Zircon Generations Research Articles

Paleoarchean tonalite-trondhjemite complex in the northwestern part of the Sharyzhalgai uplift ( southwestern Siberian craton): results of U-Pb and Sm-Nd study

In the northwestern part of the Sharyzhalgai uplift of the Siberian craton (Bulun block), the earliest sialic crust (grey-gneiss complex) is composed of plagiogneisses, their migmatized varieties, and subordinate plagiogranitoids. The petrochemical, trace-element, and Sm-Nd isotope compositions of rocks were studied, and U-Pb dating of zircons (SHRIMP II) was performed. Plagiogneisses and plagiogranitoids of trondhjemite and, more seldom, tonalite compositions are predominant; their compositions are typical of rocks of Archean tonalite-trondhjemitegranodiorite (TTG) complexes (Al 2O 3 ≥ 15%, Mg# = 28–38, (La/Yb) n = 23–66, Sr/Y = 27–135, Eu/Eu ⁎ = 0.7–1.1). Plagiogneisses of meta-andesite-rhyodacite association are subordinate (SiO 2 = 59–69%, (La/Yb) n = 7–32, Sr/Y = 11–24, Eu/Eu ⁎ = 0.5–0.7). Cathodoluminescent study of zircons revealed “magmatic cores” and metamorphic rims; most of the rims differ from the cores in U and Th contents and low or greatly varying Th/U ratios. In migmatized plagiogneisses of trondhjemite composition, two zircon generations of different morphologies have been recognized. The protoliths of the grey-gneiss complex rocks formed in the Paleoarchean as a result of two discrete magmatic events, at ∼3.3 and 3.25 Ga, and their metamorphism and migmatization took place at ∼3.2 Ga. The isotopic and geochemical features of rocks evidence that the primary melts were produced mainly through the melting of metabasic sources at different depths of the thickened crust. Plagiogneisses of trondhjemite composition apparently resulted from magma generation involving ancient sialic material.

The role of heterogeneous strain in the development and preservation of a polymetamorphic record in high‐P granulites, western Canadian Shield

AbstractMafic rocks in the Chipman domain of the Athabasca granulite terrane, western Canadian Shield, provide the first well‐documented record of two distinct high‐P granulite facies events in the same domain in this region. Textural relations and the results of petrological modelling (NCFMASHT system) of mafic granulites are interpreted in terms of a three‐stage tectonometamorphic history. Stage 1 involved development of the assemblage Grt + Cpx + Qtz ± Pl (M1) from a primary Opx‐bearing igneous precursor at conditions of 1.3 GPa, 850–900 °C. Field and microstructural observations suggest that M1 developed synchronously with an early S1 gneissic fabric. Stage 2 is characterized by heterogeneous deformation (D2) and synkinematic partial retrogression of the peak assemblage to an amphibole‐bearing assemblage (M2). Stage 3 involved a third phase of deformation and a return to granulite facies conditions marked by the prograde breakdown of amphibole (Amph2) to produce matrix garnet (Grt3a) and the coronitic assemblage Cpx3b + Opx3b + Ilm3b + Pl3b (M3b) at 1.0 GPa, 800–900 °C. M1 and M3b are correlated with 2.55 and 1.9 Ga metamorphic generations of zircon, respectively, which were dated in a separate study. Heterogeneous strain played a crucial role in both the development and preservation of these rare examples of multiple granulite facies events within single samples. Without this fortuitous set of circumstances, the apparent reaction history could have incorrectly led to an interpretation involving a single‐cycle high‐grade event. The detailed P–T–t–D history constructed for these rocks provides the best evidence to date that much of the east Lake Athabasca region experienced long‐term lower crustal residence from 2.55 to 1.9 Ga, and thus the region represents a rare window into the reactivation and ultimate stabilization processes of cratonic lithosphere.

Multi-stage origin of the lower crust of the Karelian craton from 3.5 to 1.7 Ga based on isotopic ages of kimberlite-derived mafic granulite xenoliths

Mafic garnet granulite xenoliths recovered from ∼500 to 600 Ma old eastern Finland kimberlites provide direct information on the petrology and physical properties of the lower cust below the Archean Karelian craton. Mineral thermobarometry, together with isotopic, petrological and seismic velocity constraints, imply that the xenolith suite is derived from a geophysically-determined, dense, high-velocity layer at the base of the crust (40–58 km depth). Single grain zircon U–Pb dates and Nd model ages ( T DM) imply that this is a hybrid layer consisting of both Archean and Proterozoic mafic granulites. Zircon ages of up to ∼3.5 Ga and Nd T DM model ages ∼3.7 Ga of the xenoliths are equivalent to those of the oldest upper crustal assemblages. During the Proterozoic, transient heating of these Archean granulites by voluminous basic magmatic intrusions resulted in multiple younger zircon generations within individual xenoliths. The most important post-Archean lower crust growth took place during ∼1.9 Ga accretion of the Svecofennian arc complex to the craton margin, when underplating Proterozoic basaltic magmas became mingled with pre-existing Archean mafic granulites. Later, post-orogenic (∼1.80–1.73 Ga) transient heating of the lower crust occurred as a response to magmas ponded at the uppermost lithospheric mantle. In conclusion, Karelian lower crust records the geological evolution of the craton margin environment through a period of nearly 4 billion years. Our data add a new piece to the emerging picture that emphasises the global importance of the 3.5 Ga crustal growth episode. We suggest that it represents a major mantle-plume event – more widespread than previously recognized – when a significant fraction of Paleoarchean continental crust was formed.

Zircon geochemistry of different intrusive phases of Weiya pluton: implications for magma genesis

The characteristics of hosted magmas and their petrogenesis based on electron microprobe determination of trace element contents in zircons were discussed. Trace element geochemistry of zircons indicates that zircons in both gabbro and quartz syenite have two-generations. Zircons of the first generations are crystallized in the magma chamber, whereas those of the second generations are formed in supercooling environment. The former is richer in Zr, but poorer in U, Th, Hf and Y. Quartz diorite porphyrite contains zircons that can be distinguished into the early and late generations. Compared to the late generation, the early generation is richer in Zr but poorer in U, Th, Hf and Y. No conspicuous disruption of zircon evolution has been found in both biotite monzogranite and fine-grained granite. However, the content of zircon in fine-grained granite is higher in U, Th and Y and lower in Zr relative to biotite monzogranite without significant contrast in mass fraction ratio of ZrO2 to HfO2 ratio. Such differences in zircon geochemistry of various intrusive phases and the occurrence of the two zircon generations within a single intrusive phase suggest that these phases of magmas are generated from diverse sources during post-collisional continental extension. These magmas ascend rapidly and cool quickly in a short interval.

Evidence from kimberlitic zircon for a decreasing mantle Th/U since the Archean

A decoupling in MORB of measured Th/U ( κ = 2.5) from that calculated by Pb isotopes ( κ = 3.8) for the depleted asthenosphere is well established, and has been referred to as the second Pb paradox ( Kramers, J.D., and Tolstikhin, I.N., 1997. Two terrestrial lead isotope paradoxes, forward transport modelling, core formation and the history of the continental crust. Chem. Geol., 139, 75–110.) or the kappa conundrum ( Elliott, T., Zindler, A., and Bourdon, B., 1999. Exploring the kappa conundrum: the role of recycling in the lead isotope evolution of the mantle. Earth Planet. Sci. Lett., 169, 129–145.). More controversial has been the cause and timing of this phenomenon, although a higher return flux of U 6+ relative to Th 4+ and(or) the recycling of crustal Pb into the mantle have become the preferred explanations of most workers. Such a combined mechanism effectively operating over the past 2.5 Ga was modelled in plumbotectonics ( Zartman, R.E., and Haines, S., 1988. The plumbotectonics model for Pb isotopic systematics among major terrestrial reservoirs—a case for bi–directional transport. Geochim. Cosmochim. Acta, 52, 1327–1339. 709.33.), and found to be quantitatively feasible. A large TIMS, SIMS and LA-ICPMS database of Th and U concentrations for kimberlite-hosted zircon, particularly from Cr-poor megacrystic suites, now exists ( Kinny, P.D., Compston, W., Bristow, J.W., and Williams, I.S., 1989. Archaean mantle xenocrysts in a Permian kimberlite: two generations of kimberlitic zircon in Jwaneng DK2, southern Botswana. In: Ross, J., et al. (Eds.), Kimberlites and Related Rocks. Geol. Soc. Aust. Spec. Publ., vol. 146. Blackwell, Melbourne, pp. 833–842.; Berryman, A.K., Stiefenhofer, J., Shee, S.R., Wyatt, B.A. and Belousova, E.A., 1999. The discovery and geology of the Timber Creek kimberlite, Northern Territory, Australia. In: J. J. Gurney et al. (Editors), Proceedings of the VIIth International Kimberlite Conference, Cape Town, pp. 30–39.; Griffin, W.L., Pearson, N.J., Belousova, E., Jackson, S.E., van Achterbergh, E., O–Reilly, S.Y., and Shee, S.R., 2000. The Hf isotope composition of cratonic mantle: LAM–MC–ICPMS. analysis of zircon megacrysts in kimberlites. Geochim. Cosmochim. Acta, 64, 133–147.; Peltonen, P., and Manttari, I., 2001. An ion microprobe U–Th–Pb study of zircon xenocrysts from the Lahtojoki kimberlite pipe, eastern Finland. Bull. Geol. Soc. Finl., 73, Parts 1–2, 47–58.; Spetsius, Z.V., Belousova, E.A., Griffin, W.L., O'Reilly, S.Y., and Pearson, N.J., 2002. Archean sulfide inclusions in Paleozoic zircon megacrysts from the Mir kimberlite, Yakutia: implications for the dating of diamonds. Earth Planet. Sci. Lett., 175, 1–16.; Appendices A and B, this work). Six suites comprising 10 or more zircon grains with ages between 90 and 2550 Ma reveal consistent patterns when plotted on Th/U vs. U diagrams. We interpret these patterns as resulting from fractional crystallization of a melt with kimberlite affinity presumably derived from the asthenosphere, permitting the extrapolation to an initial Th/U at the time zircon crystallization began. A two-fold decrease is seen in this ratio over the past 2.5 Ga, suggesting that during this time a similar change has occurred in the parent silicate melt. Estimates of Th and U distribution coefficients between zircon and coexisting melt permit calculation of Th/U in the melt, which, for these highly incompatible elements, presumably is the same as for its mantle source rock. Kimberlitic zircon may thus indeed give evidence of a reduction in κ, tentatively calculated as from ∼ 4 to ∼ 2, since the Archean for the depleted asthenosphere.

Assigning Dates to Thin Gneissic Veins in High-Grade Metamorphic Terranes: A Cautionary Tale from Akilia, Southwest Greenland

A granodiorite from Akilia, southwest Greenland, previously suggested to date putative life-bearing rocks to ≥3·84 Ga, is re-investigated using whole-rock major and trace-element geochemistry, and detailed cathodoluminescence image-guided secondary ion mass spectrometer analyses of zircon U–Th–Pb and rare earth elements. Complex zircon internal structure reveals three episodes of zircon growth and/or recrystallization dated to c. 3·84 Ga, 3·62 Ga and 2·71 Ga. Rare earth element abundances imply a significant role for garnet in zircon generation at 3·62 Ga and 2·71 Ga. The 3·62 Ga event is interpreted as partial melting of a c. 3·84 Ga grey gneiss precursor at granulite facies with residual garnet. Migration of this 3·62 Ga magma (or melt–crystal mush) away from the melt source places a maximum age limit on any intrusive relationship. These early Archaean relationships have been complicated further by isotopic reworking in the 2·71 Ga event, which could have included a further episode of partial melting. This study highlights a general problem associated with dating thin gneissic veins in polyphase metamorphic terranes, where field relationships may be ambiguous and zircon inheritance can be expected.

Petrology and ion microprobe U‐Pb chronology applied to a metabasic intrusion in southern Sweden: A study on zircon formation during metamorphism and deformation

The Åker metabasite occupies a key position in a major tectonic lineament in southernmost Sweden, the Protogine Zone, which coincides closely with the eastern boundary of the late Mesoproterozoic Sveconorwegian orogen of southwest Scandinavia. Metamorphic reactions, associated with the transformation from isotropic gabbro to foliated garnet amphibolite, were identified from disequilibrium textures of which some involved release of zirconium (Zr) and growth of metamorphic zircon. Ion microprobe dating of igneous zircon gave 1562 ± 6 Ma, whereas metamorphic zircons yielded ages of 1437 ± 21, 1217 ± 75, and 1006 ± 68 Ma. The presence of baddeleyite pseudomorphs made up of saccharoidal zircon and a higher abundance of older rather than younger metamorphic zircons suggest redistribution of Zr into new zircon, first by the breakdown of baddeleyite (ZrO2) and later by the consumption of igneous phases containing trace amounts of Zr. Several generations of metamorphic zircon and the presence of 1.56 and 1.22 Ga mafic intrusions along the Protogine Zone call for a complex tectonic history probably reaching back to at least ∼1.56 Ga. Growth of metamorphic zircon at ∼1.44 Ga may relate to a regional, compressional event. The WNW trending deformational structures on both sides of the Protogine Zone may possibly relate to that event. The ∼1.22 Ga metamorphic zircons are coeval with the emplacement of numerous granitic, syenitic, and mafic intrusions along and parallel to the Protogine Zone. The age around 1.0 Ga, finally, marks Sveconorwegian metamorphism for which thermobarometry of the Åker garnet‐amphibolite suggests 1000–1200 MPa at 600°C–630°C. Thereafter, significant relative uplift of the rocks to the west of the Protogine Zone occurred on nearly vertical, north‐south trending deformation zones.

Lithological Features of Precambrian Gold-Bearing Rocks: Evidence from the Ukrainian Shield

Based on traditional petrochemical and nontraditional mineralogical methods (accessory zircon generation analysis), specific features of the primary composition of strongly metamorphosed rocks from some Early Precambrian Au-bearing rocks of the Ukrainian Shield (US) were studied. The confinement of several gold ore occurrences to primarily sedimentary Late Archean rocks of the Ukrainian Shield has been established and the possibility of their chemogenic origin is considered. The joint analysis of plicative tectonics and metamorphism facies in the study area demonstrated that Au-bearing primarily sedimentary (chemogenic) rocks of the Khashchevatoe–Zaval'ev Formation of the Bug Group (AR2) are confined to amphibolite-facies domains within tectonic (high-order synform) structures with a significant gold potential.

Zircons from Syros, Cyclades, Greece--Recrystallization and Mobilization of Zircon During High-Pressure Metamorphism

Zircons were studied from high-pressure/low-temperature metamorphosed meta-igneous lithologies from Syros. These rocks carry several zircon generations related to each other by dissolution---reprecipitation processes. One generation is pristine zircon that shows growth zoning, relatively elevated contents of trivalent cations and high Th/U ratios. The other end-member is a skeletal zircon generation with negligible trivalent cation contents and low Th/U ratios ( 0 1). Texturally between these two, there is a range of zircon crystals with complex inclusion populations of Y---HREE---Th phases and fluid inclusions, showing variable progress of replacement--recrystallization. Both end-members yield distinct sensitive high-resolution ion microprobe (SHRIMP) U---Pb ages. The pristine generation has an age of 80 2 1 6Ma from a metagabbro, and 76 4 2 1Ma from a meta-plagiogranite dyke. The skeletal, low-Th/U zircon generation yields an age of 52 4 0 8Ma. The older, Late Cretaceous, zircons are interpreted to date emplacement of the magmatic protoliths in a small segment of oceanic crust. The younger, Eocene, age, however, dates a zircon recrystallization event, which possibly coincides with high solubility and mobility of high field strength elements in a high-pressure aqueous fluid phase. Intergrowth relations between zircon and peak-metamorphic garnet, and excellent agreement of the U---Pb ages with white mica Ar---Ar ages for the same samples support the conclusion that Eocene is the true age of high-pressure metamorphism on Syros.

The Proterozoic Hustad igneous complex: a low strain enclave with a key to the history of the Western Gneiss Region of Norway

The Hustad Igneous Complex (HIC) is a concentrically zoned, high K calc-alkaline intrusive complex, with a core of quartz–monzonite and granite surrounded successively by monzodiorite and pyroxenite, preserved in a 10 km2 lens within the strongly ‘caledonised’ Western Gneiss Region (WGR) of Norway. Upper intercept UPb zircon ages of 1654±1 and 1653±2 Ma, obtained from the granite and the monzodiorite, respectively, overlap with a RbSr whole rock age of 1656±49 Ma (I.R.: 0.70193±0.0009, MSWD:6.6) obtained from the felsic members of the complex and a SmNd whole rock age of 1555±180 Ma (I.R.: 0.51067±13, MSWD:3.3). The UPb age of 1654±1 Ma is interpreted to date the time of intrusion. A 50 m thick dolerite dyke that can be followed for almost 4 km across the HIC yields a UPb zircon and baddeleyite age of 1251±3 Ma. The dyke also contains a secondary polycrystalline zircon generation, which together with baddeleyite defines a lower intercept age of 384±9 Ma. The Caledonian events were also marked by the emplacement of a swarm of pegmatites, which crosscut all the other units and provide a UPb zircon age of 401±1 Ma. A RbSr whole-rock age for the pegmatites is older at 452±26 Ma (I.R.: 0.7105, MSWD:4.3) indicating some initial isotopic heterogenity and formation of the pegmatites from material with a long crustal residence time. A concordant UPb monazite age of 390±2 Ma from the pegmatite and the above 384±9 Ma lower intercept age date metamorphism caused by percolating fluids. Post pegmatite deformation transformed the complex along shear zones into gneisses indiscernible from the surrounding gneisses. The HIC is characterised by high mg-numbers (0.39–0.80), high Ni and Cr values, and depletion of Nb and Ta relative to LIL elements. Phenocrysts of ortho- and clinopyroxene have high concentrations of Cr, supporting the Cr-rich nature of the complex. The suite is LREE-enriched, has a fractionated and U- shaped REE pattern (Cen 90–143, Ybn 12–40), eNd values ranging from +0.0 to +2.7, and the felsic members display weak positive Eu anomalies while the pyroxenite has a negative Eu anomaly. The complex bears a geochemical resemblance to the late-tectonic, high-BaSr granite complexes of West Scotland, and particularly to their volcanic counterparts, the Mg-rich Old Red Sandstone andesites. Overall the data suggest generation of the suite in a subduction environment from material derived by melting of a recently enriched, but previously long-term LREE-depleted mantle. The HIC is one of the few well-preserved remnants of the igneous protoliths of the 1686–1640 Ma gneisses that make up the WGR. In fact, the pyroxenite is compositionally identical to orthopyroxene eclogite found as lenses in the surrounding gneisses and forms a likely protolith to this enigmatic rock. The metamorphism had little influence on the RbSr and SmNd systems of the felsic rocks, but it appears to have affected more strongly those in the mafic rocks, probably as a result of the more intense mineralogical transformations and metasomatic loss of Rb and LREE during high-grade metamorphism.

2.9, 2.36, and 1.96 Ga zircons in orthogneiss south of the Red River shear zone in Viet Nam: evidence from SHRIMP U–Pb dating and tectonothermal implications

Orthogneissic rocks coexisting with migmatites and containing small amphibolite lenses are exposed in the center of the metamorphic belt which runs parallel to the Day Nui Con Voi–Red River shear zone in northern Viet Nam. The orthogneiss complex has given some radiogenic dates of Early Proterozoic and Late Archean, which are the oldest ages ever registered for the Southeast Asian continent. Zircon grains separated from three samples of the orthogneiss complex have been dated to establish the protolith age and the timing of high-grade tectonothermal events in the complex. Sixty-five SHRIMP U–Th–Pb analyses of these zircons define three age groups of 2.84–2.91, 2.36, and 1.96 Ga. The age groups correspond to three periods of zircon generation. The oldest ∼2.9 Ga cores indicate a minimum age for the protolith of the orthogneiss complex. Two younger generations (including ∼2.36 Ga outer-cores and ∼1.96 Ga rims) probably grew during later high-grade tectono-metamorphic events, which were previously suggested by K–Ar and 40Ar/ 39Ar cooling ages of ∼2.0 Ga for synkinematic hornblendes. An early thermal history of the orthogneiss complex has been constrained, including a primary magma-crystallization stage starting at ∼2.9 Ga, followed by two Early Proterozoic (∼2.36 and ∼1.96 Ga) high-grade tectonothermal events. The ca. 2.9 Ga protolith age of the orthogneiss complex documented in this study provides new convincing evidence for the presence of Archean rocks in Indochina, and clearly indicates that the crustal evolution of northern Viet Nam started as early as Late Archean time.

Pre-eruptional magmatic zircon, Neogene Alborán volcanic province, SE Spain

Hornblende–biotite– (?)pyroxene dacite from Cerro Morrón de Mateo, Sierra de Gata, SE Spain has two types of zircon grains, a major, younger group with a weighted average ion microprobe U–Pb zircon age of 11.69 ± 0.15(2 σ ) Ma and a minor, older one of 13.58 ± 0.31(2 σ ) Ma. The younger crystals have mostly euhedral habit and zoning, and vary in size from c. 50 to 650 μm; a few show signs of late stage resorbtion in the form of glass filled embayments. The older crystals have anhedral zoning and irregular, corroded outlines. Considering its eruption age of 10.70 ± 0.15(2 σ ) Ma ( 40 Ar/ 39 Ar biotite plateau ages), these two zircon generations suggest a gestation period of at least 1 (–3) Ma for the Morrón de Mateo magma. Like zircon, quartz, plagioclase, biotite and hornblende, major phenocryst phases, also show variable resorbtion effects. That suggests that the erupted dacitic magma body consisted of several batches with somewhat different history, which were brought together some time before extrusion.

Composition and U–Th–total Pb model ages of polygenetic zircons from the Vånga granite, south Sweden: An electron microprobe study

U–Th–total Pb dating of zircons from the Vånga granite, south Sweden, by electron microprobe (EMP) revealed three different zircon generations. U–Th–Pb analyses of finely zoned zircons, existing both as rounded cores and overgrowth rims, yielded a well defined U–Th–total Pb model age of 1448±25 Ma (1[sgrave]). This age is interpreted to date the intrusion of the Vånga granite, which is notably older than previously suggested. Hf- and F-rich overgrowths yielded a U–Th–total Pb model age of 1128±51 Ma (1[sgrave]). Based on the Hf- and F-enrichment and morphological features, it is suggested that the overgrowths formed by a dissolution–precipitation process in a hydrothermal environment. In few cases magmatic zircon overgrows clear, unzoned cores, which could hardly be dated by EMP due to low U, Th, and Pb contents. A single analysis of one of these cores, however, yielded a U–Th–total Pb model age of c. 1700 Ma, which fits with a previously determined age of refractory zircons in a c. 1445–1465 Ma old Karlshamn-type granite. This agreement supports the previous interpretation that the Vånga granite crystallized from a residual liquid separated from the Karlshamn granite magma. The new intrusion age of c. 1450 Ma indicates that the Vånga granite intruded contemporaneously with an extensive thermomagmatic activity further west in the Eastern Segment of the Sveconorwegian orogen. The c. 1130 Ma age of zircon overgrowths has tectonic implications on the N–S trending Protogine Zone; a major shear zone that marks the eastern boundary of Sveconorwegian reworking. Since equally old syenites and dolerites occur within the Protogine Zone, the age of these hydrothermal overgrowths indicates that the crust affected by Protogine Zone deformation is not exotic relative to the crust of Blekinge, nor is it conceivable to relate the metamorphic contrast across the Protogine Zone to deformation east thereof.

Zircon in Polydeformed and Metamorphosed Precambrian Granitoids from the White Sea Tectonic Zone, Russia: Morphology, Cathodoluminescence, and U-Pb Chronology

This paper presents U-Pb isotopic data and information on the morphology of zircons from the White Sea tectonic zone, Baltic Shield. These zircons were taken from early Precambrian intrusive rocks and their metamorphic products and have revealed patterns in zircon behavior and processes of repeatedly imposed deformation and metamorphism. This study is of particular interest to those involved with U-Pb dating of zircons from repeatedly deformed and metamorphosed igneous rocks. Zircon crystals from the igneous rocks were transformed from primary prismatic forms into ovoid-isometric shapes during deformation processes under granulite and high-temperature amphibolite facies. Crystal transformation occurred just after zircon crystallization during the intrusion of hypersthene diorites at 2728 Ma and the intrusion of hornblende granodiorites at 2416 Ma. Formation of new zircon generations in the hypersthene diorites is connected with fluid mobility as the result of granodiorite intrusion at 2416 Ma and ultramet...

Applications of the SHRIMP I ion microprobe to the understanding of processes and timing of diamond formation

The ion microprobe is a highly versatile analytical instrument capable of a wide variety of in situ isotopic and trace element studies of solid materials, with a spatial resolution of 15 to 25 mm, consuming only 2 to 5 ng of sample per determination. In a study combining uranium-lead isotope dating, rare earth element fingerprinting, and sulfur plus lead isotope tracing, it has been possible to address critical questions concerning the timing of, sources of materials for, and processes involved in diamond formation in African kimberlites. The zircon uranium-lead data indicate that kimberlites may host two generations of minerals: one crystallizing at the time of kimberlite emplacement and the other significantly older, perhaps relict from a previous kimberlite magma-generating event. Rare earth element analyses of the same zircons suggest that the two generations of zircons are different from each other and both are quite distinct from crustal zircons. Since zircon is not a common phase in the mantle, it may be that some mantle metasomatic event is required to form these minerals and may actually trigger kimberlite generation. The sulfur isotope data from sulfide inclusions in diamonds suggest that two different reservoirs are being sampled during diamond growth: one with essentially pristine mantle characteristics and another with variability more reminiscent of a low-temperature or more oxidizing, possibly crustal, environment. Paragenetic data, based on nickel contents of the sulfides, indicate that it is the eclogitic diamonds which record the crustal signatures. Lead isotope variability parallels that found in sulfur, with perdiotitic sulfides having apparently normal mantle lead and eclogitic sulfides having much more radiogenic lead which in some cases requires very high U/Pb ratio growth environments, more consistent with a crustal source. The data from this study and others, including the carbon isotope heterogeneity of eclogitic diamonds, can be reconciled with a single model in which introduction of crustal material into the mantle triggers both generation of the kimberlite and diamond growth. This makes plate tectonics of paramount importance in locating economic reserves of diamonds.

Search for Archaean basement in the Caledonian fold belt of North-East Greenland

SHRIMP U-Pb isotope data on zircon crystals from a gneiss sample near Danmarkshavn, where the presence of Archaean rocks has earlier been documented, show that the rock has undergone a complex history of igneous and metamorphic zircon growth. At least three generations of zircon are present with ages of c. 3000 Ma, c. 2725 Ma and 1967 ±8 Ma (2 α). Apparently the rock was formed from an Archaean protolith which underwent high grade metamorphism during the early Proterozoic. Another sample from the easternmost exposures of the Caledonian basement, collected further north, yielded only early Proterozoic zircons with an age of 1963 ± 6 Ma. Together with a SHRIMP U-Pb zircon age of 1974 ± 17 Ma reported earlier, these results give evidence of a major igneous and metamorphic event in North-East Greenland about 1965 Ma ago.

Ion probe U–Th–Pb zircon dating of polymetamorphic orthogneisses from northern Labrador, Canada

Rounded zircons included within igneous zircons from the Uivak I gneisses, northern Labrador, have ages of up to 3863 ± 12 Ma (2σ), showing the (former) existence of rocks that predate the Uivak gneisses by more than 100 Ma. The igneous zircons themselves have experienced varying amounts of early Pb loss, but age estimates based on maximum 207Pb/206Pb from three separate gneiss samples agree to within error at 3732 ± 6 Ma (2σ), which is taken as the age of emplacement of the igneous precursor to the dominant component of the Uivak gneisses. A new generation of zircons was formed during migmatization at ca. 3620 Ma. The volumetrically less important Lister gneiss was emplaced at 3235 ± 8 Ma (2σ). In the late Archaean, different parts of the gneiss complex were subjected to different grades of metamorphic overprinting. The zircon chronology of gneisses affected by granulite facies metamorphism suggests a rapid sequence of events, with simultaneous recrystallization of old zircons and growth of new U-poor zircons at 2766 ± 17 Ma (2σ), followed by partial melting at 2744 ± 4 Ma (2σ), and further recrystallization and growth of U-poor zircons afterwards. No late Archaean zircon growth has been recorded from orthogneisses unaffected by granulite fades metamorphism.

UPb single-zircon age for the Tinissaq gneiss of southern West Greenland: A controversy resolved

Ion probe UPb dating of single zircons from a layered gneiss from Tinissaq, southern West Greenland, resolves a long standing controversy about the age of the oldest recognizable orthogneiss unit within the partly retrogressed granulite-facies area south of Ameralik. In spite of whole-rock isotopic evidence for late Archaean ages for these gneisses, some workers have used field criteria to correlate them with the early Archaean (> 3.5 Ga) Amitsoq gneisses of the Godthåbsfjord area farther north and explained the isotopic characteristics in terms of various secondary element migrations. Ages obtained on well-preserved igneous zircons from the Tinissaq gneiss suggest a 2922 ± 7 Ma (2σ) emplacement age for the igneous precursor to the gneiss, ruling out any correlation with the Amitsoq gneisses. A younger generation of igneous zircons may have formed at 2852±9 Ma (2σ), interpreted as the time of a migmatization event. All igneous zircons recrystallized to varying extent as massive U-poor zircons during granulite-facies metamorphism at 2819±23 Ma (2σ). Younger granitoid gneisses that break up the 2922-Ma gneiss were most probably emplaced approximately contemporaneously with this recrystallization event. There is little direct geological correlation across the amphibolite-granulite facies boundary between Tinissaq and Nuuk (Godthåb), in agreement with recent suggestions that the boundary is a major tectonic break.

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".

Inverse age stratification in the archaean crust of the superior province: Evidence for infra- and subcrustal accretion from high resolution U-Pb zircon and monazite ages

Abstract New U-Pb mineral ages from the Agawa migmatite terrain in the southern Superior Province, Canada, together with published ages from neighbouring high- and low-grade terrains, reveal a pattern of progressive younging of plutonic and metamorphic processes with depth and provide insights into the mechanisms controlling late Archaean cratonisation. The following ages are reported: (1) about 2670 Ma for a first zircon generation, about 2660 Ma for a second zircon generation and 2649 ±2 Ma for monazite, all from a leucocratic layer in migmatites; (2) 2666 ± 2 Ma for a first zircon generation, 2663 ± 1.5 Ma for a second zircon generation and 2642 ± 3 Ma for titanite, all from an amphibolite; (3) 2654 ± 4 Ma for monazite from a granodiorite and (4) 2637 ± 2 Ma for monazite from a granite. Together with petrographic observations and chemical and morphological features of the zircons, the ages indicate the occurrence of a protracted period of magmatism and metamorphism commencing with migmatisation at 2670-2665 Ma, followed by injection of calc-alkalic melts and metasomatism at 2663-2660 Ma and concluded by the intrusion of granites and pegmatites and extensive hydrothermal activity between 2660 and 2635 Ma. The timing and the style of plutonism and metamorphism in the Agawa migmatite terrain contrast with the situation observed in the adjacent but tectonically higher-level Batchawana greenstone belt where major metamorphism occurred about 2675 Ma ago and plutonism was concluded at 2668 Ma ago. On the other hand, it resembles the evolution of the high-grade portion of the Kapuskasing Zone where the orogenic activity continued until after 2630-2620 Ma ago. These age relationships are interpreted as reflecting downward vertical accretion of the Archaean crust. The lower crust experienced magmatism and metamorphism long after conclusion of these processes at shallower crustal levels. The late tectonic regime produced granulites at depth, transferring granitic material rich in H 2 O and heat producing elements to intermediate crustal levels. These processes were probably accompanied by crustal thickening through the addition of mantle-derived material.