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1.

The Early Devonian Bindook Volcanic Complex consists of a thick silicic volcanic and associated sedimentary succession filling the extensional Wollondilly Basin in the northeastern Lachlan Fold Belt. The basal part of the succession (Tangerang Formation) is exposed in the central and southeastern Wollondilly Basin where it unconformably overlies Ordovician rocks or conformably overlies the Late Silurian to Early Devonian Bungonia Limestone. Six volcanic members, including three new members, are now recognised in the Tangerang Formation and three major facies have been delineated in the associated sedimentary sequence. The oldest part of the sequence near Windellama consists of a quartz turbidite facies deposited at moderate water depths together with the shallow‐marine shelf Windellama Limestone and Brooklyn Conglomerate Members deposited close to the eastern margin of the basin. Farther north the shelf facies consists of marine shale and sandstone which become progressively more tuffaceous northwards towards Marulan. The Devils Pulpit Member (new unit) is a shallow‐marine volcaniclastic unit marking the first major volcanic eruptions in the region. The overlying shallow‐marine sedimentary facies is tuffaceous in the north, contains a central Ordovician‐derived quartzose (?deltaic) facies and a predominantly mixed facies farther south. The initial volcanism occurred in an undefined area north of Marulan. A period of non‐marine exposure, erosion and later deposition of quartzose rocks marked a considerable break in volcanic activity. Volcanism recommenced with the widespread emplacement of the Kerillon Tuff Member (new unit), a thick, non‐welded rhyolitic ignimbrite followed by dacitic welded ignimbrite and air‐fall tuff produced by a large magnitude eruption leading to caldera collapse in the central part of the Bindook Volcanic Complex, together with an additional small eruptive centre near Lumley Park. The overlying Kerrawarra Dacite Member (new unit) is lava‐like in character but it also has the dimensions of an ignimbrite and covers a large part of the central Bindook Volcanic Complex. The Carne Dacite Member is interpreted as a series of subvolcanic intrusions including laccoliths, cryptodomes and sills. The Tangerang Formation is overlain by the extensive crystal‐rich Joaramin Ignimbrite (new unit) that was erupted from an undefined centre in the central or northern Bindook Volcanic Complex. The volcanic units at Wombeyan and the Kowmung Volcaniclastics in the northwestern part of the complex are probably lateral time‐equivalents of the Tangerang Formation and Joaramin Ignimbrite. All three successions pre‐date the major subaerial volcanic plateau‐forming eruptions represented by the Barrallier Ignimbrite (new unit). The latter post‐dated folding and an extensive erosional phase, and unconformably overlies many of the older units in the Bindook Volcanic Complex. This ignimbrite was probably erupted from a large caldera in the northern part of the complex and probably represents surface expressions of part of the intruding Marulan Batholith. The final volcanic episode is represented by the volcanic units at Yerranderie which formed around a crater at the northern end of the exposed Bindook Volcanic Complex.  相似文献   

2.
A fossil fish assemblage associated with marine invertebrates from the Coonardoo Sandstone (Wallingalair Group) at Boor Hill (eastern limb of Tullamore Syncline) contains phyllolepid and bothriolepid placoderms of probable Late Devonian age. An angular unconformity with the overlying Hervey Group indicates erosion and folding during the Middle – Late Devonian, and evidently younger than the main Tabberabberan orogenic event. Invertebrate remains demonstrate a Late Devonian marine interval, not previously recognised as far west as the Tullamore Syncline, and assumed to represent the global maximum sea-level in the late Frasnian immediately preceding the Frasnian – Famennian extinction event. A phyllolepid placoderm plate from a sedimentary interbed of the Dulladerry Volcanics in the Hervey Syncline compares with abundant phyllolepid material from the Merriganowry Shale Member of the Dulladerry Volcanics near Cowra, and similar occurrences in the Comerong Volcanics and Boyd Volcanic Complex in southeastern New South Wales. Biostratigraphic data suggest a late Middle Devonian (Givetian) age for the Merriganowry Shale Member of the Dulladerry Volcanics, which appears conformable beneath the Upper Devonian Hervey Group.  相似文献   

3.
U–Pb (SHRIMP) detrital zircon age patterns are reported for 12 samples of Permian to Cretaceous turbiditic quartzo‐feldspathic sandstone from the Torlesse and Waipapa suspect terranes of New Zealand. Their major Permian to Triassic, and minor Early Palaeozoic and Mesoproterozoic, age components indicate that most sediment was probably derived from the Carboniferous to Triassic New England Orogen in northeastern Australia. Rapid deposition of voluminous Torlesse/Waipapa turbidite fans during the Late Permian to Late Triassic appears to have been directly linked to uplift and exhumation of the magmatically active orogen during the 265–230 Ma Hunter‐Bowen event. This period of cordilleran‐type orogeny allowed transport of large volumes of quartzo‐feldspathic sediment across the convergent Gondwanaland margin. Post‐Triassic depocentres also received (recycled?) sediment from the relict orogen as well as from Jurassic and Cretaceous volcanic provinces now offshore from southern Queensland and northern New South Wales. The detailed provenance‐age fingerprints provided by the detrital zircon data are also consistent with progressive southward derivation of sediment: from northeastern Queensland during the Permian, southeastern Queensland during the Triassic, and northeastern New South Wales — Lord Howe Rise — Norfolk Ridge during the Jurassic to Cretaceous. Although the dextral sense of displacement is consistent with the tectonic regime during this period, detailed characterisation of source terranes at this scale is hindered by the scarcity of published zircon age data for igneous and sedimentary rocks in Queensland and northern New South Wales. Mesoproterozoic and Neoproterozoic age components cannot be adequately matched with likely source terranes in the Australian‐Antarctic Precambrian craton, and it is possible they originated in the Proterozoic cores of the Cathaysia and Yangtze Blocks of southeast China.  相似文献   

4.
The Hianana Volcanics consist of bedded tuff and dacitic lava that form a locally mappable unit within the extensive, Late Permian silicic volcanic sequence of northeastern New South Wales. Principal components of the bedded tuff are crystal and volcanic lithic fragments ranging from coarse ash to lapilli, accompanied by variable amounts of fine ash matrix. Well denned plane parallel thin bedding is characteristic. Sandwave bed forms, including low‐angle cross‐beds and wavy beds, are confined to an area of 2–3 km2 coinciding with the thickest sections (70 m) of bedded tuff. A high‐aspect ratio flow of porphyritic dacitic lava overlies the bedded tuff in the same area. The setting, lithofacies, extent and geometry of the bedded tuffs of the Hianana Volcanics are comparable with modern tuff rings which are composed of the deposits from base surges generated by explosive phreatomagmatic eruptions at primary volcanic vents. Many of these have also discharged lava late in their activity. Proximal parts of the Hianana tuff ring were buried by the porphyritic lava after the phreatomagmatic eruptions had ceased. In more distal sections, the bedded tuff is less than 10 m thick and dominantly comprises fine grained, plane parallel, very thin beds and laminae; these features suggest an origin by fallout from ash clouds that accompanied the phreatomagmatic eruptions. The distal ash was covered and preserved from erosion by a layer of welded ignimbrite, the source of which is unknown.  相似文献   

5.

The Mt Wright Volcanics are located in the Wonominta Block of northwestern New South Wales. Detailed regional mapping has shown that the block is a composite tectonic unit and that the metavolcanic rocks described as the Mt Wright Volcanics may have been emplaced at different time from Late Proterozoic (northern section: Packsaddle, Nundora) to Early Cambrian (southern section: Mt Wright). Geochemical investigations, including major and trace elements, as well as analyses of relic clinopyroxene, show that the rocks have affinities with alkali basalt with light‐rare‐earth‐element‐enriched compositions. An intra‐plate extensional environment (such as rift‐ and/or plume‐related) is considered most likely for the formation of the rocks. Though metamorphosed to various degrees, the rocks apparently retain much of their primary Sr isotopic character (initial 87Sr/86Sr about 0.7032) and, apart from their age, resemble the Tertiary intraplate volcanism in eastern Australia. The Nd isotope analyses yield remarkably similar results between the two sections of the Mt Wright Volcanics, with 143Nd/144Nd between 0.51260 to 0.51271 and εNd(T) 4.7 ±0.4 (calculated to 525 Ma). A kaersutite‐bearing xenolith found in the northern section of the volcanic sequence has a Nd isotope composition more depleted than its hosts with εNd(T) of 7.7. The isotope results suggest that the Mt Wright Volcanics were derived from a depleted mantle source without significant crustal contribution. It is proposed that the Mt Wright Volcanics possibly represent the products of a rifting event that led to the breakup of the Proterozoic supercontinent during Early Cambrian in eastern Australia.  相似文献   

6.
The Ural Volcanics are a early Devonian, submarine, felsic lava-sill complex, exposed in the western central Lachlan Orogen, New South Wales. The Ural Volcanics and underlying Upper Silurian, deepwater, basin-fill sedimentary rocks make up the Rast Group. The Ural Range study area, centrally located in the Cargelligo 1:100 000 map sheet area, was mapped at 1:10 000 scale. Seventeen principal volcanic facies were identified in the study area, dominated by felsic coherent facies (rhyolite and dacite) and associated monomictic breccia and siltstone-matrix monomictic breccia facies. Subordinate volcaniclastic facies include the pumice-rich breccia facies association, rhyolite – dacite – siltstone breccia facies and fiamme – siltstone breccia facies. The sedimentary facies association includes mixed-provenance and non-volcanic sandstone to conglomerate, black mudstone, micaceous quartz sandstone and foliated mudstone. The succession was derived from at least two intrabasinal volcanic centres. One, in the north, was largely effusive and intrusive, building a lava – sill complex. Another, in the south, was effusive, intrusive and explosive, generating lavas and moderate-volume (～3 km3) pyroclastic facies. The presence of turbidites, marine fossils, very thick massive to graded volcaniclastic units and black mudstone, and the lack of large-scale cross-beds and erosional scours, provide evidence for deposition in a submarine environment below storm wave-base. The Ural Volcanics have potential for seafloor or sub-seafloor replacement massive sulfide deposits, although no massive sulfide prospects or related altered zones have yet been defined. Sparse, disseminated sulfides occur in sericite-altered, steeply dipping shear zones.  相似文献   

7.
The Ordovician mafic volcanic rocks in the Parkes region of New South Wales occur as three distinct packages of volcaniclastic and coherent volcanic rocks and minor limestone that formed part of an oceanic island arc succession. The oldest package is the Early Ordovician Nelungaloo Volcanics and overlying Yarrimbah Formation. These formations consist of volcanic siltstone, sandstone, polymictic breccia, conglomerate facies interpreted as moderately deep-water turbidites and coarser grained debris-flow deposits emplaced in the medial to distal part of a subaqueous volcaniclastic apron flanking an active volcanic centre(s). Broadly conformable massive to brecciated andesites in the apron deposits are interpreted as synsedimentary sills and/or lava flows. A hiatus in volcanism occurred between the Bendigonian and early Darriwilian (ca 476 – 466 Ma). Deposition of the second package, which produced the Middle to Late Ordovician Goonumbla Volcanics, Billabong Creek Limestone and Gunningbland Formation, commenced with shallow-water limestones and minor volcaniclastic rocks. During an approximately 15 million years period, a thick sequence of bedded volcanic sandstone, limestone and minor siltstone and volcanic breccia were deposited in very shallow to moderate water depths. The top of this package is marked by thick volcanic conglomerate and sandstone mass-flow deposits and approximately coeval basaltic andesite lavas and sills sourced from a nearby volcano. The upper age limit of this package is constrained as approximately 450 Ma by Ea3/4 fossils and monzodiorite that intrudes the Goonumbla Volcanics. The lower limit of the third package, which constitutes the Wombin Volcanics, is poorly constrained and the duration of the hiatus that separates the Goonumbla and Wombin Volcanics is unknown but may be as long as 10 million years. The Wombin Volcanics record development of a thick, proximal volcaniclastic apron flanking a compositionally more evolved volcanic edifice in the immediate Parkes area. Thick crystal-rich turbiditic sandstones of mafic provenance are intercalated with polymictic volcanic breccias and megablock breccias that are interpreted as proximal subaqueous debris-flow and debris-avalanche deposits, respectively. The sequence also includes numerous trachyandesite bodies, many of which were emplaced within the volcaniclastic apron as synsedimentary sills. No evidence was found at Parkes to support the existence of a previously proposed 22 km diameter collapse caldera and the source volcanoes for the Ordovician are envisaged as complex stratovolcanoes.  相似文献   

8.
Zircon U–Pb ages measured on four small intrusions into the succession of Ordovician volcanic rocks that hosts Northparkes Cu–Au mine northwest of Parkes, New South Wales, place limits on the age of the volcanic sequence. The basal Nelungaloo Volcanics are constrained by a cross‐cutting monzodiorite to be ≥484.3 ± 2.9 Ma (Early Ordovician). Similarly, the overlying basal Goonumbla Volcanics are constrained by another cross‐cutting monzodiorite to be ≥450.8 ± 4.2 Ma (Middle Ordovician). A later generation of monzonites intruded into the middle and upper Goonumbla Volcanics yield ages of 439.1 ± 4.5 and 438.9 ± 4.7 Ma (Siluro‐Ordovician). These various ages are consistent with the ages of fossiliferous sediments within the volcanic sequence, and indicate that both the intrusive and volcanic rocks span an appreciable period of time—neither are the product of a single magmatic episode. Intrusion of the youngest monzonites and mineralisation was virtually contemporaneous.  相似文献   

9.

The Middle Devonian to Early Carboniferous Campwyn Volcanics of coastal central Queensland form part of the fore‐arc basin and eastern flank of the volcanic arc of the northern New England Fold Belt. They consist of a complex association of pyroclastic, hyaloclastic and resedimented, texturally immature volcaniclastic facies associated with shallow intrusions, lavas and minor limestone, non‐volcanic siliciclastics and ignimbrite. Primary igneous rocks indicate a predominantly mafic‐intermediate parentage. Mafic to intermediate pyroclastic rocks within the unit formed from both subaerial and ?submarine to emergent strombolian and phreatomagmatic eruptions. Quench‐fragmented hyaloclastite breccias are widespread and abundant. Shallow marine conditions for much of the succession are indicated by fossil assemblages and intercalated limestone and epiclastic sandstone and conglomerate facies. Volcanism and associated intrusions were widely dispersed in the Campwyn depositional basin in both space and time. The minor component of silicic volcanic products is thought to have been less proximal and derived from eruptive centres to the west, inboard of the basin.  相似文献   

10.
The stratigraphic nomenclature for the Lamington Volcanics in the southern part of the Tweed Shield Volcano is revised. An alkaline formation (the Kyogle Basalt) containing discontinuous rhyolitic‐agglomerate (the Homeleigh Agglomerate Member) and rhyolite flow (the Georgica Rhyolite Member) horizons is the basal unit of the volcanic succession. The Kyogle Basalt thins eastward where it is overlain by a subalkaline sequence comprising the Lismore Basalt, Nimbin Rhyolite, and Blue Knob Basalt respectively. The Lismore Basalt is composed predominantly of tholeiitic andesite with minor icelandite and sporadic flows of mildly alkaline rocks; tholeiitic andesites predominate in the Blue Knob Basalt. The Nimbin Rhyolite consists of rhyolite flows, and a relatively minor pyroclastic component (principally lapilli tuff).  相似文献   

11.
Based on textural and geochemical evidence, the existence of Late Precambrian rhyolitic volcanism related to the Cape Granite Suite is illustrated. Recrystallised rhyolitic ignimbrites represent the volcanic phase of a subvolcanic to volcanic sequence composed of rhyodacite to rhyolitic magma. Textural features including faimme, pumice fragments and glass shards provide unquivocal evidence for the volcanic origin of these rocks.  相似文献   

12.
Detrital volcanic and vein quartz, accompanied by felsic volcanic debris, occur as minor constituents in the Ordovician subduction‐related mafic volcanics of the Molong Volcanic Belt. In the western province of the Molong Volcanic Belt, detrital quartz is present in the three episodes of the mafic Volcanics. Volcanic quartz occurs in allochthonous limestone blocks in the Bendigonian Hensleigh Siltstone overlying the Mitchell Formation. The second volcanic episode (the Fairbridge Volcanics) commenced after a hiatus of approximately 20 million years and lasted around 10 million years from Darriwilian to Gisbornian time. Locally derived vein quartz, volcanic quartz and felsic detritus are concentrated at the bases of autochthonous Wahringa and Yuranigh Limestone Members of the volcanics and are extensive and abundant in basal beds of the regional Eastonian limestone body that transgressed over an eroded volcanic centre at Cargo. This early Eastonian debris, deposited early in an 8 million‐year volcanic hiatus preceding the final Ordovician Bolindian volcanism, establishes a pre‐Eastonian age for mineralisation at Cargo. It is inferred that the pauses in volcanism were preceded by magmatic fractionation, intrusion and hydrothermal activity and followed by erosion, subsidence and deposition of autochthonous limestones. Minor occurrences of vein and volcanic quartz are found in Bolindian volcanogenic sediments of the third volcanic phase. It is concluded that hydrothermal vein formation (and mineralisation by inference) was associated with pauses in volcanic activity throughout the Middle to early Late Ordovician over a wide area in the western province, culminating in the mineralisation at Cargo and Copper Hill near Molong. Volcanism in the eastern province of the Molong Volcanic Belt was continuous from at least Darriwilian to latest Ordovician time. Here, detrital hydrothermal vein quartz and volcanic quartz and felsic detritus are distributed through late Middle and early Late Ordovician turbidites of the Weemalla Formation. The possible existence of cycles in the source area like those of the Fairbridge Volcanics is masked by the distal nature of these deposits. Vein formation occurred in both provinces from late Middle Ordovician to early Late Ordovician, long before the formation of the world‐class mineral deposit at Cadia associated with the latest Ordovician Cadia Monzonite.  相似文献   

13.
本文对右江盆地北缘晚二叠世大隆组、早三叠世罗楼组中的凝灰岩夹层开展地质分析研究。镜下岩石学鉴定显示其为一套玻屑、晶屑凝灰岩;而其主量元素特征分析表明属流纹质、安山质凝灰岩,具高硅、高铝、高钾特征。结合稀土元素球粒陨石标准化分布型式图及微量元素原始地幔标准化蛛网图形态特征,Y、Nb、Rb、La、Pb及Ce等稀土、微量元素的比值特征,认为凝灰岩的岩浆来源与该时期发生在凭祥一带的中—酸性火山弧活动密切相关。构造背景属于古特提斯洋向北往华南陆块之下俯冲时,形成活动大陆边缘岛弧,弧后陆壳发生伸展活动形成右江盆地。岩浆来源于洋壳俯冲发生部分熔融,且尚未与地幔发生作用,在上升过程中与陆壳物质发生混染,由火山弧的活动喷发而来。  相似文献   

14.
内蒙古东乌旗巴彦敖包一带出露的宝力高庙组地层可划分为三段,可对应于查干敖包剖面二、三、四段。二段为一套杂色中酸性火山熔岩和火山碎屑岩类,火山喷发以喷溢相为主;三段为一套灰黄、灰白色火山-沉积碎屑岩类和陆源沉积岩,含Annularia植物化石,为火山间歇期沉积;四段为一套灰色、浅灰色酸性熔岩夹少量火山-沉积碎屑岩,火山喷发以溢流相为主。四段紫红色含晶屑流纹岩和二段灰黑色流纹岩锆石LA-ICP-MS U-Pb年龄分别为(305±4.1)Ma和(315.2±4.6)Ma,结合所含化石确定地层时代为晚石炭晚期—早二叠世早期。宝力高庙组中酸性火山岩为安山质、英安质、流纹质高钾钙碱性-钾玄岩系列火山岩组合,岩石呈现富硅、碱,贫钙、镁,A/NK值和Fe Ot/Mg O值高,富Rb、Nb,贫过渡族元素,并具有显著的铕负异常,为A2亚类花岗岩类,其可能是幔源岩浆底侵形成的晚古生代新生下地壳物质部分熔融而成,岩浆演化经历了钛铁矿物、铁镁矿物及斜长石分离作用。宝力高庙组中酸性火山岩形成于后碰撞构造背景环境,其特征揭示兴蒙造山带内蒙古东乌旗地区晚石炭世—早二叠世已处于同碰撞-碰撞后阶段,早二叠世已经进入碰撞后伸展阶段。  相似文献   

15.
Ignimbrites in the Devonian and Late Paleozoic volcanic belts in central Kazakhstan were produced in various geotectonic environments and are diverse in composition. The bulk composition of the Devonian ignimbrites is rhyolitic. The Eifelian rocks of the Chingiz island-arc system belong to the calc-alkaline series and are enriched in Zr, Nb, Y, and REE (predominantly LREE). The Frasnian ignimbrites that were formed in unusual island arcs of the Mediterranean type are ultrapotassic. Compared to the Eifelian ignimbrites, they bear lower concentrations of Zr, Nb, Y, and REE but are richer in Rb and Ba. Both rock varieties show clearly pronounced Eu minima and Ce anomalies. The Carboniferous and Permian ignimbrites were generated within a volcanic belt in a continental margin. The Carboniferous ignimbrites are mostly of dacite-rhyolite and sometimes of dacitic andesite composition. Compared to the Devonian ignimbrites, they are depleted in Zr, Nb, and Y at higher concentrations of Ba and low REE sums, which are notably dominated by LREE; their Eu minima are small, and they have no Ce anomalies. The Permian ignimbrites are predominantly of rhyolite composition. The Early Permian rocks have REE sums close to those in the Carboniferous rocks, but the former have clearly pronounced Eu minima and Ce anomalies. The Late Permian ignimbrites have total REE concentrations close to those in the Devonian ignimbrites, but the former are strongly enriched in LREE and have prominent Eu minima and Ce anomalies. The major-and trace-element composition of fiamme in all ignimbrite varieties varies depending on the relative age of the fiamme. The REE patterns of the fiamme differ from massif to massif, but their systematic changes from older to younger fiamme are similar. Along with the identity of the isotopic characteristics of whole-rock ignimbrite samples and fiamme of different ages in them, this testifies that the ignimbrites were formed not via the mixing of various melts but by the systematic evolution of a parental melts, which were different for different massifs.  相似文献   

16.
The orogenic Balkanid belt, which developed between the Moesian Plate and the Moravian-Rhodopi-Thracian Massifs, was affected by the Late Carboniferous and Early Permian opening of W-E oriented graben structures. The progressive tectonic rejuvenation of the basins is demonstrated by the deposition of repeated regional sedimentary cycles, associated with volcanism that was mostly localised along the tectonic boundaries, in an intramontane setting.The Late Carboniferous volcanism is represented by rhyodacitic explosive products and hyaloclastites, and by andesitic flows. During the Early Permian, subvolcanic rhyodacitic and rhyolitic bodies and the explosive products prevailed in the western sectors, whereas rhyolitic ignimbrites occur to the east.The tectonically active basins are interpreted due to late orogenic collapse, and the alternation of extensional tectonics and minor compressional phases is consistent with the regional transtensional regime, active along the Variscan suture of Pangaea. The volcanic activity associated with the evolution of the basins matches the petrogenetic features and the evolution from early dacitic – andesitic to late rhyolitic activity in the Southern European segment of the Variscan system.These Late Carboniferous-Early Permian sedimentary and tectono-magmatic events in Bulgaria are characterized, and compared with the homologous Permo-Carboniferous sequences along some western European segments of the Variscan belt.  相似文献   

17.
The Paleoproterozoic basalts of the Eastern Creek Volcanics are a series of continental flood basalts that form a significant part of the Western Fold Belt of the Mt Isa Inlier, Queensland. New trace-element geochemical data, including the platinum-group elements (PGE), have allowed the delineation of the magmatic history of these volcanic rocks. The two members of the Eastern Creek Volcanics, the Cromwell and Pickwick Metabasalt Members, are formed from the same parental magma. The initial magma was contaminated by continental crust and erupted to form the lower Cromwell Metabasalt Member. The staging chamber was continuously replenished by parental material resulting in the gradual return of the magma composition to more primitive trends in the upper Cromwell Metabasalt Member, and finally the Pickwick Metabasalt Member formed from magma dominated by the parental melt. The Pickwick Metabasalt Member of the Eastern Creek Volcanics has elevated PGE concentrations (including up to 18 ppb Pd and 12 ppb Pt) with palladium behaving incompatibly during magmatic fractionation. This trend is the result of fractionation under sulfide-undersaturated conditions. Conversely, in the basal Cromwell Metabasalt Member the PGE display compatible behaviour during magmatic fractionation, which is interpreted to be the result of fractionation of a sulfide-saturated magma. However, Cu remains incompatible during fractionation, building up to high concentrations in the magma, which is found to be the result of the very small volume of magmatic sulfide formation (0.025%). Geochemical trends in the upper Cromwell Metabasalt Member represent mixing between the contaminated Cromwell Metabasalt magmas and the PGE-undepleted parental melt. Trace-element geochemical trends in both members of the Eastern Creek Volcanics can be explained by the partial melting of a subduction-modified mantle source. The generation of PGE- and copper-rich magmas is attributed to melting of a source in the subcontinental lithospheric mantle below the Mt Isa Inlier which had undergone previous melt extraction during an older subduction event. The previous melt extraction resulted in a sulfur-poor, metal-rich metasomatised mantle source which was subsequently remelted in the Eastern Creek Volcanic continental rift event. The proposed model accounts for the extreme copper enrichment in the Eastern Creek Volcanics, from which the copper has been mobilised by hydrothermal fluids to form the Mt Isa copper deposit. There is also the potential for a small volume of PGE-enriched magmatic sulfide in the plumbing system to the volcanic sequence.  相似文献   

18.
In western Anatolia, a thick volcanic succession of andesitic to rhyolitic lavas and volcaniclastic rocks crops out extensively. On Foça Peninsula, the westernmost part of the region, a dominantly rhyolitic sequence is exposed where massive rhyolites occur as dome or domelike stubby lava flows. These rhyolite domes vertically and laterally pass into blanketing volcaniclastic sequences. The gradational boundary relations and the facies characteristics of the surrounding volcaniclastic sequences indicate that the silicic domes directly intruded a subaqueous environment and were shattered upon sudden contact with water to form hyaloclastic blankets.

In and around these rhyolite domes, we have defined six different volcanic and volcaniclastic facies, consisting of: (1) massive rhyolite; (2) massive perlite; (3) hyaloclastic breccias; (4) rhyolite pumice and lithic fragment-bearing volcaniclastic rocks; (5) subaqueous welded ignimbrites; and (6) brecciated perlite. The massive rhyolite facies have distinct structures from the centers to the peripheries of the domes and stubby lava flows. Massive lava facies gradually pass into hyaloclastic breccias and massive perlite facies, indicating water-magma interaction during the emplacement. Phreatomagmatic explosive activity and doming caused the subaqueous pyroclastic flows on the flanks of the volcanic center. Welding in the upper parts of these pyroclastic flow deposits indicates the high-temperature emplacement of the pyroclastic material and relatively slow cooling caused by the cushioning effect of the gas-vapor mixture and rapid deposition of younger pyroclastic units.  相似文献   

19.
Upper Devonian continental and subaqueous sedimentary rocks and bimodal volcanic rocks of the Boyd Volcanic Complex of the south coast of New South Wales were deposited in a rapidly subsiding, 330°‐trending, transtensional basin. Structural analysis of synvolcanic and synsedimentary deformational structures indicate that basin formation is related to a 330°‐orientated subhorizontal σ1 and a 060°‐orientated subhorizontal σ3, which account for the development of the observed intrusion and fracture orientations. Rhyolitic, basaltic and associated clastic dykes are preferentially intruded along extensional 330°‐trending fractures, subordinately along sinistral, transtensional 010°‐trending fractures and along 290°‐trending fractures. One of the implications of such a palaeotectonic reconstruction is that the so called north‐trending Eden‐Comerong‐Yalwal Late Devonian rift does not represent a simple, single palaeobasin entity, but is presently a north‐trending alignment of exposures of sedimentary and volcanic rocks probably emplaced in different basins or sub‐basins, mildly folded during the Carboniferous Kanimblan compression (which also formed the north‐trending Budawang synclinorium) and then extended to the east by the Tasman Sea opening during the Jurassic. The development of scattered, rapidly subsiding, basins characterised by bimodal volcanism during the Late Devonian throughout the Lachlan Fold Belt, can be interpreted in terms of extensional collapse of a forming mountain belt contemporaneous with a sharp decrease of compressional stress after the Middle Devonian Tabberabberan orogenic event. This would promote a reorientation of σ3 and transition from a compressional to a transtensional tectonic environment, which could also favour block rotation and formation of release basins.  相似文献   

20.
The Late Silurian to Middle Devonian Calliope Volcanic Assemblage in the Rockhampton region is deformed into a set of northwest‐trending gently plunging folds with steep axial plane cleavage. Folds become tighter and cleavage intensifies towards the bounding Yarrol Fault to the east. These folds and associated cleavage also deformed Carboniferous and Permian rocks, and the age of this deformation is Middle to Late Permian (Hunter‐Bowen Orogeny). In the Stanage Bay area, both the Calliope Volcanic Assemblage and younger strata generally have one cleavage, although here it strikes north to northeast. This cleavage is also considered to be of Hunter‐Bowen age. Metamorphic grade in the Calliope Volcanic Assemblage ranges from prehnite‐pumpellyite to greenschist facies, with higher grades in the more strongly cleaved rocks. In the Rockhampton region the Calliope Volcanic Assemblage is part of a west‐vergent fold and thrust belt, the Yarrol Fault representing a major thrust within this system.

A Late Devonian unconformity followed minor folding of the Calliope Volcanic Assemblage, but no cleavage was formed. The unconformity does not represent a collision between an exotic island arc and continental Australia as previously suggested.  相似文献   

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