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1.

The Cobar Basin in central western New South Wales is a mineral‐rich Early Devonian basin typical of those that characterize the Siluro‐Devonian history of the Lachlan Orogen of southeastern Australia. One hundred and seventy kilometres of seismic profiling in three lines across the basin have shown it to be asymmetrical in shape with an east‐dipping western margin that is steeper than the moderately west‐dipping eastern margin. Maximum basin thickness is around 6 km, but there are significant thickness changes, especially from south to north, which reflect the effect of synsedimentary faulting. Seismic profiling suggests that the basin deformed by thin‐skinned tectonics; postulated strike‐slip effects were not visible on the sections. The seismic profiling has, for the first time, imaged the western synrift basin margin which is generally not exposed. Strain variations during deformation along this edge were taken up by the formation of a major jog ('dog‐leg') which has propagated into the basin as a tear fault. Intrabasinal tears, as well as thrusts, which link into one or more detachments, provide potential pathways for mineralizing fluids during basin inversion.  相似文献   

2.

Eastern Australian xenolith suites and lithospheric transition zones are re‐evaluated using new mineral analyses and thermo‐barometry. Some suites, including that defining the southeastern Australian geotherm, are not fully equilibrated. New pressure‐temperature estimates, based on experimental calibrations that allow for Cr and Ti in pyroxenes, differ from earlier results by up to 0.6 GPa and 250°C. The preferred Brey and Köhler 1990 thermo‐barometer indicates a shallower cooler garnet lherzolite transition under Mesozoic New South Wales (50 km depth at 980° C) than for Tertiary Tasmania (60 km depth at 1090°C).

Deviations between palaeogeotherms may reflect: (i) higher temperature gradients for Tasmania and New South Wales (by 100°C/0.1 GPa) related to abnormally hot mantle; (ii) higher temperature gradients linked to more voluminous magmatism, largely Cenozoic in age; and (iii) complex temperature perturbations linked to different levels of magmatic intrusion.

These deviations blur reconstructions of lithospheric assemblages, where temperature is determinable and pressure comes from an assumed geotherm. Potential errors in locating spinel lherzolite and crust‐mantle transition assemblages may reach 15 km in depth. The highest Tertiary geothermal gradients in Tasmania and northeastern New South Wales match those from regions of active lithospheric extension. The young southeastern Australian geotherm is decaying from a higher temperature equilibration, based on experimental work, and Mesozoic New South Wales geotherms trend towards the lower gradients of bounding cratons.  相似文献   

3.
The Yeoval porphyry copper prospect lies in a complex of dioritic rocks which form part of the eastern margin of the Yeoval Batholith in central‐western New South Wales. Rocks of the batholith are mainly granite and adamellite whose age is about 370 m.y. The diorite complex, (411 m.y.) is composed of rocks ranging from granodiorite to gabbro and pyroxenite.

Hydrothermal alteration of granodiorite in the Yeoval Mine area, 3.5 km north of Yeoval, is associated with disseminated and stockwork‐veinlet copper‐sulphide‐bearing zones. Alteration assemblages are similar to those described from some disseminated or porphyry copper/molybdenum deposits of southwestern USA.

The ubiquity of potassic zones in veinlet alteration envelopes and the poor development of sericitic and argillic zones suggest that the Yeoval prospect formed at or below the level of the Ajo deposit, Arizona, and the Los Loros deposit, Chile, which formed some 5 km below surface near the base of the ‘porphyry system’.

High Rb and Ba contents in the Yeoval diorites and their associated andesitic volcanics, and the presence of garnet‐bearing rhyodacite of similar age, imply that the Yeoval area was part of an Andean type of continental margin in the middle Palaeozoic.  相似文献   

4.

From the early Late Permian onwards, the northeastern part of the Sydney Basin, New South Wales, (encompassing the Hunter Coalfield) developed as a foreland basin to the rising New England Orogen lying to the east and northeast. Structurally, Permian rocks in the Hunter Coalfield lie in the frontal part of a foreland fold‐thrust belt that propagated westwards from the adjacent New England Orogen. Thrust faults and folds are common in the inner part of the Sydney Basin. Small‐scale thrusts are restricted to individual stratigraphic units (with a major ‘upper decollement horizon’ occurring in the mechanically weak Mulbring Siltstone), but major thrusts are inferred to sole into a floor thrust at a poorly constrained depth of approximately 3 km. Folds appear to have formed mainly as hangingwall anticlines above these splaying thrust faults. Other folds formed as flat‐topped anticlines developed above ramps in that floor thrust, as intervening synclines ahead of such ramp anticlines, or as decollement folds. These contractional structures were overprinted by extensional faults developed during compressional deformation or afterwards during post‐thrusting relaxation and/or subsequent extension. The southern part of the Hunter Coalfield (and the Newcastle Coalfield to the east) occupies a structural recess in the western margin of the New England Orogen and its offshore continuation, the Currarong Orogen. Rocks in this recess underwent a two‐stage deformation history. West‐northwest‐trending stage one structures such as the southern part of the Hunter Thrust and the Hunter River Transverse Zone (a reactivated syndepositional transfer fault) developed in response to maximum regional compression from the east‐northeast. These were followed by stage two folds and thrusts oriented north‐south and developed from maximum compression oriented east‐west. The Hunter Thrust itself was folded by these later folds, and the Hunter River Transverse Zone underwent strike‐slip reactivation.  相似文献   

5.
An intense Bouguer anomaly ‘high’ of about 30 milligals amplitude has been delineated to the east of Cootamundra, New South Wales. It is correlated with a north‐trending belt of basic metamorphic rocks of probable Upper Silurian age. The subsurface shape of the belt is deduced by qualitative and quantitative analytical techniques applied to the gravity data; it is inferred that the boundary on the western side is near‐vertical, while that on the eastern side is believed to have been overthrust by the Middle Devonian Young Granite. It appears also that there are no horizontal density variations below a depth of 6–7 km, implying that the base of the Young Granite lies at about this depth.  相似文献   

6.
The Wyangala Batholith, in the Lachlan Fold Belt of New South Wales, is pre‐tectonic with respect to the deformation that caused the foliation in the granite, and was emplaced during a major thermal event, perhaps associated with dextral shearing, during the Late Silurian to Early Devonian Bowning Orogeny. This followed the first episode of folding in the enclosing Ordovician country rocks. Intrusion was facilitated by upward displacement of fault blocks, with local stoping. Weak magmatic flow fabrics are present. After crystallization of the granite, a swarm of mafic dykes intruded both the granite and country rock, possibly being derived from the same tectonic regime responsible for emplacement of the Wyangala Batholith. A contact aureole surrounding the granite contains cordierite‐biotite and cordierite‐andalusite assemblages. Slaty cleavage produced in the first deformation was largely obliterated by recrystallization in the contact aureole.

Postdating granite emplacement and basic dyke intrusion, a second regional deformation was accompanied by regional metamorphism ranging from lower greenschist to albite‐epidote‐amphibolite facies, and produced tectonic foliations, termed S and C, in the granite, and a foliation, S2, in the country rocks. Contact metamorphic rocks underwent retrogressive regional metamorphism at this time. S formed under east‐west shortening and vertical extension, concurrently with S2. C surfaces probably formed concurrently with S and indicate reverse fault motion on west‐dipping ductile shear surfaces. The second deformation may be related to Devonian or Early Carboniferous movement on the Copperhannia Thrust east of the Wyangala Batholith.  相似文献   

7.
Contact metamorphism has been recognized along a 4 km wide belt adjacent to the shallow‐dipping eastern margin of the Arthursleigh Tonalite, an Early Devonian pluton of the Marulan Batholith, eastern New South Wales. In Ordovician psammitic and pelitic rocks three zones of progressive contact metamorphism range from muscovite + biotite + chlorite assemblages in the outer zone to K‐feldspar + cordierite assemblages adjacent to the pluton and in metasedimentary xenoliths. Retrograde phenomena include extensive replacement of metamorphic minerals by ‘sericite’ and chlorite. Calcareous metasediments adjacent to the tonalite typically contain assemblages of quartz + calcic plagioclase + ferrosalite + sphene, or wollastonite + calcite + diopside with minor grossularite and vesuvianite. Thermal effects in volcanic rocks along the western margin of the pluton are confined to recrystallization of the groundmass.

The regional geology indicates confining pressures of approximately 1 kbar at the time of emplacement of the tonalite. Contact metamorphic temperatures were estimated from two‐feldspar geothermometry to attain a maximum of approximately 590°C for rocks in the innermost zone of the aureole and 700°C for the xenoliths. Fluid compositions attending progressive contact metamorphism were water‐rich (Xco2<0.2) and, during cooling, these fluids probably account for the extensive retrograde hydration observed in the aureole.  相似文献   

8.
In this initial systematic study of Carboniferous spores from New South Wales, Australia, fifteen species (all but one of them new) are formally described and are distributed among eight established genera and two new genera (Rattiganispora, a distally annulate trilete form, and Psomospora, an inaperturate or proximally hilate form). The species were selected as being the most characteristic and distinctive forms found in the Italia Road Formation at its well‐exposed type section in the Hunter Valley, east‐central New South Wales. The formation is a cyclical non‐marine unit, over 300 metres (1,000 ft) thick, consisting of lithic arenites together with carbonaceous shales, claystones, and siltstones; its age is regarded as West‐phalian‐Stephanian. The microfiora is compared with those known from sediments of similar age elsewhere and its place in the Australian Palaeozoic palynostratigraphic record is discussed.

New specific institutions are as follows: Punctatisporites lucidulus, P. sub‐tritus, Verrucosisporites aspratilis, V. italiaensis, Raistrickia accincta, R. radiosa, Reticulatisporites asperidictyus, R. magnidictyus, Foveosporites pellucidus, Rattiganispora apiculata (type species), Kraeuselisporites kuttungensis, Grandispora maculosa, Psomospora detecta (type species), and Wilsonites australiensis.  相似文献   

9.
Devonian rocks occur in northeastern Australia within the ‘Tasman Geosyncline’ in three major tectonic divisions—(a) a very broad mobile platform related to the last stages of stabilisation of the Lachlan Geosyncline, marginal to which is found, (b) the volcanic‐rich New England Geosyncline, and (c) a contrasting region in northern Queensland where complex marine to continental sedimentation occurred on cratonic blocks while non‐volcanic flysch‐like sedimentation occurred in the marginal Hodgkinson Basin.

The tectonic setting was governed by differences in the nature of the continental margin, so that the New England Geosyncline and Hodgkinson Basin, which developed along the eastern margin of the continent from the earliest Devonian to the late Palaeozoic, show correspondingly different sedimentation and deformation histories.

An integrated account of the Devonian geology of these regions is given, leading to.an interpretation of the environments of the Devonian in terms of plate‐tectonic movements, generally from the east.

Postulated tectonic zones within the New England Geosyncline region include pre‐Devonian deep ocean deposits with mild high‐pressure low‐temperature meta‐morphism, and Devonian volcanic arc and marginal sea volcanic‐derived deposits. Within the mobile platform to the west, variable marine and continental deposits are associated with volcanicity in the zone transitional to the New England Geosyncline. In the northern region, rifting of the craton and development of an Atlantic‐type margin was followed by subduction with folding and metamorphism at the end of the Devonian.

The Devonian rocks are strongly affected by intense late Palaeozoic tectonic and igneous activity in the eastern marginal regions, but only minor effects are seen to the west.  相似文献   

10.

Devonian and Carboniferous (Yarrol terrane) rocks, Early Permian strata, and Permian‐(?)Triassic plutons outcrop in the Stanage Bay region of the northern New England Fold Belt. The Early‐(?)Middle Devonian Mt Holly Formation consists mainly of coarse volcaniclastic rocks of intermediate‐silicic provenance, and mafic, intermediate and silicic volcanics. Limestone is abundant in the Duke Island, along with a significant component of quartz sandstone on Hunter Island. Most Carboniferous rocks can be placed in two units, the late Tournaisian‐Namurian Campwyn Volcanics, composed of coarse volcaniclastic sedimentary rocks, silicic ash flow tuff and widespread oolitic limestone, and the conformably overlying Neerkol Formation dominated by volcaniclastic sandstone and siltstone with uncommon pebble conglomerate and scattered silicic ash fall tuff. Strata of uncertain stratigraphic affinity are mapped as ‘undifferentiated Carboniferous’. The Early Permian Youlambie Conglomerate unconformably overlies Carboniferous rocks. It consists of mudstone, sandstone and conglomerate, the last containing clasts of Carboniferous sedimentary rocks, diverse volcanics and rare granitic rocks. Intrusive bodies include the altered and variably strained Tynemouth Diorite of possible Devonian age, and a quartz monzonite mass of likely Late Permian or Triassic age.

The rocks of the Yarrol terrane accumulated in shallow (Mt Holly, Campwyn) and deeper (Neerkol) marine conditions proximal to an active magmatic arc which was probably of continental margin type. The Youlambie Conglomerate was deposited unconformably above the Yarrol terrane in a rift basin. Late Permian regional deformation, which involved east‐west horizontal shortening achieved by folding, cleavage formation and east‐over‐west thrusting, increases in intensity towards the east.  相似文献   

11.
Abstract

Edenopteron, with a lower jaw some 48?cm long, and total length perhaps exceeding 3 m, is the largest Devonian lobe-fin known from semi-articulated remains. New material described from the type locality (Boyds Tower, south of Eden) includes three slightly smaller articulated skulls and jaws, and additional bones of the shoulder girdle. Another articulated skull roof, shoulder girdle and palate is described from a second locality (Hegarty Bay), about 10?km south of Boyds Tower. Both localities represent the upper part of the Worange Point Formation, of late Famennian age (uppermost Upper Devonian). The new morphological evidence supports a close relationship to the tristichopterids Mandageria and Cabonnichthys, from the slightly older (Frasnian, Upper Devonian) fossil fish assemblage at Canowindra, New South Wales. Features of the shoulder girdle (supracleithrum, anocleithrum) suggest that Edenopteron is more closely related to Mandageria than Cabonnichthys. Eight characters are used to define a tristichopterid subfamily Mandageriinae, to which Notorhizodon from the Middle Devonian of Antarctica is also referred. The Mandageriinae is endemic to East Gondwana (Australia–Antarctica). In combination with possibly the most primitive tristichopterid, Marsdenichthys from the Frasnian of Victoria, these distributions implicate East Gondwana as a likely place of origin for the entire group. This relates to the major but unresolved question of a possible Gondwana origin for all the land vertebrates (tetrapods).

  • An endemic Gondwanan sub-group (Mandageriinae) of the Devonian fishes closest to land animals (tetrapodomorph tristichopterids) is confirmed.

  • Retention of primitive features (e.g. accessory vomers) points to an earlier origin of the Mandageriinae in East Gondwana, consistent with the Victorian occurrence of another primitive tristichopterid (Marsdenichthys).

  • Edenopteron is confirmed from a second south coast fossil site, and new characters indicate its closest relative is Mandageria from Canowindra, NSW.

  • Congruent evidence of older Gondwanan occurrences in other groups (basal tetrapodomorphs, rhizodontids, canowindrids), and previously dismissed trace fossil evidence (Grampians trackways), implicate South China and East Gondwana as the likely place of origin for all land vertebrates.

  相似文献   

12.
Fission‐track ages have been determined on sphene and apatite from 28 granitic intrusions across the western half of Victoria. The sphene ages compare closely with independent K‐Ar biotite ages for the same intrusions, where these are available, and are invariably older than apatite ages by 35 to 135 m.y. This is in accord with the effective geological track annealing temperatures for these two minerals which are estimated to be 260 ± 20°C and 80 ± 10°C respectively. Both sphene and apatite ages decrease from west to east across western Victoria, the sphenes ranging from 470 ± 28 to 355 ± 19 m.y. The Wando Vale granodiorite and Dergholm granite from the Dundas Tableland of far‐western Victoria have sphene ages of 470 ± 28 m.y. and 452 ±16 m.y. respectively, clearly suggesting a relationship to the Ordo‐vician granitic rocks of southeastern South Australia. Fission‐track ages from the numerous post‐tectonic granites in the Ballarat Trough fall into two distinct groups. Rocks from the western area have sphene ages in the relatively narrow range 393 ± 14 m.y. suggesting emplacement in the Early Devonian time whereas those in the east have sphene ages of 362 ± 7 m.y. (near the Devonian‐Carboniferous boundary). Over the temperature interval recorded by sphene‐apatite pairs, cooling of the granitic rocks was very slow ranging from 0.8 to 5.3°C/m.y. Cooling in this range was probably controlled by uplift and erosion of overburden during a long period of post‐tectonic relaxation. Corresponding uplift rates are estimated to be 0.03 to 0.18 km/ m.y. assuming a normal continental geothermal gradient of 30°C/km. Below 80°C average cooling and uplift rates were probably about l°C/m.y. and 0.03 km/m.y. respectively so that cooling was essentially complete within about 80 m.y. of the apatite ages.  相似文献   

13.

Several Late Palaeozoic granites which intrude strata of the Silurian‐Devonian Hodgkinson Province, north Queensland, display pronounced west‐northwest‐east‐southeast orientations, as do a suite of brittle structures that have affected both the plutons and country rocks. These features define a 20 km‐wide, west‐northwest‐trending zone, here named the Desailly Structure, which traverses the Hodgkinson Province and extends west across the Palmerville Fault into the Proterozoic Yambo Inlier. Deformation within the Desailly Structure was heterogeneously partitioned into zones of west‐northwest‐east‐southeast faulting separated by tracts of competent country rock. The latter contain a pervasive north‐south‐trending structural grain which locally controlled pluton emplacement and resulted in a meridional orientation of many granitoid bodies. Initiation of the Desailly Structure is attributed to have occurred syn‐ to post‐D2 of the regional deformation history. It was reactivated in the Hunter‐Bowen Orogeny (D4), with the zone expressing an overall sinistral sense of displacement.  相似文献   

14.

Gold mineralisation at the Dobroyde prospect in central New South Wales is hosted by a zoned alteration system characterised by peripheral propylitic alteration, grading inwards through argillic and advanced argillic alteration to a siliceous altered core. Overprinting textures indicate that propylitic, argillic, advanced argillic and siliceous assemblages were successively superimposed on each other. Au grades between 0.3–0.8 ppm are associated with siliceous alteration and cross‐cutting pyrite veinlets. Higher Au grades are associated with barite veins that cut the pyrite veinlets. Native Au, native Te, Au, Pb and Hg tellurides, Pb selenide, chalcopyrite, Zn‐sphalerite and tennantite‐tetrahedrite occur in the barite veins. Microscopic pyrophyllite shears cut the barite veins. The location of the Dobroyde prospect, the orientation of its internal alteration zonation and the orientation of auriferous barite veins in the core of the prospect are controlled by a 330°‐striking fault. Movement on this fault, synchronous with hydrothermal activity, at some time between the Late Ordovician and mid‐Devonian controlled the development of successive phases of brecciation, siliceous alteration, pyrite and later barite‐Au veining in the prospect core. The restricted distribution of auriferous barite veins within the siliceous altered core of the prospect is inferred to be controlled by the relatively brittle rheology of this assemblage during deformation, and its location on the fault that formed the main hydrothermal fluid conduit. Alteration zones distal from this fault remain unmineralised. The Dobroyde prospect may be a product of the same Early Devonian metallogenic epoch as the paragenetically similar Temora and Peak Hill deposits. All three deposits/prospects appear to be localised in splays of either the Gilmore Fault Zone or the Parkes Thrust.  相似文献   

15.

Three suites of alkaline granite can be recognised in the Narraburra Complex at the triple junction of the Tumut, Giralambone‐Goonumbla and Wagga Zones, central southern New South Wales. On the basis of K2O/Na2O ratios, biotite and hornblende‐biotite potassic I‐type granites have been assigned to the Gilmore Hill (K2O/Na2O 1.00) and Barmedman Suites (K2O/Na2O > 1.2). These are metaluminous to weakly peraluminous suites that crystallised from high‐temperature,reduced magmas with the least fractionated members of each suite having high Ba and low Rb abundances compared to other Lachlan Fold Belt granites. Fractionated members of these suites have high abundances of high‐field‐strength elements, similar to those observed in A‐type granites. Arfvedsonite and aegirine‐arfvedsonite granites have been assigned to the peralkaline Narraburra Suite. Granites from this suite have chemistry consistent with them being the intrusive equivalents of comendites and they are also similar in some respects to A‐type granites: they have, for example, particularly high abundances of Zr. The A‐type signature is, however, at least in part the result of strong fractionation. Total‐rock Rb–Sr isotopic analyses from both I‐type suites plot on the same isochron, giving an age of 365 ± 4 Ma (Srl = 0.70388 ± 53). A total‐rock isochron for the peralkaline Narraburra Suite gives a less well‐defined age of 358 ± 9 Ma (Srl = 0.7013 ± 80). The Late Devonian Rb–Sr ages may be emplacement ages or a result of resetting during fluid‐rock interaction. Although granites of the Narraburra Complex have geochemical affinities with alkaline granites formed late in orogenic cycles, they post‐date arc magmatism by at least 75 million years and they formed in a within‐plate setting. Magmatism was related to localised reactivation of major faults (Gilmore Fault and the Parkes Thrust) in the region, and to partial melting involving both enriched mantle and Ordovician shoshonitic crustal components. Emplacement of the Narraburra Complex was contemporaneous with magmatism in the Central Victorian Magmatic Province and A‐type magmatism in eastern New South Wales. Collectively, all these magmatic events were related to extension post‐dating amalgamation of the western and central/eastern subprovinces of the Lachlan Fold Belt.  相似文献   

16.
The Lower Devonian (Lochkovian‐Emsian) Cosheston Group of south Pembrokeshire is one of the most enigmatic units of the Old Red Sandstone of Wales. It consists of a predominantly green, exceptionally thick succession (up to 1.8 km) within the red c. 3 km‐thick fill of the Anglo‐Welsh Basin, but occupies a very small area (27 km2). Four formations—Llanstadwell (LLF), Mill Bay (MBF), Lawrenny Cliff (LCF) and New Shipping (NSF)—group into lower (LLF + MBF) and upper (LCF + NSF) units on stratigraphical and sedimentological criteria. Two palynostratigraphic associations (Hobbs Point and Burton Cliff) are recognised in the LLF. Overall, the Cosheston succession comprises a fluvial, coarsening‐upward megasequence, mostly arranged in fining‐upward rhythms. It is interpreted as the fill of an east‐west graben bounded by faults to the north and south of the Benton and Ritec faults, respectively. Both ‘lower Cosheston’ formations were deposited by east‐flowing, axial river systems draining a southern Irish Sea landmass. Drainage reversal, early in the deposition of the LCF, resulted in ‘upper Cosheston’ lateral, SW‐flowing rivers which carried predominantly second‐ and multi‐cycle detritus. The ‘lower Cosheston’ is characterized by an abundance of soft‐sediment deformation structures, probably seismically triggered by movements along the graben's northern bounding fault. A minimum average (≥ mesoseismic) earthquake recurrence interval of c. 4000 yr is estimated for the MBF. This and the correlative Senni Formation of south‐central Wales form a regionally extensive green‐bed development that represents a pluvial climatic interval. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

17.
In the Buckambool area, Cobar, New South Wales, the boundary between dominantly shallow‐water, shelf sediments of the Winduck Group and fluviatile sediments of the Mulga Downs Group has been established as a small hiatus not resolvable by available fossil age data. Although dips are parallel over much of the area, disconformable and locally angular unconformable relations are present. This hiatus, late‐Early to Middle Devonian in age, marks a period of uplift, localised folding and erosion. These reflect movement of basement blocks along major fractures that are now revealed as lineaments.

Terminal deformation in the area, reflected by folding and re‐activation of lineaments, postdated deposition of the Mulga Downs Group, and is probably Carboniferous in age.  相似文献   

18.

The Timbarra Tablelands pluton is an extensive (～550 km2) complexly zoned intrusion forming one of many predominantly monzogranite I‐type plutons, which constitute the Moonbi Supersuite in northern New South Wales, Australia. It comprises an outer rim of Rocky River monzogranite (Zones 1–3), an intermediate zone of Sandy Creek syenogranite (Zones 4A–4C), surrounding a core of Surface Hill syenogranite (Zones 5–7). The suite is calc‐alkaline, high‐K, and varies from mildly metaluminous to weakly peraluminous with increasing fractionation. Average Rb/Sr ratios range from 0.4 in the least evolved very coarse‐grained monzogranite (Zone 3) to 46 in the most evolved very fine‐grained biotite microgranite (Zone 6). Trace‐element modelling indicates that the observed compositional variation could have been produced by crystal fractionation. New bulk rock major‐ and trace‐element data for 71 samples are presented, and indicate that a compositional continuum exists that varies between 63 and 78 wt% SiO2. Importantly, there is no systematic chemical variation with spatial distribution of samples from the core of the pluton to its margin, requiring multiple separate pulses of an evolving magma to explain compositional discontinuities. The pluton is interpreted to have been emplaced at mesozonal levels (～180 ± 60 MPa, 5–10 km depth) and crystallised at temperatures between 620 and 820°C under moderately oxidising conditions (log fO2 = ‐11.5 to ‐19). The association of gold‐molybdenite mineralisation at Timbarra with moderately oxidised I‐type magmas is consistent with fractionation‐redox controls on ore‐element behaviour in magmatic systems in other studies.  相似文献   

19.
Extrusive and high level intrusive Early Devonian keratophyres are the oldest in situ igneous rocks in the Tamworth Block of the New England Fold Belt of eastern Australia. They show extensive evidence of degradation, including the destruction of magmatic phases, the growth of low grade metamorphic minerals, and changes in composition involving the dilution of elemental abundances in response to silica addition. Relations between the less mobile minor and trace elements, and limited data on clinopyroxene compositions, lead to the conclusion that these Early Devonian volcanic rocks are mostly calc‐alkaline volcanic arc andesites with minor dacite. These rocks unconformably overlie a sequence of Early Palaeozoic forearc basin deposits, indicating that the Early Devonian marks a period of readjustment of tectonic elements within the New England Fold Belt, associated with a marked east‐directed stepping out of the magmatic arc. Generation of the keratophyres in a subduction zone environment limits the position of the trench to 100 km east of the Peel Fault System.  相似文献   

20.

Nd isotope studies of the oldest metasedimentary rocks from the Wonominta Block, western New South Wales reveal that these samples have a model age (TDM) of 1780–2010 Ma, slightly younger than that of low‐grade Willyama Supergroup metasediments (1920–2160 Ma), and significantly younger than those ages previously reported from high‐grade rocks of the Broken Hill Block (2200–2300 Ma). These differences have important implications for tectonic reconstruction in this region and support a model of transitional tectonics from the Broken Hill to Wonominta Blocks, as suggested by earlier geochemical studies of mafic rocks. Those studies revealed that the mafic rocks from the basal sequence of the Wonominta Block may have formed in a back‐arc basin, developed from a propagating rifting, an environment contiguous to that in which Willyama Supergroup was deposited. These results also carry significant implications for tectonic reconstruction of eastern Australia.  相似文献   

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