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1.
K‐Ar ages of biotite and hornblende from undeformed granodiorite plutons and of slaty and phyllitic rocks, ranging from prehnite‐pumpellyite metagreywacke to greenschist fades, have been determined in an attempt to define the age of orogenesis in the eastern part of the Nambucca Slate Belt. The plutons have K‐Ar ages of 226–227 m.y. (biotite) and 228–231 m.y. (hornblende) that provide a younger age limit for deformation. The lower grade metamorphic rocks yield a range of ages including some comparable with the depositional age of the rocks as indicated by fossils. Rocks of pumpellyite‐actinolite and greenschist facies give a more coherent group of ages which suggest orogenesis at about 250–255 m.y. Specimens of these latter rocks that have been affected by a later structural episode than that during which slaty cleavage formed, yield slightly older ages, which may result from the inclusion of minor amounts of environmental excess 40Ar. Support for the 250–255 m.y. age comes from previously determined radiometric ages from the western part of the Slate Belt, although the presence of granitic bodies perhaps as old as 289 m.y., some closely associated with high‐grade regional metamorphic rocks, may indicate the presence of additional earlier orogenic movements in this region. 相似文献
2.
Thirty K‐Ar dates on Cainozoic volcanic rocks lying at the north end of the Bowen Basin suggest that several episodes of volcanism took place at major structural weaknesses. The oldest volcanism (ca 54 m.y.) was located outside the basin structure. The main volcanism (Nebo and East Clermont Provinces) extended from early Oligocene (34–35 m.y.) to mid‐Cainozoic time (21–22 m.y.?). Isolated Pliocene activity is tentatively suggested by dates on Mt St Martin (ca 3 m.y.). Dating of the Nebo central volcano (31–33 m.y.) supports the model of Wellman &; McDougall, with volcanic activity related to migration of Australia northwards over a mantle magma source. Consideration of the Nebo dates with those of other central volcanoes in north Queensland, suggests that central felsic activity was surrounded by broad zones of peripheral eruptives, petrologically zoned from outer undersaturated basalts to inner saturated basalts. These zones (super provinces) delineate the size and profile of underlying magma sources and appear to trend back in time and space to sea‐floor spreading episodes in the Coral Sea—southeastern Papua region (55 m.y.). The basalt dates also assist in fixing periods of lateritization (mid‐Oligocene) and in determining approximate minimum erosion rates in the northern Bowen Basin since the Eocene (3–5m/m.y.). 相似文献
3.
Sedimentological, geochemical and tectonic studies have been carried out on the Glyde Sub‐basin, a fault‐bounded depocentre adjacent to the margin of the Batten Trough, 80 km south of the HYC Pb‐Zn‐Ag ore deposit, in the mid‐Proterozoic McArthur Basin. Although it is unmineralized, the basin is, in some aspects, morphologically similar to the HYC Sub‐basin and provides an insight into processes which occurred coevally along strike from a giant shale‐hosted base‐metal deposit. The geometry of the sub‐basin supports an origin in a releasing bend of the Emu Fault during oblique right‐lateral extension of the Emu Fault Zone, resulting in the deposition of a very thick sequence of below wave‐base Barney Creek Formation carbonaceous siltstone. Prior to sub‐basin development the area was covered by hypersaline carbonate tidal flats of the Coxco Dolomite Member of the Teena Dolomite. Internal syn‐sedimentary normal faulting fractured the sub‐basin into seven major blocks, establishing a basic geometry of northern and southern depressions, into which the W‐Fold and HYC Pyritic Shale Members were successively deposited, separated by a non‐depositional horst. During the subsequent deposition of undivided Barney Creek Formation the horst was submerged and greater water circulation was established. The horst continued to be an east‐west barrier to clastic and volcaniclastic gravity flows, evidenced by the confinement to the northern depocentre of prograding easterly‐derived carbonate‐dominated turbidites. Rhyolitic volcanism in the Glyde Sub‐basin commenced in the W‐Fold Shale Member, and became common in the overlying Barney Creek Formation. The measurable volcanic component increases from 4.4 to 17.5% of the total sediment package southwards over 18 km, implying a southern rhyolitic source within 6–30 km. A geochemical comparison of these relatively unaltered tuffs with those intercalated in the HYC ore sequence identified a comagmatic relationship on the basis of immobile element contents, supporting a common volcanic source. This conclusion was only possible after a preliminary study found Ti, Zr, Y and Nb to be relatively immobile in the severely potassium‐altered tuff of the HYC hydrothermal ore environment. Low‐grade (as distinct from high temperature hydrothermal) potassium‐alteration of felsic tuff throughout the McArthur Basin may have resulted from diagenetic interaction with very evolved lacustrine saline brines, whereas brines in the diagenetic environment of the Glyde Sub‐basin, in which unaltered or sodically‐altered tuff predominates, were comparatively less evolved. 相似文献
4.
The southern part of the Sydney Basin of New South Wales is comprised mainly of Permian and Triassic marine to freshwater clastic sedimentary rocks. Within this sequence there are six latite extrusive units, several medium‐sized monzonite intrusions and a large number of small to medium‐sized basic to intermediate intrusions. Thin basaltic flows were extruded onto the Tertiary topographic surface. All of these rocks are relatively undeformed. Radiometric (K‐Ar) dating has previously been carried out on Mesozoic and Tertiary intrusions and flows of the southwestern portion of the Sydney Basin. However, relatively few Permian, and no post‐Permian, K‐Ar dates have been published for the southeastern portion of the basin. The present investigation provides nine K‐Ar dates from the latter area. Four extrusive and intrusive units have been confirmed as Permian in age (238 ± 6; 241 ± 4; 245 ± 6; and 251 ± 5 m.y.). Five post‐Permian (on stratigraphic criteria) intrusions yielded Tertiary ages (26.2 ± 3.0; 47.9 ± 2.5; 49.0 ± 4.0; 49.4 ± 2.0; and 58.8 ± 3.5 m.y.). The Permian ages agree with previously published K‐Ar data from the southeastern Sydney Basin, and the Tertiary ages complement and extend the data from the southwestern portion of the basin. However, no Mesozoic K‐Ar dates were obtained from the southeastern Sydney Basin. The Tertiary intrusions may have been emplaced as a result of rifting between Australia and New Zealand, or between Australia and Antarctica, or both. 相似文献
5.
40Ar‐39Ar age spectra on minerals from granitic, metamorphic and hydrothermal rocks confirm that the Early Proterozoic Tennant Creek Block was affected by two thermal events during its evolution. Although extensive alteration of biotite and feldspar within the granites precludes the direct determination of their cooling history, 40Ar‐39Ar analyses for hydrothermal muscovite from several nearby gold‐copper deposits indicate that regional cooling to below ～ 300°C was not prolonged. Flat, uniform muscovite age spectra were obtained from gold deposits east of the Tennant Creek town site and indicate a minimum age of 1825–1830 Ma for their formation. These ages are within error of those for the felsic volcanism of the Flynn Subgroup, and a genetic relationship between the two may exist. Samples from gold deposits elsewhere in the area indicate disturbance of the K‐Ar isotope system. The second thermal event to affect the region occurred at around 1700 Ma, and is confirmed by the 40Ar‐39Ar muscovite ages for the ‘Warrego’ granite (1677 ± 4 Ma) and for the metamorphism of the Wundirgi Formation (1696 ± 4 Ma). 相似文献
6.
Dov Bahat 《Australian Journal of Earth Sciences》2013,60(1):93-102
In ternary feldspars of essentially one phase, calcium content has a dominant influence on the optic axial angle. In such feldspars and also in binary feldspars from solvsbergite rocks, variations of cooling histories do not significantly affect the optic axial angle. In ternary feldspars which are unmixed into two or three prominent phases, Al/Si ordering has an important effect on the 2V value. A recent suggestion of several writers that in feldspars the alkali structural site may be partially occupied by (H3O)+ ions is applied as a possible way to explain a correlation observed between petrographical features of the rocks and the optic axial angles of their feldspar phenocrysts. 相似文献
7.
《International Geology Review》2012,54(6):502-519
The Alagoinhas pluton is a member of the widespread high‐K calc‐alkaline association of northeastern Brazil. Some authors suggest that this region represents an amalgamation of distinct tectonic terranes assembled during the Brasiliano (Pan‐African) orogeny. Our work compares geochemical data (major, trace and REE) of the Alagoinhas with other plutons of same petrotectonic association (Caruaru‐Arcoverde batholith). These plutons apparently intrude several distinct tectonic terranes, separated by a major E‐W dextral transcurrent system, the East Pernambuco shear zone (EPSZ). Anisotropy of magnetic susceptibility and structural data for the Alagoinhas pluton are used to compare tectonic regimes across the EPSZ. The results indicate that the Caruaru‐Arcoverde batholith and the Alagoinhas pluton evolved from similar sources and were subjected to the same tectonic regime during emplacement, placing severe restrictions on use of the EPSZ as a suture zone between distinct tectonic terranes. 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.
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
10.
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
11.
Potassium‐argon measurements have been carried out on the separated micas of 27 samples, principally granites, from the Mount Isa‐Cloncurry region of north‐western Queensland. There is evidence for at least two tectonic periods within the “Lower Proterozoic” of the area. The first is represented only in the north‐western portion, with ages greater than 1,770 m.y. on the Ewen Granite, and on the granites of the Nicholson River area to the far north‐west. The second at 1,400–1,450 m.y. is manifested only to the south and east of the Kalkadoon‐Leichhardt complex, and including the Sybella Granite. The results may be further interpreted as lending support to the concept of a possible “metamorphic discontinuity” along the western flank of the Kalkadoon‐Leichhardt complex, postulated by Carter, Brooks and Walker; as suggesting possible contemporaneity of the Cliffdale Volcanics and the Argylla Formation; and as giving further evidence for the antiquity of stromatolites. Comparison with earlier work suggests that some deposits in this region may be contemporaneous with some of the Agicondian sediments of the Katherine‐Darwin area, and that the second tectonic period corresponds with the K‐Ar ages obtained on the Davenportian granites of Central Australia. 相似文献
12.
J. Ayo Olatunji 《Australian Journal of Earth Sciences》2013,60(1-2):15-18
This short note presents the results of triclinicity determination of purified K‐feldspars from three spatially related granitic bodies (Kalunga Granodiorite, Watsonville granite, and Elizabeth Creek granite, ECG) in west Herberton, Queensland. The systematic decrease of triclinicity values (as 5 index) with increasing weight percent K2O and differentiation indices, decreasing colour indices, and K/Rb ratios is considered to provide an additional line of evidence for a possible co‐genetic relationship of the intrusives. The second phase of the Elizabeth Creek Granite (ECG II) is a late stage intrusive and its emplacement may explain the close spatial association of ECG members and the economic mineralization in the district. 相似文献
13.
14.
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
15.
Marion Michael 《Australian Journal of Earth Sciences》2013,60(3):211-222
Graphs of the yearly number of volcanoes in eruption from 1900 through 1968 for 21 volcanic areas of the circum‐Pacific belt show fluctuations with statistically significant amplitudes in eight of the areas, and some indication of auto‐correlation and periodicity in 14 of them. Thus, in a total of 16 out of the 21 areas studied, these volcanic pulses appear to be statistically significant. They are best defined in the New Guinea‐Solomons area, Santa Cruz‐New Hebrides‐Matthew Island area, and the West Indies where the numbers of volcanoes in eruption have significant fluctuations in amplitude, are auto‐correlated and tend to be periodic. The periods (around 17 years) of the western Pacific areas are, with few exceptions, much less than those of the eastern Pacific areas suggesting an overall first‐order cause for these volcanic fluctuations which, in each area, seem to be due to the volcanoes’ responding to regional stress pulses. These may be caused by variations in stress owing to sea‐floor spreading on the East Pacific Rise, the dichotomy of periods being due to a different mantle‐flow regime on either side of it, resulting from the proximity of the Rise to the eastern Pacific margin. An examination of the years for which the maximum or one less than the maximum number of volcanoes are in eruption suggests that large pulses of volcanic activity tend to migrate southwards. There are six main pulses in the western Pacific and three in the eastern. The latter are better‐defined and more widely spaced in time. The pulse occurring in the first decade of this century is simultaneous on both sides of the Pacific, a reflexion of the global tectonic instability of this period. The reason for this southward migration of the volcanic pulses is not yet known. It may be a strain‐release phenomenon. 相似文献
16.
In this paper, we continue our analysis of upwind‐mixed methods for advection–diffusion equations, which have been developed
and analyzed by the first author over the past several years. In previous work, our analysis has been limited to low order
approximating spaces, positive definite diffusion coefficients and Dirichlet boundary conditions. In this paper, we extend
our results to higher order approximating spaces, possibly zero diffusion, and more physically realistic boundary conditions.
Moreover, unlike previous papers, we avoid the use of Gronwall's Inequality, which can result in extremely large constants
in the stability and error bounds. Numerical results are presented for constant, linear and quadratic approximating spaces.
This revised version was published online in July 2006 with corrections to the Cover Date. 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.
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
19.
20.
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