Anorthosite-adamellite-troctolite layering in the Barth Island structure of the Nain complex, Labrador

The Barth Island structure is an oval, 9 by 6 km body of layered igneous rocks. The central part contains a rhythmically layered troctolite-adamellite sequence. The sequence is repeated outward, but appears overturned, strongly sheared and reduced in thickness. Outward follows an adamellite-anorthosite sequence in which the effects of shear gradually diminish. In both sequences cryptic layering is evidenced by gradual change in mineral compositions. Isopleth maps based on the compositions of plagioclase, orthopyroxene and olivine show a circular high of An, En and Fo percentages in troctolite, gradually decreasing inward to a low in adamellitic rock. Between the highs in troctolite and the surrounding anorthosite occurs a low-temperature trough where both sequences grade into adamellitic rock. The structure may represent a deformed portion of pre-existing igneous layering. An alternative explanation would involve mixing of successive magma batches of gradually more sodic and ferrous composition.     

Regional Geobarometry in the Contact Aureoles of the Anorthositic Nain Complex, Labrador

The 1.4 Gyr anorthositic Nain complex was emplaced by multipleintrusion into Archean and Aphebian rocks whose metamorphicgrades range regionally from greenschist to granulite and whosetemperature at the time of emplacement was probably no higherthan 200–300 °C. Using experimental data on Ga-Cdand Hy-Ol-Qz equilibria, compositional studies allow the determinationof contact metamorphic pressures and temperatures for samplesover an area > 10,000 km². The results indicate that forthese specimens temperatures ranged from 645° to 915 °Cand pressures ranged from about 3.7 to 6.6 kbar. Locally, originalgarnet porphyroblasts have partially reacted to produce symplectitesof cordierite+hypersthene, resulting in a corona-like texture.These textures and associated mineral zoning indicate that someof the specimens have experienced a polymetamorphic history.This poly metamorphism presumably resulted from the multipleintrusions of magmas of the Nain complex. The regional distributionof pressures appears systematic and suggests an arch in theancient pressure surfaces (paleo-isobars). This postulated arch,which trends WNW-ESE, roughly coincides with a proposed grabenstructure in the Conrad discontinuity based on gravity studies(Stephenson, 1974). The association of these two features canbe rationalized with a mechanism of incipient continental rifting.The emplacement of the unmetamorphosed northern Labrador anorthositemassifs in a non-orogenic environment of low temperature andmoderate depth suggests that the metamorphosed anorthositessouth of the Grenville Front may have been accidental participantsin a post-anorthosite Grenville orogeny and metamorphism.     

Pyroxene-Olivine-Quartz Assemblages in Rocks Associated with the Nain Anorthosite Massif, Labrador

Unusually iron-rich pyroxene and olivine occur in rocks associatedwith the Nain anorthosite massif, Labrador. Adamellite and granodioritecontain orthopyroxene (inverted from pigeonite) as iron-richas Ca₆Fe₈₂Mg₁₂; comparison with experimental data suggests aminimum pressure of crystallization of 5 kb. Some of these iron-richpyroxene crystals have broken down, apparently upon decreasingpressure, to yield intergrowths of less iron-rich orthopyroxene(near Ca₇Fe₇₂Mg₂₁), ferroaugite, fayalite (near Fo₉), and quartz.Other rocks, monzonites, contain pyroxenes with calcium-poorcores and ferroaugite rims, as well as crystals composed ofbroad lamellae of ferroaugite and orthopyroxene in sub-equalproportions. Analysis of one such crystal with unusually thinand closely spaced lamellae yielded a bulk composition of Ca₂₄Fe₅₈Mg₁₈.Such pyroxenes probably crystallized near or above the crestof the augite-pigeonite two-phase region, probably above 925°C. This high temperature suggests that the monzonites crystallizedfrom relatively dry magmas. If they represent a residual fractionderived from the same magma as the anorthosite, then that magmamust have been nearly anhydrous. Pigeonite rather than orthopyroxene was the primary magmaticCa-poor pyroxene in most of the Nain rocks studied here. Nucleationrates apparently were low under subsolidus conditions, and low-Capigeonite (Ca₂Fe₇₈Mg₂₀) is present in grains where orthopyroxenedid not nucleate as pigeonite cooled and exsolved ferroaugite.Iron-rich orthopyroxene (Ca₂Fe₇₉M₁₉) crystallized instead ofpigeonite in a Greenland quartz syenite that contains more abundanthydrous phases.     

Dry granulite mineral assemblages in the contact aureoles of the Nain complex,Labrador

Thermal aureoles surrounding intrusions of the Nain complex, Labrador, contain many unique or unusual mineral assemblages in aluminous gneisses and granulites, ironstones, and ultramafic rocks. Some of the limiting assemblages are (in addition to feldspars±pyrrhotite±ilmenite±graphite ±biotite±magnetite): Ga-Ol-Hy-Sp, Ga-Cd-Hy-Sp, Ga-Hy-Ol-Qz, Cd-Hy-Os-Qz, Ga-Cd-Sp-Si, Cd-Sp-Co-Si, Ga-Ol-Hy-Sp, Ga-Cd-Hy-Sp, Hy-Ol-Qz-Aug, and Ol-Hy-Sp(±Chl±Ca-amph±Aug). On the basis of some of these assemblages are (in addition to feldspars±pyrrhotite±ilmenite±graphite from 3.7–6.6 kbar and temperatures from 645–915 ° C. The paucity of hydrous phases, the preponderance of Ksp-Plag-Qz or even Cd-Ksp-Qz without evidence of a melt at these temperatures, the stable occurrence of osumilite, and the common presence of graphite suggest that was extremely low. The presence of graphite-pyrrhotite, the compositions of ilmenites, the compositions of coexisting Fe-Ti oxides, and other mineralogical data indicate the was relatively low and, despite the wide range in bulk compositions and rock types, may have followed an approximately buffered trend. It is possible that the magmas of the Nain complex may have acted as an external buffer for the aureole rocks. The unique nature of these mineral assemblages appears to be a function of the extremely low .     

Fractionation and Liquid Immiscibility in an Anorthositic Pluton of the Nain Complex, Labrador

Fine-grained anorthositic dikes are associated with a massiveleuconorite pluton (Cl = 15) which is exposed over an area ofabout 200 km². Internally, the pluton shows little compositionalvariation; average plagioclase composition ranges from An₅₂to An₄₈. The dikes are nearly uniform in composition and similarto the estimated bulk composition of the pluton (55 per centSiO₂). They therefore appear to represent the parental magmaof the leuconorite pluton. A small body of granite (10 km²) was emplaced within and priorto the complete solidification of the leuconorite. The graniticintrusion caused local deformation of the leuconorite and filter-pressingof its late stage interstitial liquids. These liquids occurin the younger hydrous granite as very finegrained, chilledpillows of nearly anhydrous Fe-rich diorite and granite. Mostof the pillows are diorites with approximately 55 per cent SiO₂.On oxide plots these lie approximately on a plagioclase controlline passing through the composition of the leuconorite dikes.The entire group of chilled pillows ranges in composition from45 to 71 per cent SiO₂ with a gap between 57 and 63 per centSiO₂. On oxide plots they produce a smooth trend which is obliqueto and truncates the plagioclase control line. Variation inthe pillows can best be explained by late-stage liquid immiscibility. Fractionation in the interstitial magma was controlled earlyby crystallization of plagioclase and later by plagioclase pluspyroxene. Very late stage differentiation was controlled mainlyby liquid immiscibility and produced FeO- and SiO₂-rich liquids.     

Chemical and petrologic trends in anorthositic and associated rocks of the Nain massif, Labrador

Two rock series are distinguished in the Nain anorthosite massif: the anorthosite-adamellite suite, occupying most of the massif, and the troctolite-syenite suite, which occurs in layered intrusions. Field relations show that troctolitic rocks intrude anorthositic rocks, but are intruded by adamellitic rocks.

The following evolutionary model is suggested for the Nain massif. Fractional crystallization in a parental magma resulted in a density-stratified magma mass in which plagioclase crystals accumulated by suspension at a level where the density of crystal and liquid matched. Thus, a plagioclase cumulate formed, which was underlain by heavier troctolitic magma and overlain by lighter acidic magma. Settling of the plagioclase mass caused local intrusion of troctolitic magma, which differentiated in members of the troctolite-syenite suite. Subsequent disturbance of the magma chamber distorted the solidified portion, and caused local intrusive relationships between it and the acidic rest magma, which further differentiated in members of the adamellite series.     

Fractional crystallization and magma mixing in the Tigalak layered intrusion,the Nain anorthosite complex,Labrador

The Tigalak intrusion is a dominantly dioritic layered body, about 80 km² in area, which ranges in composition from norite to granodiorite. Local areas of the layered rocks display upward fractionation from norite to ferrodiorite. Periodic reversals of mineral composition trends record the emplacement of less fractionated dioritic magma. Heterogeneous mixtures of dioritic and granodioritic rocks occur widely in mappable lenses and layers that alternate up section and along the strike with more uniformly layered rocks. In these mixtures, chilled dioritic pillows occur abundantly in a hybrid cumulate matrix of granodiorite to diorite composition. Cross-cutting granodioritic dikes grade upward into stratigraphically-bound lensoid masses of the hybrid cumulates. It appears that the hybrid rocks formed as a result of the emplacement of the granodioritic magma through lower cumulates into the dioritic magma chamber and that the dioritic pillows represent chilled bodies of Ferich dioritic magma that commingled with cooler granodioritic magma and settled to the floor of the Tigalak magma chamber. The restricted distribution of these mixtures of hybrid cumulates and chilled pillows indicates that mixing between granodioritic and dioritic liquids was limited in time and lateral extent. Periodic injections of granodioritic liquids may have collected as a separate layer below the roof of the magma chamber and above dioritic magma.     

Structural and Magmatic Evolution of a Magma Chamber: The Newark Island Layered Intrusion, Nain, Labrador

The Newark Island Layered Intrusion occurs in the ProterozoicNain anorthosite complex of Labrador. It contains an exceptionalsuite of cumulates ranging from troctolites and gabbros to quartzmonzonites and intermediate hybrid rocks. These layered rocksformed in a chamber that was periodically fed by a wide rangeof basic and acid magmas, the compositions of which are preservedin numerous feeder dikes. Where basic magmas commingled withcooler granitic magma, they commonly formed chilled pillows.Because of periodic injections of both acid and basic magmasthe magma chamber was compositionally stratified for much ofits existence. At times, granitic cumulates formed along thechamber walls while mafic to intermediate hybrid cumulates formedon the floor. Stratigraphic and structural relations indicatethat the magma chamber grew upward during deposition, and thatit evolved from a west-dipping sheet to a north-plunging synform.Three major episodes of expansion can be linked to injectionsof large (e.g., 20km³) volumes of acid magma. The entry of thisacid magma into the chamber disrupted previously formed cumulates,creating enlarged feeders down which resident basic magma collapsed.The resultant structures (troughs) contain strongly chilledpillows of resident basic magma that existed near the bottomof the chamber at the time of acid replenishment.     

Slow,dense replenishments of a basic magma chamber: the layered series of the Newark Island layered intrusion,Nain, Labrador

The Newark Island layered intrusion is a composite layered intrusion within the Nain anorthosite complex, Labrador. The intrusion comprises a lower layered series (LS) dominated by troctolites, olivine gabbros and oxide-rich cumulates and an upper hybrid series (HS) characterized by a wide range of mafic, granitic and hybrid cumulates and discontinuous layers of chilled mafic rocks (Wiebe 1988). The HS crystallized from a series of replenishments of both silicic and basic magmas. The LS crystallized from periodically replenished basic magmas. The LS has a lower zone that consists mainly of olivine-plagioclase cumulates and contains minor cryptic reversals in mineral compositions that resulted from replenishments of relatively primitive magma. An upper zone is dominated by olivine-plagioclaseaugite-ilmenite cumulates. Cumulus titanomagnetite and pyrrhotite occur within some oxide-rich cumulates, and the stratigraphically highest layers contain cumulus apatite. At intermediate levels in the sequence, cumulus inverted pigeonite occurs in place of olivine. Several prominent regressions in the stratigraphy of the upper zone are marked by fine-grained troctolitic layers with much higher Mg no. [100 MgO/(MgO+FeO)] and anorthite than underlying cumulates. These layers coarsen upward and grade back to oxide-bearing olivine gabbros within thicknesses ranging from 10 cm to 15 m. Dikes that cut the LS have major- and trace-element compositions that strongly suggest that they are feeders for the replenishments. In the lower zone when olivine and plagioclase were the only cumulus phases, replenishments were less dense than the resident magma and rose as plumes and mixed with it. Precipitation of cumulus oxides in the upper zone lowered the density of resident magma so that subsequent replenishments were more dense than resident magma. Replenishments that occurred after oxides began to precipitate had small injection velocities. These post-oxide injections flowed along the interface between resident magma and the cumulate pile and precipitated flow-banded, fine-grained troctolites.     

Mineral variation in anorthositic, troctolitic, and adamellitic rocks of the Barth Island layered structure in the Nain anorthosite complex, Labrador

The Barth Island layered structure is an oval, 6 by 9 km body, consisting of rhythmically layered adamellitic rock in the center which grades outward through jotunite into troctolite. Farther outward the sequence repeats itself in reversed order, strongly reduced in magnitude and finer grained; the adamellitic zone is followed by jotunite which grades into coarse-grained leuconorite and into anorthosite of the Nain complex. The Barth Island structure, having an inverted conical base topped by a hemispherical depression, seems to represent a distorted sequence of rock layers with troctolite at the bottom, grading upward into adamellitic rocks which grade into anorthosite at the top. Trend-surface analysis demonstrates the regional variation of plagioclase and orthopyroxene compositions in the troctolite—adamellite sequence of the central part of the structure. The fits for the second- and third-degree surfaces are good and significant at the 99 percent level. The regression line for compositional variation in coexisting plagioclase and orthopyroxene in all analyzed rocks has a correlation coefficient of r² = 0.78. The difference between the trends in the troctolite—adamellite sequence and the anorthosite—adamellite sequence is insignificant. The regression curve for compositional variation in coexisting orthopyroxene and olivine has a correlation coefficient of r² = 0.98. The curve shows good correlation with the experimentally established partitioning curve of Medaris, which indicates that equilibrium conditions prevailed during formation of the olivine—ortho-pyroxene pairs. The results suggest that the troctolite—adamellite sequence and the anorthosite—adamellite sequence are products of fractional crystallization, possibly from the same parental magma.     

Alkalic to transitional ferrogabbro magma associated with paleohelikian anorthositic plutons in the Flowers River area,southeastern Nain igneous complex,Labrador

Fine grained gabbroic chilled margins and crosscutting dikes are associated in space and time with three ca. 1400 Ma anorthositic plutons in the Flowers River area, southeastern Nain igneous complex. Both the anorthositic and gabbroic rocks have distinctive compositions compared to rocks of similar age and lithology elsewhere in northcentral Labrador. The anorthositic rocks contain olivine and augite rather than orthopyroxene, and Fe-Ti oxides, apatite and orthoclase are unusually abundant. Cumulus plagioclase is abnormally enriched in incompatible elements. Most of the gabbroic rocks are uniform in composition, although the effects of contamination and fractionation are evident in some places. They define a transitional to alkalic ferrogabbro magma that is strongly enriched in K, P and incompatible trace elements. The chemical characteristics of the ferrogabbro magma imply derivation from enriched mantle or involvement of a significant crustal component. A parent-daughter relationship between the ferrogabbro magma and anorthositic rocks is suggested by their compositional similarities and the fact that the gabbroic chilled margins and plagioclase-rich pluton interiors appear to be completely gradational in composition and texture.Geological Survey of Canada Contribution No. 54686.     

Prograde, peak, and retrograde P–T paths from aluminium in orthopyroxene: High-temperature contact metamorphism in the aureole of the Makhavinekh Lake Pluton, Nain Plutonic Suite, Labrador

Static heating during intrusion of the Makhavinekh Lake Pluton (MLP) caused replacement of garnet in the adjacent country rocks (Tasiuyak Gneiss) by coronal assemblages of orthopyroxene + cordierite. Thermometry based on Al solubility in orthopyroxene, applied to relict garnet and neighbouring orthopyroxene, preserves a temperature gradient from 700 to 900 °C at distances between 5750 and 20 m from the intrusion, reaffirming the robustness of this thermometry technique. Intracrystalline and intergranular variations of Al zoning in orthopyroxene are well‐preserved, suggesting that little diffusional modification of Al growth zoning occurred. Maximum Al₂O₃ in orthopyroxene ranges from c. 2.0 wt% at 5750 m from the intrusion to a maximum of 4.3 wt% at the contact. Individual orthopyroxene grains show decreasing Al from core to rim in samples < 500 m from the intrusion, while those at greater distances show an increase from core to rim. These features are interpreted with the aid of numerical models for conductive heat flow in the aureole. Coronas in samples close to the intrusion grew at high temperatures and along T‐t paths dominated by cooling, so maximum Al content in orthopyroxene in these samples occurs in the cores of grains that grew during the earliest stages of garnet consumption. In contrast, the corona‐forming reactions in rocks further from the contact proceeded along prograde heating paths, so maximum Al content in orthopyroxene occurs in the rims of grains that grew during the final stages of garnet consumption. These results document the ability of Al‐in‐orthopyroxene thermometry to preserve a detailed record of thermal histories in contact‐metamorphic granulites; they suggest that similar intracrystalline and intergranular variations of Al zoning in orthopyroxene in regional granulites may also preserve portions of both the prograde and peak‐T evolution.     

Petrogenesis and geodynamic evolution of the Kajan Neogene subvolcanic rocks,Nain, Central Iran

Kajan subvolcanic rocks in the Urumieh–Dokhtar magmatic arc (UDMA), Central Iran, form a Late Miocene-Pliocene shallow-level intrusion. These subvolcanics correspond to a variety of intermediate and felsic rocks, comprising quartz diorite, quartz monzodiorite, tonalite and granite. These lithologies are medium-K calc-alkaline, with SiO₂ (wt.%) varying from 52% (wt.%) to 75 (wt.%). The major element chemical data also show that MgO, CaO, TiO₂, P₂O₅, MnO, Al₂O₃ and Fe₂O₃ define linear trends with negative slopes against SiO₂, whilst Na₂O and K₂O are positively correlated with silica. Contents of incompatible trace elements (e.g. Ba, Rb, Nb, La and Zr) become higher with increasing SiO₂, whereas Sr shows an opposite behaviour. Chondrite-normalized multi-element patterns show enrichment in LILE relative to HFSE and troughs in Nb, P and Ti. These observations are typical of subduction related magmas that formed in an active continental margin. The Kajan rocks show a strong affinity with calc-alkaline arc magmas, confirmed by REE fractionation (La_N/Yb_N = 4.5–6.4) with moderate HREE fractionation (Sm_N/Yb_N = 1.08–1.57). The negative Eu anomaly (Eu/Eu* <1), the low to moderate Sr content (< 400 ppm) and the Dy/Yb values reflect plagioclase and hornblende (+- clinopyroxene) fractionation from a calc-alkaline melt Whole–rock Sr and Nd isotope analyses show that the ⁸⁷Sr/⁸⁶Sr initial ratios vary from 0.704432 to 0.705989, and the ¹⁴³Nd/¹⁴⁴Nd initial ratios go from 0.512722 to 0.512813. All the studied samples have similar Sr-Nd isotopes, indicating an origin from a similar source, with granite samples that has more radiogenic Sr and low radiogenic Nd isotopes, suggesting a minor interaction with upper crust during magma ascent. The Kajan subvolcanic rocks plot within the depleted mantle quadrant of the conventional Sr-Nd isotope diagram, a compositional region corresponding to mantle-derived igneous rocks.     

Petrology of a xenolith of ferro-aluminous gneiss from the Nain complex

Several gneissic xenoliths occur in granite which has been intruded along the contact between an anorthosite body and gneisses in the Nain complex, Labrador. One of these xenoliths is a ferro-aluminous gneiss with ovoids, 1–3 cm in diameter, consisting of an intimate mixture of fine, granular cordierite-olivine-spinel-orthopyroxene-plagioclase-ilmenite-pyrrhotite enclosed by poikilitic grains of garnet and orthoamphibole. Biotite occurs as both granular and poikilitic grains. The textures indicate a two-stage contact metamorphic history characterized by the simplified reaction: cordierite+olivine+spinel+plagioclase +orthopyroxene→garnet+orthoamphibole Although, on the basis of texture, the reactants represent the early stage assemblage and the products represent the final stage, the distribution coefficient data for Fe-Mg indicate that, except for orthopyroxene, all of the minerals were stable and in equilibrium during the final stage. The coexistence of cordierite and olivine indicates that pressure was less than 3 kbar. The phase relationships agree well with the experimental work of Hsu and Burnham (1969) on the almandine-pyrope join at 2 kbar and indicate a temperature of 800° C for the first stage and 615° C for the second stage of contact metamorphism. This close agreement also suggests that the ovoids at one time may have been garnet porphyroblasts that were decomposed during the first stage. These data correlate well with the field relations: the first-stage, high-temperature effects were likely caused by the intrusion of the anhydrous anorthositic body and the second-stage, low-temperature effects by the invasion of the volatile-rich granite.     

Geophysical transects of the Labrador Sea: Labrador to southwest Greenland

In 1977 the Federal Institute for Geosciences and Natural Resources, Hannover, carried out a large scale multichannel reflection seismic survey in the Labrador Sea. This survey provided an opportunity for the direct comparison of the geologic structure of the Labrador and Greenland margins. The seismic records across the Labrador Shelf show a thick, prograding sedimentary wedge consisting of several seismic sequences onlapping an acoustic basement that dips steeply seaward. The surface of the acoustic basement is irregular below the continental slope, indicating Late Cretaceous—Early Tertiary faulting. The thick sedimentary section below the slope is divided by an unconformity, tentatively identified as Late Tertiary in age, into two seismic megasequencies which can be subdivided. The acoustic basement on the Greenland side is also strongly faulted but is overlain, in the south, by a thin sedimentary section. The sediment cover thickens on the Greenland Shelf to the north as the shelf becomes wider.As with more southerly parts of the western Atlantic margin, a positive free-air anomaly (30–50 mgal) lies landward of the shelf break off Labrador and a smaller negative anomaly follows the base of the slope. Similar, but generally narrower features are observed along the Greenland margin. West of the negative anomaly off the Greenland slope a narrow band of lower amplitude positive anomalies tends to be associated with an acoustic basement high observed in the reflection profiles. A landward negative gradient in the simple Airy isostatic anomaly across this margin suggests that the ocean—continent boundary is related to this high.Detailed magnetic measurements across the northern Labrador margin show that well-developed oceanic anomalies trending north-northwest lie east of the large Labrador Shelf gravity high, beyond the 2000 m isobath. Landward of these magnetic anomalies is a quiet magnetic zone within which the linear gravity high is parallel to the shelf break and correlates with a deep, sediment-filled basin. It is inferred that oceanic-type crust or greatly-attenuated continental crust underlies this basin and that continental crust thickens markedly westward of the gravity high over a distance of about 50 km.     

Mineralogy and Petrology of the Metamorphosed Wabush Iron Formation, Southwestern Labrador

The Wabush Iron Formation, of late Precambrian (Proterozoic)age is part of the Labrador Trough in southwestern Labrador,Canada. It is the regionally metamorphosed equivalent of lowgrade metamorphic (chlorite zone) iron-rich sediments of thecentral part of the Labrador Trough. The metamorphic grade iskyanite-staurolite zone, as concluded from conformably underlyingpelitic schist assemblages. Sedimentary textural features suchas very pronounced banding and a very rare occurrence of relicgranules are still preserved. The iron formation consists mainly of quartz, specularite, magnetite,cummingtonite-grunerite, and ferrodolomite-ankerite. Less commonare actinolite, anthophyllite, riebeckitetremolite, magnesioriebeckite,ferrosalite, orthopyroxene, aegirine-augite, aegirine, rhodonite,garnet (almandine, spessartine, calderite), siderite, rhodochrosite,calcite, and kutnahorite. Conventional wet chemical analyses or electron microprobe analyseshave been made of thirty-four phases belonging to the abovelist. Six additional electron probe analyses have been madeof phases from the underlying pelitic schists. All conventionallyanalyzed phases are characterized by complete optical, unitcell parameter, and density measurments. The analyzed assemblages from the silicate and silicate-carbonateiron formation include grunerite-ferrosalite, grunerite-eulite-siderite,grunerite-actinolite, grunerite-almandine, cummingtonite-spessartine,rhodonite-kutnahorite-calderite, aegirine-augite-riebeckite-tremolite,magnesioriebeckite-cummingtonite-rhodonite, aegirine-augite-rhodonite-rhodo-chrosite,and aegirine-rhodonite-calderite-rhodochrosite. The assemblages are concluded to be equilibrium assemblages.Of the volatile components, O₂, CO₂, and H₂O, O₂, is concludedto have behaved as an inert (buffered) component. Variationsin the activity of CO₂ are concluded to have existed betweensilicate-oxide and carbonate-oxide members of the iron formation.It is not clear, however, whether CO₂ has acted as a perfectlymobile component with strong aco₂ gradients throughout the area,or as an inert component in some parts of the area. H₂O is consideredto have been perfectly mobile. An increase in Mg/(Mg+Fe) ratioin ferromagnesian silicates is correlated with an increase inthe oxidation state of the assemblage. A similar increase in(Mg+Mn)/(Mg+Mn+Fe) is found in manganoan ferromagnesian silicateswith increasing activity of O₂. A number of ferromagnesian silicatescontain large amounts of Na⁺ and Fe³⁺ as a result of the verylow Al₂O₃ content of the iron formation. The P and T conditionsof metamorphism are deduced from experimental studies applicableto the underlying pelitic schists.     

Aspects of strain history in folded sediments from the Schefferville Mining District, Labrador Trough, Canada

Preferred orientations of phyllosilicate grains in the shaley rocks of the Labrador Trough were studied using a pole-figure goniometer. Transmitted X-rays permit determination of the basal planes of chlorite and muscovite. From their preferred orientation, strain is calculated according to March's theory. By choosing samples in successively more strongly deformed domains, a strain history can be recognized which started with compaction by loss of pore volume under an overburden and was later followed by an early homogeneous tectonic shortening parallel to bedding and at a right angle to the fold axis. Buckling occurred next, with strain in the limbs becoming different from that in the hinges; the limbs were lengthened parallel to bedding and nearly normal to the fold axis. Continuing appression, finally, led to pervasive shortening normal to the axial plane with concurrent stretching both updip along the axial plane and along the fold axis. This latter stretching may have occurred when variable-plunge basin and dome structures of the Labrador Trough were formed.     

Monazite and xenotime petrogenesis in the contact aureole of the Makhavinekh Lake Pluton,northern Labrador

High-temperature (700–900°C) metamorphism in the contact aureole of the Makhavinekh Lake Pluton (MLP), northern Labrador, led to the growth of monazite and xenotime during progressive replacement of regional garnet-bearing assemblages (M1) by lower-pressure symplectitic coronas of orthopyroxene + cordierite (M2). In the inner aureole (<500 m from the contact), where M1 garnet is strongly resorbed, high-Y+HREE monazite (X_Y+HREE 0.14–0.18) occurs as small isolated grains and as discontinuous rims on partially resorbed pre-M2 monazites that were liberated from garnet. Xenotime also occurs as small isolated grains within M2 coronas. Ion-microprobe dating of thin, high-Y rims indicates that new monazite growth occurred during M2. Monazite–xenotime miscibility-gap temperatures are consistent with Al-solubility-in-orthopyroxene thermometry estimates, indicating that peak temperatures in the inner aureole are accurately recorded and preserved by monazite. M2 monazite records, therefore, the temperature and timing of M2 metamorphism. Two net-transfer reactions, modelled using singular value decomposition in the system P-Y-HREE-LREE, are proposed to account for the growth of M2 phosphates: (1) 38 Grt1 + 1 Mnz1 = 1.13 Mnz2 and (2) 737 Grt1 + 1 Ap = 1 Mnz2 + 3.4 Xno2. Reaction (1) conserves P and gave rise to locally coronitic high-Y overgrowths on partially resorbed pre-M2 monazite, whereas reaction (2) accounts for the growth of small new monazite and xenotime grains. Both reactions were highly localized within individual M2 coronas due to slow intergranular diffusion accompanying fluid-undersaturated metamorphism in the MLP aureole. Similar monazite-forming reactions are expected in other polymetamorphosed granulites.This revised version was published online in November 2004 with corrections to the reference Pyle JM et al. (2002).     

Textural development in sulfide-matrix ore breccias in the Voisey's Bay Ni-Cu-Co deposit,Labrador, Canada

Magmatic Ni-Cu sulfide ores at Voisey’s Bay contain complex assemblages of extremely heterogeneous rocks. These range from polymict breccias, with rock fragments in sulfide-rich and/or sulfide-poor matrices, to heterogeneous “vari-textured” gabbros with rapid short range variations in grain size and content of hydrous phases. Rock fragment populations in the breccias include endogenous olivine gabbros (cumulate and non-cumulate) and cumulate peridotites along with extensively depleted plagioclase-hercynite gneisses interpreted as restites from extensive partial melting of country rock quartzo-feldspathic paragneisses. Using a combination of desk-top microbeam XRF mapping at cm scale and 3D X-ray tomography, we show that both sulfide-poor and sulfide-rich breccias comprise heterolithic assemblages of clasts within a matrix of olivine gabbro. This matrix is characterised by an interconnected 3D framework of plagioclase crystals, highly variable in grain size at mm to cm scale, with interstitial olivine and poikilitic clinopyroxene, and is texturally indistinguishable from clast-free olivine gabbro. Sulfide forms interconnected networks at cm to dm scale and possibly larger. Much of the plagioclase developed by outgrowth from the margins of paragneiss xenoliths when the porosity was occupied by silicate melt. The observed range of textures is explained by a model of percolation of molten sulfide through variably crystalline inter-clast matrix, displacing the silicate melt to leave the refractory plagioclase-olivine or in some cases plagioclase-only component, now entirely within a sulfide matrix. The process is analogous to that believed to have formed interspinifex ore in komatiite-hosted deposits. Biotite rims on plagioclase enclosed in sulfide are interpreted as the result of reaction between plagioclase, olivine and a hydrous component derived from the sulfide melt itself, with a possible component of migrating residual silicate melt wicking along sulfide-silicate contacts. This sulfide infiltration model offers an alternative to the current model for upward emplacement of a slurry of silicate melt, sulfide melt and breccia fragments as a late stage injection into the dyke-sill complex. The preserved range of textures is interpreted as being due to gravity-driven percolation of sulfide liquid through a pre-existing partially molten intrusion breccia. In this model, the breccia serves as a physical trap site, accumulating downward migrating sulfide liquid. However, the invariable close mutual association of sulfide and rock fragments at Voisey’s Bay implies a common derivation.     

加拿大拉布拉多地区主要金属矿床地质特征及其时空分布

拉布拉多是加拿大大西洋成矿域中具地质多样性的最大的成矿省,区域成矿地质条件优越,产有Superior型含铁建造、岩浆硫化物、铂族元素、火山成因块状硫化物、铀、稀有稀土等多种金属矿床。文章介绍了拉布拉多铁、铀、稀有稀土、镍多金属等优势金属矿床的成矿背景、成矿特征、矿床(点)分布、典型矿床地质特征及最新勘探成果,分析了区域地质演化作用与区域内主要金属矿床的时空分布。