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Palaeozoic arc magmatism in the Central Asian Orogenic Belt of Kazakhstan SHRIMP zircon ages and whole-rock Nd
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]
A.Kroner
、
E.Hegner
、
B.Lehmann
、
J.Heinhorst
、
M.T.D.Wingate
、
D.Y.Liu
、
P.Ermelov
Early Palaeozoic tonalite to granodiorite intrusions in northern Kazakhstan are associated with lode gold mineralization and have SHRIMP zircon ages of 457.3 ± 6.6 Ma (Aksu), 452.9 ± 5.6 Ma and 447.4 ± 5.4 Ma (both Zholymbet). The Stepnyak intrusion contains large xenoliths with an age of 480.6 ± 5.0 Ma. One early Palaeozoic zircon from a porphyritic diorite at Stepnyak has a core with a nearconcordant 207Pb/206Pb age of 3888 ± 1.5 Ma, whereas other xenocrystic grains are between 983 and 2698 Ma 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 ± 4.9 Ma, whereas the youngest sample from the Topar Massif has a late Carboniferous emplacement age of 314.1 ± 5.1 Ma. Initial eNd values range from +5 to -1 corresponding to Nd-model ages of 1.1–0.6 Ga. 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 eNd values with corresponding Nd mean crustal residence ages in the range of 0.6 1.1 Ga 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 (εNd = +2 to +8) which reflects melting of juvenile, mantle-derived lower crust.
The 880-864 Ma granites of the Yenisey Ridge, western Siberian margin Geochemistry, SHRIMP geochronology, and tectonic implications
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]
V.A.Vernikovsky
、
A.E.Vernikovskaya
、
M.T.D.Wingate
、
N.V.Popov
、
V.P.Kovach
The results of geological, petrological, geochemical, and geochronological investigations of the Teya, Kalami, andYeruda granites are discussed in the context of the formation the western margin of the Siberian craton and the Central Asian Orogenic Belt. These granites occur within Meso- to Neoproterozoic greenschist and amphibolite facies metamorphic rocks of the Central Angara terrane in the Yenisey Ridge region. Based on previous geochronology, these rocks have been implicated in alleged late Mesoproterozoic (Grenville-age) collision events along the western Siberian craton margin during the assembly of Rodinia. In all three plutons, the granites have mostly subaluminous—slightly peraluminous chemical compositions, and are characterized by variable normative corundum (1.1–3.3%) and Na2O/K2O (0.4–1.8). Sm–Nd and Rb–Sr isotopic data show that they could be formed from continental crust magmatic sources with Paleoproterozoic model ages (TNd(DM-2st) = 1.8–2.1 Ga). SHRIMP U–Pb zircon ages of 868–864 Ma were obtained for three samples from the Teya granite and 875 Ma for a sample from the Kalami granite. Together with the previously dated 878 Ma Yeruda granite, these intrusive rocks are the most ancient granites in the Transangara region of the Yenisey Ridge.It is likely that the Teya, Kalami, and Yeruda granites were part of the Central Angara terrane prior to its collision with Siberia at 760–720 Ma.
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