The Cooma Metamorphic Complex, a low-P, high-T (LPHT) regional aureole beneath the Murrumbidgee Batholith

The Cooma Complex of the Lachlan Fold Belt, south‐eastern Australia, is characterised by a large (c. 10 km wide) low‐P, high‐T metamorphic aureole surrounding a small (3 × 6 km) granite pluton. The aureole extends northward to envelop the eastern lobe of the Murrumbidgee Batholith and progressively narrows to a kilometre wide hornfelsic aureole some 50 km north of Cooma. At its northern extremity, the batholith has intruded its own volcanic cover. These regional relations suggest that the Murrumbidgee Batholith is gently tilted to the north, with the Cooma Complex representing the aureole beneath the batholith. Two main deformation events, D3 and D5, affected the aureole. The inner, high‐grade migmatitic domain contains upright F5 folds defined by a composite, transposed S3/S0 fabric and S3/S0 concordant leucosomes. The folded stromatic migmatites define the western limb of a F5 synform, with its axis located in the batholith. Lenses and sheets of the Murrumbidgee Batholith intruded along S3 but also preserve S3 as a strong, solid‐state foliation. S3 and the granite sheets but are also folded by F5, outlining a fanning positive flower structure. These relations indicate that most of the batholith was emplaced before and during D3, but intrusion persisted until early syn‐D5. Formation of the Cooma Granodiorite occurred post‐D3 to early syn‐D5, after formation of the wide metamorphic aureole during early syn‐D3 to early syn‐D5. The Murrumbidgee Batholith was emplaced between pre‐D3 to early syn‐D5, synchronous with the formation of the Cooma Complex. The structural and metamorphic relations indicate that the Murrumbidgee Batholith was the ultimate heat source responsible for the Cooma Metamorphic Complex. D3 structures and metamorphic isograds are subparallel to the batholith margin for over 50 km. This concordance probably extends vertically, suggesting that the isograds also fan outward from the batholith margin. This implies an inverted metamorphic sequence focused on the Murrumbidgee Batholith, although the base has been almost completely removed by erosion in the Cooma Complex. The field evidence at Cooma, combined with previous thermal modelling results, suggest that extensive LPHT metamorphic terranes may represent regional metamorphic aureoles developed beneath high‐level granitic batholiths.     

Ordovician and Silurian lithofacies and base‐metal deposits of the Lachlan Fold Belt

Two lithofacies maps of the Lachlan Fold Belt, one for the Ordovician and one for the Silurian, are illustrated. Both maps indicate shorelines in western New South Wales, Victoria and Tasmania.

The Ordovicoan map suggests open‐sea conditions eastwards from the shoreline with one major and two minor andesitic volcanoes (or volcanic centres). The Silurian map suggests segmentation of the Lachlan Fold Belt into the Melbourne Basin, Omeo Land, Newell Basin, and Budawang Land. The Newell Basin displays a nearshore (Louth‐Mitta Mitta) coarse clastics facies and an offshore (Wellington‐Cooma) platform carbonate facies. Acid volcanism was widespread over the Newell Basin in Silurian time, but did not occur in the Melbourne Basin.

The Louth‐Mitta Mitta and Wellington‐Cooma facies boundary coincides with the position of the Coolac‐Honeybugle Serpentine Belt and the outcrop area of the Girilambone Beds, suggesting that these features were already in some way prominent during the Silurian Period: the Serpentine Belt may have been a fault, and the Girilambone Beds may have been land.

The origin of base‐metal deposits in the Silurian rocks is thought to be somehow related to the heat generated in the subsurface during Silurian time as is indicated by the volcanism and granite intrusion; and also to the fact that the deposits occur in a transgressive sequence which contains the first phase of acid volcanism in the known geological history of the Lachlan Fold Belt.     

The Berridale Wrench fault: A major structure in the Snowy Mountains of New South Wales

One of the most significant, but poorly understood, tectonic events in the east Lachlan Fold Belt is that which caused the shift from mafic, mantle‐derived calc‐alkaline/shoshonitic volcanism in the Late Ordovician to silicic (S‐type) plutonism and volcanism in the late Early Silurian. We suggest that this chemical/isotopic shift required major changes in crustal architecture, but not tectonic setting, and simply involved ongoing subduction‐related magmatism following burial of the pre‐existing, active intraoceanic arc by overthrusting Ordovician sediments during Late Ordovician — Early Silurian (pre‐Benambran) deformation, associated with regional northeast‐southwest shortening. A review of ‘type’ Benambran deformation from the type area (central Lachlan Fold Belt) shows that it is constrained to a north‐northwest‐trending belt at ca 430 Ma (late Early Silurian), associated with high‐grade metamorphism and S‐type granite generation. Similar features were associated with ca 430 Ma deformation in east Lachlan Fold Belt, highlighted by the Cooma Complex, and formed within a separate north‐trending belt that included the S‐type Kosciuszko, Murrumbidgee, Young and Wyangala Batholiths. As Ordovician turbidites were partially melted at ca 430 Ma, they must have been buried already to ～20 km before the ‘type’ Benambran deformation. We suggest that this burial occurred during earlier northeast‐southwest shortening associated with regional oblique folds and thrusts, loosely referred to previously as latitudinal or east‐west structures. This event also caused the earliest Silurian uplift in the central Lachlan Fold Belt (Benambran highlands), which pre‐dated the ‘type’ Benambran deformation and is constrained as latest Ordovician — earliest Silurian (ca 450–440 Ma) in age. The south‐ to southwest‐verging, earliest Silurian folds and thrusts in the Tabberabbera Zone are considered to be associated with these early oblique structures, although similar deformation in that zone probably continued into the Devonian. We term these ‘pre’‐ and ‘type’‐Benambran events as ‘early’ and ‘late’ for historical reasons, although we do not consider that they are necessarily related. Heat‐flow modelling suggests that burial of ‘average’ Ordovician turbidites during early Benambran deformation at 450–440 Ma, to form a 30 km‐thick crustal pile, cannot provide sufficient heat to induce mid‐crustal melting at ca 430 Ma by internal heat generation alone. An external, mantle heat source is required, best illustrated by the mafic ca 430 Ma, Micalong Swamp Igneous Complex in the S‐type Young Batholith. Modern heat‐flow constraints also indicate that the lower crust cannot be felsic and, along with petrological evidence, appears to preclude older continental ‘basement terranes’ as sources for the S‐type granites. Restriction of the S‐type batholiths into two discrete, oblique, linear belts in the central and east Lachlan Fold Belt supports a model of separate magmatic arc/subduction zone complexes, consistent with the existence of adjacent, structurally imbricated turbidite zones with opposite tectonic vergence, inferred by other workers to be independent accretionary prisms. Arc magmas associated with this ‘double convergent’ subduction system in the east Lachlan Fold Belt were heavily contaminated by Ordovician sediment, recently buried during the early Benambran deformation, causing the shift from mafic to silicic (S‐type) magmatism. In contrast, the central Lachlan Fold Belt magmatic arc, represented by the Wagga‐Omeo Zone, only began in the Early Silurian in response to subduction associated with the early Benambran northeast‐southwest shortening. The model requires that the S‐type and subsequent I‐type (Late Silurian — Devonian) granites of the Lachlan Fold Belt were associated with ongoing, subduction‐related tectonic activity.     

Emplacement and deformation of the Wyangala Batholith,New South Wales

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.     

Structural and metamorphic evolution of the Moornambool Metamorphic Complex,western Lachlan Fold Belt,southeastern Australia

The wedge‐shaped Moornambool Metamorphic Complex is bounded by the Coongee Fault to the east and the Moyston Fault to the west. This complex was juxtaposed between stable Delamerian crust to the west and the eastward migrating deformation that occurred in the western Lachlan Fold Belt during the Ordovician and Silurian. The complex comprises Cambrian turbidites and mafic volcanics and is subdivided into a lower greenschist eastern zone and a higher grade amphibolite facies western zone, with sub‐greenschist rocks occurring on either side of the complex. The boundary between the two zones is defined by steeply dipping L‐S tectonites of the Mt Ararat ductile high‐strain zone. Deformation reflects marked structural thickening that produced garnet‐bearing amphibolites followed by exhumation via ductile shearing and brittle faulting. Pressure‐temperature estimates on garnet‐bearing amphibolites in the western zone suggest metamorphic pressures of ～0.7–0.8 GPa and temperatures of ～540–590°C. Metamorphic grade variations suggest that between 15 and 20 km of vertical offset occurs across the east‐dipping Moyston Fault. Bounding fault structures show evidence for early ductile deformation followed by later brittle deformation/reactivation. Ductile deformation within the complex is initially marked by early bedding‐parallel cleavages. Later deformation produced tight to isoclinal D₂ folds and steeply dipping ductile high‐strain zones. The S₂ foliation is the dominant fabric in the complex and is shallowly west‐dipping to flat‐lying in the western zone and steeply west‐dipping in the eastern zone. Peak metamorphism is pre‐ to syn‐D₂. Later ductile deformation reoriented the S₂ foliation, produced S₃ crenulation cleavages across both zones and localised S₄ fabrics. The transition to brittle deformation is defined by the development of east‐ and west‐dipping reverse faults that produce a neutral vergence and not the predominant east‐vergent transport observed throughout the rest of the western Lachlan Fold Belt. Later north‐dipping thrusts overprint these fault structures. The majority of fault transport along ductile and brittle structures occurred prior to the intrusion of the Early Devonian Ararat Granodiorite. Late west‐ and east‐dipping faults represent the final stages of major brittle deformation: these are post plutonism.     

Tectonic imprints within a granite exposed near Srinagar,Rajasthan, India

Partial melting in the middle to lower crustal level produces melts of granitic composition during orogeny. Thrusts play a vital role in their exhumation after consolidation of these granitic melts. In this paper we focus on one such granite along the eastern margin of the Delhi Fold Belt (DFB) rocks near Srinagar, Rajasthan, India. This is the first report of granite within the area and holds a key stratigraphic position in the entire rock package. The said granite is found to be intrusive to the DFB metasediments as well as their basement popularly known as the Banded Gneissic Complex (BGC). We disentangle the deformation fabrics seen within the granite and associated DFB metasediments, suggesting that subsequent to emplacement and consolidation, the granite has co-folded along with the country rocks. Three deformational events could be identified within the DFB metasediments namely, D_1D, D_2D and D_3D. The peak metamorphism was achieved in the D_1D event. The granite magma is generated and emplaced late syn-kinematic to D_1D and thereafter is deformed by D_2D and D_3D producing D_1G and D_2G structural fabrics. These compressive deformations resulted in the collapse of the basin; the combined package of DFB rocks and the granite was thrusted eastwards over the basement rocks. The tectonic transport direction during thrusting is suggested eastwards from our structural analysis. Transverse faults developed perpendicular to the length of the granite have led to partitioning of the strain thereby showing a heterogeneity in the development of fabric within it.     

Deformation history of the Naraku Batholith,Mt Isa Inlier,Australia: Implications for pluton ages and geometries from structural study of the Dipvale Granodiorite and Levian Granite

Plutons of the Naraku Batholith were emplaced into Proterozoic metasediments of the northern portion of the Eastern Fold Belt of the Mt Isa Inlier during two intrusive episodes approximately 200 million years apart. Structural relationships and geochronological data suggest that the older plutons (ca 1750 Ma) are contemporaneous with granites of the Wonga Batholith to the west. The Dipvale Granodiorite and the Levian Granite represent these older intrusive phases of the Naraku Batholith, and both contain an intense tectonic foliation, S₁, which is interpreted to have formed during the north‐south shortening associated with D₁ of the Isan Orogeny. The geometry of S₁ form surfaces at the southern end of the Dipvale Granodiorite, and of the previously unrecognised sheeted contact, defines a macroscopic, steeply south‐southwest‐plunging antiform, which was produced by the regional D₂ of the Isan Orogeny. S₁ form surfaces in the Levian Granite define open F₂ folds with wavelengths of several hundred metres. The structural age of emplacement of the Dipvale Granodiorite and the Levian Granite is interpreted to be pre‐ or syn‐ the regional D₁. An intense foliation present in some of the younger (ca 1505 Ma) granites that comprise the bulk of the Naraku Batholith is interpreted to represent S₃ of the Isan Orogeny. Foliations commonly have similar styles and orientations in both the pre‐D₁ and younger plutons. This emphasises the simplicity with which regional fabrics can be, and probably have been, miscorrelated in the Eastern Fold Belt, and that the classification of granites in general on the basis of structural and geometric criteria alone is fraught with danger.     

Discussion: Macrofaunal zonation of Middle Permian coal sequences in Sydney Basin

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 (Sr_l = 0.70388 ± 53). A total‐rock isochron for the peralkaline Narraburra Suite gives a less well‐defined age of 358 ± 9 Ma (Sr_l = 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.     

The chemistry and origin of zincian spinel associated with the Aggeneys Cu-Pb-Zn-Ag deposits,Namaqualand, South Africa

Zincian spinel or gahnite [(Zn,Fe,Mg)Al₂O₄] occurs in metamorphosed sulphide-rich rocks, garnet quartzites, quartz-magnetite rocks, aluminous metasediments, barite-magnetite rocks, quartz veins, and pegmatites associated with the Aggeneys base metal deposits, Namaqualand, South Africa. Zincian spinel in, sulphide-bearing rocks, is considered to have formed predominantly by desulphurization reactions involving a member of the system Fe-S-O and sphalerite with sillimanite or garnet. Gahnite in sulphide-free garnet quartzites, quartz-magnetite rocks and barite-magnetite rocks probably formed from Zn and Al that were hydrothermally derived whereas gahnite in aluminous metasediments was derived from the metamorphism of metalliferous shales, in which Zn may originally have been linked to organic material. Gahnite is Zn-rich in sulphide-bearing rock, but is Fe-rich in sulphide-free garnet quartzites and quartz-magnetite rocks. Although Zn-rich spinels represent guides to ore in the Aggeneys area and elsewhere in the Namaqualand Metamorphic Complex, Fe-rich spinels should not be discounted because Zn-rich and Fe-rich spinels occur within metres of sulphides at Aggeneys.     

Crustal structure of the Ordovician Macquarie Arc,Eastern Lachlan Orogen,based on seismic‐reflection profiling

In the Eastern Lachlan Orogen, the mineralised Molong and Junee‐Narromine Volcanic Belts are two structural belts that once formed part of the Ordovician Macquarie Arc, but are now separated by younger Silurian‐Devonian strata as well as by Ordovician quartz‐rich turbidites. Interpretation of deep seismic reflection and refraction data across and along these belts provides answers to some of the key questions in understanding the evolution of the Eastern Lachlan Orogen—the relationship between coeval Ordovician volcanics and quartz‐rich turbidites, and the relationship between separate belts of Ordovician volcanics and the intervening strata. In particular, the data provide evidence for major thrust juxtaposition of the arc rocks and Ordovician quartz‐rich turbidites, with Wagga Belt rocks thrust eastward over the arc rocks of the Junee‐Narromine Volcanic Belt, and the Adaminaby Group thrust north over arc rocks in the southern part of the Molong Volcanic Belt. The seismic data also provide evidence for regional contraction, especially for crustal‐scale deformation in the western part of the Junee‐Narromine Volcanic Belt. The data further suggest that this belt and the Ordovician quartz‐rich turbidites to the east (Kirribilli Formation) were together thrust over ?Cambrian‐Ordovician rocks of the Jindalee Group and associated rocks along west‐dipping inferred faults that belong to a set that characterises the middle crust of the Eastern Lachlan Orogen. The Macquarie Arc was subsequently rifted apart in the Silurian‐Devonian, with Ordovician volcanics preserved under the younger troughs and shelves (e.g. Hill End Trough). The Molong Volcanic Belt, in particular, was reworked by major down‐to‐the‐east normal faults that were thrust‐reactivated with younger‐on‐older geometries in the late Early ‐ Middle Devonian and again in the Carboniferous.     

The relationship of structures across the Lambian unconformity near Taralga,New South Wales

The structures across the Lambian Unconformity near Taralga show evidence of two, and possibly three, significant episodes of folding. The first, Early to Middle Silurian folding is poorly defined, but may be responsible for initial dips that are reflected in the more complex deformation patterns in the Late Ordovician than in the overlying younger rocks. The second, mid‐Devonian folding produced upright folds trending 10° west of north, and the last, latest Devonian to Early Carboniferous folding produced the meridional Cookbundoon Synclinorium and the regional cleavage. No cleavage was associated with the first two episodes of folding in the area studied. The angular discordance across the Lambian Unconformity caused by mid‐Devonian folding is much greater than in the northeastern Lachlan Fold Belt, and reflects the increasing intensity of mid‐Devonian folding southward. The tight, slightly overturned profile of the Cookbundoon Synclinorium reflects an intensity of latest Devonian to Early Carboniferous folding similar to that found in the northeastern Lachlan Fold Belt, but the intensity of this folding decreases further south.     

Lower Paleozoic rifting event in Central Iberian Zone (central-north Portugal): Evidence from elemental and isotopic geochemistry of metabasic rocks

The occurrence of Lower Paleozoic mafic magmatic rocks in the Central Iberian Zone (CIZ) of the Variscan Orogen is rare. Amphibolites and metagabbros embedded in the metasediments of the Douro-Beiras Supergroup outcrop at Farminhão, Viseu (central-north Portugal). The protoliths of these two rock types are tholeiites presenting different isotopic signatures (εNd₄₈₀ = +4.63 to +4.93 and +5.74 to +7.67) and incompatible element ratios (normalized La/Lu up to 4.5 and down to 0.7), respectively which suggests they are not cogenetic. The closely related meta-ultramafic rocks are considered as cumulates generated from the magmas that originated the metagabbros. The elemental and isotopic features of the metagabbros are similar to those reported for amphibolitic occurrences in Tenzuela (near Segovia, Spain) which allows the proposition of a similar age for the mafic rocks of Farminhão. Despite the depleted characteristics of the studied rocks, they are interpreted as having been formed during a continental rifting process characterized by variable degrees of stretching, some 100 Ma after the end of the deposition of the Douro-Beiras Supergroup. This rifting event marks the onset of the Variscan Cycle (s.l.) in the Central Iberian Zone. The occurrence of these metabasic rocks near the confluence between the Porto-Viseu Metamorphic Belt and the Juzbado-Penalva do Castelo Shear Zone suggests that these first-order structures may have worked as weakness zones constraining the ascent of magmas during the Ordovician. The Lower Ordovician metabasic rocks here studied are chemically similar to the abundant lower to medium Cambrian magmatic rocks of the Ossa Morena Zone, also in Iberia, further reinforcing the diachronous character of the opening of the Rheic Ocean that later propagated to the eastern sectors of the European Variscan Belt.     

“构造杂岩”及其地质意义——以西准噶尔为例

构造杂岩是构造地层学的重要研究内容之一。以西准噶尔为例,三个不同时期形成的构造杂岩:科克沙依杂岩、玛依勒杂岩和达拉布特杂岩,代表了古生代不同时期洋盆与火山弧的残迹。现今西准噶尔的构造格局,可能是多个地体的拼合。     

The Gundahl Complex of the New England fold belt,Eastern Australia: a tectonic mélange formed in a palaeozoic subduction complex

The New England Fold Belt of eastern Australia preserves a Palaeozoic fore-arc terrain with a magmatic arc, fore-arc basin and a subduction complex. The Gundahl Complex is a tectonic mélange of regional extent in the subduction complex. The matrix and slabs of the Gundahl Complex have six mappable lithofacies: argillite, greywacke-argillite, greywacke, argillite-tuff, bedded chert and greenstone. The argillite matrix is pervasively sheared with many slickensided shear fractures. Locally the matrix is formed by highly sheared greenstone. Greywacke and greenstone blocks are affected by internal shear zones and the blocks themselves pinch and swell. Folds, in places with axial-surface spaced cleavage, are common within those slabs comprised of well-bedded sequences. Bedding-plane shear and faulting at a high angle to bedding also occur in these slabs. On a map-scale much of the Gundahl Complex comprises slabs up to 10 km long in imbricate fault-bounded slices which repeat the disrupted pre-mélange stratigraphic sequence. Elsewhere there are lithologically distinctive blocks containing thick coherent sequences which are structurally incorporated into the Gundahl Complex. The unit is believed to have formed by accretion, imbrication and subsequent tectonic disruption of arc-derived sediments and less abundant pelagic sediment and greenstone in an ancient subduction complex.     

Geochemistry of a peraluminous granitoid suite from North-eastern Victoria,South-eastern Australia

The Koetong Suite of Silurian, 2-mica granitoids was derived from a metasedimentary source and emplaced into Ordovician sediments and metasediments along the eastern margin of the Western Metamorphic Belt of South-eastern Australia. Whole-rock geochemical considerations preclude derivation of the magmas represented by the granitoids from exposed Ordovician metasediments. The magmas were generated by partial melting of material similar in composition to garnet-cordierite gneisses exposed in the adjacent metamorphic belt. Melting at pressures in excess of 5 Kb and temperatures about 750°C produced peraluminous magmas and, when the degree of partial melting approached 25–30%, these magmas became mobile and moved vertically into the overlying Ordovician sediments. During movement from the source region to the zone of emplacement, separation of the melt and refractory residue components of the magma resulted in a range of compositions so that whole-rock analyses of the granitoids are linearly related on major and trace element variation diagrams. Processes such as crystal fractionation and crystal accumulation may have operated locally. The magmas were largely composed of solid material throughout their emplacement histories and the amount of melt may not have exceeded 30–45% at any stage. Metasedimentary inclusions are a reflection of source heterogeneity.After emplacement of the magmas, in situ crystallization of a relatively anhydrous assemblage of minerals led to water contents in residual, intercrystalline, melts sufficiently high for muscovite to begin crystallization at pressures around 4 Kb. Subsequent saturation of intercrystalline residual melt and loss of the resultant volatile phase caused the development of eutectoid intergrowths involving muscovitebiotite-quartz and alkali feldspar.     

Ordovician Intrusive-related Gold-Copper Mineralization in West-Central New South Wales, Australia

Three major types of Ordovician intrusive-related gold-copper deposits are recognized in central-west New South Wales, Australia: porphyry, skarn and high sulphidation epithermal deposits. These deposits are mainly distributed within two Ordovician volcano-intrusive belts of the Lachlan Fold Belt: the Orange-Wellington Belt and the Parkes-Narromine Belt. Available isotopic age data suggest that mineralization of the three types of deposits is essentially coeval with the Ordovician intrusive rocks (480-430 Ma).Porphyry gold-copper deposits can be further divided into two groups. The first group is associated with monzonite showing shoshonitic features, represented by Cadia and Goonumbla. The second group is associated with diorite and dacite, including the Copper Hill and Cargo gold-copper deposits. Gold skarn is associated with Late Ordovician (430-439 Ma) monzonitic intrusive complexes in the Junction Reefs area (Sheahan-Grants, Frenchmans, and Cor-nishmens), Endeavour 6, 7 and 44, Big and Little Cadia     

Geochemical features,depositional settings,and provenances of Lower Paleozoic rocks in the Mamyn terrane,Central Asian Fold Belt

New data on geochemical features of the Lower Paleozoic terrigenous rocks in the Mamyn terrane (eastern Central Asian Fold Belt) and U–Pb geochronological studies of the detrital zircon from these rocks are presented. The obtained results suggest the following conclusions. 1. At present, the Kosmataya sequence includes different age Lower Cambrian terrigenous–carbonate and Lower Ordovician terrigenous rocks or represents Lower Ordovician olistostromes including limestone blocks with the Lower Cambrian fauna. Lower Ordovician terrigenous rocks were formed in an island arc or active continental margin, mainly, owing to the erosion of Cambrian–Early Ordovician plutons and volcanics that are widespread in structures of the Mamyn terrane and weakly reworked by the chemical weathering. 2. The Silurian Mamyn Formation was developed at a passive continental margin. The main sources of clastic material for this formation were the same Cambrian–Early Ordovician igneous rocks as for the Cambrian sequence, with the participation of Early Silurian and Vendian igneous complexes. The obtained data significantly refine concepts about the geological structure of the Mamyn terrane, which is a member of the Argun Superterrane, one of the largest tectonic structures in the eastern Central Asian Fold Belt.     

Evolution of the early devonian bindook volcanic complex,wollondilly basin,eastern lachlan fold belt

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.     

Response of detrital zircon and monazite,and their U–Pb isotopic systems,to regional metamorphism and host‐rock partial melting,Cooma Complex,southeastern Australia

Progressive Early Silurian low‐pressure greenschist to granulite facies regional metamorphism of Ordovician flysch at Cooma, southeastern Australia, had different effects on detrital zircon and monazite and their U–Pb isotopic systems. Monazite began to dissolve at lower amphibolite facies, virtually disappearing by upper amphibolite facies, above which it began to regrow, becoming most coarsely grained in migmatite leucosome and the anatectic Cooma Granodiorite. Detrital monazite U–Pb ages survived through mid‐amphibolite facies, but not to higher grade. Monazite in the migmatite and granodiorite records only metamorphism and granite genesis at 432.8 ± 3.5 Ma. Detrital zircon was unaffected by metamorphism until the inception of partial melting, when platelets of new zircon precipitated in preferred orientations on the surface of the grains. These amalgamated to wholly enclose the grains in new growth, characterised by the development of {211} crystal faces, in the migmatite and granodiorite. New growth, although maximum in the leucosome, was best dated in the granodiorite at 435.2 ± 6.3 Ma. The combined best estimate for the age of metamorphism and granite genesis is 433.4 ± 3.1 Ma. Detrital zircon U–Pb ages were preserved unmodified throughout metamorphism and magma genesis and indicate derivation of the Cooma Granodiorite from Lower Palaeozoic source rocks with the same protolith as the Ordovician sediments, not Precambrian basement. Cooling of the metamorphic complex was relatively slow (average ～12°C/10⁶y from ～730 to ～170°C), more consistent with the unroofing of a regional thermal high than cooling of an igneous intrusion. The ages of detrital zircon and monazite from the Ordovician flysch (dominantly composite populations 600–500 Ma and 1.2–0.9 Ga old) indicate its derivation from a source remote from the Australian craton.     

Petrology and isotope geology of mafic to ultramafic metavolcanic rocks of the Brusque Metamorphic Complex,southern Brazil

The Brusque Metamorphic Complex (BMC) is one of the main units of the Tijucas Terrain within the Dom Feliciano Belt, located in the state of Santa Catarina in southern Brazil. In the Itapema region, the BMC is composed chiefly of metasediments, including subordinate metabasalts, meta-ultramafic rocks, and clinoamphibole schists. The metavolcanic rocks form 4 m-thick lenses interlayered with metapelites and calc-silicate schists. Based on the observed textures and the associated structural, bulk-rock geochemical, and mineral chemical data, these metamafites and ultramafites were ancient lava flows of tholeiitic basalts and ultramafic cumulates. The mineral parageneses of the metabasalts are albite?+?actinolite?+?chlorite?+?epidote?+?titanite?+?magnetite and oligoclase?+?hornblende?+?epidote?+?titanite?+?magnetite, indicating progressive transformations produced under greenschist to amphibolite facies conditions. Volcanogenic metasediments show the same geochemical patterns as the metabasalts, whereas the metamorphosed ultramafic rocks consist of cumulates generated by crystal fractionation and flow segregation. The studied rocks show similar rare-earth element (REE) patterns, characterized by clearly higher normalized contents of light REEs compared with heavy REEs, without Eu anomalies in the metabasalts and positive Eu anomalies in meta-ultramafic rocks and volcanogenic metasediments. In accordance with the trace element contents that indicate a within-plate nature, the corresponding mafic melts apparently formed in the mantle by partial fusion and were subsequently enriched with crustal components during ascent into the sialic crust. The analysed ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr ratios lie between 0.5123 and 0.5126 and 0.7067 and 0.7086, respectively, and are thus typical of tholeiitic basalts of the continental plateau type. Initial ?Nd₍₉₃₆₎ values and derived model ages (T _DM) between 1028 and 1762 million years support a mantle source or sources, with extraction and emplacement in the Neoproterozoic. Field relations and geochemical data (including isotopic data) indicate the generation of the studied mafic and ultramafic rocks in a continental rift. In the regional geologic context, the formation of the BMC volcanic and metasedimentary units marks a period of fragmentation of the Palaeoproterozoic continental crust. This extensional event is preserved regionally in gneisses from the Santa Catarina Granulitic Complex and the Camboriú Complex.