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Abstract: Early Palaeozoic tonalite to granodiorite intrusions in northern Kazakhstan are associated with lode gold mineralization and have SHRIMP zircon ages of 457.3±6.6Ma (Aksu), 452.9±5.6Ma and 447.4±5.4Ma (both Zholymbet). The Stepnyak intrusion contains large xenoliths with an age of 480.6±5.0Ma. One early Palaeozoic zircon from a porphyritic diorite at Stepnyak has a core with a near-concordant 207Pb/206Pb age of 3888±1.5Ma, whereas other xenocrystic grains are between 983 and 2698Ma old. The early Archaean age is probably inherited from unexposed basement of the Kokchetav Massif and represents the oldest crustal material so far known from the Asian continent. It appears that the Aksu, Zholimbet and Stepnyak granitoids were emplaced in the late Ordovician in an Andean- or Japan-type continental arc environment on the margin of the Kokchetav Massif. Late Palaeozoic granitoids in central Kazakhstan have Devonian zircon ages of 407.1±3.9, 381.1±3.1 and 369.2±4.9Ma, whereas the youngest sample from the Topar Massif has a late Carboniferous emplacement age of 314.1±5.1Ma. Initial ε Nd values range from +5 to −1 corresponding to Nd-model ages of 1.1–0.6Ga. The isotopic data are similar to those of other Phanerozoic granitoids of the Central Asian Orogenic Belt and corroborate melting of predominantly juvenile crustal protoliths. Our data also confirm that Devonian and Carboniferous arc magmatism in central Kazakhstan assimilated older and already accreted island arc systems, yet is derived from primitive sources, probably in a subduction setting. The range of ε Nd values with corresponding Nd mean crustal residence ages in the range of 0.6–1.1Ga for most samples indicates variable recycling of late Proterozoic largely juvenile continental crust in the formation of the Palaeozoic magmatic arcs of central Kazakhstan. The orogenic evolution terminated with Permian anorogenic, rift-related alkaline granite magmatism (ε Nd =+2 to +8) which reflects melting of juvenile, mantle-derived lower crust. [Copyright &y& Elsevier]

Abstract: Geochemical and Nd-isotopic data are reported for 24 shale and sandstone samples comprising the Upper Ordovician to Lower Cretaceous Sekondian Group, southern Ghana. The data are interpreted in terms of the provenance of these siliclastic sediments and the Paleozoic and Mesozoic geologic development of southern Ghana. The sandstones and shales generally have trace element characteristics typical of material eroded from the upper continental crust. Cr and Ni concentrations are low suggesting their derivation from dominantly felsic sources. There appear to be chemical and Nd-isotopic differences between the lower formations (i.e., Ajua Shale and Elmina Sandstone) and the uppermost formation (i.e., Essikado Sandstone) on one hand, and the upper formations (i.e., Takoradi Sandstone, Takoradi Shale, Effia Nkwanta Beds and Sekondi Sandstone) on the other. The upper formations are characterized by evolved major element compositions, high large-ion lithophile element abundances, large negative Eu-anomalies (0.58–0.73), and low ε Nd values (−10.8 to −4.1), indicating stable craton settings. On the other hand, the Ajua Shale, Elmina Sandstone, and Essikado Sandstone formations have less evolved major element compositions, lower large-ion lithophile abundances, and higher Eu-anomalies (0.69–0.88), and this may reflect an active margin setting of the source (juvenile-type Birimian felsic volcanics and/or granitoids) rather than Phanerozoic active continental settings. The upper formations have T DM model ages of 1.8–2.3Ga indicating that the Paleoproterozoic Birimian rocks are the ultimate sources, whereas the lower formations and uppermost formation have slightly younger T DM values of 1.6–1.9Ga suggesting of a component from the Pan-African mobile belt. These differences in model ages may, however, be related to lithology (rather than stratigraphy) where the shales (1.6–2.3Ga; mean, 1.9Ga) typically give older ages than the sandstones (1.6–1.9Ga; mean 1.7Ga). [Copyright &y& Elsevier]

Large volumes of ∼880–1100 Ma calc-alkaline granitoid rocks in the Wanni and Vijayan crustal provinces of Sri Lanka make it likely that these domains were produced in active margin settings, probably Grenville-age magmatic arcs. We report new single zircon evaporation ages and Nd isotopic systematics for dioritic to granodioritic gneisses of the Kadugannawa Complex of central Sri Lanka which record a period of magmatic arc activity between 1006 and 881 Ma and show this complex to be part of the Wanni domain. Both provinces were probably generated in arc-related settings at the outer margin of Rodinia, but this cannot be ultimately proven on the basis of the currently available data. In spite of a strong Pan-African tectono-thermal overprint at ∼600–550 Ma that we relate to involvement of Sri Lanka in the amalgamation of the Gondwana Supercontinent, rare ∼1000–900 Ma structures are preserved in the gneisses of the Kadugannawa Complex that may constitute remnants of the accretion history of Rodinia. We further suggest that the basement rocks of Sri Lanka do not correlate with crustal domains in southern India or in East Africa, since these were not part of Rodinia. [Copyright &y& Elsevier]

In this paper we present new data for the metamorphic Garm Block (150 × 40 km) that was traditionally shown on regional maps as fragment of a Precambrian basement. The Garm Block comprises migmatized gneisses and schists for some of which peak metamorphic conditions (600–660 °C at 7–9 kbar) characteristic for medium P/T series metamorphic belts were determined. U–Pb ages of overgrown rims of zircon grains and newly grown magmatic zircon grains from four migmatitic samples indicate that melting occurred during peak metamorphic conditions between 299 and 290 Ma. Thus the Garm Block is the first Early Permian regional metamorphic terrane established in the southern part of the Central Asian Orogenic Belt. A crosscutting granite intrusion yields a concordia age of 290 Ma identical to the age of metamorphism and similar to ages of post-collisional intrusions elsewhere in the Tien Shan. Detrital zircon grains from the Garm samples yield an age spectrum with major peaks at 700–550 Ma and 1150–700 Ma, and smaller ones at ca. 2.7–2.4 Ga and 3.5–3.3 Ga. The youngest grains define an Ediacarian maximum depositional age of the sedimentary protoliths. The presence of Grenvillian ages of 1150–800 Ma, which are characteristic for the Tarim Craton, and the lack of ca. 1.6–1.2 Ga ages, characteristic for the Northern Tien Shan, suggest an origin of the Precambrian zircon grains from southern continental blocks similar to the Tarim Craton. Six granitoid samples yield negative εNd values of −10.6 to −3.7 consistent with the reworking of crustal material with 1.8–1.1 Ga average crustal residence times and the formation of the South Tien Shan belt on a continental basement with Paleoproterozoic or older crust. The Garm Block formed as a consequence of lithospheric thickening during the Late Carboniferous collision of two continental blocks that created the South Tien Shan fold belt. Lateral crustal melt flow coupled with extension may have resulted in exhumation of the Permian metamorphic rocks. [ABSTRACT FROM AUTHOR]

Abstract: The Antarctic Peninsula has been part of a magmatic arc since at least Jurassic times. The South Shetland Islands archipelago forms part of this arc, but it was separated from the Peninsula following the Pliocene opening of the Bransfield Strait. Dikes are widespread throughout the archipelago and are particularly accessible on the Hurd Peninsula of Livingston Island. The host rocks for the dikes are represented by the Miers Bluff Formation, which forms the overturned limb of a large-scale fold oriented 63/23 NW. The orientation of minor structures indicates a fold axis oriented NNE–SSW (24/0). Structural analysis of the dikes and their host rocks shows that the tectonic regime was similar to other parts of the archipelago and that only minor changes of the stress field occurred during dike emplacement. Based on crosscutting field relationships and geochemical data, six early Paleocene to late Eocene intrusive events can be distinguished on Hurd Peninsula. In contrast to calc-alkaline dikes from other parts of the South Shetland Islands, the majority of the Hurd Peninsula dikes are of tholeiitic affinity. Nd and Pb isotope data indicate a significant crustal component, particularly during initial magmatic activity. Plagioclase 40Ar/39Ar and whole rock K–Ar ages show that dike emplacement peaked during the Lutetian (48.3±1.5, 47.4±2.1, 44.5±1.8 and 43.3±1.7Ma) on Hurd Peninsula and also further northeast on King George Island. Dike intrusion continued on Livingston Island at least until the Priabonian (37.2±0.9Ma). The type of magma sources (mantle, slab, crust and sediment) did not change, though their relative magmatic contributions varied with time. During Cretaceous and Early Paleogene times, the Antarctic Peninsula including the South Shetland Islands was situated southwest of Patagonia; final separation from South America occurred not before the Eocene. Thus, the geological evolution of Livingston Island is related as much to the development of Patagonia as of Antarctica, and needs to be considered within the history of southernmost South America. [Copyright &y& Elsevier]