Michael T.D. Wingate

Models of Rodinia assembly and fragmentation

Abstract Amongst existing palaeogeographic models of the Rodinia supercontinent, or portions thereof, arguments have focused upon geological relations or palaeomagnetic results, but rarely both. A new model of Rodinia is proposed, integrating the most recent palaeomagnetic data with current stratigraphic, geochronological and tectonic constraints from around the world. This new model differs from its predecessors in five major aspects: cratonic Australia is positioned in the recently proposed AUSMEX fit against Laurentia; East Gondwanaland is divided among several blocks; the Congo-São Francisco and India-Rayner Cratons are positioned independently from Rodinia; Siberia is reconstructed against northern Laurentia, although in a different position than in all previous models; and Kalahari-Dronning Maud Land is connected with Western Australia. The proposed Rodinia palaeogeography is meant to serve as a working hypothesis for future refinements.

Early Neoproterozoic magmatism (1000–910 Ma) of the Zadinian and Mayumbian Groups (Bas-Congo): onset of Rodinia rifting at the western edge of the Congo craton

Abstract New ion microprobe U–Pb zircon ages, as well as some geochemical and isotopic analyses, for key igneous units within the central part of the West Congo belt are integrated with geological information to provide an updated geological map (1:1 000 000 scale) and a synthetic type cross-section of the belt, as well as an updated lithostratigraphic chart of the ‘West Congo Supergroup’. Three Neoproterozoic units are recognised, from oldest to youngest, the Zadinian, Mayumbian and West Congolian ‘Groups’. Emplacement of early Zadinian peralkaline granites (Noqui massif, 999±7 Ma) and rhyolites (Palabala) was accompanied by incipient rift sedimentation, corresponding to the onset of transtensional rifting, preferentially in a transverse mega-shear setting along the margin of the Congo craton. Subsequent upper Zadinian magmatism produced a thick (1600–2400 m) basaltic sequence (Gangila), which has geochemical characteristics typical of continental flood basalts (CFBs). The Gangila basalts, associated with major pull-apart rifting, were followed rapidly by the 3000–4000 m thick Mayumbian rhyolitic lavas, dated at 920±8 Ma at the base and 912±7 Ma at the top. The felsic lavas are intruded by coeval high-level (micro)granites, whose emplacement is dated at 924±25 Ma (Mativa body) and at 917±14 Ma (Bata Kimenga body) in the Lufu massif. This voluminous bimodal magmatic province is similar to the Parana and Deccan provinces, and shares similar lithospheric sources. It corresponds to the initial, transtensional rifting stage along the western edge of the Congo craton before Rodinia breakup. The early Neoproterozoic rocks of the West Congo Supergroup rest unconformably on a ca. 2.1 Ga Palaeoproterozoic polycyclic basement (Kimezian Supergroup). No Mesoproterozoic events are recorded in the area. Following the initial, transtensional early Neoproterozoic (ca. 1000–910 Ma) rifting stage, Bas-Congo behaved as a passive margin of the Congo craton, as indicated by deposition of ca. 4000 m of Neoproterozoic (pre-Pan-African) platform sediments (lower part of West Congolian Group) preceding ca. 2000 m of Pan-African molasse-type sediments (upper part of West Congolian Group). In the late Neoproterozoic, during Pan-African assembly of Gondwanaland, the Bas-Congo passive margin, which was largely protected by thick lithosphere of the Congo craton, collided with a western active margin to form the Brasiliano-Aracuai belt, now preserved adjacent to the Sao Francisco craton of Brazil. This collision, which ended in Bas-Congo at ca. 566 Ma, induced relatively limited effects in the West Congo belt, which experienced no late Neoproterozoic magmatic activity.

Warakurna large igneous province: A new Mesoproterozoic large igneous province in west-central Australia

Coeval mafic igneous rocks emplaced rapidly over;1.5 3 10 6 km 2 in western and central Australia represent the erosional rem- nants of a late Mesoproterozoic large igneous province, named here the Warakurna large igneous province. SHRIMP U-Pb dating of rocks separated by as much as 1500 km indicates that the main episode of magmatism occurred between 1078 and ca. 1070 Ma. The Warakurna large igneous province includes layered mafic- ultramafic intrusions and mafic to felsic volcanic rocks and dikes in central Australia, a 1000-km-long mafic sill province in Western Australia, and several swarms of mafic dikes. The large areal ex- tent and short duration imply emplacement above a mantle-plume head. Despite their wide separation, the mafic rocks have similar mid-oceanic-ridge basalt-normalized trace element patterns and rare earth element characteristics. West-directed paleocurrents, westward-radiating dike swarms, and the occurrence of high-Mg rocks indicate that the center of the plume head was located be- neath central Australia. Other late Mesoproterozoic large igneous provinces, in the Laurentia and Kalahari cratons, appear to be significantly older than the Warakurna large igneous province in Australia and thus are unlikely to be related to the same mantle- plume head.

Lithostratigraphic and geochronological constraints on the evolution of the Central Asian Orogenic Belt in SW Mongolia: Early Paleozoic rifting followed by late Paleozoic accretion

New SHRIMP U-Pb and evaporation Pb-Pb zircon ages, together with a revision of the lithostratigraphy of “suspect” terranes in SW Mongolia, suggest that the collage of continental and oceanic units in this region resulted from recurrent magmatic reworking and deformation of Silurian–early Devonian proximal and distal passive margin sequences of the Paleo-Asian Ocean. The zircon ages from early Ordovician volcaniclastic rocks and syntectonic felsic dikes reveal an heterogeneous stretching of the Precambrian Dzabkhan microcontinent (Lake Zone basement) during the Ordovician, followed by the development of a carbonate platform on a proximal margin (Gobi-Altai Zone), serpentinite breccias and Silurian chert sequences on a distal margin and possibly also the formation of oceanic crust. The assumed early Neoproterozoic South Gobi continental zone may either represent an allochthonous block detached from Dzabkhan or, less likely, the conjugate margin of a Paleo-Asian continental rift. Early Devonian volcanism subsequently affected both types of margins with back-arc spreading centers and arcs located in the core of the future Trans-Altai Zone. During the late Devonian to early Carboniferous a Japan-type magmatic arc developed on the previously stretched continental crust of the Gobi-Altai Zone. This event was associated with shortening of the entire domain, exhumation of the deep arc core and formation of intramontane basins with Devonian and Carboniferous detrital zircons of the adjacent Lake Zone continent. Clastic, flysch-type sedimentation occurred on the former distal margin and in oceanic areas. During this early Carboniferous contraction event the continental and oceanic units were imbricated and accreted to the continent in the north. Subsequently, late Carboniferous volcanic arc sequences and a Japan-type magmatic arc developed on the Trans-Altai oceanic crust and the southern South Gobi Zone, respectively. Finally, a Permian thermal event was localized in the Gobi-Altai–Lake Zone contact domain and was responsible for formation of Permian grabens, bimodal volcanism and substantial melting of the accreted crust.

Crystal orientation effects during ion microprobe U–Pb analysis of baddeleyite

Abstract We show that 206 Pb/ 238 U ratios measured in baddeleyite by ion microprobe vary significantly (up to ±10% or more) and systematically with the relative orientation of the baddeleyite crystal structure and the primary ion beam. Low dispersion between multiple analyses of single crystals demonstrates that it is possible to measure undispersed 206 Pb/ 238 U ratios in multi-grain baddeleyite samples if all material is in a single orientation. Owing to the small size of most crystals and their ubiquitous polysynthetic {100} twinning, however, it is extremely difficult to arrange a reasonable number of crystals in identical orientations for analysis. These orientation effects constitute an intractable problem, and place unacceptable limits on the accuracy and precision that can be obtained for 206 Pb/ 238 U in baddeleyite by ion microprobe techniques. Comparisons with isotope dilution measurements indicate that any Pb isotope discrimination during analysis of baddeleyite by ion microprobe is negligible, and there is no evidence that Pb isotope ratios vary with crystal orientation. Although the precision of 207 Pb/ 206 Pb ages decreases for younger samples, rocks younger than 1 Ga can be dated precisely if the baddeleyite is enriched sufficiently in uranium and if enough analyses are performed. We speculate that orientation effects in baddeleyite might involve channelling of primary ions into the crystal, emission of secondary ions along preferred directions, and/or differential ionisation of secondary species. No orientation-related differences in 206 Pb/ 238 U were detected during ion microprobe analysis of zircon or monazite.

Age, geochemistry, and tectonic significance of Neoproterozoic alkaline granitoids in the northwestern margin of the Gyeonggi massif, South Korea

Alkaline meta-granitoids, ranging in composition from syenite to alkali granite, occur in the northwestern Gyeonggi massif. Ion microprobe U–Pb zircon analyses indicate that the granitoids were emplaced at 742±13 Ma, and are corroborated by a Rb–Sr whole rock age of 770 ± 40 Ma. Major and trace element characteristics, together with Sr and Nd isotopic data, suggest that the granitoid magma was derived from ancient (TDM = 2.6–2.2 Ga) continental crust with addition of juvenile mantle-derived basaltic magma. The generation of the alkaline granitoid is attributed to crustal thinning induced by deep-seated thermal activity such as mantle upwelling or mafic magma influx. Alkaline igneous activity at 742 Ma is coeval with Neoproterozoic rift-related magmatism prevalent in South Korea and the South China Block but lacking in the North China Block. Thus, we suggest that the Gyeonggi massif is correlative with the South China Block and has experienced a rifting event during Rodinia breakup.

Untying the Kibaran knot: A reassessment of Mesoproterozoic correlations in southern Africa based on SHRIMP U-Pb data from the Irumide belt

The Irumide belt is part of a network of late Mesoproterozoic Kibaran-age orogens in south-central Africa. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages for gneisses, migmatites, and granitoids indicate that peak Irumide metamorphism was ca. 1020 Ma and that this was associated with widespread granitic magmatism at 1050–950 Ma. Pre-Irumide protoliths are dominated by 1650–1519 Ma granitic gneisses. These data provide the first robust constraint on the timing of Irumide tectonism and show that previous estimates of ca. 1350 and 1100 Ma are incorrect, thereby negating previously proposed correlations of the Irumide belt with nearby Kibaran-aged tectonism. The correlation between the Irumide belt and Choma-Kalomo block of southern Zambia has had a major influence on models for the tectonic assembly of southern Africa because it required that the intervening Neoproterozoic Zambezi belt was intracratonic and associated with minimal horizontal displacements. Our data indicate that both terranes have distinct histories, consistent with lithologic and metamorphic evidence of Neoproterozoic ocean closure along the Zambezi belt. This implies that the Kalahari and Congo cratons assembled during the Neoproterozoic and not during Kibaran-age tectonism, as previously believed. This new outlook on regional African tectonics supports a configuration of the Rodinia supercontinent that places the Congo craton well away from the Kalahari craton ca. 1000 Ma.

Palaeomagnetic constraints on the position of the Kalahari craton in Rodinia

Abstract A comparison of late Mesoproterozoic palaeomagnetic poles from the Kalahari craton and its correlative Grunehogna craton in East Antarctica shows that the Kalahari–Grunehogna craton straddled the palaeo-Equator and underwent no azimuthal rotation between ca. 1130 and 1105 Ma. Comparison of the Kalahari palaeopoles with the Laurentia APWP between 1130 and 1000 Ma shows that there was a latitudinal separation of 30±14° between Kalahari and the Llano–West Texas margin of Laurentia at ca. 1105 Ma. The Kalahari craton could have converged with southwestern Laurentia between 1060 and 1030 Ma to become part of Rodinia by 1000 Ma. In Rodinia, the Kalahari craton lay near East Antarctica with the Namaqua–Natal orogenic belt facing outboard and away from the Laurentian craton.

Petrology, geochronology, and tectonic implications of c. 500 Ma metamorphic and igneous rocks along the northern margin of the Central Asian Orogen (Olkhon terrane, Lake Baikal, Siberia)

A significant portion of the continental crust of northern Eurasia is thought to have formed during the evolution of the Central Asian Orogenic Belt at the time of accretion of continental terranes and island arcs. Records of this event are well preserved within the Siberian craton–Central Asian Orogenic Belt transition zone in Lake Baikal region, particularly in the Olkhon terrane. Our results establish granulite-facies conditions for peak metamorphism in the Olkhon terrane, and indicate that the granulites were derived from island arc mafic volcanic rocks and back-arc basin sediments. Sensitive high-resolution ion microprobe dating of metamorphic zircons from two mafic granulites yielded 238U/206Pb ages of 507 ± 8 and 498 ± 7 Ma, and magmatic zircons from syntectonic syenite yielded an age of 495 ± 6 Ma. The main metamorphic event occurred at about 500 Ma, and was probably related to collision of the Barguzin microcontinent with the Siberian craton. Ages from 535 to 2750 Ma for detrital zircon cores in early Palaeozoic metasediments of the Olhkon terrane were obtained. Archaean ages of detrital zircons in such metasediments suggest that the Barguzin microcontinent was originally part of the Aldan Province of the Siberian craton that was detached in late Mesoproterozoic, and reattached to the craton during early Palaeozoic collision.

Zircon Ages from the Baydrag Block and the Bayankhongor Ophiolite Zone: Time Constraints on Late Neoproterozoic to Cambrian Subduction- and Accretion-Related Magmatism in Central Mongolia

Central Mongolia represents a heterogeneous crustal domain of the Central Asian Orogenic Belt and is composed of contrasting lithotectonic units with distinct preorogenic histories. We report single‐zircon evaporation and SHRIMP ages for high‐grade rocks of the Neoarchean‐Paleoproterozoic Baydrag block and for metaigneous rocks of the junction between the late Neoproterozoic Bayankhongor ophiolite zone (BOZ) and the Baydrag block. Zircon ages for metamorphic rocks of the Baydrag block indicate a major tectonothermal event between 1840 and 1826 Ma, coeval with the emplacement of granitic rocks at middle‐crustal level dated at 1839 Ma. A granite‐gneiss yielded a much younger crystallization age of 1051 Ma, the first Grenvillian age reported for this region. Together with predominantly Mesoproterozoic detrital zircon ages for a quartzite lens from the Burd Gol accretionary complex, these data attest to the heterogeneity and long Precambrian history of the Baydrag block. Crystallization ages for granite‐gneisses from the northeastern margin of the Baydrag block indicate prolonged plutonic activity between 579 and 537 Ma, probably related to southward subduction of the Bayankhongor oceanic crust. A syntectonic granite vein yielded a crystallization age of 519 Ma, probably linked to accretion of the BOZ onto the northeastern active margin of the Baydrag block. Lastly, a felsic metavolcanic rock from the southeastern termination of the BOZ yielded a crystallization age of 472 Ma and suggests that punctuated volcanic centers developed during the early Ordovician in response to protracted convergence.