Pinjarra Orogen Research Articles

Subduction initiation of the Proto-Tethys Ocean that facilitated climate change and biodiversification

The effect of plate interactions on surface processes, environmental changes of the earth and their contributions to biodiversification during Gondwana assembly has been hotly debated in the last decades. The late Neoproterozoic-early Paleozoic was a key period in the tectonic assembly of Gondwana, global climate change and metazoan evolution. During this interval, the amalgamation of Greater Gondwana was succeeded by the subduction initiation of oceanic slabs in the Proto-Tethys Ocean. Here, we use whole-rock geochemical data, detrital zircon U-Pb and Hf isotopic of Cambrian and Ordovician sedimentary rocks from the Baoshan block in the Southeastern Tibetan Plateau combined with existing data in the literature to constrain the timing of subduction initiation of the Proto-Tethys Ocean at 540-500 Ma. The results show that the Baoshan block, adjacent to the Pinjarra Orogen on the northern margin of India, witnessed the transition from passive to active continental margin in East Gondwana. After the subduction initiation, the source of the sedimentary rocks experienced more intense weathering, causing a higher sediment flux. Contemporaneously, the Cambrian Explosion and stepwise increase in atmospheric oxygen accompanied the Greater Gondwana assembly at the late Neoproterozoic-early Paleozoic boundary. Subduction of the Proto-Tethys Ocean and occurrence of peripheral orogens further increased atmospheric O2 content, transported nutrients to the ocean and reduced the greenhouse effect, which provided major propulsion for biological evolution.

Late Paleozoic detrital history of eastern Gondwanaland: petrofacies and detrital geochronology of Permo-Carboniferous intracratonic sequences of the northwest Bengal Basin

ABSTRACTPermo-Carboniferous Gondwanan sequences have been reported from several isolated basins of Peninsular India. These siliciclastic sequences were preserved in several intracratonic basins in northwest Bangladesh. Sandstone petrography, heavy-mineral assemblages, mineral chemistry, and 40Ar/39Ar geochronology of sediment cores were used in this study to decipher the provenance history of Gondwanan sediments at two localities (Khalashpir and Barapukuria). Petrographic studies suggest that these sequences are mostly immature and poorly sorted arkosic sandstones (Khalashpir-Qt60F27L13, Barapukuria-Qt52F31L17), with compositions ranging from quartzarenite to litharenite. Among lithic fragments, sedimentary types are abundant. Heavy minerals are volumetrically rare and of low diversity in sediments of northwest Bangladesh. Garnet geochemistry indicates that metamorphic grades in the source terranes were of the amphibolite to granulite facies. Laser 40Ar/39Ar ages for single crystals of detrital muscovite from the deepest drilled Gondwanan sequences yielded the broadest age range, with a dominant mode at circa 515 Ma and lesser clusters of ages at circa 550, 570, and 600 Ma. The other two shallower samples are dominated by ages with similar single modes at circa 495–500 Ma. The oldest muscovite crystals may have been derived from the adjacent Indian craton and/or the Meghalayan craton. Younger muscovite crystals may have been contributed from the Pinjarra Orogen, formed during episodes of Neoproterozoic to early Paleozoic collision among India, Antarctica, and Australia.

Tectonic controls on sediment provenance evolution in rift basins: Detrital zircon U–Pb and Hf isotope analysis from the Perth Basin, Western Australia

The role of tectonics in controlling temporal and spatial variations in sediment provenance during the evolution of extensional basins from initial rifting to continental breakup and passive margin development are not well established. We test the influence of tectonics in a rift basin that has experienced minimal uplift but significant extension throughout its history: the Perth Basin, Western Australia. We use published zircon U–Pb and Hf isotope data from basin inception through to continental drift and complement this with new data from samples deposited synchronously with the continental breakup of eastern Gondwana. Three primary source regions are inferred, namely the Archean Yilgarn Craton to the east, the Paleo- and Mesoproterozoic Albany–Fraser–Wilkes Orogen to the south and east, and the Mesoproterozoic and Ediacaran–Cambrian Pinjarra Orogen underlying the rift basin and comprising the dominant crustal components to the west and southwest. From mid-Paleozoic basin inception to Early Cretaceous breakup of eastern Gondwana, drainage in the Perth Basin was primarily north- to northwest-directed as evidenced by the dominant Mesoproterozoic detrital zircon cargo, paleodrainage patterns and paleocurrent directions. Thus, provenance was primarily parallel to the rift axis and perpendicular to the extension direction, particularly during periods of thermal subsidence. During episodes of mechanical extension, detrital zircon ages are polymodal and consistently dominated by Paleo- and Mesoproterozoic grains derived from the Albany–Fraser–Wilkes Orogen, but with significant Archean and Neoproterozoic inputs from the rift margins. It is inferred that during mechanical extension the rate of subsidence exceeded sediment supply, which generated basin-margin scarps and enhanced direct input from the rift shoulders. Detrital zircon spectra from temporally-equivalent samples at the rift margin and in the rift axis reveal that distinct sedimentary routing operated on the flanks. In summary, sediment provenance in the Perth Basin (and probably other rift basins) is tectonically controlled by: (1) extension direction, (2) episodes of mechanical extension (rift) or thermal subsidence (post-rift), and (3) proximity to rift axis or rift margin.

Variations in Zircon Provenance Constrain Age and Geometry of an Early Paleozoic Rift in the Pinjarra Orogen, East Gondwana

AbstractThe Tumblagooda Sandstone in Western Australia documents Early Paleozoic rifting in East Gondwana and provides the earliest evidence for terrestrial activity of multicellular animals on Earth. We constrain the provenance of this sequence using 737 concordant U–Pb ages of detrital zircons from two stratigraphic wells in the northern Perth Basin and Southern Carnarvon Basin and outcrops of the type sections near Kalbarri. Detrital zircon age signatures are linked to infrared spectral data and stratigraphic logs. These ages span 3,312–466 Ma, including major Precambrian age peaks at 1,079–1,544 Ma, 1,695–2,403 Ma, and 2,640–2,879 Ma, consistent with igneous sources in the West Australian Craton. Significant Early Paleozoic age peaks at 499–541 Ma suggest a North Indian Orogen source. The maximum depositional age is constrained by the youngest detrital zircon, which yields an age of 466 ± 8 Ma. Our age constraints imply that terrestrial activity of multicellular animals on Earth may not be older than Middle Ordovician (Darriwilian). Rifting resulted in the exposure of the Yilgarn Craton and the segmentation of the Pinjarra Orogen. The Northampton Complex segment of the Pinjarra Orogen constituted a basement high that separated subbasins during the onset of rifting. Discordant Archean zircons provide a consistent record of radiogenic Pb loss at ~470 Ma, which we interpret as being related to the denudation of the Yilgarn Craton. The Pb loss event suggests that intracratonic rifting in the Pinjarra Orogen was initiated in the Middle Ordovician, after the Kuunga Orogeny completed the final amalgamation of Gondwana.

East Antarctic sources of extensive Lower–Middle Ordovician turbidites in the Lachlan Orogen, southern Tasmanides, eastern Australia

ABSTRACTLower to upper Middle Ordovician quartz-rich turbidites form the bedrock of the Lachlan Orogen in the southern Tasmanides of eastern Australia and occupy a present-day deformed volume of ∼2–3 million km3. We have used U–Pb and Hf-isotope analyses of detrital zircons in biostratigraphically constrained turbiditic sandstones from three separate terranes of the Lachlan Orogen to investigate possible source regions and to compare similarities and differences in zircon populations. Comparison with shallow-water Lower Ordovician sandstones deposited on the subsiding margin of the Gondwana craton suggests different source regions, with Grenvillian zircons in shelf sandstones derived from the Musgrave Province in central Australia, and Panafrican sources in shelf sandstones possibly locally derived. All Ordovician turbiditic sandstone samples in the Lachlan Orogen are dominated by ca 490–620 Ma (late Panafrican) and ca 950–1120 Ma (late Grenvillian) zircons that are sourced mainly from East Antarctica. Subtle differences between samples point to different sources. In particular, the age consistency of late Panafrican zircon data from the most inboard of our terranes (Castlemaine Group, Bendigo Terrane) suggests they may have emanated directly from late Grenvillian East Antarctic belts, such as in Dronning Maud Land and subglacial extensions that were reworked in the late Panafrican. Changes in zircon data in the more outboard Hermidale and Albury-Bega terranes are more consistent with derivation from the youngest of four sedimentary sequences of the Ross Orogen of Antarctica (Cambrian–Ordovician upper Byrd Group, Liv Group and correlatives referred to here as sequence 4) and/or from the same mixture of sources that supplied that sequence. These sources include uncommon ca 650 Ma rift volcanics, late Panafrican Ross arc volcanics, now largely eroded, and some <545 Ma Granite Harbour Intrusives, representing the roots of the Ross Orogen continental-margin arc. Unlike farther north, Granite Harbour Intrusives between the Queen Maud and Pensacola mountains of the southern Ross Orogen contain late Grenvillian zircon xenocrysts (derived from underlying relatively juvenile basement), as well as late Panafrican magmatic zircons, and are thus able to supply sequence 4 and the Lachlan Ordovician turbidites with both these populations. Other zircons and detrital muscovites in the Lachlan Ordovician turbidites were derived from relatively juvenile inland Antarctic sources external to the orogen (e.g. Dronning Maud Land, Sør Rondane and a possible extension of the Pinjarra Orogen) either directly or recycled through older sedimentary sequences 2 (Beardmore and Skelton groups) and 3 (e.g. Hannah Ridge Formation) in the Ross Orogen. Shallow-water, forearc basin sequence 4 sediments (or their sources) fed turbidity currents into outboard, deeper-water parts of the forearc basin and led to deposition of the Ordovician turbidites ∼2500–3400 km to the north in backarc-basin settings of the Lachlan Orogen.

Early Cambrian metamorphic zircon in the northern Pinjarra Orogen: Implications for the structure of the West Australian Craton margin

The Mesoproterozoic Pinjarra Orogen formed when present-day India and Australia amalgamated to form Gondwana. Outcrop of the Pinjarra Orogen is limited to the Leeuwin, Mullingarra, and Northampton Complexes, which are exposed as basement inliers in the Paleozoic to Mesozoic Perth and Southern Carnarvon Basins. We used U-Pb zircon geochronology to date Pinjarra Orogen basement rocks from the Wendy-1 drill core, which intersects the Paleozoic Tumblagooda Sandstone and its underlying paragneiss basement east of the Northampton Complex. Our results suggest an Early Cambrian (526.3 ± 12 Ma) metamorphic age for this basement domain, which is uncharacteristic for the nearby Northampton Complex, but correlates well with the much more distant Leeuwin Complex. Detrital ages between 1120 Ma, 1210 Ma, and 1530 Ma dominate the zircon cargo of this basement sample, which may have been sourced from the Albany-Fraser Orogen to the south and east. An Archean detrital zircon component is also identified from one concordant analysis, and from radiogenic Pb-loss modeling. These results have important implications for the crustal architecture of the western margin of the West Australian craton and for correlating domains of the Mesoproterozoic Pinjarra Orogen in reconstructions of Gondwana. Our data suggest that the basement below the Perth Basin is more segmented than previously assumed. Evidence for a common Indian-Australian tectonometamorphic event in the Late Neoproterozoic to Early Cambrian is not limited to the Leeuwin Complex in the southwest corner of present-day Western Australia but also now identified in basement rocks in the Northampton area. These results confirm the in situ formation of Pinjarra Orogen basement complexes in the Mesoproterozoic with a metamorphic reactivation in the Neoproterozoic–Cambrian during the collision with present-day India.

Western Australia-Kalahari (WAlahari) connection in Rodinia: Not supported by U/Pb detrital zircon data from the Maud Belt (East Antarctica) and the Northampton Complex (Western Australia)

The Kalahari Craton is an important building block of the supercontinent Rodinia, but its position with respect to other cratons is still controversially discussed. The Maud Belt in East Antarctica is part of the extensive Namaqua-Natal-Maud Orogen along which Kalahari collided with another continent during Rodinia assembly. One of the continents that have been suggested as collision partners for Kalahari is Western Australia, with the Pinjarra Orogen as the counterpart to the Maud Belt. We investigate this connection from a geochronological point of view. SHRIMP U/Pb zircon analyses of three metasedimentary samples from the Maud Belt date Grenville-age metamorphism within the orogen at ca. 1100–1060Ma. One sample was later affected by Pan-African metamorphism at ca. 540Ma. A second sample is interpreted as a molasse of the Maud Belt and was deposited in the Neoproterozoic. Detrital zircons from all three samples are consistent with derivation of the sediments predominantly from within the Namaqua-Natal-Maud Belt, with minor contributions from the Kalahari Craton. No clear Western Australian fingerprint could be detected in the detrital ages and a direct comparison between detrital zircon ages from the Maud Belt and the Northampton Complex (Pinjarra Orogen, Western Australia) showed distinct differences in the age spectra. Altogether, we consider a collision between Kalahari and south-western Laurentia a more likely scenario.

Geoscience Australia seismic survey 310: revealing stratigraphy and structure of the outer northern Perth Basin margin

Geoscience Australia acquired seismic survey GA 310 in 2008–09, across the southwest margin of Australia, as part of the Australian government’s Energy Security Program. Deep reflection seismic and potential field data were recorded across sparse 2D grids located on the Wallaby Plateau in the north, Mentelle Basin in the south, and the intervening Houtman and Zeewyck sub-basins of the northern Perth Basin. The offshore northern Perth Basin extends for about 700 km along the Western Australia margin, from the towns of Carnarvon in the north to Cervantes in the south. The largely Paleozoic-Mesozoic tectonostratigraphic framework is dominated by Permian and Early-Middle Jurassic rifting, followed by Late Jurassic-Early Cretaceous rifting leading to Valanginian breakup between Australia and Greater India. Underlying Precambrian Pinjarra Orogen structuring, in conjunction with rifting, has resulted in the development of several complex depocentres and basement highs. A recent re-evaluation of the offshore northern Perth Basin well-based lithostratigraphy into a new chronostratigraphic sequence framework has been carried outboard, on the GA 310 seismic lines, into the margin bounding the Zeewyck and northern Houtman sub-basins. The main sequences hosting source rocks—Kockatea and Cattamarra—are widely present in the expansive northern Houtman Sub-basin, and are likely to be present in the deep Zeewyck Sub-basin. The mapping of a thick Late Jurassic Yarragadee Sequence in the Zeewyck Sub-basin indicates a major pre-breakup locus of relatively rapid deposition. The structural interpretation across the sub-basin highlights breakup-drift unconformities and strike-slip faulting and suggests a probable along-margin sheared crustal sliver; tectonic elements commensurate with an evolving rift-shear breakup margin.

U–Pb ages of metamorphic monazite and detrital zircon from the Northampton Complex: evidence of two orogenic cycles in Western Australia

The Pinjarra Orogen, stretching along the western coast of Western Australia, is one of the least understood orogens on the continent. Exposure is limited to fault-bound basement blocks within the Phanerozoic Perth Basin, which have been interpreted as either remnants of a Mesoproterozoic orogen or as allochthonous blocks within a Neoproterozoic orogen. Thus even the timing of orogenesis is controversial. However, since the orogen is in a critical position for reconstructions of both Rodinia and Gondwana, a better understanding is highly desirable. New U–Pb ages of detrital zircon cores, metamorphic zircon rims and metamorphic monazites from the Northampton Complex, the northernmost and largest of the exposed basement blocks, provide better insights into the sedimentary provenance and metamorphic history of the metasediments exposed in this part of the orogen. Both the monazite ages and the youngest metamorphic zircon rims constrain metamorphism in the Northampton Complex to 1090–1020Ma. Additionally, a large number of older metamorphic zircon rims pre-date this event and were inherited from the Albany-Fraser Orogen in southwestern Australia, which is also identified as the main source region for the detrital zircon cores. Additional sources of detrital zircons are the Mawson Continent (the South Australian Craton and parts of East Antarctica) and possibly the North Australian Craton, as well as synsedimentary volcanic rocks. However, the combination of ages from the cores and inherited metamorphic rims shows that zircons from the Mawson Continent/North Australian Craton were not directly transported to the Northampton Complex but arrived there after being incorporated into the Albany-Fraser Orogen and then eroded from there. The dataset records therefore two orogenic cycles: (1) erosion of the Mawson Continent/North Australian Craton, deposition along the craton margins, collision between the Mawson Continent and North-West Australian Craton and metamorphism in the Albany-Fraser Orogen and (2) erosion of the Albany-Fraser Orogen, deposition along Australia's western margin and metamorphism during the Pinjarra Orogeny. This highlights the care that must be taken in detrital zircon studies, since many of the detrital cores record a second-order provenance. It also explains how zircons from the Mawson Continent could be transported across the Albany-Fraser Orogen that should normally have been a barrier to northwest-directed sediment transport. Finally, in light of these new and previously published data, we support the interpretation that the Pinjarra Orogen is a Mesoproterozoic orogen formed along Australia's western margin.

Zircon U-Pb Chronology of the Nyingtri Group, Southern Lhasa Terrane, Tibetan Plateau: Implications for Grenvillian and Pan-African Provenance and Mesozoic-Cenozoic Metamorphism

The Nyingtri Group of the Lhasa terrane in southern Tibet consists dominantly of metasedimentary rocks and orthogneiss. These rocks have a similar mineral paragenesis of plagioclase + K-feldspar + biotite + quartz ± sillimanite ± garnet ± staurolite ± muscovite ± amphibole, indicating amphibolite-facies metamorphic conditions. Inherited detrital zircons from the metasedimentary rocks show magmatic features and yield widely variable 206Pb/238U ages ranging from 3300 to 50 Ma. The data define two prominent age populations, 1200–1000 and 600–500 Ma, indicating that the source of the Nyingtri Group preserves the records of both Grenville and Pan-African magmatic-thermal events. Inherited magmatic zircon cores from the orthogneiss yield a crystallization age of 496 Ma, limiting the depositional age of the metasedimentary sequence to Cambrian or older. Overgrowth rims on the detrital zircons from one metasedimentary rock yield a metamorphic age of 32 Ma. On the basis of these results, together with the regional comparison, we infer that the Nyingtri Group was formed during or before the Cambrian, with a potential provenance from the Pinjarra Orogen of Western Australia–East Antarctica. This rock group, together with the Tethyan Himalayan Sedimentary Sequence, represents an early Paleozoic sedimentary cover of the northern margin of the Gondwana supercontinent that was intruded by Cambrian granites during the circum-Gondwana Andean-type orogeny. Along with published data, this study demonstrates that the Nyingtri Group was metamorphosed during Mesozoic and Cenozoic, as against the previous notion of a Precambrian metamorphic basement for the Lhasa terrane.

Upper mantle seismic anisotropy of South Victoria Land and the Ross Sea coast, Antarctica from SKS and SKKS splitting analysis

SUMMARY We determine shear wave splitting parameters of teleseismic SKS and SKKS phases recorded at 43 broad-band seismometers deployed in South Victoria Land as part of the Transantarctic Mountain Seismic Experiment (TAMSEIS) from 2000 to 2003. We use an eigenvalue technique to linearize the rotated and shifted shear wave particle motions and determine the best splitting parameters. The data show a fairly consistent fast direction of azimuthal anisotropy oriented approximately N60 ◦ E with splitting times of about 1 s. Based on a previous study of the azimuthal variations of Rayleigh wave phase velocities that show a similar fast direction, we suggest that the anisotropy is localized in the uppermost mantle, with a best estimate of 3 per cent anisotropy in a layer of about 150 km thickness. We suggest that the observed anisotropy near the Ross Sea coast, a region underlain by thin lithosphere, results either from upper mantle flow related to Cenozoic Ross Sea extension or to edge-driven convection associated with a sharp change in lithospheric thickness between East and West Antarctica. Both hypotheses are consistent with the more E–W fast axis orientation for stations on Ross Island and along the coast, subparallel to the extension direction and the lithospheric boundary. Anisotropy in East Antarctica, which is underlain by cold thick continental lithosphere, must be localized within the lithospheric upper mantle and reflect a relict tectonic fabric from past deformation events. Fast axes for the most remote stations in the Vostok Highlands are rotated by 20 ◦ and are parallel to splitting measurements at South Pole. These observations seem to delineate a distinct domain of lithospheric fabric, which may represent the extension of the Darling Mobile Belt or Pinjarra Orogen into the interior of East Antarctica.

Neoproterozoic reworking of the Palaeoproterozoic Capricorn Orogen of Western Australia and implications for the amalgamation of Rodinia

Abstract Argon isotopic data from mica from the southern Capricorn region of Western Australia record complex intra- and inter-grain systematics that reflect modification due to a range of processes. However, 40 Ar/ 39 Ar age distributions, though complex, generally show early Neoproterozoic ages in the west, increasing to Mesoproterozoic ages in the east. Palaeoproterozoic ages associated with cooling after the c. 1.8 Ga Capricorn Orogen or c. 1.6 Ga Mangaroon Orogen are not preserved. These data reflect cooling from a c . 300 °C thermal overprint that took place prior to 960 Ma that is related to the enigmatic Edmundian Orogeny. These data, combined with sediment provenance data from the Early Neoproterozoic Officer Basin and U–Pb age data from the nearby Pinjarra Orogen, indicate that the late Mesoproterozoic–Neoproterozoic Pinjarra and Edmundian events are dynamically linked and reflect tectonic activity on the western margin of the amalgamated West Australian Craton. The temporal framework for this event suggest a link to the evolving Rodinian supercontinent and reflect the oblique collision of either Greater India or Kalahari cratons with the West Australian Craton. These results illustrate that the temporal evolution of poorly preserved orogens can be constrained by low-temperature thermochronology in the adjacent cratons.

Assembling Australia: Proterozoic building of a continent

The western two-thirds of Australia is underlain by Precambrian rocks that are divisible into three Archean to Paleoproterozoic cratons, the West Australian, North Australian and South Australian cratons, separated by Paleoproterozoic to Mesoproterozoic orogens. Prior to about 1500 Ma, the North Australian and South Australian (along with extensions into Antarctica) cratons show a similar geological history and are herein assumed to have evolved as a single entity, termed the Diamantina Craton. The temporal and spatial record of Proterozoic rock units and orogenic events documents accretion and assembly of Precambrian proto-Australia. The Archean Yilgarn and Pilbara cratons were assembled into the West Australian Craton along the Capricorn Orogen during the late Paleoproterozoic (2000 Ma) Glenburgh Orogeny, which then combined with the North Australian segment of the Diamantina Craton along the Paterson Orogen during the 1800–1765 Ma Yapungku Orogeny to form proto-Australia. It was bounded throughout most of the late Paleoproterozoic to earliest Mesoproterozoic along its south western and probably its north eastern margins by subduction zones such that much of the craton occupied an upper plate, backarc basin environment. After ∼1500 Ma, proto-Australia differentiated into two cratonic masses, the combined North Australian and West Australian cratons and the Mawson Craton, consisting of the South Australian Craton and extensions into Antarctica. This breakup, through rotation and lateral translation of the Mawson Craton, resulted in convergence and collisional suturing with the West Australian Craton along the 1345–1140 Ma Albany–Fraser Orogen. The integrated continental assemblage of West Australian, North Australian and South Australian (including parts of Antarctica) cratons are herein referred to as the Great Southern Continent, which persisted until final breakup of Pangea associated with the current northward drift of Australia. The Pinjarra Orogen developed along the margin of the West Australian Craton and records late Mesoproterozoic to Neoproterozoic strike-slip juxtaposition of India within an assembling Gondwana. The Neoproterozoic record of the Terra Australis Orogen, which extends along the eastern side of Precambrian Australia, records rifting and continental breakup within the supercontinent of Rodinia. Australian Proterozoic rocks host significant mineral resources, including world class banded iron-formations in the West Australian Craton (Hamersley), and iron oxide copper gold deposits (Olympic Dam), Pb–Zn–Ag systems (Mount Isa and Broken Hill) and uranium deposits in the Diamantina Craton.

Commentary on the position of higher Himalayan basement in Proterozoic Gondwanaland

Until recently the Higher Himalayan Crystalline Sequence (HHCS) was generally regarded to be the basement of the Himalayan Orogen. Our recent field work in the Kali Gandaki area of western Central Nepal showed that the HHCS is likely continuous with the overlying Tethyan Sedimentary Sequence (TSS) stratigraphically and structurally, and therefore does not form the basement of the Himalayan Orogen (Yoshida et al. 2004); a similar view was already mentioned in some earlier works (e.g., Le Fort 1975, Gehrels et al. 2003). Many workers have placed thrust or normal fault, and even unconformity between the HHCS and TSS; but based on our observation it is difficult to confirm any of these relationships. Our observation also conforms to the geochronologic work by DeCelles et al. (2000) that the HHCS is younger than ca 800 Ma and older than ca 480 Ma; this age-range naturally shows the geochronologic identity or continuation of the HHCS with the so far known base of the TSS. Although the basement of the Himalayan Orogen is not recognized anywhere, its paleoposition can be estimated by identifying the provenance of sediment deposited on the basement. Examination of zircon U-Pb data from HHCS gneisses by DeCelles et al. (2000) showed that the ages of detritus zircons have a range of ca 800~2700 Ma with a main peak at ca 954 Ma; based on these, they suggested the provenance to be the ArabianNubian Shield of the East-African Orogen lying to the west of the Proterozoic Himalayan Basin (Indian coordinate within the East Gondwana assembly, e.g., Yoshida et al. 2003). Robinson et al. (2001) showed the Nd model ages of HHCS to lie between 1.362.29 Ga (with an exceptional age of 2.85 Ga) and eNd (0) is moderately minus (-7.6~19.9, average–13.8); their neodymium isotopic data indicate the source area to have somewhat long crustal history. Recent compilation of geochronologic data from the Arabian-Nubian Shield (Johnson and Woldehaimanot 2003) shows zircon ages as well as neodymium model ages from the Arabian-Nubian Shield are a little too younger when compared with those of HHCS, and even bearing many positive eNd values. Although there are some exceptional data showing older neodymium model with highly minus eNd (0) values for rocks from some small areas of older crustal blocks within the ArabianNubian Shield, it is difficult to expect considerable amount of material supply from these blocks, since they are thought to lie, if any, below the younger geologic units and only a small amount could have exposed. Gehrels et al. (2003) recently reported somewhat older ages of detritus zircons from HHCS than those reported by DeCelles et al. (2000), showing a major peak range of ca 900-1300 Ma with a main peak of ca 1050 Ma. The wide range of Palaeoproterozoic and late Archaean zircons were also reported, although they form minor populations except those of ca 2500 Ma, which forms the second major peak. Thus, the ArabianNubian Shield may not constitute the source for the HHCS. We propose the Pinjarra Orogen (Western Australia, Fitzsimons 2003) lying to the east of the ancient Himalayan basin in the Proterozoic East Gondwana assembly, to be a more possible candidate for the source area that provided material to HHCS, based on their similar geochronologic and crustal signatures and suitable location near by the Himalayan basin. The Pinjarra Orogen suffered peak metamorphism of ca 1000–1100 Ma with later granitic activity of ca 800-650 Ma and superposed by ca 550– 500 Ma Pan-African deformation and metamorphism. Material of rocks in the orogen were derived from Mesoproterozoic Albany-Flaser Orogen of southwestern Australia, in which a variety of older crustal component were incorporated. The ages of rocks from the Pinjarra Orogen are quite conformable with recent zircon data from HHCS (Gehrels et al. 2003). Sporadically known Nd model ages from the Pinjarra Orogen lie between ca 1.6-2.2 Ga, with few exceptions of ca 1.1-1.6 Ga. Thus, it is most likely that the Pinjarra Orogen provided source material for the HHCS. Our conclusion does not support a recent model denoting the Pan-African assembly of East Gondwana (Pisarevsky et al. 2003) in which India and Western Australia do not juxtapose during the Neoproterozoic, but is in favour of the classical model of East Gondwana assembly through the Circum-East Antarctic Orogen of ca 1000 Ma (Yoshida et al. 2003) where Western Australia juxtaposes with northern India.

The seismic structure of Precambrian and early Palaeozoic terranes in the Lambert Glacier region, East Antarctica

The Lambert Glacier region of East Antarctica encompasses the proposed boundary between three of the ancient continents that formed East Gondwana: Indo-Antarctica, the central East Antarctic Craton and a proposed extension of the Pinjarra Orogen of Australia. The only area of extensive rock exposure in central East Antarctica, it uniquely allows the seismic structure to be linked to surface geology. New broadband seismic stations were established at the remote sites of the SSCUA deployment, which ran between the austral summers of 2002/2003 and 2004/2005. Recorded energy from distant earthquakes is used to calculate receiver function waveforms that are then modelled to deduce the seismic structure of the upper lithosphere. The results of this study are two-fold. Firstly, seismic structure and crustal depth are determined beneath the Lambert Glacier region providing constraints on its tectonic evolution. A significant contrast in crustal depth is found between the Northern and Southern Prince Charles Mountains that may indicate the location of a major tectonic boundary. Secondly, baseline seismic receiver structures are established for the Rayner, Fisher and Lambert terranes that may be traced beneath the Antarctic ice sheet in the future.

Pan-Gondwanaland detrital zircons from Australia analysed for Hf-isotopes and trace elements reflect an ice-covered Antarctic provenance of 700–500 Ma age, TDM of 2.0–1.0 Ga, and alkaline affinity

Eastern Australian sediments of Cambrian, Ordovician, Silurian–Devonian, Triassic, and Neogene ages are known to be dominated by zircons dated 700–500 Ma (“Southwest Pacific–Gondwana igneous component”) by the U–Pb SHRIMP method, and thought to be derived from Antarctica, as suggested also by paleogeographical evidence. To extend the characteristics of the provenance we subjected SHRIMPed zircons from the Middle Triassic Hawkesbury Sandstone and four Neogene beach sands to LAM–ICPMS analysis for rock type and Hf-isotope T DM model ages. These data confirm the demonstration (from ages alone) that the beach sands were recycled from the Hawkesbury Sandstone. All five samples have a substantial fraction of 700–500 Ma zircons derived from alkaline rocks with T DM of 2.0–1.0 Ga. We analysed zircons from the nearest exposed alkaline rock of appropriate age in Antarctica: the 550–500 Ma Koettlitz Glacier Alkaline Province of the Ross orogen of the Transantarctic Mountains, emplaced during contemporary transtension. The rock types and T DM of the Koettlitz Glacier Alkaline Province zircons match those of the eastern Australian samples but only over the restricted range of 550–500 Ma. Rocks of 700–550 Ma age and alkaline type are unknown in Antarctic exposures. Rocks of this age and type, however, abound in the Pan-Gondwanaland orogens. The nearest of these is the Leeuwin Complex of the Pinjarra orogen of southwestern Australia, which connects through the Prydz–Leeuwin Belt with the Mozambique Orogenic Belt. In turn, the Leeuwin Complex is dominated by 700–500 Ma alkaline rocks generated by contemporary transtension. We find that zircons from a beach sand at Eneabba, 450 km north of the Leeuwin Complex, are dominated by alkaline types, so confirming demonstration by age alone that the sand came from the Leeuwin Complex. An important inference is that Pan-Gondwanaland terranes are potential provenances of sediment with zircons of ages 700–500 Ma, model ages 2.0–1.0 Ga, and alkaline affinity. We compiled the age spectra of our samples with others of the Southwest Pacific–Gondwana igneous component in southern Australia and reviewed their paleogeographical data. The Early Cambrian turbidites of the Kanmantoo Group were deposited from an axial northward paleocurrent that flowed across adjacent Antarctica in Wilkes Land. The Ordovician turbidite was deposited in fore-arc and abyssal fans likewise from a northward paleocurrent that flowed parallel to the uplifted Ross–Delamerian orogen. The Ordovician–Silurian–Devonian Mathinna Supergroup was deposited in a submarine fan system that prograded to the east-northeast from high ground in Antarctica. The Hawkesbury Sandstone was deposited in a braided-river system on a surface that sloped to the north-northeast from high ground in Antarctica. Together with the zircon data, the paleogeographical evidence in the Cambrian, Ordovician, Ordovician–Silurian–Devonian, and Triassic all point to a provenance in Wilkes Land, Antarctica of Pan-Gondwanaland (700–500 Ma) terranes with alkaline rocks.

Provenance of the Earaheedy Basin: implications for assembly of the Western Australian Craton

Abstract The Palaeoproterozoic Earaheedy Basin in Western Australia lies at the southern end of the Capricorn Orogen. The Earaheedy Group, the sole lithostratigraphic unit comprising the basin fill, is a 5 km thick succession of shallow-marine siliciclastic and carbonate sedimentary rocks. It consists from oldest to youngest of the Yelma, Frere, Windidda, Chiall, Wongawol, Kulele and Mulgarra formations. U–Pb age data for 312 detrital zircon grains from seven siliciclastic samples from the Yelma Formation, the Wandiwarra and Princess Ranges members of the Chiall Formation and the Mulgarra Sandstone reveal the presence of both Archaean (3.4–2.5 Ga) and Palaeoproterozoic (2.4–1.8 Ga) detritus. Zircon grains of Archaean age are dominantly in the range 2.7–2.6 Ga, similar to those of the Yilgarn Craton, which unconformably underlies the southern margin of the Earaheedy Basin. Age peaks in the range 2.3–2.2, 2.0 and 1.8 Ga, characterise the Palaeoproterozoic detritus. The two younger ages overlap with those of the adjoining Gascoyne Complex of the Capricorn Orogen to the west of the basin. Source components with ages in the range 2.3–2.2 Ga are extremely rare in Western Australia. Undeformed granite in the Mullingarra Complex within the Pinjarra Orogen to the west of the Darling Fault yielded an age of around 2.2 Ga. In addition, rare rims on detrital zircon grains from gneisses within basement of the Glenburgh Terrane of the Gascoyne Complex as well as a few detrital grains in metamorphosed quartz arenites of the terrane have yielded ages of around 2.4–2.2 Ga, suggesting early Palaeoproterozoic activity in this region. Detrital grains as young as 1800 Ma, combined with deposition of the unconformably underlying strata in the Yerrida Basin at c. 1840 Ma and deformation of the Earaheedy Group at around 1750 Ma, provide a relatively tight constraint on the depositional age of the Earaheedy Group.

Proterozoic basement provinces of southern and southwestern Australia, and their correlation with Antarctica

Abstract Three Precambrian basement provinces extend from the southern coast of Australia into East Antarctica when reconstructed in a Gondwana configuration. These are, from east to west, the Mawson Craton, and the Albany-Fraser and Pinjarra Orogens. The Mawson Craton preserves evidence for tectonic activity from the late Archaean until the earliest Mesoproterozoic. It is exposed in the Gawler Craton of South Australia, the Terre Adélie and King George V Land coastline of East Antarctica, and the Miller Range of the central Transantarctic Mountains. It may form a significant part of the ice-covered East Antarctic Shield, although insufficient data are available to constrain its lateral extent. The Mawson Craton underwent late Palaeoproterozoic tectonism along its eastern margin (the Kimban Orogeny) and the occurrence of c. 1700Ma eclogites in the Transantarctic Mountains implies that this was, in part, a collisional event, although elsewhere it was characterized by low P/T metamorphism. The western margin of the Mawson Craton collided with a continental fragment comprising the Nawa Domain of the Gawler Craton, the Coompana Block and the Nornalup Complex of Western Australia at c. 1560Ma during the Kararan Orogeny. The western edge of the Nornalup Complex later collided with the Biranup and Fraser Complexes and Yilgarn Craton to form the Albany-Fraser Orogen during two stages of tectonism at c. 1350–1260 and 1210–1140Ma. The Pinjarra Orogen truncates the western margin of the Yilgarn Craton and Albany-Fraser Orogen, and contains allochthonous 1100–1000Ma gneissic blocks transported along the craton margin during at least two stages of Neoproterozoic transcurrent movement. It divides East Gondwana into Australo-Antarctic and Indo-Antarctic domains, which are distinct continental fragments with different Proterozoic histories that were juxtaposed by oblique collision at 550–500Ma during the assembly of Gondwana. The path taken by the Pinjarra Orogen beneath the Antarctic ice sheet is unknown, but it is of similar width and length to the East African Orogen, and must have been a fundamental Neoproterozoic boundary of critical importance to supercontinent assembly and breakup.

Monazite “in situ” 207Pb/ 206Pb geochronology using a small geometry high-resolution ion probe. Application to Archaean and Proterozoic rocks

This paper reports the application of secondary ion mass spectrometry (SIMS) using a small geometry Cameca IMS4f ion probe to provide reliable in situ 207Pb/ 206Pb ages on monazite populations of Archaean and Proterozoic age. The reliability of the SIMS technique has been assessed on two samples previously dated by the conventional ID-TIMS method at 2661±1 Ma for monazites extracted from a pelitic schist from the Jimperding Metamorphic Belt (Yilgarn Craton, Western Australia) and 1083±3 Ma for monazites from a high-grade paragneiss from the Northampton Metamorphic Complex (Pinjarra Orogen, Western Australia). SIMS results provide 207Pb/ 206Pb weighted mean ages of 2659±3 Ma ( n=28) and 1086±6 Ma ( n=21) in close agreement with ID-TIMS reference values for the main monazite growth event. Monazites from the Northampton Complex document a complex history. The spatial resolution of about 30 μm and the precision achieved successfully identify within-grain heterogeneities and indicate that monazite growth and recrystallisation occurred during several events. This includes detection of one inherited grain dated at ca. 1360 Ma, which is identical to the age of the youngest group of detrital zircons in the paragneiss. Younger ages at ca. 1120 Ma are tentatively interpreted as dating a growth event during the prograde stages of metamorphism. These results demonstrate that the closure temperature for lead diffusion in monazite can be as high as 800 °C. At last, ages down to ca. 990 Ma are coeval with late pegmatitic activity and may reflect either lead losses or partial recrystallisation during the waning stages of metamorphism. A third unknown sample was analysed to test the capability of the in situ method to date younger monazite populations. The sample, a pelitic metatexite from Northwestern Hoggar (Algeria), contains rounded metamorphic monazites that provide a 207Pb/ 206Pb weighted mean age of 603±11 Ma ( n=20). This age is interpreted as recording emplacement of a gabbronoritic body during amphibolite facies regional metamorphism and is representative of the late pulse of the Pan-African tectonometamorphic evolution in the western part of the Tuareg shield. In situ SIMS analyses using a widely available, small geometry ion probe, can thus be successfully used to accurately determine ages for complex Precambrian monazite populations.

Provenance record of a rift basin: U/Pb ages of detrital zircons from the Perth Basin, Western Australia

Abstract U/Pb dating of 588 detrital zircons by ion microprobe from Lower Triassic, Permian and lower Paleozoic sandstone samples from the Perth Basin yield ages ranging from Archean to early Paleozoic. The detrital age spectrum of Lower Triassic samples differ from underlying units in lacking Archean and post-Mesoproterozoic detritus. The Archean Yilgarn craton has previously been envisaged as the main source of detritus in the Perth basin. The bulk of the detritus, however, is of Mesoproterozoic age, which together with the overall broad spectrum of age ranges within individual samples indicates that the detritus was derived from multiple sources. Potential source regions recognized include the Yilgarn craton, the Mesoproterozoic Albany Fraser Orogen, the Meso- to Neoproterozoic Leeuwin complex of the Pinjarra orogen, and probably also the late Mesoproterozoic Northampton complex of the Pinjarra orogen. In addition, source terranes possibly associated with Greater India and East Antarctica, which originally formed the western and southern margins of the basin, may have also contributed detritus accounting for the input of material whose age cannot be related to the known age ranges in currently exposed bounding source blocks. The multi-sourced nature of the detritus suggests the Perth basin was not a simple half-graben structure down stepping to the west but contained a variety of uplifted blocks with an overall longitudinal supply of detritus from the south. The distribution of exposed source blocks changed dramatically at the start of the Triassic with the cessation of input from Archean and Neoproterozoic source regions. The differing age spectra between Triassic and Permian strata and between Late Permian and older sequences suggest any reworked detritus is of extra-basinal origin and not related to intra-basin erosion of underlying rock units. Potential sources for this reworked multicycle detritus are the metasedimentary sequences within the bounding source terranes.