The petrology and mineralogy of the pegmatite complex at Bismuth,Torrington, N.S.W.

The pegmatite complex of epi‐Permian age at Bismuth near Torrington, N.S.W., consists of an elongated intrusion of a granitoid quartz‐topaz rock (silexite) together with a series of pegmatites of varying composition. The principal pegmatite consists of orthoclase, biotite, quartz and beryl with concentric zoning passing outwards into fine‐grained biotite‐beryl rock containing a number of ore minerals: arsenides of Co, Fe and Ni, wolframite, bismuth, bismuthinite, molybdenite, joseite, cassiterite, rutile, uraninite and monazite. Small pegmatite veins issuing from this main body contain, in addition to the silicate minerals, high temperature tetrahedrite, chalcopyrite and sphalerite. A second group is characterised by quartz, orthoclase and beryl with occasional patches of tourmaline.

Emplacement at no great depth is indicated by breccia veins and stock‐works filled with pegmatite.

The origin of a silica hydromagma is considered in terms of existing experimental work and in terms of field occurrence. Structural evidence suggests that the quartzose intrusion preceded the injection of the pegmatite fluids, both being derived from the parent Mole biotite granite.     

Plagioclase-spinel intergrowths in alkali basaltic rocks from the Southern Highlands,N.S.W.

Dominant plagioclase (An₅₉) and minor spinel form possible cumulates in alkali basaltic flows from the Southern Highlands, N.S.W. The mode of occurrence of the spinel, and the chemical composition of both the spinel and the plagioclase suggest the spinel originated by exsolution from plagioclase formed at high pressure.     

Geochemisty and tectonic setting of igneous rocks in the Glenrock Station area,N.S.W.

In the Upper Murray Valley, Victoria, Late Silurian, high‐Si igneous rocks, which are closely associated with alkalic, basaltic dykes, were emplaced at high crustal levels following the peak of the Benambran Orogeny, which deformed and metamorphosed the Wagga Zone in Late Ordovician‐Early Silurian times. These rocks, which are informally termed ‘the Upper Murray high‐Si magmatic suite’, include leucogranites, rhyolite dykes and flows, and ash‐flow tuffs characterised by the following features. They are transitional from mildly peraluminous to mildly metaluminous; they represent relatively anhydrous magmas, in which halides were important volatile constituents; they have high Si, total alkalies, Rb, Th, U, Nb, Sn and heavy rare earth elements; and they are relatively repleted in Mg, Ca, Sr, Eu, V, Cr and Ni. In these respects and in their post‐orogenic setting and close association with alkalic basalts, they resemble many post‐orogenic granitoids from elsewhere. Such granitoids appear to have formed as partial melts during crustal extension following major episodes of deformation and high‐Si magmatism. A residual granulitic crust, from which an earlier generation of granitoid magmas had been extracted, is argued to be the source rock‐type for these post‐orogenic magmas. Tectonic extension, affecting such a crust, was accompanied by deep fracturing and basaltic vol‐canism. Mantle‐derived, CO₂‐ and halide‐rich fluids moved into the residual crust, causing widespread metasomatism, and emplacement of basaltic magma caused temperatures to rise until melting took place and a second group of magmas was produced. This model explains most aspects of the trace and major element chemistry of post‐orogenic, high‐Si igneous rocks and, for the Upper Murray high‐Si suite it also provides an explanation for variations in trace elements and isotopic characteristics. Other processes, such as crystal fractionation, magma mixing, thermogravi‐tational diffusion, and separation and loss of a volatile phase, provide explanations for variations within individual units of the suite, but they do not explain overall variations or the highly fractionated nature of the suite.     

The origin of the albite-rich rocks enclosing the cobaltian pyrite deposit at Thackaringa,N.S.W., Australia

A stratabound disseminated and massive cobaltian pyrite deposit at Thackaringa, 30 km SW. of Broken Hill occurs in banded albite-quartz-biotite rocks which are conformable with regional structure and stratigraphy. The albite rocks are associated with pelitic and psammitic metasediments, amphibolite and minor quartzgahnite rocks. The deposit has undergone granulite facies metamorphism and there is no evidence that pyrite has reacted to form pyrrhotite. It is suggested that the albite rocks were an analcimized tuffaceous rock in which disseminated pyrite formed by a volcanic exhalative process. A massive cobaltian pyrite body is enclosed by a metamorphosed hydrothermal alteration zone characterised by an increase in quartz, magnesian fluorobiotite and Rb.     

Geomorphology and tectonics of the Dorrigo Plateau,N.S.W.

The Dorrigo Plateau is covered by basalt, which is a remnant of the 18 Ma old Ebor Volcano. The centre of this volcano is an intrusion in the Bellinger Valley. The volcano was erupted on a palaeoplain of moderate relief. Subsequent uplift and tilting led to erosion of the Nambucca Beds, together with much of the volcano, and creation of a major escarpment, part of the Great Escarpment of eastern Australia. In this area the Great Escarpment is younger than 18 Ma.     

Petrology of the Nandewar Volcano,N.S.W., Australia

The Nandewar Mountains, N.S.W., Australia, are the remains of a Miocene continental alkaline volcano whose products range from olivine basalts to comendites and alkali rhyolites. Intermediate hawaiites, mugearites and benmoreites predominate in the shield, in which olivine basalts are rare, and the trachytic rocks form many intrusions into the shield. The Nandewar alkaline series shows extreme fractionation of a relatively differentiated alkali olivine basalt magma, saturated with silica, to yield extremely oversaturated peralkaline comendites and peraluminous alkali rhyolites. The nature of the ferromagnesian phases forming was controlled by low oxygen fugacities. Throughout the series clinopyroxenes range from diopsidic augite, through sodic ferrohedenbergites to hedenbergite-acmite solid solutions. Riebeckite-arfvedsonite solid solutions appear in the trachytes and comendites, and aenigmatite appears in some of the peralkaline rocks. The feldspars in the series fractionate from calcic labradorite through potash oligoclase and calcic anothoclase towards the minimum melting alkali feldspar composition, Ab₆₅Or₃₅. The compositions of the alkali rhyolites approach the minimum in the system SiO₂-KAlSi₃O₈-NaAlSi₃O₈. All the mineralogical and chemical evidence points to the development of the Nandewar series by the processes of extreme crystallization differentiation of an alkali olivine basalt parent magma. No significant contamination occurred, xenoliths and xenocrysts are absent, and volatile transfer and metasomatism played a minor role.     

Structural analysis of Bermagui area,N.S.W.

The Ordovician rocks exposed along the N.S.W. coast, near Bermagui, comprise a sequence of alternating greywacke and shale and a less abundant sequence of alternating chert and detrital beds. The only lithological boundary that can be mapped is the contact between the two sequences and it sheds little light on the large scale structure. However, due to continuity of outcrop, well‐defined vergence zones and abundant younging evidence it is possible to interpret the regional structure.

Two generations of folds (B1 and B2) are recognized and the regional folds, a N/S trending anticlinorium to the east and synclinorium to the west, are interpreted as second generation structures (B2). First generation folds (B1) are refolded by B2 on the limbs of the large B2 structures and are commonly recumbent. In the hinges of the regional B2 folds, B1 axial planes are steeply dipping and the folds instead of being refolded by B2, are more tightly appressed than elsewhere. A model is described to explain these observations.     

Platinum-group elements in rocks from the voikar-syninsky ophiolite complex,Polar Urals,U.S.S.R.

Analyses of platinum-group elements (PGE) in rocks collected from the Voikar-Syninsky ophiolite in the Polar Urals suggest that the distribution and geochemistry of PGE in this Paleozoic ophiolite are similar to those in Mesozoic ophiolites from elsewhere. Chondrite-normalized PGE patterns for chromitite, the tectonite unit, and ultramafic and mafic cumulate unit have negative slopes. These results are similar to those found for chromitites from other ophiolites; stratiform chromities show positive slopes. If the magmas that form both types of chromitite originate from similar mantle source material with respect to PGE content, the processes involved must be quite different. However, the distinct chondrite-normalized PGE patterns may reflect differing source materials.     

The shoshonite porphyry Cu-Au association in the Goonumbla District,N.S.W., Australia

Summary The Goonumbla porphyry copper-gold deposit in N.S.W., Australia, is hosted by late Ordovician (439.2 ± 1.2 Ma)shoshonitic igneous rocks. In terms of their petrography, the rocks vary from andesitic to dacitic lavas and tuffs which are partly intruded by monzonite stocks; they are characterized by high and variable Al₂O₃ (13.4–19.9 wt%), very high K₂O values (up to 6.8 wt%), and high K₂O/Na₂O ratios (0.58–1.48), which are typical for the shoshonite association. The rocks also have enriched LILE concentrations (Ba up to 1200 ppm, Sr up to 1350 ppm), low HFSE (TiO₂ < 0.67 wt%, Zr < 125 ppm, Nb < 10 ppm, Hf < 3.4 ppm), and very low LREE (La < 22.4 ppm, Ce < 31 ppm), which are typical for potassic volcanic rocks formed in alate oceanic-arc setting.Mineral chemistry of selected magmatic mica and apatite phenocrysts from host rocks reveals relatively high SrO and BaO contents (micas: 0.15 wt% and up to 0.28 wt%, respectively; apatites: up to 0.28 wt% and 0.19 wt%, respectively) and very high halogen concentrations. Micas are characterized by up to 3.9 wt% F and 0.14 wt% Cl, whereas apatites have up to 3.6 wt% F and 0.68 wt% Cl. These very high halogen contents compared to those from barren intrusions imply that the shoshonitic magmatism was the source of mineralization.Copper-gold mineralization consists mainly of bornite, chalcopyrite, chalcocite and minor pyrite and tetrahedrite. Native gold occurs mainly as minute grains within silicates of the host rocks, and more rarely as fine inclusions in the sulphides. Mineralization is accompanied by wallrock alteration comprising a spatially restricted potassic type and a regional propylitic alteration type.Thus, the porphyry copper-gold deposit in the Goonumbla district can be viewed as an additional example of a worldwide association between potassic/shoshonitic magmatism and base- and precious-metal mineralization. More specifically, it appears to be the oldest recorded example of a shoshontie-associated porphyry Cu-Au deposit from a late oceanic-arc setting, a possible modern analogue being Ladolam at Lihir Island, Papua New Guinea

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Die Shoshonit Porphyry Cu-Au Assoziation im Goonumbla Distrikt, N.S.W., Australien

Zusammenfassung Die Porphyry Cu-Au Vererzung im Goonumbla Distrikt in New South Wales, Australien, sitzt in oberordovizischen (ca. 439.2 ± 1.2 Ma) Shoshoniten auf. Das petrographische Spektrum dieser Gesteine reicht von andesitischen bis dazitischen Laven und Tuffen, die lokal von Monzonit-Stöcken intrudiert werden; die Gesteine besitzen hohe, aber variable Al₂O₃ Gehalte (13.4–19.9 Gew%), sehr hohe K₂O Gehalte (bis zu 6.8 Gew%) und hohe K₂O/Na₂O Verhältnisse (0.58–1.48), die typisch sind für Shoshonite. Außerdem weisen sie hohe Konzentrationen an LILE Elementen (Ba bis 1200 ppm, Sr bis 1350 ppm) auf und geringe Konzentrationen an HFSE (TiO₂ < 0.67 Gew%, Zr < 125 ppm, Nb < 10 ppm, Hf < 3.4 ppm) sowie an LREE (La < 22.4 ppm, Ce < 31 ppm), die als typisch gelten für potassische Vulkanite von ozeanischen Plattengrenzen.Die Mineralchemie von repräsentativen Glimmer- und Apatit-Phänokristallen ist charakterisiert durch hohe SrO und BaO Gehalte (Glimmer: 0.15 Gew%, bzw. bis 0.28 Gew%; Apatite: bis 0.28 Gew%, bzw. 0.19 Gew%). Sie enthalten ferner sehr hohe Halogen-Konzentrationen. Die Glimmer enthalten beispielsweise bis zu 3.9 Gew% F und 0.14 Gew% Cl, während Apatite bis zu 3.6 Gew% F und 0.68 Gew% Ci aufweisen. Dies erscheint nicht ungewöhnlich, weil Glimmer und Apatite von vererzten Mag matiten zumeist deutlich höhere Halogengehalte besitzen, als solche von unvererzten Magmatiten. Die hohen Halogen-Gehalte in Phänokristallen aus den Shoshoniten legen nahe, die Vulkanite als den Ursprung der Vererzung zu interpretieren.Die Cu-Au Vererzung besteht überwiegend aus den Sulfiden Bornit, Kupferkies, Kupferglanz und vereinzelt auftretendem Pyrit und Tetrahedrit. Gediegen Gold wird in der Regel nur als kleine Partikel innerhalb von Silikaten der shoshonitischen Wirtsgesteine und seltener als feine Einschlüsse in Sulfiden gefunden.Die Vererzung wird von hydrothermaler Alteration der Wirtsgesteine begleitet und zwei Alterationsarten lassen sich unterscheiden: eine potassische sowie eine regional zu beobachtende propylitische Alteration.Die Porphyry Cu-Au Lagerstätte im Goonumbla Gebiet ist ein Beispiel für die weltweit beobachtete Assoziation von Bunt- und Edelmetallvererzungen und potassisch/shoshonitischem Magmatismus. Der Goonumbla Distrikt stellt die älteste bisher bekannte Porphyry Cu-Au Lagerstätte aus einerspätgenetischen ozeanischen Plattengrenze dar. Einmodernes Beispiel für eine Cu-Au Lagerstätte vergleichbaren Typs ist Ladolam auf Lihir Island, Papua New Guinea.

     

Resolution of foraminiferal biotopes in Broken Bay,N.S.W.

A recent benthonic foraminiferal population consisting of 144 species from an interacting marine‐estuarine environment has been analysed by Q‐mode cluster analysis to reveal biotopes. Simplification of the total populations, both in terms of abundance and occurrence, discloses that the more significant subpopulation is composed of the widely‐occurring species. Subpopulations based on rarely occurring but locally abundant species do not define meaningful biotopes.     

Stratiform and stratabound tungsten mineralisation in the Broken Hill Block,N.S.W.

The Early‐Middle Proterozoic Broken Hill Block contains three types of W occurrences, which show close stratigraphic control. All three types occur within a relatively narrow stratigraphic interval (the ‘Mine Sequence’ Suite of Stevens et al., 1980) comprising a highly variable group of metamorphosed silicic and mafic volcanics, clastic sediments, and exhalative and chemical sediments containing base metals. The first type includes occurrences of W and base metals in bedded calc‐silicate rocks. In the second type, W occurs in layered to non‐layered calcsilicate rocks associated with amphibolite; these are intimately associated in a narrow stratigraphic interval containing abundant, small, Broken Hill type deposits. The third type comprises stratabound, W‐bearing pegmatites, which have been remobilised from quartz‐feldspar‐biotite gneiss and bedded quartz‐tourmaline rocks. Tungsten has been mined only from the third type and only in small quantities. The three types of tungsten deposits show a close spatial relationship with stratiform and stratabound Pb‐Zn mineralisation, including the Broken Hill type. The Pb‐Zn and W deposits are inferred to be genetically related.     

Polymetamorphism of the sulphide ores of Broken Hill,N. S. W., Australia

Two main periods of metamorphism have effected the Broken Hill base metal deposit. The first, at granulite grade, occurred at 1,700 m.y., the second, at lower amphibilite grade, occurred at 500 m.y. The earlier metamorphism correlates with two stages of intense regional folding; the latter occurs as narrow shears across the orebody. The prograde metamorphism caused intense brecciation, development of an ore mush with ore movement, formation of ore-bearing parapegmatites and boudins and much recrystallization of ore and gangue minerals. The orebody parallels an axial plane schistosity in the wall rocks with numerous ore piercement structures causing local discordancies. Ore in retrograde zones is again brecciated with galena further recrystallizing after destruction of prograde recrystallization. Gangue minerals remain essentially as brecciated fragments without further recrystallization. Secondary hydrothermal veins with rare silver minerals derived from the orebody transect the retrograde zones. Galena is plastically injected into fractures in the retrograde wall rock schists. Prograde ore shows co-recrystallization of various sulphides and gangue minerals yielding characteristic annealed textures. Quartz, garnet, hedenbergite, roepperite and apatite co-recrystallize with galena, sphalerite and chalcopyrite with balanced surface tensions. Retrograde ore shows mainly fragments of gangue and sphalerite set in a matrix of further recrystallized galena or schistose galena with a superimposed sub-grain structure. The significance of the ubiquitous sub-structures within galena is considered in terms of retrograde effects upon high grade metamorphic textures.

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Zusammenfassung Das Erzvorkommen in Broken Hill, N.S.W., Australien, unterlag zwei Hauptphasen regionaler Metamorphose. Die erste Phase, die eine Granulitfacies erreichte, fand vor 1700 Millionen Jahren statt; die zweite Phase, vor 500 Millionen Jahren, zeigt einen Amphibolit-Grad. Die frühere Metamorphose korreliert mit zwei Stufen intensiver Faltungen, die spätere trat in Form enger Scherungen quer durch den Erzkörper auf. Die prograde Metamorphose verursachte intensive Breccienbildung, die Entwicklung eines Erzbreies mit begleitender Erzbewegung, die Bildung erzführender Parapegmatite und Boudinagen sowie reichliche Rekristallisation des Erzes und der Gangmineralien. Der Erzkörper liegt parallel eine axial-plane-schistocity im Nebengestein mit zahlreichen Erzdurchdringungen, die zu örtlicher Diskordanz führen. In retrograden Zonen wurde das Erz ein zweites Mal brecciert, was von einer weiteren Rekristallisation des Bleiglanzes unter Zerstörung der prograden Rekristallisation begleitet wurde. Die Gangmineralien bleiben in der Hauptsache im breccierten Zustand ohne weitere Rekristallisation. Sekundäre hydrothermale Erzgängchen mit seltenen Silbermineralien, die vom Erzkörper stammen, durchschneiden die retrograde Zone. Bleiglanz ist in plastischem Zustand in die Spalten des retrograden Nebengesteines (Schist) hineingedrückt worden. Progrades Erz zeigt Co-rekristallisation der verschiedenen Sulphide und Gangmineralien mit charakteristischen Temperungs gefügen. Quarz, Granat, Hedenbergit, Roepperit und Apatit co-rekristallisieren mit Bleiglanz, Zinkblende und Kupferkies mit ausgeglichenen Oberflächenspannungen. Retrogrades Erz zeigt hauptsächlich zerstückelte Gangmineralien und Zinkblende in einer Grundmasse weiter rekristallisierten oder schiefrigen Bleiglanzes mit einer überprägten Sub-grain Textur. Die Bedeutung der allgegenwärtigen Sub-Textur im Bleiglanz wird im Sinne eines retrograden Effektes auf hochgradige metamorphe Strukturen gedeutet.

     

Some Feldspars, Nephelines and Analcimes from the Square Top Intrusion, Nundle, N.S.W.

The Square Top intrusion, near Nundle, New South Wales, revealsin its mineralogy and chemistry the transition of analcime-olivinetheralite to analcime tinguaite. Chemical, X-ray, and opticaldata are presented on 2 plagioclases, 9 alkali feldspars, 2nephelines and 3 analcimes, from various members of the differentiationsequence. With differentiation, plagio-clase becomes enrichedin Ab and, to a lesser extent, in Or. The coexisting alkalifeldspars initially reveal enrichment in Ab and trend from sanidineto lime anorthoclase; subsequently in the tinguaites, the sanidinesbecome enriched in Or. Nephelines in the lower theralites havehighly sodic compositions and become slightly enriched in Kin the later differentiates. Replacement of Si by NaAl in theanalcimes decreases with progressive differentiation. The rapidcooling of this relatively small intrusion assisted fractionalcrystallization, and resulted in the preservation of disorderedplagioclase and alkali feldspars. The Square Top feldspar crystallizationtrends are compared with trends presented by Tuttle and Bowen(1958). The location of the feldspar boundary curve in the Ab–An–Orsystem and some general relationships in the undersaturatedportion of the Ne–Ks–Qz system, including the heteromorphicfelsic assemblages developed from the lowest temperature liquids,are also discussed.     

Abiogenic Fischer–Tropsch synthesis of hydrocarbons in alkaline igneous rocks; fluid inclusion, textural and isotopic evidence from the Lovozero complex, N.W. Russia

A detailed fluid inclusion study has been carried out on the hydrocarbon-bearing fluids found in the peralkaline complex, Lovozero. Petrographic, microthermometric, laser Raman and bulk gas data are presented and discussed in context with previously published data from Lovozero and similar hydrocarbon-bearing alkaline complexes in order to further understand the processes which have generated these hydrocarbons. CH₄-dominated inclusions have been identified in all Lovozero samples. They occur predominantly as secondary inclusions trapped along cleavage planes and healed fractures together with rare H₂O-dominant inclusions. They are consistently observed in close association with either arfvedsonite crystals, partially replaced by aegirine, aegirine crystals or areas of zeolitization. The majority of inclusions consist of a low-density fluid with CH₄ homogenisation temperatures between −25 and −120 °C. Those in near-surface hand specimens contain CH₄+H₂ (up to 40 mol%)±higher hydrocarbons. However, inclusions in borehole samples contain CH₄+higher hydrocarbons±H₂ indicating that, at depth, higher hydrocarbons are more likely to form. Estimated entrapment temperatures and pressures for these inclusions are 350 °C and 0.2–0.7 kbar. A population of high-density, liquid, CH₄-dominant inclusions have also been recorded, mainly in the borehole samples, homogenising between −78 and −99 °C. These consist of pure CH₄, trapped between 1.2 and 2.1 kbar and may represent an early CH₄-bearing fluid overprinted by the low-density population. The microthermometric and laser Raman data are in agreement with bulk gas data, which have recorded significant concentrations of H₂ and higher hydrocarbons up to C₆H₁₂ in these samples. These data, combined with published isotopic data for the gases CH₄, C₂H₆, H₂, He and Ar indicate that these hydrocarbons have an abiogenic, crustal origin and were generated during postmagmatic, low temperature, alteration reactions of the mineral assemblage. This would suggest that these data favour a model for formation of hydrocarbons through Fischer–Tropsch type reactions involving an early CO₂-rich fluid and H₂ derived from alteration reactions. This is in contrast to the late-magmatic model suggested for the formation of hydrocarbons in the similar peralkaline intrusion, Ilímaussaq, at temperatures between 400 and 500 °C.     

Rare earth element investigation of the Cliefden Outcrop,N.S.W., Australia

The effect of low grade hydrous burial metamorphism (prehnite-pumpellyite facies) upon the rare earth elements (REE) has been studied by using samples from the Cliefden Outcrop, New South Wales. The REE, together with other reputedly immobile elements, have been mobilised during the metamorphism. Although mobile, the REE have behaved remarkably coherently with little light rare earth (LREE) fractionation. This is reflected in the chondrite normalised patterns which are sub-parallel to parallel in shape. High correlations of REE with other elements can be used to predict the maximum likely variation of these elements in the studied outcrop. The high correlations do not necessarily mean that, for similarly metamorphosed terrains, crystallisation-differentiation processes have operated but may rather have resulted from strong geochemical coherence during post-crystallisation elemental redistribution. The REE do not appear to be strongly domain controlled within the Cliefden Outcrop.     

Geological interpretation of ply structure of the Bulli Seam,Sydney Basin,N.S.W.

The ply structure of the Bulli seam in the Southern Coalfield of the Sydney Basin persists over the main part of the coalfield. Lateral variations were investigated using both petrographic profiles and swelling index profile sections. The results indicated that the thickness pattern of the whole Bulli seam was affected by three independent factors: (1) post‐depositional loss of the top portion of the seam; (2) pre‐depositional topography; and (3) differential subsidence during deposition. The lack of definite correlation between seam thickness and present geological structure suggests that no local tectonic movement took place during deposition of the seam. Tectonic movement cannot, therefore, be the main cause of variation in seam thickness. Subsidence during deposition of the Bulli seam in the northeastern margin of the coalfield appears to have been too rapid, and near the western margin too slow, to form a thick coal seam. However, in the main part of the coalfield, the rate of subsidence was suitable for peat accumulation and a thick seam was deposited under stable conditions, as indicated by the persistent ply structure.     

Geochemistry and origins of the annagh division of the precambrian erris complex,N.W. county Mayo,Ireland

A geochemical study of 29 acid gneisses from the Annagh Division of the Erris Complex has shown that all lithologies are chemically distinct and that further distinction can be made between different areas. The acid and intermediate gneisses, which in the field have been regarded as orthogneisses associated with widespread metasomatism, have orthogneiss chemistry and show the effects of considerable local metasomatism. Isotopic age dating records that the latest major pre-Caledonian event in the Annagh Gneiss is of Grenvillian age. The Annagh Division gneiss differs chemically from Laxford gneiss but is similar to reworked Lewisian inliers seen in the N.W. Highlands of Scotland.All analysed basic rocks fall into three categories: early basic and ultrabasic pods and metadolerite dykes. All originated as minor intrusives of tholeiitic affinities.     

Kyanite-bearing rocks from the Hackett River area,N.W.T.: Implications for Archean geothermal gradients

The increase in metamorphic grade toward the Hackett River gneiss dome indicates that the structural dome is also a metamorphic culmination. In pelitic rocks east of the dome, the prograde sequence is chlorite, biotite, staurolite-cordierite, sillimanite. To the southwest the sequence is andalusite-staurolite-cordierite, sillimanite. In quartzofeldspathic gneisses which are closer to the dome than the sillimanite isograd, kyanite occurs as corroded relics, cross-cut by sillimanite and rimmed by cordierite and plagioclase.The pelites were metamorphosed under regional low-pressure conditions at the same time the quartzo-feldspathic rocks underwent higher-pressure metamorphism. A lateral variation in geothermal gradient during metamorphism is postulated to account for the change in facies-series. High heat flow beneath oceanic crust produced the low-pressure assemblages in the pelites; higher-pressure assemblages formed in a region of suppressed isotherms around a relatively cool, proto-continental trondhjemitic body. Low-pressure conditions were imposed on the kyanite-bearing gneisses during continuing metamorphism and diapiric uplift.Spatial association of late Archean kyanite-bearing rocks with early Archean sodic proto-cratons has also been noted in the Churchill and Superior Provinces of Canada as well as the Rhodesian Craton.