Richard J. Squire

Middle and Late Ordovician magmatic evolution of the Macquarie Arc, Lachlan Orogen, New South Wales

Early Ordovician (Phase 1) magmatism in the Macquarie Arc was followed by a magmatic hiatus of ∼9 million years, between late Bendigonian and early Darriwilian (i.e. between ca 475 and ca 466 Ma). Resumption of magmatism in the Middle Ordovician produced Phase 2 rocks, recorded by three major rock suites: (i) medium-K calc-alkaline lavas in the Cargo block (Molong Volcanic Belt) have primitive Nd values (+6.9 to +7.8) and volcanic facies suggesting eruption in an intra-oceanic arc stratovolcano; lavas in the fault-bounded Parkes Volcanics in the Junee – Narromine Volcanic Belt are compositionally identical to those in the Cargo block, suggesting that similar Phase 2 Middle Ordovician arc-type lavas may underlie the Cowra Trough; (ii) medium- to high-K dioritic to monzodioritic intrusions in the Narromine and Cowal Igneous Complexes of the Junee – Narromine Volcanic Belt have ages that cluster in the 470 – 460 Ma interval, and intrude presumed Phase 1 lavas and volcaniclastics; and (iii) in all three main volcanic belts, Middle Ordovician lavas range from medium-K to dominantly high-K calc-alkaline compositions with a clear trend to shoshonitic compositions late in the Phase 2 magmatic episode. Phase 2 units in the Molong Volcanic Belt (lower Blayney, Byng and lower Fairbridge Volcanics) and Rockley – Gulgong Volcanic Belt (Rockley and lower Sofala Volcanics) are dominated by significantly more unfractionated high-MgO lava compositions than contemporaneous lavas in the Cargo block or Junee – Narromine Volcanic Belt, suggesting that rifting of the arc had occurred by this time, and that the main extensional zone lay along the eastern side of the Macquarie Arc. Identical compositions of unusual shoshonitic ultramafic lavas in the Byng Volcanics of the Molong Volcanic Belt and the Rockley Volcanics of the Rockley – Gulgong Volcanic Belt provide strong evidence that these volcanic belts were once contiguous and were disrupted during Silurian – Devonian opening of the Hill End Trough. Phase 3 magmatism in the Macquarie Arc is represented by a widespread but relatively small volume magmatic event, dominated by shallow intrusive rocks of the Copper Hill Suite, emplaced in the Eastonian – Bolindian, between 456 and 441 Ma. These distinctive porphyritic dacites and associated holocrystalline diorites and granodiorites show medium-K calc-alkaline compositions, and their emplacement was intimately linked to an episode of regional uplift, erosion and limestone deposition in the Junee – Narromine Volcanic Belt and western Molong Volcanic Belt. Phase 4 magmatism extended from late Eastonian or Bolindian until Early Silurian time, and was dominated by relatively evolved (compared with Phase 2 lavas) shoshonitic lavas until the end of the Bolindian and porphyries in the Early Silurian. Collision-related shut-down of the arc, and initiation of arc extension and dismemberment, occurred around 438 Ma in the latest Ordovician. Post-arc magmatism during the Early Silurian is represented by high-Th, high-Nb lavas of the shoshonitic Nash Hill Volcanics in the Junee – Narromine Volcanic Belt, and Alaskan-type zoned ultramafic intrusions of the Fifield complexes farther west. The latter were emplaced through deformed Ordovician turbidites of the Girilambone Group, and their radiogenic isotope signatures show significant crustal involvement.

Interaction between collisional orogenesis and convergent-margin processes: evolution of the Cambrian proto-Pacific margin of East Gondwana

Difficulties in correlating the Cambrian magmatic, depositional, structural and metamorphic events along the proto-Pacific margin of East Gondwana have led to subdivision of the region into the Delamerian, Lachlan and Tyennan Orogens in Australia, the Ross Orogen in Antarctica and the Takaka Terrane in New Zealand. As a result, the Cambrian tectonic evolution of the region is poorly understood. We present here a revised lithostratigraphic section from the late Early to Mid-Cambrian rocks exposed near Stawell in western Victoria, which is used as the basis for correlating geological events in East Gondwana. These data show that the Cambrian tectonic evolution of East Gondwanas >4000 km long proto-Pacific margin involved predominantly compressional orogenesis separated by major short-lived extensional events at c. 516–514 and 504–500 Ma. The most significant extensional event, at c. 516–514 Ma, involved extensive slab rollback along the proto-Pacific margin in response to major changes in global plate motions and plate-boundary stresses following the termination of East–West Gondwana collision. Partial subduction of a ribbon of buoyant continental crust led to localized subduction-zone failure and obduction of the young hot forearc lithosphere in Tasmania at c. 510 Ma. Collision of the continental ribbon also significantly modified the architecture of the proto-Pacific margin and ultimately controlled the extent of the second major extensional event associated with slab rollback at c. 504–500 Ma. Tectonic evolution of the proto-Pacific margin of East Gondwana thus involved the complex interaction between convergent-margin processes and collisional orogenesis.

Hydrothermal alteration at the Magdala gold deposit, Stawell, western Victoria

The Magdala deposit in the Stawell goldfield in western Victoria was formed during the 440 Ma gold event of the Lachlan Orogeny and is hosted by Cambrian quartz-rich turbiditic sedimentary rocks (Albion Formation) that onlap a thick pile of tholeiitic basaltic lavas (Magdala Basalt). Detailed petrographic and geochemical analyses suggest that the host-rock (Stawell Facies) was originally a turbiditic sedimentary rock that was hydrothermally altered in response to seawater interaction with the hot basaltic pile. Subsequent regional greenschist metamorphism and ductile deformation that lasted at least 10 million years culminated in the formation of the Magdala mineralised system and produced a complex pattern of hydrothermal alteration. Evolution of this alteration occurred over six stages: Stage 1, Fe-enrichment of sedimentary rock adjacent to the basalt pre-D1; Stage 2, chlorite (metamorphism), orbicular carbonate and pyrite, syn-D2; Stage 3, muscovite, siderite, ankerite and pyrrhotite, syn-D3 – D4a – b; Stage 4, stilpnomelane, siderite, pyrrhotite, arsenopyrite and pyrite, syn-D4c; Stage 5, silica, minnesotaite and magnetite, post-D4c – pre-D5; and Stage 6, Fe-rich chlorite, muscovite, calcite, arsenopyrite and pyrite, syn-D5. Comparisons with other turbidite-hosted gold deposits in Victoria (e.g. Bendigo and Ballarat) highlight four major differences: (i) presence of a tholeiitic basaltic pile; (ii) ductile deformation (D1 – 4) over at least 60 million years prior to gold mineralisation; (iii) highly evolved hydrothermal alteration; and (iv) source of sulfur. Of these differences the key element is the basaltic pile and its associated heat, which may have promoted the growth of micro-organisms in, and alteration of, the onlapping sedimentary rocks, thereby creating a basis from which an unusual turbidite-hosted orogenic-gold deposit was formed.

Synchronous compression and extension in East Gondwana: Tectonic controls on world-class gold deposits at 440 Ma

The collision of a seamount or microcontinental block with an arc may produce complex strain distributions and changes in the tectono-magmatic setting. Slab rollback and associated upwelling of asthenosphere occurred ca. 440 Ma near the margin of East Gondwana in response to subduction zone lock-up following arc collision ca. 455 Ma. World-class porphyry copper gold deposits (e.g., Cadia) formed in the backarc during extension and magmatism associated with slab rollback. Upwelling of the asthenosphere also provided the heat engine for generation of hydrothermal fluids. The result was elevated fluid pressures and compressional brittle deformation associated with orogenic gold mineralization (e.g., Bendigo) inboard of the East Gondwana margin.

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.

Volcanology of the Archaean Lunnon Basalt and its relevance to nickel sulfide‐bearing trough structures at Kambalda, Western Australia

The Lunnon Basalt is an Archaean, subaqueous, tholeiitic metabasalt succession, with a minimum inferred thickness of 1750 m. It forms the oldest exposed stratigraphic unit at Kambalda in the nickel sulfide‐rich Norseman‐Wiluna Greenstone Belt, Western Australia, and is dominated by variable proportions of massive basalt, pillow basalt, and basalt breccia. These facies form intimately inter‐layered massive, pillow and complex lava flow units, with an average thickness of about 20 m. The stratigraphy of the basalt can be subdivided into a ‘lower’ MgO‐rich member and an ‘upper’ less MgO‐rich member, these being separated by a sedimentary horizon. Of the possible palaeovolcanic and tectonic settings (layer 2 of oceanic crust, a large shield volcano, or a tectonically‐topographically ponded sea‐floor lava field), the Lunnon Basalt appears to be the remains of a ponded (?rift) lava succession. The presence of inherited zircon xenocrysts indicates that the subjacent crust at the time of formation was sialic, Arc...

Acritarchs in polydeformed and highly altered Cambrian rocks in western Victoria

The paucity of microfossils in the Cambrian volcanic and sedimentary succession west of the Avoca Fault in western Victoria is generally attributed to their low preservation-potential in the polydeformed and highly altered early Palaeozoic host rocks. The resulting lack of biostratigraphic age control has made temporal correlations between Cambrian successions difficult, leading to a relatively poor understanding of the Cambrian palaeogeographic and palaeotectonic evolution of the region. Here, we present evidence of a diverse range of acritarchs and a rare possible conodont from siliceous mudstone (chert) of the middle Botoman (ca 515 Ma) Albion Formation at Stawell and the middle Middle Cambrian (ca 505 Ma) Glenronald Shale Member near Mt Stavely. Acritarchs are polyphyletic organisms that include the vegetative and resting cysts of unicellular protists, mainly phytoplankton, which underwent rapid diversification during the Early Cambrian. The morphologically heterogeneous populations of Early Cambrian acritarchs enable them to be used as biostratigraphic indicators. Therefore, the discovery of several populations of acritarchs in successions that have relatively good age control represents an important advance, because these fossils may be used to assist with temporal correlations of successions in southeastern Australia. Moreover, the well-preserved nature of these microfossils means that a variety of biota may be preserved in chert in highly deformed and altered Cambrian successions elsewhere in East Gondwana that currently have poorly constrained ages.

Structural and lithological controls on the high-grade Hangingwall Reef quartz – gold veins, Stawell, Victoria

The Hangingwall reefs are high-grade auriferous quartz lodes that occur in the upper levels of the Magdala mine at Stawell, and thus represent an important deposit style in western Victoria. Structural relationships indicate that the Hangingwall Reefs formed coincident with the same metallogenic event that produced the adjacent Magdala deposit. However, the host-rocks to the quartz – gold veins that make up the Hangingwall Reefs display markedly different structural and stratigraphic relationships. Hangingwall Reef mineralisation occurred during east – west shortening in a muscovite-altered turbidite sequence that had little prior iron alteration (cf. Magdala deposit). Lithological and structural data show that the distribution of the quartz – gold veins is related to the geometry of the Stawell Fault and its associated fault splays, with the quartz veining being localised where the faults are discordant with the pre-existing structural fabrics. This discordant relationship produced the dilational sites and jogs in which the auriferous quartz veins formed. Results from 3D numerical modelling show that dilation along the Stawell Fault provided the conduit for fluid flow.