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The Cambrian-Ordovician transition was the time of several key events in the history of Central Asia. They were the accretion of Mariana-type island arc systems to the Siberian continent, the related large-scale orogeny and intrusions of basaltic and granitic magma and the formation of a huge turbidite basin commensurate with the Bengal Gulf basin in the western part of the Central Asian orogenic belt (CAOB). The structure of the basin, as well as the sources and environments of deposition remain open to discussion. This paper presents new major- and trace-element data on Late-Cambrian-Early Ordovician turbidites from different parts of the Russian Altai and a synthesis of Nd isotope composition and ages of detrital zircons. The turbidites share chemical similarity with material shed from weathered continental arcs. Broad variations of CIA (39–73) and ICV (0.63–1.66) signatures in sandstones suggest origin from diverse sources and absence of significant sorting. Trace elements vary considerably and have generally similar patterns in rocks from different terranes. On the other hand, there are at least two provinces according to Nd isotope composition and age of detrital zircons. Samples from eastern Russian Altai contain only Phanerozoic zircons and have Nd isotope ratios similar to those in Early Cambrian island arcs (eNdt + 4.4… + 5.4; TNd(DM)-2-st = 0.8–0.9 Ga). Samples from central, western, and southern parts of Russian Altai contain Precambrian zircons (some as old as Late Archean) and have a less radiogenic Nd composition (eNdt up to -3.6; TNd(DM)-2-st up to 1.5 Ga). The chemical signatures of Late Cambrian to Early Ordovician turbidites indicate a provenance chemically more mature than the island arc rocks, and the presence of zircons with 510–490 Ma ages disproves their genetic relation with island arcs. The turbidite basin formed simultaneously with peaks of granitic and alkali-basaltic magmatism in the western Central Asian orogen and resulted from interplay of plate tectonic and plume tectonic processes.

Major-element and trace-element compositions, as well as Nd isotope ratios and protolith model ages of Early to Middle Paleozoic granitoids from the Russian Gorny Altai are summarized and discussed in terms of the compositions and tectonic settings. The observed geographic variations of compositions in the granitoid assemblages correlate with three groups of late Neoproterozoic to Middle Paleozoic terranes corresponding to oceanic, island arc, and turbidite basin settings. Plutonic rocks in the oceanic and island arc terranes are commonly low-alkali metaluminous tonalite and trondhjemite that formed by melting of oceanic crust. The island arc terranes also contain tonalite–trondjemite–granodioritic and tonalite–granodiorite–granitic rocks, with higher alkali and potassium contents, which result from anatectic melting of volcanics and sediments. Granitoids in the turbidite basins are potassic peraluminous monzogranite produced by anatectic melting of upper crustal turbidites, as well as subalkaline tonalite, granodiorite, and granite derived from upper crustal metasediments and metamorphosed mafic oceanic crust underlying the turbidite basins. Alkaline granitoids emplaced during two major tectonomogamatic events (Early and Late Devonian) and are present in terranes of all three settings. Their source is of crustal origin, like in other groups of granitoids, but it was a mixture of crustal and mantle material. Thus the compositional trends of Early and Middle Paleozoic granitoids in the Gorny Altai record the history of continental crust.