High-grade Metamorphic Overprint Research Articles

The Mesoarchean Amikoq Layered Complex of SW Greenland: Part 2. Geochemical evidence for high-Mg noritic plutonism through crustal assimilation

AbstractWhole-rock major- and trace-element data are presented on a sample collection from the >3 Ga Amikoq Layered Complex (ALC), and hosting amphibolites within the Mesoarchean Akia terrane, SW Greenland. The lithologies range from leuconorite to melanorite/feldspathic orthopyroxenite, orthopyroxenite to harzburgite through to dunite, and tholeiitic basaltic–picritic mafic host rocks. The Amikoq Layered Complex samples are primitive (Mg#: 65–89) with elevated Ni and Cr contents. However, the absence of troctolitic lithologies and the presence of two orthopyroxene compositional trends, suggests that the successions might not be comagmatic. On the basis of trace-element cumulate models, relatively low Ni contents and minor negative Sr-Eu anomalies in some high-Ti ultramafic rocks, it is not possible to exclude a petrogenesis related to a melt similar to that of the mafic host-rocks. Ultramafic samples with U-shaped trace-element distribution patterns are petrogenetically related to the noritic sequences, either through cumulus mineral accumulation or melt-rock reactions. Assimilation-fractional-crystallisation modelling of melanorites nevertheless require the parental melt to have been contaminated/mixed with a component of island-arc-like tholeiite affinity. A boninite-like parental melt might have been derived from the subcontinental lithospheric mantle of the Akia terrane, or alternatively via assimilation of an ultramafic parental melt with island-arc-like tholeiite. Given the complex geological evolution and high-grade metamorphic overprint of the Amikoq Layered Complex, we are unable to differentiate between the two models.

Evidence of late Palaeozoic and Middle Triassic magmatism in the Sakar-Strandzha Zone, SE Bulgaria: Regional geodynamic implications

ABSTRACT Late Palaeozoic granitoids, meta-granitoids and meta-volcanic rocks predominate in the metamorphic basement of the northern and western parts of the Sakar-Strandzha Zone (SASTZ) in southeast Bulgaria, together with the subsidiary Triassic meta-granitoids and meta-volcanic rocks. Generally, igneous minerals and textures are preserved, except in the meta-granitoids and meta-volcanic rocks that experienced a low- to high-grade metamorphic overprint. The volcanic rocks have a peraluminous and high-K calc-alkaline composition, and the granitoids range between I- to S-type compositions, typical of volcanic arcs and syn-collisional settings. LILE and LREE-enrichment and Nb-Ta anomalies characterize the intrusive and extrusive rock suites. U-Pb zircon geochronology has yielded crystallization ages between 245 and 237 Ma for the majority of the studied igneous rocks, and between 297 and 295 Ma for a small group of igneous rocks. Early Permian and Middle Triassic igneous suites of the northern and western SASTZ have similar compositions and a similar tectonic setting when compared to Late Carboniferous-Early Permian intrusive and extrusive suites of the adjacent Sakar unit of the SASTZ, confirming a common regional late Palaeozoic-early Mesozoic tectono-magmatic event. As the Late Carboniferous-Permian to Middle Triassic magmatic arc components extend across the SASTZ, they trace the time-correspondent active continental margin along the Eurasian plate during subduction of the Paleotethys oceanic lithosphere. The late Palaeozoic Eurasian active continental margin magmatic arc evolution of the SASTZ can be linked with the Serbo-Macedonian-Rhodope zones to the southwest, where coeval meta-granitoids document the same geodynamic context. By contrast, the Triassic igneous suite of the SASTZ is unrelated to the Serbo-Macedonian-Rhodope zones, where Triassic meta-ophiolite and meta-granitoids record Neotethys rifting.

How diverse is the source? Age, provenance, reworking, and overprint of Precambrian meta-sedimentary rocks of West Gondwana, Cameroon, from zircon U-Pb geochronology

The Nyong Complex and the Yaoundé Group represent two crustal sequences of West Gondwana in Cameroon. We use U-Pb zircon geochronology on meta-sedimentary rocks to analyze their depositional age, provenance, and age of the high-grade metamorphic overprint. Due to the undoubtedly Precambrian age of these rocks, we also pursue questions regarding the veiling of the provenance signal by subsequent overprints, the diversity of the sources, and the role of crustal reworking in such an old setting. Although both the Nyong and the Yaoundé meta-sedimentary rocks were strongly affected by the Pan-African orogeny, Discordia analysis and the evaluation of remnant concordant zircons allow addressing these classic provenance questions with the LA-ICP-MS U-Pb zircon technique. We show that the Nyong Complex meta-sedimentary rocks likely have a <2.4 and> 2.2 Ga depositional age (∼Rhyacian, early-middle Paleoproterozoic) and were locally sourced from the Meso-Neoarchean Congo Shield (∼2.8 Ga); they were overprinted during the Paleoproterozoic Eburnean orogeny (∼2.2–2.0 Ga). The Yaoundé Group meta-sedimentary rocks likely have a <0.7 Ga depositional age (Cryogenian, middle Neoproterozoic); they lack a major Archean source, and have distinct, likely locally-sourced Eburnean (∼2.0 Ga), and Tonian-Cryogenian (∼1.0 and 0.7 Ga) sources. The meta-sedimentary rocks of both units share the Pan-African (∼0.6 Ga) orogenic overprint. Our study highlights the limited source diversity of these old basin deposits with a provenance from the few existing crustal masses.

Chromitites from an Archean layered intrusion in the Western Dharwar Craton, southern India

Chromite deposits hosted in a layered ultramafic–mafic intrusion in the Archean Chithradurga greenstone belt are part of a suture that divides the Western and Central Dharwar craton blocks in southern India. Serpentinised ultramafic rocks including dunite, peridotite, and phlogopite-rich clinopyroxenite occur with massive chromitite. The olivine compositions show two distinct groups, one with Fo ~91–92 olivines in the massive chromitite, and another group with olivine Fo values of ~95–96, in the serpentinised dunite. The chromite occurs as massive chromitite and as an accessory phase in dunite. The former is characterized by high Cr# (Cr/(Cr + Al) × 100 = 67–70) and moderate to high Mg# (Mg/(Mg + Fe) = 52–66); The latter shows a larger variation in Cr which is likely related to sub-solidus re-equilibration of the chromite during the serpentinization event. In the massive chromitites, sub-solidus re-equilibration of chromite is less significant or absent. Bulk-rock major and trace element analyses of the peridotites reveal a composition less magnesian than depleted mantle rocks, with low TiO2, Na2O, CaO and high Cr, Ni content, a general depletion in Sr and enrichment in Th, U. The chromitites contain variable PGE abundances (ΣPGE = 8-244 ppb) with enrichments of Os, Ir and Ru, relative to Pt and Pd . These features suggest that the chromitites are cumulates formed from a highly magnesian parental magma with komatiitic affinity, derived by large degrees of partial melting, , which underwent crustal contamination. The ReOs isotope data demonstrate a minimum Re-depletion model age (TRD) of 2895 ± 24 Ma for the serpentinized dunite (MY11/3), and a more robust age of 3120 ± 12 Ma for two chromitite samples, which we interpret as the best estimate of the magmatic age of this ultramafic intrusion. Abundant zircon grains also occur within these ultramafic rocks with UPb ages spanning from ca. 2.9 Ga to ca. 2.4 Ga, with a dominant age population in the range 2.8–2.6 Ga. We interpret the upper intercept age at ca. 2.9 Ga to represent the initial crystallization of zircon in the ultramafic rocks, potentially as a result of an early stage of metamorphism and hydrothermal alteration. The 2.5–2.4 Ga zircon ages reflect a late-stage high-grade metamorphic overprint.

Deciphering deformation in ultramafic intrusions via magnetic fabric (AMS) and palaeomagnetic studies, Savannah Ni-PGE camp, NW Australia

Knowledge of the internal architecture of mafic-ultramafic intrusions is critical to efficient targeting of magmatic Ni-PGE mineralisation. However, the visual homogeneity of such rocks in drill-core, coupled with poor outcrop in many areas can make their structure difficult to decipher using standard techniques. In this study we use measurements of anisotropy of magnetic susceptibility (AMS) and remanent magnetisation to examine the structural history of four adjacent, temporally equivalent, ultramafic intrusions from the east Kimberley Province, NW Australia. Whilst temporally equivalent, the intrusions have contrasting shape including bladed dyke, sill and stock geometries, and are hosted in rocks of contrasting rheology, including mafic granulite and meta-sedimentary migmatite. Early palaeomagnetic signatures present at Savannah and Savannah North originally had shallow NNE- to ENE-oriented inclinations, consistent with acquisition at low palaeolatitudes at ca 1800 Ma. However, these primary magnetisations were subsequently rotated during the ca. 1810 Ma Mount Stafford Event and/or ca. 1400–1200 Ma Yampi Orogeny, demonstrating that significant deformation occurred post-emplacement. Magmatic fabrics observed in AMS data for Savannah, Savannah North and Dave Hill all show evidence of NE or SW rotation during the same events. However, whilst there are similarities in the implied shortening directions across all four intrusions, they each preserve a unique deformation style, which is attributed to the unique shape of each intrusion coupled with the contrasting rheology of their respective host rocks. Each intrusion preserves a common dominant palaeomagnetic signature which is consistent with a high-grade metamorphic overprint at ca. 1200 Ma during the latter stages of the Yampi Orogeny. The lack of subsequent rotation of this dominant palaeomagnetic signature demonstrates that the majority of deformation pre-dates the Yampi event. This study demonstrates that even where tectonic conditions are similar, that intrusion shape and host rock rheology exert strong control on their deformation style.

Neoproterozoic geodynamic evolution of easternmost Kalahari: Constraints from U-Pb-Hf-O zircon, Sm-Nd isotope and geochemical data from the Schirmacher Oasis, East Antarctica

Late Tonian (ca. 785–760 Ma) granodioritic to granitic orthogneisses of the Schirmacher Oasis region in Dronning Maud Land (DML), East Antarctica, are interpreted as recording an active continental margin setting at the periphery of Kalahari and Rodinia. The rocks probably represent exposures of a significant tectonic province hidden beneath the ice, the erosional remnants of which are recorded as detrital zircons in late Tonian-Cryogenian metasedimentary rocks throughout central and eastern DML, as well as in ice-rafted debris from recent sediments offshore Dronning Maud Land. The orthogneisses have single-stage Sm-Nd model ages of ca. 1.3–1.5 Ga and zircon Hf-signatures (εHft = +2 – +5), indistinguishable from the adjacent Grenville-age basement rocks of easternmost Kalahari. Their geochemistry suggests that they evolved in the late stages of a continental margin magmatic arc and possibly within a roll-back tectonic framework, suggestive of subduction of relatively old oceanic lithosphere. The eastern Kalahari continental arc is one of a number of continental arcs that characterize the western part of the fragmenting Rodinia and document the supercontinent “turning inside out” after its formation at ca. 1000 Ma and a period of relative tectonic quiescence between ca. 900 and 800 Ma.The rocks show an ultra-high temperature metamorphic overprint that was accompanied by syn-tectonic magmatism from ca. 650 to 600 Ma. The high temperature metamorphism is interpreted to relate to back-arc extension that also led to major anorthosite magmatism elsewhere, prior to continental collision in the region. The rocks lack the subsequent widespread high-grade metamorphic overprint at ca. 590–500 Ma which occurs in the adjacent regions due to Himalayan-style continental collision along the East African-Antarctic Orogen during Gondwana assembly.

Coeval high Ba-Sr arc-related and OIB Neoproterozoic rocks linking pre-collisional magmatism of the Ribeira and Araçuaí orogenic belts, SE-Brazil

Two arc systems have been described within the Neoproterozoic Ribeira Belt: the inner continental magmatic arc and the outer intra-ocean magmatic arc. New geochemical and U-Pb and Lu-Hf (LA-ICPMS) data from zircons of the Marceleza-Leopoldina stocks, belonging to the inner magmatic arc system, in the boundary region between the Ribeira and Araçuaí orogenic belts are presented. The intermediate to felsic magmatic rocks have high Ba, Sr, K/Rb, LILE and LREE contents. Low Rb, Th, U, Nb, and very low Y and HREE contents point to their generation in a supra-subduction tectonic scenario, from melting of the asthenospheric wedge above a subducting oceanic plate. Zircon U-Pb (LA-ICPMS) data indicates crystallization ages between ca. 650 and 600 Ma, with high-grade metamorphic overprint at ca. 582 Ma. Tonian inherited zircons were found in the older rocks, suggesting that the development of the continental magmatic arc might have begun earlier. Associated basic (gabbros and monzogabbros) to intermediate (diorites and monzodiorites) rocks are geochemically heterogeneous. Some represent less evolved calc-alkaline basaltic magmas, while relatively more enriched varieties are intraplate basalts with OIB components. Together with lithogeochemical data, Lu-Hf signatures showing TDM model ages between ca. 1.7 to 1.3 Ga, with initial ɛHf values ranging from slightly negative values (−0.9 to −3.75) to strongly negative (up to −10.4) indicate a magmatic arc developed on continental margin environment (i.e., continental-type arc). The studied rocks represent the Rio Doce arc in the studied region and are correlated with other arc segments located from the north, in the Araçuaí Orogen, to the southernmost Ribeira Orogen, defining a c. 1000 km long continental-type magmatic arc. This inner arc system first collided against the São Francisco craton during the Neoproterozoic Brasiliano orogeny around 580 Ma. The geochemical diversity of coeval intermediate to felsic rocks and the variability of associated basic rocks are suggestive of a complex evolution of the subduction zone system. Possible mechanisms for the enrichment in incompatible elements of magmatic products may have involved slab roll-back and subduction of fore-arc rocks, and the influx of ascending asthenosphere associated with slab windows (slab break off, ridge subduction) prior to collision.

Mesoarchaean collision of Kapisilik terrane 3070 Ma juvenile arc rocks and >3600 Ma Isukasia terrane continental crust (Greenland)

The Mesoarchaean Kapisilik and Eoarchaean Isukasia terranes in the Nuuk region of southern West Greenland were tectonically juxtaposed in the Archaean. The north of the Isukasia terrane is distal from the Kapisilik terrane and has only rare growth of ∼2690Ma metamorphic zircon and no 2980–2950Ma metamorphic zircon. The southern part of the Isukasia terrane lies between two ∼2690Ma shear zones, and has locally preserved high pressure granulite facies assemblages and widespread growth of 2980–2950Ma metamorphic zircon and also sporadic growth of ∼2690Ma metamorphic zircon. Within this southern part of the Isukasia terrane there is a folded klippe of mylonitised Mesoarchaean detrital meta-sedimentary rocks (carrying >3600 and ∼3070Ma detrital zircons), mafic and ultramafic rocks, with ∼2970Ma metamorphic zircon overgrowths. South of the Isukasia terrane is the Kapisilik terrane, containing ∼3070Ma arc-related volcanic rocks, gabbro-anorthosites and meta-tonalites, intruded by 2970–2960Ma granites. Zircons of an Ivisârtoq supracrustal belt ∼3075Ma intermediate volcanic rock have initial ɛHf values of +2 to +5 thus are juvenile crustal additions. ∼3070Ma tonalites along the northern edge of the Kapisilik terrane have whole rock positive initial ɛNd values and thus are also juvenile crustal additions. In contrast, igneous zircons in 2960Ma granites intruded into juvenile ∼3075Ma supracrustal rocks of the Kapisilik terrane have initial ɛHf values of −5 to −10, and must have involved the partial melting of >3600Ma Isukasia terrane rocks.The integrated structural and zircon U-Th-Pb-Hf isotopic data show that at 2980–2950Ma the Kapisilik terrane juvenile arc components collided with, and over-rid, the Isukasia terrane. The southern edge of the Isukasia terrane came to lie in the deep crust under the Ivisârtoq supracrustal belt and melted at 2970–2960Ma to produce granites. These granites derived from ancient crust rose into the upper crust, where they intruded the overlying allochthonous juvenile ∼3075Ma Ivisârtoq supracrustal belt arc assemblages. The southern edge of the Isukasia terrane is interpreted as an interior nappe of Eoarchaean basement rocks interfolded with a klippe of Mesoarchaean metasedimentary and mafic/ultramafic rocks, both of which are affected by 2980–2950Ma metamorphism. The mixed Eoarchaean–Mesoarchaean detrital provenance suggests that the klippe could be dismembered components of an accretionary prism or forearc crust. The northern part of the Isukasia terrane is interpreted as foreland, free of 2980–2950Ma high-grade metamorphic overprint. This shows that the Isukasia terrane is not a coherent block, but contains ancient rocks that are parautochthonous or allochthonous to each other, with contrasting later metamorphic history.At ∼2690Ma the crustal architecture arisen from Mesoarchaean collision between an older continental block and an island arc was reworked along intra-crustal shear zones, coeval with amphibolite facies metamorphism. This reworking followed on from major terrane assembly at 2710–2700Ma in the southern part of the Nuuk region, when the Eoarchaean Færingehavn terrane was juxtaposed with 2840–2825Ma arc rocks. Thus the 2980–2950Ma assembly of the Isukasia and Kapisilik terranes is distinct from the later 2710–2700Ma terrane assembly further south in the Nuuk region.

Decompressional Heating of the Mahalapye Complex (Limpopo Belt, Botswana): a Response to Palaeoproterozoic Magmatic Underplating?

Results of petrological and geochronological investigations indicate that during the Palaeoproterozoic the Mahalapye Complex of the Limpopo Belt experienced a prograde decompression-heating P–T path from 650°C/6·5 kbar to 800°C/5 kbar, followed by a retrograde decompression-cooling P–T path to 650°C/3 kbar. This P–T–time path is constrained by detailed petrographic observations in concert with P–T pseudosection calculations applied to Si-undersaturated metapelitic rocks, U–Pb ages between 2046 ± 9 and 2025 ± 18 Ma obtained by in situ dating of monazite, xenotime and zircon grains from metasediments, and a garnet Lu–Hf isochron age of c. 2040 Ma. Additionally, combined U–Pb and Lu–Hf isotope data from magmatic zircon grains indicate that the inferred metamorphic evolution was accompanied by the intrusion of granitic rocks derived from both crust and mantle sources. Diorites, which intruded the Mahalapye Complex between 2061 ± 6 and 2040 ± 18 Ma (i.e. prior to or during the metamorphic peak) are characterized by εHft values between –0·5 and –3·5, whereas granodiorites and granites with intrusion ages between 2039 ± 9 and 2026 ± 10 Ma have significantly lower εHf2·04Ga between –9·4 and –12·6. These data suggest that the diorites were derived predominantly from a Palaeoproterozoic mantle source, whereas the granites were formed by re-melting of Archaean crust. This history is consistent with a model suggesting that the Mahalapye Complex was underplated and intruded by mantle-derived mafic melts between 2·06 and 2·04 Ga and that these melts caused partial melting of the adjacent continental crust. Formation and ascent of voluminous crustal melts caused heat transfer into the middle and upper crust and led to the observed high-grade metamorphic overprint. In a wider geological context, the inferred magmatic underplating at 2·06–2·04 Ga was caused by mantle melting beneath the Mahalapye Complex, which was related to the formation of the Bushveld Igneous Complex and/or to subduction processes during the Kheis–Magondi Orogeny.

Neoarchaean high-grade metamorphism in the Central Zone of the Limpopo Belt (South Africa): Combined petrological and geochronological evidence from the Bulai pluton

The Bulai pluton represents a calc-alkaline magmatic complex of variable deformed charnockites, enderbites and granites, and contains xenoliths of highly deformed metamorphic country rocks. Petrological investigations show that these xenoliths underwent a high-grade metamorphic overprint at peak P–T conditions of 830–860 °C/8–9 kbar followed by a pressure–temperature decrease to 750 °C/5–6 kbar. This P–T path is inferred from the application of P–T pseudosections to six rock samples of distinct bulk composition: three metapelitic garnet–biotite–sillimanite–cordierite–plagioclase–(K-feldspar)–quartz gneisses, two charnoenderbitic garnet–orthopyroxene–biotite–K-feldspar–plagioclase–quartz gneisses and an enderbitic orthopyroxene–biotite–plagioclase–quartz gneiss. The petrological data show that the metapelitic and charnoenderbitic gneisses underwent uplift, cooling and deformation before they were intruded by the Bulai Granite. This relationship is supported by geochronological results obtained by in situ LA-ICP-MS age dating. U–Pb analyses of monazite enclosed in garnet of a charnoenderbite gneiss provide evidence for a high-grade structural-metamorphic–magmatic event at 2644 ± 8 Ma. This age is significantly older than an U–Pb zircon crystallisation age of 2612 ± 7 Ma previously obtained from the surrounding, late-tectonic Bulai Granite. The new dataset indicates that parts of the Limpopo's Central Zone were affected by a Neoarchaean high-grade metamorphic overprint, which was caused by magmatic heat transfer into the lower crust in a ‘dynamic regional contact metamorphic milieu’, which perhaps took place in a magmatic arc setting.

Time constraints for Mesoproterozoic upper amphibolite facies metamorphism in NW Namibia: a multi-isotopic approach

This study presents the chronological evolution of the upper amphibolite facies Orue Unit in NW Namibia. Metasedimentary and meta-igneous rocks of the Orue Unit were investigated using the Pb–Pb stepwise leaching technique on garnet and rutile, U–Pb multi-grain analysis on rutile, Sm–Nd–Lu–Hf leaching technique on garnet, SHRIMP analysis on zircon and Ar–Ar dating on amphibole. Each of these techniques pertains to different processes that occurred before, during, or after the metamorphic peak. Our age data can be integrated with petrological constraints to provide a more complete understanding of the metamorphic cycle. Our pre-peak metamorphic zircon ages, peak metamorphic garnet ages and peak to late peak metamorphic amphibole 39Ar– 40Ar ages bracket the upper amphibolite facies metamorphic event including hydration or dehydration processes into a time span of only ca. 20 Ma. The age data obtained by peak metamorphic mineral analyses cluster around 1340–1320 Ma. Based on age data and field observation, we interpret the upper amphibolite facies metamorphism as a large-scale regional mid-crustal event. Spot analyses of inherited zircon cores obtained by SHRIMP reflect the sedimentary origin of the respective rocks of the Orue Unit and derivation from Palaeoproterozoic protoliths. The metamorphic rocks south of the anorthositic Kunene Intrusive Complex (KIC) have previously been ascribed to the Palaeoproterozoic Epupa Complex at the SW margin of the Congo craton and were thus thought to be older than the Mesoproterozoic KIC. Our data show that the high-grade metamorphic overprint took place 30–50 Ma after emplacement of the KIC. Rutile growth ages of 1248 Ma in one sample reflect fluid activity which seems to be a local phenomenon since there is no other evidence of geological activity throughout the Orue Unit at that time. The rutile ages predate the emplacement of satellite intrusions in that area by 30 Ma and there is no causal relation between these two events.

ISLAND ARC ORIGIN OF THE VARIEGATED FORMATIONS FROM THE EAST RHODOPE, BULGARIA - IMPLICATIONS FOR THE EVOLUTION OF THE RHODOPE MASSIF

The Variegated Formations (VF) of the eastern Rhodope Mountains (SE Bulgaria) form part of the pre-Alpine basement of the region. They are composed of alternating sediments and igneous rocks with a high-grade metamorphic overprint. Numerous ophiolitic slivers are associated with the VF and include metamorphosed peridotites, ultramafic cumulates, and amphibolitized eclogites. The dismembered ophiolites usually form the base of the VF successions. The metasedimentary rock types contain terrigenous materials (metapsammites and quartzites) that frequently alternate with metapelites and marbles. The nature of this sedimentary package, along with the field relations and sedimentary features, reflects its flysch character. The metaigneous rocks of the VF occur either as layers interbedded with the metasediments, or as intrusive bodies that intersect the ultramafic rocks. The principal mineral phases in the metabasites are amphibole + plagioclase + quartz + epidote ± garnet ± chlorite. We calculate temperatures of 630°C to 520°C at 6-2 kbar pressures, indicating moderate amphibolite facies metamorphism. Major rock-forming minerals (amphibole, plagioclase, and garnet) exhibit zoning typical of retrograde P-T conditions. When plotted on tectonic-setting discrimination diagrams, the metabasic rocks of the VF fall mainly in the fields of modern boninites and arc tholeiites. They show low Ti and Zr contents and key elemental ratios of CaO/TiO2, Al2O3/TiO2, Ti/Zr, Ti/Y and Zr/Y, all transitional between island arc tholeiites and boninites. Chondrite-normalized REE patterns reveal the existence of two different trends: U-shaped REE patterns (for the majority of samples) and LREE depleted patterns. Regardless of the existence of these two trends, the [La/Sm]N ratios of the metabasites perfectly coincide with the same ratios for many Cenozoic boninite series. The flysch character of the sedimentary sequences, as well as the clear supra-subduction zone affinities of the igneous rocks, indicates that the VF formed in an oceanic island-arc setting. The boninitic affinities of the meta-igneous rocks indicate possible origin in an immature arc. The character of the VF and its association with the dismembered ophiolite slivers shows the presence of a suture zone. The East Rhodope suture zone distinguishes the VF from the rocks structurally below it, which consist of orthogneisses typical of continental crust. Existing U-Pb zircon data indicate that the orthogneisses are of Variscan age. New U-Pb zircon age data for the VF suggest Late Neoproterozoic ages for some protoliths. Based on regional correlations, the interpretation of the VF as a fossil accretionary prism can be useful for elucidating the structure of the whole Rhodope composite terrane, and for tracing the suture itself to the Central and Western parts of the Massif.

Crustal evolution and age of thermotectonic reworking in the western hinterland of the Trans-Hudson Orogen, northern Saskatchewan

The Hearne province in northern Saskatchewan forms the western hinterland of the Trans-Hudson Orogen. It is a fault-bounded terrane, lying structurally and geographically between the Rae province to its west and the Wathaman batholith and orogenic internides to its east. The Hearne province is comprised primarily of high-grade metamorphic Archean rocks structurally overlain by Archean or Proterozoic metasedimentary rocks and intruded by Paleoproterozoic granites, whereas the Rae terrane on its west boundary is comprised primarily of still higher-grade Archean rocks. The calc-alkaline Wathaman batholith was intruded at ca. 1.855 Ga, and the orogenic internides to the east of it include mostly high-grade mafic and intermediate volcanic rocks crystallized between 1.92 and 1.86 Ga. Terminal collision and orogenesis in the internides have been constrained to be later than 1.82 Ga and earlier than 1.78 Ga. Mineral U/Pb data presented here unequivocally demonstrate that high-grade metamorphism and large-volume anatexis across the southern Hearne province were approximately synchronous with terminal collision and high-grade metamorphism across the Trans-Hudson Orogen. Zircon data for most samples form complex arrays in concordia space, but demonstrate a high-grade metamorphic overprint at 1810–1800 Ma. Zircons typically have inherited components of Archean age, suggesting the presence of extremely old material in the Hearne and a potential Archean connection with the Rae province. Sm–Nd data indicating ca. 3500 Ma model ages for similar parageneses in both terranes further support the proposed Archean connection between Hearne and Rae province and the great antiquity of Hearne crust. Sm/Nd data also elucidate the similarity of the ca. 1.80 Ga anatectic granites of the Hearne to the 1.82 Ga Junction granite, which intruded the Virgin River shear zone and constrains the timing of final motion on that structure. Monazite 207Pb/ 206Pb ages are not older than 1812 Ma or younger than 1769 Ma, and rutile 207Pb/ 206Pb ages and U/Pb ages are tightly constrained at about 1750 Ma. These data, in combination with results of standard thermobarometric analysis, permit determination of post-orogenic cooling and denudation rates. High cooling (7°C/Ma) and denudation (240 m/Ma) rates suggest fault-accomodated uplift during post-orogenic collapse.

Tectonothermal history of the western part of the Limpopo Belt: tectonic models and new perspectives

The westernmost part of the Central Zone of the Limpopo Belt in Botswana is commonly interpreted as a frontal ramp of a westward extruding block or nappe sheet, related to a late Archæan Limpopo Orogeny. In this paper the structural observations from this area are reviewed and new geochronological results presented, in order to test the existing tectonic models. Two tectonometamorphic events can be distinguished. Neoarchæan granulite metamorphism (M 1) is associated with the generation of voluminous granitic bodies at ca 2.6 Ga. The nature of the Archæan tectonometamorphism is difficult to interpret because of a major high-grade metamorphic overprint at 2.0 Ga, which is characterised by a complicated succession of ductile deformational phases. The structural patterns indicate that during the 2.0 Ga evolution of the Limpopo Belt convergence directions changed from north to northwest and west. During the same period metamorphic conditions gradually decreased from upper amphibolite-facies to greenschist-facies. The structural features in the western part of the Central Zone are not compatible with a frontal ramp geometry. Models proposing a single Neoarchæan Limpopo Orogeny do not account for the polyphase tectonometamorphic evolution in the Central Zone and are also rejected. This study suggests that a Proterozoic orogeny involving transpression best explains the geometries encountered in the western Limpopo Belt.

Crustal history of the Rae and Hearne provinces, southwestern Canadian Shield, Saskatchewan: constraints from geochronologic and isotopic data

The Cree Lake Zone (Hearne Province) and Western Granulite domain (Rae Province), in northern Saskatchewan, are contrasting parts of the variably reworked, (?) upper plate western hinterland of the Paleoproterozoic Trans-Hudson Orogen. Their junction is defined by the Virgin River Shear Zone, a part of the more extensive Snowbird “tectonic zone”. Subsurface extension of the ∼ 2.0 Ga Thelon Orogen lies west of the Western Granulite domain. We present UPb zircon ages, SmNd and RbSr isotopic data from the Cree Lake Zone and Western Granulite domain, and from the Junction Granite which intrudes the Virgin River Shear Zone. Data from the Western Granulite domain indicate that prevalent granulite facies metamorphism occurred between ∼ 2.3 and 2.0 Ga, but is imposed on Archean protoliths ∼ 2.8−3.0 Ga old. Low amphibolite facies retrogression, which increases toward the Virgin River Shear Zone, probably postdates 2.1 Ga. Data from the Cree Lake Zone also yield Archean protolith ages as old as 3.0 Ga, and some rocks yield interpreted high-grade metamorphic overprint ages of ∼ 2.3−2.0 Ga . However, two samples yield data indicating “Hudsonian” high-grade metamorphic overprint ∼ 1.87 to 1.81 Ga . The Junction Granite has a UPb zircon age of 1.82 ± 0.30 Ga , but relict zircons and SmNd data show that it was derived by melting of Archean crust. High-grade mid-crustal thermal resetting in the Western Granulite domain may be coeval with the 1.9–2.0 Ga Thelon Orogen, although the data do not preclude an earlier event or events. Similar, ∼ 2.3−2.1 Ga reworking extends into the Cree Lake Zone, but high-grade “Hudsonian” (1.87-1.84 Ga) reworking is also evident in parts of this zone well to the west of the Wollaston domain, and may extend entirely across the zone to the Virgin River Shear Zone. Relative importance of the “Thelon” and “Hudsonian” overprints in Cree Lake Zone is unknown. Data from the Junction Granite and Western Granulite domain suggest that all recorded high-strain events in the Virgin River Shear Zone postdate ∼ 2.3 Ga and are thus unrelated to deep-crustal Archean granulite facies mylonites in the Tantato-Black Lake segment of the Snowbird tectonic zone, north of the Athabasca basin.

Late Proterozoic outer shelf manganese and iron deposits at Otjosondu (Namibia) related to the Damaran oceanic opening

The Otjosondu manganese mining area is located at the eastern exposed extent of the inland branch of the late Proterozoic Damara orogen. In spite of intense deformation and a high-grade metamorphic overprint, the sedimentological interpretation of the manganeserich rocks and their associated lithologies has led to a paleoenvironmental model which gives insight into ore genesis. At least two manganese silicate ore horizons (manganese formations) are associated with banded iron-formations. This sequence is sandwiched between supermature metaquartzarenites interpreted as a shoreface deposit derived from a cratonic basement and deposited under high-energy conditions.Manganese ore varieties and their country rocks have been classified using their texture, mineralogy, and geochemistry and have thus been interpreted in sedimentological and facies terms. The evaluation of the vertical facies sequences, their lateral distribution and their contact relationships renders a spatial reconstruction of the environmental setting for the manganese ores possible. The development of facies from a near-shore clastic regime to manganese-bearing hemipelagic sediments and ultimately to pelagic manganostones and ironstones reflects a zonation on a marine shelf. The facies evolution indicates transgressive conditions on a large scale onto a tectonically stable but subsiding shelf, starting with the transgressive quartzarenites overlying an unconformity to the pre-Damara basement, followed by chemical sediments of the ore-bearing horizon and overlying carbonates, and ending up with pelitic schists. On a smaller scale both transgressive and regressive conditions prevailed during manganese precipitation. Small-scale sedimentary cycles are interpreted as due to glaciogene-influenced fluctuations of the sea level.The source of manganese and iron is proposed to have been situated in the deeper oceanic parts of the associated basin. Initial spreading in the Khomas sea and the formation of oceanic crust is documented by tholeiitic volcanics and associated base metal orebodies representing more proximal manifestations of the exhalative activity. These base metal sulfide deposits are associated with pyrite-bearing graphitic pelagics and this was the first site to partition iron and other base metals from manganese. Upwelling along the shelf edge was involved and additional separation of Mn and Fe was due to a change in Eh conditions associated with a redox interface intersecting with the outer shelf environment. The evolution of the Khomas sea provided the ore-forming solutions and also initiated the subsidence of the passive shelf margin. Fluctuations in sea level were due to the basin development on one hand and glacial-interglacial cycles on the other. The latter fluctuations controlled the amount of terrigenous input into an otherwise pelagic regime.