Événements panafricains dans l'Adrar Souttouf (Sahara marocain)

The Adrar Souttouf belt (Western Sahara) lies in the northern part of the Mauritanide belt and was considered as a part of the Hercynian Mauritanian-Appalachian belt. Recently the presence of a Panafrican protolith (595 Ma) was evidenced by the age of inherited zircons from an eclogitic unit and led to consider complex Panafrican events included in the Hercynian belt, as in the central and southern Mauritanide belt. Our new field survey completed by ten KAr dating (whole rocks and separate minerals) confirmed that multiphased Panafrican formations outcrop within the center of the Adrar Souttouf massif. Diverse events ranged from ca. 1000 to 500 Ma, some of them being indicative of an oceanic type environment and metamorphism. The 1000 Ma metamorphic event is recorded for the first time in the West African craton. A large Hercynian remobilisation is also evidenced. To cite this article: M. Villeneuve et al., C. R. Geoscience 338 (2006).     

Geodynamic evolution of the mauritanide,bassaride, and rokelide orogens (West Africa)

The Mauritanide, Bassaride and Rokelide orogens occur along the western edge of the West African Craton. These record a polyphase tectonothermal evolution, including Pan African I (c. 650 Ma) and Pan African II (c. 550 Ma) events together with local Hercynian (late Paleozoic) overprinting. Pan African I activity is most penetratively recorded in the Bassarides, and resulted from late Proterozoic collision of a western continental structural block. Pan African II orogenesis increases in intensity from the southern Mauritanides through the Bassarides and dominates the Rokelides. This tectonothermal activity appears to reflect collision of the West African and Guyanean Cratons during assembly of Gondwana. Hercynian activity is concentrated along the margin of a western continental block which underwent relative eastward translation during collision of Gondwana and Laurentia. This resulted in extensive thrusting of intracontinental foreland sequences (external nappes) and more ductile imbrications of pre-deformed and metamorphosed late Proterozoic rift sequences and western calc-alkaline igneous successions (internal nappes).     

Proterozoic links between Northeastern Brazil and West Africa: A plate tectonic model based on gravity data

Using the Bullard predrift reconstruction for the southern Atlantic, analysis of the major gravimetric trends of the West African craton (2000 Ma) and the surrounding mobile belts allows us to delineate the upper Proterozoic shield boundary and to put forward an evolutionary model for late Precambrian time.Over the cratonic area gravimetric anomalies are related to a basement framework or to sedimentary basins whose geometry is linked to reactivation of ancient faults. Gravity anomalies and LANDSAT imagery show a NE-SW lineament in West Africa which can be correlated with a variety of geologic features, the largest being the late Precambrian Gourma aulacogen.At the margins of the craton the deformation patterns and gravity signatures associated with the 600 Ma orogeny were largely controlled by the geometry of the stable craton itself. On the eastern edge (Pan-African belt and Brazilian belt east of the São Luis and Amazonian cratons), the distribution of the anomalies and deduced structures can be explained by a continental collision process. On the western and southern edge (Brazilian belt between the São Luis and Amazonian cratons, Rokelide belt, Mauritanide belt) the gravity signature is different, suggesting a large amount of intensely tectonized basic rocks. A limited spreading process followed by an intense compressional episode is suggested.     

Neoproterozoic to Early Ordovician Evolution of the Paleo-Asian Ocean: Implications to the Break-up of Rodinia

The paper reviews and integrates the recent geological and geochronological data, which allow us to recognize three stages of the evolution of the Paleo-Asian Ocean.The opening of the Paleo-Asian Ocean at 970-850 Ma is dated by the Nersin Complex in the Aldan shield, plagiogranites of the Sunuekit massif, enderbites of the Sludinsk Lake area, and passive margin sediments of the Patoma or Baikal series. The initial subduction (850-700 Ma) is marked by volcanic rocks, trondjemite and gabbro of the Sarkhoy island arc series. Collisions of microcontinents with Siberia at 660 to 620 Ma are evidenced by the exhumation of Muya eclogites (650 Ma), formation of migmatites and amphibolites of the Njurundukan belt (635 and 590 Ma), metamorphic units of the Near-Yenisei belt (640-600 Ma), and orogenic molasse (640-620 Ma). The Paleo-Asian Ocean maximally opened at 620-550 Ma, because at that time a long island arc composed of boninite volcanic rocks was formed. Primitive island arcs of that age have been reconstructed in Kazakhstan, Gorny Altai, West and East Sayan, and North Mongolia. HP and UHP rocks formed in two stages at 550-520 and 520-490 Ma. At 550-490 Ma oceanic islands and Gondwana-derived microcontinents (Kokchetav, Tuva-Mongolian, Central Mongolian and others) collided with the Cambrian-early Ordovician island arc of the Siberian continent. As a result, the island-arc system was extensively modified. Collision occurred twice at 550-520 and 520-490 Ma during which many HP and UHP rocks formed. At that time, the new oceans - the Junggar, Kazakhstan and Uralian - with an Ordovician island arc were formed.     

Sur la présence d'une série molassique (de type série pourprée) au Sud-Est de l'Ahaggar (In Guezzam,Ahaggar, Algérie)Presence of molassic series (‘Série pourprée’ type) in the Southeast of the Ahaggar (In Guezzam,Ahaggar, Algeria)

In the western Ahaggar shield, the transition between the Precambrian and the Ordovician units is characterised by thick volcano-sedimentary series (‘Série pourprée’ of the Ahnet). This series, in part of Cambrian age, results from the demolition of the Panafrican belt. Similar series were known in grabens located between the West African craton and the Ahaggar, from the ‘Adrar des Iforas’, in the south, to the Ougarta belt, in the north. We describe in this study a new formation identical to the ‘Série pourprée’ of the Ahnet, but cropping out in the far Southeast of the Ahaggar (In Guezzam). This new datum improves the Panafrican belt configuration. To cite this article: H. Djellit et al., C. R. Geoscience 334 (2002) 789–794.     

A late Neoproterozoic paleomagnetic pole for the Congo craton: Tectonic setting, paleomagnetism and geochronology of the Nola dike swarm (Central African Republic)

New structural, geochronological and paleomagnetic data were obtained on dolerite dikes of the Nola region (Central African Republic) at the northern border of the Congo craton. In this region, metavolcanic successions were thrust southward onto the craton during the Panafrican orogenic events. Our structural data reveal at least two structural klippes south of the present-day limits of the Panafrican nappe suggesting that it has once covered the whole Nola region, promoting the pervasive hydrothermal greenschist metamorphism observed in the underlying cratonic basement and also in the intrusive dolerite dikes. Paleomagnetic measurements revealed a stable dual-polarity low-inclination magnetization component in nine dikes (47 samples), carried by pyrrhotite and magnetite. This component corresponds to a paleopole at 304.8°E and 61.8°S (dp = 5.4, dm = 10.7) graded at Q = 6. Both metamorphism and magnetic resetting were dated by the ⁴⁰Ar/³⁹Ar method on amphibole grains separated from the dikes at 571 ± 6 Ma. The Nola pole is the first well-dated paleomagnetic pole for the Congo craton between 580 and 550 Ma. It marks a sudden change in direction of the Congo craton apparent polar wander path at the waning stages of the Panafrican orogenic events.     

Australian island arcs through time: Geodynamic implications for the Archean and Proterozoic

The operation and extent of modern-style plate tectonics in the Archean and Paleoproterozoic are controversial, although subduction and terrane accretion models have been proposed for most Archean cratons in the world, including both the Yilgarn and Pilbara Cratons of Western Australia. The recognition of ancient island arcs can be used to infer convergent plate margin processes, and in this paper we present evidence for the existence of several intraoceanic island arcs now preserved in Australia. Beginning in the Archean, Australia evolved to its present configuration through the accretion and assembly of several continental blocks, by convergent plate margin processes. In Australia, possibly the best example of an Archean island arc (or primitive continental arc) is preserved within the Mesoarchean (ca. 3130–3112 Ma) Whundo Group in the Sholl Terrane of the West Pilbara Superterrane. Two younger, Neoarchean, island arc terranes, and associated accretion, have also been proposed for the Yilgarn Craton: the Saddleback island arc (ca. 2714–2665 Ma) in the southwest Yilgarn Craton and the Kurnalpi island arc (ca. 2719–2672 Ma) in the eastern Yilgarn Craton. In the early Proterozoic, in the Central Zone of the Halls Creek Orogen, northern Western Australia, the Tickalara Metamorphics (ca. 1865–1850 Ma) have been interpreted to represent an island arc. In the southwest Gawler Craton in South Australia, the St Peter Suite (ca. 1631–1608 Ma), of juvenile I-type calcalkaline tonalite to granodiorite, possibly represents an island arc. In the Musgrave Province in central Australia, age and geochemical constraints are poor due to later overprinting tectonic events, but felsic orthogneisses (ca. 1607–1565 Ma) possibly represent juvenile felsic crust which was emplaced though subduction-related processes into an oceanic island arc. The arcs are volumetrically insignificant, but important, in that they separate much larger tracts of, usually older, continental crust, often of different composition and geological history. The arcs were sutured to continental crust during arc–continent collisional events, which eventually resulted in the assembly of much of present-day Australia. The arcs, thus, indicate lost oceanic crust. The recognition of island arcs in the ancient rock record indicates that subduction processes, similar in many ways to modern day processes at convergent plate margins, were operating on Earth by at least 3100 Ma ago.     

Timing of the Erlangping back‐arc basin in the Qinling orogen,central China and its tectonic significance

Back‐arc basins hold the key in understanding the geodynamics of orogenic processes. The Qinling–Dabie orogenic belt in central China is one of the most important orogenic belts constraining the tectonic framework of eastern Asia. However, its Palaeozoic accretionary processes remain equivocal, mainly derived from the age uncertainty of the back‐arc basin in the Qinling orogen. We carried out zircon U–Pb geochronology for two pyroclastic volcanic rocks intercalated within the Erlangping back‐arc basin basalts. They yield U–Pb ages of 435.8 ± 4.2 Ma and 435.7 ± 3.8 Ma, which precisely constrain the timing of the back‐arc basin opening. The opening of the Erlangping back‐arc basin might have been triggered by the rollback of the Proto‐Tethyan oceanic slab due to the southward migration of arc magmatism at ca. 440 Ma. The Palaeozoic tectonic evolution and orogen‐scale geodynamic processes of the Qinling orogen are thus reconstructed.     

Evolution of the Mayo Kebbi region as revealed by zircon dating: An early (ca. 740 Ma) Pan-African magmatic arc in southwestern Chad

The Mayo Kebbi region in SW Chad is part of the NNE-SSW trending Neoproterozoic Central African Fold Belt (CAFB) and is made up of three calc-alkaline granitoid suites emplaced into a metavolcanic–metasedimentary sequence. The first suite is represented by mafic to intermediate rocks (gabbro-diorite and metadiorite) emplaced between 737 and 723 Ma during early Pan-African convergence. The second consists of the Mayo Kebbi batholith and includes tonalites, trondhjemites and granodiorites, emplaced during several magmatic pulses between 665 and 640 Ma. The third suite includes porphyritic granodiorite and hypersthene monzodiorite dated at ca. 570 Ma. The Mayo Kebbi domain extends southward into Cameroon and is interpreted as a middle Neoproterozoic arc stabilized at ca. 650 Ma. This study also revealed a diachronous evolution between Mayo Kebbi and western Cameroon (e.g., the Poli region). The overall evolution of this part of the CAFB is interpreted as the result of successive development of magmatic arcs, since ca. 740 Ma, and tectonic collage of three different domains (Adamawa-Yade, Mayo Kebbi, and West Cameroon) which, after suturing, were intruded by post-collisional granitoids (<600 Ma).     

Re-assessment of the northern Guinean “Koubia–Lessere unconformity” (KLU): Consequences on the geological correlations throughout West Africa

New radiometric data from recent cartographic projects (2007, 2010) lead to a re-interpretation of the “Koubia–Lessere Unconformity” (KLU). This unconformity is evidenced in the Bassaride Ranges (northern Guinea) and in two western inliers (Tominé and Kemberra). The KLU was previously correlated with the main unconformity (Jbeliat unconformity [JU]) separating the lower “super group 1” from the middle “super group 2” of the large Taoudeni basin. Consequently, the brecciated level (partly glaciogenic) that underlines the “KLU” was correlated with the “tillitic” level located at the base of the super group 2. The latter is linked to the worldwide “Marinhoan” glacial event (650–635 Ma). However, 32 recently published U/Pb age data on detrital or new formed zircons show clearly that the Bassarides groups (ante KLU), deformed and slightly metamorphosed by the Panafrican-II orogen, belong to the super group 2 of the Taoudeni basin. Consequently, the overlying “Koubia–Lessere unconformity” (KLU) cannot be linked to the “Jbeliats unconformity” (JU) that separates the super groups 1 and 2 in the Taoudeni basin. The KLU is younger than the Panafrican-II orogen and probably younger than the last volcanic basaltic flows (492 Ma) outcropping in the Bassaride belt. Consequently, the glaciogenic levels that outcrop in the KLU should be distinguished from the numerous “Marinhoan” glacial levels of West Africa. This new KLU interpretation implies some important changes in the stratigraphic correlations and interpretations all over the West African domain.     

The Rio Capim volcanic–plutonic–sedimentary belt,São Francisco Craton,Brazil: Geological,geochemical and isotopic evidence for oceanic arc accretion during Palaeoproterozoic continental collision

The collision of oceanic arcs with continents is a common plate tectonic process in the Phanerozoic, but its recognition in the Precambrian is hampered by deformation and metamorphism. The Rio Capim volcanic–plutonic–sedimentary belt lies in sharp tectonic contact with Archaean rocks of the Uauá block in the northern part of the São Francisco craton. Field relationships and high-precision geochronology indicated that the Rio Capim basalts, gabbros, diorites, and dacites were emplaced approximately at 2148–2143 Ma, and later intruded by 2128 Ma-old diorite to tonalite plutons. All rocks were metamorphosed under amphibolite to granulite facies conditions mainly between 2080 Ma and 2070 Ma, but deformation may have lasted until about 2040 Ma as estimated from syn-deformation zircon and titanite grains. The association of basalt, andesite, dacite, and their plutonic counterparts, combined with their positive ε_Nd(t) values and incompatible trace element geochemical signatures similar to island arc magmas, support the proposition that the Rio Capim belt was a Palaeoproteorozoic intra-oceanic arc sequence that collided with a continent, of which the Mesoarchaean Uauá block is a remnant. The implications for the regional evolution and metallogenesis are also discussed.     

青藏高原中的古特提斯体制与增生造山作用

青藏高原古特提斯体系的特征表现为古特提斯洋盆中多条状地体的存在,多俯冲、多岛弧增生体系的形成和多地体汇聚、碰撞造山的动力学环境,其构架包括4条代表古特提斯洋壳残片的蛇绿岩或蛇绿混杂岩(昆南-阿尼玛卿蛇绿岩带、金沙江-哀牢山-松马蛇绿岩带、羌中-澜沧江-昌宁-孟连蛇绿岩带和松多蛇绿岩带)、5条火山岩浆岛弧带(布尔汗布达岛弧岩浆带、义敦火山岩浆岛弧带、江达-绿春火山岛弧带、东达山-云县火山岛弧带和左贡-临沧岛弧-碰撞岩浆带)、4个陆块或地体(松潘-甘孜地体、羌北-昌都-思茅地体、羌南-保山地体)、3条洋壳深俯冲形成的高压-超高压变质带(金沙江得荣高压变质带、龙木错-双湖高压变质带、松多高(超)压变质带),以及5条弧前增生楔或增生杂岩(西秦岭增生楔、巴颜喀拉-松潘-甘孜增生楔、金沙江增生楔、双湖-聂荣-吉塘-临沧增生楔、松多增生杂岩)。古特提斯洋盆的俯冲增生造山作用普遍存在于青藏高原古特提斯复合造山体中,构成与多条古特提斯蛇绿岩带(缝合带)相伴随的俯冲增生杂岩带(链)。古特提斯俯冲增生杂岩带包括由弧前强烈变形的沉积增生楔、以及高压变质岩、岛弧岩浆岩、蛇绿岩和外来岩块组成的混杂体,代表在洋盆俯冲过程中的活动陆缘的地壳增生。     

Neoproterozoic glacial deposits from the Araçuaí orogen,Brazil: Age,provenance and correlations with the São Francisco craton and West Congo belt

Glacigenic diamictite successions of the Macaúbas Group are widespread in the western domain of the Araçuaí orogen, east of the São Francisco craton (Brazil). Diamictites also occur on this craton and in the African counterpart of the Araçuaí orogen, the West Congo belt. Detrital zircon grains from the matrix of diamictites and sandstones from the Macaúbas Group were dated by the U–Pb SHRIMP technique. The geochronological study sets the maximum depositional age of the glacial diamictites at 900 Ma, and indicates multiple sources for the Macaúbas basin with ages ranging from 900 to 2800 Ma. Sm–Nd T_DM model ages, determined on whole rock samples, range from 1.8 Ga to 2.5 Ga and get older up-section. Comparison of our data with those from the cratonic area suggest that these glacial deposits can be correlated to the Jequitaí and Carrancas diamictites in the São Francisco craton, and to the Lower Mixtite Formation of the West Congolian Group, exposed in Africa. The 900–1000 Ma source is most probably represented by the Zadinian–Mayumbian volcanic rocks and related granites from the West Congo belt. However, one of the most voluminous sources, with ages in the 1.1–1.3 Ga interval, has not been detected in the São Francisco-Congo craton. Possible sources for these grains could occur elsewhere in Africa, or possibly from within the Brasília Belt in western central Brazil.     

Revised Mesozoic–Cenozoic orogenic architecture and gold metallogeny in the northern Circum-Pacific

The orogenic collages of the northern Circum-Pacific between Japan and Alaska revealed an endowment of about 450 Moz Au in various deposit types and diverse Mesozoic–Cenozoic tectonic settings. The area consists of predominantly late Paleozoic to Cenozoic turbidite to island arc terranes as well as Precambrian cratonic terranes that can be grouped into the Kolyma–Alaska, Kamchatka–Aleutian, and Nipponide collages. The latter can be linked via the Mongol–Okhotsk suture with the late Paleozoic to early Mesozoic terranes in the Mongolides.The early Yanshanian magmatic arc terranes in the fossil Kolyma–Alaska collage host copper–gold porphyry deposits, which have only recently received much attention. Exploration has revealed a large and growing gold endowment of more than 30 Moz Au in some individual deposits, with smaller role of epithermal deposits. This mineralization, formed at 140–125 Ma, is partly coeval with the collisions of magmatic arcs with the passive margin sequences of the Siberian craton and related granitoid magmatism. About 200 Moz of gold is known in the Kolyma–Alaska collage in the Mesozoic orogenic gold deposits and related Quaternary placers. The Central Kolyma, Indigirka, South Verkhoyansk, and North Chukotka subprovinces of the collage revealed an endowment of more than 10 Moz Au each. A similar and coeval event in the Mongolides in relation to the collision between Siberia and North China is largely reflected in still poorly dated intrusion-related gold deposits clustered along the Mongol–Okhotsk suture.The overlapping Yanshanian magmatic arcs in Transbaikalia and northeast China and the Okhotsk–Chukotka magmatic arc in the Russian Far East stitch the Kolyma–Alaska collage with the Paleozoic Central Asian supercollage and adjacent cratons. While the Okhotsk–Chukotka arc reveals a relatively simple and broad oroclinal pattern, the Yanshanian arcs in Mongolia, and NE China form a tightly deformed giant Z-shaped feature that was bent in response to the southward movement of the Siberian craton and northward translation of the Nipponides and North China craton to close the Mongol–Okhotsk suture in late Jurassic to Cretaceous times. The Yanshanian arcs host mostly small to medium-sized 100–70 Ma Au–Ag deposits, with the largest endowment discovered in the Baley district in Transbaikalia and at Kupol in the northern part of the Okhotsk–Chukotka arc. Some intrusion-related gold deposits were formed synchronously with this arc magmatism, with the largest known examples in the Tintina belt in Alaska formed at 104 and 93–91 Ma.The Kamchatka–Aleutian collage is still evolving in front of the westward-subducting Pacific plate. It's late Cretaceous to Paleogene magmatic arc rocks form immature island arc terranes, extending from the Aleutian islands towards the Nipponides via Kamchatka peninsula, Kuril islands and eastern Sakhalin. However, in the Nipponides, the Sikhote–Alin portion of the magmatic arc overlaps the Mesozoic turbidite terranes. The oroclinal pattern of this more than 8000 km-long magmatic arc indicates its westward translation in agreement with the movement of the Pacific plate so that the arc is presently colliding with itself along the island of Sakhalin, a seismically active intraplate lineament and a boundary between the Nipponide and Kamchatka–Aleutian collages. This magmatic arc is usually interpreted to be of intra-oceanic origin, with subsequent docking to Asia from the south; however, presence of the Sea of Okhotsk cratonic terrane between Sakhalin and Kamchatka suggests that it may be rather considered as an external arc system that separated from the rest of Asia due to backarc spreading events, therefore, forming the most external arc system at the active margin with the Pacific plate. The subduction-related events in the collage produced numerous late Mesozoic to Cenozoic 1–3 Moz gold epithermal deposit in Kamchatka and Sikhote–Alin as well as Au–Cu porphyry deposits, with currently largest gold endowment in the pre-Tertiary Pebble Copper deposit in Alaska. The westward translation of the Kamchatka–Aleutian collage might have controlled the emplacement of this porphyry deposit, as well as up to 30 Moz into intrusion-related gold deposits at 70–65 Ma in the Kuskokwim belt, immediately north from the porphyry cluster.     

Deep inside a neoproterozoic intra-oceanic arc: growth,differentiation and exhumation of the Amalaoulaou complex (Gourma,Mali)

We show here that the Amalaoulaou complex, in the Pan-African belt of West Africa (Gourma, Mali), corresponds to the lower and middle sections of a Neoproterozoic intra-oceanic arc. This complex records a 90–130-Ma-long evolution of magmatic inputs and differentiation above a subducting oceanic slab. Early c. 793 Ma-old metagabbros crystallised at lower crustal or uppermost mantle depths (25–30 km) and have geochemical characteristic of high-alumina basalts extracted from a depleted mantle source slightly enriched by slab-derived sedimentary components ((La/Sm)_N < 1; ε_Nd: +5.4–6.2; ⁸⁷Sr/⁸⁶Sr: 0.7027–0.7029). In response to crustal thickening, these mafic rocks were recrystallised into garnet-granulites (850–1,000°C; 10–12 kbar) and subject to local dehydration–melting reactions, forming trondhjemititic leucosomes with garnet–clinopyroxene–rutile residues. Slightly after the granulitic event, the arc root was subject to strong HT shearing during partial exhumation (detachment faults/rifting or thrusting), coeval with the emplacement of spinel- and garnet-pyroxenite dykes crystallised from a high-Mg andesitic parental magma. Quartz and hornblende-gabbros (700–660 Ma) with composition typical of hydrous volcanic rocks from mature arcs ((La/Sm)_N: 0.9–1.8; ε_Nd: +4.6 to +5.2; ⁸⁷Sr/⁸⁶Sr: 0.7028–0.7031) were subsequently emplaced at mid-arc crust levels (~15 km). Trace element and isotopic data indicate that magmas tapped a depleted mantle source significantly more enriched in oceanic sedimentary components (0.2%). Exhumation occurred either in two stages (700–660 and 623 Ma) or in one stage (623 Ma) with a final exhumation of the arc root along cold P-T path (550°C, 6–9 kbar; epidote–amphibolite and greenschist facies conditions) during the main Pan-African collision event (620–580 Ma). The composition of magmas forming the Cryogenian Amalaoulaou arc and the processes leading to intra-arc differentiation are strikingly comparable to those observed in the deep section of exposed Mezosoic oceanic arcs, namely the Kohistan and Talkeetna complex. This evolution of the Amalaoulaou oceanic arc and its accretion towards the West African craton belong to the life and closure of the Pharusian Ocean that eventually led to the formation of the Greater Gondwana supercontinent, a similar story having occurred on the other side of the Sahara with the Mozambique Ocean.     

Geochronology of the Birim Supergroup of the West African craton in the Wa-Bolé region of west-central Ghana: Implications for the stratigraphic framework

The Birim rocks of the West African craton comprise belts of greenschist- to amphibolite-grade gneiss and schist, and subparallel basins of greenschist-grade phyllite of volcaniclastic and epiclastic origin, which were intruded by igneous rocks. The granitoids intruded between 2213 and 2060 Ma and overlap with the volcaniclastic units dated between 2211 and 2064 Ma. The simultaneous occurrence of the magmatic events and irregular distribution of the rock ages hamper the formulation of a stratigraphic succession. SHRIMP spot analyses were done on older cores, crystals and rims from 23 rocks from the Bolé-Wa region in west-central Ghana. The crystallization ages range from 2195 to 2118 Ma, the inherited ages from 2876 to 2130 Ma, and metamorphic ages from 2114 to 2090 Ma. Aided by metamorphic, structural and chemical studies an older geotectonic cycle (2195–2150 Ma), containing the Dole and Guropie Suite and Bolé Group, was established. These units were subjected to several orthogonal and shear deformation events. These events were followed by the contemporaneous Sawla calc-alkaline monzonitic plutonism (2132–2126 Ma) and deposition of the epiclastic Maluwe Group (2137–2125 Ma) of calc-alkaline felsic to tholeiitic volcanic origin. Deformation of the basin beds was succeeded by the intrusion of the Tanina Suite granitoids of 2122–2120 Ma, which, themselves, were deformed prior to 2119 Ma. At 2118 Ma syenite and gabbro intruded along conjugate extension fractures. The gabbro and syenite of the Wakawaka Suite were only affected by three events of brittle strike-slip faulting. The first had significant displacement along NNE- to NE-directed shear zones, while the latter only formed conjugate joint systems with limited transport. Palaeo- to Neoarchaean cores, the oldest yet reported in the Baoulé Mossi domain, are restricted to the gneissic Dole Suite biotite granites. The presence of Dole-, Guropie-, Sawla-, and Tanina-aged older cores and grains in younger rocks reflects continuous reworking of the developing crust during successive magmatic episodes. Zircon rim growth between 2105 and 2090 Ma indicates posttectonic crustal thickening. The low Rb/Sr R_o of ～0.7032 of gabbro and monzonite, and the recycling of the Birim-age crust confirm the primary and juvenile nature of the West African craton after ～2195 Ma. With the various ages obtained, it was possible to link deposition, magmatism and deformation to crustal processes, and establish the cyclic geotectonic evolution in the West Africa craton (individual basin opening and closure) over time as part of an intraoceanic arc–back-arc basin system.     

An evolutionary model of the western churchill province and western margin of the superior province in Canada and the North-Central United States

Regional-scale geophysical information, which includes aeromagnetic, gravity, seismic refraction, multi-channel seismic reflection and electromagnetic induction data, is used to extend our knowledge of the Canadian Shield beneath the Phanerozoic Williston basin of south-central Canada and the north-central United States. A new tectonic map based on this information shows the Proterozoic Flin Flon-Snow Lake and La Ronge-Lynn Lake volcanic island arcs and their associated fore-arc (Kisseynew belt) and back-arc (Reindeer-South Indian Lakes belt) basins wedged between the Archean Superior craton on the east and the Archean parts of the Churchill and Wyoming cratons on the west. Along the western margin of the Superior craton the Thompson nickel belt, including its extension southwards beneath the Williston basin, is interpreted to have been successively the site of continental rifting and rupturing, an evolving continental margin, a continent-volcanic island arc “suture” zone and eventually a continental-scale strike-slip fault. The North American Central Plains electrical conductivity anomaly and closely related seismic low-velocity zones are explained by the presence in the lower crust of buried slices of hydrated oceanic-type material, situated within the southward extension of the Reindeer-South Indian Lakes remnant back-arc basin and adjoining tectonic units. A new plate tectonic model is proposed for this region that involves the rifting and rupturing of the Archean continents and the opening and closing of one or more oceanic basins. This model is shown to be consistent with most of the geological, geophysical and geochronological data that pertains to the Proterozoic evolution of the exposed Shield and similar geophysical data and subsurface geochronological information from further south.     

苏门答腊岛（印尼）岩浆旋回中的成矿作用

本文阐述了东苏门答腊地体和西苏门答腊地体白海西期岩浆旋回以来各自的金属矿产分布特征。海西期东苏门答腊地体以裂陷盆地的层控型铅-锌矿为主,而矽卡岩型Ag、cu和Pb．zn矿化产于西苏门答腊地体。印支期sn矿成矿作用主要与s-型花岗岩类（220～195Ma）侵入和苏门答腊岛中部的梅迪亚苏门答腊深大断裂走滑活动有关。燕山早期铜．金成矿作用为陆缘夭折古裂谷和岛弧环境。燕山晚期为弧-陆碰撞的火山弧的Sn-Au-Ag成矿作用。喜山期发育的岩浆弧Au—Ag成矿与苏门答腊深大断裂活动和巴厘散构造带有关,归因于印度-澳大利亚洋壳斜向俯冲于苏门答腊岛之下。     

Palaeozoic multiphase magmatism at Barleik Mountain,southern West Junggar,Northwest China: implications for tectonic evolution of the West Junggar

The West Junggar lies in the southwest part of the Central Asian Orogenic Belt (CAOB) and consists of Palaeozoic ophiolitic mélanges, island arcs, and accretionary complexes. The Barleik ophiolitic mélange comprises several serpentinite-matrix strips along a NE-striking fault at Barleik Mountain in the southern West Junggar. Several small late Cambrian (509–503 Ma) diorite-trondhjemite plutons cross-cut the ophiolitic mélange. These igneous bodies are deformed and display island arc calc-alkaline affinities. Both the mélange and island arc plutons are uncomfortably covered by Devonian shallow-marine and terrestrial volcano-sedimentary rocks and Carboniferous volcano-sedimentary rocks. Detrital zircons (n = 104) from the Devonian sandstone yield a single age population of 452–517 million years, with a peak age of 474 million years. The Devonian–Carboniferous strata are invaded by an early Carboniferous (327 Ma) granodiorite, late Carboniferous (315–311 Ma) granodiorites, and an early Permian (277 Ma) K-feldspar granite. The early Carboniferous pluton is coeval with subduction-related volcano-sedimentary strata in the central West Junggar, whereas the late Carboniferous–early Permian intrusives are contemporary with widespread post-collisional magmatism in the West Junggar and adjacent regions. They are typically undeformed or only slightly deformed.

Our data reveal that island arc calc-alkaline magmatism occurred at least from middle Cambrian to Late Ordovician time as constrained by igneous and detrital zircon ages. After accretion to another tectonic unit to the south, the ophiolitic mélange and island arc were exposed, eroded, and uncomfortably overlain by the Devonian shallow-marine and terrestrial volcano-sedimentary strata. The early Carboniferous arc-related magmatism might reflect subduction of the Junggar Ocean in the central Junggar. Before the late Carboniferous, the oceanic basins apparently closed in this area. These different tectonic units were stitched together by widespread post-collisional plutons in the West Junggar during the late Carboniferous–Permian. Our data from the southern West Junggar and those from the central and northern West Junggar and surroundings consistently indicate that the southwest part of the CAOB was finally amalgamated before the Permian.     

Diachronous evolution of volcano-sedimentary basins north of the Congo craton: Insights from U–Pb ion microprobe dating of zircons from the Poli, Lom and Yaoundé Groups (Cameroon)

Ion microprobe U–Pb dating of zircons from Neoproterozoic volcano-sedimentary sequences in Cameroon north of the Congo craton is presented. For the Poli basin, the depositional age is constrained between 700–665 Ma; detrital sources comprise ca. 920, 830, 780 and 736 Ma magmatic zircons. In the Lom basin, the depositional age is constrained between 613 and 600 Ma, and detrital sources include Archaean to Palaeoproterozoic, late Mesoproterozoic to early Neoproterozoic (1100–950 Ma), and Neoproterozoic (735, 644 and 613 Ma) zircons. The Yaoundé Group is probably younger than 625 Ma, and detrital sources include Palaeoproterozoic and Neoproterozoic zircons. The depositional age of the Mahan metavolcano-sedimentary sequence is post-820 Ma, and detrital sources include late Mesoproterozoic (1070 Ma) and early Neoproterozoic volcanic rocks (824 Ma). The following conclusions can be made from these data. (1) The three basins evolved during the Pan-African event but are significantly different in age and tectonic setting; the Poli is a pre- to syn-collisional basin developed upon, or in the vicinity of young magmatic arcs; the Lom basin is post-collisional and intracontinental and developed on old crust; the tectono-metamorphic evolution of the Yaoundé Group resulted from rapid tectonic burial and subsequent collision between the Congo craton and the Adamawa–Yade block. (2) Late Mesoproterozoic to early Neoproterozoic inheritance reflects the presence of magmatic event(s) of this age in west–central Africa.