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1.
Malcolm T. McCulloch William Compston Dereck Froude 《Australian Journal of Earth Sciences》2013,60(1-2):149-153
Sm‐Nd and Rb‐Sr isotopic data for Archaean gneisses from three localities within the eastern Yilgarn Block of Western Australia indicate that the gneisses define a precise Rb‐Sr whole rock isochron age of 2780 ± 60 Ma and an initial 87Sr/86Sr of 0.7007 ± 5. The Sm‐Nd isotopic data do not correspond to a single linear array, but form two coherent groups that are consistent with a c. 2800 Ma age of crust formation, with variable initial Nd. These results indicate that the gneiss protoliths existed as continental crust for a maximum period of only c. 100 Ma, and probably for a much shorter time, prior to the formation of the 2790 ±30 Ma greenstones. 相似文献
2.
Investigations described from three Victorian damsites indicate that weathering of Ordovician bedrock is increased where it is overlain by basalt, even though the basalt is little weathered. Sub‐basaltic weathering may account for the fact that weathered rock beneath a Jurassic lava flow gives a weathering remagnetization age of Mid‐Cainozoic, consistent with palaeomagnetically determined ages of lateritization from elsewhere in Australia. 相似文献
3.
The Tari‐Koroba district is at an altitude of about 1,500 m in the Southern Highlands of Papua in an imbricate province in which uplift and faulting occurred in Pliocene times. In the southeast of the area Miocene limestones are overlain by volcanic rocks erupted about 0.85 m.y. ago, as determined from K‐Ar measurements. These middle Pleistocene flows crossed the course of the Tagari River, damming its waters to form Lake Haibuga. Sediments accumulated in this and neighbouring basins. At one site near Pureni, diprotodontid remains were found; wood associated with the deposit gave a 14C age of 32,700 yrs B.P. Pollen from this fossiliferous horizon and from another section nearby (14C age of 38,600 yrs B.P.) indicate that the climate was cooler than at present and perhaps equivalent to that experienced at about 600 m higher elevation today. Pollen from stratigraphically lower samples indicate still colder conditions and may reflect a glacial phase in the highlands earlier than the last glaciation. 相似文献
4.
The Proterozoic Soldiers Cap Group, a product of two major magmatic rift phases separated by clastic sediment deposition, hosts mineralised (e.g. Pegmont Broken Hill‐type deposit) and barren iron oxide‐rich units at three main stratigraphic levels. Evaluation of detailed geological and geochemical features was carried out for one lens of an apatite‐garnet‐rich, laterally extensive (1.9 km) example, the Weatherly Creek iron‐formation, and it was placed in the context of reconnaissance studies of other similar units in the area. Chemical similarities with iron‐formations associated with Broken Hill‐type Pb–Zn deposit iron‐formations are demonstrated here. Concordant contact relationships, mineralogy, geochemical patterns and pre‐deformational alteration all indicate that the Soldiers Cap Group iron‐formations are mainly hydrothermal chemical sediments. Chondrite normalised REE patterns display positive Eu and negative Ce anomalisms, are consistent with components of both high‐temperature, reduced, hydrothermal fluid (≥250°C) and cool oxidised seawater. Major element data suggest a largely mafic provenance for montmorillonitic clays and other detritus during chemical sedimentation, consistent with westward erosion of Cover Sequence 2 volcanic rocks, rather than local mafic sources. Ni enrichment is most consistent with hydrogenous uptake by Mn‐oxides or carbonates. Temperatures inferred from REE data indicate that although they are not strongly enriched, base metals such as Pb and Zn are likely to have been transported and deposited prior to or following iron‐formation deposition. Most chemical sedimentation pre‐dated emplacement of the major mafic igneous sill complexes present in the upper part of the basin. Heating of deep basinal brines in a regional‐scale aquifer by deep‐seated mafic magma chambers is inferred to have driven development of hydrothermal fluids. Three major episodes of extension exhausted this aquifer, but were succeeded by a final climactic extensional phase, which produced widespread voluminous mafic volcanism. The lateral extent of the iron‐formations requires a depositional setting such as a sea‐floor metalliferous sediment blanket or series of brine pools, with iron‐formation deposition likely confined to much smaller fault‐fed areas surrounded by Fe–Mn–P–anomalous sediments. These relationships indicate that in such settings, major sulfide deposits and their associated chemical sediment marker horizons need not overlie major igneous sequences. Rather, the timing of expulsion of hydrothermal fluid reflects the interplay between deep‐seated heating, extension and magmatism. 相似文献
5.
The Coolgarra Batholith in north‐east Queensland is composed of the Go Sam and Nettle granite suites which were emplaced approximately 314 Ma ago. Initial 87Sr/86Sr ratios of the Nettle Suite appear to be marginally above 0.710, a value characteristic of a wide range of Late Palaeozoic felsic igneous rocks in the region. This suite appears to have been derived from an extensive isotopically homogeneous crustal source distinct from rocks which crop out in the Palaeozoic Hodgkinson Basin or Precambrian Georgetown Inlier. Higher and more variable initial 87Sr/86Sr ratios (0.727–0.744) of the Go Sam Suite are attributed to a combination of magmatic and hydrothermal processes. Isotopic data, thus, cannot be used to constrain the nature of the Go Sam source rocks. Tungsten (with or without tin) mineralization is associated with the Nettle Suite plutons, with which it shares a common age, whereas the Go Sam Suite is characterized by prominent tin mineralization. One tin occurrence is the same age as, or only slightly younger than, the Nettle Suite tungsten (with or without tin) mineralization. Another tin deposit yields an age of 301 Ma, suggesting the presence of a significantly younger granite at depth. 相似文献
6.
The geochemistry of two deerite‐bearing meta‐ironstones from the Emo Metamorphics of Papua New Guinea suggests that they were deposited as metalliferous cherts enriched in manganese and iron by hydrothermal exhalative activity in an ocean ridge system. Subsequent blueschist facies metamorphism resulted in the formation of the assemblage deerite‐quartz‐albite‐iron oxides‐alkali amphiboles‐spessar‐tine‐stilpnomelane‐apatite, with calcium‐manganese carbonates in one specimen. Assemblages in associated metabasites suggest P‐T conditions of 7 kb at 320°C, which overlap with the P‐T field defined by one of the meta‐ironstones. Oxygen fugacity was probably an important control in determining variations in mineralogy and mineral chemistry. Preliminary data on the trace element geochemistry of associated metabasites are consistent with an ocean ridge environment for the formation of the meta‐ironstones. 相似文献
7.
Hydrogen‐ and oxygen‐isotope analyses of biotite (19), sericite (8), chlorite (2), quartz (27), and total rocks (37) from the Panguna porphyry‐copper deposit on Bougainville Island, place important constraints on the origin of the hydrothermal fluids responsible for mineralization and alteration in the mine region. Early high‐temperature amphibole‐magnetite alteration resulted from magmatic‐hydrothermal fluids. Several lines of evidence indicate 500°C as a realistic average temperature for mineralization, development of quartz veins, and biotitization processes. On the basis of mineral isotope data, responsible fluids could represent either 18O‐shifted ground‐waters or magmatic‐hydrothermal fluids at submagmatic temperatures. Independent evidence, as well as total‐rock 18O data, support the magmatic‐hydrothermal model. Late‐stage sericitization processes probably resulted from fluids produced by 18O shifting of groundwaters during the evolution of the propylitic zone. Outermost quartz veins and biotitization conceivably resulted from fluids similar to those that caused sericitization, indicating that some interaction between relatively cool, 18O‐poor meteoric waters and the ore fluids occurred near the margins of the deposit. The origin of the chlorite‐sericite alteration cannot be resolved solely by isotope studies. 相似文献
8.
R. W. Page 《Australian Journal of Earth Sciences》2013,60(3-4):141-164
A detailed Rb‐Sr total‐rock and mineral and U‐Pb zircon study has been made on suites of Proterozoic silicic volcanic rocks and granitic intrusions, from near Mt Isa, northwest Queensland. Stratigraphically consistent U‐Pb zircon ages within the basement igneous succession show that the oldest recognized crustal development was the outpouring of acid volcanics (Leichhardt Metamorphics) 1865 ± 3 m.y. ago, which are intruded by coeval, epizonal granites and granodiorites (Kalkadoon Granite) whose pooled U‐Pb age is 1862 +27 ‐21 m.y. A younger rhyolitic suite (Argylla Formation) within the basement succession has an age of 1777 ± 7 m.y., and a third acid volcanic unit (Carters Bore Rhyolite), much higher again in the sequence, crystallized 1678 ± 1 m.y. ago. All of these rocks are altered in various degrees by low‐grade metamorphic events, and in at least one area, these events were accompanied by, and can be partly related to, emplacement of a syntectonic, foliated granitic batholith (Wonga Granite) between 1670 and 1625 m.y. ago. Rocks that significantly predate this earliest recognized metamorphism, have had their primary Rb‐Sr total‐rock systematics profoundly disturbed, as evidenced by 10 to 15% lowering of most Rb‐Sr isochron ages, and a general grouping of many of the lowered ages (some of which are in conflict with unequivocal geological relationships) within the 1600–1700 m.y. interval. Such isochrons possess anomalously high initial 87Sr/86Sr ratios, and some have a slightly curved array of isotopic data points. Disturbance of the Rb‐Sr total‐rock ages is attributed primarily to mild hydrothermal leaching, which resulted in the loss of Sr (relatively enriched in 87Sr in the Sr‐poor (high Rb/Sr) rocks as compared with the Sr‐rich rocks). 相似文献
9.
Ground‐penetrating radar surveys across the southern end of the Boco Plain, western Tasmania, revealed a complex sequence of Quaternary glacial and non‐glacial sediments. The subsurface imaging supported previous suggestions of a complex Boco Plain palaeotopography that incorporates a range of depositional environments and multiple constructional events. The ground‐penetrating radar technique enabled imaging of the sediments to 20 m depth, and permitted identification of different sedimentary facies and constructional events due to the significant contrast in dielectric constant within and between the sediments and bedrock. The bedrock and sediment stratigraphy are in broad agreement with drillcore records from the southern end of the Boco Plain and indicate the utility of the method in the initial stages in the investigation of Pleistocene sedimentary sequences of this type. 相似文献
10.
《International Geology Review》2012,54(1):77-93
In the Lycian Basin (SW Turkey), the Miocene Karabay?r and Karaku?tepe formations consist of algal limestone, conglomerate, sandstone, shale and limestone. Total organic carbon (TOC) analysis of the Miocene units show that these formations are poor in organic matter. TOC values are generally between 0.02 and 0.51%, but reach 3.47% in the Karabay?r Formation. Hydrogen indices (HI) are mostly below 600 mgHC/gTOC, increasing to 1200 mgHC/gTOC in the Karabay?r Formation. S2 vs. TOC diagrams are used to evaluate the sedimentary environments and hydrocarbon potential of the Lower–Middle Miocene sediments (the Isparta, Bucak and Korkuteli‐Elmal? areas). The organic material contains about 63 (type I), 35 (type II) and 29 (type II/III) pyrolysable hydrocarbons, respectively. The dominant organic matter is type II kerogens, and hydrocarbon generating potential is quite low. A positive x‐intercept has been calculated in analysed samples according to S2 vs. TOC diagrams; this value shows a rock‐matrix effect. Clay is the main agent of adsorption. Biomarker characteristics also verify these results. Isoprenoid rates are Pr/Ph: 2, Pr/n‐C17: 1.9, and Pr/n‐C18: 0.5, and a high Pr/Ph ratio (pristane/phytane) indicates an oxic environment; the terpane C29 NH/C30 H ratio is >1 for the Karabay?r Formation, and this value indicates a carbonate lithology. On the other hand, the C25 NH/C30 H ratio is <1 for the Karaku?tepe Formation; this indicates that the hydrocarbons were derived from terrestrial organic matter. According to m/z 191 mass fragmentograms, the Miocene units contain oleanane, indicating a Tertiary age. The abundance of sterane C29>C28>C27 shows that the kerogens formed from algal organic matter. 相似文献
11.
12.
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. 相似文献
13.
K. A. W. Crook 《Australian Journal of Earth Sciences》2013,60(1):195-202
Two approaches to the time‐stratigraphic subdivision of the Australian Pre‐cambrian are possible. In one, bodies of rock are referred, by means of radio‐metric dates, to periods defined by chosen points on the absolute time‐scale. In the second, the boundaries of time‐rock units are defined at selected reference points on the ground, and these reference points are dated as accurately as possible by radiometric means. Geographically distant bodies of rock are correlated with “type” time‐rock units by radiometric dates. The second approach is preferred since: (a) it leads to a geological time‐scale homogeneous in concept; (b) it implies that there is no difference in principle between Precambrian and Phanerozoic time‐stratigraphy; and (c) the time‐rock units so created are directly comparable with those that might be established by lithological, palaeontological, magnetometric, or climatic‐litho‐logical correlation. Criteria and procedures for the definition of time‐rock units and their boundaries are suggested. 相似文献
14.
Measurements of 18 closely spaced beach profiles spanning Warilla Beach, N.S.W., over a 5‐year period have been examined for alongshore exchanges in beachface configuration. Horizontal slices of beach corresponding with berm, upper swash, upper and lower intertidal zones were examined by empirical orthogonal function (EOF) analysis. The first four EOF's for each of the four slices contain over 90% of the total variance for each slice. In each instance the first eigen‐function mode represents the onshore‐offshore component of beach response, and accounts for 50 to 60% of the variance. Subsequent modes describe alongshore exchange of sediment associated with large, standing, cellular water circulations of the embayment. Sediment shunts along each horizontal slice are determined by identification of nodal points in the eigen‐functions. These represent pivotal points through which sediment is transferred. The patterns of exchange vary considerably from slice to slice, reflecting change in the dominant beach processes from swash processes in the top of the beach to wave, tidal and current processes on the lower beach slope. Despite their disparity, similar periodicities are determined for the sediment exchanges in each slice. The amplitude spectra of the time series associated with each eigen‐function indicate that the exchanges take place at 24, 12 and 6 monthly periods. 相似文献
15.
Natural Hazards - Drought is considered to be one of the most devastating natural disasters. In recent years, determination of historical droughts has gained more importance. This can be attributed... 相似文献
16.
C. L. Fergusson 《Australian Journal of Earth Sciences》2013,60(4):475-490
Palaeolatitude data obtained from palaeomagnetic studies of Australian formations are described and compared with the palaeoclimatic zones inferred from geological observations. The two techniques produce results which agree for most of the Palaeozoic. Only for the Early Cambrian (and late Proterozoic) and Mesozoic do the climatic indicators appear to contradict the palaeolatitude evidence. It is pointed out that each of these geological intervals follows immediately a period of widespread glaciation. 相似文献
17.
At present, most, if not all zinc is being produced from the mineral sphalerite. With the exception of its piezoelectricity and in some cases its density, sphalerite has no salient physical properties to routinely allow its direct detection by geophysical methods. Although the mineral usually occurs with other sulfides, which do respond to various geophysical techniques, detection of zinc‐bearing deposits is often difficult. In this paper, physical property measurements from numerous, mainly Australian, deposits are presented, together with examples from various geophysical surveys. The results are used to suggest the most effective geophysical technique for the direct detection of the different types of zinc‐bearing deposits. Induced polarisation is probably the most effective technique for unmetamorphosed sediment‐hosted deposits and for carbonate‐hosted deposits, whereas electromagnetics is usually the most appropriate method to use for volcanic‐hosted deposits and for sediment‐hosted deposits that have been strongly metamorphosed. Magnetics is the best technique for skarn and ‘Cobar‐style’ deposits. However, it must be emphasised that usually more than one geophysical technique is required before a prospect can be considered to have been thoroughly explored. 相似文献
18.
《International Geology Review》2012,54(2):116-132
Lower Miocene Boyalik volcanic rocks, situated approximately 80 km south of Ankara, exhibit both alkaline and calc‐alkaline characteristics. Alkaline products are trachybasaltic and trachyandesitic, whereas calc‐alkaline products are dacitic. The phenocrysts in the dacites consist primarily of plagioclase and hornblende, with lesser amounts of biotite. The groundmass contains plagioclase and quartz microcrysts. Trachyandesites are mainly composed of plagioclase and biotite phenocrysts with a groundmass of alkali feldspar microlites and minor clinopyroxene microcrysts. Trachybasalts are mainly composed of olivine and plagioclase phenocrysts, with minor clinopyroxene phenocrysts associated with alkali feldspar, plagioclase and clinopyroxene microlites and microcrysts in the groundmass. Oxides are common accessory phases in all products. Boyalik volcanic rocks have essentially homogeneous incompatible trace element patterns with variable Nb and Th anomalies, enrichment in Rb, Ba, K, La, Ce and Nd, and positive Sr anomalies. Some trace element ratios (e.g. Ba/Ta, Ba/Nb, Th/U and Ce/Pb) are variable among the series. For instance, dacites and trachyandesites have higher Ba/Ta (724–2509), Ba/Nb (45–173) and Th/U (3.5–8.7) and lower Ce/Pb (7.1–3.9) values than the trachybasalts. Trace element data indicate that the series are chemically distinct but probably were derived from a common lithospheric mantle source via variable degrees of partial melting. The magmas then underwent a process of evolution involving assimilation and fractional crystallization (AFC) during ascent to the surface. Although trachyandesites and dacites were generated from a lithospheric mantle source via ～1% and ～1.5% to ～5% degrees of partial melting, respectively, trachybasalts were derived from the same source via higher degrees of partial melting (～20%) with neglegible crustal contamination. Boyalik volcanism is linked to an intracontinental transpressional setting. However, the overall geochemical features are consistent with derivation from a mantle source that records earlier Eocene subduction between the Sakarya continental fragment and the K?r?ehir block during time. 相似文献
19.
We compare two methods of evaluating head covariance for two‐dimensional steady‐state flow in mildly heterogeneous bounded
rectangular aquifers. The quasi‐analytical approach, widely used in stochastic subsurface hydrology, is based on the Green's
function representation, and involves numerical four‐fold integration. We compare this approach with a numerical solution
of the two‐dimensional boundary‐value problem for head covariance. We show that the finite differences integration of this
problem is computationally less expensive than numerical four‐fold integration of slowly‐convergent infinite series.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
The oxide mineralogy and rock magnetic properties of unmineralised banded iron‐formations in selected portions of four drillholes in the Hamersley Basin, Western Australia are reviewed. In all four drillholes, petrographic studies indicate that primary euhedral to subhedral hematite is partially replaced by magnetite as a result of subsolidus reduction. All drillholes show partial recrystallisation of the secondary magnetite, suggesting that early subsolidus reduction was probably a regional event occurring during prograde metamorphism. Incomplete replacement of primary hematite by magnetite within and between sedimentary band structures indicates that equilibration in the magnetite stability field was not reached even at the mesoband scale. Subsequent subsolidus oxidation of magnetite and the formation of a second‐generation hematite are documented in only two of the drillholes. Goethite‐filled veins and thick selvages of goethite around some veins reflect movement of circulating oxidising fluids. The absence of goethite and second‐generation hematite in two of the drillholes indicates that subsolidus oxidation is not a regional event, but very much localised. Rapid changes in down‐hole susceptibility measurements correlate directly with detailed petrographic results as susceptibility readings change with the hematite/magnetite ratio on a mesoband scale. Acquisition of the main remanence correlates with the formation of hematite as the primary oxide phase followed by partial replacement by magnetite as a result of subsolidus reduction, supporting regional models requiring pre‐folding remanence. The strong orientation of the primary hematite parent parallel to band structures in the banded iron‐formations has influenced the direction of crystallisation remanent magnetisation during subsolidus reduction to the magnetite daughter. The strong planar alignment has also produced a planar magnetic fabric and marked anisotropy of magnetic susceptibility. A natural remanent magnetisation overprint and reduction in anisotropy of magnetic susceptibility are only recorded in samples that have undergone subsolidus oxidation and the recognition of localised post‐metamorphic oxidation overprinting can also explain ore deposit models requiring post‐folding remanence. The relative timing of and between oxidising fluid events is not known, but both petrographic and rock magnetic evidence to date suggests that there was at least one and probably two post‐folding oxidising events in the area of study. 相似文献