摘要:

The Charleston Granite from the Gawler Craton, South Australia, has been dated by the ion‐microprobe U‐Pb zircon method at 1585 ± 5 Ma (2σ). This confirms previous interpretations of population‐style U‐Pb zircon analyses which record a slightly older age due to the presence of inherited zircon. Inherited cores are present in many zircon crystals, and while the age of some cores can not be accurately determined due to extreme loss of radiogenic Pb, others have ages of ∼ 1780, ∼ 1970, and > 3150 Ma. These cores record a diverse crustal heritage for the Charleston Granite and indicate that ancient crustal material (> 3150 Ma) is present at depth in the Gawler Craton. This is also suggested by available Nd isotopic data for both the Charleston Granite and other Gawler Craton Archaean rocks. The Rb‐Sr and K‐Ar biotite ages from the Charleston Granite of 1560 to 1570 Ma are close to the U‐Pb zircon crystallization age and suggest that the granite has not experienced sustained thermal disturbance (>250° C) since emplacement and cooling. However, a much younger Rb‐Sr total‐rock age of 1443 ± 26 Ma probably reflects low‐temperature disturbance to the Sr isotope system in feldspar.

ISSN:0812-0099    

DOI:10.1080/08120099308728101

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Sedimentological, geochemical and tectonic studies have been carried out on the Glyde Sub‐basin, a fault‐bounded depocentre adjacent to the margin of the Batten Trough, 80 km south of the HYC Pb‐Zn‐Ag ore deposit, in the mid‐Proterozoic McArthur Basin. Although it is unmineralized, the basin is, in some aspects, morphologically similar to the HYC Sub‐basin and provides an insight into processes which occurred coevally along strike from a giant shale‐hosted base‐metal deposit. The geometry of the sub‐basin supports an origin in a releasing bend of the Emu Fault during oblique right‐lateral extension of the Emu Fault Zone, resulting in the deposition of a very thick sequence of below wave‐base Barney Creek Formation carbonaceous siltstone. Prior to sub‐basin development the area was covered by hypersaline carbonate tidal flats of the Coxco Dolomite Member of the Teena Dolomite.

ISSN:0812-0099    

DOI:10.1080/08120099308728102

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Isolated outcrops of coarse‐ and medium‐grained, quartz‐feldspar porphyry occur for some 60 km from Nobles Nob to near Warrego in the Early Proterozoic Tennant Creek Block of the central Northern Territory. The outcrops appear to be part of an approximately conformable sheet of variable thickness (several tens to a few hundred metres), enclosed by sedimentary rocks of the Warramunga Group. The porphyry is characterized by euhedral, complete, evenly distributed crystals up to 2 cm across, in a microcrystalline groundmass. In some samples the groundmass shows relict perlitic cracks and was formerly coherent glass. In detail, upper and lower contacts of the porphyry sheet are highly irregular: porphyry contains wisps, blocks and large rafts of sandstone, and sandstone encloses single euhedral crystals and blebs, tongues and lobes of porphyry. Clasts of each rock type commonly have fluidal shapes. Sedimentary rocks at both the upper and the lower contacts are indurated and silicified. Close to the contacts, bedding is not easily identified, or else is disturbed. The quartz‐feldspar porphyry is interpreted to have cooled and solidified from phenocryst‐rich magma that intruded approximately parallel to bedding in the enclosing sedimentary sequence. The peperite margins of the sill suggest that the porphyritic magma invaded relatively weak, poorly consolidated, wet sediments rather than solid sedimentary rock. Rapid expansion and movement of heated pore fluid would have temporarily disrupted particle packing in the sediments at the contacts, and allowed intricate penetration of magma accompanied by quenching and fragmentation. In addition to relatively short‐term and local effects on the pore fluid chemical and physical properties, the intrusion was probably responsible for premature and permanent dewatering of the adjacent sediments. The sill together with its bordering zone of peperite and indurated sedimentary rocks may have constituted a significant physical barrier to any subsequent fluid transport in the enclosing sedimentary sequence.

ISSN:0812-0099    

DOI:10.1080/08120099308728103

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Ion probe U‐Th‐Pb dating of zircons from the Late Archaean granites of the Norseman region of the southeastern Yilgarn shows the existence of two distinct magmatic episodes. Large regional tonalite and granodiorite plutons were emplaced between 2685 and 2690 Ma, whereas large regional granite, and small tonalite and leucogranite plutons that intrude the greenstones have ages of 2660–2665 Ma. A small body of granite that intrudes the western edge of the greenstones has an inferred emplacement of 2672 ± 7Ma, and contains inherited zircon that is ∼2800 Ma. The monzogranite core from a second pluton in a similar structural position also contains ∼2800 Ma zircon; this age is similar to published Sm‐Nd and Rb‐Sr whole rock ages for banded gneisses associated with other members of this suite of domal plutons and is interpreted as representing the age of a significant component within the source region for these distinctive rocks.

ISSN:0812-0099    

DOI:10.1080/08120099308728104

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The Coopers Creek Limestone represents an Early Devonian redeposited carbonate accumulation and records the evolution of a carbonate slope in the southeastern portion of the Melbourne Trough. During the earliest Devonian, the underlying Boola Formation was deposited, probably as turbidites, in a moderately deep‐water setting. The presence of chert and greenstone clasts in the top of the formation indicates exposure of an area of Cambrian greenstones in this part of the Melbourne Trough, as a result of uplift associated with the earliest Devonian Bowning Orogeny. This uplift provided ideal conditions for carbonate production along the margin of the exposed landmass. The periodic transportation of carbonate material downslope resulted in the accumulation of the Coopers Creek Limestone. Initially, in the early Pragian, turbidity currents deposited clayey biomicrites and biopelmicrites on a relatively gentle slope. However, the rapid build‐up of carbonate sand banks at the shelf margin steepened the gradient from the shelf into the basin and a bypass margin began to develop. Grainflows deposited pelsparites and biopelsparites and the presence of debris flow breccias indicates erosion of lithified limestone by channelling. Continued carbonate build‐up led to the development of a rimmed reef margin in the earliest Emsian, with a steep fore‐reef gradient. Large blocks of reefal limestone fell or rolled to the base of the slope, to accumulate as reefal megabreccias at the top of the Coopers Creek Limestone. Carbonate production abruptly ceased in the early Emsian, due to the uplift of a quartzo‐micaceous source to the east during the initial stages of the Tabberabberan Orogeny. This uplift supplied abundant terrigenous material into the Melbourne Trough to be deposited as the turbidites of the Walhalla Group, which deeply buried the limestone accumulation.

ISSN:0812-0099    

DOI:10.1080/08120099308728105

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

ISSN:0812-0099    

DOI:10.1080/08120099308728106

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor