摘要:

The Heathcote Greenstone Belt is composed mainly of Lower Cambrian metavolcanic rocks and is one of three outcropping belts of the apparent basement to the Lachlan Fold Belt in SE Australia. The greenstones may be assigned to two broad magma series. A younger tholeiitic series with mid‐ocean ridge basalt (MORB) affinities has intruded through, and been erupted upon low‐Ti, intermediate SiO2lavas. The latter were originally boninites (both clinoenstatite‐phyric and more fractionated orthopyroxene‐phyric varieties) and plagioclase‐phyric, low‐Ti andesites. They have partially re‐equilibrated to the lower greenschist facies and outcrop mainly in the central segment of the Heathcote Greenstone Belt, where deeper stratigraphic levels are exposed. Tholeiitic lavas and sills metamorphosed to the prehnite‐pumpellyite facies dominate the northern and southern segments. As the association boninite/low‐Ti lavas/MORB is known only from modern West Pacific‐type settings involving island arcs and backarc basins, the early history of the Lachlan Fold Belt is inferred to have taken place in a similar setting.

ISSN:0812-0099    

DOI:10.1080/08120098408729290

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

The Ediacara mineral field is situated 30 km W of Beltana on the western margins of the Flinders Ranges, South Australia, and consists of silver‐lead and copper deposits in lower Cambrian carbonate rocks that contain anomalous base‐metal contents throughout the Adelaide Geosyncline. The lower Cambrian rocks, which consist of the basal Parachilna Formation and overlying Ajax Limestone, rest disconformably on the Precambrian, and at Ediacara occupy a shallow N‐S elongate syncline near the hinge zone of the Adelaide Geosyncline. The main primary ore minerals of the silver‐lead mineralization are galena and pyrite, with very minor chalcopyrite and sphalerite, and rare tetrahedrite and pearceite. The gangue consists mainly of silica (both chalcedony and quartz), with minor dolomite and rare barite. The mineralization is stratabound and occurs in conformable zones, the lowest of which commences about 30–50 m above the base of the Cambrian sequence. The host to the silver‐lead mineralization, the Ajax Limestone, can be subdivided into three units which represent a set of lithologies, structures and organic traces indicative of a shallow near‐shore carbonate environment. The silver‐lead mineralization is mainly present in sandy and laminated dolomites which were deposited in an environment ranging from sub‐tidal to bar and channel and tidal flat, respectively. Four types of mineralization have been recognized; disseminated sulphides of syngenetic and/or diagenetic origin and epigenetic concentrations along stylolites, in veins and as breccia fillings. Post‐depositional solution activity has affected a large proportion of the carbonate sequence. The effects of this activity range from stylolites through stylobreccias to solution collapse breccias. The epigenetic concentrations of mineralizations have apparently been formed by the remobilization of the disseminated sulphides during solution activity. The ore and gangue minerals of the epigenetic mineralization display both euhedral forms and distinct colloform banding, and framboidal textures have also been observed in both pyrite and galena. There is evidence of repeated episodic precipitation and no simple paragenetic sequence can be recognized. Fluid inclusions in silica and dolomite associated with the epigenetic mineralization have homogenization temperatures of 159 to 199°C and freezing temperatures that indicate the fluids to be saline brines containing NaCl with CaCl2and/or MgCl2. Sulphur isotope analyses show a range of 834S values from ‐12.5 to +8.6 per mil, with no evidence of significant differences between the four types of mineralization. The data suggest deposition of the disseminated sulphides as a result of biological reduction of seawater sulphate in a system partially open with respect to sulphate supply. Subsequent remobilization of sulphides apparently involved little or no sulphur isotope fractionation. The Ediacara silver‐lead deposits have many features in common with Mississippi Valley‐type lead‐zinc deposits and appear to have similarities in terms of genesis, in that the epigenetic mineralization has been formed as a result of post‐depositional solution activity during diagenesis in a sedimentary basin. The scale of transport of the metals deposited as the epigenetic mineralization at Ediacara appears, however, to have been very much less than that of the metals in other Mississippi Valley‐type deposits.

ISSN:0812-0099    

DOI:10.1080/08120098408729291

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

Well laminated piemontite‐bearing rocks occur as a local manganiferous unit within a metamorphosed, dominantly albite‐rich volcano‐sedimentary sequence forming part of the Olary Block of the Willyama Orogenic Domain. The fine to medium grained rocks contain piemontite, spessartine‐rich garnet, albite, phengite, manganoan tremolite and phlogopite, quartz and Mn‐bearing hematite. Distinctive mineralogical layering is characteristic and appears to represent original sedimentary bedding. Bulk chemical compositions indicate that the piemontite‐bearing rocks represent a Mn (‐Fe‐Ca‐K)‐enriched variant of the regionally widespread albite‐rich rocks;Piemontite and garnet exhibit wide compositional ranges (Pm3.6–35.1PS2.5–25.1CZ51. 4–75.3 and Sp59.2–82.2 Ad6.6–34.6Gro‐16.7Cto‐10. 7, respectively) and associated tremolite and phlogopite are highly magnesian, with Mn > Fe. The coexisting metamorphic assemblage of piemontite, garnet, tremolite, phlogopite and hematite indicate that during crystallization, oxygen fugacity was above the hematite‐magnetite buffer (10∼22—10∼15bars). Estimates of metamorphic temperatures are loosely constrained between 400 and 500°C. The manganiferous rocks possibly represent an original exhalative chemical sediment rich in MnO2 and Fe2O3 within reworked felsic‐intermediate (‐alkaline) tuffs. High oxygen fugacity conditions were maintained during metamorphism, resulting in the piemontite‐bearing assemblages.

ISSN:0812-0099    

DOI:10.1080/08120098408729292

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

The Coffs Harbour Block is a major part of the Tablelands Complex in the SE New England Fold Belt. Complexly deformed rocks of the central Coffs Harbour Block are subdivided into three tectono‐stratigraphic units. The youngest unit is the ?Late Carboniferous Coffs Harbour sequence which consists mainly of turbidites. Greywackes of this unit were derived from the erosion of a silicic volcanic terrain. To the NE of the Coffs Harbour sequence is the Gundahl Complex, a tectonic melange of regional extent. This unit consists of slabs of chert, greenstone, massive greywacke and bedded turbidite sequences embedded in sheared argillite matrix. Greywackes of the Gundahl Complex are oolith‐bearing and rich in intermediate to silicic volcanic rock fragments, characteristics which imply an Early Carboniferous age for greywacke‐bearing blocks. To the NE of the Gundahl Complex are the Willowie Creek beds. These consist mainly of tuffaceous mudstone and fine‐grained intermediate to mafic igneous rocks with minor greywacke, argillite, conglomerate, limestone breccia, chert and jasper. The Siluro‐Devonian Willowie Creek beds are part of a suspect terrain that was accreted to the ancient eastern Australian margin in the ?Middle Devonian. During the Carboniferous all three tectono‐stratigraphic units constituted a subduction complex paralleling the Tamworth‐Yarrol shelves (forearc basin) and the Connors‐Auburn arches (magmatic arc).

ISSN:0812-0099    

DOI:10.1080/08120098408729293

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

U‐Pb isotopic systems of zircons from the Boobina and Spinaway Porphyries from the Precambrian Pilbara Block of Western Australia indicate ages of 3307± 19 Ma and 2768 ± 16 Ma, respectively. The Boobina Porphyry intrudes upper members of the Archaean greenstones of the Warrawoona Group. The Spinaway Porphyry intrudes basal units of the unconformably overlying volcanics and sediments of the Mt Bruce Supergroup. The age of the Boobina Porphyry, together with previous zircon U‐Pb and whole rock Sm‐Nd age determinations on stratigraphically older units, indicate that early Archaean volcanism in the Pilbara took place between 3560 Ma and 3300 Ma. On the basis of the age determination of the Spinaway Porphyry, and the chronometric definition of 2500 Ma for the Archaean—Proterozoic boundary, by the International Subcommis‐sion on Precambrian Stratigraphy (James H. L. 1978,Precambrian Res.7, 193–204), the lower units of the Mt Bruce Supergroup should now be assigned to the Archaean.

ISSN:0812-0099    

DOI:10.1080/08120098408729294

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

摘要:

The IUGS Subcommission on the Systematics of Igneous Rocks recommends that volcanic rocks should be classified on a non‐genetic basis using the total alkali silica (TAS) diagram. This is divided into 14 fields, three of which contain two root names which are separated according to other chemical criteria, giving the following 17 root names: basalt, basaltic andesite, andesite, dacite, rhyolite, alkali rhyolite, trachybasalt, trachyandesite, trachyte, alkali trachyte, picrobasalt, basanite, tephrite, phonotephrite, tephriphonolite, phonolite and foidite. Using other simple parameters, trachybasalt may be further divided into the sub‐root names of hawaiite and potassic trachybasalt, and trachyandesite may be further divided into the sub‐root names of mugearite, shoshonite, benmoreite and latite.

ISSN:0812-0099    

DOI:10.1080/08120098408729295

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor

ISSN:0812-0099    

DOI:10.1080/08120098408729289

出版商:Taylor & Francis Group    

年代:1984

数据来源: Taylor