G. M. Gibson

Age constraints on metamorphism and the development of a metamorphic core complex in Fiordland, southern New Zealand

Fiordland, southern New Zealand, contains two primary geologic components: an elongate core comprising high-pressure (>12 kbar) granulite facies orthogneisses (western Fiordland orthogneiss) and a structurally overlying mid-Paleozoic plutonic-metasedimentary complex with subordinate metavolcanics (Tuhua Sequence). The contact between these two components is a major structural discontinuity or decollement hitherto defined as the Doubtful Sound thrust, but reinterpreted here as a ductile shear zone separating lower and upper crustal plates generated during continental extension. Strongly lineated Lower Cretaceous mylonites occur throughout this zone; they formed prior to separation of New Zealand from Australia (at ∼80 Ma). Continental rifting was accompanied by increased heat flow, silicic to mafic magmatism, and the widespread resetting of mineral isotopic systems. Thus, in terms of its overall tectonic setting and tectonothermal history, Fiordland bears a striking resemblance to the metamorphic core complexes of the North American Cordillera.

Discussion on detachment faulting and bimodal magmatism in the Palaeoproterozoic Willyama Supergroup, south-central Australia: keys to recognition of a multiply deformed Precambrian metamorphic core complex

C. H. H. Connor, W. V. Pries, R. W. Page, B. P. J. Stevens, I. R. Plimer and P. M. Ashley write: Gibson & Nutman (2004) postulate that the Willyama Supergroup in the southern Curnamona Province contains a 1690–1670 Ma metamorphic core complex, and imply that this is relevant to the genesis of the Broken Hill Pb–Zn–Ag deposit. We contend that this is a model-driven, speculative interpretation predicated upon unsupported assertions that conflict with substantial stratigraphic and geochronological data. Not only does the paper fail to demonstrate an early high-grade event and formation of a metamorphic core complex at 1690–1670 Ma, but it is also factually incorrect in several critical geological and geochronological aspects. The detachment concepts of Gibson & Nutman may appear persuasive, but only by omitting reference to a huge body of published and recent mapping and laboratory research, by companies, universities, and geological surveys, some of which are cited here. Surprisingly, Gibson & Nutman ignore much recent work (e.g. Page et al . 2004) to which one of them has contributed. The essence of the model is that a low-angle, extensional ‘detachment’ formed along the boundary between the Broken Hill and Sundown Groups in the Broken Hill Domain, and between the Bimba and Plumbago Formations in the Olary Domain (Gibson & Nutman, fig.1). The ‘detachment’ was alleged to have occurred as part of a sillimanite-grade D1 deformation between 1690 and 1670 Ma, and to have provided a channel for hydrothermal fluids that could have been responsible for regional scale Na (–Fe) alteration and formation of Pb–Zn mineral deposits, including the Broken Hill orebody. As pointed out below, most of the evidence is misinterpreted, misleading and/or ambiguous, and the geochronological case advanced by Gibson & Nutman for such an early (1690–1670 Ma) high-grade event has no foundation. …

New SHRIMP geochronology for the Western Fold Belt of the Mt Isa Inlier: developing a 1800 – 1650 Ma event framework ∗

The integration of detrital and magmatic U – Pb zircon SHIRMP geochronology with facies analysis has allowed the development of a chronostratigraphic framework for the Leichhardt and Calvert Superbasins of the Western Fold Belt, Mt Isa Inlier. This new event chart recognises three supersequences in the Leichhardt Superbasin: the Guide, Myally and Quilalar Supersequences. The Guide Supersequence spans the interval ca 1800 – 1785 Ma and includes the Bottletree Formation and the Mt Guide Quartzite. Sequence relationships suggest that this sedimentary package represents an asymmetric second-order cycle, recording a thickened transgressive suite of deposits and a comparatively thin second-order highstand. The overlying Myally Supersequence spans the interval ca 1780 – 1765 Ma and includes the Eastern Creek Volcanics and syndepositional Lena Quartzite, and the Myally Subgroup. This package represents a second-order supersequence cycle in which mafic volcanism was initiated during a phase of east – west extension. Following the cessation of volcanism, transgression led to the deposition of the Alsace Quartzite and deeper water Bortala Formation. An increase in the rate of sediment supply over accommodation resulted in progradation and deposition of the Whitworth Quartzite and redbed playa facies of the Lochness Formation as accommodation closed. The Quilalar Supersequence spans the interval ca 1755 – 1740 Ma. Sequence analysis in the eastern part of the Leichhardt River Fault Trough identifies a transgressive suite of facies at the base of this supersequence. Black shales from the upper part of the transgressive deposits characterise the condensed section for this supersequence. Facies analysis indicates that deposition took place in a series of storm-, tide- and wave-dominated shelfal marine depositional systems. Although there are no new depositional age constraints for the younger Bigie Formation, field relationships suggest that it is coeval with, or immediately preceded, the ca 1710 Ma Fiery magmatic event. Therefore, a separate supersequence is defined for the Bigie Formation, the Big Supersequence, even though it may be more genetically related to the Fiery magmatic event. The Big Supersequence, together with the ca 1690 Ma Prize Supersequence, comprise the Calvert Superbasin. The evolution of the Leichhardt and Calvert Superbasins are temporally and spatially related to magmatism. In particular, the new maximum depositional ages for the Guide and Myally Supersequences refine the age of the Eastern Creek Volcanics to ca 1780 – 1775 Ma. The new age for the Weberra Granite is within error of the age for the Fiery Creek Volcanics, indicating that they are both part of the ca 1710 Ma Fiery event. New ages for the Sybella Granite confirm that magmatism associated with this magmatic event is refined to 1680 – 1670 Ma, and is followed by deposition of the Gun Supersequence. Combining the new geochronological constraints with previous work now provides a detailed stratigraphic event framework between 1800 and 1575 Ma for the Western Fold Belt of the Mt Isa Inlier, and allows detailed comparisons and correlations with the Eastern Fold Belt and other Proterozoic terranes.

Black Giants Anorthosite, New Zealand: A Paleozoic analogue of Archean stratiform anorthosites and implications for the formation of Archean high-grade gneiss terranes

The Black Giants Anorthosite, a mid-Paleozoic (349 ± 5 Ma U-Pb zircon age) layered anorthosite complex in Fiordland, New Zealand, bears striking compositional and lithologic similarities to Archean stratiform anorthosites and, like many of its Archean counterparts, occurs within a high-grade gneiss terrane, preserving a record of metamorphism at mid-crustal depths followed by higher-pressure metamorphism and burial to lower-crustal levels. These and other similarities point to formation of the Black Giants Anorthosite and its Archean equivalents in comparable tectonic environments, most likely a subduction-related magmatic arc which, in the case of Fiordland, resulted from plate convergence along the Pacific margin of Gondwana.

Pre-existing basement structure and its influence on continental rifting and fracture zone development along Australia’s southern rifted margin

Palaeogeographical reconstructions of the Australian and Antarctic margins based on matching basement structures are commonly difficult to reconcile with those derived from ocean-floor magnetic anomalies and plate vectors. Following identification of a previously unmapped crustal-scale structure in the southern part of the early Palaeozoic Delamerian Orogen (Coorong Shear Zone), a more tightly constrained plate reconstruction for these margins is proposed. This reconstruction places the Coorong Shear Zone opposite the Mertz Shear Zone in Antarctica and lends itself to a revised interpretation of continental rifting along Australia’s southern margin in which rift basin architecture, margin segmentation and the formation of ocean-floor fracture zones are all linked to pre-existing basement structure and the reactivation of a few deep-rooted crustal structures inherited from the Delamerian Orogeny in particular. Reactivation of the Coorong Shear Zone and other basement structures (Avoca–Sorell Fault Zone) during the earlier stages of rifting was accompanied by the partitioning of extensional strain and formation of late Jurassic–Early Cretaceous normal faults and half-graben in the Bight and Otway basins with opposing NE–SW and NW–SE structural trends. Previously, the Mertz Shear Zone has been correlated with the Proterozoic Kalinjala Mylonite Zone in the Gawler Craton but this positions Australia 300–400 km too far east relative to Antarctica prior to breakup and fails to secure an equally satisfactory match in both basement geology and the superimposed extension-related structures.

New SHRIMP age constraints on the timing and duration of magmatism and sedimentation in the Mary Kathleen Fold Belt, Mt Isa Inlier, Australia

Paleoproterozoic magmatic rocks from the Mary Kathleen Fold Belt of the Mt Isa Inlier record different magmatic textures and variations in tectonic strain associated with extension and the development of crustal-scale detachment zones. New SHRIMP U–Pb zircon geochronology for magmatic rocks, combined with field relationships, refine the duration of this extension to between 1780 and 1740 Ma. The initial stages of this tectonomagmatic event are coincident with mafic magmatism, basin formation and rapid sedimentation of the ∼1780–1765 Ma Myally Supersequence of the Leichhardt Superbasin in the adjacent Leichhardt River Fault Trough. The Ballara Quartzite and Corella Formation represent a period of sag-phase sedimentation during the later part this event, and facies models, sequence-stratigraphic interpretations and detrital-zircon geochronology data confirm the time equivalence of these units to the Quilalar Supersequence of the Leichhardt River Fault Trough. These correlations permit the Eastern and Western Successions of the Mt Isa Inlier to be correlated from 1780 Ma. Locally, the Corella Formation is intruded by 1740 Ma granites, suggesting that at least the lower parts of this package were deposited during the 1780–1740 Ma extensional event. By linking deep-crustal extension processes in the Mary Kathleen area with near-surface basin formation in the adjacent Leichhardt River Fault Trough, it is possible to develop crustal-scale architecture models that provide insights into the development and migration of ore-bearing fluids.

Depositional systems in the Mt Isa Inlier from 1800 Ma to 1640 Ma: Implications for Zn–Pb–Ag mineralisation

Two facies models are proposed to explain siliciclastic and carbonate depositional systems of 1800 Ma to 1640 Ma age in the Western Fold Belt of the Mt Isa Inlier. Both models record the response of depositional systems to storm-driven processes of sediment transport, dispersal and deposition on a shallow water shelf. The same suite of facies belts can also be identified in sedimentary successions of the Eastern Fold Belt. Slope driven processes of sediment transport and dispersal characterise turbidite and debrite deposits of the Soldiers Cap Group and Kuridala Formation and provide evidence for significantly greater water depths in this part of the basin from ca 1685 Ma. Through the recognition of unconformity surfaces, their correlative conformities, maximum flooding and ravinement surfaces, the facies belts are packaged into seven supersequences for the interval ca 1800–1640 Ma. The new correlations are shown in an event chart that correlates linked depositional systems across the entire Mt Isa Inlier. Thick successions of turbidite and debrite deposits are restricted to the eastern parts of the Mt Isa Inlier and do not occur in the Western Fold Belt. A major phase of extension and rifting commenced at ca 1740 Ma and by ca 1690 Ma led to significant crustal thinning and increased rates of accommodation over an area east of the Mount Dore Fault and Burke River Structural Belt. In the Mitakoodi Anticline and Kuridala-Selwyn Block, the rapid transition from shallow-water shelf depositional systems of the Prize Supersequence to significantly deeper-water slope environments of the Gun Supersequence coincided with the development of a platform margin, the deposition of turbidite and debrite deposits in deep water on the continental slope and the intrusion of mafic sills and dykes. Turbidite and debrite depositional systems of the Soldiers Cap Group and Kuridala Formations are restricted to a lowstand wedge of siliciclastic facies deposited basinward of a platform margin. Basin geometries and sediment architectures associated with this extensional event and recorded in the Gun Supersequence (ca 1685 Ma to 1650 Ma) provide an explanation for the geographic separation and fluid evolution pathways responsible for the Mt Isa Type and Broken Hill Type Zn–Pb–Ag deposits.