R. Offler

Trace element distribution, Co:Ni ratios and genesis of the Big Cadia iron-copper deposit, New South Wales, Australia

Analyses of pyrite, chalcopyrite and magnetite from the volcanic-hosted Big Cadia stratabound iron-copper deposit in Central Western New South Wales show considerable variation in the minor elements Mn, Ba, Ag, Pb, Zn, Cd, Se, Co and Ni. The preferential concentration of Co and Cd in pyrite, Zn and Ag in chalcopyrite and Mn in magnetite can be attributed to variations in activities of the ions in the hydrothermal fluid at the time of crystallisation of the mineral phases, or in cases such as the concentration of Co in pyrite, dependent on compatible electronic spin states between Co2+ and Fe2+. Trace element concentrations, especially Co and Ni contents and Co:Ni ratios in pyrite (average Co:Ni ratio=17.1) support a volcanic exhalative origin of mineralisation at Big Cadia. Differences in trace element composition such as higher Ni contents in pyrite in contrast with other volcanic-hosted ores may reflect the more basic character of volcanic rocks underlying the Big Cadia deposit.

A synthesis of folding and metamorphism in the Mt Lofty Ranges, South Australia

Abstract The Upper Precambrian and Lower Palaeozoic Rocks in the Mt Lofty Ranges, South Australia, have been subjected to at least three phases of folding. The first involved the formation of inclined folds and less common reclined folds. These structures are overprinted by usually upright, moderately tight, second and third generation folds which may show a well developed axial plane crenulation cleavage. The metamorphism commenced prior to the appearance of penetrative structures and continued in many areas until after the third phase of deformation. It appears to have had its greatest effect during the static period following the first phase of folding. Mineral assemblages of the pelitic rocks indicate that the metamorphism is of the low pressure‐intermediate type and that there are at least four progressive zones of metamorphism, namely, chlorite, biotite, andalusite‐staurolite, and sillimanite. Cordierite occurs in the sillimanite zone and kyanite is sporadically distributed in the andalusite‐staurol...

Timing and development of oroclines in the southern New England Orogen, New South Wales

40Ar/39Ar dating of slate and phyllite in the Nambucca Block, southern New England Orogen, indicates that deformation took place between 264 and 260 Ma. These data further show that indentation of the Early Permian turbidite successions by the southward-moving Coffs Harbour Orocline ceased at this time. Folding of the Sakmarian to Sarginian sequences in the Silver Spur Basin (294.6 – 275.6 Ma) during the megafolding event constrains the formation of the Texas Orocline to be post-ca 276 Ma. The presence of brachiopod faunas indicates that orocline formation commenced in the early Kungurian (ca 273 Ma) during the early stages of the Hunter – Bowen Orogeny. Thus, development of the Texas and Coffs Harbour Oroclines took place within a period of 13 million years, from 273 to 260 Ma. Oblique convergence is believed to be responsible for the formation of the Texas and Coffs Harbour Oroclines and associated constrictional structures during the Hunter – Bowen Orogeny. Slip on a dextral, strike-slip fault formed during convergence, buckled and rotated the meridional-trending arc – forearc – subduction complex to form north-northwest-trending, Z-shaped megafolds.

Structural-metamorphic evolution of the tia complex, new england fold belt; thermal overprint of an accretion-subduction complex in a compressional back-arc setting

Rocks in the Tia Complex underwent a simple, clockwise P-T-t-deformation path, moving from mid-crustal levels at blueschist facies conditions (D1, D2; 6 kbar, 200°C) to upper-crustal levels at high-T amphibolite facies conditions (D4, D5; 2.5 kbar, 600°C), in a relatively short time (∼ 15 Ma). The rocks remained under these conditions for approximately 40 Ma (D6) before they cooled and were brought to the surface (D7, D8). During D6,Permian rift basins were opening as thermal gradients in the Tia Complex reached a maximum. D1–2 deformation was associated with accretion-subduction processes and was followed by progressively less intense, mainly E–W oriented compression and uplift during D3–5, bringing the rocks to within 2 kbars of the surface. D6 is characterized by limited extensional deformation, and final uplift took place during D7–8, along shear zones that have orientations similar to D3–5 thrusts. The shift from subduction-dominated processes to thermally-dominated processes occurred during D3–4 uplift of ∼4 kbar, around 310-300 Ma. This coincided with a change in tectonic setting of the Tia Complex from an accretionary prism to a back-arc position, as the subduction zone stepped eastward. An increased heat flow, associated with back-arc processes, gradually caused thermal weakening of the lithosphere, which, due to external, subduction-related, compressive forces, resulted in uplift (D3). With continued heating of the crust, thermal doming resulted and uplift stopped (D4–5), whilst thermal weakening led to limited crustal collapse as Permian basins opened (D6). The extremely high heat flow that was maintained for ∼40 Ma. suggests a heat source external to the crust, possibly associated with thermal relaxation, dehydration and detachment of the remnant stab left beneath the New England Orogen after the subduction zone shifted east. A stable crustal, thermal structure was re-established by 260 Ma, as the crust strengthened and could further support subduction-induced, compressional structures (D7–8).

Carboniferous to Lower Permian stratigraphy of the southern Tamworth Belt, southern New England Orogen, Australia: Boundary sequences of the Werrie and Rouchel blocks

Carboniferous to Lower Permian successions along the western border of the Tamworth Belt between Wallabadah and Muswellbrook were remapped to clarify the stratigraphy and establish a boundary between the Werrie and Rouchel blocks. The boundary, located at the Waverley Fault, separates Carboniferous sequences containing different formations and volcanic members. SHRIMP AS3 dating of volcanic members indicates that successions within the Rouchel and Gresford blocks were deposited, uplifted and eroded at different times. The lacustrine Woodton Formation in the Werrie block, previously considered Carboniferous, is earliest Permian (Asselian) from palaeobotanical and SHRIMP AS3 evidence. Stratigraphic and palaeoenvironmental differences between the Werrie and Rouchel blocks suggest that they were not directly juxtaposed during the greater part of the Carboniferous, supporting palaeomagnetic evidence that blocks within the Tamworth Belt are allochthonous. Superposition of the western extremity of the Waverley Fault by Lower Permian (Sakmarian) formations and intrusion of folded and faulted Devonian to Lower Permian successions by the Barrington Tops Granodiorite (ca 280 Ma) indicate that the Werrie, Rouchel and Gresford blocks were subjected to tectonism before the Late Permian Hunter – Bowen Orogeny.

Exhumation of late Paleozoic blueschists in Queensland, Australia, by extensional faulting

Blueschists in southeastern Queensland, Australia, record a Carboniferous history of subduction and metamorphism at >6 kbar. A later thermal overprint associated with intrusion of S type granitoids ca. 306 Ma and regional greenschist facies metamorphism accompanied formation of ductile fabrics in the lower plate of an oceanic-protolith metamorphic core complex. Late Carboniferous extensional deformation in this part of the New England orogen may have accompanied outstepping of the subduction trench, or rollback and steepening of the paleo-Pacific plate during development of a wide zone of asthenospheric upwelling and continental back-arc extension. -Authors

Tectonic implications of RbSr biotite ages for the Hillgrove Plutonic Suite, New England Fold Belt, N.S.W., Australia

Abstract The Late Carboniferous (∼ 300 Ma) Hillgrove Plutonic Suite is a deformed, fault-bound suite of S-type granitoids in the southern New England Fold Belt of eastern Australia. Large-scale, W-over-E, high-angle, reverse faulting along major crustal shear zones has exhumed high-grade metamorphic terranes such as the Tia and Wongwibinda migmatite complexes, thereby exposing a tilted section of crust. Exhumed lower amphibolite facies mylonite zones within granitoids have been dated by biotite RbSr using both neocrystallized and relict magmatic biotite. Combined, the analyses define a narrow age range of 266-258 Ma, which indicates that faulting and uplift occurred during the Late Permian. Although the initial temperature conditions during shearing were well above the biotite RbSr closure temperature, these mylonites cooled rapidly, as indicated by identical RbSr ages for relict muscovite and biotite in the adjacent Wongwibinda Complex. RbSr biotite ages for granites not directly affected by ductile shearing range from 296 to 256 Ma, the oldest of which is within error of the ∼ 300 Ma emplacement ages given by UPb zircon dating. RbSr ages obtained from relict biotite in lower-grade, greenschist facies mylonites define a wide range of ages (290-271 Ma), similar to the range acquired for the unsheared granites. The data suggest that shearing in the low-grade mylonites took place at, or just below, the biotite RbSr closure temperature. This range of biotite ages, which varies systematically across the fault blocks, reflects the combined effects of slow cooling from 300 Ma, followed by rapid uplift and tilting of the terrane at ∼ 260 Ma. Biotite RbSr ages of less than 250 Ma are only recorded where samples are affected by local static thermal recrystallization associated with intrusion of Triassic granites. The biotite RbSr data place important chronological constraints on the tectonic evolution of the southern New England Fold Belt. Up to 12 km of Late Permian (268-256 Ma) uplift was the culmination of major E–W compression, which involved large-scale dispersal, oroclinal bending and tilting of crustal blocks.

Unconformities as mineralogical breaks in the burial metamorphism of the Andes

Stratigraphical-structural units separated by regional unconformities in the Andes of Peru and Chile, display a pattern of low grade burial metamorphism. Each stratigraphical-structural unit shows a particular facies series covering part or all the range between the zeolite and the greenschist facies. These facies series were episodically generated as part of the geological evolution of each unit prior to its own folding. Mineralogical breaks are found to coincide with the regional unconformities and often cases of higher grade assemblages on top of lower grade ones occur. This pattern may be explained by a process of “ sealing” of each unit after its particular metamorphic episode took place. Porosity and permeability conditioning Pf, as demonstrated for individual lava flows, are the significant controlling factors in the production of the metamorphic assemblages.

Upper Carboniferous to Lower Permian volcanic successions of the Carroll‐Nandewar region, northern Tamworth Belt, southern New England Orogen, Australia

Geological mapping in the Carroll‐Nandewar region near the northwestern margin of the Tamworth Belt has identified an Upper Carboniferous to Lower Permian succession, the Willuri Formation, containing coarse volcaniclastic sediments and interbedded dacitic to rhyolitic pyroclastics, lavas and a possible dome or coulée. The Currangandi Group (new) accommodates Upper Carboniferous Volcaniclastic formations in the northern Tamworth Belt, including the Willuri Formation. Volcanic rocks in the Willuri Formation are distributed in four stratigraphically distinct, intertonguing or overlapping packages: the Kaputar, Piney Range, Tulcumba and Gunnan packages. According to SHRIMP (AS3) dating most of the succession ranges in age from ca 320 to ca 308 Ma (Namurian‐Westphalian). Dacitic and rhyolitic lavas, apparently in sequence with the succession on the northwestern part of Tulcumba Ridge, are 297–287 Ma or younger (Early Permian). They appear to disconformably overlie the Carboniferous succession and could be either part of the Willuri Formation or related to the Boggabri Volcanics. Volcanic centres near Maules Creek (Kaputar centre) and Wean (Wean centre) appear to have been the sources for volcanics in the Kaputar package and most parts of the Rocky Creek region to the northeast, and the Tulcumba and probably the Piney Range packages, respectively. In the Gunnan package a single major ignimbrite was derived from a volcanic centre in the Werrie pyroclastic field, but the provenance of other pyroclastics is unknown. Correlations based on SHRIMP ages and the distribution of pyroclastics indicate that the Willuri Formation is equivalent to most of the Currabubula Formation in the Werrie Syncline, and to the Rocky Creek Conglomerate and Lark Hill Formation in the Rocky Creek region. Boundaries of the Werrie and Rocky Creek pyroclastic fields are adjusted to incorporate a new pyroclastic field based on the Wean volcanic centre and covering the Tulcumba and Piney Range packages of the Willuri Formation.

Devonian‐Carboniferous stratigraphy of the southern Hastings Block, New England Orogen, eastern Australia

Devonian and Carboniferous rocks within the Hastings Block represent an arc‐related basin fill located, out of place, on the outboard margin of the subduction complex and juxtaposed along strike from the fore‐arc basin of the southern New England Orogen. The stratigraphic sequence, structural style and history of rocks in the southern part of the Hastings Block differs from that in the northern part of the block. The stratigraphy of the southern part of the Hastings Block consists of a succession of nine formations (eight new) ranging in age from Late Devonian (Frasnian) to Late Carboniferous (Namurian). A further five formations (three new) ranging in age from Siluro‐Devonian to Late Devonian (Famennian) are confined to fault‐bounded blocks. Three of the latter are probably correlative with parts of the continuous Frasnian‐Namurian succession. The Port Macquarie Block is considered part of the southern Hastings Block because it contains two closely comparable formations. Late Devonian formations include ...