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
1560 9 40 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 D
1
and metamorphosed to upper amphibolite facies M
1
, 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 1330 9 14 and 1314 9 21 Ma
Nelson et al., 1995. The younger xenocrystic population identified in sample 9509243 Fig.
3c 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from Point Malcolm Fig. 2 have ages ranging between 1221 and 1334 Ma Nelson, unpublished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
M
1
event. While not constraining the timing of M
1
, the data suggest that D
1
– M
1
only marginally preceded the intrusion of the Recherche Granite.
Subsequent to the emplacement of Recherche Granite plutons and prior to the intrusion of
post-D
3
aplites at
1313 9 16 Ma
samples 95091214 and 9509243, the Nornalup Complex
was twice again pervasively deformed D
2
and D
3
under waning M
1
thermal conditions. Whereas D
2
produced an essentially sub-horizontal fabric, D
3
resulted in substantial horizontal NW-SE shorten- ing, producing steeply dipping fabrics. D
3
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 ranging 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 1321 9 24 Ma detrital
zircon population identified near the base of the Mount Ragged metasedimentary rocks sample
9510101, Fig. 3e 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- D
3
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 D
4a
and metamor- phosed under granulite facies conditions M
2a
, 800°C and \ 5 kbar before 1214 9 8 Ma sam-
ple 9611201, Fig. 3d. Peak assemblages and decompression textures are overprinted by D
4b
deformation dated at 1182 9 13 Ma sample 96110201, relating to the uplift and eventual
westward transport of the terrain. The rutile age obtained from the Mount Ragged metasedimen-
tary rocks sample 9510092 indicates that these rocks were buried and metamorphosed M
2b
by 1154 9 15 Ma. The major deformation in the
Mount Ragged metasedimentary rocks D
4b
in- volved horizontal NW-SE shortening and pre-
dates the rutile age. Discrete D
4b
shear zones in the Malcolm Gneiss were active in a dip-slip
capacity prior to the intrusion of pegmatite dykes at 1165 9 5 Ma sample 9411112. We propose
that the formation of these shear zones coincided with the burial and deformation of the Mount
Ragged metasedimentary cover rocks, and that both events resulted from the overthrusting of the
Salisbury Gneiss, on what we have termed the Rodona fault Fig. 5d.
The position of the Rodona Fault is tentatively fixed by the reported occurrence of unretrogressed
granulites on Pasley Island correlatable to those on Salisbury Island Peers, 1967; Fig. 2. Gran-
ulite facies metamorphism on the northwestern side of the structure M
1a
predates that in rocks less than 5 km away on the southeastern M
2a
side by 100 million years. Significant movement must also have been accommodated in the other
major non-outcropping faults during the second stage of the Albany – Fraser Orogeny to account
for the difference in apparent D
4
strain between the folded Mount Ragged metasedimentary cover
rocks and underlying basement. Stage II of the Albany – Fraser Orogeny drew to
a close with the intrusion of high level felsic plutons into the Mount Ragged metasedimentary
rocks and gneissic basement rocks. Nelson et al. 1995 present SHRIMP age data for two of these
late, largely undeformed intrusions 1138 9 38 Ma and 1135 9 56 Ma from Esperance Harbour and
Balladonia Rock, respectively. These ages are within error of a Rb-Sr age of 1165 9 25 Ma
obtained on the granite outcropping at Esperance Harbour Doepel, 1975. While the data are too
imprecise to uniquely define a lower age bound for Stage II tectonism, they are consistent with D
4
deformation having ceased by c. 1160 – 1140 Ma.
Fig. 5. Simplified tectonic evolution of the eastern Albany – Fraser Orogen extrapolated from the Nornalup Complex. a Convergence between the Western Australian Craton and the East Antarctic ShieldSouth Australian Craton. The absence of
post-Archaean granitoids intruding the Yilgarn Craton margin suggests southeasterly dipping subduction. It is unclear whether the Archaean and Palaeoproterozoic rocks of the Biranup Complex resided on the leading edge of the advancing South AustralianEast
Antarctic craton at this time or had previously accreted to the Yilgarn Craton margin. b Stage I of the Albany – Fraser Orogeny c. 1345 – 1260 Ma: collision of the continental masses resulting in crustal thickening, high-grade deformation and metamorphism,
and syn-orogenic felsic plutonism. c Uplift and erosion. Sediments shed off the orogen are deposited in shallow basins unconformably overlying c. 1300 Ma basement gneisses. d Stage II of the Albany – Fraser Orogeny c. 1214 – 1140 Ma:
intracratonic reactivation of the mobile belt resulting in a second episode of high-grade deformation and metamorphism, crustal thickening and the intrusion of syn- to late-orogenic felsic plutons.
4
.
2
. A re6ised Mesoproterozoic geochronology for the eastern Albany – Fraser Orogen and tectonic
implications The evolutionary path deduced in the previous
section for the Nornalup Complex, combined with published data from the Biranup and Fraser
complexes, is used to construct a model for the tectonic evolution of the eastern part of the Al-
bany – Fraser Orogen. The main events in the model are summarised schematically in Fig. 5.
No unequivocal evidence has thus-far been recognised for magmatism during the convergent
stage Pre-D
1
of the Western Australian and South Australian – East Antarctic cratons. The ab-
sence of Mesoproterozoic plutons intruding the southeast margin of the Yilgarn Craton suggests
that subduction during convergence occurred be- neath the South Australian – East Antarctic craton
Fig. 5a. In the eastern Nornalup Complex, Recherche Granite plutons are associated with the
formation of recumbent structures under low- pressure high-grade metamorphic conditions and
crustal thickening during D
2
and are therefore probably related to the collision and subsequent
extension, rather than convergence Fig. 5b. Geochemical data from two such plutons Nelson
et al., 1995, Table 2 are not indicative of a unique tectonic setting.
Continental collision Fig. 5b initiated large fault structures with dextral displacements Fig. 2
that facilitated moderate crustal thickening c. 50 – 60 km total crustal thickness and the even-
tual westward transport of the orogen onto the Yilgarn Craton margin. High-grade deformation
and plutonism relating to the collision had ceased by 1313 9 16 Ma in the eastern Nornalup Com-
plex. The average age of four syn-collisional Recherche Granite intrusions dated in the Bi-
ranup Complex Nelson et al., 1995 is 1292 Ma. This is significantly younger than the two
Recherche Granite plutons dated by the same study 1330 9 14 and 1314 9 21 Ma, and the
1313 9 16 Ma estimate for cessation of deforma- tion, east of the Tagon Fault. Furthermore, the
granulite facies layered mafic intrusions of the Fraser Complex Myers, 1985; Fig. 1b crys-
tallised at 1291 9 21 Ma Fletcher et al., 1991. This suggests an orogenic front that migrated
westward over a period of c. 30 – 40 million years, culminating in high-grade deformation of the Bi-
ranup Complex at c. 1300 Ma and transport of Fraser Mobile Belt terrains onto the granites and
greenstones of the Yilgarn Craton before c. 1285 – 1260 Ma Fletcher et al., 1991; Nelson et al.,
1995. A significantly deeper level of the crust is exposed in the eastern Biranup Complex com-
pared to the Nornalup Complex e.g. maximum pressure estimates for metamorphism in the
Fraser Complex of 9 kbar Clarke et al., 1995, c.f. 4 – 5 kbar in the Malcolm Gneiss. This
implies greater vertical transport of the Fraser Complex. Biranup Complex gneisses outcropping
west of the Fraser Complex appear not to have experienced similarly extreme conditions Myers,
1995a.
Rapid uplift and exhumation terminated Stage I of the Albany – Fraser Orogeny Fig. 5c. The
Fraser Complex, and by association, the sur- rounding Biranup Complex gneisses, had cooled
below Rb-Sr mica blocking temperatures by c. 1260 – 1270 Ma Bunting et al., 1976; Fletcher et
al., 1991. The intrusion of dolerite dykes in the Nornalup Complex at this time is consistent with
upper crustal extension, which probably also cre- ated basins in which sediments shed off the erod-
ing orogen were deposited.
The intracratonic second stage of the Albany – Fraser Orogeny began before 1214 9 8 Ma with
granulite facies metamorphism and high-grade de- formation in rocks east of the Rodona Fault Fig.
2. Shortly thereafter c. 1182 9 13 Ma, the Salis- bury Gneiss was thrust upwards and westwards
along the Rodona Fault, burying the Mount Ragged metasedimentary rocks \ 1154 9 15 Ma
and reactivating the Malcolm Gneiss basement in discrete dextral shear zones Fig. 5d. A folded
pegmatite dated at 1187 9 12 Ma Nelson et al., 1995 from the southern Biranup Complex sug-
gests that coeval tectono-thermal activity was oc- curring in the western parts of the eastern
Albany – Fraser Orogen. K-Ar and Rb-Sr isotopic systems in the Fraser Complex Bunting et al.,
1976; Baksi and Wilson, 1980; Fletcher et al., 1991 do not record reactivation during this event,
implying that the rocks west of the Coramup
Fault were high in the crust. The coincidence of shortening directions both stages of the Albany –
Fraser Orogeny suggests that, although the two stages are temporally distinct, a single continent-
scale convergent regime was probably responsible for the entire Albany – Fraser Orogeny.
Subsequent to D
4
, rocks of the Nornalup Com- plex were folded around temporally uncon-
strained large-scale open upright D
5
structures consistent with NE-SW shortening. This fold gen-
eration is also identified in the Biranup Complex the Munglinup Terrane of Harris 1995, sug-
gesting the deformation is regionally significant. Shortening along this axis is incompatible with
the Albany – Fraser Orogeny, but occurred in the Northampton Block Bruguier et al., 1994 and in
the Musgrave Block Sun et al., 1996 at c. 1080 Ma and again in the Musgrave Block at c. 550
Ma Maboko et al., 1992; Camacho and Fanning, 1995; Clarke et al., 1995. Which, if either, of
these events relates to D
5
in the Albany – Fraser Orogen remains unclear.
5. Regional correlations on the basis of geochronology
Kategori