ISSN:0016-7614    

DOI:10.1080/00167618108729149

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor

摘要:

A review of the large number of stratigraphically controlled isotopic age determinations relevant to the Triassic shows that the ages of the Triassic‐Jurassic and Permian‐Triassic boundaries are approximately 200 Ma and 245 Ma, respectively. Estimates for the period boundaries within the Triassic are as follows: Norian‐Carnian—215 Ma; Carnian‐Ladinian—225 Ma; Ladinian‐Anisian—235 Ma; Anisian‐Scythian—240 Ma. Because of the many uncertainties inherent in these estimations, both stratigraphic and radiometric, the ages of the Triassic system and stage boundaries given above are assigned an error of ± 5 Ma.

ISSN:0016-7614    

DOI:10.1080/00167618108729150

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor

摘要:

The nomenclature of replacement deposits is reviewed in terms of two processes, namely those involving the reaction between: (a) carbonate and siliceous rocks and (b) carbonate and unusual solutions, commonly, but not always, related to granitoids. ‘Wrigglite’ skarn is an example of a replacement deposit produced by process (b). It is highly characteristic rock consisting of alternating light layers of fluorite ± other F‐rich minerals and dark layers composed usually of Fe‐rich minerals or, rarely, Be‐rich minerals. The layers formed parallel or sub‐parallel to fractures, which are now veins, by a Leisegang‐type diffusion process. Repeated periods of fracturing, with growth away from sequentially produced fractures, produced an apparently chaotic ‘wrigglite’ skarn. The skarn is invariably enriched in Fe, F, Sn, Be, W, Bi and Zn. For ‘wrigglite’ to form, fluorine must be greater than 9 weight percent in the rock, and the intrusion of a Sn‐leucogranite ('A‐type') high into the crust is needed. Recognition of ‘wrigglite’ is important because (1) in some areas it is an ore of Sn (Mt Bischoff, Tas.), rare earth metals (Bayan Obo, China) or Be (Lost River, Alaska); (2) it indicates a high‐level environment of emplacement of a Sn‐granitoid; and (3) it indicates clearly that intricate layering can be produced by a diffusional process and not only by primary sedimentation.

ISSN:0016-7614    

DOI:10.1080/00167618108729151

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor

摘要:

Green to blue‐green secondary hornblendes, reportedly common in many granulite facies terrains, occur, predominantly as partial mantles around hypersthenes, in the Mann Ranges, Australia. Electron probe analyses of six such hornblendes show that they have abundant sodium and aluminium and can be classified as ferroan pargasites. Though they have an overall ‘high‐grade’ chemistry, the titanium contents are markedly low, and it is argued that hornblendes of similar major element composition are rich in titanium only when prograde metamorphic reactions promote it. Textural evidence shows that the hornblendes formed later than the hypersthenes and are ‘retrograde’. The hornblende also formed later than ‘retrograde’ biotite. Possibly reduced pressure and temperature, consequent on uplift, facilitated the secondary mineral development but, alternatively, increased availability of water, accompanying abundant shear‐type strain at high temperatures, was responsible.

ISSN:0016-7614    

DOI:10.1080/00167618108729152

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor

摘要:

Application of sedimentological, geochemical and discriminant analysis techniques to the engineering geological investigation of damsites assists in understanding the variation of rock types, stratal correlation, porosity, folding and faulting, through studying the history of depositional and diagenetic environments.

ISSN:0016-7614    

DOI:10.1080/00167618108729153

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor

摘要:

Lord Howe Island is the eroded remnant of a large shield volcano. Tholeiitic lavas of the North Ridge Basalt comprise the main shield building phase and were erupted about 6.9 Ma ago. The Boat Harbour Breccia probably formed within the throat of the volcano and, together with the North Ridge Basalt, is intruded by numerous basaltic dykes, which grade into a cone sheet complex near the main vent. Large scale collapse of the summit area of the volcano produced a caldera which was filled rapidly by lavas of the Mount Lidgbird Basalt some 6.4 Ma ago, bringing to a close the volcanic history of Lord Howe Island. The shield volcano thus was built during a short interval in the late Miocene.

ISSN:0016-7614    

DOI:10.1080/00167618108729154

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor

摘要:

The Lachlan Fold Belt has the velocity‐depth structure of continental crust, with a thickness exceeding 50 km under the region of highest topography in Australia, and in the range 41–44 km under the central Fold Belt and Sydney Basin. There is no evidence of high upper crustal velocities normally associated with marginal or back‐arc basin crustal rocks. The velocities in the lower crust are consistent with an overall increase in metamorphic grade and/or mafic mineral content with depth. Continuing tectonic development throughout the region and the negligible seismicity at depths greater than 30 km indicate that the lower crust is undergoing ductile deformation.

ISSN:0016-7614    

DOI:10.1080/00167618108729155

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor

摘要:

Two bores were drilled in the weathered zone of a Precambrian dolomite at Fowler's Gap, 100 km N of Broken Hill, New South Wales. Five weathering units have been distinguished: fresh dolomite; weathered dolomite, type B; weathered dolomite, type A; ferricrete; and surface hardened dolomite. Physical and mechanical properties show variation throughout the weathering profile, and are related to the fabric of the rock, especially to the fracturing intensity, which is a function of degree of weathering; and to secondary surface silicification. A tentative engineering classification of the weathered dolomite is proposed.

ISSN:0016-7614    

DOI:10.1080/00167618108729156

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor

摘要:

It is difficult to determine the ages of thick stratigraphic units in different parts of the Tablelands Complex in New England, because of a scarcity of fossils and the effects of superposed deformations. Many problems disappear if it is assumed that the Silverwood Group is in part Lower Permian and that its Ordovician and Devonian fossils are in olistoliths derived from the Thanes Creek Slate to the W of the Con‐damine Fault. With that assumption it is possible to divide the Tablelands Complex into four main layers, which in upward order consist of the following sedimentary associations: (1) Woolomin (?Devonian), (2) Sandon and Beenleigh (Late Devonian to Carboniferous), (3) Nambucca, Silverwood and Coffs Harbour (?Late Carboniferous to Early Permian), and (4) Dummy Creek (Late Permian). The four layers can be extended to include sets of stratigraphic units in the Tamworth Belt and Gunnedah Basin.

ISSN:0016-7614    

DOI:10.1080/00167618108729157

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor

摘要:

Woodlark Island, the largest above‐sea portion of the Woodlark Rise, has an exposed basement of pre‐Miocene (?Cretaceous‐Eocene) low‐K tholeiitic basalt and dolerite, and minor sediments. The basement is unconformably overlain by Early Miocene limestone and volcaniclastic sediments and later Miocene high‐K volcanics and comagmatic intrusives. Pleistocene to Recent sediments partly blanket the Tertiary sequence. Basement low‐K tholeiites vary only slightly in composition and are interpreted as ocean floor or possible marginal basin material. The high‐K suite appears to be chemically similar to late Tertiary to Recent high‐K igneous rocks of mainland Papua New Guinea. It includes porphyritic hornblende‐, clinopyroxene‐, biotite‐ and magnetite‐bearing shoshonite, latite and toscanite, and intrusive equivalents that range from olivine normative to strongly quartz normative compositions (S1Q2 46% to 75%). Computer mixing models indicate that separation of the pheno‐crysts in the shoshonites, particularly pargasitic hornblende, is a feasible mechanism for producing the more silica‐rich monzonites and latites.

ISSN:0016-7614    

DOI:10.1080/00167618108729158

出版商:Taylor & Francis Group    

年代:1981

数据来源: Taylor