C.J.L. Wilson

Early Paleozoic tectonism within the East Antarctic craton: The final suture between east and west Gondwana?

New U-Pb SHRIMP ages from East Antarctica point to the existence of a laterally continuous orogenic belt that bisects the East Antarctic craton. This orogenic belt juxtaposes Archean crust to the south and east against Neoproterozoic metamorphic rocks to the north and west. It defines the margin of a separate lithospheric block that consists of a large section of East Antarctica and India that did not form part of east Gondwana or Rodinia as they are currently reconstructed. Instead, this Indo-Antarctic continent accreted with west Gondwana along the Mozambique suture shortly before collision and suturing along a second “Pan-African” suture now cropping out in the southern Prince Charles Mountains and Prydz Bay regions of Antarctica. This scenario is consistent with (1) the abrupt termination of ca. 990–900 Ma tectonism recognized in the northern Prince Charles Mountains–Rayner Complex–Eastern Ghats against Paleozoic orogenic belts, (2) the lack of terranes of equivalent age found elsewhere in either Antarctica or other previously adjacent continents, and (3) the distinct detrital-zircon populations obtained from either side of this proposed suture.

Neoproterozoic deformation in the Radok Lake region of the northern Prince Charles Mountains, east Antarctica; evidence for a single protracted orogenic event

Abstract Ion microprobe dating of structurally constrained felsic intrusives indicate that the rocks of the northern Prince Charles Mountains (nPCMs) were deformed during a single, long-lived Neoproterozoic tectonic event. Deformation evolved through four progressively more discrete phases in response to continuous north–south directed compression. In the study area (Radok Lake), voluminous granite intrusion occurred at ∼990 Ma, contemporaneous with regionally extensive magmatism, peak metamorphism, and sub-horizontal shearing and recumbent folding. Subsequent upright folding and shear zone development occurred at ∼940 Ma, while new zircon growth at ∼900 Ma constrains a final phase of deformation that was accommodated along low-angle mylonites and pseudotachylites. This final period of deformation was responsible for the allochthonous emplacement of granulites over mid-amphibolite facies rocks in the nPCMs. The age constraints placed on the timing of deformation by this study preclude the high-grade reworking of the nPCMs as is postulated in some of the recent literature. Furthermore, 990–900 Ma orogenesis in the nPCMs is at least 50 Myr younger than that recognised in other previously correlated Grenville aged orogenic belts found in Australia, east Africa and other parts of the Antarctic. This distinct age difference implies that these belts are probably not correlatable, as has been previously suggested in reconstructions of the supercontinent Rodinia.

Timing of the progress granite, Larsemann hills: Additional evidence for early Palaeozoic orogenesis within the east Antarctic Shield and implications for Gondwana assembly

The Progress Granite is one of numerous S‐type granitoid plutons exposed in the Larsemann Hills region, southwest Prydz Bay, east Antarctica. The granite was emplaced into a migmatitised pelitic to felsic paragneiss sequence during a regional high‐grade transpressional event (D2) that pre‐dates high‐grade extension (D3). SHRIMP (II) U‐Pb dating for two occurrences of the Progress Granite from D2 and D3 structural domains gives 206Pb/238U ages of 516.2 ± 6.8 Ma and 514.3 ± 6.7 Ma, respectively. These ages are interpreted as crystallisation ages for the Progress Granite and confirm Early Palaeozoic orogenesis in the Larsemann Hills region. This orogen appears to have evolved during continental convergence and is probably responsible for widespread radiogenic isotopic resetting and the near‐complete exhumation of the adjacent northern Prince Charles Mountains which evolved during a ca 1000 Ma event. The identification of a major Early Palaeozoic orogen in Prydz Bay allows tentative correlation of other domai...

Pan-African intraplate deformation in the northern Prince Charles Mountains, east Antarctica

New structural and metamorphic data coupled with U–Pb SHRIMP zircon and Rb–Sr step-leach biotite ages help constrain a period of Early Palaeozoic (Pan-African) deformation recognised in the northern Prince Charles Mountains, east Antarctica. This period of deformation is accommodated along discrete northeast trending mylonites that preserve up-dip reverse kinematics with dominantly southeast over northwest vergence. Ambient P–T conditions of 524±20°C and 7.6±4 kbar accompanied deformation. This phase of deformation post-dated the intrusion of planar felsic dykes that yield ages of c. 550 Ma and pre-dated Rb–Sr biotite ages of c. 475 Ma that record cooling of the terrane below c. 300°C. These mylonites are identical in age to continental collisional events recognised in the southern Prince Charles Mountains and Prydz Bay, which lie to the south and east of the northern Prince Charles Mountains, and similar in age to orogenesis recognised to the west in Lutzow-Holm Bay. These belts represent sutures between the component lithospheric blocks of east and west Gondwana. The northern Prince Charles Mountains lie between these sutures. Consequently, the mylonites we report here are interpreted to have formed in an intraplate setting and developed in response to stresses applied along the plated margins as a consequence of continental collision during the amalgamation of Gondwana.

Geologic framework and tectonic evolution in Western Victoria, Australia

Abstract 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.

Deformation of biotite and muscovite: Tem microstructure and deformation model

Abstract Growth and deformation defects have been observed in 1M and 2M 1 biotites and 2M 1 muscovites using transmission electron microscope (TEM) techniques. These have been related to optical observations. In deformed biotite, the dislocation density increases as the strain increases. The density of stacking faults also increases with strain. The spatial ordering of stacking faults decreases as the strain increases. In muscovites selected from the same areas as the biotites, stacking fault and dislocation densities show little or no variation with deformation. The different responses of biotite and muscovite to deformation are explained, in part, by the physical and chemical differences between 1M biotite and 2M 1 muscovite. There is strong evidence, in both the biotites and muscovites, that stacking faults may be result of deformation as well as a crystal growth process. The grain size of the deformed mica aggregates is reduced by the nucleation of new micas. There is also evidence for a grain size reduction mechanism (segmentation) in biotites. This mechanism appears to achieve the same result as recovery-recrystallization mechanisms, but not by the same method. Segmentation may be a mechanism which is favoured in strongly anisotropic minerals with a limited number of slip-systems. A model is proposed that attempts to correlate response of the biotite to deformation. The deformation is synchronous with, and post-dates, the nucleation of the new metamorphic micas.

Intracrustal detachments and implications for crustal evolution within the Lachlan fold belt, southeastern Australia

Seismic reflection profiling in the southern Lachlan fold belt, Australia, indicates geometrically complex mid- to lower crustal detachments possibly associated with crustal thickening by tectonic underplating of collided microplates and/or oceanic plateaus. Low-angle detachments either intersect or sole into the crust-mantle boundary, forming a crustal-stacking wedge typical of lithospheric- or A-type subduction. Upper crustal reflections show that major, moderately to steeply dipping reverse faults exposed at the surface flatten with depth and have listric form. The chevron-folded quartz-rich turbidite sequence of the Lachlan fold belt is therefore allochthonous with respect to the lower crust. Like the northern Appalachian maritimes, the Lachlan fold belt has a composite crust with diverse lower crustal blocks overlain by allochthonous, imbricated terranes of the surface rocks.

Kinks in mica: Role of dislocations and (001) cleavage

Abstract Kinking in natural and experimentally deformed biotite and muscovite has been investigated by transmission electron microscopy (TEM). It is found that complex microstructures exist in the vicinity of kink band boundaries (KBBs). Gentle bending of (001) planes plays an important role in the kinking process; where (001) planes are bent several degrees, edge dislocations are observed in complex walls between active slip planes together with broad bands of dislocations parallel to (001). Microfracturing and dilation occur parallel to (001) planes and the KBB. A model is presented to explain the origin of observed KBBs which may be applicable to other crystalline materials with only one active slip plane.

Stawell gold deposit: a key to unravelling the cambrian to early devonian structural evolution of the western Victorian goldfields

Major 440 Ma orogenic-gold deposits in the western Victorian goldfields formed during east – west shortening but have markedly different structural complexity. These deposits occur in: (i) a Cambrian Delamerian basement block that was substantially reworked and reactivated during the Lachlan Orogeny (Stawell); and (ii) Ordovician turbidites deformed solely by Lachlan-aged deformation (Bendigo, Ballarat, Castlemaine). This produced different structural histories prior to mineralisation, although gold deposits have been localised at the top of regional domal culminations. At Stawell, the 440 Ma gold event reactivated a strike-change along a pre-existing Cambrian fault system above a major lithospheric boundary. In the Bendigo Zone, 440 Ma orogenic-gold deposits have a trend oblique to the dominant structural grain and parallel to the western edge of an inferred crystalline basement block (the Selwyn Block). This gold trend is parallel to metamorphic field gradients and pluton age boundaries, which suggests an underlying basement control on the localisation of these orogenic-gold deposits, even though the exact basement architecture is still unresolved. Major variations in the regional stress fields occurred between 425 and 370 Ma, with large gold deposits [>62 t (2 million ounces) endowments] forming at ca 380 – 370 Ma. These events are not deposit-scale structural anomalies as they also regionally affect overlying cover sequences (e.g. the Grampians Group). Gold deposits that formed in the 425 – 400 Ma period have small endowments, but introduce a marked amount of structural and mineralogical complexity to the gold province. The 425 – 400 Ma period preserved at Stawell records sinistral wrenching associated with gold mineralisation, southeast-directed faulting, intrusion-related gold mineralisation and extensive high-level Early Devonian plutonism.

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.