Geochronological constraints for a two-stage history of the
Albany – Fraser Orogen, Western Australia
D.J. Clark
a, B.J. Hensen
a,* , P.D. Kinny
baDepartment of Applied Geology,Uni
6ersity of NSW,Sydney2052,Australia bTectonics Special Research Centre,School of Applied Geology,Curtin Uni
6ersity of Technology,
GPO Box U1987Perth6845,Australia
Received 4 March 1998; accepted 16 January 2000
Abstract
Based on structural, petrographic and geochronological work (SHRIMP zircon, monazite and rutile), the Mesoproterozoic Albany – Fraser Orogeny is divided into two discrete thermo-tectonic stages, between c. 1345 and 1260 Ma (Stage I) and c. 1214 and 1140 Ma (Stage II). The existence of a two-stage history is confirmed by the discovery of 1321924 Ma detrital zircons and 1154915 Ma metamorphic rutiles in metasedimentary rocks from Mount Ragged. The detrital zircons demonstrate that the Mount Ragged metasedimentary rocks unconformably overly, and were derived from, Stage I basement. Metamorphic rutile formed as a consequence of overthrusting by high-grade early-Stage II rocks along an inferred NE-SW striking structure (the Rodona Fault). This interpretation is supported by zircon geochronology, which demonstrates that granulite facies metamorphism on the northwestern side of the structure predates that on the southeastern side by100 Ma. Rocks to the northwest record a low-grade imprint relating to the younger (Stage II) event. The two-stage thermo-tectonic history of the Albany – Fraser Orogen correlates with adjacent Grenville-age orogenic belts in Australia and East Antarctica, implying that Mesoproterozoic Australia assembled in two stages subsequent to the amalgamation of the North Australian and West Australian cratons. Initial collision between the combined West Australian – North Australian craton and the South Australian – East Antarctic continent at c. 1300 Ma was followed by intracratonic reactivation affecting basement and cover at c. 1200 Ma. Two comparable and contemporaneous compressional orogenies controlled the formation of the Kibaran Belt in Africa and the Grenville Belt in Canada, suggesting that tectonic events in Mesoproterozoic Australia follow a similar pattern to that recognised for Rodinia amalgamation world-wide. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Grenvillian; Rodinia; Albany – Fraser Orogen; Geochronology; Plate tectonics
www.elsevier.com/locate/precamres
1. Introduction
By the early Mesoproterozoic, Australia con-sisted of three relatively stable regions; the North, South and West Australian cratons (Myers et al.,
* Corresponding author.
E-mail address:b.hensen@unsw.edu.au (B.J. Hensen)
1996; Fig. 1(a)). At this time the South Australian Craton was contiguous with the East Antarctic Shield (Fanning et al., 1996). Tectonic activity between c. 1300 and 1000 Ma (Chin and de Laeter, 1981; Pidgeon, 1990; Black et al., 1992a; Bruguier et al., 1994; Camacho and Fanning, 1995; Nelson et al., 1995) led to the assembly of Proterozoic Australia as a component of the su-percontinent Rodinia. The sutures between the three Australian cratons are defined by ‘Grenville-age’ orogenic belts (i.e. broadly correlatable to the c. 1300- to 950-Ma Grenville belts of the North American Shield) containing high-temperature, medium to low-pressure polymetamorphic rocks with complex histories. By studying these oro-genic belts an understanding can be gained of the tectonothermal processes by which Mesoprotero-zoic Australia assembled, and their timing.
This contribution focuses primarily on the Al-bany – Fraser Orogen, which defines the suture between the West Australian and combined South Australian – East Antarctic Craton (Fig. 1(a)). The principal aims of this study were two-fold: firstly,
to gain an understanding of the thermo-tectonic history of the Albany – Fraser Orogen through a detailed structural, metamorphic and geochrono-logical study of a key area in the eastern part of the orogen, within a unit called the Nornalup Complex (Myers, 1990); and secondly, to investi-gate whether the sequence of events recognised in the Albany – Fraser Orogen is of local significance only, or reflects processes on a larger scale.
Reconnaissance fieldwork and a supporting geochronological study conducted by the Geologi-cal Survey of Western Australia in the eastern part of the Albany – Fraser Orogen (Nelson et al., 1995; Myers, 1995a) identify the eastern Nornalup Complex as of major significance to the develop-ment of the orogen. The Nornalup Complex is geologically complicated, comprising three or more distinct fault-bounded rock units and sev-eral felsic intrusive suites (Myers, 1995a; Fig. 2). Thus far, dating has been mainly limited to the major felsic intrusive suites (Nelson et al., 1995), with the result that many of the relationships between geological units remain propositional,
Fig. 2. (a) Basement geology of the eastern Nornalup Complex showing major lithological units and structures (adapted after Myers, 1995a). The Rodona Fault is inferred from the findings of this study. Rocks to the southeast of this fault are denoted the Salisbury Gneiss. Numbers represent sample localities discussed in the text and presented in Table 2. (b) Schematic NW-SE cross-section showing the relationships between the major lithological units. Dimensions are to scale. The dips and movement senses of major non-outcropping faults are inferred from sympathetic structures within the lithological units.
derived by correlation of isolated outcrops across large non-outcropping fault structures. Of partic-ular interest to the present study were the ages of a sequence of quartzose cover rocks near the eastern margin of the orogen (around Mount Ragged), and high-grade rocks occurring on is-lands off the eastern coast (the Salisbury Gneiss; Fig. 2), which remained completely unknown.
Geometric considerations (Fig. 1(a)) suggest that the events described in the Albany – Fraser Orogen should be manifest in some form in other Australian and East Antarctic Grenville-age orogenic belts. The second part of this study
presents a review of Mesoproterozoic geochrono-logical data from contiguous Australian and East Antarctic orogenic belts in order to estab-lish the degree of correlation with the Albany – Fraser Orogen. The Albany – Fraser Orogen, as part of Mesoproterozoic Australia, is placed into the context of the supercontinent Rodinia and
the worldwide Grenville-age orogenesis that
In this paper we describe the structural and metamorphic history of the Mesoproterozoic rocks of the Nornalup Complex in the eastern-most Albany – Fraser Orogen (Figs. 1 and 2) using age constraints provided by U-Pb geochronology. On the basis of SHRIMP II ion microprobe data the Albany – Fraser Orogeny is divided into two distinct periods of tectonothermal activity similar to those previously identified by Myers (1995a,b) and Nelson et al. (1995). Here, in addition, we recognise an episode of intracratonic sedimenta-tion and dyke intrusion between the two periods of tectonothermal activity, and determine a more detailed sequence of time constrained events. Cor-relation of the Albany – Fraser Orogen with con-tiguous Australian and East Antarctic orogenic belts, and with Grenville-age belts worldwide, provides insight into the scale and nature of the tectonic processes responsible for the assembly of the Mesoproterozoic supercontinent Rodinia.
2. The Nornalup Complex and its regional context
The Albany – Fraser Orogen (Myers, 1990) is a Proterozoic orogenic belt outcropping along the southern and southeastern margins of the Ar-chaean Yilgarn Craton in Western Australia (Fig. 1(b)). Myers (1990) subdivided the orogen into Biranup and Nornalup complexes (Fig. 1(b)), defined mainly on the basis of apparent differ-ences in structure shown by regional aeromagnetic data (Myers, 1990, 1995a,b) and brief observation of discontinuous coastal outcrop in the western part of the orogen. The division was subsequently substantiated (Myers, 1995a) following further fieldwork supported by a U-Pb zircon geochrono-logical study of major felsic intrusives in the east-ern part of the orogen (Nelson et al., 1995). Both complexes are dissected by a number of non-out-cropping structures, which are defined by aero-magnetics and are inferred to have formed as thrust faults (Myers, 1995a,b; Fig. 1(b) and Fig. 2).
In the eastern part of the orogen, the Biranup Complex comprises strongly deformed Archaean, Palaeoproterozoic and Mesoproterozoic felsic
plu-tons and minor metasedimentary gneiss (Myers, 1995a; Nelson et al., 1995). The northeastern part of the Biranup Complex is dominated by a tecton-ically disrupted layered basic intrusion called the Fraser Complex (Myers, 1985; Fig. 1(b)), which is Mesoproterozoic in age (Fletcher et al., 1991) and is recrystallised in granulite to garnet amphibolite facies. The Mesoproterozoic Nornalup Complex is dominated by scattered outcrops of Recherche Granite (Myers, 1995a) and comprises several in-ferred tectonostratigraphic units (Fig. 2). The Malcolm Gneiss, in the far east of the complex, comprises highly deformed ortho- and parag-neisses intruded by Recherche Granite and nu-merous generations of felsic and mafic dykes. The Mount Ragged metasedimentary rocks, formerly called the Mount Ragged Beds (Lowry and Doe-pel, 1974) and the Mount Ragged schist (Myers, 1995a), are a sequence of massive quartzites and subordinate metapelites recrystallised in upper
greenschist/lower amphibolite facies that outcrop
northwest of the Malcolm Gneiss (Fig. 2). Off-shore islands southeast of the Malcolm Gneiss form a unit here called the Salisbury Gneiss, which we infer to be separated from rocks on the mainland by a fault (the Rodona Fault), based upon the geochronological data presented in this paper. Late- to post-tectonic Esperance Granite plutons (Myers, 1995a) outcrop throughout the Nornalup Complex.
U-Pb zircon geochronological studies con-ducted in the early 1990s (Pidgeon, 1990; Black et al., 1992a; Nelson et al., 1995) succeeded in brack-eting the major period of tectonothermal activity in the Albany – Fraser Orogen, denoted the Al-bany – Fraser Orogeny, to between c. 1300 and 1100 Ma. Based on this information, Myers
(1995a) introduced a structural/metamorphic
framework for the Albany – Fraser Orogen. Three major Mesoproterozoic tectonothermal episodes
(D1– D3/M1– M3) were proposed. D1and D2 were
considered to have occurred under granulite facies
conditions (M1 and M2) at c. 1300 Ma, resulting
in pervasive fabric formation, crustal thickening
and thrust stacking. D3– M3 was broadly
under both ductile and brittle deformation conditions.
2.1. A synopsis of the structural/metamorphic framework for the eastern Nornalup Complex
In order to put the new geochronological data into context, an overview of the structural and metamorphic history of the area illustrated in Fig. 2 is presented in this section. The structural and metamorphic framework developed for the Al-bany – Fraser Orogen by Myers (1995a) provides a foundation for the present study of the eastern part of the Nornalup Complex. A more detailed analysis of the structural and metamorphic data will be published elsewhere.
Five distinct deformation episodes are recog-nised in the Nornalup Complex (Table 1): (1) formation of a first gneissic fabric in the Malcolm
Gneiss (D1); (2) transposition of that fabric into a
second composite recumbent fabric and the
folia-tion of Recherche Granite plutons (D2); (3) open
upright folding (D3); (4) high grade deformation
and metamorphism of the Salisbury Gneiss, folia-tion development and folding of the Mount Ragged metasedimentary rocks, and reactivation of the Malcolm Gneiss in discrete shear zones
(D4); and (5) a further generation of open folding
(D5).
2.1.1. D1
A pervasive layer-parallel foliation (S1) is
devel-oped in both metasedimentary and meta-igneous
rocks of the Malcolm Gneiss. S1-parallel
stro-matic migmatites occur locally, suggesting upper
amphibolite facies conditions (M1) prevailed
dur-ing D1 deformation. Pelitic mineral assemblages
are characterised by the stability of garnet, biotite and sillimanite. Textural and mineralogical evi-dence from migmatites, inferred to have formed by melting reactions involving muscovite and
bi-otite, suggest peak M1conditions in the vicinity of
750°C and 4 kbar.
2.1.2. D2
Further deformation of the Nornalup Complex
occurred during D2, after the intrusion of the
Recherche Granite (two nearby plutons are dated
at 1330914 and 1314921 Ma, Nelson et al.,
1995). The S1 fabric in the Malcolm Gneiss was
isoclinally folded and transposed into a second
planar, recumbent S1/S2 fabric, containing
root-less intrafolial isoclinal folds. D2 did not result in
the formation of a new axial planar fabric.
Post D1, pre- to syn-D2 Recherche Granite
plutons commonly outcrop as elongate trains of prominent hills. There is a progressive increase in strain intensity from weakly foliated cores to moderately gneissic margins. The first-formed
fab-ric in these rocks (S2) is defined by oriented biotite
and amphibolite boudins, and is associated with a
variably-oriented stretching lineation (L2).
Late-plutonic aplite dykes are tightly folded (F2) with
their axial planes parallel to S2. F2 fold axes
coincide with L2. The S2 fabric is truncated by
numerous small ductile shears (S2b). These
struc-tures rarely exceed a centimetre in width and are generally continuous over a metre or two. Both sinistral and dextral shear sets are represented,
across which S2 is offset up to 15 cm.
Quart-zofeldspathic leucosomes commonly occupy the
plane of S2bshears. Shear orientations are
consis-tent with extension parallel to the S2 fabric.
2.1.3. D3
D3 is characterised by significant horizontal
shortening. F3 folds are ubiquitous at all scales,
ranging in style from open kilometre-scale struc-tures to tighter, shorter wavelength folds. Dextral
asymmetry of F3 folds is consistent with
approxi-mately NW-SE bulk shortening during D3. Axial
planes trend northeasterly and dip steeply to the southeast. An axial planar fabric is best developed in mica-rich metasedimentary rocks. Fold axes trend generally NE-SW and plunge variably
ac-cording to their position on later folds. F3 folds
commonly have thickened hinges and attenuated limbs. Clearly recrystallised boudin necks on
at-tenuated F3 limbs preserve amphibolite facies
mineral assemblages.
Subsequent to D3, vertical NE-trending dolerite
dykes intruded the Malcolm Gneiss and
Recherche Granite. These intrusions are typically no more than 2 m in width and may be continu-ous over hundreds of metres. They provide a
D
.
J
.
Clark
et
al
.
/
Precambrian
Research
102
(2000)
155
–
183
Table 1
Summary of the structural/metamorphic framework proposed by this studya
Myers (1995a)
Deformation Metamorphism Structure U-Pb age (Ma)
equivalent event (this
paper)
Salisbury Gneiss Mt Ragged Mseds. Recherche Granite
Malcolm Gneiss
D1 Gneissosity (S1)+
c. 1330 D1 Peak M1: upper
stromatic amphibolite facies
migmatites
D2 Retrograde M1: Isoclinal folding Foliation (S2)+ con-c. 1330–1310
(F2)+transposition
amphibolite facies jugate shears (S2b) of S1(S1/S2)
D3 D2 Retrograde M1: Open-tight NW-SE Open NW-SE fold-amphibolite facies folding (F3) ing (F3)
n.r. Gneissosity (S1S)+ n.r.b
D4a n.r.
c. 1215-1180 M2a: granulite
isoclinal folding facies
(F1S) NE-trending thrust
?D3 M2b: greenschist to
D4bD4c NE-trending thrust NW-SE open fold- Layer parallel
c. 1180-1140
zones (S4b) strike- ing (F2S)+shear shearing (S1R)+ zones (S4b)
strike-amphibolite facies
fabric (S2S) slip reactivation
slip reactivation NW-SE open
fold-(S4c)
(S4c) ing (F2R)+cleavage
(S2R)
Open NE-SW fold- Open NE-SW
fold-D5 n.r. n.r.
? M3? open NE-SW
fold-ing (F5), local ing (F5) ing (F5) crenulation
cleav-age (S5)
2.1.4. D4
The D4 deformation episode heterogeneously
affected the eastern part of the Nornalup Com-plex and comprises several distinct phases of
de-formation (D4a,b,c). During these events strain was
partitioned into large fault structures transecting the complex (e.g. Tagon, Wininup and Rodona faults, Fig. 2), which acted as thrusts. Rocks of the Malcolm Gneiss and Recherche Granite were
reactivated in discrete northeasterly-trending
zones of intense shearing deformation during D4.
L4 lineations are steep in these subvertical to
southeast-dipping zones, indicating a dominant vertical component to motion, probably related to the larger thrusts. Mineral assemblages in the shear zones are recrystallised in mid-amphibolite facies.
2.1.5. D4a
New age constraints presented in the following section (see also Fig. 4) suggest that the first deformation fabric recognised in the Salisbury
Gneiss (S1S) formed at the onset of the D4
struc-tural episode. S1S is a pervasive foliation in all
Salisbury Gneiss lithologies and is defined by
peak-M2amedium-pressure granulite facies
assem-blages characterised by the stable coexistence of biotite, sillimanite and garnet in metapelites and the occurrence of orthopyroxene in metabasic rocks. S-planes trend to the northeast and dip subvertically. In migmatitic metapelitic rocks,
leu-cosomes are oriented within S1S. Thin leucosomes
are often isoclinally folded on a centimetre-scale
(F1S), reflecting progressive non-coaxial
deforma-tion during D4a. F1S axial planes are sub-vertical
and fold axes plunge moderately to the northeast
parallel to a pervasive sillimanite lineation (L1S).
Coronitic textures consistent with decompression
from peak conditions (800°C and \5 kbar)
formed in metapelites subsequent to D4a. The S1S
fabric is folded around a later generation of open
asymmetric folds (F2S), which are associated with
shearing deformation (D4b), and overprint the
decompression textures. Biotite, sillimanite and quartz are stable within the sheared matrix.
2.1.6. D4b
We propose that the deformation in the Mount
Ragged metasedimentary rocks can be correlated
to D4b in the Salisbury Gneiss (Table 1, Fig. 4).
Metapelitic layers contain a pervasive
bedding-parallel schistosity (S1R) defined by
monomineral-lic quartz segregations, sheets of opaque minerals
and oriented mica. The development of S1R
pre-dates the growth of randomly-oriented porphy-roblastic minerals (e.g. andalusite, gahnite-rich spinel) and is related to strain partitioning into the more micaceous rocks in preference to in-terbedded massive quartzites in the early stages of
D4b. Continued NW-SE horizontal shortening
folded S0 and S1R around open NW-verging F2R
similar folds, which are ubiquitous at all scales.
F2Rfold axes plunge shallowly towards the
north-east in the northern parts of the Mount Ragged metasedimentary rocks and plunge shallowly southwestwards in the south. An axial planar
fracture cleavage (S2R) is pervasively developed
throughout the Mount Ragged metasedimentary
rocks. In schistose layers, bedding and the S1R
fabric are crenulated by S2R. These planes trend
northeasterly and dip steeply to the southeast. The peak, uppermost greenschist-lower
amphibo-lite facies metamorphic paragenesis (M2b)
post-dates deformation and comprises the assemblage muscovite, chlorite, margarite, quartz and rare kyanite overprinting andalusite. Aluminosilicate-bearing quartz-mica veins postdate the formation
of S2R and commonly occupy the plane of small
shears with centimetre-scale offsets. Viridine an-dalusite is the stable aluminosilicate polymorph in most veins, but sillimanite has been observed in the southernmost exposures of the Mount Ragged metasedimentary rocks. Bodies of undeformed granite and pegmatites, which correlate with the c. 1140-Ma Esperance Granite, intrude the Mount Ragged metasedimentary rocks.
2.1.7. D4c
The evidence for thrust movement in D4 shear
zones is typically obscured by pervasive
reactiva-tion (D4c) at a lower metamorphic grade, typically
upper greenschist to lower amphibolite facies.
Shear sense indicators in reactivated D4 shear
to the southwest, suggesting a dominant horizon-tal component of movement.
2.1.8. D5
Fold axes vary systematically in plunge
throughout the mainland exposures of the Nor-nalup Complex, consistent with folding by a later
generation of regional-scale folds (F5) with
half-wavelengths in the order of 20 km. F5 folds
plunge shallowly to the northwest and reflect moderate NE-SW horizontal shortening. A
crenu-lation cleavage (S5) related to this deformation is
developed in amphibolites in the Malcolm Gneiss.
3. Geochronology
3.1. Pre6ious geochronology
A comprehensive review of geochronological investigation in the Albany – Fraser Orogen is pre-sented in Nelson et al. (1995). A summary of the major studies and their implications, including recent work, is presented below.
3.1.1. Eastern Albany–Fraser Orogen
With the exception of several studies conducted in the Fraser Complex region (Bunting et al., 1976; Baksi and Wilson, 1980; Fletcher et al., 1991), the age structure of the eastern part of the Albany – Fraser Orogen was unknown prior to a reconnaissance SHRIMP U-Pb zircon study by the Geological Survey of Western Australia (Nel-son et al., 1995). The Biranup Complex was found to comprise Late Archaean (c. 2595 – 2640 Ma) basement intruded by c. 1600 – 1700 Ma and c. 1300 Ma felsic plutonic rocks. In the Nornalup Complex, pre-orogenic basement rocks outcrop only in the Malcolm Gneiss. Zircons from a metasedimentary gneiss from near Point Malcolm yielded a wide spectrum of detrital ages. Two
distinct populations at 1560940 and 1807935
Ma, and single grain analyses ranging in age from 2033 to 2734 Ma, suggest that the sedimentary precursors to these rocks were not derived from the vicinity of the Albany – Fraser Orogen (Nelson
et al., 1995). The 1560940 Ma population
pro-vides a maximum estimate for the age of deposi-tion of the precursor sediments.
Two major felsic intrusive events relating to the Albany – Fraser Orogeny were identified by Nel-son et al. (1995). Six samples of granitic gneiss representative of the widely-distributed Recherche Granite (Myers, 1990; Fig. 2) yielded crystallisa-tion ages of between c. 1330 and 1283 Ma. These rocks intruded during a period of high-grade metamorphism and intense deformation recog-nised throughout the eastern Albany – Fraser Oro-gen (Myers, 1995a; Nelson et al., 1995). The layered basic rocks of the Fraser Complex crys-tallised under granulite facies conditions during this event, as constrained by an Sm-Nd isochron
age of 1291921 Ma (Fletcher et al., 1991). On
the basis of this age, and Rb-Sr and Ar-Ar cool-ing ages of between 1285 and 1262 Ma (Buntcool-ing et al., 1976; Baksi and Wilson, 1980; Fletcher et al., 1991), Fletcher et al. (1991) argued that the Fraser Complex intruded, was metamorphosed, and was subsequently tectonically emplaced into
the upper crust all in a period of 30 Ma. The
cooling ages for the Fraser Complex have also been interpreted to date the termination of high-grade metamorphism throughout the eastern part of the orogen (Nelson et al., 1995).
Two outcrops of undeformed granite (Esper-ance Granite, Myers, 1995a) gave imprecise U-Pb
zircon crystallisation ages of 1138938 and
1135956 Ma (Nelson et al., 1995). These ages
were interpreted by Myers (1995a) as dating a second period of tectonism and metamorphism correlating to the major c. 1190 – 1170-Ma oro-genic episode identified in the western part of the orogen. Although Esperance Granite plutons have not been recognised in the Biranup Complex, a
folded 1187912 Ma pegmatite dyke intruding
Palaeoproterozoic gneisses at Lake Gidong (Nel-son et al., 1995) may be related to this second thermo-tectonic event.
3.1.2. Western Albany–Fraser Orogen
1995b; Fig. 1(b)). U-Pb zircon crystallisation ages of six representative plutons range between c. 1170 and 1190 Ma (Pidgeon, 1990; Black et al., 1992a). Granite intrusion was preceded by high-grade metamorphism and deformation, dated by U-Pb in zircon at about 1190 Ma (Black et al., 1992a).
On the basis of inherited Archaean zircons (c. 3100 Ma) within felsic orthogneiss, Black et al. (1992a) interpreted much of the Biranup Complex to represent reworked Yilgarn Craton crust. These authors found no evidence for thermo-tec-tonic activity relating to c. 1300 Ma events in the eastern part of the orogen (Fletcher et al., 1991), and therefore interpreted the c. 1190 – 1170-Ma event to be the principal period of orogenesis in the western part of the orogen. However, a
1289910 Ma crystallisation age on an enderbitic
pluton outcropping near Albany (Pidgeon, 1990; Fig. 1(b)) suggests the existence of an earlier event. A recent U-Pb SHRIMP study by Clark
(1995) identified 130495 and 116997 Ma
meta-morphic zircon populations in a granulite facies metasedimentary migmatite from near the c. 1289-Ma enderbite, indicating that the western part of the Albany – Fraser Orogen did experience high-grade metamorphism and deformation at c. 1300 Ma.
K-Ar ages of 1160 – 1060 Ma obtained on horn-blende from metamorphosed basic rocks are inter-preted to date late granite emplacement and post-metamorphic uplift and cooling of the west-ern part of the orogen (Stephenson et al., 1977).
3.2. Metamorphic and structural context of the dated samples
Six rocks samples with well-defined
relation-ships to the structural/metamorphic history of the
Nornalup Complex were selected for age
determi-nation (Fig. 2, Table 2): (1) a post-D3, pre-D4
aplite dyke from the Malcolm Gneiss; (2) a
syn-D4 pegmatite dyke from the Malcolm Gneiss; (3)
a post-D2, pre-D4 aplite dyke from a Recherche
Granite pluton; (4) a syn-M2a leucosome layer
from a granulite facies metapelite from the Salis-bury Gneiss; and samples of quartzite (5) and schist (6) from the Mount Ragged metasedimen-tary rocks. Zircon was chosen to date the igneous crystallisation ages for samples (1) and (3), leuco-some formation in (4) and provenance ages for sample (5). Monazite was used to provide an igneous age for sample (2) as zircon was unavail-able. Metamorphic rutile crystals were dated in number (6). The morphological and internal char-acteristics of radiogenic minerals separated from the samples are described together with the iso-topic results in Section 3.4.
3.2.1. Post-D3, pre-D4 aplite dyke (95091214,
Malcolm Gneiss)
This metre-thick dyke cross-cuts F3 folds
defined by a pervasive S1 gneissosity in granitic
and metasedimentary rocks at Point Malcolm (Fig. 2). The dyke is linear but shows signs of
tectonic attenuation resulting from D4
deforma-tion. It contains an annealed assemblage of two
Table 2
Brief description of samples used for geochronology (sample localities are shown in Fig. 2)a
Mineral assemblage Sample Locality AMG coords Lithology Structural/Metm
context
Point Malcolm WC704600
(1) 95091214 Aplite dyke Post-D3pre-D4 Qtz-Bt-Kfs-Pl-Mag (2) 9411112 Little Bellinger WC648573 Pegmatite dyke Syn-D4 Qtz-Grt-Bt-Ms-Kfs-Pl-Mag
Aplite dyke WC148366
Cape Arid
(3) 9509243 Post-D2pre-D4 Qtz-Bt-Kfs-Pl-Mag-Hbl-Ttn
Qtz-Grt-Spl-Crd-Sil-Bt-Kfs-Pl (4) 9611201 Salisbury Island WB502982 Migmatitic Syn-M2a
paragneiss
Mt. Ragged WD437001 Quartzite Post-D3 Qtz-Bt-Ms-Chl-Hem (5) 9510101
Mt. Ragged WC431971 Mica schist Syn-M2b Qtz-Ms-Chl-Mrg-And-Hem-Rt9Ky (6) 9510092
feldspars, quartz, and biotite. Biotite is weakly oriented sub-parallel to the margins of the dyke, which is oblique to the tectonic fabrics
pre-served in the host rocks (e.g. S1/S2, S3, S4b). The
fabric is interpreted to result from igneous flow. Zircon and titanite occur as accessory minerals.
3.2.2. Syn-D4 pegmatite (9411112, Malcolm
Gneiss)
Sample 9411112 is from a pegmatite dyke
hosted by a large D4 shear zone outcropping
midway between Point Malcolm and Cape
Pasley (Fig. 2). The shear zone contains an am-phibolite facies mineral assemblage comparable
in grade to an M2b overprint recognised in the
metasedimentary rocks through which the shear
zone cuts. The dyke cross-cuts the S4b fabric in
the shear zone and is boudinaged by later
duc-tile movement along the shear planes (late-D4b
or D4c). Its mineralogy comprises garnet, two
feldspars, biotite, muscovite and quartz. Monaz-ite occurs as an accessory mineral but zircon was not found in either thin section or sepa-rates. The pegmatite contains a fabric parallel to
S4b/cdefined by oriented biotite.
3.2.3. Post-D2, pre-D4 aplite dyke(9509243,
Recherche Granite)
This aplite intrudes into a pluton of gneissic Recherche Granite outcropping at Cape Arid (Fig. 2). It comprises an annealed assemblage of two feldspars, quartz, hornblende, biotite, titan-ite and zircon. The dyke is linear over several
hundred metres of outcrop and cuts the S2 and
S2b fabrics in the host gneiss. F3 folding in the
area is at kilometre-scale so the timing of the
dyke relative to D3 is unclear. A shear fabric is
developed within the dyke oblique to its margins
consistent with deformation during D4.
3.2.4. Syn-D4a migmatitic leucosome (9611201,
Salisbury Gneiss)
This sample was collected from an S1S
concor-dant granitic leucosome outcropping on Salis-bury Island (Fig. 2). Leucosomes formed by extensive biotite dehydration partial melting of
metapelitic rocks during M2a and comprise
mesoperthitic feldspars, quartz and minor
gar-net9cordierite. Thin restitic schlieren rich in
garnet, biotite, sillimanite, spinel and cordierite separate leucosome layers. Zircon and monazite are present in both leucosome and mesosome layers. The leucosomes are everywhere
concor-dant with S1S, and are locally disharmonically
folded, suggesting that their formation occurred
synchronous with D4a. M2a garnets in leucosome
areas proximal to mesosome schlieren are
man-tled by cordierite9spinel coronas, which
con-tain abundant small zircon grains.
3.2.5. Post-D3 quartzite (9510101, Mount
Ragged)
Sample 9510101 was taken from near the ex-posed base of the Mount Ragged metasedimen-tary rocks at Mount Ragged. The quartzite consists almost entirely of a coarse-grained gra-noblastic aggregate of recrystallised quartz. A
weak annealed S1R foliation is defined by
ori-ented haematite and minor muscovite, chlorite and biotite. In section, haematite grains com-monly form rings up to several centimetres in diameter enclosing many small quartz grains,
suggesting the recrystallisation of originally
much coarser grains. Zircons are sporadically distributed along the boundaries of these relic grains.
3.2.6. Syn-D4 mica schist(9510092, Mount
Ragged)
This sample was taken from a thin pelitic lens intercalated with massive quartzite in the same vicinity as sample 9510101. The rock contains the assemblage muscovite, chlorite, margarite, andalusite, haematite and rutile. Ilmenite, spes-sartine garnet, gahnite-rich spinel, kyanite and epidote occur as accessory phases. Small euhe-dral rutile needles post-date garnet growth and
form part of a near-peak retrograde-M2b
meta-morphic paragenesis in this rock. Delicate knee-bend twins are common, suggesting that rutile
growth post-dates D4b shearing deformation in
3.3. Methodology and analytical procedures for isotopic analysis
The majority of U-Th-Pb isotopic measure-ments were made using the sensitive high resolu-tion ion microprobe (SHRIMP-II) at Curtin University of Technology, Perth. Zircons from
sample 9611201 were analysed using the
SHRIMP II facility at the Australian National University, Canberra, with the assistance of Dr R. Armstrong. All sample minerals were ex-tracted from the disaggregated rock samples and mounted in epoxy discs before being polished, gold coated and imaged. Before SHRIMP analy-sis all zircon grains were imaged by
cathodolu-minescence (CL) using a Cambridge S360
scanning electron microscope, with an operating voltage of 20 kV, equipped with a polychro-matic CL detector. The SEM is located in the Electron Microscope Unit at the University of New South Wales.
Procedures for SHRIMP U-Th-Pb isotopic analysis of zircon follow those originally de-scribed by Compston et al. (1984) and Williams et al. (1984), with subsequent modifications to analytical routines and data reduction methods
outlined by Williams and Claesson (1987),
Compston et al. (1992) and Williams (1998). For zircon analyses undertaken using the Perth SHRIMP-II, U-Pb ratios and U and Th concen-trations were determined relative to Sri Lankan
zircon standard CZ3 (564 Ma, 206Pb*
/238U
=
0.0914, Nelson (1997)). For analyses undertaken in Canberra, U-Pb ratios were determined
rela-tive to the Duluth Complex gabbroic
anorthosite standard AS3 (1099.1 Ma, 206Pb*/
238
U=0.1859, Paces and Miller (1989)), whilst
U and Th concentrations were determined rela-tive to ANU zircon standard SL13.
The procedure for monazite analysis by
SHRIMP followed the method outlined by Kinny (1997), which differs somewhat from that of Williams et al. (1996), and Ireland and
Gib-son (1998) in that the calibration of Pb/U ratios
is based upon a plot of ln(206Pb*/UO) versus
UO2/UO (calibration slope 0.7), with data for
unknowns normalised to Madagascan monazite
standard MAD (514 Ma, 206Pb*/238U=0.0830,
based on TIMS analyses of L.M. Heaman). An-other difference in the monazite analytical pro-cedure of Kinny (1997) is that, prior to being
used for common Pb correction, 204Pb counts
are corrected for a background interference of scattered ions the size of which is directly pro-portional to the Th content of the sample.
Pb/U ratios for rutiles were determined
rela-tive to 2625 Ma-old rutile from the Windmill Hill quartzite, Jimperding metamorphic belt,
Western Australia (206
Pb*/238
U=0.5025, based
on TIMS analyses by L.M. Heaman). Norm-alisation of rutile unknowns was based on
an observed linear covariation between
206Pb*/UO and UO2/UO for the standard, slope
1.17.
Common Pb corrections were applied using
the 204Pb correction method (Compston et al.,
1984), assuming the isotopic composition of Broken Hill ore Pb, except for zircon sample 96110201 which contained very low Th and so
was corrected via the 208Pb method (Compston
et al., 1984), using a modelled common Pb iso-topic composition appropriate to its age, and for the Mount Ragged rutile sample which con-tained no detectable Th. For rutile data in
which the measured 208Pb peak is entirely
non-radiogenic, a simplified 208Pb correction method
was applied, whereby the proportion of
non-ra-diogenic 206Pb, denoted f206, is given by:
f206%=100×(208Pb/206Pb)measured/(208Pb/206Pb)common
For both monazite and rutile analyses, the composition of the common Pb component was modelled upon that of contemporary terrestrial lead. Reproducibility of the U-Pb ratios of the standards on both machines was better than
92.1% in all cases. Elemental concentrations in
the monazite and rutile analyses were calculated by assuming a similar sensitivity of ionising spe-cies for standards and unknowns, and are
accu-rate to approximately 920%. The decay
constants used are those recommended by
3.4. Isotopic results and age constraints on field relationships
The processed U-Pb data are presented in Ta-bles 3 – 6. Results are presented on conventional concordia plots in Fig. 3(a – f). Errors given on individual analyses in the data tables and on
concordia plots are at 1slevel. They are based on
counting statistics, uncertainty in the common Pb
correction and, in the case of Pb/U ratios, the
uncertainties associated with normalisation to the standards. Pooled ages quoted in the text are
weighted means and their errors are given attsor
95% confidence level. Brief descriptions of the morphology of the analysed grains and, in the case of zircon, the internal characteristics as
re-Fig. 3. Concordia diagrams for dated samples; error boxes shown are 1s. Inset diagrams illustrate the structural context of the
samples. (a) Sample 95091214, Point Malcolm. The hatched analysis has not been used to determine the crystallisation age of this sample. (b) Sample 9411112, Malcolm Gneiss. Concordia diagrams for dated samples; error boxes shown are 1s. Inset diagrams
illustrate the structural context of the samples. (c) Sample 9509243, Cape Arid. Hatched analyses have not been used in determining the crystallisation age of this sample. The xenocrystic population is interpreted to be inherited from the Recherche Granite, and is quoted at 1slevel. (d) Sample 9611201, Salisbury Island. The two groups main groups represent zircon growth under metamorphic
conditions. Analyses in black do not fall into either population and have been excluded from age calculations. Concordia diagrams for dated samples; error boxes shown are 1s. Inset diagrams illustrate the structural context of the samples. (e) Sample 9510101,
Fig. 3. (Continued)
vealed by CL imaging are provided before the results for each sample.
3.4.1. Post-D3, pre-D4 aplite dyke(95091214,
Malcolm Gneiss)
Zircons extracted from this sample are colour-less, euhedral and squat to elongate. They range
in length from 150 to 200mm and in length/width
ratio from 1.5 to 2.5. Delicate oscillatory
growth zoning is evident in most grains. CL inten-sity ranges from slightly darker cores to brighter rims. Hourglass and sector zoning is prominent in many grains. Crystals are bounded by large prism and pyramid faces (notably {211}). Grains in this sample commonly have thin rims with dark CL response. Rims, ranging in width from 5 to 20
mm, are typically concordant to the internal
zona-tion of the grains, but sometimes form embay-ments transgressive into core material, truncating core zonation.
Seventeen zircon analyses fall within error of a
mean207Pb
/206Pb age of 1313916 Ma (Fig. 3(a)).
The remaining analysis (3.43) is significantly dis-cordant (9%) and so has been excluded from the age calculation. The analyses contain 79 – 204 ppm
U, 50 – 245 ppm Th (Table 3) and Th/U ratios
clustering closely around an average of 0.8. The ubiquitous presence of oscillatory and sector zonation, the abundance of {211} pyramid faces,
and the moderate Th/U ratios strongly suggest
that this zircon has a primary igneous origin. The age recorded by this zircon population is therefore interpreted to date the crystallisation of the aplite,
D
Geochronological results obtained on zircons from samples 95091214 and 9509243
207Pb/
95091214Malcolm Gneiss aplite dyke
0.00091 0.2909 0.0024
3.1 185 180 49 0.11 0.08363 0.2236 0.0037 2.579 0.0536 101 1284 21
0.00185 0.2595 0.0044 0.2238 0.0037 2.628 0.0762
0.08519 99
131 1320 42
3.2 116 34 0.11
0.08451
104 60 25 0.31 0.00144 0.1688 0.0031 0.2237 0.0038 2.607 0.0661 100 1304 33 3.3
0.08 0.08544 0.00093 0.1982 0.0021 0.2263 0.0037 2.665 0.0556 99 1326 21
3.8 180 121 45
0.00130 0.2323 0.0031 0.2209 0.0037 2.595 0.0622
0.08523 97
28 0.10 1321 29
3.1 111 87
0.08562
119 78 30 0.31 0.00152 0.1958 0.0035 0.2276 0.0038 2.687 0.0697 99 1330 34 3.14
0.08349
182 157 49 0.21 0.00102 0.2608 0.0025 0.2294 0.0038 2.640 0.0575 104 1281 24 3.19
0.00187 0.1763 0.0042 0.2237 0.0038 2.711 0.0780
0.08792 94
3.22 99 59 24 0.26 1381 41
0.18 0.08391 0.00142 0.1998 0.0032 0.2333 0.0039 2.698 0.0683 105 1290 33
3.28 118 83 31
0.00183 0.1834 0.0042 0.2262 0.0038 2.493 0.0751
0.07993 110
101 1195 45
3.37 64 25 0.34
0.08602
195 244 50 0.57 0.00128 0.3252 0.0032 0.2081 0.0034 2.468 0.0580 91 1339 29 3.43
0.08428
154 150 42 0.28 0.00140 0.2856 0.0035 0.2285 0.0038 2.656 0.0663 102 1299 32 3.46
0.00220 0.1791 0.0050 0.2242 0.0038 2.562 0.0852
0.08289 103
3.5 80 50 20 0.59 1267 52
0.00129 0.2794 0.0032 0.2266 0.0038 2.680 0.0639
3.54 119 113 32 0.10 0.08580 99 1334 29
0.00079 0.2552 0.0020 0.2287 0.0038 2.705 0.0536
0.08579 100
54 0.04 1333 18
3.62 203 171
0.08451
204 188 55 0.09 0.00098 0.2680 0.0024 0.2291 0.0038 2.670 0.0570 102 1304 23 3.63
0.08144
145 103 37 0.37 0.00125 0.2037 0.0029 0.2280 0.0038 2.560 0.0615 107 1232 30 3.66
0.00105 0.2694 0.0026 0.2294 0.0038 2.747 0.0598
0.08684 98
3.67 127 115 34 0.09 1357 23
9509243Recherche Granite aplite dyke
0.00063 0.0877 0.0012 0.2266 0.0037 2.687
113 0.0500
492 150 0.21 0.08600 98 1338 14
1.1a
377 21 82 0.08685 0.00062 0.0175 0.0009 0.2286 0.0037 2.737 0.0509 98 1357 14
1.2b 0.03
154 175 45 0.09065 0.00103 0.2735 0.0025 0.2464 0.0041 3.080 0.0654 99 1439 22
1.2a 0.23
0.00145 0.3902 0.0040 0.2263 0.0038 2.663 0.0673
0.08536 99
1.8 105 138 30 0.11 1324 33
0.00133 0.1744 0.0029 0.2251 0.0037 2.609
1.15 118 70 29 0.30 0.08405 0.0634 101 1294 31
0.00120 0.4352 0.0033 0.2192 0.0036 2.526 0.0583
0.08358 100
69 0.45 1283 28
1.2 239 351
0.08112
129 88 33 0.86 0.00202 0.1887 0.0046 0.2277 0.0038 2.546 0.0805 108 1224 49 1.22b
0.08540
69 46 18 0.46 0.00211 0.1886 0.0048 0.2284 0.0039 2.689 0.0854 100 1325 48 1.23
0.00111 0.1908 0.0025 0.2281 0.0038 2.659 0.0597
0.08457 101
1.27 130 80 32 0.00 1306 26
0.00089 0.2793 0.0023 0.2251 0.0037 2.662 0.0547 98
1.31 179 166 47 0.01 0.08576 1333 20
0.00084 0.2209 0.0019 0.2261 0.0037 2.694 0.0541
0.08641 98
223 1347 19
1.33 166 57 0.06
0.08296
96 63 24 0.26 0.00144 0.1900 0.0033 0.2233 0.0037 2.555 0.0654 102 1268 34 1.41
0.08364
167 176 46 0.15 0.00102 0.3153 0.0027 0.2259 0.0037 2.606 0.0565 102 1284 24 1.44
0.00106 0.2347 0.0025 0.2250 0.0037 2.661 0.0579
0.08578 98
1.49 196 154 50 0.14 1333 24
0.00090 0.4004 0.0024 0.2320 0.0038 2.674 0.0546 105 1284
1.5 267 363 80 0.33 0.08362 20
0.00140 0.2610 0.0033 0.2087 0.0034 2.393 0.0595
0.08317 96
64 1.17 1273 33
1.52 254 236
0.08571
247 275 68 0.05 0.00082 0.3247 0.0022 0.2274 0.0037 2.688 0.0536 99 1332 18 1.54
0.00094
1.4 181 130 45 0.07 0.08673 0.2111 0.0021 0.2245 0.0037 2.684 0.0560 96 1355 21 0.00106 0.3062 0.0027 0.2194 0.0036 2.558 0.0560 98 1305
J
.
Clark
et
al
.
/
Precambrian
Research
102
(2000)
155
–
183
169
Table 4
Geochronological results obtained on monazites from sample 9411112
91s 208Pb/206Pb 91s 206Pb/238U 91s 207Pb/235U 91s % conc.b 207Pb/206Pb Age
Label U (%) Th (%) Pb (%) f206%a 207Pb/206Pb 91s
9411112Malcolm Gneiss pegmatite
0.00019 0.67741 0.00082 0.2094 0.0043 2.297
8.75 3.06 0.048 103 1186 5
3.09
dan.3 1.21 0.07955
0.00011 0.69294 0.00074 0.1998 0.0041 2.166
dan.4 3.69 10.58 3.23 0.02 0.07860 0.045 101 1162 3
0.00022 1.25280 0.00109 0.2011 0.0041 2.184 0.046
0.07877 101
1.22 1166 5
dan.5 3.59 21.71 3.42
0.00007 1.42859 0.00101 0.2089 0.0043 2.259 0.046
dan.6 3.12 18.97 3.16 0.02 0.07842 106 1158 2
0.00013 0.30047 0.00045 0.2020 0.0041 2.213 0.046
0.07947 100
dan.7 3.92 5.09 3.83 0.74 1184 3
0.00010 0.60103 0.00061 0.1999 0.0041 2.171 0.045 101
dan.8 4.13 11.74 3.72 0.09 0.07879 1167 3
0.00010 0.58745 0.00065 0.2086 0.0043 2.264 0.047
0.07870 105
dan.9 3.13 7.63 2.88 0.22 1165 3
0.00007 0.89487 0.00072 0.2073 0.0042 2.243 0.046
dan.10 3.48 14.03 3.22 0.05 0.07848 105 1159 2
0.00012 0.42808 0.00053 0.1979 0.0040 2.145 0.044
0.07863 100
dan.11 4.44 8.05 3.71 0.03 1163 3
0.00041 2.94523 0.00485 0.2109 0.0043 2.284 0.050 106 1161
dan.12 0.69 9.48 0.90 1.49 0.07855 10
0.00014 1.44305 0.00144 0.2011 0.0041 2.183 0.045
0.07872 101
0.28 1165 4
dan.13 2.57 16.90 2.62
0.00008 0.67851 0.00063 0.2051 0.0042 2.229 0.046 103 1168
dan.14 3.91 10.76 3.67 0.08 0.07884 2
0.00014 0.70253 0.00092 0.1962 0.0040 2.144 0.044 98 1179
0.07926 3
dan.15 2.32 8.05 2.01 0.24
af206%
=100×(common206Pb
/total206Pb). b
% conc=100×(206
Pb/238
U age)/(207
Pb/206
D
Geochronological results obtained on zircons from sample 96110201
f206%a
96110201Salisbury Gneiss migmatitic leucosome
0.00073 – – 0.2104
96-1.2 735 34 145 1.40 0.08071 0.0024 2.342 0.0364 101 1214 18
0.00042 – – 0.2049
96-2.1 611 22 117 0.10 0.08076 0.0024 2.282 0.0301 99 1216 10
0.00048 – – 0.2023 0.0023 2.253 0.0306
0.08076 98
0.10 1216 12
96-3.1 687 27 130
0.00063 – – 0.1981 0.0025 2.190 0.0345
96-4.2 468 20 87 0.17 0.08019 97 1202 16
0.00048 – – 0.2069 0.0023 2.328 0.0308
0.08159 98
96-5.2 854 48 167 2.69 1236 12
0.00050 – – 0.2118
96-6.1 854 31 170 0.05 0.08233 0.0025 2.404 0.0334 99 1253 12
0.00028 – – 0.2107 0.0025 2.352 0.0295
0.08095 101
96-8.1 726 28 144 0.08 1220 7
0.00044 – – 0.2000 0.0022 2.209 0.0287
96-9.1 434 20 82 0.20 0.08010 98 1199 11
0.00056 – – 0.2075 0.0024 2.322 0.0334
0.08116 99
96-10.1 508 18 99 0.15 1225 14
0.00034 – – 0.2176 0.0026 2.455 0.0321
96-12.1 775 23 158 0.03 0.08183 102 1242 8
0.00053 – – 0.2064 0.0025 2.297 0.0327
0.08069 100
0.05 1214 13
96-13.1 565 20 109
0.00038 – – 0.2042 0.0023 2.253 0.0289
96-14.1 604 9 115 0.10 0.08003 100 1198 9
0.00054 – – 0.1993 0.0024 2.173 0.0311
0.07911 100
96-15.1 488 17 91 0.23 1175 13
0.07 0.08097 0.00042 – – 0.2077 0.0025 2.319 0.0312 100 1221 10 34
96-16.1 805 157
0.00074 – – 0.2144 0.0027 2.389 0.0396
0.08082 103
0.07 1217 18
96-17.1 533 21 107
0.00063 – – 0.2045 0.0026 2.300 0.0360
96-18.1 668 32 129 0.32 0.08159 97 1236 15
0.00102 – – 0.1875 0.0027 2.048 0.0419
0.07924 94
96-18.2 497 15 87 6.22 1178 26
0.00125 – – 0.1855 0.0105 2.101 0.1268
96-19.1 783 34 137 6.71 0.08212 88 1248 30
0.00058 – – 0.2165 0.0027 2.434 0.0371
0.08155 102
0.02 1235 14
96-20.1 704 47 144
0.00050 – – 0.2126 0.0054 2.351 0.0633
96-5.3 799 26 159 0.08 0.08021 103 1202 12
Rims
0.00032 – – 0.2005 0.0023 2.182 0.0278
0.07894 101
96-1.1 473 15 89 0.28 1171 8
0.00036 – – 0.2038 0.0022 2.232 0.0277
96-4.1 480 15 92 0.05 0.07941 101 1182 9
0.00046 – – 0.2030 0.0027 2.206 0.0333
0.07882 102
96-5.1 695 21 132 0.13 1168 12
0.00034
96-7.1 488 27 95 0.11 0.07980 – – 0.2062 0.0023 2.269 0.0279 101 1192 8
0.00033 – – 0.2062 0.0023 2.270 0.0275
0.07987 101
0.06 1194 8
96-11.1 779 13 150
0.00065
96-16.2 574 19 115 0.17 0.07867 – – 0.2145 0.0027 2.326 0.0373 108 1164 16
af206%
=100×(common206Pb
D
Geochronological results obtained on zircons from sample 9510101 and rutiles from sample 9510092
207Pb/
9510101Mt Ragged Quartzite
0.00104 0.3236 0.0027 0.2193
1.1a 253 279 68 0.13 0.08727 0.0055 2.639 0.077 94 1367 23
0.00102 0.1998 0.0022 0.3207 0.0080 4.798 0.133
0.10853 101
361 1775 17
1.7a 257 134 0.82
0.08597
181 133 46 0.01 0.00104 0.2143 0.0024 0.2273 0.0057 2.694 0.079 99 1337 23 1.12
0.00221 0.5947 0.0063 0.3095
1.13 87 179 40 0.26 0.10879 0.0080 4.643 0.161 98 1779 37
0.00073 0.1967 0.0016 0.2018 0.0050 2.390 0.065
0.08592 89
135 0.18 1336 16
1.14 603 374
0.10803
187 269 45 0.72 0.00204 0.5124 0.0055 0.1688 0.0043 2.514 0.084 57 1767 34 1.15
0.00049 0.3696 0.0013 0.3097 0.0077 4.680 0.121
1.16 659 846 260 0.04 0.10961 97 1793 8
0.00173 0.2769 0.0040 0.3134 0.0080 4.550 0.146
0.10531 102
93 1720 30
1.2 93 35 0.34
0.10120
72 92 25 0.65 0.00252 0.3683 0.0063 0.2691 0.0070 3.754 0.143 93 1646 46 1.24
0.28 0.08392 0.00107 0.6240 0.0038 0.2224 0.0056 2.573 0.076 100 1291 25
1.25 231 483 76
0.00102 0.2097 0.0022 0.2177 0.0054 3.275 0.091
0.10911 71
346 1785 17
1.26 204 86 0.37
0.10749
244 239 92 0.22 0.00097 0.2771 0.0023 0.3145 0.0079 4.661 0.129 100 1757 16 1.29
0.00456 0.1554 0.0103 0.1096 0.0028 1.413 0.082
1.39 172 71 23 3.80 0.09352 45 1498 92
0.00131 0.2047 0.0030 0.2250 0.0057 2.564 0.080
0.08266 104
195 1261 31
1.49 139 49 0.28
0.08330
226 135 42 0.47 0.00155 0.1676 0.0035 0.1680 0.0042 1.930 0.064 78 1276 36 1.51
0.08590
49 29 12 0.23 0.00359 0.1835 0.0082 0.2209 0.0058 2.616 0.136 96 1336 81 1.52
0.00119 0.3026 0.0029 0.3199 0.0081 4.783 0.138
0.10844 101
60 0.20 1773 20
1.63 153 165
0.08768
502 451 86 0.95 0.00126 0.2844 0.0030 0.1412 0.0035 1.707 0.052 62 1375 28 1.68
0.08593
238 174 57 0.08 0.00122 0.2079 0.0028 0.2129 0.0053 2.522 0.077 93 1336 27 2.2
0.00234 0.4108 0.0058 0.2966 0.0076 4.692 0.162
0.11475 89
2.3 107 130 42 0.36 1876 37
0.08426
356 180 83 0.19 0.00090 0.1444 0.0019 0.2186 0.0055 2.539 0.072 98 1299 21 2.8
0.00235 0.1322 0.0051 0.2247 0.0058 2.595 0.105
0.08377 102
18 0.33 1287 55
2.3 74 34
0.10937
649 1139 121 0.89 0.00115 0.2872 0.0027 0.1514 0.0038 2.284 0.065 51 1789 19 2.36
0.00098 0.4184 0.0027 0.3159 0.0080 4.773 0.133 99
2.49 180 261 75 0.18 0.10959 1793 16
9510092Mt Ragged mica schist
121 nd 24 0.07797 0.00080 – – 0.1912 0.0044 2.055 0.054 99 1146 20
mr1.1 0.62
116 nd 23 0.07630 0.00085 – – 0.1931 0.0045 2.031 0.055 103 1103 22
mr1.2 1.09
0.00100 – – 0.1962 0.0046 2.089 0.059
0.07723 103
21 0.75 1127 26
mr1.3 95 nd
0.07713
122 nd 24 0.67 0.00079 – – 0.1926 0.0045 2.048 0.054 101 1124 20
mr1.4
0.07955
77 nd 24 2.83 0.00087 – – 0.2025 0.0047 2.221 0.060 100 1186 22
mr10.1
0.00109 – – 0.2007 0.0047 2.191 0.063
0.07920 100
mr11.1 93 nd 25 5.25 1177 27
0.00077 – – 0.1958 0.0045 2.072 0.055 104
mr12.1 83 nd 28 2.61 0.07676 1115 20
0.00106 – – 0.1825 0.0043 1.976 0.056
0.07856 93
0.00069 – – 0.1862 0.0043 2.010 0.052
mr4.1 80 nd 29 1.85 0.07827 96 1154 17
0.00141 – – 0.1983 0.0047 2.143 0.068
0.07840 101
mr7.1 121 nd 25 3.06 0.07851 – – 0.1862 0.0043 2.015 0.056 95 1160 25
0.00110 – – 0.1919 0.0045 2.090 0.060
0.07903 97
23 3.49 1173 28
mr8.1 109 nd
0.07892
109 nd 23 3.16 0.00108 – – 0.1910 0.0045 2.079 0.059 97 1170 27
mr8.2
0.00143
mr8.3 76 nd 20 4.73 0.07836 – – 0.1943 0.0047 2.100 0.067 99 1156 36
0.00119 – – 0.1904 0.0045 2.085 0.061 95 1183
dark CL rims observed on these grains proved too thin to analyse.
3.4.2. Syn-D4 pegmatite (9411112, Malcolm
Gneiss)
Honey-yellow monazites from this pegmatite are subhedral to anhedral and equant, with
length/width ratios typically less than 2. Whole
grains range from 100 to 250mm in diameter and
are unzoned in transmitted and reflected light. SHRIMP analyses define an approximately concordant population with excess scatter in
207Pb
/206Pb around a mean age of 116595 Ma
(x2=8.5). Although the data may be divided into
two statistically valid populations on the basis of counting statistics, we see no geological
justifica-tion to do so. The largex2of the population may
be attributable to an underestimation of the errors in the individual measurements in monazite analy-ses (Ireland and Gibson, 1998). In the present instance, the effect on the age uncertainty does not influence the geological significance of the age (Fig. 4).
Th contents of the population range widely,
from 5.1 to 21.7%, whilst Th/U ratios range from
1.3 to 13.7, and average 4.4 (Table 4). Typical Th
contents in monazite range from 4 to 12% ThO2
(Watt, 1995) but Th-rich monazite (up to 30%
ThO2) has been recorded from pegmatitic rocks
(Bowles et al., 1980). Th and U enrichment in such rocks has generally been considered to be controlled, at least in part, by processes involving magmatic fluids (Watt, 1995). This suggests that the monazite grains from which the population of analyses were derived crystallised from the host pegmatitic melt. Mineral assemblages preserved in
D4 shear zones suggest the Malcolm Gneiss
ter-rain was at upper greenschist to lower amphibo-lite-facies temperatures at the time of intrusion of the 9411112 pegmatite. This corresponds to tem-peratures much less than the estimated 725°C closure temperature for U-Pb diffusion in monaz-ite (Mezger et al., 1993). The pooled age of
116595 Ma therefore records the age of
crystalli-sation of the host pegmatite and provides an
estimate for the timing of movement in D4 shear
zones in the Malcolm Gneiss.
3.4.3. Post-D3, pre-D4 aplite dyke (9509243,
Recherche Granite)
Most zircons from this sample are uniform in
morphology and range from 150 to 280 mm in
length. Length/width ratios range from 2.5 to
4. Crystals are well-faceted, inclusion-free and colourless. Prominent steep {211} pyramidal faces are common. CL imaging reveals bold oscillatory growth zoning with dark cores grading into bright
rims. Very thin (B10 mm) dark rims are often
present. They are concordant with the oscillatory zonation of the cores. Two grains in this sample are morphologically distinct from the remainder (1.1 and 1.2). They are squat and contain rounded, irregular cores showing signs of metam-ictisation (patchy CL response). The outer mar-gins of core regions appear strongly resorbed. The
cores are enveloped by thick (20 – 60 mm)
euhe-dral, faceted rims, which show bold concentric zonation consisting of broad bands of bright and dark CL response. Thin outer rims of dark CL response envelope the thicker inner-rims, without truncation of zonation, and may represent contin-uous growth.
Fourteen zircon analyses define the main popu-lation in this sample and scatter about a mean 207
Pb/206
Pb ratio corresponding to an age of
1313916 Ma (Fig. 3(c)). Most analyses contain
95 – 270 ppm U and 60 – 370 ppm Th (Table 3).
Th/U ratios range from 0.5 to 1.5 with a
cluster-ing of seven analyses around 0.7. All the analysed grains from this population are elongate, are bounded by well-developed crystal faces, and show prominent oscillatory zoning. This habit is consistent with their growth in a magma (Vavra,
1994). The pooled age of 1313916 Ma is
there-fore interpreted to represent the igneous crystalli-sation age for the host aplite dyke. Analyses 1.22b and 1.52 are discordant (8 and 4%, respectively) and have a relatively high f206% (0.86 and 1.17%, respectively) and so were excluded from the age calculations.
Rim analyses (1.1a and 1.2b) on the two
mor-phologically distinct grains have much lower Th/
Fig. 4. Time-space diagram constructed for the Nornalup Complex. The four units compared are shown in Fig. 2. The height of an ‘event block’ is schematic; large boxes for intrusive events represent larger volumes of magma, cf. smaller boxes. Question marks indicate uncertain interpretations. The S0symbol represents the formation of bedding surfaces. SHRIMP error bars are at 95% confidence levels for pooled analyses and 1sfor single analyses. Geochronological data from Nelson et al. (1995) and Myers (1995a)
included in the diagram are mentioned in the text. Four of the five single grain xenocrystic analyses shown for the Recherche Granite were obtained from a gneissic sample of Recherche Granite located at Cape Arid (Clark, D., unpublished data).
chemical environment of different Th/U
composi-tion to the main populacomposi-tion of analyses. The two
analyses have a pooled207Pb/206Pb age of 13459
1330914 Ma crystallisation age obtained on the Recherche Granite pluton that the dyke intrudes (Nelson et al., 1995). Thin, dark CL rims on these grains too thin to analyse may represent a second period of zircon growth within the aplitic magma. Analysis 1.2a from the irregular core of grain 1.2
contains 175 ppm Th, 154 ppm U and a Th/U
ratio of 1.1. A distinct207Pb/206Pb ratio consistent
with an age of 1442922 Ma suggests that this
core is xenocrystic to the aplite.
3.4.4. Post-D3 migmatitic leucosome (9611201,
Salisbury Island)
Two distinct morphological types of zircons occur in this sample. Elongate grains averaging
150 – 250mm long occur dispersed throughout the
leucosome portion of the migmatite. Length/
width ratios range from 1.5 to 3.5. These zircons are clear, subhedral and are strongly oscillatorily zoned. The second group consists of clear equant
grains (soccerballs) averaging 150 – 200 mm in
di-ameter. The grains are bounded by many high-or-der facets. This group is also abundant in the leucosome and shows strong oscillatory zoning. Grains from both groups are typically mantled by
unzoned rims (530 – 50 mm thick) of slightly
brighter CL-response than the cores. Core zona-tion is truncated by rim material in rare instances. Very thin bright-CL outer rims truncate zonation in some grains.
Eighteen zircon analyses of cores from both morphological groups in this sample define a
pop-ulation with a mean 207Pb/206Pb age of 121498
Ma (Fig. 3(d)). Uranium contents range from 434 to 854 ppm and average 637 ppm (Table 5). Thorium contents are low and range from 9 to 48
ppm, averaging 26 ppm. Th/U ratios are
ex-tremely low (0.01 – 0.07), which is consistent with the coeval growth of monazite with this zircon. The well-developed planar boundaries and oscilla-tory zonation exhibited by these grain cores
sug-gests that they formed within the granitic
leucosome. The 121498 Ma age is therefore
in-terpreted to date the onset of crystallisation of the
leucosome and thus constrains the timing of M2a.
Six rim analyses from both elongate and
equant grains fall within error of a mean 207Pb/
206Pb age of 1182913 Ma (Fig. 3(d)). Uranium
and thorium contents are lower but comparable
to the core material (Table 5), while Th/U ratios
range from 0.02 to 0.06, averaging 0.03. Similar chemistry and the absence of zonation is reported to be consistent with zircon growth under meta-morphic conditions (Williams et al., 1996). Fraser et al. (1997) demonstrated that zircon growth in high-grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown of Zr-bearing phases such as garnet. Fluid-present
D4b shearing, which resulted in the extensive
re-placement of peak assemblages by M2b biotite+
sillimanite+quartz, provides a likely candidate
for such a zirconium-liberating event. Hence, the
1182913 Ma rim age is interpreted to record the
timing of D4b shearing, which then provides a
lower age bound for high-grade activity and sub-sequent decompression in the Salisbury Gneiss.
Two analyses (6.1 and 12.1) fall statistically outside the two major populations in this sample
(based on a x2-test) and so were not included in
age calculations.
3.4.5. Post-D3 quartzite (9510101, Mount
Ragged)
Zircons in this sample range from subhedral
elongate grains (up to 320 mm in length)
exhibit-ing strong concentric zonation in transmitted light
and CL, to rounded grains (\100 mm in length)
filled with apatite inclusions. Many are metamict to varying degrees and are brown in colour, while others are colourless. All show pitting and have irregular surfaces, consistent with detrital trans-port. CL imaging reveals a surprising conformity of internal zonation patterns. Most grains pre-serve concentric oscillatory zoning with no evi-dence of inherited cores. The zonation commonly truncates against fracture surfaces. The intensity of the CL response varies markedly between grains. No grains preserve evidence for more than one major period of zircon growth, although some grains show evidence of small palaeofrac-tures having healed.
upon is the present, indicating recent lead loss.
Primary 207Pb/206Pb ratios are therefore retained.
Percentages of common 206
Pb are generally less than 0.5% for these analyses (Table 6).
The younger population, comprising nine
analyses, has 207
Pb/206
Pb ratios, which are within error of a single value and indicate an age of
1321924 Ma. Analyses contain between 54 and
600 ppm U, and 34 – 484 ppm Th (Table 6). U and Th contents average 257 and 207 ppm,
re-spectively. Apart from a few outliers the Th/U
ratios of analyses from this population cluster fairly closely around a mean value of 0.8. To-gether with the oscillatory zonation noted in CL images, these data suggest that the analyses sample zircon formed in an igneous rock.
The older population, comprising seven
analy-ses, forms a discrete group with a pooled 207
Pb/
206Pb age of 1783912 Ma. Two analyses (1.24
and 2.3) fall outside the older population. Both are discordant and were not included in age cal-culations. Analyses from this population are more heterogeneous with respect to chemistry. Uranium concentrations range from 71 to 660 ppm and thorium concentrations from 93 to 1126 ppm (Table 6). The average U and Th concentrations (261 and 322 ppm, respectively) do not differ significantly from those of the
younger population. Th/U ratios show no
sig-nificant cluster and vary from 0.6 to 2.1. The chemistry of the zircons does not provide con-clusive evidence as to their origin but as oscilla-tory zonation is present in the majority of grains an igneous origin is most plausible.
The rounded, fractured and abraded surfaces of zircons from both populations indicates detri-tal transport. The zircons are therefore inter-preted to be detrital grains in the sedimentary precursor to the quartzite and have the U-Pb isotopic characteristics of their igneous source
rocks. The younger population of 1321924 Ma
sets a maximum age for the deposition of the sediments that formed the protolith of the quartzite. The dominantly clean quartzitic na-ture of the Mount Ragged metasedimentary rocks precludes a volcanic or volcanoclastic
origin and instead suggests granitic source rocks shed off the uplifted and eroding Albany – Fraser
Orogen. The high oxidation state of the
metasedimentary rocks, characterised by the sta-bility of haematite, suggests deposition in a shal-low and oxygenated environment.
3.4.6. Syn-D4 mica schist(9510092, Mount Ragged)
Rutile crystals from this sample are a lustrous brown-red colour, euhedral in shape and vary
from elongate crystals (length/width 4 – 7) up
to 500 mm in length to equant plates 250 mm
in length. The width of needles varies from
50 mm in the most elongate grains to several
hundreds of micrometres. The grains show no evidence of zonation in transmitted light.
All 18 rutile analyses from this sample fall
within error of a mean 207Pb/206Pb ratio
corre-sponding to an age of 1154915 Ma (Fig. 3(f)).
The percentage of common 206
Pb in the analyses is high (ranging from 0.6 to 6.0%, see Table 6) but is lower than usual for this mineral by virtue of atypically high uranium concentrations, which range from 76 to 122 ppm and average 98 ppm (Table 6). The metamorphic mineral as-semblage in the schist, the abundance of planar crystal faces on rutile grains, and their tendency to form delicate knee-bend twins suggests that they grew as a part of a post-kinematic parage-nesis, which slightly post-dates peak
metamor-phism. Based on considerations of mineral
assemblage, peak metamorphic temperatures are unlikely to have far exceeded 500°C. The cool-ing rate for the metasedimentary rock is un-known but can be assumed to be in the order of several degrees or more per million years. At this cooling rate, and for rutile grains of the size analysed, the closure temperature must be in ex-cess of 420°C (Mezger et al., 1989). The rutiles therefore crystallised near to their closure tem-perature, so it is expected that the age recorded is close to the actual crystallisation age. The
1154915 Ma age therefore provides a minimum
4. Discussion
4.1. Chronology of major e6ents in the Nornalup
Complex
The chronological data obtained by this study, together with relevant age data collected by Nel-son et al. (1995), have been used to construct a metamorphic and structural history of the Nor-nalup Complex, summarised on a space-time dia-gram (Fig. 4, see also Table 1). The new age constraints confirm the assertion of Myers (1990) that, although significantly older rocks exist in the Albany – Fraser Orogen, the major tectono-meta-morphic features of the Nornalup Complex formed during the Mesoproterozoic. Further-more, Fig. 4 shows that data relating to orogenic events (plutonism and metamorphism) fall into two fairly well-defined bands within the period c. 1345 – 1140-Ma. We denote these periods of thermo-tectonic activity Stages I and II of the Albany – Fraser Orogeny.
The oldest rocks recognised in the Nornalup Complex occur in the Malcolm Gneiss. The sedi-mentary precursors of the Malcolm Gneiss are constrained to have been deposited between
1560940 Ma (detrital population, Nelson et al.,
1995) and the intrusion of granitic and granodi-oritic rocks that make up the second major com-ponent of the terrain at c. 1450 Ma (Myers, 1995a). These rocks and abundant intercalated mafic rocks of unknown age and origin were
strongly deformed (D1) and metamorphosed to
upper amphibolite facies (M1, 750°C and 4
kbar) early in Stage I of the Albany – Fraser Orogeny, prior to the intrusion of numerous Recherche Granite plutons. Two outcrops of Recherche Granite proximal to the Malcolm
Gneiss have ages of 1330914 and 1314921 Ma
(Nelson et al., 1995). The younger xenocrystic population identified in sample 9509243 (Fig. 3(c)) suggests that plutonism related to the Al-bany – Fraser Orogeny may have initiated as early as c. 1345 Ma. Three analyses on high uranium zircon grains from a c. 1450 Ma granitic gneiss from Point Malcolm (Fig. 2) have ages ranging between 1221 and 1334 Ma (Nelson, unpublished data), consistent with either the isotopic resetting
of older grains, or zircon formation, during the
M1 event. While not constraining the timing of
M1, the data suggest that D1– M1only marginally
preceded the intrusion of the Recherche Granite. Subsequent to the emplacement of Recherche Granite plutons and prior to the intrusion of
post-D3 aplites at 1313916 Ma (samples
95091214 and 9509243), the Nornalup Complex
was twice again pervasively deformed (D2and D3)
under waning M1thermal conditions. Whereas D2
produced an essentially sub-horizontal fabric, D3
resulted in substantial horizontal NW-SE
shorten-ing, producing steeply dipping fabrics. D3 is the
last deformation phase associated with the first stage of the Albany – Fraser Orogeny recognised in the Nornalup Complex. Rb-Sr and Ar-Ar cool-ing ages rangcool-ing from c. 1285 to 1260 Ma (Bunting et al., 1976; Baksi and Wilson, 1980; Fletcher et al., 1991) for the Fraser Complex, adjacent to the western boundary of the Nornalup Complex, are consistent with rapid exhumation and cooling following Stage I.
Although no contact relationships are exposed, angular relationships between bedding in the Mount Ragged metasedimentary rocks and the more complex fabrics in the basement gneisses suggest that the cover rocks unconformably
overly the basement. The 1321924 Ma detrital
zircon population identified near the base of the Mount Ragged metasedimentary rocks (sample 9510101, Fig. 3(e)) confirms this interpretation and is consistent with the local derivation of the precursor sediments. Uplift and erosion of the Albany – Fraser Orogen must therefore have oc-curred prior to Stage II of the Albany – Fraser Orogeny, with deposition of mature quartzose sediments into shallow intracratonic basins during the c. 65-million year interval separating the two stages. The intrusion of northeast-trending
post-D3 dolerite dykes into the Nornalup Complex
may reflect the same period of extension which facilitated basin formation.
The first evidence for Stage II of the Albany – Fraser Orogeny is recorded in the Salisbury Gneiss (Fig. 4). These rocks east of the Rodona
Fault were strongly deformed (D4a) and
metamor-phosed under granulite facies conditions (M2a,
