showing the data points of the various zircon fractions. The symbols are the same as those in Fig.4. The horizontal lines (dashed) represent the concordia-intercept ages of the best-fit lines T and P in Fig.4. The ages of 421 and 455 m.y. indicate times o f possible zircon new-growth. The array of the data points is approximated by a convex, band-like area (shaded) that shows a g e n eral decrease in the 2°6pb/238U ratios with increasing 238U concentrations. A and B indicate mixing lines; they are discussed in the text.

Prior to a closer look at some details, the data shall be discussed as a whole. From the linear patterns on the concordia diagram in Fig.4, a two-stage development of the zircons of the Phoenix and Towson dome may be assumed as a first approximation. The 206 Pb/ 238U ratios and the respective ages of the concordia intercepts of the two best-fit lines P and T are indicated in Fig.5 by dashed horizontal lines. Since in this type of diagram mixing lines are straight lines and since we may reasonably assume from the observations with the microscope and the electron microprobe that the U concentration of zircon phase three, the conceivable Paleozoic component, is below 400 ppm, we may conclude that the relative U distribution observed is not due to mixing of Precambrian with early Paleozoic zircon phases. In addition, Fig.5 clearly demonstrates that the isotopic data of the points that fall below line B cannot be explained by a mixing model of Precambrian with Paleozoic zircon alone. As the maximum U concentration in the cores may be reasonably assumed to be 10,000 ppm (point E in Fig.5), and as the U concen-

tration of phase three is 400 ppm (point F) or less, the U-Pb systems of fractions that plot below line B must have experienced an additional disturbance. This is most likely an episodic loss of radiogenic Pb, because from the concordia plot and the cathode luminescence patterns continuous Pb loss and episodic U gain (without new zircon growth) are not apparent. Furthermore, Fig.5 allows one to evaluate directly the maximum contribution of a Paleozoic zircon component to the age discordancy of the more discordant zircon fractions. In case of point J (zircon fraction BAL-5-7) the maximum contribution by mixing with an overgrown component having 400 ppm U (point F) is given by the ratio of the distances (G-H)/(G-J). This means that for the most discordant data points (samples BAL-5-7, BAL-6-2, BAL-12-2) the effect of episodic Pb loss during an early Paleozbic event accounts for at least 85 % of the age discordancy.

The isotopic pattern of zircon sample BAL-5 In Fig.4 the best-fit lines have been calculated for all zircon fractions of the same location. A close look at the regression line to the data points of zircon from the Phoenix dome, however, reveals that the data plot would be better described by a narrow band or wedgelike area. Particularly the points of sample BAL-5 (see also the enlarged detail in Fig.6) show that the rectilinear regression is a gross simplification. Only the fact that the flanks of the narrowly bent sequence of points are symmetrically parallel to line P suggests that the pattern may be basically attributed to the same causes as the overall pattern. In comparison with the systematics generally observed among presumably cogenetic fractions of magmatic zircon [25], the pattern of sample BAL-5 shows several peculiarities: (1) The U concentration increases with increasing grain size. (2) In the smaller size fractions BAL-5-1 to 5-3, the U concentration decreases with decreasing U-Pb apparent ages. (3) Extrapolation of both flanks of the bent pattern yield intersections with the positive ordinate axis of the concordia diagram. These peculiarities in the systematics of sample BAG 5 suggest several conclusions:

U-Pb SYSTEMATICSIN HETEROGENEOUSZIRCONPOPULATIONS (1) The development of the zircon crystals comprises more than two stages of crystal growth, which is consistent with the conclusions from the morphologic and electron-microprobe studies. (2) Tile decrease of the U-Pb apparent ages with decreasing U concentrations for the three finest size fractions BAL-5-I to 5-3 (see also line A in Fig.5) is indicative of a relatively U-poor, young zircon new-growth among an older, relatively U-rich population. Similar systematics have been observed also in magmatic rocks of presumably palingenetic origin [14] and in anatectic paragneisses [26] where the young, U-poor, and concordant zircon component could be analyzed separately. From the cathode luminescence studies it is obvious that the effect of mixing with U-poor, younger zircon (zircon phase three) is present also in all the other zircon fractions of the Baltimore Gneiss, but is generally not perceptible on the concordia plot because it is overridden by the effect of episodic Pb loss as a function of the radiation damage. (3) The deviation of the data points from the bestfit line P indicates that prior to the Paleozoic metamorphism the data points of the Precambrian zircon component had already a distinct spread due to the presence of possibly much older inherited components and/or multi-stage zircon growth during the Grenville orogenic cycle.

5. Geologic conclusions The fact that variable amounts of inherited zircon can be incorporated in the zircon fractions suggests that the ages obtained by upward and downward extrapolations to the concordia curve should be interpreted with great caution. The results - particularly those from sample BAL-5 - clearly show the risk one takes if one depends only on the zircon data of a single rock sample or even on analyses of unsplit zircon populations. However, despite the uncertainty due to the presence of inherited material, I believe that the difference in the upper intercept ages of about 100 m.y. between the data of the Phoenix and Towson dome reflects not only the effects of different amounts of inherited zircon. Thus it seems likely that the Precambrian metamorphic history is not the same for the various gneiss domes. Rb-Sr whole-rock analyses [21 ] on the same sam-

245

pies from the Phoenix dome which were used for the zircon separation did not yield an isochron: all data points fall on or above a 1.1 b.y. reference isochron (X87 Rb = 1.47 X 10-11yr-1) and plot also above an isochron of 1050 -+ 100 m.y. (X87Rb = 1.39 × 10-11 yr -1 ) that Wetherill et al. [20] have obtained on wholerock samples from the Phoenix, Towson, and Woodstock domes. This may be due to later, post-Grenville disturbances of the Rb Sr systems or because isotopic homogenization among the various whole-rock systems has never been reached. In any case, neither the U-Pb nor the Rb-Sr data can support the assumption that the rocks of the two localities studied were metamorphosed during the same event, and that the Sr isotopes were ever homogenized between the two areas. From a comparison with zircon ages of the Precambrian basement of the West Chester Prong in the Pennsylvanian Piedmont [27], it seems that the Grenville metamorphism in the gneiss domes near Baltimore is some 50-200 m.y. older than the final stage of metamorphism in the Baltimore Gneiss rocks of the West Chester Prong, where the extrapolated zircon ages of granulite-facies rocks yielded intercept ages between 980 and 1060 m.y.*. As to the lower concordia-intercept ages, the values of 421 and 455 m.y. appear to be consistent with the results of Rb-Sr analyses on post-Taconic pegmatites [19]. From those data Wetherill et al. [19] concluded that the major plutonism, deformation, and metamorphism associated with the early Paleozoic orogeny was completed 425 +- 20 m.y. ago (X87Rb = 1.39 X 10 -11 yr -1 ). However, on the grounds of the same uncertainties mentioned above, this consistency in the ages should also be considered with caution: from the U-Pb data alone, one cannot exclude the existence of a minor post-Taconic (e.g. Acadian) disturbance of the U-Pb systems, superimposed on the effects of the Taconic metamorphism.

6. Summary of the conclusions (1) More than half of the zircon grains of each population reflect more than one stage of crystal growth. * The U-Pb analyses on the zircons from Pennsylvania (Grauert et al. [27] ) were performed with the same combined 2°spb-235U tracer used for this study.

246

B. GRAUERT

Sieve Uranium fraction (ppm)

z°6Pb Zo~pb

(pro)

9~o~) >" o16o

o ,~/77"/S-3 r ,. -/

/~

""

53-75 75100

5-4

s-2

/

. . . . 146

. . . . . 150

48,000 27,000

40-53

866

~3000

100160

1162

7,790

>150

1342

1,940

<40

642

47500

>150"

2776

4,614

0145 [ / ~

890 973

2-7j 155 2O7p b 23sU

Ilseparated by handpicking

Fig.6. Enlarged detail of Fig.4 showing the data plot of samples BAL-5-1 to 5-6, the zircon fractions of a migmatitic biotite gneiss with intense K-feldspar blastesis. P indicates the bestfit line to all zircon fractions of the Phoenix dome. The dashed lines connect fractions of increasing grain size and U concentration. The error in the 2°Tpb/2°6pb ratios increases with decreasing 2°4pb/2°6pb ratios, which is due to the increasing uncertainty in the common-Pb correction. (2) The bulk of the zircon substance crystallized most likely during high-grade metamorphism, migmatization, and granitization of the rocks in the course of the Grenville orogeny. (3) Roundish cores in many zircon crystals may either represent an earlier stage of zircon formation during the same orogeny or may be relics of inherited grains of possibly much higher "primary" ages. (4) Clear, U-poor overgrowths are probably the result of a general recrystallization during or after the Taconic (and Acadian?) deformation of the rocks. The "new" zircon substance could partly be remobilized material which was previously corroded from the same crystals. (5) The U - P b systematics in the zircon populations reflect essentially two episodes which may be correlated with major metamorphic events during the Grenville and Taconic (and Acadian?) orogeny, respectively. (6) Models of continuous loss of radiogenic Pb cannot adequately explain the data pattern: (7) The discordant age pattern is essentially the result of a superposition of episodic Pb loss and zircon overgrowth during the Taconic (and Acadian?) metamorphisms. Zircon overgrowth appears to be present in all zircon fractions but its effect on the U - P b systematics is generally overridden by the effect of episodic Pb loss. For the fractions showing the most discordant ages, the contribution of episodic Pb loss to

the age discordancy is at least 85 %. Evidence for episodic U gain has not been found. (8) Considering the zircon data of Baltimore Gneiss rocks of the West Chester Prong, Pennsylvania, one may conclude that the Grenville metamorphic history in the Piedmont of Maryland and Pennsylvania lasted from at least 1200 m.y. until about 980 m.y. (9) The Precambrian zircon-forming events in the Phoenix and Towson domes seem to be 5 0 - 2 0 0 m.y. older than the granulite-facies metamorphism in the West Chester Prong.

Acknowledgements This study was carried out during a North Atlantic Treaty Organization fellowship. I wish to thank Drs. A. Hofmann, St.R. Hart, T.E. Krogh, R.H. Steiger, and V. K6ppel for helpful discussions and critical comments and suggestions on an earlier version of the manuscript. Also the comments made by the reviewers are gratefully acknowledged. | am thankful to Mr. K.D. Burrhus and Dr. St.R. Hart, who took care for the high performance of the mass spectrometer. Mr. J. Sommerauer kindly called my attention to the cathode luminescence study of zircon. The electron-microprobe studies were made at the Institut fiir Kristallographie und Petrographie, Ziirich.

Appendix Analytical Techniques The zircon separation and the mass spectrometric technique have been described previously [30]. The method developed by Krogh [31 ] was used for the extraction of U and Pb. Sample weights were 7 - 6 0 rag. Aliquots of all samples were spiked with the same combined 208pb-235U tracer. Common-Pb contamination during the analyses was on the order of 0 . 5 - 2 . 0 ng. Ages were calculated with the following decay constants: X238U = 1.5513 X 10 -10 yr -1 [29], X235U = 9.8485 X 10 -10 yr -1 [29].

Sample locations Phoenix gneiss dome Samples BAL-2, BAL-3, BAL-4, BAL-5, and BAL-6

U-Pb SYSTEMATICS IN HETEROGENEOUS ZIRCON POPULATIONS were collected within 130 m from outcropping rocks at Piney Creek (Hereford quadrangle, Maryland, Baltimore Co., 356850/4380650, 1000-m grid). It is the same locality as that designated B4 by Tilton et al. [16] and Wetherill et al. [20]. See also the sketch map fig. 18 in Hopson [18]. BAL-2: fine-grained, dark biotite-hornblende gneiss with some light veins (conf. [18], plate 1, fig.2). Modal analyses similar to Hopson [18; table 9, N o . 6 ] . BAL-3: light, b a n d e d gneiss showing alternating more or less biotite-rich layers 0 . 5 - 5 cm thick. BAL-4." fine-grained, dark, and homogeneous biotite gneiss with some h o r n b l e n d e . BAL-5: medium-grained migmatitic biotite gneiss showing intense K-feldspar blastesis, b u t no veins (leucosomes). BAL-6." K-feldspar-rich aplitic neosome about 2 - 5 cm thick o f distinctly veined gneiss like those shown in Hopson [18; plate 1, riga or plate 2, figs.1 and 2].

Towson gneiss dome Samples BAL-11, BAL-12, and BAL-13 were collected within 12 m from a blasted outcrop at Gunpowder Falls (Towson quadrangle, Maryland, Baltimore Co., 369420/4364470, 1000-m grid).

BAL-11: coarse-grained augen gneiss (Hartley augen gneiss) with large crystals of microcline [18; p.43 and plate 5, figs.1 and 2 ] . BAL-12: migmatitic intercalation of biotite gneiss in the augen gneiss. BAL-13: medium-grained granitic gneiss from about 40 cm thick vein in the augen gneiss.

References

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247

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