Early–Middle Paleozoic granitoids in Gorny Altai, Russia: Implications for continental crust history and magma sources

Abstract

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.