摘要:

Quartz‐andalusite‐fuchsite rocks in an Archaean greenstone belt at Menzies, Western Australia, are described in their geological setting. They are shown to have developed from intense metasomatism of layered rocks of komatiitic composition. Ratios of the immobile components A12O3, TiO2, Cr, V and Zr are consistent, despite wide variations in their absolute values, and compare closely with those of an underlying komatiite suite. Marked depletion of Ca, Na, Mg and Fe has led to a strongly peraluminous composition and enhanced Cr values. Silicification and introduction of K has also occurred. Most of the metasomatism took place before peak metamorphism, and the mineral assemblage is now dominated by andalusite, fuchsite, and recrystallized quartz. Schlieren or vein‐like segregations rich in andalusite, chromite, rutile and minor sulphides and tourmaline are interpreted as the original fluid pathways, where concentration of immobiles was achieved through solution of mobile components, and volume loss. They have been termed residual veins. Metasomatism is believed to have taken place by synvolcanic processes analogous to those operating in modern hot spring systems. The rocks were severely modified by metamorphism and tectonism. The fuchsitic rocks at Menzies are compared to similar rocks in other areas.

ISSN:0812-0099    

DOI:10.1080/08120098608729346

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Environmental radiocarbon was used to study aspects of groundwater origin, transport and mixing in the aquifer system underlying northern Victoria. The clearing of native vegetation and the advent of irrigation on the northern plains have upset the hydrological balance, caused water tables to rise and in some areas caused the waterlogging and salination of the land. Groundwater shallower than 30 m only yielded modern radiometric dates. Groundwater in the ‘Calivil/Renmark Aquifer’ in the Loddon and Goulburn Valleys has yielded ages up to 20 000 years and has a flow rate between 0.13 and 0.01 m day‐1. These values agree with the mean pore velocity calculated using Darcy's Law.

ISSN:0812-0099    

DOI:10.1080/08120098608729347

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Faulting and folding are interspersed and locally interrelated in the Lake Moondarra area. Minor faulting occurred during deposition, after which the region was deformed by three folding events, developing weak slaty cleavages and regional folds. Geometrical constraints and overprinting relations between associated quartz veins have established the relative timing of cleavages. Following F1 folding, major E‐W trending faults formed with dip‐slip motion followed by an interval of strike‐slip movement. The second deformation produced N‐S folds in S0 and S1, and a slaty cleavage, S2. Some faults were dilated and infilled locally with quartz and country rock fragments during the second deformation or slightly earlier. The fault‐filling material contains a foliation, with about the same orientation as S2 throughout the region, and also a mineral‐elongation lineation of aligned quartz fibres and deformed elongate quartz grains. There was a further component of strike‐slip movement, the sense of which changes across and was apparently controlled by F2 folds. The third deformation caused NNW‐SSE folds and an associated cleavage. During D3 some faults underwent further extension and dilation, parallel and perpendicular to the maximum finite extension direction (i.e. L33), after which another group of faults formed at a low angle to S3 and subsequently underwent a component of sinistral strike‐slip displacement.

ISSN:0812-0099    

DOI:10.1080/08120098608729348

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

Lake Tyrrell, a saline playa in semi‐arid north‐western Victoria, records a long history in which a succession of lacustrine and aeolian environments can be related to past hydrologic variations. Cores through the saline evaporitic facies reveal a vertical pattern reflecting cyclic changes through time. Detrital clastics predominated during deep‐water lacustral phases; evaporites were deposited during drying phases. A model, depicting surface‐groundwater interaction during discrete stages of a typical cycle, relates changes in water depth, salinity and typical depositional facies. On the drying trend, the sequence evolves through carbonate to sulphate deposition. Progressive reduction in water level results in partial drying and production of the groundwater outcrop playa stage. Salt efflorescence and production of detrital pelletal clays provide parent materials for aeolian transport and dune building. Any additional fall in watertable permits downward leaching of salts, plant colonization of the lake floor and associated soil development. Facies variations reveal four hydrologic cycles within this saline sequence reflecting major climatic events in the Brunhes normal chron. The modern playa‐salina phase represents a relatively late stage development. The mineral suite, including dolomitic carbonate, sulphide and gypsum‐clay laminites, formed in this continental setting closely resembles facies described from coastal sebkhas, highlighting the problem of distinguishing between the origins of similar deposits in the ancient record.

ISSN:0812-0099    

DOI:10.1080/08120098608729349

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

The Coppin Gap copper‐molybdenum mineral deposit is situated in the NE of the Pilbara region, near the northern margin of the Mount Edgar Batholith. The prospect is associated with felsic intrusives occupying a position near the top of a suite of tholeiitic basalts and extends through an overlying mixed assemblage of lavas and tuffs. A Rb‐Sr isochron age of 3234 ± 117 Ma (with an initial ratio of 0.7027 ± 0.0012), is reported for a suite of feldspar porphyry and granodiorite samples from the mineralized region. This age is consistent with reported Rb‐Sr ages from the Mount Edgar Batholith, and suggests a close association between the mineralization at Coppin Gap and the batholith. The reported age represents a minimum age for the mineralization at Coppin Gap, and suggests that this deposit is one of the oldest ‘porphyry style’ copper‐molybdenum deposits in the world.

ISSN:0812-0099    

DOI:10.1080/08120098608729350

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

The Broken Hill Block, in far western New South Wales, comprises complexly folded and metamorphosed Proterozoic metasediments and metavolcanics of the Willyama Supergroup, and pre‐ and post‐folding intrusives. The history of the Block is described in four tectonic stages. The precratonic stage commenced with separation of crustal material from the mantle at 2100–2300 Ma. Deposition of the Willyama Supergroup occurred at about 1820 Ma. These rocks were metamorphosed to amphibolite‐granulite facies and deformed twice at about 1660 Ma, in the Olarian Orogeny. A third deformation with accompanying retrograde metamorphism occurred soon after. Before 1570 Ma, retrograde schist zones had formed, some with associated kyanite and staurolite bearing metamorphic assemblages. The transitional tectonic stage commenced at 1490 ± 20 Ma, with the emplacement of Mundi Mundi type granites and minor muscovite pegmatites. Between 1490 Ma and about 1100 Ma, the presently exposed rocks were elevated from 13 to 20 km below the surface, to surface and near surface levels. During the epicratonic stage (1100–500 Ma), continental and shallow marine sediments of the Adelaide System plus minor basalt were deposited in grabens or half‐grabens on Willyama basement. Substantial erosion of the Willyama Supergroup occurred. Alkaline ultramafic plugs and dykes were intruded at 561 ± 7 Ma, and a thermal pulse at 520 ± 20 Ma marks the Delamerian Orogeny. This was accompanied by low grade metamorphism, re‐activation of retrograde schist zones, intrusion of dolerite plugs, dykes, and zoned pegmatites, and gentle folding of the Adelaidean rocks. Peridotite stocks and dykes may have accompanied the ultramafic intrusions at 561 ± 7 Ma, or the dolerite intrusions at 520 ± 20 Ma. During the cratonic stage (500 Ma to present), faulting occurred, fluorite bearing veins and siderite rich Pb‐Ag veins formed. At about 280 Ma, either a thermal event occurred, or substantial erosion dropped the temperature of the presently exposed rocks to less than 100°C. Between 280 Ma and the present there was substantial weathering, and significant uplift and erosion. Silcrete and ferricrete were formed locally, possibly in the Oligocene (36–23 Ma). Specific events of uplift and erosion probably occurred in the Palaeocene to Mid Eocene (65–45 Ma) and Late Miocene to Pliocene (10–2 Ma). These left the Broken Hill Block in its present state, a series of slightly uplifted, tilted and incised sub‐blocks, with adjacent alluvial fans. The Willyama Supergroup contains the Broken Hill Pb‐Zn‐Ag orebody and abundant minor deposits. The top of the orebody was removed by erosion and it is possible that other orebodies have been eroded away. The main period of erosion of potentially ore‐bearing Willyama Supergroup was between 1490 Ma and 1100 Ma. Substantial erosion also occurred between 800 Ma and 600 Ma, between 520 Ma and 280 Ma, or at about 280 Ma, and at intervals during the Cainozoic. It seems likely that the Broken Hill orebody was partly eroded at 280 Ma, and possibly also during the Cainozoic.

ISSN:0812-0099    

DOI:10.1080/08120098608729351

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

摘要:

A tectonic analysis of the faulting and slickensides at the Woodsreef Asbestos Mine has resulted in the preliminary determination of the principal axes of Early Permian deformation that may have been responsible for the syntectonic emplacement of the serpentinite and the local development of the faults, fractures, schistosities and the asbestos fibres: lengthening (X)—18°, 258; shortening (Z)—72°, 097; and intermediate (Y)—4°, 348. These values are compatible with the known geometries of the structures in the Woodsreef Serpentinite and the X and Y directions are subparallel to the orientation of the asbestos fibres. These results can also be utilized to demonstrate the transtensional nature of the Peel Fault during the Early Permian.

ISSN:0812-0099    

DOI:10.1080/08120098608729352

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor

ISSN:0812-0099    

DOI:10.1080/08120098608729345

出版商:Taylor & Francis Group    

年代:1986

数据来源: Taylor