摘要:

A discrete, roughly circular, steep‐sided magnetic anomaly outlines a buried trachyandesite plug at Gearless Well in the Murchison Province of the Yilgarn Block. Agglomeratic texture indicates high level emplacement and probable venting. The mildly alkaline, silica‐saturated, heterogeneous trachyandesite is markedly alumina‐deficient and enriched in large ion lithophile elements and, to a lesser extent, high field strength elements. A seven‐point whole rock linear alignment on the Rb/Sr isochron diagram of samples from one drill hole is interpreted as a two‐component mixing line of meaningless age. Samples from another drill hole indicate the presence of a third component. Two two‐point biotite‐whole‐rock measurements yield similar ages which pool to give 2188 ± 11 Ma, the estimated time of emplacement of the trachyandesite plug. Samples defining the seven‐point line were produced by small to moderate amounts of partial melting of LIL‐enriched (large ion lithophile elements) upper mantle, and the line is interpreted as representing a mixture of depleted and fertile mantle material. The group of samples reflecting the third component is believed to be derived by a larger degree of partial melting of the same LIL‐enriched upper mantle source. The scatter observed in this group is probably due to interaction of the volatile and fluid‐charged magma with wall‐rock during ascent through the crust.

ISSN:0812-0099    

DOI:10.1080/08120098908729492

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The Siluro‐Devonian, volcano‐sedimentary rift basin sequence at Breadalbane, NSW, has undergone three phases of deformation, with biotite grade metamorphism occurring during the first two events. A peak metamorphic temperature of 425 ± 30°C is indicated by calcite‐dolomite geothermometry, and a pressure of 4.0 ±0.5 kbar is estimated from the Si content of white mica. These P/T estimates give a geothermal gradient of 30°C/km, consistent with the inferred tectonic environment in the Lachlan Fold Belt during the Devonian.

ISSN:0812-0099    

DOI:10.1080/08120098908729493

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Four types of Pleistocene clastic dykes are present in a variety of sediments in the King Valley. They include till dykes injected into bedrock fractures produced by overriding ice; gravel dykes in bedrock openings that appear to be eroded and stream‐filled bedding planes; gravel dykes in weathered limestone formed as fillings of dolines and solution tunnels; and gravel dykes in unconsolidated Pleistocene deposits. Gravel‐filled, wedge‐shaped structures in Pleistocene tills and outwash gravels are the most numerous dykes and most occur in three distinct swarms. The dykes of one swarm have along‐slope strikes and are formed on laminated glacial lake sediments that have been subjected to landsliding; the dyke structures apparently formed as tensions cracks caused by the landslides. The other two swarms have downslope strikes and are associated with sediments deposited in ice contact environments. Although these dykes resemble ice wedge casts, they probably formed syndepositionally by the collapse and deformation of the sediments as buried ice melted.

ISSN:0812-0099    

DOI:10.1080/08120098908729494

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Nauru Island, in the central Pacific Ocean, is a raised atoll capping a volcanic seamount rising from an ocean floor depth of 4300 m. The land area is 22 km2, and the island rises to 70 m above sea level. Drilling has proved dolomitized limestone of Late Miocene or younger age to a depth of 55 m below sea level. Gravity and magnetic surveys indicate that the substructure of Nauru is approximately radially symmetrical. Bouguer anomaly increases by 18 mgal from the coast to the centre of the island. The magnetic field has a range of 830 nT with a negative anomaly located over the north‐northwest coastline and a large east‐west‐elongated positive anomaly over the southern sector of Nauru. Modelling of the magnetic field suggests that the island is underlain by a reversely magnetized volcanic core with magnetization 1.5–1.9 A/m and depth of 500 m. The orientation of the magnetization vector gives the palaeomagnetic age of the core as mid‐Eocene to Oligocene. This age range for the evolution of Nauru is confirmed by an estimate based on island subsidence. Nauru may have been constructed by volcanism associated with Cainozoic transit across the Samoan hot spot. Gravity modelling indicates a density of 2.5 t/m3for the island pedestal. The calculated values for density and magnetization are typical for Pacific seamounts, implying that basement beneath Nauru's carbonate platform is composed mainly of basaltic lavas.

ISSN:0812-0099    

DOI:10.1080/08120098908729495

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

The regional pattern of deformation in the Leonora district is the result of three main episodes of deformation, each of which produced distinctive structures and regional features. The earliest episode was nappe‐like, and produced gently dipping mylonite, orthogneiss and schist in discrete zones of deformation. These zones are easterly striking in areas not affected by later deformation events, but movement directions are not consistent throughout the region, being south‐directed in the west and north‐directed in the east. The second episode resulted in a widespread north to northwest‐striking crenulation of the early schistosity and long wavelength, open, upright regional folds. These folds affected the early orthogneiss. Regional northwest‐striking strike‐slip faults, dominated by the sinistral Mt George shear zone, with an attendant cleavage and sub‐horizontal stretching lineation, are the product of a third episode of deformation. In the east a southeasterly striking, widely spaced crenulation is evidence of a fourth episode, whereas in the west, east‐northeast‐striking faults cross‐cut all earlier structures. Mineral deposits are invariably associated with deformational structures in the Leonora district. The larger deposits (e.g. Sons of Gwalia) lie within early shear zones up to 0.5 km wide, which can be traced through a number of prospects to the north. Later large shear zones are largely unmineralized, although small fourth generation faults are the site of small prospects. A model of main mineralization during early deformation with remobilization of ore into later structures is consistent with the regional structural evolution of the district.

ISSN:0812-0099    

DOI:10.1080/08120098908729496

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Recent work in the Mount Isa Inlier has suggested that the Mount Albert Group forming the Deighton ‘Klippe’ (or outlier) is the stratigraphic equivalent of, but now thrust over, the Mary Kathleen Group. New mapping indicates that three other outliers of the Mount Albert Group are underlain conformably and gradationally by the Mary Kathleen Group on their western sides, but are vertically or normally faulted against older rocks on their eastern or northeastern sides. One of the normal faults is clearly pre‐ or early D2, but post‐D2 movement on the other two has obliterated any evidence of an earlier history. The Deighton Outlier is in largely bedding‐parallel fault contact with the underlying Mary Kathleen Group, but the fault is folded by and therefore pre‐dates D1. Excision of stratigraphic units along the eastern limb of the outlier indicates the extensional nature of the fault contact.

ISSN:0812-0099    

DOI:10.1080/08120098908729497

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

A short wavelength (<250 km) residual Bouguer gravity image of eastern Australia emphasizes anomalies and anomaly patterns related to continental crustal structure in comparison with a total‐field gravity image. Filtering removes the masking effect of long wavelength anomalies, whether they have a shallow origin (e.g. extensive platform cover) or a deep source (e.g. mantle). Short wavelength anomalies are mainly caused by density differences within the crust and give a clear indication of source shape. The filtered image is particularly useful for providing basement information for large regions concealed beneath Late Palaeozoic, Mesozoic and Cainozoic cover where subsurface information is limited.

ISSN:0812-0099    

DOI:10.1080/08120098908729498

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

摘要:

Low grade metasediments and metavolcanics of the Hill End Synclinorial Zone within the Rockley district, NSW have experienced two phases of macroscopic folding (D1 and D2), both of which are post‐latest Silurian in age. No hiatus is evident between D1 and D2. D1 produced large Fi folds (λ/2 usually > 2 km) lacking mesoscopic elements and having variable axial trends. D2 was associated with the development of regional slaty cleavage (S2) and mesoscopic folds which are parasitic on plunging macroscopic F2 folds (λ/2=0.4–2 km). D2 strain is variable, being most intense in the north of the district where slaty cleavage and tight mesoscopic F2 folds are well developed, and weakest in the south where mesoscopic folds are absent or usually gentle and cleavage is often feebly developed even in mica‐rich rocks, which are stratigraphic equivalents to slates and schists in the north. The F1 fold mechanism may involve multiple folding, simultaneous folding in more than one direction, or complex buckling of layers of variable thickness. D1 and D2 are tentatively correlated with folding events elsewhere in the Hill End Synclinorial Zone.

ISSN:0812-0099    

DOI:10.1080/08120098908729499

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

ISSN:0812-0099    

DOI:10.1080/08120098908729500

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor

ISSN:0812-0099    

DOI:10.1080/08120098908729501

出版商:Taylor & Francis Group    

年代:1989

数据来源: Taylor