The Early Cambrian Age and Crustal Sources of Granitoids of the Goryachinskiy Pluton (Northern Baikal): Geodynamic Implications

Doklady Earth Sciences - New results of U–Pb zircon and Nd–isotope analyses of the Goryachinskiy Pluton adakite granites have been obtained. The pluton is exposed along the Baikal coast...     

Stenian granitoids of the west Kyrgyz Ridge (North Tien Shan): Position, structure, and age determination

In the structure of west Kyrgyz Ridge (North Tien Shan), a great role is played by complexly dislocated Upper Precambrian-Cambrian terrigenous-carbonate and shale strata, as well as by granitoids that comprise several coupled WNW-striking synforms and antiforms, the largest of which is the Makbal antiform. Southeast of the core of this antiform, granitoids comprise the large Kara Dzhilga massif and several massifs that are of lesser size and have tectonic correlations with the hosting terrigenous-carbonate strata. In the Kara Dzhilga massif, the rocks of three penetration phases are distinguished; contacts between rocks are often of tectonic character. The early phase is presented by monzonite and monzodiorite; the main one, by large-porphyric biotitic granites; and the additional one, by aplitic granites and pegmatites. By the chemical composition, granites of Kara Dzhilga massif of the main phase correspond to subalkaline granites of high-potassium calc-alkali series. The age of their crystallization (zircon-based U-Pb method) is 1131 ± 4 Ma (Stenian). The formation of Stenian granitoids in the North Tien Shan may be related to development of Grenville fold belts, whose fragments were identified in the units of the Central Asian Belt. Tectonic correlations between these granitoids and hosting terrigenous-carbonate strata appeared as a result of immersion to significant depths and subsequent exhumation into the upper crustal horizons in the Early Ordovician.     

Late Carboniferous Monzonite–Granosyenite Magmatism in the Northern Balkhash Region (Central Kazakhstan)

U–Pb dating of the Torangalyk Complex (Northern Balkhash) yielded a Late Carboniferous age of 305 ± 2 Ma. Taking into account the previous data, a new scheme for Late Paleozoic granitic magmatism in this region has been proposed. It includes the Early Carboniferous granite–granodiorite Balkhash Complex, Late Carboniferous monzonite–granosyenite Kokdombak and Torangalyk complexes, and the Late Carboniferous–Early Permian granite–leucogranite Akchatau Complex.     

Vendian stage in formation of the Early Caledonian superterrane in Central Asia

Granitoids and metamorphic rocks of the Baidarik basement block of the Dzabkhan microcontinent are studied in terms of geology, geochronology (U-Pb dating of zircon microfractions and individual grains) and Nd isotopic-geochemical systematics. As is established, the formation history of metamorphic belt (disthene-sillimanite facies) in junction zone of the Baidarik block and Bayankhongor zone of the Late Riphean (～665 Ma) ophiolite association characterizes development of the Vendian (～560–570 Ma) active continental margin. The high-P metamorphic rocks of that time span evidence formation of structures with the Earth’s crust of considerable thickness. In Central Asia, events of the Vendian low-gradient metamorphism are established also in the Tuva-Mongolian massif, Kan block of the East Sayan Mountains, and South Chuya inlier of the Caledonides in the Altai Mountains. Based on these data, it is possible to distinguish the Late Baikalian stage in development of the Early Caledonian superterrane of Central Asia, which antedated the subsequent evolution of this structure during the Late Cambrian-Ordovician. The high-gradient metamorphism that affected most intensively the southeastern part of the Baidarik block can be correlated with the Early Paleozoic (525–540 Ma) evolution of active continental margin and associated development of the Vendian oceanic basins and island arcs of the Ozernaya zone.     

New Data on the Age of the Tonalite–Trondhjemite Orthogneisses of the Olekma Complex of the Central Part of the Chara–Olekma Geoblock,Aldan Shield

New U–Pb zircon (TIMS) results allow dating of protoliths of tonalite–trondhjemite orthogneisses of the Olekma Complex in the central part of the Chara–Olekma Geoblock (Aldan Shield) to 2825 ± 3 Ma and 2994 ± 3 Ma. Together with the results of previous geochronological studies, this proves that the Olekma Complex comprises heterochronous igneous rocks intensively reworked under amphibolite facies conditions and formed during different stages of geological evolution of the Aldan Shield.     

Ophiolite sections of the Dzhalair-Nayman zone,South Kazakhstan: Their structure and age substantiation

The ophiolite complexes outcrop extending more than 700 km in the Dzhalair-Nayman zone located between the Chuya-Kendyk Tas (in the southwest) and Aktau-Dzhungar (in the northeast) Precambrian sialic massifs in South Kazakhstan. The most complete ophiolite sections are described in the central (Andassai massif) and southeastern (Dulankara massif) parts of the zone. Plagiogranites occur in a sheeted dyke complex in the upper part of ophiolite sections. A bimodal volcanic series occurs atop the section. The U/Pb zircons ages for the Andassai and Dulankara plagiogranites are 519 ± 4 and 521 ± 2 Ma, respectively. Ophiolites associated with bimodal volcanic series are also found in the north and northeast of Kazakhstan. They may form, together with the Lower Cambrian complexes of the Dzhalair-Nayman zone, a single paleostructure, whose fragments are well-traceable for over 2000 km