Evolution of the southeastern Lachlan Fold Belt in Victoria

The Benambra Terrane of southeastern Australia is the eastern, allochthonous portion of the Lachlan Fold Belt with a distinctive Early Silurian to Early Devonian history. Its magmatic, metamorphic, structural, tectonic and stratigraphic histories are different from the adjacent, autochthonous Whitelaw Terrane and record prolonged orogen‐parallel dextral displacement. Unlike the Whitelaw Terrane, parts of the proto‐Benambra Terrane were affected by extensive Early Silurian plutonism associated with high T/low P metamorphism. The orogen‐parallel movement (north‐south) is in addition to a stronger component of east‐west contraction. Three main orogenic pulses deformed the Victorian portion of the terrane. The earliest, the Benambran Orogeny, was the major cratonisation event in the Lachlan Fold Belt and caused amalgamation of the components that comprise the Benambra Terrane. It produced faults, tight folding and strong cleavage with both east‐west and north‐south components of compression. The Bindian (= Bowning) Orogeny, not seen in the Whitelaw Terrane, was the main period of southward tectonic transport in the Benambra Terrane. It was characterised by the development of large strike‐slip faults that controlled the distribution of second‐generation cleavage, acted as conduits for syntectonic granites and controlled the deformation of Upper Silurian sequences. Strike‐slip and thrust faults form complex linked systems that show kinematic indicators consistent with overall southward tectonic transport. A large transform fault is inferred to have accommodated approximately 600 km of dextral strike‐slip displacement between the Whitelaw and Benambra Terranes. The Benambran and Bindian Orogenies were each followed by periods of extension during which small to large basins formed and were filled by thick sequences of volcanics and sediments, partly or wholly marine. Some of the extension appears to have occurred along pre‐existing fractures. Silurian basins were inverted during the Bindian Orogeny and Early Devonian basins by the Tabberabberan Orogeny. In the Melbourne Zone, just west of the Benambra Terrane, sedimentation patterns in this interval, in particular the complete absence of material derived from the deforming Benambra Terrane, indicate that the two terranes were not juxtaposed until just before the Tabberabberan Orogeny. This orogeny marked the end of orogen‐parallel movement and brought about the amalgamation of the Whitelaw and Benambra Terranes along the Governor Fault. Upper Devonian continental sediments and volcanics form a cover sequence to the terranes and their structural zones and show that no significant rejuvenation of older structures occurred after the Middle Devonian.     

Metallogeny and tectonic development of the tasman fold belt system in tasmania

In western Tasmania, Precambrian sedimentary sequences form the basement for narrow trough accumulations of Eocambrian and younger sequences. The main trough, the meridional Dundas Trough, is flanked to the west by the Rocky Cape region of Precambrian rocks within which major, apparently stratiform, exhalative magnetite-pyrite deposits are intercalated with metabasaltic volcanics and ultramafic bodies.The Eocambrian-Cambrian troughs apparently developed during extension of Precambrian continental crust. Early shallow-water deposition includes thick dolomite units in some troughs. Deepening of the troughs was accompanied by turbidite sedimentation, with minor limestone, and submarine basaltic volcanism with associated minor disseminated native copper. Ultramafic and related igneous rocks were tectonically emplaced in some troughs during a mild compressional phase. They contain only minor platinoids, copper-nickel sulphides and asbestos, but are source rocks for Tertiary secondary deposits of platinoids, chromite and lateritic nickel.In the Dundas Trough, Eocambrian-Early Cambrian rocks are separated by an inferred erosional surface from structurally conformable overlying Middle to Late Cambrian fossiliferous turbidite sequences. The structural conformity continues through overlying Ordovician to Early Devonian terrestrial and shallow-marine stable shelf deposits.A considerable pile of probable Middle Cambrian felsic volcanics accumulated between the sedimentary deposits of the Dundas Trough and the Tyennan region of Precambrian rocks to the east. A lava-dominated belt within the volcanics hosts major volcanogenic massive sulphide deposits, including those of the exhalative type, which in the south are enriched in copper, gold and silver, whereas in the north they are rich in zine, lead, copper, gold and silver. Cambrian movements along faults near the margin of the Tyennan region resulted in erosion of the mineralized volcanics, locally exposing sub-volcanic granitoids. Above the local unconformities occur unmineralized volcaniclastic sequences that pass conformably into Ordovician to Early Devonian shelf deposits. Ordovician limestone locally hosts stratabound disseminated and veined base metal sulphide deposits.Pre-Middle Devonian rocks of western Tasmania differ, for most part, from those in the northeast where deeper marine turbidite quartz-wacke sequences were deposited during the Ordovician and Early Devonian.The Eocambrian to Early Devonian rocks of Tasmania were extensively deformed in the mid-Devonian. The Precambrian regions of western Tasmania behaved as relatively competent blocks controlling early fold patterns. In northeastern Tasmania, folding is of similar age but resulted from movements inconsistent with those affecting rocks of equivalent age in western Tasmania.The final metallogenic event is associated with high-level granitoid masses emplaced throughout Tasmania during the Middle to Late Devonian. In northeastern Tasmania, extensive I-type granodiorite and S-type granite, with alkali-feldspar granites, are associated with mainly endogranitic stanniferous grelsens and wolframite ± cassiterite vein deposits. In contrast, scheelite-bearing skarns and cassiterite stannite pyrrhotite carbonate replacement deposits are dominant in western Tasmania, associated mainly with S-type granites. Several argentiferous lead-zinc vein deposits occur in haloes around tin-tungsten deposits. A number of gold deposits are apparently associated with I-type granodiorite, but some have uncertain genesis.The contrasting regions of western and northeastern Tasmania have probably been brought together by lateral movement along an inferred fracture. Flat-lying, Late Carboniferous and younger deposits rest on the older rocks, and the only known post-Devonian primary mineralization is gold associated with Creta ceous syenite.     

Mid to late Paleozoic shortening pulses in the Lachlan Orogen,southeastern Australia: a review

In the late Silurian, the Lachlan Orogen of southeastern Australia had a varied paleogeography with deep-marine, shallow-marine, subaerial environments and widespread igneous activity reflecting an extensional backarc setting. This changed to a compressional–extensional regime in the Devonian associated with episodic compressional events, including the Bindian, Tabberabberan and Kanimblan orogenies. The Early Devonian Bindian Orogeny was associated with SSE transport of the Wagga–Omeo Zone that was synchronous with thick sedimentation in the Cobar and Darling basins in central and western New South Wales. Shortening has been controlled by the margins of the Wagga–Omeo Zone with partitioning along strike-slip faults, such as along the Gilmore Fault, and inversion of pre-existing extensional basins including the Limestone Creek Graben and the Canbelego–Mineral Hill Volcanic Belt. Shortening was more widespread in the late Early Devonian to Middle Devonian Tabberabberan Orogeny, with major deformation in the Melbourne Zone, Cobar Basin and eastern Lachlan Orogen. In the eastern Melbourne Zone, structural trends have been controlled by the pre-existing structural grain in the adjacent Tabberabbera Zone. Elsewhere Tabberabberan deformation involved inversion of pre-existing rifts resulting in a variation in structural trends. In the Early Carboniferous, the Lachlan Orogen was in a compressional backarc setting west of the New England continental margin arc with Kanimblan deformation most evident in Upper Devonian units in the eastern Lachlan Orogen. Kanimblan structures include major thrusts and associated fault-propagation folds indicated by footwall synclines with a steeply dipping to overturned limb adjacent to the fault. Ongoing deformation and sedimentation have been documented in the Mt Howitt Province of eastern Victoria. Overall, structural trends reflect a combination of controls provided by reactivation of pre-existing contractional and extensional structures in dominantly E–W shortening operating intermittently from the earliest Devonian to Early Carboniferous.     

Pre‐Tabberabberan deformation in eastern Tasmania: A southern extension of the Benambran Orogeny

Recumbent folding in eastern Tasmania affected turbidites containing Lower to Middle Ordovician (Bendigonian Be1 to Darriwilian Da3) fossils, but not stratigraphically overlying turbidites containing Silurian (Ludlow) graptolites, and is of a timing consistent with Ordovician to Silurian Benambran orogenesis on the Australian mainland. Two subsequent phases of upright folding post‐date deposition of turbidites containing Devonian plant fossils but pre‐date intrusion of Middle Devonian granitoids, and are of Tabberabberan age. A closely spaced disjunctive cleavage (S₂), associated with the first phase of Tabberabberan folding, everywhere cuts a slaty cleavage (S₁) associated with the earlier formed recumbent folds. However, refolding associated with development of S₂ is not always clear in outcrop and it is proposed that coincident tectonic vergence between the two events has resulted in reactivation of recumbent D₁ structures during the D₂ event. The transition to rocks not affected by recumbent folding coincides with a marked change in sedimentology from shale‐ to sand‐dominated successions. This contact does not outcrop but, from seismic data, appears to dip moderately to the east, and can only be explained as an unconformity. The current grouping of all pre‐Middle Devonian turbidites in eastern Tasmania into the one Mathinna Group is misleading in that the turbidite sequence can be subdivided into two distinct sedimentary packages separated by an orogenic event. It is proposed that the Mathinna Group be given supergroup status and existing formations placed into two new groups: an older Early to Middle Ordovician Tippogoree Group and a younger Silurian to Devonian Panama Group.     

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt,southeastern Australia

The Lachlan Fold Belt of southeastern Australia developed along the Panthalassan margin of East Gondwana. Major silicic igneous activity and active tectonics with extensional, strike-slip and contractional deformation have been related to a continental backarc setting with a convergent margin to the east. In the Early Silurian (Benambran Orogeny), tectonic development was controlled by one or more subduction zones involved in collision and accretion of the Ordovician Macquarie Arc. Thermal instability in the Late Silurian to Middle Devonian interval was promoted by the presence of one or more shallow subducted slabs in the upper mantle and resulted in widespread silicic igneous activity. Extension dominated the Late Silurian in New South Wales and parts of eastern Victoria and led to formation of several sedimentary basins. Alternating episodes of contraction and extension, along with dispersed strike-slip faulting particularly in eastern Victoria, occurred in the Early Devonian culminating in the Middle Devonian contractional Tabberabberan Orogeny. Contractional deformation in modern systems, such as the central Andes, is driven by advance of the overriding plate, with highest strain developed at locations distant from plate edges. In the Ordovician to Early Devonian, it is inferred that East Gondwana was advancing towards Panthalassa. Extensional activity in the Lachlan backarc, although minor in comparison with backarc basins in the western Pacific Ocean, was driven by limited but continuous rollback of the subduction hinge. Alternation of contraction and extension reflects the delicate balance between plate motions with rollback being overtaken by advance of the upper plate intermittently in the Early to Middle Devonian resulting in contractional deformation in an otherwise dominantly extensional regime. A modern system that shows comparable behaviour is East Asia where rollback is considered responsible for widespread sedimentary basin development and basin inversion reflects advance of blocks driven by compression related to the Indian collision.     

Towards understanding the early Gondwanan margin in southeastern Australia

This review synthesizes the Proterozoic and early Paleozoic geology of Tasmania, Bass Strait and western and central Victoria. We examine the many different conflicting hypotheses that have been proposed to solve the paradoxical relationships between Tasmanian geology and that of mainland Australia, most notably the prevalence of Proterozoic basement of western and central Tasmania, while immediately across Bass Strait evidence of Proterozoic rocks is much more cryptic. We conclude that the Selwyn block model is the most satisfactory hypothesis to date, since it fits best with the obvious patterns in the magnetic and gravity data. This model proposes that the central Victorian Melbourne Zone is underlain by the northern extension of thin Tasmanian Proterozoic and Cambrian crust under Bass Strait, and that the Silurian to Middle Devonian Melbourne Zone was shortened along a décollement during the Tabberabberan Orogeny. The Ordovician rocks of eastern Tasmania correlate more closely with the Tabberabbera Zone than the Melbourne Zone in Victoria; however the Silurian and Devonian correlations are less certain. Major unresolved issues are the origins of the Proterozoic and Early Cambrian lithostratigraphic packages, tectonic models for their assembly during the Tyennan Orogeny, and how these models fit with those for mainland Australia.     

Exotic crustal block accretion to the eastern Gondwanaland margin in the Late Cambrian–Tasmania,the Selwyn Block,and implications for the Cambrian–Silurian evolution of the Ross,Delamerian, and Lachlan orogens

Reconstructions of the Cambrian–Silurian tectonic evolution of eastern Gondwanaland, when the Australian Tasmanides and Antarctic Ross Orogen developed, rely on correlation between structural elements in SE Australia and Northern Victoria Land (NVL), Antarctica. A variety of published models exist but none completely solve the tectonic puzzle that is the Delamerian–Lachlan transition in the Tasmanides. This paper summarizes the understanding of Cambrian (Delamerian) to Silurian (Lachlan) geological evolution of the eastern Tasmanides, taking into account new deep seismic data that clarifies the geological connection between Victoria and Tasmania — the ‘Selwyn Block’ model. It evaluates previous attempts at correlation between NVL, Tasmania and Victoria, and presents a new scenario that encompasses the most robust correlations. Tasmania together with the Selwyn Block is reinterpreted as an exotic Proterozoic microcontinental block – ‘VanDieland’ – that collided into the east Gondwanaland margin south of western Victoria, and north of NVL in the Late Cambrian, perhaps terminating the Delamerian Orogeny in SE Australia. Subsequent north-east ‘tectonic escape’ of VanDieland in the Early Ordovician explains the present-day outboard position of Tasmania with respect to the rest of the Delamerian orogen, the origin of the hiatus that separates the Delamerian and Lachlan orogenic cycles in Australia, and how western Lachlan oceanic crust developed as a ‘trapped plate-segment’. The model establishes a new structural template for subsequent Lachlan Orogen development and Mesozoic Australia–Antarctica separation.     

Variable hangingwall palaeotransport during Silurian and Devonian thrusting in the western Lachlan Fold Belt: Missing gold lodes,synchronous Melbourne Trough sedimentation and Grampians Group fold interference

The western margin of the Lachlan Fold Belt contains early ductile and brittle structures that formed during northeast‐southwest and east‐west compression, followed by reactivation related to sinistral wrenching. At Stawell all of these structural features (and the associated gold lodes) are dismembered by a complex array of later northwest‐, north‐ and northeast‐dipping faults. Detailed underground structural analysis has identified northwest‐trending mid‐Devonian thrusts (Tabberabberan) that post‐date Early Devonian plutonism and have a top‐to‐the‐southwest transport. Deformation associated with the initial stages of dismemberment occurred along an earlier array of faults that trend southwest‐northeast (or east‐west) and dip to the northwest (or north). The initial transport of the units in the hangingwall of these fault structures was top‐to‐the‐southeast. ‘Missing’ gold lodes were discovered beneath the Magdala orebody by reconstructing a displacement history that involved a combination of transport vectors (top‐to‐the‐southeast and top‐to‐the‐southwest). Fold interference structures in the adjacent Silurian Grampians Group provide further evidence for at least two almost orthogonal shortening regimes, post the mid‐Silurian. Overprinting relationships, and correlation with synchronous sedimentation in the Melbourne Trough, indicates that the early fault structures are mid‐ to late‐Silurian in age (Ludlow: ca 420–414 Ma). These atypical southeast‐vergent structures have regional extent and separate significant northeast‐southwest shortening that occurred in the mid‐Devonian (‘Tabberabberan orogeny’) and Late Ordovician (‘Benambran orogeny’).     

Evolution of geochemistry and depositional settings of Early Palaeozoic siliciclastics of the Barrandian (Teplá-Barrandian Unit,Bohemian Massif,Czech Republic)

Cambrian and Ordovician-Middle Devonian sequences of two successive Early Palaeozoic basins of the Barrandian unconformably overlie Cadomian basement in the Bohemian Massif NW interior (Teplá-Barrandian unit) which is the easternmost peri-Gondwanan remnant within the Variscides. Correlation of stratigraphy and geochemistry of the Early Palaeozoic siliciclastic rocks elucidated sediment provenances. Sandstones of the Middle Cambrian Píbram-Jince Basin were derived from a Cadomian Neoproterozoic island arc. The source area of the Ordovician shallow-marine siliciclastics of the successor Prague Basin is a dissected Cadomian orogen. Late Cambrian acid volcanics of the Barrandian and Cambrian (meta)granitoids emplaced in the W part of the Teplá-Barrandian Cadomian basement are also discernible in these sediments. Old sedimentary component increased during the Ordovician. Early Llandovery siliciclastic rocks show characteristics of an abruptly weakened supply of terrigenous material and an elevated proportion of synsedimentary basic volcanics as a result of Silurian transgression. Emsian siliciclastics (intercalated in the Late Silurian to Early Devonian limestone suite) presumably comprise an addition of coeval basic/ultrabasic volcaniclastics. Middle Devonian flysch-like siliciclastics indicate reappearance of Cadomian source near the Barrandian during early Variscan convergences of Armorican microplates that preceeded accretion of the Teplá-Barrandian unit within the Bohemian Massif terrane mosaic.Dr. Patoka deceased in July 2004.     

The Berridale Wrench fault: A major structure in the Snowy Mountains of New South Wales

One of the most significant, but poorly understood, tectonic events in the east Lachlan Fold Belt is that which caused the shift from mafic, mantle‐derived calc‐alkaline/shoshonitic volcanism in the Late Ordovician to silicic (S‐type) plutonism and volcanism in the late Early Silurian. We suggest that this chemical/isotopic shift required major changes in crustal architecture, but not tectonic setting, and simply involved ongoing subduction‐related magmatism following burial of the pre‐existing, active intraoceanic arc by overthrusting Ordovician sediments during Late Ordovician — Early Silurian (pre‐Benambran) deformation, associated with regional northeast‐southwest shortening. A review of ‘type’ Benambran deformation from the type area (central Lachlan Fold Belt) shows that it is constrained to a north‐northwest‐trending belt at ca 430 Ma (late Early Silurian), associated with high‐grade metamorphism and S‐type granite generation. Similar features were associated with ca 430 Ma deformation in east Lachlan Fold Belt, highlighted by the Cooma Complex, and formed within a separate north‐trending belt that included the S‐type Kosciuszko, Murrumbidgee, Young and Wyangala Batholiths. As Ordovician turbidites were partially melted at ca 430 Ma, they must have been buried already to ～20 km before the ‘type’ Benambran deformation. We suggest that this burial occurred during earlier northeast‐southwest shortening associated with regional oblique folds and thrusts, loosely referred to previously as latitudinal or east‐west structures. This event also caused the earliest Silurian uplift in the central Lachlan Fold Belt (Benambran highlands), which pre‐dated the ‘type’ Benambran deformation and is constrained as latest Ordovician — earliest Silurian (ca 450–440 Ma) in age. The south‐ to southwest‐verging, earliest Silurian folds and thrusts in the Tabberabbera Zone are considered to be associated with these early oblique structures, although similar deformation in that zone probably continued into the Devonian. We term these ‘pre’‐ and ‘type’‐Benambran events as ‘early’ and ‘late’ for historical reasons, although we do not consider that they are necessarily related. Heat‐flow modelling suggests that burial of ‘average’ Ordovician turbidites during early Benambran deformation at 450–440 Ma, to form a 30 km‐thick crustal pile, cannot provide sufficient heat to induce mid‐crustal melting at ca 430 Ma by internal heat generation alone. An external, mantle heat source is required, best illustrated by the mafic ca 430 Ma, Micalong Swamp Igneous Complex in the S‐type Young Batholith. Modern heat‐flow constraints also indicate that the lower crust cannot be felsic and, along with petrological evidence, appears to preclude older continental ‘basement terranes’ as sources for the S‐type granites. Restriction of the S‐type batholiths into two discrete, oblique, linear belts in the central and east Lachlan Fold Belt supports a model of separate magmatic arc/subduction zone complexes, consistent with the existence of adjacent, structurally imbricated turbidite zones with opposite tectonic vergence, inferred by other workers to be independent accretionary prisms. Arc magmas associated with this ‘double convergent’ subduction system in the east Lachlan Fold Belt were heavily contaminated by Ordovician sediment, recently buried during the early Benambran deformation, causing the shift from mafic to silicic (S‐type) magmatism. In contrast, the central Lachlan Fold Belt magmatic arc, represented by the Wagga‐Omeo Zone, only began in the Early Silurian in response to subduction associated with the early Benambran northeast‐southwest shortening. The model requires that the S‐type and subsequent I‐type (Late Silurian — Devonian) granites of the Lachlan Fold Belt were associated with ongoing, subduction‐related tectonic activity.     

The Narooma Terrane offshore: a new model for the southeastern Lachlan Orogen using data from rocks dredged from the New South Wales continental slope

Eight dredges from the southern New South Wales continental slope sampled the offshore extension of the Lachlan Orogen. Two rock suites were recovered: (1) lower greenshist facies limestones, felsic volcanics, sandstones, mudstones and Moruya Suite granodiorite correlate with the onshore Silurian to mid-Devonian orogenic phase; and (2) a strongly deformed greenschist to lower amphibolite facies mafic volcanics, cherts, marbles, pelites and serpentinites correlate in part with the Cambro-Ordovician Wagonga Group of the Narooma Terrane. The mafic volcanic rocks have ocean island, tholeiitic and boninitic basalt affinities. The offshore distribution of ocean island basalt that correlates with medial Cambrian basalt breccias at Batemans Bay suggests a large seamount or seamount complex. The boninites, tholeiites and ultramafics could be part of a forearc-generated ophiolite. The Narooma Terrane basement is interpreted as the part of the bonititic arc postulated to have collided with Vandieland in late early Cambrian time. Mid-Cambrian rifting of the oceanward part of this arc remnant, generated the Albury–Bega Terrane oceanic basement exposed in the Howqua Valley in the west and Melville Point in the east. Overlying are upper–mid-Cambrian to lowermost Ordovician black shale and chert, Lower Ordovician to Gisbornian Adaminaby Group quartz turbidites and Gisbornian to lower Bolindian Bendoc Group black shales. Batemans Bay exposures are reinterpreted as a dismembered basin margin succession onlapping the west-facing attenuated flank of the Narooma Terrane. The Narooma Cambro-Ordovician cherts and mudstones were initially deposited outboard on the more elevated seamount flank elevated above the clastic-filled basin to the west. Benambran deformation commenced in latest Ordovician time uplifting the outer Narooma Terrane, shedding debris from the seamount and its flanks, culminating in allochthonous displacement of chert masses to the basin's eastern margin to Narooma, and emplacing them as a succession of thrust sheets. Contemporaneously, silt and mud of the Bogolo Formation, deposited from the west, were mixed with olistostomal basalt and chert debris from the east. Early Silurian westward tectonic transport of the Narooma Terrane ruptured the Albury-Bega basin floor at Batemans Bay, thrusting it and its sedimentary cover over its eastern margin as a series of thrusts each floored by melange (mapped Bogolo Formation), derived from the slope debris and its overpressured sedimentary cover. Offshore, the metamorphosed Benambran phase rocks are unconformably overlain by Tabberabberan cycle sediments and volcanics intruded by granodiorite. Our interpretation of the boundary between the Albury-Bega and Narooma terranes as a thrusted passive margin accumulation is incompatible with models of a Narooma Accretionary Complex formed by the subduction of the Paleopacific Plate.     

Evolution of a reworked orogenic zone: The boundary between the delamerian and lachlan fold belts,southeastern Australia *

⁴⁰Ar/³⁹Ar age data from the boundary between the Delamerian and Lachlan Fold Belts identify the Moornambool Metamorphic Complex as a Cambrian metamorphic belt in the western Stawell Zone of the Palaeozoic Tasmanide System of southeastern Australia. A reworked orogenic zone exists between the Lachlan and Delamerian Fold Belts that contains the eastern section of the Cambrian Delamerian Fold Belt and the western limit of orogenesis associated with the formation of an Ordovician to Silurian accretionary wedge (Lachlan Fold Belt). Delamerian thrusting is craton-verging and occurred at the same time as the final consolidation of Gondwana. ⁴⁰Ar/³⁹Ar age data indicate rapid cooling of the Moornambool Metamorphic Complex at about 500 Ma at a rate of 20 – 30°C per million years, temporally associated with calc-alkaline volcanism followed by clastic sedimentation. Extension in the overriding plate of a subduction zone is interpreted to have exhumed the metamorphic rocks within the Moornambool Metamorphic Complex. The Delamerian system varies from a high geothermal gradient with syntectonic plutonism in the west to lower geothermal gradients in the east (no syntectonic plutonism). This metamorphic zonation is consistent with a west-dipping subduction zone. Contrary to some previous models involving a reversal in subduction polarity, the Ross and Delamerian systems of Antarctica and Australia are inferred to reflect deformation processes associated with a Cambrian subduction zone that dipped towards the Gondwana supercontinent. Western Lachlan Fold Belt orogenesis occurred about 40 million years after the Delamerian Orogeny and deformed older, colder, and denser oceanic crust, with metamorphism indicative of a low geothermal gradient. This orogenesis closed a marginal ocean basin by west-directed underthrusting of oceanic crust that produced an accretionary wedge with west-dipping faults that verge away from the major craton. The western Lachlan Fold Belt was not associated with arc-related volcanism and plutonism occurred 40 – 60 million years after initial deformation. The revised orogenic boundaries have implications for the location of world-class 440 Ma orogenic gold deposits. The structural complexity of the 440 Ma Stawell gold deposit reflects its location in a reworked part of the Cambrian Delamerian Fold Belt, while the structurally simpler 440 Ma Bendigo deposit is hosted by younger Ordovician turbidites solely deformed by Lachlan orogenesis.     

Early island-arc granitoids of the Shchuchinskaya zone of the Polar Urals: U–Pb (SIMS) zircon isotope data

Granitoids of the Rechnoy and Yalya-Pe paleovolcanoes, which were ascribed to the Silurian Khoimpe complex during a geological mapping, and granitoids of the Nganotsky-1 and Nganotsky-2 plutons that were ascribed to the Early Devonian Yunyaga complex were studied in the Shchuchinskaya zone of the Polar Urals. It was established that according to the mineral and chemical compositions the rocks of the plutons studied correspond to island-arc granitoids of I-type. Zircons from granitoids of the Rechnoy and Yalya-Pe paleovolcanoes and the Nganotsky-1 pluton yielded concordant U–Pb (SIMS) isotope ages of 456 ± 6, 454 ± 4, and 463 ± 3 Ma, respectively, which indicates the existence of an island arc within the Shchuchinskaya zone starting from the Middle–Late Ordovician. Based on the obtained zircon ages of granitoids, the country volcanics were ascribed to the Syaday Formation; the upper stratigraphic boundary of their formation was specified as the Middle–Upper Ordovician.     

Late Paleozoic – Mesozoic subduction-related magmatism at the southern margin of the Siberian continent and the 150 million-year history of the Mongol-Okhotsk Ocean

The paper reviews geological, geochronological and geochemical data from the Late Paleozoic – Mesozoic magmatic complexes of the Siberian continent north of the Mongol-Okhotsk suture. These data imply that these complexes are related to the subduction of the Mongol-Okhotsk Ocean under the Siberian continent. We suggest that this subduction started in the Devonian, prior to the peak of magmatic activity. Studied magmatic complexes are of variable compositions possibly controlled by changes of the subduction regime and by possible input from enriched mantle sources (hot spots).The oceanic lithosphere of the Mongol-Okhotsk Ocean had shallowly subducted under the Siberian continent in the Devonian. Steeper subduction in the Early – Late Carboniferous led to switching from an extensional to compressional tectonic regime resulting in fold-thrust deformation, to the development of duplex structures and finally to the thickening of the continental crust. This stage was marked by emplacement of voluminous autochthonous biotite granites of the Angara-Vitim batholith into the thickened crust. The igneous activity in the Late Carboniferous – Early Permian was controlled by the destruction of the subducted slab. The allochthonous granitoids of the Angara-Vitim batholith, and the alkaline granitoids and volcanics of the Western Transbaikalian belt were formed at this stage. All these complexes are indicative of extension of the thickened continental crust. A normal-angle subduction in the Late Permian – Late Triassic caused emplacement of various types of intrusions and volcanism. The calc-alkaline granitoids of the Late Permian – Middle Triassic Khangay batholith and Late Triassic Khentey batholith were intruded near the Mongol-Okhotsk suture, whereas alkaline granitoids and bimodal lavas were formed in the hinterland above the broken slab. The Jurassic is characterized by a significant decrease of magmatic activity, probably related to the end of Mongol-Okhotsk subduction beneath the studied area.The spatial relationship of the Late Permian – Middle Triassic granitoids, and the Late Triassic granitoids is typical for an active continental margin developing above a subduction zone. All the Late Carboniferous to Late Jurassic mafic rocks are geochemically similar to subduction-related basalts. They are depleted in Nb, Ta, Ti and enriched in Sr, Ba, Pb. However, the basaltoids located farther from the Mongol-Okhotsk suture are geochemically similar to a transition type between island-arc basalts and within-plate basalts. Such chemical characteristics might be caused by input of hot spot related enriched mantle to the lithospheric mantle modified by subduction. The Early Permian and Late Triassic alkaline granitoids of southern Siberia are of the A₂-type geochemical affinities, which is also typical of active continental margins. Only the basaltoids generated at the end of Early Cretaceous are geochemically similar to typical within-plate basalts, reflecting the final closure of the Mongol-Okhotsk Ocean.     

Proterozoic – Early Palaeozoic rocks and the Tyennan Orogeny in central Victoria: The Selwyn Block and its tectonic implications

Aeromagnetic and field data suggest that meta‐igneous rocks exposed on the south coast of central Victoria at Waratah Bay, Phillip Island, Barrabool Hills and inland near Licola, are continuous—beneath Bass Strait—with Proterozoic/Cambrian igneous rocks in King Island and Tasmania. This correlation is supported by a pre‐Early Ordovician unconformity above gabbro protomylonite at Waratah Bay, age equivalent to the Tasmanian Tyennan unconformity. Cambrian volcanics at Licola and unusual features of the Melbourne Zone sequence indicate that Tyennan continental crust extends north as basement to the central Victorian portion of the Lachlan Fold Belt. In contrast, adjacent parts of the Lachlan Fold Belt in Victoria contain conformable sea‐floor sequences that span the Early Cambrian to Late Ordovician, with no evidence of either Cambrian deformation or underlying continental basement. The block of Tyennan continental crust beneath central Victoria—the Selwyn Block—is fundamentally different, and has influenced temporal and spatial patterns of sedimentation, deformation, metamorphism and plutonism. Palaeogeographical reconstructions suggest that the block was a submarine plateau that lay outboard of the Australian craton, upon which a condensed Ordovician sequence was deposited. The sequence above the Selwyn Block unconformity at Waratah Bay is similar to widespread post‐Tyennan sediments in western Tasmania. During Late Ordovician and Early Silurian deformation, the Selwyn Block protected much of the overlying sedimentary sequence. Instead, shortening was focused into the Stawell and Bendigo Zones to the west. These zones were sandwiched between the Selwyn Block and the Australian craton in a ‘vice’ scenario reminiscent of some Appalachian orogenic events. The region above the Selwyn Block was downwarped adjacent to the overthrust Bendigo Zone as a foreland deep, into which a conformable clastic wedge of sediment was deposited in Late Ordovician to Devonian time, prior to final Middle Devonian deformation. The Selwyn Block includes the Cambrian calc‐alkaline Licola and Jamieson Volcanics that are correlated with the Tasmanian Mt Read Volcanics. In Victoria, these form a basement high controlling the unusual down‐cutting thrusts in the overlying Melbourne Zone and explaining the major structural vergence reversal between the Melbourne and Tabberabbera Zones. The Selwyn Block has exerted some control on the timing, chemistry and distribution of post‐orogenic granites, and on central Victorian gold mineralisation. Reactivated faults in the block influenced deposition, and continue to control the deformation of the portions of the Otway and Gippsland Basins that lie above it.     

Provenance and structure of the Yancannia Formation,southern Thomson Orogen: implications for the tectono-stratigraphic evolution of the Cambro-Ordovician western Tasmanides

Abstract

The upper Cambrian Yancannia Formation is a small and isolated basement exposure situated in the southern Thomson Orogen, northwestern New South Wales. Understanding the geology of the Yancannia Formation is important, as it offers a rare glimpse of the composition and structure of the mostly covered basement rocks of the southern Thomson Orogen. It consists of deformed fine-grained, lithic-rich, turbiditic metasediments, suggesting deposition in a proximal, low-energy deep-marine environment. A 497 ± 13 Ma U–Pb detrital zircon date provides its maximum depositional age, the same as previously published for a tuff horizon in a correlative unit. Analysis of sedimentological, geochronological and geophysical data confirms the Yancannia Formation belongs to the Warratta Group. The Warratta Group exhibits many similarities to the Teltawongee Group in the adjacent Delamerian Orogen, including similar provenance, sedimentology and deep-water turbiditic depositional environment. Additionally, there is no sedimentological evidence for deposition of the Warratta Group following the ca 500 Ma Delamerian Orogeny, which suggests that the Warratta Group is syn-Delamerian. However, no geochronological or structural evidence for Delamerian orogenesis was observed in the Warratta Group, suggesting that the group was either unaffected by Delamerian orogenesis, or that no conclusive record remains. The provenance signature of the Warratta Group also bears strong similarities with the upper Cambrian Stawell Zone Saint Arnaud Group in the western Lachlan Orogen. Units east of Yancannia have similar provenance signatures to the Lower Ordovician Girilambone Group of the Lachlan Orogen, suggesting equivalents exist in the southern Thomson Orogen. These are likely to be the Thomson beds, deposited in a deep-marine setting outboard of the Delamerian continental margin. Structural analysis from a ～10 km, semi-continuous, across-strike section indicates a major, kilometre-scale, upright, shallow northwest-trending, doubly plunging anticline dominates the Yancannia region. This D₁ structure was associated with tight-to-isoclinal folding, penetrative cleavage and abundant quartz veining of probable Benambran age. Later dextral transpressional deformation (D₂) produced a sporadic, weak cleavage and dextral faulting, possibly of Bindian age. Major south-directed thrusting (D₃) on the adjacent Olepoloko Fault occurred in the early Carboniferous and appears to pre-date a later deformation event (D₄), which was associated with kink folding.     

Structure of the Upper Devonian Boyd Volcanic Complex,south coast New South Wales: Implications for the Devonian‐Carboniferous evolution of the Lachlan Fold Belt

Upper Devonian continental and subaqueous sedimentary rocks and bimodal volcanic rocks of the Boyd Volcanic Complex of the south coast of New South Wales were deposited in a rapidly subsiding, 330°‐trending, transtensional basin. Structural analysis of synvolcanic and synsedimentary deformational structures indicate that basin formation is related to a 330°‐orientated subhorizontal σ₁ and a 060°‐orientated subhorizontal σ₃, which account for the development of the observed intrusion and fracture orientations. Rhyolitic, basaltic and associated clastic dykes are preferentially intruded along extensional 330°‐trending fractures, subordinately along sinistral, transtensional 010°‐trending fractures and along 290°‐trending fractures. One of the implications of such a palaeotectonic reconstruction is that the so called north‐trending Eden‐Comerong‐Yalwal Late Devonian rift does not represent a simple, single palaeobasin entity, but is presently a north‐trending alignment of exposures of sedimentary and volcanic rocks probably emplaced in different basins or sub‐basins, mildly folded during the Carboniferous Kanimblan compression (which also formed the north‐trending Budawang synclinorium) and then extended to the east by the Tasman Sea opening during the Jurassic. The development of scattered, rapidly subsiding, basins characterised by bimodal volcanism during the Late Devonian throughout the Lachlan Fold Belt, can be interpreted in terms of extensional collapse of a forming mountain belt contemporaneous with a sharp decrease of compressional stress after the Middle Devonian Tabberabberan orogenic event. This would promote a reorientation of σ₃ and transition from a compressional to a transtensional tectonic environment, which could also favour block rotation and formation of release basins.     

加里东运动构造古地理及滇黔桂盆地的形成--兼论滇黔桂盆地深层油气勘探潜力

中国南方的滇黔桂地区,早古生代与晚古生代之交曾经发生过较为强烈的加里东运动,包括三个幕:寒武纪末期的郁南运动,中、奥陶世末期的都匀运动以及志留纪末期的广西运动;奥陶系与志留系的残留不全和晚奥陶世至志留纪大片古陆———滇黔桂古陆的展布是加里东运动的重要体现。志留纪末期的广西运动之后,在大致相当于早古生代“滇黔桂古陆”分布的地区形成一个特殊的“滇黔桂盆地”,而且在滇黔桂盆地的主体部位常常是泥盆系直接覆盖在寒武系之上。寒武系,特别是下寒武统,由于寒武纪初期的快速海侵作用而在研究区域普遍发育烃源岩系;研究区域的泥盆系,特别是中泥盆统,在台间盆地中发育优质烃源岩。因此,巨大的构造古地理演变和海陆变迁,形成了一个晚古生代的泥盆系优质烃源岩与早古生代的下寒武统优质烃源岩的空间叠合区域,该叠合区域的加里东运动不整合面上、下的储集体即成为该地区的深层油气勘探对象,预示着滇黔桂盆地的深层存在较大的油气勘探潜力。     

Thermal Metamorphism in the trial harbour district,Tasmania

A Devonian granite complex intrudes Precambrian and Silurian siltstones and sandstones as well as (?) Cambrian volcanics and dunite.

Metamorphism of the Precambrian sediments is slight, and an andalusite‐bearing, pelitic hornfels is the only characteristic assemblage. The (?) Cambrian volcanics give rise to a variety of assemblages; (1) lime‐ and ferromagnesia‐rich (hypersthene — cummingtonite — labradorite; diopside — hornblende — labradorite); (2) magnesia‐rich (cordierite — hypersthene; cordierite — anthophyllite); (3) ultrabasic (olivine and/or pleonaste). Biotite (or phlogopite) is an almost invariable component, and garnet may also be present in these groups.

No significant metamorphism of the dunite is evident; minor development of veins and segregrations of aragonite, magnetite, phlogopite, brucite, chalcedony and antigorite may result from low‐grade hydrothermal activity Metamorphic assemblages in calcareous Silurian siltstones contain garnet, diopside, calcite and epidote.

A characteristic feature of the contact metamorphic aureole is the occurrence of diopside‐rich bodies in granite, volcanic hornfels, quartzite and dunite host rocks.