Reworking of Archaean and Early Proterozoic components during a progressive, Middle Proterozoic tectonothermal event in the Albany Mobile Belt, Western Australia

Precambrian Research, 59 (1992) 95-123

95

Elsevier Science Publishers B.V., Amsterdam

Reworking of Archaean and Early Proterozoic components during a progressive, Middle Proterozoic tectonothermal event in the Albany Mobile Belt, Western Australia Lance P. Blacka, Lyal B. Harris b and Claude P. Delor b,1 aBureau of Mineral Resources, G.P.O. Box 378, Canberra, A. C. T. 260 I, Australia bDepartment of Geology, University of Western Australia, Nedlands, W.A. 6009, Australia (Received September 25, 1990: accepted after revision January 22, 1992)

ABSTRACT Black, L.P., Harris, L.B. and Delor, C.P., 1992. Reworking of Archaean and Early Proterozoic components during a progressive, Middle Proterozoic tectonothermal event in the Albany Mobile Belt, Western Australia. Precambrian Res., 59: 95--123. Isotopic data, derived mainly from U - P b zircon ion-microprobe analyses, are presented for each of the three tectonic domains of the Albany Mobile Belt, Western Australia. They show that not only the Northern Domain, but also parts of the Central domain originally crystallised about 3100 Ma ago; they represent reworked Archaean rocks similar to those of the adjoining Yilgarn Block. On the basis of Sm-Nd model ages, a somewhat younger, less precisely defined crustal formation event of probable Early Proterozoic ( ~ 2100 Ma ) age can be inferred for the Southern Domain and the remainder of the Central Domain. The younger component of the Albany Belt is older than some previous estimates, being of similar initial age to most of the Early Proterozoic orogenic provinces of northern and central Australia. Interpretation of the StaNd data is not strictly based on conventional model ages, for it appears that significant fractionation of the rare-earth elements occurred during Middle Proterozoic crustal melting. Pegmatites and granites dated by U - P b on zircon provide a temporal framework for the Middle Proterozoic evolution of the Albany Mobile Belt. Progressive deformation, incorporating foliation development, up to four superimposed fold generations, late-tectonic granite intrusion and conjugate shear zone arrays took place under a consistent orientation of maximum compressive stress over a geologically brief time span, about 1190 Ma ago. Dextral transpression and thrusting at 1190 Ma postdates granulite facies metamorphism and major deformation events in the Fraser Mobile Belt. The widespread 1190 Ma rocks are of comparable age to intrusives rocks emplaced during a similar high-grade metamorphic event in the Bunger Hills of Antarctica (a region that is commonly juxtaposed with the Albany Mobile Belt in Gondwana reconstructions).

Introduction Many, if not most, Precambrian terrains are characterised by complex geological histories, which incorporate a series of tectonothermal events. Whilst it is not u n c o m m o n for as many Correspondence to: Dr. L.P. Black, Bureau of Mineral Resources, G.P.O, Box 378, Canberra, A.C.T. 2601, Australia. 1Present address: D6partement des cartes et synth6ses, BRGM, Orl6ans, France.

as five such events to be reported from a single terrain (e.g., Black et al., 1979 ), other terrains may portray a progressive evolution of structural overprinting (which may be associated with changes in metamorphic grade) where successive stages in a single tectonic episode may be recognised (Brun and Choukroune, 1981 ). Critical to our understanding of the geological evolution of these regions, especially in developing tectonic models, is the timing of relative tectonothermal sequence.

96

The main purpose of the present research was to find the absolute ages of successive stages of foliation development, folding, metamorphism and intrusive activity during an intense tectonothermal event which fashioned the Proterozoic Albany Mobile Belt (AMB) of Western Australia. In addition, studies were undertaken to determine the age of precursor components to the AMB. This information was needed to expand our knowledge of regional geological history, and to determine whether a progressive model, suggested by interpretations of a coherent stress regime, is indeed applicable, or whether several events, separated by large time intervals, can be determined. It also allows comparisons to be made with Gondwana correlatives in Antarctica and India, and is of general importance to many crustal evolution studies. The ages of the different foliations were determined by dating the emplacement of pegmatite and granitoid bodies with different structural timing relationships. These ages are considerably at variance with earlier deductions by Beeson et al. ( 1988 ), which were based on previously reported ages for the AMB and correlations with older ages from the adjacent Fraser Mobile Belt. In order to circumvent the many potential difficulties associated with the dating of complex Precambrian terrains, and to maximise the chance of obtaining igneous crystallisation ages, this investigation was primarily designed around a U - P b zircon study on the ANU ionmicroprobe SHRIMP. Some Rb-Sr and SmNd work was also done. Controlled redundancies were built into the sampling program, which was closely integrated with structural and metamorphic field observations. Geological setting The E-W to ENE-WSW trending AMB, together with the NE-SW trending Fraser Mobile Belt (FMB) comprise the Albany-Fraser Province, an arcuate Proterozoic orogenic belt

L.P. BLACK ET AL.

immediately to the south and southeast of the Archaean Yilgarn Block in southern Western Australia. In Gondwana reconstructions, the AMB marks the tectonic contact between the Yilgarn Block and terrains within East Antarctica. Its structural evolution should thus record the relative displacements between these two Precambrian Shields. The location of the AMB sample sites examined in this study and other place names referred to in the text are shown in Fig. 1. The AMB is about 1000 km long and up to 200 km wide. It is composed of highly deformed orthoand (less commonly), para-gneisses (mainly of felsic composition), mafic and felsic granulites, and syn- to late- tectonic granitoids. The Mount Barren group metasediments crop out in the eastern, ENE-striking sector of the AMB. The deformation regime in the AMB is a function of orientation against the Yilgarn craton margin: progressive folding and shear zone development indicates NW-directed thrusting for granulite facies gneisses and the Mount Barren Group metasedimenty rocks. Similarly, a dextral transpressional regime is indicated for the remainder of the belt. Both regimes indicate NW-directed convergence between East Antarctica and the West Australian Shield and show up to four "generations" of folds (based on overprinting criteria), with the formation of shear fabrics in high strain zones. The first two generations of folds are generally tight to isoclinal, with the third upright to NW-verging phase generally tight to open. Discrete dextral (dominant) and minor sinistral conjugate ductile shear zones, bisected by pegmatites, formed late in the deformation history at a higher structural level, again indicating a continued NW-SE maximum compressive stress orientation. Warping of foliations due to movement along late shear zones gave rise locally to another generation of open folds. Based essentially on aeromagnetic properties and structural/metamorphic criteria, the

P R O T E R O Z O I C EVENTS IN THE ALBANY MOBILE BELT

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Fig. 1. Map of the Albany Mobile Belt showing the principal geological domains and the location of all sample sites. ND=Northern Domain, CD=Central Domain, SD= Southern Domain of the Albany Mobile Belt; WGT=Western Gneiss Terrain and B= Balingup Metamorphic Belt of the Archaean Yilgarn Block; PB=Phanerozoic sediments of the Perth Basin. Inserts show regional location.

AMB was divided by Beeson et al. (1988) into the following domains: The Northern Domain (about 25 km wide) consists of reworked Archaean orthogneisses of the Yilgarn Block, together with minor E - W trending, ~ 2.5-2.1 Ga dolerite dykes. Primary Archaean textures are progressively overprinted to the south by non-coaxial deformation and prograde amphibolite-facies metamorphism. The Central Domain is separated from the Northern Domain by a dextrally oblique thrust zone. This Domain consists of felsic granulites (70%), quartz-magnetite gneiss, including metamorphosed iron formations (25%), and mafic granulite (5%). In the northern Central Domain, retrogression to amphibolite-facies

assemblages and development of a new foliation occurred within wide, ductile shear zones (oblique-thrusts). Granulite-facies metamorphism elsewhere in the Central Domain continued during the dominant fold phases. The Central Domain grades transitionally into the Southern Domain, which is characterised by voluminous granitoids and orthogneisses with retrograde, amphibolite-facies assemblages containing enclaves (dominantly mafic gneisses) in which granulite mineralogy is preserved. Two suites of granitoids can be distinguished from digitally-processed aeromagnetic images (Tucker et al., 1986). Those granitoids intruding the Central and Southern Domains, such as Mount Chudalup, the Porongurups and Mount Manypeaks (all show-

98

ing localised, discrete shear zones or a flattening fabric) have high magnetic susceptibilities due to high magnetite content. In contrast, the granitoids of the Burnside Batholith (including Mount Franklin and the Albany Adamellite, (which show only slight, local foliation development), have low magnetic susceptibilities. The magnetic granitoids also tend to be spatially associated with late cross-cutting NWstriking ductile dextral shear zones (some of which were subsequently reactivated during young brittle events) that are discernible on the regional aeromagnetic image. Samples of both types ofgranitoid in the Southern Domain were analysed to see if there is an appreciable age difference between them. Published Rb-Sr and Sm-Nd data (e.g., Rosman et al., 1980; Fletcher et al., 1983 ) suggest that the Central and Southern Domains are Proterozoic, rather than being derived from an Archaean precursor, and isotope studies were also carried out to investigate this issue.

Previous geochronology The best summaries of geochronological information for the Albany Mobile Belt are to be found in the work by Page et al. (1984) and Pidgeon (1990). Although a reasonable number of geochronological studies have been made, most of these have not been sufficiently rigorous to provide meaningful ages for precise correlation with known geological events, and are therefore only of interest in an historical context. A Rb-Sr age of 1070 __+50 Ma was obtained for the post-kinematic Albany Adamellite (Turek and Stephenson, 1966 ) and KAr ages of seven hornblende separates range from 1060 to 1160 Ma (Stephenson et al., 1977 ). Stephenson et al. (1977) argued that the minimum age for high-grade metamorphism in the region was 1160 Ma. The current study shows that this estimate is remarkably accurate. Only very recently has U - P b zircon dating been applied to the Albany Mobile Belt. Pid-

L.P. BLACK ET AL.

geon (1990) presents a combination of multigrain and single grain analyses for three different rock types. A coarse-grained enderbite from Albany township yields an age of 1289 _+10 Ma, which is interpreted as dating igneous emplacement. The nearby Albany Adamellite yields an age of 1174+ 12 Ma. An age of 1177___4 Ma was derived for an Adamellite from Mount Chudalup, 200 km to the west. The Adamellite ages were interpreted by Pidgeon as dating emplacement, thus providing a minimum estimate for the age of the last regional-scale Proterozoic tectonism to have affected the area. That minimum estimate is close to, but a little older than, the value of I 160 Ma derived from earlier studies.

Summary of the Rb-Sr data Rb-Sr determinations (Tables 1, 2 and Appendix B ) were made on ten rocks (from nine separate sites), including each of the six rocks investigated below by U-Pb. A surprising feature of the Rb-Sr data for the gneisses from AFP 2 and AFP 3 is the lack of isotopic equilibration achieved at about 1190 Ma, even though there appears to have been a fluid phase capable of extensive elemental transportation at that time. In terms of the primary aims of this study, the Rb-Sr data show that tectonothermal activity is at least as old as I 164 + 27 Ma (and is perhaps as old as 1230+50 Ma, noting the precision limits) and that the third folding phase in the eastern AMB occurred before 1140 Ma. Although these conclusions are consistent with the igneous crystaUisation ages determined below from U-Pb analyses of zircon, they nevertheless document yet another instance of the Rb-Sr system underestimating igneous crystallisation ages in a polymetamorphic, deformed terrane.

U-Pb zircon data

Sample AFP 1, GroperBluff granulite (118°54'E, 34°30'S)

PROTEROZOIC EVENTS IN THE ALBANYMOBILEBELT

99

TABLE 1 Rb-Sr isotopic composition of rocks from the Albany Mobile Belt Sample No.

Phase

Sr (~tg/g)

878r/86Sr

358

0.70365

0.0050

39.1 39.5 68.3 30.8 35.8 35.3

503 478 401 501 490 472

0,71142 0.71151 0.71853 0.70992 0.71102 0.71138

0.2247 0.2247 0.4922 0.1778 0.2108 0.2161

73,9 51.4 87.9 88.2 55.0 79.2

362 371 376 380 351 365

0,72516 0.72086 0.72438 0.72569 0.72345 0.72604

0.5907 0.4011 0.6753 0.6715 0.4533 0.6283

164 209 169 8.01 158 243 141 162 202 205 9.87 158

0.80019 0.83970 0.76574 8.6089 0.74759 0.73837 0.84223 0.74695 0.74960 0.72696 5.2252 0.74912

2.277 4.540 0.3220 494.5 1.740 1.020 3.030 1.794 1.861 0.5602 284.2 1.255

652 635 644 623 648 657 658 652

0.71227 0.71589 0.71636 0.71150 0.71508 0.71430 0.71270 0.71660

0.2242 0.4537, 0.4759 0.1933 0.4054 0.3594 0.2487 0.4986

59.3 59.1 49.0 52.8 59.2 76.3 85.0 100.5

1.4435 1.4329 1.5919 1.5412 1.4507 1.1867 1.1935 1.2793

Rb (/~g/g)

87Rb/86Sr

Age (Ma)

Pegmatite, Groper Bluff AFP 1 A

rock

0.62

Felsic gneiss, Palhnup Estuary AFP2 AFP2 AFP2 AFP2 AFP2 AFP2

A B C D E F

rock rock rock rock rock rock

Pegmatite, Pallinup Estuary AFP2 AFP2 AFP2 AFP2 AFP2 AFP2

G H 1 K L M

rock rock rock rock rock rock

Banded gneiss, Hassel Highway AFP3 C AFP3 (" AFP3 C AFP3 C AFP3 K AFP3 L AFP3 N AFP3 O AFP3 0 AFP30 AFP30 AFP3 P

rock K-feldspar plagioclase biotite rock rock rock rock K-felspar plagioclase biotite rock

128.1 324 18.8 774 94.6 85.5 146 107 130 39.7 674 68.3

Orthopyroxene-bearing pegmatite, Fishery Beach AFP4 AFP4 AFP4 AFP4 AFP4 AFP4 AFP4 AFP4

A B C E G H I J

rock rock rock rock rock rock rock rock

50.6 99.7 106.1 41.7 90,9 81.7 56.7 112.4

Biotite-bearingpegmatite, Fishery Beach AFP5 AFP5 AFP5 AFP5 AFP5 AFP5 AFP5 AFP5

A B C D E F H 1

rock rock rock rock rock rock rock rock

874 869 872 877 889 742 840 1148

45.72 45.47 55.80 51.85 46,30 29,43 29.90 34.85

1108

1108

100

L.P. BLACK ET AL.

TABLE 1 (continued) Sample No.

Phase

Rb (/~g/g)

Sr (/zg/g)

aTSr/S6Sr

STRb/86Sr

154.0 85.0 120.1 71.3 49.2 118.2 40.4 11.5

319 314 322 298 296 279 287 263

0.73816 0.72789 0.73280 0.72637 0.72276 0.73470 0.72131 0.71682

1.396 0.7846 1.079 0.6932 0.4807 1.230 0.4068 0.1264

184 444 16.6 908

338 578 241 47.0

0.73562 0.74605 0.71353 1.6137

1.574 2.226 0.1991 60.76

181 401 41.6 876 23.0

397 680 367 17.98 62.6

0.73126 0.73696 0.71539 3.6529 0.72697

1.323 1.706 0.3276 181.3 1.063

240

253

0.75635

2.744

Age ( Ma )

Orthopyroxene-bearingpegmatite, Short Beach AFP6 AFP6 AFP6 AFP6 AFP6 AFP6 AFP6 AFP6

B C D E F G H J

rock rock rock rock rock rock rock rock

Aplite, PorongurupRange AFP8 AFP8 AFP8 AFP8

A A A A

rock K-feldspar plagioclase biotite

1037

Augen gneiss, PorongurupRange AFP9 AFP9 AFP9 AFP9 AFP9

A A A A A

rock K-feldspar plagioclase biotite hornblende

1134

Porphyritic granite, Burnside Batholith MF2

rock

Procedures used for Rb--Sr analysis are described in Williams et al. (1976).

TABLE 2 Summary of Rb-Sr isochron parameters Sample

Rock type

Isochron type

No. of phases

MSWD

Age (Ma)

Initial STSr/S6Sr

Comments

AFP2 AFP2 AFP3 AFP3 AFP3 AFP3 AFP3 AP'P4 AFP5 AFP6 AFP8 AFP8 AFP9 AFP9

pegmatite gneiss gneiss feisic layer felsic layer mafic layer mafic layer pegmatite pegmatite pegmatite aplite aplite gneiss gneiss

rock rock rock minerals minerals minerals minerals rock rock rock minerals minerals minerals minerals

6 6 6 4 3 4 3 8 8 8 4 3 5 4

170 7

1040_+730 1870 + 150

0.716_+0.006 0.7053 + 0.0006

287 < 1 95 8 < 1 6 7 120
1109 _+ 15 1230 + 50 1108-+15 1109 + 42 1140_+ 40 1084_+30 1164 + 27 1040 + 16 1120 _+60 1133 + 11 1100 _+25

0.764_+ 0.009 0.760_+ 0.001 0.719_+0.003 0.718 _+0.008 0.7085 _+0.0002 0.73+0.02 0.7147 + 0.0002 0.712 -+0.003 0.710 _+0.001 0.7097 + 0.0006 0.7103 _+0.0004

Too scattered to be useful Doubtful significance Extremely scatttered Minimum age for D2 Regression excluding biotite Minimum age for D2 Regression excluding K-feldspar Minimum age for D 1 Minimum age for DI Minimum age for D 1 Minimum age for D2 Regression excluding biotite Minimum age for D2 Regression excluding biotite

The regressions listed above are by the method of Mclntyre et at. (1966). MSWD is a measure of the goodness of fit, and should ideally be less than unity. The ages are derived from the decay constants recommended by Steiger and Jager (1977). All uncertainties are reported at the 95% confidence level.

PROTEROZOIC EVENTS IN THE ALBANY MOBILE BELT

AFP 1 is a 30 cm wide quartz-plagioclaseorthopyroxene-hornblende vein from Groper Bluff, in the Central Domain about 120 km northeast of Albany (Fig. 1 ); a description of this area is given by Beeson et. al. ( 1988 ). This pegmatite slightly cross-cuts the regional foliation (Fig. 2d) and was emplaced early in the structural history. The pegmatite is well endowed with coarse-grained zircon (averaging about 300 by 1000 #m), most of which is indisputably co-magmatic. The grains show no signs of zonation (presumably due to their low U and Th contents, Table 3 ) and are bounded by well crystallised first order prismatic and pyramidal faces. Some of these grains have nucleated around older zircon crystals which have either euhedral or partially resorbed boundaries. Five analyses were made on three of these cores. Correction for common Pb was by the 2°Spb method (Compston et al., 1984) because the analyses exhibit no significant evidence of disturbance to the U - T h - P b system, and because this method is potentially more capable of precisely resolving common Pb components than the 2°4pb method. The sixteen analyses of comagmatic zircon form a tight isotopic grouping (Fig. 3 ). However, the mean value is a little reversely discordant; the 2°6pb/23sU age is 1196+-8 Ma and the 2°Tpb/2°6pb age is 1165 +_28 Ma. The older mean age is favoured as the time of magmatic crystaUisation for two reasons. First, it can be relatively difficult to determine precisely radiogenic 2°7pbin low U ( 35-111 #g/g) grains, even when they are Late Precambrian in age. Second, it is not common to find relatively young, low U grains lying above concordia (see discussion below for the explanation of that phenomenon). It is likely that this circumstance is only apparent, arising from a slight over-correction for common Pb in the estimation of radiogenic Pb compositions. The analysed cores (Table 3; included with the old grains of AFP 2 in Fig. 7) are clearly much older than their host rock, and tend to

101

have higher U and Th (140+_32 and 52+_27 #g/g compared with 62 +- 23 and 31 _ 16 #g/g, respectively) than the co-magmatic zircon. In common with general laboratory practice, based on an increasing probability of disturbance to the U - T h - P b system in progressively older rocks, radiogenic Pb of these Archaean grains was determined by the 2°4pb correction; however, the 2°Spb method produces indistinguishable results. The five imperfectly aligned analyses ( M S W D = 12) of these three cores form an array which intersects concordia at 2o~n+ Af~(~+ 400 . . . . 160 12o and !,-,,,,,_7oo Ma. Although the younger value is consistent with the crystallisation age of the pegmatite (suggesting that the Pb was lost from these cores to the pegmatite magma), its imprecision renders it of little use. The older intercept age should approximate the original age of the zircon cores. It can be somewhat refined on the basis that even the oldest preserved 2°7pb/E°6pbage (2933 + _ 16 Ma for grain l 0.1 ) should define a younger limit for the time of zircon growth. The cores in AFP 1 are therefore likely to have crystallised between 2917 and 3110 Ma ago.

Sample MF 2, Mount Franklin granite (116°47'E, 34°50'5) MF 2 is a late-tectonic, porphyritic granite forming part of the Burnside Batholith about 200 km west of Albany (Fig. l ) in the Southern Domain. Zircons from MF 2 are of typical igneous form. Most have length: breadth ratios of 3 or 4: l, average about 400 #m in length, and are doubly terminated. Simple first order prismatic and pyramidal faces dominate. Silicate minerals and iron-oxides commonly occur as inclusions. Some grains are optically homogeneous or faintly zoned; others are strongly zoned. Many grains have an optically distinct, generally rounded, zircon core. The 2°4pb and 2°gPb common Pb correction methods yield indistinguishable radiogenic Pb components for the MF 2 analyses; the 2°Spb method was used, for the same reasons given

I-

.r-

t,~

| 03

PROTEROZOIC EVENTS IN THE ALBANY MOBILE BELT

for the young AFP 1 zircons. Grain 8.1 is the only isotopic outlier on the concordia diagram (Fig. 4). Its position is readily explicable in terms of abnormally high U and Pb (2775 and 2462 #g/g, respectively). Kinny (1987) and Harrison et al. (1987) reported that high-U parts of zircon crystals may actually lose some of that U, and be displaced above concordia because of this factor, not because of Pb gain. Alternatively, the high U contents of these grains might have produced a metamict structure which cannot be strictly calibrated in terms of P b / U ratio with the lattice of the standard zircon grain (I.S. Williams, pers. commun. 1989). The analysis in question (8.1) represents a core, but unlike the other nine analysed cores, has different 2°8pb-corrected 2°7pb/2°6pb from that of the obviously syn-magmatic zircon in this rock. Presumably this means that at least most of the cores represent either an early stage of magmatic crystaUisation, or that they were derived from an already crystallised rock (possibly the immediate source of the granite itself) only marginally older than the granite. After exclusion of grain 8. l, the data configuration falls within experimental error of constant 2°7pb/2°6pb and 2°7pb/235U ratios, but it is a factor of two greater than the expected error for 2°6pb/238U. As all the data lie on or below concordia, this is probably due to recent loss of Pb from the zircons. In this circumstance, the age of crystallisation is given by the intersection of the data array with concordia, which corresponds in this instance with the mean 2°7pb/2°6pb age (because the lower intersection is within error of a zero age). The crystallisation age of this granite is therefore taken to be 1189 + 9 Ma.

Sample AFP 8 AFP 8 is an undeformed, vertical, 110 °striking aplite dyke (containing minor amphibole) cutting the late-tectonic Porongurups granite (at Serena Park, 50 km north of Albany; see AFP 9 below), which in turn intrudes gneisses of the Southern Domain. Zircons from AFP 8 have typical igneous form, unmodified by later events. First order prismatic and pyramidal faces dominate. Length: breadth ratios average about 3:1 to 4:1, but range up to 6:1 (in 6 0 0 / l m long grains). Silicate inclusions are common. Most grains contain a discrete, optically distinct, markedly elongated core which is generally more highly zoned than the homogeneous to faintly zoned rims. Some cores appear to be metamict, presumably as a consequence of high U and Th content. Because of the likelihood of high common Pb levels and loss of radiogenic Pb, such cores were not analysed. The isotopic array (corrected by the 2°Spb method) on Fig. 5 reveals only one aberrant analysis (6.1), though its position is not governed by obvious factors such as abnormally high U or Th (Table 3). The analysed core ( 1.2 ) is the same age as the rimming syn-magmatic zircon. It might therefore have formed during the early crystallisation of this aplite. It is important to decide whether the very slight discordance of the cluster of analyses on Fig. 5 (individual analyses are within error of concordia, but the group as a whole plots a little below it ) is due to a marginal under-correction of the common Pb component, or whether it results from recent Pb loss. The spread of 2°7pb/2°6pb values over twice the experimentally expected range (compared with the sta-

Fig. 2 (a) Strongly banded gneiss AFP 3 (Hassel Highway) showing the locations of each hand specimen. (b) 035 °striking pyroxene-bearing pegmatite (AFP 4) cut by 135 °-striking biotite pegmatite (AFP 5) at Fishery Beach, Bremer Bay. (c) Sample locality for augen orthogneiss AFP 9 (Porongurups) showing the main 100 °-striking shear foliation cut by a 118 °-striking secondary shear zone. (d) 30 cm-wide, syn-D, pegmatite A FP I, the strike of which is slightly discordant to the regional foliation at Groper Bluff.

104

L.P. BLACKETAL.

TABLE 3 Ion-microprobe U-Th-Pb data for zircons from the Albany Mobile Belt Grain area

U (/zg/g)

Th (/zg/g)

Th/U

2°6pb/ 2°4pb

f206

2°6pb/23sU +- laerror

2°Tpb/235U _+laerror

2°7pb/2°6pb _+la error

AFP 1 (pegmatite, Groper Bluff) 1.1 1.2 2.1 2.2 ~ 2.3 ~ 3.1 4.1 5.1 6.1 7.1 8.1 9.1 9.2' 10.1 ~ 10.21 11.1 11.2 12.1 13,1 14.1 15.1

58 87 36 116 166 52 111 69 90 44 73 47 178 130 105 45 42 80 86 42 35

24 53 18 17 89 17 65 41 43 18 46 19 56 63 37 19 21 40 45 16 14

0.417 0.605 0.498 0.146 0.537 0.334 0.586 0.590 0.477 0.414 0.637 0.406 0.316 0.485 0.350 0.428 0.508 0.503 0.528 0.392 0.403

595 1048 482 3950 3477 645 685 694 1675 714 1704 1008 4740 4240 2577 r/96 668 1920 1103 447 611

0.0166 0.0133 0.0282 0.0032 0.0037 0.0108 0.0187 0.0138 0.0088 0.0151 0.0096 0.0141 0.0027 0.0030 0.0049 0.0141 0,0183 0.0047 0.0076 0.0184 0.0123

0,198_+0.002 0,203+0.002 0.205-+0.003 0.475_+0.005 0.451+_0.005 0.208+_0.003 0.207_+0.002 0.198_+0.002 0.200-+0.002 0.203_+0.003 0.203-+0.002 0.201 +0.003 0.392+0.004 0.571 +_0.007 0.505_+0.006 0.215+-0.003 0.206_+0.003 0.209_+0.002 0.211_+0.002 0.201 +-0.003 0.201 _+0.003

2.22+_0.07 2.20_+0.06 2.16_+0.09 13.10_+0.18 12.24-+0,16 2.34+0.07 2.29+_0.06 2.10+_0.07 2.11 _+0.05 2.25_+0.08 2.23_+0.07 2.20_+0.07 9.12+0.12 16.82+0.22 13.67-+0.20 2.33+-0.08 2.17_+0.08 2.27_+0.06 2.33_+0.06 2,16_+0.08 2.17_+0.07

0.0810_+0.0021 0.0785+0.0019 0.0762+_0.0029 0.1998_+0.0012 0.1966+_0.0010 0.0814-+0.0019 0.0803-+0.0019 0.0766_+0.0021 0.0762_+0.0017 0.0805+0.0024 0.0795+0.0021 0.0791 +0.0022 0.1687_+0.0009 0.2136_+0.0011 0.1961 +0.0014 0.0785_+0.0024 0.0767+-0.0026 0.0786+0.0017 0.0802+-0.0017 0.0778_+0.0024 0.0785_+0.0024

0.176 0.136 0.159 0.383 0.454 0.416 0.653 0.898 0.201 0.405 0.835 0.172 0.149 0.538 0.611 0.287 0.146

11293 12530 8506 1022 1172 1663 2124 5169 5513 1978 3186 33168 33124 405 14296 2301 19209

0.0014 0.0015 0.0014 0.0125 0.0109 0.0077 0.0060 0.0025 0.0023 0.0064 0.0040 0.0006 0.0004 0.0187 0.0029 0.0050 0.0006

0.196+0.002 0.192+0.002 0.197+0.002 0.545+0.009 0.519-+0.008 0.527_+0,007 0.525+_0.007 0.564+0.007 0.478+0.006 0.562+-0.009 0.550+-0.007 0.206+0.002 0.203+0.002 0.190+0.003 0.195+0.002 0.191 +_0.002 0.200-+0.002

2.14+0.03 2.11 _+0.03 2.14+0.03 16.45+0.37 15.08+-0.29 15.50-+0.27 15.30_+0.24 16.79+0.23 12.60+0.17 17.38+0.34 16.52+-0.24 2.26+0.03 2.22±0.03 2.03+0.08 2.11_+0.03 2.08_+0.03 2.19_+0.03

0.0791_+0.0003 0.0796+0.0003 0.0789+-0.0003 0.2189+0.0030 0.2108_+0.0023 0.2t33+_0.0017 0.2114+_0.0013 0.2160-+0.0008 0.1910_+0.0008 0.2244+0.0021 0.2t77_+0.0011 0.0795+-0.0002 0.0794+0.0002 0.0777_+0.0025 0.0785+0.0007 0.0790+0.0005 0.0792-+0.0002

5304 2190 2840 4427 4736 4682 2379 888

0.0024 0.0058 0.0045 0.0029 0.0027 0.0027 0.0054 0.0144

0.439+0.006 0.437+0.006 0.470+0.006 0.458+0.006 0.552+0.007 0.452+0.006 0.485+0.007 0.505+-0.008

10.59+0.15 10.37+0.16 11.69+0.18 11.01 + - 0 . 1 6 16.33+0.24 10.99+0.16 13.19+0.22 14.26__+0.35

0.1750+_0.0008 0.1722+0.0012 0.1804+-0.0011 0.1745_+0.0008 0.2145+0.0010 0,1764+-0.0008 0,1972+0.0015 0.2047_+0.0033

AFP2 (pegmatite, Pallinup Estuary) 1.12 2.12 3.12 4.1 5.1 6.1 22.1 23.1 24.1 25.1 26.1 27.12 27.22 28.12 29.12 30.12 31.12

1447 1482 1103 29 48 67 100 153 178 36 120 2873 2124 37 508 493 1593

255 202 176 11 22 28 65 137 36 15 100 494 316 20 310 142 233

AFP 20Celsicgneiss, Pallinup Estuary) 7.1 8.12 9.12 10.12 11.1 12.1 13.1 14.1

198 125 126 189 115 178 74 28

82 32 30 58 19 63 28 13

0.417 0.258 0.241 0.306 0.164 0.356 0.383 0.465

PROTEROZOIC EVENTSIN THE ALBANYMOBILEBELT

Grain area

U (#g/g)

Th (/1g/g)

Th/U

15.13 16.13 16.23 17.13

684 157 155 340 208 79 52 290 208 256 42 105

679 61 37 32 30 42 14 80 87 69 17 101

0.993 0.390 0.237 0.095 0.145 0.537 0.276 0.277 0,419 0.271 0.410 0.968

18.13 19.13 20.13 21.13 32.1 33.1 34.1 35.1

2°6pb/ 2°4pb

105

.~o6

z°6Pb/238U +_ let error

2°7Pb/235U _+ l a error

2°7pb/2°6Pb _+ lcrerror

343 3142 2342 4844 5558 2187 1165 4986 2079 8163 1934 8330

0.0372 0.0041 0.0054 0.0026 0.0023 0.0058 0.0109 0.0026 0.0061 0.0016 0.0066 0.0015

0.195_+0.002 0.433+_0.006 0.364_+0,005 0.371 _+0,005 0.469_+0.006 0,468_+0.006 0.449+_0.007 0.474 _+0.006 0.434_+0.005 0.468_+0.006 0.553_+0.008 0.561 _+0,007

3.82_+0.06 11.60_+0.17 9.84_+0.15 10.66_+0.14 13.25_+0.18 14.17_+0.23 12.03_+0.24 13.69 + 0.18 10.52+0.15 11.30-+0.15 16.55_+0.30 17.10_+0.25

0.1423+_0.0014 0.1942_+0.0011 0.1960_+0.0012 0.2083_+0.0007 0.2047_+0.0007 0.2194_+0.0016 0.1945_+0.0024 0.2094 _+0.0006 0.1759_+0.0009 0.1751 _+0.0006 0.2169_+0.0018 0,2210_+0,0009

0.906 0.357 0.743 0,373 0.572 0.598 0.885 0.828 0.616 0.056 0.723 0,483 0.221 1.067 0.421 0.604 0.486 0.610 1.190 0.552

3188 29197 2513 3379 4611 2796 4168 6250 2296 13556 4700 1931 12960 5134 3954 7111 14370 5195 4889 2138

0,0038 0.0015 0.0061 0.0034 0.0072 0.0056 0.0055 0.0159 0.0077 0.0007 0.0054 0.0048 0.0006 0.0076 0.0022 0.0021 0.0010 0.0023 0.0001 0.0042

0,198+0.002 0,199+_0.002 0.196_+0.002 0.200_+0.002 0.199_+0.002 0.196-+0.002 0.199+0.002 0,223 _+0.002 0.201_+0,002 0.204-+0.002 0.197-+0.002 0.186_+0.002 0.196_+0.002 0.194_+0.002 0.196+0.002 0,191 _+0.002 0.198_+0.002 0.196-+0,002 0.199_+0.002 0.198_+0.002

2.15_+0.05 2.18-+0.03 2.11 _+0.05 2.20_+0.03 2.14_+0.04 2.15_+0.04 2.11 _+0.05 2.09_+ 0.03 2.15_+0.04 2.27+0.03 2.12+0.05 2.08_+0,05 2.16+_0.03 2.04_+0.06 2.18+_0.04 2.09_+0.04 2.18_+0.03 2.19_+0.05 2.31_+0.08 2.13_+0.05

0.0789-+0.0014 0.0793_+0.0004 0.0783_+0.0017 0.0796_+0.0008 0.0781 _+0,0010 0.0793-+0.0012 0.0769_+0.0014 0.0682_+ 0.0004 0.0774_+0.0012 0.0804_+0.0004 0.0779_+0.0014 0,0808_+0.0015 0.0802_+0.0004 0.0765_+0.0019 0.0805_+0.0009 0.0794_+0.0010 0.0799_+0.0005 0.0808_+0.0013 0.0842_+0.0027 0.0778+_0.0013

0.620 0.824 0.482 0.628 1.414 0.342 0.890 1.138 0.604 0.828 0.583 0,996 0.628 0.670

62150 7586 76453 21505 56980 774 270 18149 15555 5464 45413 9591 24588 131579

0.0006 0.0075 0.0002 0.0017 0.0004 0.0277 0.0817 0.0039 0.0021 0.0065 0.0032 0.0027 0.0035 0.0003

0,203__+0,004 0.198+0,004 0.199-+0,004 0.202+0,004 0.202_+0,004 0.198_+0.004 0,182_+0.004 0.202-+0.004 0.202+0.004 0.202-+0.004 0.198-+0.004 0.203+0.004 0,205+__0.004 0,204 + 0.004

2.24_+0.05 2.12-+0.07 2,24__+0.05 2.23+0,05 2.34_+0.08 2.10_+0.05 1.62_+0.07 2.18-+0.08 2.20_+0.05 2.15_+0.08 2.15-+0.06 2.19-+0.09 2.21 -+0.06 2.34 _+0.05

0.0799+__0.0009 0.0778__+0.0020 0.0816__+0.0007 0.0801 -+0.0008 0.0839_+0.0022 0.0768-+0.0008 0.0647__+0.0024 0.0784-+0.0024 0.0791 _+0.0009 0.0771__+0.0023 0.0788_____0.0014 0.0784+0.0025 0.0784-+0.0012 0.0835 _+0,0009

MF 2 (Burnside Granite, Mount Franklin) 1.1 2.1 3.1' 4.1 5.1 6.11 7.1 ~ 8.1 ~ 9.1 f 9.2 10.1 11.11 I 1.2 12,11 13.1' 14.1 ~ 15.1 16.11 17.1 18.1

323 1278 201 492 375 256 274 2975 323 1001 212 144 945 227 369 313 1135 194 116 176

293 456 149 184 214 153 243 2462 199 56 153 70 209 242 156 189 552 119 137 97

AFP 8 (aplite, Porongurup Range) 1.1 1.21 2.1 3.1 4.1 5.1 6.1 7.1 8.1 9.1 10.1 11.1 12.1 13.1

553 133 614 631 241 484 210 132 502 112 203 111 332 517

343 109 296 396 341 166 187 150 303 93 118 111 209 346

106

L.P. BLACK ET AL.

TABLE 3 (continued) Grain area

U (/~g/g)

Th (/zg/g)

Th/U

1.1 1.2 2.1 3.1 3.2 4.1 4.2' 5.1 6.1 7.1 8.1 9.1 10.1 11.1 ~ 12.1' 13.1 ~ 14.1 15.1 16.1 17.1 18.1

369 366 47 287 366 82 71 431 128 185 290 408 343 224 21 290 95 87 193 360 189

154 163 58 229 292 96 57 352 151 157 131 320 170 195 28 271 111 112 149 159 154

0.417 0.446 1.233 0.798 0.798 1.176 0.802 0.817 1.181 0,853 0.454 0.785 0.496 0.867 1.329 0.936 1.174 1.290 0.775 0.442 0.811

2°6pb/ 2°4pb 6402 8700 477 1860 9279 246 80 5317 11325 6666 4408 8035 15995 4986 906 5300 1330 2253 9178 13057 3349

,~o6

2°6pb/238U + I tr error

0.0026 0.0019 0.0350 0.0090 0.0018 0.0680 0.2075 0.0031 0.0015 0,0025 0.0038 0.0021 0.0010 0.0033 0.0184 0.0031 0.0125 0.0074 0.0018 0.0013 0.0050

0.191_+0.003 0.202+_0.003 0.188+0.004 0.187_+0.003 0.201 _+0.003 0.175+0.003 0.149_+0.003 0.199-+0.003 0.206+0.004 0.200-+0,003 0.202_+0.003 0.204_+0.004 0.210_+0.004 0.202_+0.004 0.203-+0.004 0.192_+0.003 0.191_+0.003 0.198-+0.004 0.204-+0.004 0.203-+ 0.004 0.195_+0.003

2°7pb/235U + 1a error 2.1t_+0.04 2.23-+0.04 1.65_+0.16 2.01 _+0.05 2.18_+0.04 1.72_+0.19 0.70-+0.29 2.21 -+0.05 2.35-+0.06 2.21 -+0.05 2.18+0.05 2.23+0.04 2.31 -+0.05 2.19-+0.05 2.46_+0.19 2.08_+0.05 1.99_+0.08 2.09+0.08 2.29-+0.05 2.21 + 0.04 2.15+0.05

2°TPbF°6Pb -+ I a error 0.0798_+0.0006 0.0802_+0.0006 0.0636_+0.0058 0.0779_+0.0014 0.0786_+0.0007 0,0711 _+0.0074 0,0341 _+0.0142 0,0805+__0.0007 0.0829_+0.0013 0.0803±0.0011 0.0785-+0.0010 0.0792_+0.0007 0.0797+0.0006 0.0789-+0.0010 0.0881 _+0.0063 0.0785+0.0009 0.0759_+0.0027 0.0764-+0.0024 0.0816_+0.0011 0.0790 +_0.0007 0.0798-+0.0013

The analyses in this table were generally made on zircon of igneous origin that crystallised during the emplacement of the rock or its precursor. Exceptions, or special cases, include the following. ' Core. Some of these are indistinguishable in age from zircon which crystaUised during the emplacement of these rocks. In other instances the cores are considerably older (see text ). 2 Equant multifaceted grains of possible metamorphic deviation. 3 Cracked, euhedral igneous grains. The remaining igneous zircon grains in the AFP 2 gneiss are essentially uncracked; metamorphism and/or deformation have resulted in substantial rounding of tbeir original form. Each of these two igneous zircon types in AFP 2 has a distinctive isotopic signature. f2o6 is the proportion of common Pb to total Pb in terms of mass 206. The analytical procedures used for the SHRIMP ionmicroprobe are described in Compston et al. (1984) and Williams et al. (1984). The instrument was operated at a mass resolution in excess of 6500 to eliminate significant spectral interferences. Minor changes in inter-element fractionation were monitored by repeated analyses of a standard zircon fragment (SL3 Pl ). Absolute values for Pb/U and Pb/Th are referenced to a 2°6pb/ 238U ratio in that standard of 0.0928, equivalent to an age of 572 Ma. Systematic inter-element fractionation was compensated by the use of a quadratic relationship determined between Pb ~"/U + and UO + / U + from the standard zircon. In order to obtain better analytical precision, correction for common Pb was based on the 2°8pb isotope for all of the Late Proterozoic zircons, with the exception of those in AFP 9, which were corrected by the 204 Pb method due to disturbance of the U/Th-2°6pb/2°Spb system. In common with regular practice, all of the Archaean zircons were corrected by means of the :°4Pb isotope. The uncertainties given in the above table represent ltr. However, ages in diagrams and in the text are given at the 95% confidence level.

tistically tight grouping of 2°6pb/23sU analyses) supports the former option. Thus, the 2°6pb/23sU age of 1182 + 12 Ma is favoured for the age of this rock over the 2°7pb/2°6pb age ( 1194 + 16 Ma), but both are within error of each other. Sample AFP 9

Emplacement of the Porongurups Granite (cut by the AFP 8 dyke) post-dated oblique

thrusting of the Central Domain over the Northern Domain: granite generation may have therefore been generated from such crustal thickening. The granite locally displays a weak flattening (S) foliation striking 60-70 °, with local dextral ductile shear zones striking about E-W (i.e. consistent with E-W dextral displacements parallel to the belt). An additional set of conjugate ductile shear zones (ENE sinistral and ESE dextral ) is bisected by

PROTEROZOICEVENTSINTHEALBANYMOBILEBELT

107

0.22

/

AFP1 (young grains)

/

/ ~

Ma

oJ 13,..

g 04 0.20

/[

/[------~

1100Ma ~

//--//

[1196_+8M a j

1150Ma

•~ 1

207

0.18 1.8

I 2.0

~

I 2.2

Pb/

235

U

I 2.4

I

Fig. 3. 2°7pb/235U-2°6pb/238U concordia diagram for 16 comagmatic zircon grains from pegmatite AFP 1 ( 87010001 ) from Groper Bluff. The 2°6pb/238U age of 1196 + 8Ma, which is insensitive to uncertainties associated with c o m m o n Pb correction, is ascribed to magmatic crystallisation (see text). The analyses of the (Archaean) cores within these zircon grains, are depicted in Fig. 6.

MF2

i_:,2(3

C, ' ,..:

1 :!,

i

I

~

[ 0

i

I

J

~•

Fig. 4.2°Tpb/235U-2°6pb/238U concordia diagram for porphyritic granite M F 2 (87010010) from 200 km west of Albany. Only the shaded analysis ( # 8.1 ) is discarded from the pooled results, the pattern of which is suggestive of recent Pb loss (see text), The 2°Tpb/2°6pb data yield an age of 1189 4- 9 Ma for the crystallisation of the granite. The exclusion of grain 8. I from the pooled regression can be justified on the basis of its abnormally high U content.

pegmatites (equivalent in strike and structural style to sample AFP 5 ). At Serena Park, the main shear foliation (C) at the sample locality strikes 100 °, with a broad, several metres-long secondary shear

zone (C') striking 118 ° (Fig. 2c). C/C' relationships at the metric and centimetric scale, along with the asymmetry of foliation and pressure shadows around feldspars, indicate dextral displacements.

108

L.P. BLACK ET AL.

022

~D

02o,

o

÷

I

UI7 ] ,

Fig. 5.2°7pb/235U-2°6pb/23su concordia diagram for aplite AFP 8 (87010008 ) from the Porongnrup Range. The isotopically distinct analysis 6.1 (shaded) is omitted from grouped age calculations, but no independent criteria could be found to justify its exclusion. The spread in 2°Tpb/2°6pb suggests that the isotopic array derives from uncertainties associated with the correction for common Pb. The time that this rock crystaUised is thus given by the pooled 2°~pb/2asU age of 1182+ 12 Ma. 0.22

AFP 9

¢,j

0_

d

t'M 0.18

1000 Ma , , ~ /

4.2 900

4.1

M/

207

!

pb/235 U

0.14

0.4

1.2

2.0

2.8

Fig. 6.2°TPb/23sU-2°6pb/23su concordia diagram for augen gneiss AFP 9 (87010009) from the Porongurup Range. The four shaded analyses are not used to obtain the pooled age. Both analyses of grain 4 have much higher common Pb than the other grains. However, no independent criteria could be found for the exclusion of grains 2.1 and 6.1. The remaining 17 analyses are charactedsed by constant 2°~pb/2°~pb and a range of Pb/U, features consistent with recent Pb loss. the preferred (2°Tpb/2°~pb) age for the igneous crystallisaion of the gneiss precursor is 1184 + 1 ! Ma.

109

PROTEROZOIC EVENTS IN THE ALBANY MOBILE BELT

Zircons from AFP 9 are about three times as long (500/~m) as they are broad. These doubly terminated grains are of igneous, albeit slightly rounded, appearance. Quite commonly the zircons are deeply embayed, presumably as a result of partial solution during the post-crystallisation high grade tectonothermal event that is recorded in this rock. Some silicate inclusions are present. Perhaps 30% of the grains contain elongated, rounded cores of older zircon. Most of the syn-magmatic zircon is faintly to moderately zoned. Cores are generally more intensely zoned and darker, implying that they contain higher levels of U (and probably also Th). In contrast to the other Late Proterozoic zircons of this study, radiogenic Pb has been derived using the 2°4pb method, because the U Th-Pb systematics of AFP 9 have been modified since crystallisation. On the basis of its high common Pb content and low radiogenic P b / U ratio (Table 3, Fig. 6), grain 4 (analyses 4.1 and 4.2 ) apparently exchanged radiogenic Pb for common Pb in relatively recent times. The location of analysis 4.2 to the left of concordia presumably results from the associated difficulty of determining its radiogenic Pb content. These factors make grain 4 unsuitable for age calculation. Two other analyses (2.1 and 6.1) also lie outside the main analytical grouping, though no independent justification for their exclusion could be found. The remaining 17 analyses have a uniform 2°Tpb/ 2°6pb ratio, but a range of 2°6pb/238U and 2°Tpb/235U ratios, probably resulting from a small proportion of recent Pb loss. The 2°Tpb/ 2°6pb age of 1 184+__ 1 1 Ma (indistinguishable from the 2°Spb/232U age of 1194+_22 Ma), which is derived from cores, rims and discrete syn-magmatic grains, is therefore ascribed to the igneous crystallisation of the granitoid precursor. Although,derivation of the preferred age has involved treatment of the data in a way that cannot be independently justified, the various manipulations have no significant effect on mean z°Tpb/2°6pb age.

Sample AFP 2 AFP 2 samples are considered last because of their complexity. They come from felsic gneisses (in which quartz, plagioclase, biotite and garnet dominate) of the Central Domain, adjacent to its boundary with the Northern Domain at the Pallinup Estuary, about 120 km northeast of Albany. Early-formed granulitefacies assemblages show retrogression at amphibolite facies within an oblique, thrust zone emplacing the Central Domain over the Northern Domain at a stucturally later stage in the deformation history. The intensity of the second event in this area is exemplified by the progressive rotation of the axes of minor folds within the gneissic layering towards parallelism with the movement direction in shear zones (D2 structures ofBeeson et al., 1988). Two different lithologies were sampled from a 1 m 3 rock volume: a micaceous gneiss of probable igneous origin and which is composed of felsic and mafic layers, and a crosscutting pegmatitic body of similar mineralogical composition to the felsic layers within the gneiss. Both rocks contain a penetrative amphibolite facies foliation (which overprints granulite facies foliations) and strikes 070 °. This feature and their partly conformable, partly discordant nature indicate that the pegmatitic phases probably formed during the latter stages of the oblique thrusting stage. Similarly shaped zircons occur in both the gneiss and pegmatite. They have a range of forms, but two main types appear to be present. One of these consists of elongated grains (with length:breadth ratios up to 6: 1, and generally 200 to 500 #m long) of obvious igneous origin. Marked zonation is present in many of these grains, particularly towards their outer margins (some grains have a thin, optically distinct zircon rim ). There are two main subgroups within this zircon type. One consists of grains that are quite heavily cracked and which preserve pointed terminations. The other grains are essentially free of cracks, and

1 10

L.P. BLACK ET AL,

AFP1and2

30~.~

(old grains)

0.55

28ooM~>,....~-,,-~>-"

/

0.35

•

207 a

0.15

.

t

[

j

7

235 Pb/ t

I

11

U I

~5

Fig. 7.2°7pb/235U-2°6pb/238Uconcordia diagram for cores within AFP 1 (87010001, from Groper Bluff) and old grains from AFP 2 (87010002, from the Pallinup Estuary). The analyses from these two samples have been combined on this diagram because of their isotopic similarity, the relative proximity of the two sites, and because the pegmatite occurs in the same sequence as the rocks from the AFP 2 site. On regression, the analyses of the five cores from AFP 1 (indicated by arrows) yield concordia intercept ages of 2950_~2o+ ,6o Ma and 1400_+40o 700 Ma (not shown ). The younger value is similar to the crystallisation age of the pegmatite itself, and probably signifies that This AFP 1 zircon population as a whole lost Pb during the processes associated with the production and emplacement of the pegmatitic magma. The older age is taken to be the time of original zircon crystallisation within the source rocks from which the pegmatite was derived. The remaining anlyses on this diagram relate to AFP 2. Eight of these analyses (indicated by the letter a) were derived from morphologically distinct zircon grains, which have well formed igneous euhedral faces and are highly cracked. Their position below the other analyses is presumably a consequence of an increased vulnerability to recent Pb loss. Eighteen of the remaining 20 analyses (from both the gneiss and pegmatite) perfectly conform to an alignment with concordia intercepts at 31 .~,-45~ n +*~ Ma and 2000~'9 a2_ Ma. The older value defines the crystallisation age of the igneous precursor to the gneiss (and pegmatite). The younger age is probably a meaningless, composite value (see text). their original terminations are well rounded. The remaining zircon type comprises roughly equant, relatively homogeneous, multifaceted grains, o f possible m e t a m o r p h i c origin. A F P 2 zircons define two isotopic groupings. One o f these has a limited range o f compositions close to the 1200 M a point o f c o n c o r dia (Fig. 8). The other grouping is not as coherent, and encompasses analyses with 2°7pb/2°rpb ages ranging from about 2600 to 3000 M a (Fig. 7). In c o m m o n with the general practice used above, the ~ 1200 M a zircon analyses are corrected for c o m m o n P b by the z°Spb m e t h o d (as U - T h - P b systematics have not been significantly disturbed), whereas the Archaean analyses have been corrected by means o f 2°4pb.

The older grouping is c o m p o s e d o f analyses o f both elongated and equant grains from the gneiss and o f elongated grains from the pegmatite. Most o f the analyses closely approximate a straight line (Fig. 7 ). The eight exceptions are distinguishable by their morphology, being the grains (from the gneiss) with wellformed igneous crystal faces. Although three of those grains have the highest U contents o f the older grouping, this is not a feature o f the grain type as a whole and is not the primary explanation for their isotopic distinctiveness. This relates to the m a n y cracks within those grains. Whereas none of the analyses were sited on obvious cracks, the floors of the pits excavated by the ion b e a m in these particular zircon grains are now transected by cracks. Whether these

P R O T E R O Z O I C EVENTS IN T H E ALBANY MOBILE BELT

111

0.22

AFP 2 (young grains)

1250 Ma

cl 1180-!--6 Ma

I

0.20

11 O0 Ma 207 Pb/235 0.18 18

I 20

I

I 2.2

I

U I 2.4

Fig. 8.2°7pb/235U-2°6pb/238U concordia diagram for the equant grains within the pegmatitic phase ofAFP 2 (87010002, Pallinup Estuary). All nine analyses have a constant 2°7pb/2°6pb age of 1180+ 6 Ma, which is ascribed not to the segregation of this phase from the surrounding gneiss (based on the Sm-Nd data this occurred considerably earlier, probably during the Late Archaean ), but to the pervasive Late Proterozoic granulite-faciesmetamorphism and deformation. cracks were originally present as hair-line fractures which enlarged during excavation, or occurred below the original polished surface, is not known. However, because of their location below the other analyses on Fig. 7, it is highly likely that they acted as channel-ways for recent migration of radiogenic Pb from these grains. Except for 15.1, the most highly cracked and most discordant grain, c o m m o n Pb contents (Table 1) of the euhedral, cracked, grains are no higher than those of the general population, signifying that loss of radiogenic Pb was not accompanied by gain of external c o m m o n Pb. Deletion of these eight aberrant zircons and a further two analyses (32.1, 33.1 ) leaves 18 well aligned analyses with concordia intercept ages o f 311 °v _+445 7 and 20a0 u +s2 - - 9 6 Ma. The former value is taken as the original igneous crystallisation age of the precursor of this orthogneiss. The equant grains (from the gneiss) o f presumed m e t a m o r p h i c origin, which also fall on this alignment, probably signify that the igneous precursors of this gneiss were the ultimate product of a m e t a m o r p h i c event. It

should be noted that the Archaean cores from the AFP 1 pegmatite also fall on this trend, signifying a possible link between the source of that pegmatite and the AFP 2 gneiss. The even isotopic spread of these Archaean analyses does not indicate the mixing of unrelated age components, but reflects the presence of one or more episodes of post-crystallisation partial Pb loss from the zircon grains. The lower concordia intercept age of 2000_+ 82 Ma therefore designates either the age of a single Pb loss episode, or an "average" age of two or more postcrystallisation events. It is unlikely that the data define a single Pb loss event, because this rock underwent granulite-facies metamorphism and deformation at about 1200 Ma, when significant Pb loss must almost certainly have occurred. If such 1200 Ma Pb loss did occur, then in order to obtain a composite lower intercept at 2000 Ma, there must also have been an episode of Pb loss between 2000 and 3100 Ma. This inferred event might correspond with the formation of new Proterozoic crust in the Albany Belt, speculated to have formed at about 2100 Ma (Fletcher et al., 1983; this study, see

1 12

below). Alternatively, it might relate to the youngest granite-forming event in the southern part of the Yilgarn Block, which occurred at about 2600 Ma (de Laeter et al., 1981 ), or perhaps to both of those two events. The imprecise lower intercept age of 1400_+400 700 Ma derived from the five AFP 1 Archaean zircon analyses falls outside the precision limits of the AFP 2 lower concordia intercept age. This possibly reflects a greater response of the AFP 1 zircons to the 1200 Ma event (due to their presence at that time in a substantially greater body of melt). It is often misleading, however, to attach geological significance to lower intercept ages on concordia, particularly when they are derived from a substantial extrapolation. The analyses forming the compact grouping near the 1200 Ma concordia locus (Fig. 8 ) were entirely derived from, and comprise all of the equant grains within the pegmatitic phase. These nine analyses have a constant 2°TPb/ 2°6pb, but a range of 2°6pb/238Uand 2°TPb/ 235U, a trend that is indicative of recent Pb loss. The 2°7pb/2°6pb age of 1180_+6 Ma, rather than the 2°6pb/23sU and 2°7pb/235Uages of 1157_+22 and 1167_+15 Ma, respectively, should therefore temporally define zircon growth and, on the basis of the field relations given above, the timing of the second foliation and pegmatite formation.

Summary of the U-Pb data The U - P b zircon data show that all of the intrusives are approximately 1190 Ma old, even though they have a range of temporal relationships with respect to structural overprinting relationships, thus indicating a series of stages in a single, progressive deformation event over a short time interval at ~ I 190 Ma. Archaean zircon was identified in gneiss and pegmatite, at two neighbouring locations (AFP 1 and AFP 2) in the same sequence of the Central Domain. These 3000-3100 Ma zircon grains lost a substantial proportion of their radiogenic Pb during the 1190 Ma magmatic/

L.P. BLa.CK ET AL.

metamorphic event, and apparently also in late Archaean and/or Early Proterozoic times. These conclusions are amplified below.

Sm-Nd systematics and the origin of the intrusive rocks In order to understand better their origin and inter-relationships, the six rocks dated by U Pb zircon were also analysed for S m - N d (Table 4). The evolution of S m - N d within the analysed samples is presented in Fig. 9, where they are projected backwards in time from their present compositions (on the left hand margin) to three commonly used theoretical trajectories that are related in different scenarios to the changing composition of the Mantle. These mantle trajectories include the "chonNd dritic uniform reservoir" or TCHUR model, in which chondritic S m / N d ratios are assumed, and two "depleted mantle" (TaoM) r~d models. One of the latter is based on relative light rare earth depletion having occurred in the relevant part of the mantle at 4.5 Ga (an assumption commonly used in the calculation of model ages for Archaean rocks), and the other assumes that such depletion occurred at 2.7 Ga (a model widely used in the derivation of model ages for Proterozoic rocks). Extrapolation of composition back to the most recent melting event can be made with some certainty, but there is no guarantee that S m / N d was unchanged during that event, though it is generally considered to have remained essentially unaffected by infra-crustal melting. Perhaps the most significant feature of Fig. 9 is the marked dichotomy of extrapolated isotopic compositions at I 190 Ma, with the three rocks containing Archaean zircon (AFP 2 gneiss and pegmatite and AFP l) having distinctly lower Nd than those that do not (AFP 8, 9 and MF 2 ). There is a general consistency of model ages both between and within the gneiss and pcgmatite phases ofAFP 2 (TSARare 2.8 and 3.0 Ga; TDM r~d are 3.15 and 3.05 Ga, respectively), despite the inherent uncertainties

113

PROTEROZOIC EVENTS IN THE ALBANY MOBILEBELT TABLE 4 S m - N d composition of rocks from the Albany Mobile Belt Sample No.

Rock type

Sm (#g/g)

Nd (#g/g)

147Sm/144Nd

143Nd/144Nd

Nd TCHUR (Ga)

Nd TOM (Ga)

t tN~9o

AFP IA AFP2B AFP 2G AFP 8F AFP9E MF 2

pegmatite gneiss pegmatite aplite gneiss granite

0.301 1.996 1.025 0.470 14.11 11.43

2.148 9.749 5.671 5.108 90.78 76.97

0.08459 0.1238 0.1093 0.05559 0.09397 0.08977

0.510989+_39 0.511274+_21 0.511081 +_ 17 0.511105 ±21 0.511398+_22 0.511355 +_22

2.25 2.86 2.81 1.67 2.00 1,84

2.48 3.05 2.99 1.85 2.05 2,03

- 15.4 -15.8 -18.3 -8.7 -8.8 -9,0

S m - N d analysis was based on the method of Richard et al, (1976). Mass-spectrometric analysis was by means of a system of calibrated Faraday cup collectors. 146Nd/t44Nd values were normalised to 0.7219, The present day ~43Nd/~44Nd used to calculate all of the depleted mantle ( D M ) model ages is 0.513163. A ~47Sm/144Nd of 0.225 was used to calculate DM ages for the rocks containing no Archaean zircon (AFP 8, AFP 9 and MF 2); a 1475m/144Nd ratio of 0.2176 was used to calculate DM ages for the rocks (AFP 1 and the two AFP 2 samples) containing Archaean zircons.

associated with the calculation of such ages (i.e., those points where the evolutionary paths of the sample intersect the trajectories presumed for the Mantle). This implies that the felsic crustal precursors of these rocks formed at about 3100 Ma, as the majority of zircons in these rocks also indicates. On the basis of its TDM Nd model age of 2.55 Ga, pegmatite AFP 1 from Groper Bluff, might be considered to have formed from a younger crustal source than that of the AFP 2 rocks. However, the presence of 3100 Ma zircon cores in AFP 1 indicates that a component of similar age was at least partly involved in its origin. A strong argument for the singular participation of crust of that age can be found in the similarity of Nd isotopic compositions of the AFP 2 gneiss and the AFP 1 pegmatite at 1190 Ma, the time that the pegmatite was formed. It is unlikely that these two rocks, with markedly different Sm/Nd, would have had the same Nd isotopic composition at 1190 Ma unless the pegmatite was derived by melting of a rock similar to the gneiss (gneiss at the AFP 2 site is equivalent to that at Groper Bluff). This explanation requires the S m / N d ratio of the AFP 1 pegmatite to have been significantly altered during its generation at 1190 Ma, presumably by preferential incorporation into that magma of light rare earth-enriched minerals.

The abnormally low 1 4 7 S m / 1 4 4 N d of 0.084, which might be expected to have arisen by two stages of crustal enrichment, is consistent with this explanation. Normally a roughly 40% reduction of S m / N d occurs during felsic crust formation. A further 30% reduction appears to have accompanied the production of the AFP 1 pegmatite from its parent felsic gneiss. The low initial 87Sr/86Sr (0.70356) o f A F P 1 pegmatite compared with the contemporaneous composition o f A F P 2 gneiss (0.7079 at 1190 Ma) is consistent with the precise source of the pegmatite being more depleted in Rb due to stronger metamorphism in its early history. The remaining three samples have Sm-Nd characteristics more in keeping With Proterozoic than Archaean prehistories. The most notable feature of those samples (AFP 8 and AFP 9 and MF2 ) is a similar Nd isotopic composition at their time of emplacement ( 1190 Ma), which is once again strongly indicative of a common source. The extrapolated compositions are different prior to I 190 Ma, but this is probably once again more real than apparent, reflecting some infra-crustal fractionation of Sm/Nd. All measured ~ 4 7 5 m / ~ 4 4 N d values are lower than the average crustal value of about 0.12, and there is a also the requisite correlation (once the samples had a common 1 4 3 N d / 1 4 4 N d a t 1190 Ma) between model age

114

L.P. BLACK ET A L

20 IVlF2

0

...2...... ~ t -

° o.."

ENd

,,4,

2pg

o~

I-

b.-">"

-20 ~.jz'

-40

' ~ 1190 Ma

,

0

I

1

,

,

,

2

AGE (Ga)

i

, 4

Fig. 9. Evolution diagram showing the changing Nd isotopic composition of the Albany samples with time (present time is on the left of the diagram). The solid horizontal line represents chondritic evolution. The solid lines above it, with present day end values of + 10, represent separate scenarios for a mantle that has been depleted in the rare-earth elements. The steeper of the two trajectories depicts depletion at 2700 Ma (applicable for Proterozoic rocks); the flatter trajectory depicts depletion at 3700 Ma, a model commonly applied to Archaean rocks. The solid vertical line defines the time ( 1190 Ma) when the analysed rocks were either molten or undergoing deformation and high-gade metamorphism. Labels at the old end oftbe six sample trajectories are slightly abbreviated versions of the sample numbers. In order to discriminate between the two phases o f AFP 2, the pegmatite is labelled pg and the gneiss is labelled gn. A full description of the significance of this diagram is given in the text. In essence it shows that the three 1190 Ma granitoids which contain no Archaean zircon (denoted by the finest of the dashed lines--AFP 8, 9 and M F 2) were derived from the melting of a common source. Pegmatite AFP 1, which contains Arehaean zircon cores, had an identical composition at 1190 Ma to the nearby gneiss AFP 2. This is consistent with this pegmatite being of fairly local derivation. However, the implication of these data is that there was considerable fractionation (about 40%) of the rare earths during crustal melting at 1190 Ma. This would ensure that model ages calculated from present day Sm/Nd ratios would provide only younger limits for the mantle fractionation which gave rise to the precursors of the analysed samples. A more appropriate estimate should arise from extrapolation backwards in time from 1190 Ma along trajectories more representative of typical continental crust. The solid line of equivalent slope which emanates from the M F 2-AFP 8-AFP 9 common point at 1190 Ma suggests that the postulated common source of those rocks is about 2100-2200 Ma old. The indicated source age of the two AFP 2 phases from this diagram is the same as the age of the oldest zircons in those rocks, that is about 3100 Ma.

and 1475m/144Nd ( 1.85 and 0.056 for AFP 8; 2.03 Ga and 0.090 for MF 2; 2.05 Ga and 0.0.094 for AFP 9). The exceptionally low value for AFP 8 serves as a warning of unusual fractionation in at least this aplite. Probably the best way to derive the age of the precursors of this group of rocks is to assume varied degrees of fractionation during the generation of the 1190 Ma melts, and to apply a general crustal value of about 0.12 t o 147Sm/144Nd (typical of Australian Proterozoic rocks, e.g., Black and McCulloch, 1984, 1990) for crustal evolution prior to that time. This procedure yields preferred T~uR and TOM Nd ages of 2.1 and 2.25 Ga, respectively, for the formation of the felsic crustal precursors of AFP 8. Similar 143Nd/144Nd compositions noted above for AFP 8, AFP 9 and MF 2 at 1190 Ma are not matched by similarity of 87Sr/S6Sr at that time (Fig. 10). The values for the spatially associated AFP 8 and AFP 9 intrusives of 0.7088 and 0.7087 were mutually indistinguishable, but the geographically distant MF 2 -8 MF 2 AFP 8,9 j -12 Nd E 1190 -16

AFP 2 pg

AFP 1 AFp 2 gn

-20

t

0.702

0.706

• =

87 86 Sr / Sr

;

0.714

Fig. 10. Sr and Nd isotopic composition of selected Albany samples during the pronounced l 190 Ma tectonothermal event. The data show that although the AFP 8 and AFP 9 granitoids had similar Nd isotopic compositions to M F 2, they had significantly less radiogenic Sr. This is consistent with all three rocks being derived from a similar source, but one which had undergone a period of higher grade metamorphism (leading to Rb depletion and a consequent retardation in the development of radiogenic Sr) in the east, below the Porongurup Range. A similar explanation could apply for the isotopic differences between pegmatite AFP 1 and the AFP 2 gneiss (see text). This diagram clearly shows that the AFP 8, AFP 9 and M F 2 felsic intrusives were derived from a different source than the AFP 1 and AFP 2 felsic intrusives.

PROTEROZOIC EVENTS IN THE ALBANY MOBILE BELT

granite was more radiogenic (0.7096) in terms of Sr. This overall combination of isotopic characteristics could arise from a common deep source which experienced stronger metamorphism (well before the time of granitoid generation), and hence more Rb depletion, in the Porongurups (part of the highly magnetic suite) than in the Mount Franklin region (granites with little magnetic response ). Being more resistant to elemental fractionation, the S m - N d system would have been essentially unaffected by such an event. Model ages of approximately 2000 Ma were obtained for the AMB from the S m - N d system by Fletcher at al. (1983) and McCulloch (1987). Fletcher et al. (1983) interpret their data in terms of a major crust-forming event in the Albany Mobile Belt about 2000 Ma ago. McCulloch (1987) is more conservative, presenting two alternative hypotheses, based on an unusually large spread of S m - N d model ages for this crustal block. He proposes that either there were either two Proterozoic crustal formation events or that mixing occurred between Mid Proterozoic and Late Archaean crust. Our data support a combination of both options. Both Archaean ( ~ 3100 Ma) and Early Proterozoic ( ~ 2100 Ma) crust contributed to the formation of the Albany Belt. The spread in model ages partly reflects this complexity, but it also results from an unusually high and varied fractionation of S m / N d in the rocks we studied, presumably because of significant infra-crustal fractionation of those elements, a potential factor that is mostly ignored in the interpretation of S m - N d systematics. In terrains where such infra-crustal fractionation occurred relatively soon (e.g., within several hundred million years) after primary crustal formation, it has only a minor effect on the calculation of model ages. However, in the case of the Albany Belt granitoids, a second stage of fractionation was apparently delayed until the generation of those granitoids, approximately 900 Ma after the original crustal

| 15

emplacement of their source rocks, leading to an enhanced spread of apparent model ages.

Geological conclusions

Timing of deformation events A synthesis of the ages determined in this study is shown in Table 5 and portrayed graphically, along with other ages for the Albany Mobile Belt, in Fig. 11. The major conclusion is that shear zone development, folding and refolding, granitoid intrusion and late cross-cutting aplite and pegmatite emplacement took place within a single, brief tectonothermal event. Rb Sr data show that this event is at least 1164 + 27 Ma, and possibly up to 1230 +_50 Ma old. U - P b zircon data confirm that the similar Rb-Sr ages derived for these events are not due to isotopic resetting. Widespread felsic igneous activity with a range of temporal relationships to associated metamorphism and deformation occurred within a few million years of 1190 Ma. The structurally early pegmatite (AFP 1) crystallised during granulite facies metamorphism at 1196+8 Ma. The AFP 2 pegmatite crystallised during the emplacement ofgranulites over gneisses of the Northern Domain and was associated with retrograde, amphibolite facies metamorphism at 1180 + 6 Ma. Granites (MF 2 and AFP 9) with indistinguishable ages of 1188 _+9 and 1184 + 12 Ma, respectively, were emplaced during the late stages of this event as a result of crustal thickening. Post-tectonic aplite (AFP 8) crystallised at 1184 + 12 Ma. A summary of Proterozoic deformation events and the relative timing of granitoid intrusion can now be made. Intrusion of the Albany enderbite at 1289+_10 Ma (Pidgeon, 1990) possibly marks the onset of a new cycle of tectonism along the southern margin of the Yilgarn Block. Alternatively, it might have been emplaced during an earlier event (which pre-dates the ~ 1190-1150 Ma shear zones and post-tectonic granitoids, and for which no evi-

AFT 4: Qz-plag--opx +_bi pegmatite, Fishery Beach Bremer Bay granulites AFP 5: Qz-K-feld-plag-bi pegmatite, Fishery Beach Bremer Bay AFP 6: bi-opx pegmatit¢, Short Beach, Bremer Bay AFP 8: Aplite dyke, Serena Park, Porongnrups AFP 9: Augcn orthogneiss Screna Park, Porongnrups MF 2: Porphyritic granite

Early $ranulite-facies foliation Late penetrative foliation in retrograde shear zone

AFP 1: Qz-plag-opx-hb pegnaatite, Groper Bluff AFP 2: Layered felsic/ marie gneiss and pegmatite. Northern boundary of Central Domain APF 3: Biotite gneiss ( amphibolite facies ) in Northern Domain

Post-kinematic granitoid

13 5 °-striking tension fracture, cuts across late folds and AFP 4 Along early secondary shear band in granulites Cuts foliated granites ( including AFP 9 ) Cut by late dextral shear zone

Late folding (along axial plane) in granulites

Early penetrative foliation

Structural timing

Sample and locality

whole-rock isochron

whole-rock isochron Isochron age, all minerals Biotite age All minerals but biotite biotite age

1084+_ 30

1164+27 1121 + 57

1037 1100+-25 1134

1108+15 1109_+42 1230+50 1040 _+40

Felsic layer, all minerals Mafic layer, all minerals Mafic layer (less K-feldspar) Felsic layer (less biotite ) whole-rock isochron

Probably not valid, as is a 2 point line

Comments

1109_+ 15

( 1870+_ 150)

Age (Ma) Rb/Sr

Summary of the structural relationships and new ages for the Albany Mobile Belt

TABLE 5

1184+11 1184+11 1189+9

1182_+ 12

1196 + 8 2917-3110 3100+47,-45 2000 + 82, - 96 1180_+6

Age (Ma) U/Pb

Emplacement age Emplacement age Emplacement age

Emplacement age

Age of zircon rims Age of zircon cores Igneous crystaUisation Composite age for Pb loss Age ofequant grains in pegmatite

Comments

PROTEROZOICEVENTSIN THE ALBANYMOBILEBELT

1000 -

. . . . . . . . . . .

A G E S

117

ERROR

WITH

BARS

.....

SHRIMP ages

~11

Rb/Sr

=--

ages

U/Pb ages (Pldgeon,1990)

* Late stage pegmatite emplacement .and brittle, defor_matlon ~

*"fherma, re~tting. . . . .

~

.... !-"-?--P~-"-~::-~!!~t-~t!?-:--'z'---~-t-tt-"-!

Ud 0

1200

........................

i ......

...................................................................

* Main tectonethermalphase (D1 & D2 penetrative fabrics) ~

. . . . . . . . . . . . . . . . . . . . . .

* Early intrusive .a_ct!vit_y . . . . . . .

• !

I

i

. I

.

I

i

= ~"

•

:

....... I---,-'~

I

........

I

•

Inll

. . . . . . . . . . . . .

• i,

|-'-

I

.........

I

i I

i

== ! = I

-

......

...............................

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

_,,,,,,~._ . .~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

o~

-

1400 2o ~"

~

~

o.£

o-

o_

<80-

<

<

< .E

<

~_. <

a_

<

~o, ta.o < O.

#

LL~.

oo

(3-

t.,_ <

,,a_ <

~E

LL¢

<

Fig. 11. Summary of the Proterozoic isotopic ages obtained in this study (black bands represent U - P b ion-microprobe ages, grey bands represent relevant Rb-Sr ages) and the conventional U - P b zircon study (shown as striped bands) of Pidgeon (1990). In essence, the diagram shows that the Rb-Sr system mostly remained open over the 1 m 3 scale of sampling until about I 100 Ma, well after the time (about 1190 Ma) that the last penetrative deformation was developed in these rocks.

dence has been discovered in this study) that would equate with intense deformation, granulite facies metamorphism and charnockite intrusion in the Fraser Mobile Belt. Granulite facies metamorphism in the Central Domain is first recorded by zircon growth at 1196 + 8 Ma in pegmatite at Groper Bluff. On the basis of its structural relationships and granulite facies mineralogy, the quartz-plagioclase-orthopyroxene +_biotite pegmatite AFP4 was emplaced at an early stage in the deformation history. However, its Rb-Sr age of I 140 +_40 Ma must at least partly reflect a subsequent episode, for it is even younger than the crystallisation age of late-tectonic granitoids. The mineralogically comparable AFP 6 pegmatite yields a Rb-Sr whole-rock age ( 1164 + 27 Ma) that is within error of its early structural origin. Granitoids of the Burnside Batholith such as Mount Franklin, the Porongurups granite and the Albany Adamellite cross-cut regional up-

right, or more commonly north-west overturned folds and oblique thrusts. U - P b zircon ages show that they intruded shortly after the onset of orogenic activity in the AMB. Preliminary ion microprobe studies show that the slightly younger age of 1174 + 12 Ma derived by Pidgeon (1990) for the Albany Adamellite might merely reflect the different analytical technique; the ion-microprobe age of that rock appears to be essentially the same as those derived above for the other late- to post-tectonic granitoids. It is surprising, especially given the spread of younger Rb/Sr ages, that the undeformed aplite dyke cutting dextral shear zones in the Porongurups yields essentially the same U - P b zircon age as that derived for the igneous crystaUisation age of the granitoid itself. This indicates that granitoid intrusion, local development of foliation and discrete dextral shear zones, and subsequent post-kinematic aplite intrusion occurred within a maximum

1 18

time-span of 10 Ma (the approximate error range of zircon analyses). The younger Rb-Sr biotite ( 1140 Ma) and mineral ages (1100_+25 Ma if biotite is excluded from the regression) for the mylonitised Porongurups granitoid (AFP 9) clearly indicate that these systems record neither intrusion, deformation, nor aplite emplacement. Indeed, most Rb-Sr whole-rock and mineral isochrons (Table 2, Fig. 11 ) yield ages indistinguishable from about 1100 Ma. Examples include (i) Northern Domain gneisses (AFP 3 ); (ii) sheared Porongurups granite and crosscutting aplite dyke (AFP 9 and 8) and the late, cross-cutting pegmatite AFP 5 intruding Central Domain granulites; (iii) the Albany Adamellite (Turek and Stephenson, 1966) and (iv) the middle of the distribution of K/Ar ages for regional metamorphism also determined by Turek and Stephenson (1966). The youngest part of the possible range in ages of Central Domain pegmatite AFP 4 also falls within this field. Except for structures formed during the late pegmatite intrusion stage (intruded along extension fractures associated with conjugate shear zones), no regional penetrative deformation took place at that time. Thus, the region appears to have cooled through the average closing temperature of the Rb-Sr mineral system at about 1100 Ma, a comparatively long time after the cessation of the main stage of deformation. The young ages for AFP 5, and to some extent AFP 4, probably result from their relatively coarse grain size, and the associated difficulty of representatively sampling pegmatitic rocks. It is likely that the Rb-Sr ages of these samples do not faithfully record whole-rock isotopic systematics (for which huge specimens would need to have been collected and analysed), but partially reflect isotopic exchange between minerals, thereby recording meaningless ages between those of the two systems. To assess further the possibility for isotopic resetting, it is important to constrain temper-

L.P. BLACK ET AL.

ature estimates during deformation. Two-pyroxene thermometry (Lindsey, 1983 ) on pegmatites AFP 4 and AFP 6 give consistent temperatures of 900°C for mineral rims. The Al-enriched clinopyroxene cores in AFP 6, however, yield temperatures as high as 1000 ° C. These temperatures and the presence of pyroxene zoning indicate a re-equilibration of early assemblages at a later stage in the deformation event, but still at granulite facies conditions. The versatile calcic amphibole-plagioclase geothermometer of Blundy and Holland (1990) has been applied to AFP 4 and AFP 9 amphibole-plagioclase pairs, as the main constraints for its application are met (i.e. silica in amphibole structural formulae < 7.8, and X albite > 0.08 ). Temperatures of 800 °C + 75 ° C were obtained from both samples. These are consistent with the granulitic mineralogy of AFP 4 but are probably overestimatingthe subsolidus conditions for the syn-kinematic emplacement of AFP 9. Temperatures up to 700°C are locally obtained from garnet amphibotites along the contact between the Northern and the Central domains (Delor, 1987). Temperatures of about 650°C are deduced for the Northern Domain from garnetbiotite and garnet-amphibole pairs. This progressive drop in metamorphic temperature from south to north has also been recognised throughout the Northern Domain (Beeson et al., 1988). Late retrogression products throughout the AMB, such as clinozoisite and micas after plagioclase, are characteristic of subsequent low-T reaction. In conclusion, the major fall in temperature occurred at a late stage in the tectonothermal history; consequently, thermal perturbation during late folding and oblique thrusting were high enough to allow (at least partially) Rb-Sr isotopic exchange between minerals. The oldest biotite ages indicate that the region was uplifted to within about 10 km of the surface by 1100 Ma. The 1037 Ma biotite age (AFP 8 ) shows that there was no significant uplift ira-

PROTEROZOIC EVENTS IN THE ALBANY MOBILE BELT

mediately after that time, and that a stable Proterozoic geotherm had developed.

Evidence for Archaean and Early Proterozoic precursors Samples from two of the analysed sites reveal that the Central Domain is not entirely of Proterozoic age, parts of it having originally formed about 3100 Ma ago. Zircon cores from the pegmatite at AFP 1 signify that the precursors of the surrounding gneisses formed between 2917 and 3110 Ma because this, and other early-formed pegmatites, were generated in situ. The evidence from AFP 2 is even more direct. U - P b zircon data show that gneiss at that locality formed by the metamorphism and deformation of 311n+47,,-45Ma igneous precursors. These conclusions are supported by two Nd TDM model ages of about 3100 Ma. Windy Harbour granulite facies gneisses give Nd model age of about 3200 Ma (Fletcher a TOM et al., 1983 ). Whilst those authors consider that the Windy Harbour gneisses represent a slice of the Balingup Metamorphic Belt, putting the boundary of the AMB further to the east, their lateral continuity on digitally processed aeromagnetic data with granulites of the AMB clearly indicates that they are in fact part of the Central Domain of the AMB. Their northsouth foliation trends, which Fletcher et al. ( 1983 ) use as an argument for their inclusion in the Balingup Metamorphic Belt, result from regional folding due to sinistral transcurrent displacements along the western margin of the Yilgarn Block along the "proto-Darling Fault" (Harris, 1987). The mantle extraction model age of the Windy Harbour granulites is therefore in agreement with Archaean ages determined in this study further east in the same domain. The southern part of the Western Gneiss Terrain (WGT) is the immediate northerly neighbour of the AMB. Gneisses of ~ 3100 Ma or older occur over the entire WGT (Fletcher et al., 1983; McCulloch et al., 1983). TDMNO

119 model ages of about 3200 and 3300 Ma were obtained by Fletcher et a1.(1983) for paragneisses of the Balingup Metamorphic Belt in the southern WGT (there is also much earlier Rb-Sr work to support their data). Parts of the Central Domain of the AMB therefore represent reworked segments of the Yilgarn Block (as Beeson et al. (1988) had concluded from structural evidence from the Northern Domain, but was not recorded by previous geochronology ). As argued above, the lower concordia intercept of 2000_96+ 82 Ma for the old grains in AFP 2 is most likely to be an artifact only, with no geological significance. It was further argued, on the basis of the intensity of the 1190 Ma event, that these rocks must also have experienced a substantial tectonothermal event sometime between 3100 and 2000 Ma, but not close to either of those times. As parts of the Central Domain are now confirmed to be reworked equivalents of the Yilgarn Block, the domain was affected by suitably old events to have generated Late Archaean Pb loss. Thus, Nieuwland and Compston ( 1981 ) showed that the youngest phase of igneous activity in that region took place 2670 + 45 Ma ago. Based on model S m - N d ages, McCulloch et al. (1983) reported that this activity was also roughly synchronous with an episode of new crust formation. Early Proterozoic TOM Nd model ages were reported for the Southern Domain of the AMB by Fletcher et al. (1983): precursor rocks to sillimanite gneisses associated with a deformed quartzite west of Walpole formed at about 2300 Ma, and layered gneisses at Nornalup Inlet formed at about 2200 Ma. Granites emplaced late in the mid-Proterozoic event also give Early Proterozoic TD~ Nd model ages. For example, Fletcher et al. (1983) derived a model age of about 2200 Ma for 1177 +_4 Ma old (Pidgeon, 1990 ) Mount Chudalup granite. A porphyritic granite at Crystal Springs (part of the non-magnetic phase of the Burnside Batholith, equivalent to our Mount Franklin

120

sample which crystallised 1188 _+9 Ma ago) Nd age of 2100 Ma. The TDM yields a TDM s,t ages should approximate the age of precursor gneisses which were melted during crustal thickening to form the Burnside Batholith.

Tectonic implications and correlations with other segments of Gondwana Early geochronological investigations, mainly based on K/Ar and Rb/Sr methods, have shown the apparent protracted character of ancient orogens, with time spans of up to several hundred million years (e.g., Black and James, 1983). This has often been used as a critical argument for suggesting that fundamental differences exist concerning the duration of events in ancient orogens in comparison to their modern counterparts. The present study shows that a large number of distinct, and regionally correlatable structures and intrusive phases represent verious stages in a single, progressive deformation event without significant changes in the orientation of implied maximum compressive stress. This was accompanied by a constant granulite facies metamorphic grade during three phases of folding in the eastern sector of the Central Domain, with a change from granulite to amphibolite facies occuring in basal shear zones. Retrogresion to lower grade assemblages has occured along cross-cutting conjugate transcurrent shear zones throughout the Central Domain. The evidence that the Central Domain also represents (at least in part) reworked Archaean rocks has important repercussions for the tectonic evolution of the Albany Fraser Province and for progressive crustal accretion models as proposed by Fletcher et al. (1983 ). The Manjimup Lineament is described by Fletcher et al. (1983) as a major boundary within the Yilgarn Block a~ainst which ~ 2700 Ma material has been accreted. This study shows, however, that this lineament corresponds only to the northern margin of the Central Domain. The Pemberton Lineament lies

L.P. BLACK ET AL.

within the Central Domain. As older Archaean precursor rocks ( ~ 3100 Ma zircon cores from sample AFP 1) occur south of the eastwards continuation of the Pemberton Lineament, this structure cannot represent a second accretionary margin between a younger Archaean terrain and the Proterozoic mobile belt, as proposed by Fletcher et al. ( 1983 ). It is now possible to compare directly the geology of the AMB with once continuous Gondwana fragments by means of modern geochronological data. There is no doubt that the AMB once adjoined part of what is now Antarctica, the most popular reconstructions (e.g. Oliver et al., 1983) placing the AMB near or adjacent to the Bunger Hills of Antarctica. Currently unpublished U-Pb zircon work by the senior author reveals that the Bunger Hills region was subjected to deformation, metamorphism and granitoid emplacement over a relatively restricted range of ages slightly after 1190 Ma. Archaean components have also been recognised immediatelyto the south and to the west of the Bunger Hills, increasing the geochronological similarities of the two terrains, and thereby supporting the Oliver et al. ( 1983 ) Gondwana reconstruction.

Summary of geological history This study establishes the following chronology for the principal geological history of the AMB. (1) The southern part of the Yilgarn Block formed at about 3100 Ma. (2) Granite emplacement and metamorphism of this terrane occurred at about 2700 Ma. (3) At 2100-2200 Ma there was another primary crustal formation event. (4) Enderbite intrusion occurred at 1290 Ma (5) Regional progressive deformation reworked both of the pre-existing crustal components at 1190 Ma. New foliation, up to four superposed fold generations, late-tectonic granite and pegmatite intrusions, and conjugate shear zones were developed.

PROTEROZOIC EVENTS 1N THE ALBANY MOBILE BELT

(6) Uplift and cooling over about the next 100 Ma was followed by the development of a stable Proterozoic geotherm. Acknowledgements The structural and metamorphic aspects of this study were supported by an Australian Research Council grant and University of Western Australia Special Research grant. John Beeson is thanked for his helpful comments and assistance in sampling at Mt Franklin. Technical assistance in sample preparation was provided by L.A. Keast, B.J. McDonald, M.J. Bower and L.P.J. Kinsley. Geoff Aldridge is thanked for his help in the field. L.P. Black publishes with permission of the Executive Director, Bureau of Mineral Resources, Geology and Geophysics, Australia. Appendix A

Sampling procedures Either gelignite or feathers and plugs were used in the collection of all samples to ensure that they were as fresh as possible. Ten rock types, separated by up to 300 km, were isotopically analysed for Rb-Sr (Tables 1 and 2). Sm-Nd (Table 4) and U, T h - P b zircon (Table 3) analyses were made on six of those rocks. All samples were collected from a restricted rock volume of about 1 m 3 in order to increase the chances of obtaining tectonothermally reset Rb-Sr isochrons (Black et al., 1979 ). In order to minimise any possibility of isotopic resetting, sampling was confined to rocks with clearly established relationships, away from younger, cross-cutting, discrete ductile to brittle shear zones (Harris et al., 1989).

Appendix B

Rb-Sr analyses Sample AFP 2, Pallinup Estuary (118 ° 50' E, 34 ° 28'S) A limited range of STRb/s6Sr (0.4-0.7) and isotopic scatter (MSWD= 170, Table 2) did not allow the pegmatite to be dated by Rb-Sr. The "age" produced from the gneiss samples is also unreliable, for it is essentially derived from a two-point line which might not be an isochron at all. However, the data do show that neither the development of a late penetrative fabric at amphibolite

121 facies, nor the more intense earlier granulite-facies metamorphism produced wholescale isotopic resetting over distances of about 1 m at this site, even though the study of Black et al. (1979) would have predicted its occurrence.

Sample AFP 3, Northern Domain gneisses (118°37'E. 34°26'S) AFP 3 was collected from the Northern Domain near the Hassel Highway crossing of the Pallinup River, about 12 km west-northwest of the AFP 2 locality. This rock (Fig. 2a ) is a very strongly banded biotite-gneiss, the mafic bands of which probably represent mafic dykes tectonically interleaved with the felsic gneiss. Six samples (Table 1) failed to produce a definitive isotopic array. Major mineral phases from a dominantly felsic layer (3C) and a dominantly mafic layer (3N) were also analysed. Each specimen yields a relatively precisely-defined mineral age ( 1109 _+15 and 1108 +_15 Ma, Table 2), but only because they each include a biotite analysis. The isochrons themselves are poorly defined (MSWD = 287 and 95, respectively). However, the biotite, plagioclase and whole-rock analyses for AFP 3N yield a considerably improved analytical alignment ( M S W D = 8 ) with age of 1109_+42 Ma. K-feldspar, plagioclase and whole-rock fractions from AFP 3C conform to a perfectly-titled isochron (MSWD = 0.8) with an age of 1230 + 50 Ma, which should represent, within its error limits, a minimum estimate for the early stages of the deformation history. As biotite ages are totally reset by temperatures as low as 300°C, subsequent amphibolite-facies metamorphism is certainly no younger than 1100 Ma. Sample AFP 4, late (third stage).folding, Fishery Beach, Bremer Bay (119 ° 24' E, 34 ~26' S) AFP 4 is from Fishery Beach, Bremer Bay ( 180 km to the east of Albany) within the Central Domain. This greycoloured pegmatite contains quartz, plagioclase, K-feldspar, ortho- and clinopyroxene, magnetite and minor biotite. The pegmatite strikes 035 ° and was intruded subparallel to the axial plane of a slightly north-westerly overturned fold, representative of the third generation of folds in this area: these structures refold previously isoclinally folded compositional layering (as sampled at Groper Bluff), and a second generation of light to isoclinal folds (folding both previous structures). The mineralogy of the pegmatile shows that granulite-facies conditions continued throughout all stages of folding in the Central Domain away from oblique thrust zones. The strike of fold axial planes, along with the formation of conjugate ductile shear zones nearby, indicates a northwest-directed maximum compression direction at the time of AFP 4 pegmatite emplacement. All eight whole-rock analyses define within the limits of analytical error an age of 1140 + 40 Ma (Table 2, initial STSr/SrSr is 0.7085 + .0002 ), which is a younger limit for this generation of folds.

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Sample AFP 5, late pegmatite emplacement (119 ° 24'E, 34°26'S) The 135 °-striking AFP 5 pegrnatite has lower temperature mineralogy (quartz, K-feldspar, plagioclase, biotite) and cuts the previously discussed sample (Fig. 2b ). Pegrnatites of this generation are commonly zoned, with an inner core of clear quartz surrounded by a characteristically coarse grained, pink K-feldspar+biotite-bearing assemblage. Related pegrnatites have consistent strike elsewhere in the AMB, cross-cutting all folds and the latetectonic Porongurups granite (AFP 9). Intruded along tension fractures, their strike bisects conjugate brittleductile shear zones (which can also contain minor pegmatite). The maximum horizontal compressive stress at the time of pegmatite intrusion is deduced to have had the same northwest-southeast orientation as interpreted for previous structures, suggesting this pegrnatite represents a deformation response to continued shortening across the orogen after, or towards the cessation of thrusting. AFP 5 does not produce a comparably well defined isochron ( M S W D = 6 ) as AFP 4. The resultant age of 1084_+30 Ma (Table 2) is a minimum estimate for the time of pegmatite emplacement during the waning stages of tectonothermal activity. Sample AFP 6, early granulite facies pegmatite. Short Beach, Bremer Bay (119 ° 24'E, 34 ° 27' S) AFP 6 is a biotite, ortho- plus clinopyroxene-bearing pegmatite from Short Beach, Bremer Bay which was emplaced along a metric-scale shear band. Refolding of such structures by two subsequent fold generations indicates its early timing. Regional studies show that this sample site is on the overturned limb of a major, second phase, isoclinal fold which was subsequently refolded by third phase folds (as dated at AFP 4). Despite the clearly earlier structural timing, the age of 1164_+ 27 Ma is not significantly older than that of the other orthopyroxenebearing pegmatite (AFP 4). Sample AFP 8, late aplite dyke, Porongurups (117 ° 58'E. 34°41'S) This aplite dyke cross-cuts shear zones within the Porongnrups granite. S7Rb/S6Sr is too uniform to allow whole-rock R b - S r dating, but mineral dating was done. All phases produced a poorly-defined ischron with MSWD of 120. Deletion of the aberrant ( 1037 Ma) biotite analysis yields an analytically perfect alignment with an age of 1121 + 57 Ma, a minimum estimate for the final stage of shear zone development, which this dyke postdates. Sample AFP 9, deformed Porongurups granite (I 17 ° 58'E, 34°41'S) AFP 9 has too restricted a range in 87Rb/aTSr to permit the derivation of a whole-rock isochron. Similarly, the biotite analysis falls off the mineral isochron, which has an MSWD of 10. Regression through all mineral phases

LP. BLACK ET AL

but biotite yields an analytically perfect isochron (Table 2) with an age of 1100+25 Ma, which is a minimum estimate for structurally late transcurrent shear zones. The biotite age of 1140 Ma provides an even older temporal estimate for that stage of the deformation history., and for the intrusive age of the Porongurups granite.

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bany Fraser Province of Western Australia. IGCP Project 236. West Aust. Inst. Technol., Perth: pp 2738. Harris, L.B., 1987. A tectonic framework for the Western Australian Shield and its significance to gold mineralisation. In: S.E. Ho and D.I. Groves (Editors), Recent Advances in Understanding Precambrian Gold Deposits. Geol. Dept. and Univ. Extension, Univ. West Aust. 11: 1-27. Harris, L.B., Delor, C.P., Beeson, J, and Standing, J.G., 1989. The structural evolution of the Albany-Fraser Province and Leeuwin Block, Western Australia. Proceedings of Kangaroo Island Tectonics Conference, Geol. Soc. Aust. Abstr., 3: 67-68. Harrison, T.M., Aleinikoff, J.N. and Compston, W., 1987. Observations and controls on the occurrence of inherited zircon in Concord-type granitoids, New Hampshire. Geochim. Cosmochim. Acta, 51:2549-1558. Kinny, P,D., 1987. An ion microprobe study of uraniumlead and hafnium isotopes in natural zircon. Aust. Natl. Univ. PhD Thesis (unpubl.). Lindsey, D.H., 1983. Pyroxene thermometry. Am. Mineral.. 68: 477-493. McCulloch, M.T., 1987, S m - N d isotopic constraints on the evolution of Precambrian crust in the Australian continent. In: Proterozoic Lithospheric Evolution. Geodynamic, 17. Publ. No. 0130 Int. Lithosphere Program, pp. 115-130. McCulloch, M.T, Collerson, K.D. and Compston, W., 1983. Growth of Archaean crust within the Western Gneiss Terrain, Yilgarn Block, Western Australia. J. Geol. Soc. Aust., 30: 155-160. Mclntyre, G.A., Brooks, C., Compston, W. and Turek, A., 1966. The statistical assessment of Rb-Sr isochrons. J. Geophys. Res., 71: 5459-5468. Nieuwland, D.A. and Compston, W., 1981. Crustal evolution in the Yilgarn Block near Perth, Western Australia. In: J.E. Glover and D.I. Groves (Editors), Archaean Geology: 2nd Int. Symp., (Perth, 1980), Spec. Pubis. Geol. Soc. Aust., 7: 159-171. Oliver, R.L., Cooper. J.A. and Truelove, J.A.. 1983. Petrology and zircon geochronology of Herring Island and Commonwealth Bay and evidence for Gondwana re-

123 construction. In: R.L. Oliver, P.R. James and J.B. Jago (Editors), Antarctic Earth Science. Aust, Acad. Sci., Canberra, pp. 64-68. Page, R.W., McCulloch, M.T. and Black, L.P., 1984. Isotopic record of major Precambrian events in Australia. Proc. 27th Int. Geol. Congr. VNU Science Press, 5: 2572. Pidgeon, R.T., 1990. Timing of plutonism in the Proterozoic Albany Mobile Belt, southwestern Australia. Precambrian Res., 47:157-167. Richard, P., Shimizu, N. and Allegre, C.J., 1976. ~43Nd/ t46Nd, a natural tracer: an application to oceanic basalts. Earth Planet. Sci. Lett., 31: 269-278. Rosman, K.J.R., Wilde, S.A., Libby, W.G. and de Laeter, J.R., 1980. Rb-Sr dating of rocks in the Pemberton area. West Aust. Geol. Surv. Ann. Rep., 1979, 97-100. Steiger, R.H. and Jager, E., 1977. Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett., 36: 359-362. Stephenson, N.C.N., Russell, T.G., Stubbs, D.G. and Kalocsai, G.I.Z., 1977. Potassium-argon ages of hornblendes from Precambrian gneisses from the south coast of Western Australia. Jour. Roy. Soc. West Aust., 59: 105-109. Tucker, D.H., Anfiloff, V. and Bagliani, F., 1986. Albany (W.A.) Map Sheet: total magnetic intensity, third generation compilation. Bureau of Mineral Resources, Australia. l/l,000,000 aeromagnetic anomaly pixel map series. Turek, A. and Stephenson, N.C.N., 1966. The radiometric age of the Albany granite and the Stifling Range Beds, southwest Australia. J. Geol. Soc. Aust., 13: 449458. Williams, I.S., Compston, W., Chappell, B.W. and Shirahase, T., 1976. Rubidium-strontium age determinations on micas from a geologically controlled, composite batholith. J. Geol. Soc. Aust.. 22: 497-506. Williams, I.S., Compston, W., Black, L.P., Ireland, T.R. and Foster, J.J., 1984. Unsupported radiogenic Pb in zircon: a cause of anomalously high Pb-Pb, U - P b and Th-Pb ages. Contrib. Mineral. Petrol., 88: 322-327.