Rutile Ages Research Articles

An 40Ar–39Ar investigation of high-pressure metamorphism and the retrogressive history of mafic eclogites from the Lanterman Range (Antarctica): evidence against a simple temperature control on argon transport in amphibole

Mafic eclogites sampled from a restricted area in the Lanterman Range (Antarctica) retrogressed variably under amphibolite facies metamorphism. Assemblages range from well-preserved eclogite, with minor growth of Na–Ca amphibole, to strongly retrogressed ones with extensive development of Ca amphibole. 40Ar–39Ar furnace step-heating experiments on the different amphiboles yield results varying from plateau ages of ~498 Ma to a near-plateau age of ~490 Ma, and the greater the amphibolite retrogression, the younger the age. 40Ar–39Ar infrared laser-probe analyses on rock chips from a well-preserved eclogite and a slightly retrogressed one reveal the presence of an excess argon component. Whereas excess argon is invariably present in garnet and clinopyroxene developed under high-pressure metamorphism, it is heterogeneously distributed in amphibole on a millimetre scale. Results indicate that excess argon was incorporated during high-pressure metamorphism; this component was then lost during retrogression, while a change in composition of ambient argon to atmospheric argon occurred. New 40Ar–39Ar data and previously published Sm–Nd garnet and U–Pb rutile ages obtained from the same well-preserved eclogite sample suggest that the oldest Na–Ca amphibole age is reliable and not an artefact due to the incorporation of excess argon. The variably retrogressed eclogites are thought to derive from different parts of the enclosing metasedimentary rocks that were variably invaded by fluids during amphibolite facies metamorphism. Thus the circulation of fluids promoting (re)crystallisation, and not temperature, was the main process controlling the rate of argon transport in the studied eclogites. The different 40Ar–39Ar ages are interpreted to record diachronous amphibole growth at different crustal levels during exhumation. Data indicate that there was about a 10-Ma interval between the eclogite facies stage (at ≥1.5 GPa) and the Ca amphibole-hydration forming reaction (at 0.3–0.5 GPa); this translates into an average exhumation rate of 3–4 km/Ma.

Age and tectonic evolution of Neoproterozoic ductile shear zones in southwestern Madagascar, with implications for Gondwana studies

Southern Madagascar comprises a complex Precambrian terrain of high‐grade metamorphic rocks with a history of polyphase deformation and metamorphism. Two prominent N‐S trending late Neoproterozoic ductile shear zones, the Ampanihy and Vorokafotra shears, each with projected strike length of > 450 km and between 10 and 20 km in width, crosscut the region. A third set of en echelon shears forms part of the early Paleozoic Ranotsara Shear Zone that cuts the basement in a NW‐SE direction over a combined strike length of > 400 km. The host rocks of these shears comprise paragneisses (metasediments) with detrital zircons ranging in age between 720 and 1900 Ma. A felsic layer, interpreted as a metavolcanic rock, gives a date of 722±1 Ma. Remnants of late Archean orthogneisses in the central part of the study area may represent basement to the paragneisses. Four episodes of deformation and metamorphism have been recognized on the combined basis of field observations, petrogenesis, and U/Pb analyzes of zircons, monazites, sphenes, and rutiles. Two episodes of early simple shear deformation (D1 and D2) at midcrustal levels occurred between 627 and 647 Ma, during which northeast verging recumbent sheath folds and ductile thrusts were formed and peak prograde metamorphism reached 7–12 kbar at 750°–900°C. Early prolate mineral fabrics (L1/L2) are preserved in massif‐type anorthosite bodies and their marginal country rocks. D1 occurred between 630 and 647 Ma, while D2 occurred at 627‐628 Ma. This was followed by a 10–15 Myr period of static, annealing metamorphism until 609–614 Ma when bulk shortening (D3) took place. D2 and D3 are coaxial but are separated in time by leucocratic dykes that intruded between 610 and 620 Ma. D3 was focused zonally, forming the prominent N‐S shear zones between 607 and 609 Ma; its oblate strain resulted in a strong composite D2/D3 fabric defined by subvertical S‐tectonites and subhorizontal intersection lineations. A variety of post‐D3 pegmatites accompanied ∼85 Myr of relatively static annealing and metasomatic/metamorphic mineral growth, during which numerous occurrences of phlogopite, uranium, and rare earth elements formed. A continuum of concordant monazite dates suggests that this thermal event is part of an extended period of low‐pressure (3–5 kbar) charnockite‐producing processes between 520 and 605 Ma. The continuum, however, appears to be punctuated at ∼580, 550, and 520 Ma. Deformation (D4) recorded within the Ranotsara Shear Zone overlaps with the youngest parts of the regional metamorphic conditions between 520 and 550 Ma. Prevailing low‐pressure, high‐temperature amphibolite‐granulite facies rapidly gave way to greenschist facies conditions between 490 and 530 Ma, as is evident from overlapping ages of zircon, monazite, sphene, and rutile. We conclude that D1 to D3 represents a period of 40 Myr of compressional deformation that we interpret to be related to collisional events during the amalgamation of Gondwana. The first part of the thermal continuum between 550 and 605 Ma reflects ∼55 Myr of slow cooling and annealing at midcrustal levels, while the onset of the last episode, between 520 and 530 Ma, heralds accelerated exhumation accompanied by extensional tectonics between 490 and 520 Ma. We believe that this postcollisional time span represents a prolonged period of evolution of a Tibetan‐style plateau into an Aegean‐style extensional terrain. This ∼100 Myr event in southern Madagascar is similar to that recorded throughout large sectors of the East African Orogen between ca. 500 and 600 Ma. We believe that this type of postconvergent thermotectonism best represents the original definition of “Pan‐African” [Kennedy, 1964], which in today's terminology equates with “postorogenic extensional collapse” [Dewey, 1988], or “destabilization of an orogen” [Lipps, 1998]. Kennedy's Pan‐African was widespread throughout the interior a supercontinent, when Gondwana's periferal margins were subjected to far‐field tensional forces. This suggests that neither gravitational collapse of the Pan‐African‐Braziliano Orogens nor delamination were the sole or even the dominant driving forces for the postconvergent extension.

U–Pb dating of metamorphic minerals: Pan-African metamorphism and prolonged slow cooling of high pressure granulites in Tanzania, East Africa

U–Pb monazite and zircon ages reveal that the high pressure granulites from eastern Tanzania were metamorphosed during a Pan-African tectonothermal episode. These mineral ages range from 610 to 655 Ma and indicate that peak metamorphic conditions were diachronous in the different granulite domains. U–Pb titanite and rutile ages define integrated cooling rates of 2–5°C/Ma for all investigated granulite areas, and suggest a common process for the post-metamorphic histories of the different granulite areas. Prolonged slow cooling-rates are consistent with near-isobaric cooling in the deep crust after the metamorphic peak. The process responsible for crustal thickening during heating did not produce isostatic instability and fast erosion-driven or tectonic exhumation. The thermal history determined in this study is not consistent with the collision of East- and West-Gondwana as the cause of granulite facies metamorphism. Palaeomagnetic data have shown that this collision did not occur until 550 Ma, when the Pan-African granulites in Tanzania had already cooled below 500°C. The high pressure granulites of eastern Tanzania are thus interpreted as having attained their metamorphic peak prior to the final amalgamation of Gondwana, probably in an active continental margin setting.

Timing and characterization of recurrent pre-Sveconorwegian metamorphism and deformation in the Varberg–Halmstad region of SW Sweden

The Ätran terrane of southwestern Sweden is a consequence of westward crustal growth away from an Archean core and records both Gothian (c. 1.75–1.55 Ga) and c. 1.0 Ga Sveconorwegian (Grenvillian) tectonism. In spite of widespread consensus on the first-order aspects of this westward growth model, there is a current debate as to whether the bulk of the deformation and migmatization in the Ätran terrane should be attributed to Gothian or Sveconorwegian events. U–Pb zircon, titanite, monazite and rutile ages from gneiss samples and three tectonostratigraphic groups of dykes in the Varberg–Halmstad region constrain the timing of major tectonothermal events. A felsic orthogneiss yields a protolith age of 1664±7 Ma. Mafic dykes of a first dyke group (Steninge dykes) intruded after the first recognized deformation (D1) and were subsequently metamorphosed at 1654±9 Ma. The Steninge dykes constrain D1 to the 1664–1654 Ma interval and D2 at 1654±9 Ma, requiring the D1/D2 gneissosity to have formed during Gothian orogenesis. Steninge dykes and migmatitic D1/D2 structures in the host gneisses are cross-cut by less deformed dykes of a second dyke group, one regional granitic suite dated at 1426+9/−4 Ma and a pegmatite swarm at 1399+7/−6 Ma. The second group of dykes therefore confirms a pre-1.43 Ga age for the main D1/D2 gneiss-forming event(s) consistent with previous models of regional Gothian deformation in this terrane. Group 2 magmatism also includes the Varberg Charnockite–Granite Association, from which a sample of charnockite is now dated at 1399+12/−10 Ma. In addition, metamorphism at 1438+12/−8 Ma resulted in new zircon growth in a mafic gneiss. Together, the Group 2 magmatic rocks and metamorphic zircons represent an inter-orogenic, 1.44–1.38 Ga thermo-magmatic episode in the Varberg–Halmstad region that has c. 1.4 Ga equivalents elsewhere in southern Sweden. Previous models of high heat flow and comprehensive metamorphism during a thermo-magmatic Hallandian event at c. 1.4 Ga is thus supported. Sveconorwegian deformation and recrystallization (D3–D4) occurred before 946+6/−4 Ma, as constrained by a post-kinematic third group of dykes. U–Pb ages for monazite (948±9 Ma), titanite (935±7 Ma and 932±5 Ma) and rutile (878±9 Ma) reflect Sveconorwegian cooling after peak metamorphism. During this stage, U–Pb ages and closure temperatures suggest slow cooling rates of 5–11°C/m.y. from 948 to 932 Ma and 2.5–5°C/m.y. from 932 to 878 Ma, attributed to late-stage erosion and isostatic uplift. The lack of regional penetrative migmitization during the Sveconorwegian granulite conditions is consistent with metamorphism of previously dehydrated rocks (during Gothian and Hallandian times) during eastward Sveconorwegian thrusting of western segments over the Ätran terrane.

Metamorphic evolution of eclogite and associated garnet-mica schist in the high-pressure metamorphic Maksyutov complex, Ural, Russia

Metasedimentary garnet-mica schists are interlayered with metabasic garnet–omphacite schists and enclose eclogite boudins in the high-pressure metamorphic Maksyutov complex in the Southern Urals, Russia. These three rock types were investigated in one outcrop and compared chemographically and thermobarometrically. The Fe/Mg distributions between garnet rim–omphacite and garnet rim–phengite pairs indicate different equilibration temperatures for the three samples, with the lowest temperature (500°C, >1.5 GPa) for the eclogite boudin, an intermediate temperature (630°C, >1.7 GPa) for the foliated eclogite and the highest temperature (650°C, >1.7 GPa) for the garnet-mica schist. The garnets in garnet-mica schist enclose abundant chloritoid relics and the Fe/Mg distribution between chloritoid and garnet records an earlier high-temperature stage (650°C, >2.0 GPa) before the garnet rim–phengite temperatures were reached. Together with some minimum- and maximum-pressure estimates three different prograde pressure–temperature paths and a common retrograde metamorphic evolution are interpreted from the chemographic and thermobarometric data. The different early metamorphic evolutions and conditions confirm the variability of protoliths, which are also indicated by different U/Pb zircon and rutile ages.

Petrology and U–Pb geochronology of mafic, high-pressure, metamorphic coronites from the Tshenukutish domain, eastern Grenville Province

A suite of mafic and ultramafic rocks from the Tshenukutish domain, eastern Grenville Province, have been the subject of a detailed petrographic and geochemical study combined with U–Pb geochronology. The Tshenukutish domain is the highest tectonic level in the recently discovered, eclogite-bearing, Manicouagan Imbricate Zone. The mafic and ultramafic rocks are characteristic of two of the lithotectonic segments in the domain, the Lac Espadon segment and the Baie du Nord segment. In the Lac Espadon segment, cumulate textures and geochemical characteristics suggest that the units represent a layered, mafic and ultramafic complex and an unrelated apatite-rich, Fe–Ti mafic pegmatite (nelsonite) body. In contrast, the samples from the Baie du Nord segment represent fairly homogeneous Fe–Ti gabbros and anorthosites. The samples studied show abundant petrographic evidence for high-pressure metamorphism, for instance the widespread development of garnet, clinopyroxene and kyanite bearing metamorphic assemblages. Estimates of the conditions of high-pressure metamorphism in previous studies and melting textures in deformed and hydrated gabbro margins are in the range 750–900°C and 1600–1800 MPa. The gabbroic end-members from both segments suggest within-plate tholeiite petrotectonic signatures. REE-patterns of gabbros and cumulates in both segments suggest fractionation from a tholeiitic source. Cumulate textures, fractionation and inherited zircons suggest high-crustal level emplacement of the magmatic protoliths in the Tshenukutish domain. In the Lac Espadon segment U–Pb ages of igneous zircon and baddeleyite give an age range of 1643–1629 Ma for the emplacement of the layered complex. The nelsonite in the LES gives a poorly constrained igneous zircon age of 1596+126/−99 Ma. In the Baie du Nord segment baddeleyite gives an age of 1170±5 Ma for emplacement of the Fe–Ti gabbros. This is the youngest intrusion in the TD and thus, gives a maximum limit for the onset of Grenvillian metamorphism. Ages of high-P metamorphism are further constrained by zircon formed during partial melting of deformed and hydrated Fe–Ti gabbro margins (1046±3 Ma), overgrowth of igneous zircon (1042+22/−28 Ma) in the nelsonite, saccharoidal zircon replacing baddeleyite (1030+10/−7 Ma and 1030±12 Ma) and new growth of equant zircon (1012±12 Ma) from the undeformed interior of the gabbros. These ages indicate that the high-P metamorphism of the Tshenukutish domain was late-Grenvillian (Ottawan). Retrogression to amphibolite facies is constrained by the ages of titanite (1006–997 Ma) in deformed gabbro margins. Rutile ages range between 960 and 929 Ma and are interpreted as cooling ages. The difference between the ages of titanite and rutile suggest slow cooling of the Tshenukutish domain after initial high-pressure and temperature metamorphism and exhumation.

U-Pb zircon and rutile ages from granulite xenoliths in the Slave province: Evidence for mafic magmatism in the lower crust coincident with Proterozoic dike swarms

Zircon and rutile in two garnet-bearing mafic granulite xenoliths from a kimberlite in the Archean Slave province record Proterozoic ages similar to the 1.27 Ga Mackenzie dike swarm or similar to the ca. 2.23–2.21 Ga Malley-McKay diabase dike swarms. The xenolith sample site currently overlies thick lithosphere, more than 700 km to the southeast of the proposed Mackenzie plume center. Because lower crust was effectively shielded from the thermal effects of an upwelling plume by thick lithospheric mantle, heat was most likely advected into the lower crust by lateral intrusion of mafic sills over distances of hundreds of kilometres from magma generation zones nearer the plume center.

Mesoproterozoic sedimentation, magmatism, and metamorphism in the southern part of the Grenville Province (western Quebec): U–Pb geochronological constraints

U–Pb data provide new constraints on the age of sedimentation, metamorphism, magmatism, and deformation in the Grenville Province of western Quebec. A metapelite, an alaskitic gneiss, and an amphibolite were sampled within an area of 1 km2 in the Mont-Laurier terrane. The metapelite yielded detrital-metamorphic zircons that gave 207Pb/206Pb ages of ca. 1205–2200 Ma. The youngest detrital components, between 1210 and 1300 Ma and possibly as old as [Formula: see text] Ma, provide a maximum age range for the deposition of this rock. Data for the alaskitic gneiss suggest that it is either derived from an igneous (volcanic) protolith with a minimum age of ca. 1250 Ma and a maximum age of [Formula: see text] Ma, or is a dyke emplaced at ca. 1140–1170 Ma. The amphibolite yielded zircon interpreted as metamorphic, with a minimum age of 1118 Ma, and a maximum age not likely older than ca. 1160 Ma. Zircons from charnockites and monzonites of the Morin plutonic complex gave zircon igneous ages between ca. 1157 and 1165 Ma. High-grade metapelites of the Réservoir Cabonga terrane yielded metamorphic zircon ages of 1140-1160 Ma. Metamorphic monazites from both the Réservoir Cabonga and the Mont-Laurier terranes yielded ages of 1138−1182 Ma, interpreted as the crystallization age or the time that significant Pb loss ceased. These ages indicate that the two terranes underwent the same long-lasting metamorphic event. The overlap between ages of metamorphic zircons and monazites on the one hand and the age of anorthosite–charnockite magmatism on the other hand suggests a long-lasting high-grade metamorphism with heat contribution from crystallizing plutons. A posttectonic aplite dyke from the interior of the Mont-Laurier terrane gives a zircon minimum age of 1054 Ma, considered a minimum age for penetrative deformation in this part of the Grenville Province. Rutile ages of 945–955 Ma record cooling through about 400 °C in both the Réservoir Cabonga and the Mont-Laurier terranes.

U–Pb geochronology of the Mazinaw terrane, an imbricate segment of the Central Metasedimentary Belt, Grenville Province, Ontario

The Mazinaw terrane, in the Central Metasedimentary Belt of the Grenville Province comprises, volcanic, sedimentary, and plutonic rocks that were intensely folded and faulted, and metamorphosed to as high as upper amphibolite facies. U–Pb geochronology establishes an early period of magmatism and sedimentation at about 1280–1240 Ma, probably in a marginal basin setting, and a multistage rhetamorphic evolution in the period between 1100 and 980 Ma, which was probably related to crustal thickening by imbrication during compression and wedging of the terrane. Some of the earliest magmatism formed calc-alkalic volcanic rocks of Kashwakamak Formation at 1276 ± 2 Ma. An associated sedimentary assemblage was intruded by the Helena trondhjemite stock at 1267 ± 5 Ma. A younger succession, formed between 1250 and 1240 Ma, includes the bimodal volcanic Mazinaw Lake Formation, alaskitic granites of the Norway Lake and Abinger plutons, and a granodioritic gneiss phase of the Cross Lake pluton. These units were covered unconformably by the Flinton Group sometime after 1150 Ma, as defined by published detrital zircon data. A northern segment of the Mazinaw terrane underwent deformation and metamorphism accompanied by the emplacement of granitic pegmatites in the period between 1100 and 1050 Ma. In contrast, metamorphism and deformation occurred significantly later in central domains, as shown by several metamorphic zircon populations grown at about 1040–1030, 1020, and 1010–1000 Ma, by pegmatite emplacement at about 980 Ma, and titanite ages of 1010–960 Ma. Younger rutile ages of about 915 Ma may reflect uplift of the terrane. Some rutile also appears to record the latest Proterozoic faulting and mafic diking related to formation of the Ottawa-Bonnechere graben at about 600 Ma.

Zircon and rutile U-Pb geochronology of the Niquelândia layered mafic and ultramafic intrusion, Brazil: constraints for the timing of magmatism and high grade metamorphism

Abstract The Niquelândia intrusion is among the largest layered intrusions in the world. It is part of a 350 km long belt of large mafic-ultramafic complexes in central Brazil. Both intrusion and country rocks were subjected to highgrade metamorphism ranging from amphibolite to granulite facies, and associated ductile deformation. The first U-Pb zircon and rutile ages for this region are presented. Zircon fractions from a meta-anorthosite from the upper gabbro-anorthositic unit are variably discordant (0–28% with regard to the lower intercept) along a mixing line from 777±10 Ma lower intercept and 1583±25 Ma upper intercept. Zircon fractions from a later quartz diorite within the Central Mafic Unit are variably discordant (0–14% with regard to the lower intercept) along a mixing line from 794−6+4 lower intercept and 1565±22 upper intercept. The upper intercepts for both rocks, interpreted to be the magmatic age, overlap and suggest a Mesoproterozoic age for the intrusion. The lower intercept is interpreted to be the high-grade metamorphic event. Zircons from two quartz-rich blastomylonites representing different metamorphic domains and cutting through the layered intrusions were analyzed. A single zircon from the amphibolite facies domain blastomylonite gives a concordant (−0.3%) 207 Pb 206 Pb age of 777±3 Ma, and a single zircon from the granulite facies blastomylonite gives a 207 Pb 206 Pb age of 786±5 Ma (−0.8% discordance). Both results are consistent with the lower intercept ages and set the 770–795 Ma interval for the high-grade metamorphic event. Rutile ages (∼650 Ma) are considerably younger than zircon metamorphic ages and suggest a milder regional tectonomagmatic episode or a complex and long-lasting cooling history for the region. The U-Pb data provide evidence for an important Mesoproterozoic extensional tectonic event in central Brazil. It also sets the 770–795 Ma interval for the high-grade amphibolite to granulite facies prograde metamorphism, interpreted to represent the timing of a continental collision event.

U-Pb Garnet, Sphene, Monazite, and Rutile Ages: Implications for the Duration of High-Grade Metamorphism and Cooling Histories, Adirondack Mts., New York

U-Pb ages were determined on metamorphic garnets, monazites, sphenes, and rutiles in metapelites and marbles from the Adirondacks to determine the duration of metamorphism, the timing of mineral growth, and the cooling history. Garnet ages for the Lowlands range from 1168-1127 Ma, those from the central and southern Highlands from 1154-1013 Ma. The ages indicate that metamorphism may have begun synchronously throughout the Adirondacks, but high-grade conditions existed over a period of about 150 Ma in the Highlands and only about 40 Ma in the Lowlands. Metamorphism in the Highlands may not have occurred as a single event but rather in several discrete thermal pulses. An age of $$1153 \pm 3 Ma$$ was determined for garnets in the syn-regional metamorphic contact aureole of the Diana syenite, consistent with that of the syenite intrusion, $$1155 \pm 4 Ma$$ (U-Pb zircon, Grant et al. 1986). Garnets just outside the contact aureole give an age of $$1168 \pm 6 Ma$$. In the Lowlands, monazite yielded an age of $...

High-precision U [sbnd]Pb ages of metamorphic rutile: application to the cooling history of high-grade terranes

Metamorphic rutiles occurring in granulite and upper amphibolite facies metapelitic rocks of the Archean Pikwitonei granulite domain (Manitoba) and the Proterozoic Adirondack terrane (New York) give concordant and near concordant U Pb ages. The Pb concentrations in rutile range from 2.85 to 168 ppm, U concentrations range from 10.9 to 390 ppm and the measured 206Pb/ 204Pb ratios range from 182 to 22,100 corresponding to 238U/ 204Pb ratios of 398–75,100. The proportions of radiogenic 208Pb are very low, ranging from 0.0 to 6.9% of total radiogenic Pb. The habits of the rutile crystals range from stubby to acicular, the physical properties vary from opaque/black to transparent/reddish-brown. Separate batches of black and reddish-brown rutile grains from the same samples have similar U and Pb concentrations, Pb-isotope ratios, and yield the same U/Pb ages within analytical uncertainty. No correlation of U concentration and 206Pb/ 204Pb ratios with morphology or color of the rutiles was observed among the samples analyzed. Most rutiles yield concordant U Pb ages which are reproducible within analytical uncertainty, i.e. generally ±2 Ma. The U Pb ages for prograde rutile are younger than the time of peak metamorphism given by U Pb ages for garnet and zircon, and also younger than U Pb ages for sphene and monazite, and 40Ar/ 39Ar and K Ar ages for hornblende but older than 40Ar/ 39Ar and K Ar ages for biotite from the same area. This suggests that the rutile ages reflect cooling below closure temperatures. Within a single hand-specimen, and thus for an identical thermal history, larger rutile grains give older ages than do smaller grains. This suggests that volume diffusion is the most probable mechanism responsible for the ages being younger than the time of peak metamorphism. It also suggests that the dimensions for such diffusion are directly related to the dimensions of the rutile crystal and not to the dimensions of sub-grain domains, as is the case for Ar diffusion in hornblende and feldspar. The concordant U Pb ages as well as the similar ages over a large area suggest that rutile has a well-defined temperature of closure. At a cooling rate of about 0.5–1°C/Ma the closure temperature for U Pb diffusion in rutile is about 420°C for grains with a radius of 0.009–0.021 cm, and about 380°C for grains with a radius of 0.007–0.009 cm. U Pb rutile ages obtained on different grain sizes indicate a cooling rate of about 0.5°C/Ma for the Pikwitonei granulite domain at 2460-2300 Ma. U Pb ages for rutile thus provide high-precision ages for evaluating the cooling history of high-grade terranes and can be critical to the quantitative development of thermal models for crustal evolution.

U–Pb geochronology of reactivated Archean basement and of Hudsonian metamorphism in the northern Labrador Trough

The Labrador Trough is a Proterozoic orogenic belt bordering the eastern margin of the Archean Superior Province. The volcano-sedimentary sequences of the trough were deformed and metamorphosed during the Hudsonian Orogeny (ca. 1800 Ma). The eastern basement, present as domal inliers, was remobilized during this orogeny, whereas the western basement remained unaffected.In this study we present U–Pb ages of the western and eastern basement of the Proterozoic Labrador Trough orogen between Leaf River and Koksoak River and of the northernmost extension of the DePas batholithic complex farther east. From the western basement, the Leaf Bay granodiorite has an age of 2721 ± 3 Ma (zircon). From the gneiss domes of the eastern basement the ages are as follows: Lac Moyer—2883 ± 6 Ma (zircon), 1793 ± 5 Ma (monazite), and 1746 Ma (minimum age, rutile); Lac Boulder—an upper-intercept age of 2868 ± 8 Ma and a lower-intercept age of 1783 ± 11 Ma (zircon–titanite regression) and an age of 1740 ± 5 Ma (rutile); Lac Olmstead Lake—2721 ± 4 Ma (zircon) and 1774 ± 5 Ma (titanite); Leaf Strait—2719 ± 7 Ma (zircon) and 1783 ± 2 Ma (monazite). The DePas Batholith contains four generations of zircon: the minimum ages of the three oldest are 2688, 2779, and 2922 Ma; we also obtained a monazite age of 1808 ± 2 Ma.These results lead us to the following conclusions: (i) The age of the Leaf Bay granodiorite, 2721 ± 3 Ma, is a minimum age of the gneisses of the Minto Subprovince, (ii) The westernmost gneiss domes of the eastern basement (Lac Moyer and Lac Boulder), bounded by two major faults, contain zircon formed during metamorphic events dated at 2868 ± 8 and 2883 ± 6 Ma and may be part of an allochthon. (iii) The Lac Olmstead and Leaf Strait domes contain metamorphic zircon dated at 2719–2721 Ma. (iv) The minimum age of migmatization of the DePas complex is 2688 Ma. (v) The monazite ages date the latest phase of metamorphism in the area, (vi) The titanite ages probably represent post-deformational cooling, whereas the rutile ages could represent late-metamorphic hydrothermal activity, (vi) The available data suggest the presence of continuous Archean basement—remobilized during the Hudsonian Orogeny—between the Labrador Trough and the DePas Batholith, which could represent a collision zone. The final stages of this collision could be slightly older (1808 Ma) than the equivalent activity in the Labrador Trough. These speculations raise the possibility that the evolution of the Labrador Trough is closely related in time and space to events occurring in its "hinterland".

Fluid inclusion characteristics and U-Pb rutile age of late hydrothermal alteration and veining at the Musoshi stratiform copper deposit, Central African copper belt, Zaire

Hydrothermal veining at the Musoshi stratiform copper deposit in Zaire has caused extensive alteration in the arkosic footwall sediments and in the mineralized ore shale (argillaceous siltstone) horizon. Pervasive growth of secondary rutile, biotite, and muscovite in the wall rocks accompanied this activity, whereas hematite, biotite, calcite, K-feldspar, barite, and accessory rutile occur with quartz in footwall veins. In contrast, veins cutting the ore shale carry Cu-Fe sulfides, uraninite, and molybdenite, with quartz, calcite, biotite, K-feldspar, and minor rutile. Linear zones of particularly intense alteration and veining probably represent structurally controlled fluid conduits. Fluid inclusions in vein quartz from such zones identify an apparently halite-saturated liquid at 397 degrees + or - 5 degrees C, with approximately 39 wt percent NaCl, 15 wt percent KCl, and minor amounts of CO 2 and other components.Rutile is found pervasively, both as a wall-rock alteration phase and as large euhedra (up to 3 cm) in the quartz veins. The hydrothermal growth of this normally very insoluble mineral is explained by the high temperature and salinity of the aqueous phase.The widespread occurrence of rutile in the deposit makes it suitable for determining the age of hydrothermal alteration. Six samples of rutile picked from footwall and ore-shale veining and alteration give a U-Pb concordia upper intercept of 514 + or - 2 m.y. (2 Sigma ). This value is confirmed on a 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb plot (four of these samples contained common Pb concentrations in excess of blank), which yields a slope age of 513 + or - 5 m.y. (2 Sigma ). A seventh sample from an unusual footwall hematite vein gave a slightly younger 207 Pb/ 206 Pb age (496 + or - 1 m.y.).The 514 + or - 2 m.y. alteration event is considerably younger than the minimum age of the host sediments (602 + or - 20 m.y.), and probably also postdates ore deposition, believed to be older than 600 m.y. No unambiguous evidence was seen at Musoshi to support a relationship between hydrothermal alteration and stratiform Cu mineralization, although sulfide textures were modified at this time (as shown by intergrowths with rutile, dated here at 514 m.y.). The hydrothermal activity is concluded, therefore, to be a late overprint, unrelated to Cu ore deposition, but perhaps related to compressional deformation and metamorphism associated with the Lufilian orogeny.