Quartz-rich Turbidites Research Articles

Stratigraphic and igneous relationships west of Yass, eastern Lachlan Orogen, southeastern Australia: subsurface structure related to caldera collapse?

Volcanic rocks are widespread in middle Silurian to Lower Devonian rocks of the Lachlan Orogen in southeastern Australia. In the Yass–Canberra region, the Silurian succession consists of two S-type dominantly ignimbrite suites with the lower Hawkins/Goobarragandra and upper Laidlaw volcanics separated and overlain by clastic-carbonate intervals, and unconformably overlain by Lower Devonian silicic volcanic rocks. In the Talmo area, 30 km west of Yass, the Goobarragandra Volcanics are overlain by a clastic-carbonate succession of the Talmo Formation, which has a partly faulted boundary with the Lower Devonian Mountain Creek Volcanics containing rheomorphic welded ignimbrites. The contact of the Mountain Creek Volcanics and the Talmo Formation coincides with the margin of a prominent ovoid gravity low. Dip patterns in the Talmo Formation indicate two orientation domains divided by a northwest-trending faulted boundary. The domain to the southwest consists of gently northeast-dipping interbedded limestone and mudstone whereas the domain to the northeast has steep to moderate dips to the south and southeast with interbedded limestone and mudstone overlain by massive mudstone and quartz-rich turbidites. These contrasting domains are interpreted as a result of subsurface differential downsag, possibly a caldera, accommodated by downwarping in the adjacent Talmo Formation and accompanied by movements across a northwest-trending fault and potential ring faults.

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.

Different styles of modern and ancient non-collisional orogens and implications for crustal growth: a Gondwanaland perspective

Non-collisional, convergent margin orogens are generally called accretionary orogens, although there may not have been horizontal accretion across the plate boundary. We revive the term non-collisional orogen and use a Gondwanaland perspective to discuss different types. On the northern margin of the Australian Plate, the New Guinea non-collisional, accretionary orogen was formed by large-scale terrane accretion across an advancing plate margin. On the eastern margin, the Southwest Pacific Orogen is a non-collisional and non-accretionary orogen, involving virtually no horizontal transfer of material across its eastward-retreating plate boundary. In the Tasmanides, the Lachlan Orogen, commonly described as an accretionary orogen, is another non-collisional, non-accretionary orogen developed behind the plate margin after major Cambrian rollback, with resultant backarc basins filled mainly by quartz-rich turbidites subsequently recycled. The outboard New England Orogen is a non-collisional but accretionary orogen, marked by the frontal accretion of continental margin arc detritus, subsequently recycled into younger arcs. The Permian to Cretaceous Rangitata Orogen of New Zealand is an ?oblique non-collisional, accretionary orogen in which Permian–Triassic sediments of the accretionary wedge have no link with inboard (near) arc terranes. Late Jurassic to Cretaceous parts were sourced by a combination of first cycle volcanogenic detritus passing through the forearc basin together with recycling of the exhumed parts of the wedge. All non-collisional orogens involve continental growth, but only the New England Orogen and to a lesser extent the New Guinea Orogen involve significant crustal growth.

Refining accretionary orogen models for the Tasmanides of eastern Australia

The well-known southwest-to-northeast younging of stratigraphy over a present-day cross strike distance of >1500 km in the southern Tasmanides of eastern Australia has been used to argue for models of accretionary orogenesis behind a continually eastwards-rolling paleo-Pacific plate. However, these accretionary models need modification, since the oldest (ca 530 Ma) outcrops of Cambrian supra-subduction zone rocks occur in the outboard New England Orogen, now ∼900 km east of the next oldest (520–510 Ma) supra-subduction zone rocks. This is not consistent with simple, continuous easterly rollback. Instead, the southern Tasmanides contain an early history characterised by a westwards-migrating margin between ca 530 and ca 520 Ma, followed by rapid eastwards rollback of the paleo-Pacific plate from 520 to 502 Ma that opened a vast backarc basin ∼2000 km across that has never been closed. From the Ordovician through to the end of the Carboniferous, the almost vertical stacking of continental margin arcs (within a hundred kilometres of each other) in the New England Orogen indicates a constant west-dipping plate boundary in a Gondwana reference frame. Although the actual position of the boundary is inferred to have undergone contraction-related advances and extension-related retreats, these movements are estimated to be ∼250 km or less. Rollback in the early Permian was never completely reversed, so that late Permian–Triassic to Cretaceous arcs lie farther east, in the very eastern part of eastern Australia, with rifted fragments occurring in the Lord Howe Rise and in New Zealand. The northern Tasmanides are even more anomalous, since they missed out on the middle Cambrian plate boundary retreat seen in the south. As a result, their Cambrian-to-Devonian history is concentrated in a ∼300 km wide strip immediately west of Precambrian cratonic Australia and above Precambrian basement. The presence in this narrow region of Ordovician to Carboniferous continental margin arcs and backarc basins also implies a virtually stationary plate boundary in a Gondwana frame of reference. This bipolar character of the Tasmanides suggests the presence of a segmented paleo-Pacific Plate, with major transform faults propagating into the Tasmanides as tear faults that were favourably oriented for the formation of local supra-subduction zone systems and for subsequent intraplate north–south shortening. In this interpretation of the Tasmanides, Lower–Middle Ordovician quartz-rich turbidites accumulated as submarine fan sequences, and do not represent multiple subduction complexes developed above subduction zones lying behind the plate boundary. Indeed, the Tasmanides are characterised by the general absence of material accreted from the paleo-Pacific plate and by the dominance of craton-derived, recycled sedimentary rocks.

Detrital zircon U–Pb ages of metasedimentary rocks from Sierra de Valle Fértil: Entrapment of Middle and Late Cambrian marine successions in the deep roots of the Early Ordovician Famatinian arc

We obtained detrital zircon ages from three metasedimentary rocks with siliciclastic protoliths that were buried to paleodepths of 20–27 km during Early Ordovician Famatinian arc magmatism. In all three samples, U–Pb zircon age distributions are polymodal but the dominant peaks make well-defined clusters, matching the ages of orogenic systems that characterize West Gondwana. The most prominent peaks of constituent age populations at 520 Ma, 600 Ma, and 1050 Ma reflect derivation from sources that are characteristic of the West Gondwana margin. Moreover, grains with Meso- and Paleo-Proterozoic ages appear as small fractions (<5%). The siliciclastic protoliths to metasedimentary rocks from the deep Famatinian crust are dominantly composed of detritus derived from two sources: 1) a nearby magmatic arc or a rapidly exhuming orogen, and 2) sedimentary recycling of Neoproterozoic–Lower Cambrian sedimentary successions broadly grouped as Puncoviscana Formation. Our findings reveal that granulite–facies metasedimentary rocks in the deep crust of the Famatinian arc have detrital zircon patterns closely resembling those found in nearby Middle to Upper Cambrian quartz-rich turbidite sequences from central Famatina, the Cordillera Oriental and the Puna. At the regional scale, these sedimentary successions yield maximum depositional ages corresponding to the Middle Cambrian (<520 Ma). This indicates that metasedimentary rocks in the Sierra de Valle Fértil were rapidly buried to deep levels after deposition. Our results are also consistent with the existence of a crustal scale paleo-suture located at the western margin of the Sierras Valle Fértil and La Huerta that explains the differences of detrital zircon age patterns between metasedimentary rocks in the Valle Fértil and age-equivalent sedimentary units in the nearby Cuyania Terrane.

Controls on Devonian–Carboniferous magmatism in Tasmania, based on inherited zircon age patterns, Sr, Nd and Pb isotopes, and major and trace element geochemistry

Devonian–Carboniferous granites are widespread in Tasmania. In eastern Tasmania, Devonian granites intrude Ordovician–Early Devonian quartz-rich turbidites of the Mathinna Supergroup. The earliest (∼400 Ma) I-type granodiorites may be arc-related. Following the Tabberabberan Orogeny (∼389 Ma), more felsic and, finally, strongly fractionated I- and S-type granites were emplaced until ∼373 Ma. In contrast, western Tasmania granites intrude a more diverse terrane of predominantly marine shelf successions, with depositional ages as old as Late Mesoproterozoic. They are mostly felsic and fractionated I- and S-types emplaced from ∼374–351 Ma, possibly in response to post-collisional crustal extension following juxtaposition of the eastern and western Tasmanian terranes. Granites from the two terranes are readily distinguishable by the age spectra of their inherited zircon, which are noticeably similar to those of the detrital zircon from sedimentary successions in their respective terranes. Furthermore, within each terrane, both I and S-types yield similar inheritance patterns. This suggests a pivotal role for the sedimentary successions in the petrogenesis of both types. Western Tasmanian granites are also enriched in ∼1600 Ma zircon, which is essentially unrepresented in the exposed supracrustal succession. Subtle differences between the inheritance and detrital age spectra in eastern Tasmania probably relate to unrepresentative sampling of the supracrustal rocks. Nd, Sr and Pb isotopic characteristics of the granites are consistent with their derivation by mixing of magmas derived from the mantle, possibly the lower crust, and from supracrustal rocks. Systematic isotopic trends in some eastern Tasmanian I-types, particularly in the Scottsdale Batholith, correlate well with major and trace element geochemistry and age. The isotopes are inconsistent with simple restite unmixing or crystal fractionation in a closed magma chamber, and indicate progressive contamination by the Mathinna Supergroup, or similar rocks. The isotopic characteristics of late, strongly fractionated granites, although sometimes obscured by hydrothermal alteration, are also consistent with concurrent assimilation-fractional crystallisation processes. Together with the close association of some strongly fractionated I- and S-types, this suggests that such granites were generated directly in the lower crust, and were not derived from unfractionated parental granite magmas.

Syngenetic gold in western Victoria: occurrence, age and dimensions

In western Victoria, a widespread stratiform style of gold enrichment in Palaeozoic black mudstone and chert—clearly different from the classic mesothermal quartz vein deposits of the Victorian goldfields—has been confirmed by whole-rock geochemistry and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). This enrichment pre-dates compaction, deformation and low-grade metamorphism of the sedimentary host-rocks, and therefore possibly developed diagenetically during slow deposition of the thin carbonaceous black mudstone beds and the thinner layers of chert. These paired strata have been documented at many locations in three regional outcrop areas of chevron-folded Cambrian and Lower Ordovician metasediments in the Stawell and Bendigo Zones, where they are interbedded with quartz-rich turbidites. The layers were named ‘indicators’ by the early miners, who found locally rich nuggety gold deposits at intersections between these layers and mesothermal quartz veins. Gold is present in euhedral pyrite crystals in both black shale and chert. LA-ICPMS analysis of individual pyrite crystals in the indicator beds shows that pyrite is enriched in Au, As, Sb, Se, Te and Bi. The Au content of pyrite varies from 0.03 to 2.69 ppm with a mean of 0.58 ppm and shows a positive correlation with As, which varies from 1000 to 6000 ppm. Many pyrite crystals show enrichment of gold in their cores and depletion in their rims, confirming the likely syngenetic or syndiagenetic accumulation of gold during pyrite formation in the sediments. Prior to regional metamorphism, folding and faulting, the many indicator strata in the outcrop areas were parts of an extensive marine sequence of Late Cambrian and Early Ordovician age. The former primary source of this mineralisation is considered to have been one or more contemporaneously exhalative submarine hydrothermal systems. Thus, the older Palaeozoic sediments of the western Lachlan Fold Belt were significantly enriched in syngenetic gold in the Early Palaeozoic, at least 40 million years before emplacement of the quartz – gold vein deposits of the goldfields.

Re-evaluation of contact relationships between Ordovician volcanic belts and the quartz-rich turbidites of the Lachlan Orogen*

Some published tectonic reconstructions of the eastern Lachlan Orogen in New South Wales have shown Ordovician volcanic and volcaniclastic rocks of the Macquarie Arc conformably overlying or interfingering with a coeval Ordovician quartz-rich turbidite sequence. Re-examination of key contacts between the volcanic and quartz-rich successions has found no evidence to support this interpretation, and suggests that the two packages are separate tectonostratigraphic terranes. The contacts between these two coeval successions are generally marked by major faults containing mylonites, cataclasites and, at some locations, fragments of mid-ocean ridge-type pillow basalt and chert. The quartz-rich turbidites are generally highly deformed and of higher metamorphic grade than the adjacent volcanics. At Oberon and Mudgee, contacts are faulted but there are no mylonites or significant differences in metamorphic grade. At Palmers Oaky and Black Springs, Silurian quartz-rich sandstones overlying the Ordovician volcanics have been mistakenly assigned to the Ordovician in previous studies. Throughout the Lachlan Orogen, there is no mixing of framework grains. Quartz-rich turbidite successions are dominated by quartz with lesser feldspar and rare tourmaline, zircon and monazite derived from recycled continental sources. In contrast, the volcaniclastic sandstones contain feldspar, clinopyroxene and lithic fragments derived from subduction-related clinopyroxene-phyric basalt and plagioclase-phyric andesite. Detrital-zircon populations also differ, with separate U/Pb age populations and almost no overlap. Comparison of the Ordovician sequences of the Lachlan Orogen with modern turbidites from continental- and arc-related sedimentary basins suggests that complete separation of sedimentary sources is only possible if the sandstones were deposited hundreds of kilometres apart, in separate tectonic environments. The two sequences were juxtaposed along major faults in the Late Ordovician or Early Silurian, probably when the Macquarie Arc collided with a thick Ordovician sedimentary wedge located on the Gondwanan continental margin.

Benambran Orogeny in the Eastern Lachlan Orogen, Australia

The Benambran Orogeny reflects the accretion of the intra-oceanic Macquarie Arc to the Gondwana Plate. The ultimate cause was the northwards strike-slip transport of the allochthonous Bega Terrane along the eastern margin of Gondwana, into a forearc position outboard of the Macquarie Arc. Ensuing oblique compression in the outboard part of the Gondwana Plate drove the Macquarie Arc into and under its backarc Wagga Basin, represented by the Girilambone – Wagga Terrane, and led to a combination of thrusting and major strike-slip faulting within, inboard and outboard of the arc over 10 million years. In this time frame, the Benambran Orogeny in the Eastern Subprovince of the Lachlan Orogen in New South Wales consists of two phases of exhumation – deformation, at ca 443 Ma (late Bolindian to early Llandovery) and ca 430 Ma (late Llandovery), separated by a relaxation/extensional event. Both phases involved deformation and exhumation of the Macquarie Arc and coeval quartz-rich turbidites and black shales of the Adaminaby Superterrane. Closure of the former backarc basin was facilitated by limited east-dipping subduction that generated the short-lived Fifield arc. The second phase of the Benambran Orogeny also involved deformation and exhumation of overlying Llandovery strata (e.g. the Yalmy Group) and syn- to post-tectonic emplacement of granitoids. In both phases, deformation of Adaminaby Superterrane rocks was more intense than deformation of arc rocks.

Evidence of Early to Middle Ordovician arc volcanism in the Cordillera Oriental and Altiplano of southern Peru, Ollantaytambo Formation and Umachiri beds

Volcanic rocks of intermediate to predominantly mafic composition are exposed in the Cordillera Oriental and Altiplano of southern Peru in two localities: (1) the Ollantaytambo Formation, which crops out near Cusco, and (2) the Umachiri beds in the Departamento de Puno. The most notable unit in the Ollantaytambo Formation is an approximately 100 m thick succession of lapilli tuffs that originated by hydroclastic fragmentation. These volcanic rocks are of calc-alkaline basaltic to andesitic composition. The tuffs are overlain by fine-grained volcaniclastic sandstones and shales, which grade into quartz-rich turbidites. The Umachiri beds consist of monomict lapilli tuffs with varying lapilli abundance and size, formed by hydroclastic fragmentation, and volcaniclastic sandstones. The rocks have a uniform basaltic andesite composition and tholeiitic affinity. Chondrite-normalized element patterns of both volcanigenic units show a moderate enrichment of LREE and a weak negative Eu anomaly. Pronounced negative Nb and Ta anomalies indicate magma evolution in an arc environment. In contrast, the overlying epiclastic sandstones and shales in the Ollantaytambo Formation reflect upper crustal sources. Our data indicate that the deposition of the lapilli tuffs of the Ollantaytambo Formation and Umachiri beds likely was connected to the westward-lying Ordovician magmatic arc on the Arequipa Massif. If the Ordovician active margin in southern Peru had the same east-dipping subduction polarity as the active Gondwana margin in Argentina and Chile, the sedimentary basin in which the Ollantaytambo Formation and Umachiri beds were deposited was in a backarc position.

Cambrian backarc-basin basalt in western Victoria related to evolution of a continent-dipping subduction zone

Lower Cambrian basalts in the Stawell Zone (e.g. Magdala Basalt) are intercalated with a thick package of continentally derived quartz-rich turbidites, whereas similar basalts in the Heathcote and Mt Wellington Greenstone Belts are underlain by broadly coeval boninites and overlain by Upper Cambrian cherts. These marked differences in the magmatic – stratigraphic associations of the Early Cambrian basalts in Victoria have led to highly varied interpretations of their tectonic setting, which include subduction zones that are east-dipping, west-dipping and even a combination of east- and west-dipping systems. Here, we present new geochemical data for Cambrian basalts from the Stawell Zone and reassess the palaeogeographic and palaeotectonic setting of the region. Our geochemical data show that basalts in the Stawell Zone may be broadly divided into two units: basalts that are LREE-depleted relative to N-MORB and have low (≤3.0 ppm) Nb contents; and basalts that are slightly LREE-enriched relative to N-MORB and have relatively high (>3.0 ppm) Nb contents. The LREE-depleted Nb-poor basalts are the most abundant type in the Stawell Zone and generally occur at lower stratigraphic levels than the Nb-rich basalts. While the slightly LREE-enriched basalts are most similar to the backarc-basin basalts in the Heathcote and Mt Wellington Greenstone Belts, all basalts from the Stawell Zone display characteristics similar to modern backarc-basin basalts from the West Pacific. We propose that basaltic magmatism in the Stawell Zone was generated during a widespread backarc extension event that affected much of western Victoria. However, important stratigraphic – magmatic associations in the Stawell Zone, including the absence of forearc-generated boninites and the presence of a thick (>300 m) package of quartz-rich sandstone and mudstone underlying the Early Cambrian basalts, indicate that the Stawell Zone was relatively close to (or possibly at the outer margins of) the East Gondwanan continental margin and thus always distal to the forearc (cf. Heathcote and Mt Wellington Greenstone Belts). These characteristics indicate that basaltic magmatism in the Stawell Zone occurred in the backarc of a west-dipping subduction zone that initiated (i.e. generated) boninites farther east and southeast before undergoing rapid east-directed rollback.

Reconstructing the event stratigraphy from the complex structural – stratigraphic architecture of an Archaean volcanic – intrusive – sedimentary succession: the Boorara Domain, Eastern Goldfields Superterrane, Western Australia

Two main deformational phases are recognised in the Archaean Boorara Domain of the Kalgoorlie Terrane, Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia, primarily involving south-over-north thrust faulting that repeated and thickened the stratigraphy, followed by east-northeast – west-southwest shortening that resulted in macroscale folding of the greenstone lithologies. The domain preserves mid-greenschist facies metamorphic grade, with an increase to lower amphibolite metamorphic grade towards the north of the region. As a result of the deformation and metamorphism, individual stratigraphic horizons are difficult to trace continuously throughout the entire domain. Volcanological and sedimentological textures and structures, primary lithological contacts, petrography and geochemistry have been used to correlate lithofacies between fault-bounded structural blocks. The correlated stratigraphic sequence for the Boorara Domain comprises quartzo-feldspathic turbidite packages, overlain by high-Mg tholeiitic basalt (lower basalt), coherent and clastic dacite facies, intrusive and extrusive komatiite units, an overlying komatiitic basalt unit (upper basalt), and at the stratigraphic top of the sequence, volcaniclastic quartz-rich turbidites. Reconstruction of the stratigraphy and consideration of emplacement dynamics has allowed reconstruction of the emplacement history and setting of the preserved sequence. This involves a felsic, mafic and ultramafic magmatic system emplaced as high-level intrusions, with localised emergent volcanic centres, into a submarine basin in which active sedimentation was occurring.

Structure, detrital zircon U – Pb ages and 40Ar/39Ar geochronology of the Early Palaeozoic Girilambone Group, central New South Wales: subduction, contraction and extension associated with the Benambran Orogeny

The Girilambone Zone is located in the northern part of the central subprovince of the Lachlan Fold Belt and its bedrock exposure includes the widespread Girilambone Group. This unit consists of two main components, low-grade, metamorphosed quartz-rich turbidites and bedded chert with Middle to Late Ordovician conodont ages in the west (Ballast Formation), and low to moderate-grade schist, psammitic schist, mafic schist and phyllite that occur in the east. Detrital zircon grains from samples of a quartzite and a quartzose psammite from the eastern part of the Girilambone Group around Girilambone, have age spectra (600 – 500 Ma) typical of Ordovician turbidite successions in southeastern Australia. In the Girilambone district, an intense, steeply dipping differentiated layering (S2) formed in the D2 deformation and has largely masked the effects of earlier deformation apart from foliation (S1). S2 is overprinted by mainly north-trending, moderately to steeply west-dipping crenulation cleavage (S3) that is axial planar to abundant open to close folds (D3 deformation). Locally, S3 cleavage and fold axial planes have a low dip. In the Tottenham district, the main foliation is also a differentiated layering (S2) with low to moderate dips to the southeast in a northern domain and subhorizontal in a larger southern domain due to folding by abundant near-isoclinal, recumbent folds (F3). These reflect ductile thinning of the crust and are attributed to extensional deformation. Weak to locally intense crenulation cleavage and associated folds (D4) affect the S2 in the northern domain and are weakly developed in the southern domain. Three muscovite 40Ar/39Ar ages from both the Girilambone and Tottenham regions are ca 435 Ma consistent with deformation and metamorphism during the Late Ordovician – Early Silurian Benambran Orogeny. The Girilambone Group in the Late Ordovician was accreted in an inferred subduction complex related to an east-dipping subduction zone in the Wagga marginal sea. The subduction complex in the Early Silurian was affected by contractional and extensional deformation prior to the migration of igneous activity from the Molong island arc to the Wagga – Omeo Zone.

Evolution and Structure of the Lachlan Fold Belt (Orogen) of Eastern Australia

The Lachlan Fold Belt (Lachlan Orogen) of eastern Australia was part of a Paleozoic convergent plate margin that stretched around the supercontinent of Gondwana from South America to Australia. Lower Paleozoic (545–365 Ma) deep-water, quartz-rich turbidites, calcalkaline volcanic rocks, and voluminous granitic plutons dominate the Lachlan Orogen. These rocks overlie a mafic lower crust of oceanic affinity. Shortening and accretion of the Lachlan occurred through stepwise deformation and metamorphism from Late Ordovician (∼450 Ma) through early Carboniferous times, with dominant events at about 440–430 Ma and 400–380 Ma. The development and accretion of the Lachlan Orogen and other related belts within the Tasmanides added about 2.5 Mkm2 to the surface area of Gondwana. The sedimentary, magmatic, and deformational processes converted an oceanic turbidite fan system into continental crust of normal thickness. The addition of this recycled continental detritus and juvenile material to Australia represents an under-recognized continental crustal growth mechanism that has been important thoughout earth history.

The structural setting and contact metamorphism of the Wonga gold deposit, Victoria, Australia

Gold quartz vein deposits in the Wonga mine area are fault-related structures, occur in a low-grade, regionally metamorphosed, Cambrian quartz-rich turbidite sequence (Wonga schist) interlayered with mafic volcanics, and postdate a suite of quartz-feldspar porphyry dikes. The nonplanar geometry of these vein deposits can be explained by their location within a shear system as en echelon dilations, in combination with refraction across rheological interfaces within the sediment package. The gold mineralization and related deformation occurred under low- to midgreenschist facies conditions. The intrusion of the Early Devonian Stawell granite, at approximately 400 Ma, has contact metamorphosed the Wonga schist, a preexisting suite of porphyry dikes and the first stage of mineralization. Typically, mineralization and alteration occur in discrete dilational openings as a package of mineralized structures related to a D 4 deformation event and fluid injection. The first stage of gold mineralization and accompanying alteration was deposited in 350 degrees -trending 25 degrees to 50 degrees east-dipping fracture sets within a 320 degrees -trending shear system (Eastern and Western shears), and a 240 degrees -trending 40 degrees to 70 degrees southeast-dipping extensional fracture set (link structures).Both the Eastern and Western shears are anastomosed high-strain zones, up to 50 m in width, and are connected at a high angle by linking mineralized structures that begin to dominate as the main mineralized lodes persist with depth, where the mineralized link has an east-west trend. The lodes located within the Eastern and Western shear zones are oblique shear veins, whereas the link structures are large-scale tensile fractures in the block between the zones of shear zone-hosted mineralization. These D 4 features initiated in a regional northeast-southwest crustal shortening episode. Subsequent deformation was then accommodated by sinistral oblique slip during a north-south compressional event. The mineralized lodes are locally influenced by a set of northwest-trending, shallow northeast-dipping faults and refracted across the preexisting porphyry dikes.Postdating the Stawell granite is a second stage of hydrothermal fluid introduction along some northwest-trending, northeast-dipping faults (D 5 event). This fluid flow is focused along the reverse faults and gave rise to local retrogression of peak contact metamorphic assemblages producing local carbonate-sericite alteration and further gold mineralization. Subsequent fluid activity has produced carbonate alteration and/or minor sulfide mineralization that can be related to either the Stawell granite, late fractures, or a suite of lamprophyre dikes.

Provenance of early Palaeozoic sandstones, southeastern Australia, Part 2: cratonic to arc switching

Abstract Early Palaeozoic strata of the northeastern Lachlan Fold Belt, Australia, consist of an Early Ordovician sequence (Adaminaby Group) of quartz-rich turbidites and chert, and the conformably overlying Late Ordovician–?Early Silurian Coomber Formation, a mafic volcaniclastic sequence of lithic sandstone and mudstone with mafic volcanic and intrusive rocks. Analysis of detrital modes from the Adaminaby Group indicate a uniform quartzose petrofacies (Q 91 F 2 L 7 ) composed of deep-water quartz and sublithic sandstones derived from the west. By contrast, the lithic sandstone of the Coomber Formation represents a volcanolithic petrofacies (Q 1 F 34 L 65 ) of mainly intermediate volcanic detritus with a tendency to become more silicic up-section. These sandstones are considered to be derived from a calc-alkaline andesitic–basaltic volcanic centre in the eastern Lachlan Fold Belt. The change from a quartz-rich petrofacies to a volcanolithic petrofacies was sharp with no evidence of mixing. This change primarily reflects a rapid outpouring of mafic volcanic material in the Late Ordovician, together with an approximately concurrent eustatic rise in sea level.

Strain, cleavage and microstructure variations in sandstone: implications for stiff layer behaviour in chevron folding

Sandstone layers within tightly chevron-folded quartz-rich turbidites of the Lachlan Fold belt southeastern Australia show a fanning spaced cleavage overprinted by a non-fanning grain alignment cleavage, low internal strains recorded by quartz grain separations ( XZ ≈ 1.4−1.8) and low fold flattening strains ( XZ < 1.8). Angular relations between spaced cleavage and bedding in folds with varying tightness suggest that sandstone layers initially act as stress-supporting members with spaced cleavage development due to pressure solution taking place at limb dips up to 15°. During fold development sandstone layers undergo rigid body rotation involving interlayer slip without marked internal deformation except at fold hinges. After fold lock-up bedding-spaced cleavage angles change due to inner arc collapse by pressure solution and subsequent flattening with formation of a grain alignment cleavage and mica beard growth. Internal deformation of sandstone is characterized by microstructures related to solution-redeposition processes and growth of mica. Chevron folds form by homogeneous shortening and pressure solution due to layer-parallel shortening, tangential longitudinal strain due to buckling, fold tightening by inner arc collapse due to pressure solution and fold flattening at metamorphic peak with growth of mica. Geometric modelling of internal strains and bedding-cleavage angles requires a significant component of interlayer slip during chevron fold development under greenschist facies conditions.

Geologic framework and tectonic evolution in Western Victoria, Australia

The Stawell Zone is interpreted to be part of the eastern extension of the Adelaide Fold Belt into Victoria. The Cambro-Ordovician turbidite sequence of the Stawell and Glenelg zones rests on a pile of Late Proterozoic metavolcanic rocks, rather than Cambrian metavolcanic units that are characteristic of the Lachlan Fold Belt. Early deformation events, D 1–D 3, in the Stawell Zone pre-date granite emplacement and were synchronous with regional fold-forming events that accompany thrust movements along discrete detachment surfaces. The thrust system follows a NE-SE-trending strike, with an E-directed translation of the tectonic units. The steep (≈ 60°) predominantly W-dipping thrusts represent high-strain zones localised in relatively weak Cambro-Ordovician quartz-rich turbidites, that are sandwiched along the boundaries of the Late Proterozoic metavolcanics. Overprinting the early thrust system are D 4–D 6 deformation events that include reverse, strike-slip and normal faults. The Grampians extensional basin, overlying the Late Proterozoic to Ordovician sequence, records a significant change in the tectonic regime operating during the Late Silurian-Early Devonian. It has been subjected to a Middle Devonian compressional deformation event, D 4, with the development of thrust and fold structures. This deformation is also superimposed on the thrusted and folded (D 1 to D 3) sequence to the east of the Grampians half-graben, producing further thrusts. A gradual change from a NE-SW to a N-S stress field produces oblique strike-slip faults. Normal faults of probably Early Cretaceous age transect the entire sequence. The Proterozoic-Cambrian sequence has been affected by a low- P/high- T, mid-greenschist facies regional metamorphic event. Peak-metamorphic conditions have been inferred from metavolcanic rocks and mafic units; derived from a strongly differentiated tholeiitic suite at Stawell. These have been calculated to be 1.7 ± 0.7 (2 σ) kbar and 450 ± 20 (2 σ)° C. The advective heat input necessary to create this low- P/high- T metamorphic event is attributed to penetrative deformation and strain partitioning between the crust and the mantle lithosphere which in turn causes crustal thickening, associated compression, and the observed D 1 to D 3 structures on the surface.

Convergence and intraplate deformation in the Lachlan Fold Belt of Southeastern Australia

The Lachlan Fold Belt of southeastern Australia is dominated by a widespread oceanic association including Cambrian submarine mafic volcanics and an overlying widespread Ordovician quartz-rich turbidite and black shale succession with scattered mafic to andesitic volcanic centres. These rocks are overlain by, or in fault contact with, deep-marine to continental Silurian to Early Carboniferous successions with mafic to silicic volcanics and abundant plutonic rocks. Major deformation, achieved by thrusting and folding, affected the fold belt during the Silurian to Middle Devonian and the Early Carboniferous. Shortening estimates vary throughout the belt but the widespread Ordovician quartz turbidite succession usually has at least 60% shortening which implies an original width of the fold belt of about 1700 km. Strike-slip faulting was important on a local to regional scale, but large-scale strike-slip displacements appear unlikely. The large values of shortening are consistent with development of the belt in a convergent margin setting at least for the Silurian to Carboniferous. The nature of the lower crust of the Lachlan Fold Belt has always presented a problem of interpretation but we favour a Late Proterozoic quasi-continental lower crustal layer, that has deformed independently of the upper crust by either homogeneous flattening or crustal duplexing during Silurian-Carboniferous convergence, and provided a source for Silurian-Devonian granites.

Eocene Deep-Marine Clastic Sedimentation in Southwest Java, Indonesia: An Example of a Quartz-Rich Turbidite System on the Flank of the Sunda Shield

Eocene Deep-Marine Clastic Sedimentation in Southwest Java, Indonesia: An Example of a Quartz-Rich Turbidite System on the Flank of the Sunda Shield