Paleoproterozoic Formation Research Articles

Identifying Areas of High Gold Potential Using Geochemical Prospecting Methods: The Tenado Area in the Boromo Birimian Belt, West-Central Burkina Faso, West Africa

The investigation area is located in the central-western region of Burkina Faso, in a geological environment that incorporates the Paleoproterozoic formations of the Boromo greenstone belt. This zone already has several gold showings and an active zinc mine, but no major gold deposits. The aim of this study is to define target areas with high gold potential for further prospecting. To meet this objective, we mainly used total rock geochemistry and stream sediment geochemistry methods. The combination of stream sediment gold content and litho-geochemical data has enabled us to highlight two areas of gold (Au) and various metal anomalies. The gold content of stream sediments is mainly influenced by that of the parent rock from which they are derived. In fact, strong gold anomalies are more common in clay sediments than in sandy sediments. These clay sediments are derived from basic rocks (basalt) and intermediate rocks (andesite-dacite), while the gold-poor sandy sediments are derived from granitoids. Basic to intermediate rocks are fertile for primary gold. Anomaly zone one (01) in the northern part of the study area (Kelsio, Zoula, Poa and Dyoro) and anomaly zone two (02) in the southern part of the study area (Kwademan, Baguiomo, Bérédo, Bonga and Baganapou) show occurrences of mainly gold and various metals (Zn, Cu, Ni).

Gravity inversion with a binary structure constraint imposed by magnetotelluric data and its application to a Carlin-type gold deposit

With characteristics of low cost and high efficiency, the gravity method is one of the most important means for the deep exploration of mineral resources. However, due to its inherent physical properties, its inversion results provide a limited vertical resolution. To reduce the uncertainty of gravity inversion, it is necessary to impose constraints from other geophysical methods, drilling, or geologic models. Based on the high vertical resolution of the multifrequency magnetotelluric (MT) data, we have developed a novel 2D gravity inversion method with a binary structure constraint imposed by MT data. The guided model is the density model reconstructed by the cross-gradient joint inversion of MT and gravity data, and the target and background regions of the guided model are divided using a fuzzy c-means clustering algorithm. The obtained binary target-background model is used as a structural constraint of the gravity inversion. The gravity inversion changes the density values of the target region rather than the background region, which can significantly reduce the dimensions of the inversion solution equation. The experimental results demonstrate that, even if the resistivity model and the density model indicate an inconsistent structure, the binary structure-constrained gravity inversion can reconstruct the density model with a higher degree of resolution than the traditional gravity inversion and cross-gradient joint inversion. Finally, we apply this algorithm to density modeling for the exploration of Carlin-type gold deposits in the southeast of the Jilin province, China, where the image determines the distribution of deep Paleoproterozoic Formation and high-density deposits, providing important technical support for the exploration of Carlin-type gold deposits.

3D Phase tensor inversion and Joint inversion using impedance and phase tensor components aiming better subsurface mapping and interpretation in Dhanjori basin, Singhbhum craton, eastern India

Dhanjori formation, a significant member of Singhbhum Craton, Eastern India, is well-known for its complex mineralogical character and huge content of Au-mineralization. This 2.1 Ga old Paleoproterozoic formation is composed of siliciclastic metasedimentary rocks interlayered with mafic/ultramafic volcanic rocks. Traces of gold has been spotted by many researchers in the sulfide-hosted mafic-ultramafic volcanic rocks and quartz pebble conglomerate units by different studies. Regional Audio Magnetotelluric (AMT) survey was conducted in 10 kHz–10 Hz frequency range over various lithological units of Dhanjori and adjacent Iron Ore Group. The key challenge faced during the analysis of magnetotelluric (MT) data is due to the presence of near-surface heterogeneities. The amplitude of the MT response gets distorted by galvanic effects which further create difficulties in modelling and inversion. If the effect is not removed in advance, it might lead to erroneous subsurface modelling and interpretations. On the contrary, the phase tensor data does not show any change in values even in the presence of galvanic distortion. Though phase tensors are the computed values from impedance tensor, still the response is free from any shallow level ambiguities unlike impedance data. We have performed 3-D inversion of phase tensor data to generate a conductivity model of Dhanjori basin. The objective was to verify whether the inclusion of phase tensor components in the inversion routine was able to provide some extra information that might add value to the analysis. Model from phase tensor inversion indicates only relative variation in the subsurface conductivity structure due to lack of amplitude information in the data. Hence, we also performed 3D joint inversion including both impedance and phase tensor components for obtaining the final conductivity model of Dhanjori to recover the absolute subsurface conductivity structure unaffected by galvanic distortion. Correlation of the conductivity model with available geoscientific information in Dhanjori and adjacent geological units helped us to relate the conductors to the sulfide mineralization prospect.

About this title - Mineralization and Sustainable Development in the West African Craton: From Field Observations to Modelling

This Special Publication combines results obtained by interdisciplinary groups from numerous academic institutions working on Paleoproterozoic formations to decipher the origins of the main mineralization resources in the West African Craton (WAC) and their impacts on African economic development. Structural, geophysical, sedimentological, stratigraphical, geochemical, petrophysical and mineralogical analyses have been used to highlight the complexities involved in mineralization emplacement and its origin and evolution within the WAC. Fourteen articles contribute to new knowledge in mineral research. They show that the geodynamic evolution of the WAC is complex from one area to another: it involves subduction, collision and obduction during several deformation phases ranging from Birimian (2.3–2.0 Ga) to Pan-African (650–450 Ma) events. Various modelling techniques, when integrated, help in understanding the mechanisms of mineralization emplacement, some of which are still a matter of debate. The challenge for further studies is mitigation for sustainable development that can be appropriately used to minimize such damage.

Mineralization and sustainable development in the West African Craton: from field observations to modelling

Abstract This Special Publication combines results obtained by interdisciplinary groups from numerous academic institutions working on Paleoproterozoic formations to decipher the origins of the main mineralization resources in the West African Craton (WAC) and their impacts on African economic development. Structural, geophysical, sedimentological, stratigraphical, geochemical, petrophysical and mineralogical analyses have been used to highlight the complexities involved in mineralization emplacement and its origin and evolution within the WAC. Fourteen articles, mainly of basic research carried out in the WAC and surrounding areas, contribute to new knowledge in mineral research with updated references. They show that the geodynamic evolution of the WAC is complex from one area to another: it involves subduction, collision and obduction during several deformation phases ranging from Birimian (2.3–2.0 Ga) to Pan-African (650–450 Ma) events. Mineralization is mainly controlled by tectonics within shear zones, orogenic belts, basins and faulting systems occurring in the various corridors. Mineralized fluid circulation is stressed and injected into appropriate formations and precipitate several types of well-documented ore deposits: porphyry, metal-bearing, volcanogenic massive sulfide, sedimentary exhalative and lateritic. Various modelling techniques, when integrated, help in understanding the mechanisms of mineralization emplacement, some of which are still a matter of debate. Traditional and industrial exploitation of ore deposits, mainly gold, may inadvertently cause pollution to water tables and rivers, thus affecting the environment including watersheds. The challenge for further studies is mitigation for sustainable development that can be appropriately used to minimize such damage. The aim of this volume is thus to bring new insights to research activities on ore deposits within the WAC.

Gold mineralization related to Proterozoic cover in the Congo craton (Central African Republic): A consequence of Panafrican events

Despite its high endowments, the gold mining potential of Central African greenstone belts is seemingly underrated when compared to equivalent belts in neighbor West Africa. This is probably because, over the past half century, only minor exploration efforts were ever made in this region. In the southwest of the Central African Republic, near the locality of Moboma, gold-bearing quartz veins are hosted in greenschist facies Paleoproterozoic formations that are intruded by numerous dolerite dykes. These rocks occur in a strongly deformed terrane that marks the front of the Panafrican Oubanguides nappe, developed during an E-W regional shortening. Presence of multiple banding indicates repeated reactivation of the quartz veins and circulation of H2O–CO2–NaCl fluids, similar to those characterizing typical orogenic gold-bearing settings. Fluid inclusion petrography and microthermometry permitted to distinguish two different fluids: one, aqueous-carbonic, circulated at relatively high temperature (Th = 250–270 °C) and was responsible for the main stage of Au deposition; a second fluid of low-salinity trapped in microcracks and in a late quartz generation, interpreted as meteoric, precipitated silver-poor native gold. At a later stage, supergene alteration caused the formation of discrete gold nuggets in the upper levels of the mineralization. The competent nature of the dolerite dykes and quartzite intersected by these quartz veins contributed to focus rock fracturing, localizing fluid circulation and the mineralization. The alteration assemblage developed in the veins is equivalent to that found in the dolerite dykes, which was dated at 571 Ma, thus pointing to a Panafrican age for the mineralization at Moboma.

Isotope-geochronological (SHRIMP-RG) studies of zircon from muscovite-tourmaline granites of the Kolmozero-Voronya greenstone belt: insights for sources of rare metal pegmatites

The article provides data on the U-Pb isotope (SHRIMP RG) study of tourmaline-muscovite granites of the Kolmozero-Voronya greenstone belt. The tourmaline-muscovite granites formed in the time span of 2451±60 Ma, which corresponds with the previously obtained age of tantalite from rare metal pegmatites of the Vasin-Myl’k deposit. The obtained age indicates the Paleoproterozoic formation stage of granites and, probably, associated rare metal pegmatites.

Vanadium Mineralization in the Kola Region, Fennoscandian Shield

In the northern Fennoscandian Shield, vanadium mineralization occurs in the Paleoproterozoic Pechenga–Imandra-Varzuga (PIV) riftogenic structure. It is localized in sulfide ores hosted by sheared basic and ultrabasic metavolcanics in the Pyrrhotite Ravine and Bragino areas and was formed at the latest stages of the Lapland–Kola orogeny 1.90–1.86 Ga ago. An additional formation of vanadium minerals was derived from contact metamorphism and metasomatism produced by the Devonian Khibiny alkaline massif in the Pyrrhotite Ravine area. Vanadium forms its own rare minerals (karelianite, coulsonite, kyzylkumite, goldmanite, mukhinite, etc.), as well as occurring as an isomorphic admixture in rutile, ilmenite, crichtonite group, micas, chlorites, and other minerals. Vanadium is inferred to have originated from two sources: (1) basic and ultrabasic volcanics initially enriched in vanadium; and (2) metasomatizing fluids that circulated along shear zones. The crystallization of vanadium and vanadium-bearing minerals was accompanied by chromium and scandium mineralization. Vanadium mineralization in Paleoproterozoic formations throughout the world is briefly considered. The simultaneous development of vanadium, chromium and scandium mineralizations is a unique feature of the Kola sulfide ores. In other regions, sulfide ores contain only two of these three mineralizations produced by one ore-forming process.

Paleoproterozoic Rock Metamorphism of the Nyartin Complex, the Subpolar Urals

In the Urals, Archean and Paleoproterozoic formations are revealed as a part of some polymetamorphic complexes which crop out onto the day surface in relatively small tectonic blocks (up to the first thousand square kilometres in area). Belonging of rocks composing polymetamorphic complexes within the Western tectonic zone of the Urals, which located west of the Main Ural Fault, to the Archean-Paleoproterozoic section is most reliable. These complexes are interpreted as fragments of the heterogeneous crystalline basement of the Ural part of the East European craton. Nevertheless, belonging of only two South Ural complexes: the Taratash and Aleksandrov to the Archean-Paleoproterozoic section causes no special disputes due to the relatively weak geochronological study of the polymetamorphic formations. They are framed by the weakly metamorphosed Lower Riphean deposits; reliable geochronological data are obtained on them, which unambiguously indicate the early Precambrian age of rock metamorphism. The existing Early Precambrian dates (with the prevalence of Late Precambrian and Paleozoic ages) for other Ural polymetamorphic complexes are interpreted in different ways. Therefore, their attachment to the Lower Precambrian section is disputed by many researchers. Taking into account the already available data and the first results of mass U-Pb dating of metamorphogenic zircons from gneisses of the Nyartin polymetamorphic complex in the Subpolar Urals, the Paleoproterozoic age of the earliest stage of rock metamorphism is substantiated in the paper. This gives grounds for the conclusion that this geological object belongs, as well as the Taratash and Alexandrov complexes in the Southern Urals, to the Lower Precambrian formations involved into the Uralides’ structure.

ARCHEAN AND PALEOPROTEROZOIC HISTORY OF ROCK METAMORPHISM IN THE URALS CRUSTAL SEGMENT

In the Urals, Archean and Paleoproterozoic formations were found in several polymetamorphic complexes which crop out as relatively small tectonic blocks (within the first thousands sq. km in area). It is most likely that the rocks forming the polymetamorphic complexes located within the paleocontinental Ural area extending to the west of the Main Uralian Fault belong to the Archean-Paleoproterozoic section. These complexes are interpreted as fragments of the crystalline basement of the Ural part of the East European Craton, which are included in the structure of the Uralides. Two stages of the granulite facies metamorphism - earlier Neoarchean and later Paleoproterozoic - were established in the rocks of the polymetamorphic complexes in the paleocontinental (and partially paleooceanic) Ural region. High-pressure mineral parageneses are the products of the earliest endogenous rock transformational processes in eclogite-bearing complexes, where high-pressure metamorphism is apparently complementary to the Paleoproterozoic granulite metamorphism.

Provenance and depositional age of Paleoproterozoic metasedimentary rocks in the Kuluketage Block, northern Tarim Craton: Implications for tectonic setting and crustal growth

Precambrian geology of the Tarim Craton, especially for the crustal evolution, is poorly constrained. Paleoproterozoic metasedimentary rocks are extensively exposed in the Kuluketage Block, northern Tarim Craton, and thus have recorded abundant geological information of its tectonic history. Supracrustal rocks within the Archean basement (Tuoge Complex) and the Paleoproterozoic Xingditage Group were collected from the Kuluketage Block for geochemical study and zircon U–Pb dating. The Tuoge supracrustal rocks have low CIA values between 45 and 63, indicating weak chemical weathering. Although the Xingditage Group shows a large variation of CIA values (37–67), their low average value suggests that they underwent similar weathering to the rocks from the basement. This similarity is also supported by their respective PIA values (mostly 43–57 for the basement rocks, and mostly 37–60 for the Xingditage Group). The high ICV values (mostly >1) of both rock suites indicate an immature source that lacked alumina-rich minerals. Although these metasedimentary rocks show different REE patterns, they are all enriched in light rare earth element (LREE) and display relatively flat heavy rare earth element (HREE) patterns. The low values of ferromagnesian (e.g. Cr, Co and Ni) and high field strength elements (e.g. Zr, Hf, Nb, Ta and Y) are similar to those of graywackes from active continental margins. Therefore, the Kuluketage Block most likely evolved in an active continental margin setting during the late Paleoproterozoic. Detrital zircons from a slate sample of the Tuoge supracrustal rocks are dominated by igneous zircons, which mostly yield Paleoproterozoic 207Pb/206Pb ages between 1.72Ga and 2.08Ga. Other igneous grains give Paleoproterozoic 207Pb/206Pb ages at ∼2.2Ga and ∼2.32Ga. All these igneous zircons show large variations of ɛHf(t) values mostly between −17 and −3, and have Neoarchean crustal model ages (TDMc=2.6–3.7Ga). Schist and quartzite samples of the upper Xingditage Group show similar age spectra to the slate and contain a major igneous zircon population with Paleoproterozoic formation ages (mostly 1.8–2.1Ga) and Paleo- to Mesoarchean TDMc ages (2.7–3.1Ga). A schist sample of the lower Xingditage Group shows a much different age spectrum. Detrital zircons from this sample are dominated by Neoarchean grains (2.5–2.7Ga) with Paleoarchean TDMc ages (3.14–3.51Ga) and two of them yield the youngest ages at 1.94 and 2.09Ga. The youngest zircons from these samples, therefore, define the maximum depositional ages at 1.63, 1.94 and 1.77Ga for the Tuoge supracrustal rocks, the upper Xingditage Group and the lower Xingditage Group, respectively. On basis of these age spectra, we suggest that the Tuoge supracrustal rocks are equivalents of the upper Xingditage Group, instead of parts of the Archean basement. Overall, the Hf isotopic compositions of these detrital zircons indicate three early Precambrian crustal growth events in the Tarim Craton, which occurred in the late Paleoarchean, the early Neoarchean and at the beginning of the Paleoproterozoic.

Bryansk-Kursk-Voronezh intra-continental collisional orogen (East European craton)

The article presents new data on the deep crustal structure, origin and evolution of the Bryansk-Kursk-Voronezh orogen in the south-eastern segment of the East European craton; it is composed of the Paleoproterozoic formations and areas of reworked Archean crust. The purpose of this paper is the development and improvement of ideas on intra-continental orogens. The deep structure of the orogen is presented by the 3D model based on results of geological mapping of the Precambrian basement and interpretations of geophysical fields and seismic images of the crust along geotraverse 1-EB and profile DOBRE. It is established that the orogen originated with riftogenic extension of the crust at 2.6–2.5 Ga, that was repeated at 2.2–2.1 Ga, and formation of wide depressions that were efficiently filled in with volcanosedimentary layers including BIF, which accumulation was followed by high-temperature up to granulite facies metamorphism. Suprasubduction magmatism took place at 2.10–2.08 Ga and resulted in formation of the Lipetsk-Losevka volcano-plutonic complex. The active margin was completely formed at about 2.05 Ga. The short duration of subduction-related magmatic activity and the lack of relics of the oceanic lithosphere suggest short-term and spatially limited developing of the oceanic structure. The tectonothermal activity of collisional and postcollision stages was expressed in emplacement of alkaline ultramafic (2.1–2.0 Ga) and gabbro-syenite (1.8–1.7 Ga) complexes. It is difficult or impossible to explain specific features of the structure and evolution of the orogen in framework of the model of the Cordilleras type accretionary orogen. Mafic-ultramafic magmatism and indications of intensive heating of the crust suggest a special role of plume type processes that provided for influx of mantle heat and juvenile mantle derived matter.

The Paleoproterozoic Waterberg Group, South Africa: Provenance and its relation to the timing of the Limpopo orogeny

Four Paleoproterozoic formations of the Waterberg Group in South Africa are composed of coarse clastic detritus derived from erosion of the Limpopo Belt. Timing of the Limpopo orogeny, an event involving the collision of the Kaapvaal and Zimbabwe cratons, has long been a contentious issue. The results of point counting, major and trace element geochemistry, and U–Pb detrital zircon geochronology indicate that the Waterberg sedimentary formations in the study area were primarily sourced by siliceous (rifted margin) sedimentary and minor mafic volcanic rocks of the Archean Beit Bridge Complex, Limpopo Central Zone.The volumetrically predominant beige/brown sandstones in the four studied formations are quartz-rich with average QFR ratios of 80:7:13 (Blouberg), 70:19:11 (Setlaole), 88:5:7 (Makgabeng), and 89:3:8 (Mogalakwena). Chert and arenite account for >90% of lithic fragments in all formations, with minor siliceous gneiss fragments. Although all formations are silica enriched, the Makgabeng dune samples produce extremely high SiO2 abundances (92–99wt%), which are attributed to the presence of silica cement and quartz within rock fragments. Geochemically, the stratigraphically highest Mogalakwena Formation is unique with elevated Ti and Zr values, and intra-formational differences in REE patterns; the latter feature is consistent with a mixed provenance. Volumetrically minor green and purple sandstones in the Waterberg formations contain the greatest Cr, Ni, Ti, and V abundances, which supports localized derivation from a mafic or ultramafic source. Chemical index of alteration (CIA) values range from 57 to 89, which could indicate significant chemical weathering of the source rocks, but a plot of Th/Sc versus Zr/Sc illustrates that the sandstones have undergone recycling, which was probably responsible for enrichments in Al2O3 relative to Na2O3, CaO and K2O. Combinations of well rounded and subangular quartz grains support a recycled origin. Detrital zircons in the lowermost Blouberg Formation produced a wide array of ages ranging from ca. 3379 to 2043Ma. The youngest peak at 2046Ma is also the largest, and represents the maximum age of deposition for the formation. Additional peaks at 3281Ma, 3330Ma, and 3379Ma are consistent with ages previously determined from the Beit Bridge Complex, whereas peaks at 2578Ma and 2649Ma coincide with ages determined from gneisses of the Limpopo Central Zone.Derivation of detritus from the Beit Bridge Complex is directly indicated by sedimentary and siliceous gneiss fragments in the sandstones, subrounded quartz grains suggestive of relatively short transport distances, green and purple sandstone drapes derived from mafic volcanic units, and paleocurrent patterns consistent with south to southwest flow directions. Therefore the timing of deposition of the Blouberg Formation (2046Ma) equates to the end stages of the Limpopo orogeny. This negates previous suggestions that the Limpopo orogeny occurred only during the Neoarchean.

Structural Evolution of a Precambrian Segment: Example of the Paleoproterozoic Formations of the Mako Belt (Eastern Senegal, West Africa)

The western part of the Kedougou Kenieba Inlier is located in the West African Craton. It consists of paleoproterozoic NE-trending elongate belts (subprovinces) of metavolcanic and granitic rocks that alternate with metasedimentary belts. Major linear fault such as the MTZ which also approximate a north-easterly trend form the eastern boundaries. The field observations and geophysics analyses were completed by a microscopic study. Based on these data we define across this region four lithostructural domains from east to west. The western domain is structurally complex. The rocks of this domain have been subjected to a complex history of polyphase deformation and metamorphism. The structural analyse allow us to distinguished three deformation events. The deformation results in the formation of D1 thrust tectonic and D2 and D3 transcurrent tectonic. The structural evolution of the Mako Belt is characterized by deformation dominated by the intrusion of large TTG batholiths (D1) followed by basins formation and transpression accommodating oblique convergence and collision (D2 and D3). The change from thrusting (D1 deformation to transcurrent motion (D2 and D3) is recorded in the marginal basin of the central domain and in Tinkoto pull apart basin. The timing of these basins indicates a diachronous evolution. Deformation styles within the basin are compatible with a dextral transpression which terminated at ca 2090 Ma. Small extensional basins formed over the rocks of the Mako Belt are filled with continental detrital sedimentary rocks that show weak foliation and active felsic volcanism. We suggest that the sinistral transpressive tectonic associated with oblique subduction may have generated the pull-apart basin and subaqueous volcanism. In part these features are now related to terrain accretion, thrusting and strike slip movement during oblique convergence. The inversion of the large scale structural evolution from thrusting to strike slip is common to modern orogenies.

Nature and evolution of the Slave Province subcontinental lithospheric mantleThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.

A review of the ages determined for mantle material (xenoliths and xenocrysts entrained in kimberlite) derived from the Slave Province continental lithospheric mantle (CLM) indicates that a portion of the central Slave lithosphere may be ancient (3.5–3.3 Ga) harzburgite, but the majority of this lithosphere is much younger (2.9–2.0 Ga). Relying on the most robust chronometers, the majority of Slave lithosphere peridotite formed in the Neoarchean (peak at 2.75 Ga), whereas the majority of eclogite formed in the Paleoproterozoic (2.2–2.0 Ga). The northern Slave lithosphere contains evidence of peridotite xenolith ages that young with depth. The Paleoproterozoic eclogites may have multiple origins including remnants of subducted oceanic crust and mafic–ultramafic magmas that crystallized at great depth (100–200 km). Re–Os studies of sulfide inclusions in diamond indicate that some diamonds currently mined are ancient (∼3.5 Ga), but many Slave diamonds could be considerably younger. Most eclogitic diamonds recovered from the Slave craton are interpreted to be related to the formation of Paleoproterozoic eclogite. There is abundant evidence for Mesoproterozoic modification of the Slave lithosphere (e.g., heating by magma emplacement at great depth and metasomatism) and possible new addition to the lithosphere at that time. The Canadian Slave and African Kaapvaal lithospheres have similar peaks in cratonic peridotite formation ages at about 2.8 Ga, indicating that a large portion of the CLM in these two cratons formed and stabilized in the Neoarchean. One difference is that the Slave peridotites are much less enriched in SiO2, possibly reflecting the more metasomatized nature of the Kaapvaal CLM. The dominance of Paleoproterozoic formation ages for Slave mantle eclogites contrasts with the dominance of Neoarchean formation ages for Kaapvaal mantle eclogites.

The oldest known rocks in south–western China: SHRIMP U–Pb magmatic crystallisation age and detrital provenance analysis of the Paleoproterozoic Dahongshan Group

The Paleoproterozoic Dahongshan Group outcrops along the present day southwest margin of the Yangtze Craton. It is the oldest rock found in the region and forms the basement for the late Mesoproterozoic sedimentary successions. The Dahongshan Group is a sequence of volcanic and sedimentary rocks which have undergone structural deformation and metamorphism to upper greenschist–lower amphibolite facies. Some of the original sedimentary features, such as unimodal cross-bedding in quartzite and rounded conglomerate clasts, are still preserved, suggesting deposition in a fluvial environment. Abundant carbonate rocks, also found within the sedimentary sequence, indicate periods of shallow marine or lagoonal conditions. A weighted mean SHRIMP 206Pb/ 207Pb age of 1675 ± 8 Ma for a tuffaceous schist unit confirms the Paleoproterozoic formation age of the group. SHRIMP and LAM ICP-MS U–Pb analysis of detrital zircon grains from metasedimentary units indicate a late Archean to Paleoproterozoic (ca. 2780–1860 Ma) source region. The age spectrum of the detrital zircons is similar to that of the late Archean and early Paleoproterozoic basement rocks previously identified in the Yangtze Craton, but includes a population of Paleoproterozoic grains (ca. 2400–2100 Ma) which have no known source region in the exposed Yangtze Craton.

The provenance of fertile off-craton lithospheric mantle: Sr–Nd isotope and chemical composition of garnet and spinel peridotite xenoliths from Vitim, Siberia

Nd and Sr isotope compositions were obtained for pure garnet and pyroxene separates from coarse peridotite xenoliths from the Vitim volcanic province east of Lake Baikal. Internal Sm–Nd isochrons give ages of 24–42 Ma. These dates are older than the eruption age for the host picrite tuff (∼16 Ma) indicating that garnet and clinopyroxene may not have been in isotopic equilibrium in the mantle (at 19–23 kbar, 990–1070 °C). In spite of their fertile major element chemistry, many peridotites have extremely depleted Nd and Sr isotope compositions (calculated from mineral analyses: ɛ Nd T =+14 to +27; 87Sr/ 86Sr=0.7017–0.7028) and therefore plot outside the MORB field in the Sr–Nd isotope diagram. Sm–Nd and Rb–Sr model ages for unmetasomatised peridotites are concordant for each sample within ±0.4 Ga and suggest a depletion event of a primitive mantle ∼2 Gy ago, also consistent with Re–Os data on the same xenolith suite [Pearson, D.G., Irvine, G.J., Ionov, D.A., Boyd, F.R., Dreibus, G.E., 2004. Re–Os isotope systematics and Platinum Group Element fractionation during mantle melt extraction: a study of peridotite xenoliths from N. Lesotho and S. Namibian kimberlites, the Vitim volcanic field and massif peridotites from Beni Bousera. Chem. Geol. 208, 29–59]. This suggests that the mantle depletion event was roughly matched with formation of the mature continental crust in the region (∼2.0 Ga according to U–Pb and Sm–Nd dating) and implies that the mantle peridotites have been attached to the continental lithosphere since that time and survived several major tectono-thermal events. The Paleoproterozoic formation age for the fairly fertile Vitim mantle domain indicates that average lithospheric mantle compositions cannot be used to assess lithospheric formation ages in continental off-craton mantle.

A new metamorphic constraint for the Eburnean orogeny from Paleoproterozoic formations of the Man shield (Aribinda and Tampelga countries, Burkina Faso)

Abstract Progressive metamorphic zones developed in pelites around the Aribinda granite, Burkina Faso, indicate contact metamorphism at 5–6 kbar. Biotite, chloritoid, staurolite–kyanite zones are distributed concentrically around the Aribinda pluton on its northern margin but are displaced by later thrust faults in the south of the Aribinda area and in the nearly Tampelga area. These metamorphic and structural characteristics provide new constraints for the geodynamical evolution of the Eburnean orogeny in the North part of the Baoule-Mossi domain. They show the leading part of the granitoid emplacement in the thermic structuration of the greenstone belts and the post-metamorphic character of the tangential tectonic. In Burkina Faso, the Eburnean orogeny underwent: (1) initial greenschist-facies metamorphism; (2) emplacement of granitoids at mid- deep-crustal levels leading to unusual medium pressure metamorphism; (3) later thrusting.