Magmatism of the Devonian Altai-Sayan Rift System: Geological and geochemical evidence for diverse plume-lithosphere interactions

The geodynamic environment of the 407–392 Ma Altai-Sayan Rift System is characterized using previously published and new original data on whole rock, trace and Sr-Nd isotopic compositions, along with U-Pb zircon ages. Five magmatic associations are present: basalt (basalts and basaltic trachyandesites), continuous (basalts, andesites, dacite-rhyolites), alkaline (basalts, nephelinite, tephrite, phonotephrite, phonolite, teralite, ijolite-urthite, foyaite, nepheline and alkaline syenite), bimodal (trachybasalts, trachyrhyolites-pantellerites and peralkaline granites) and ultramafic-mafic (picrites and picrodolerites). Mafic rocks of basalt, continuous, alkaline, and bimodal associations exhibit a wide variation of TiO2 (from 1.05 to 4.05 wt%) and are compositionally intermediate between intraplate basalts of OIB type and basalts of active continental margins IAB type. The TiO2 content in these mafic rocks correlates directly with the content of large ion lithophile elements (LILE), rare-earth elements (REE), high field strength elements (HSFE), and particularly with Nb and Ta. The basaltic samples have positive εNd(395) values (+3.4 to +7.7) and a large range of εSr(395) values (−13.6 to +12.6). εSr(395) decreases with increasing TiO2 abundance. Pantellerites and alkaline granites have ore-level concentrations of Nb, Ta, Zr, Hf, REE; and they have similar Sr and Nd isotope parameters to those of the high-Ti basalts. This indicates their origin via fractionation of mantle magmas. Rhyolite samples are depleted in rare incompatible elements, but have low positive εNd(395) values (+1.5 to +1.8), and εSr(395) values (+16.6 to +20.6), and they compositionally resemble the rocks produced from anatectic magmas of crustal origin. Whole-rock elemental and isotopic data suggest that the mafic rocks were likely derived from lithospheric mantle that was metasomatized during the prior Caledonian accretion/subduction event. In combination with the field relationship and regional geology, our study suggests that the rock associations from the Devonian Altai-Sayan Rift System were derived by the activity of mantle plumes

Linking magmatism with collision in an accretionary orogen

A compilation of U-Pb age, geochemical and isotopic data for granitoid plutons in the southern Central Asian Orogenic Belt (CAOB), enables evaluation of the interaction between magmatism and orogenesis in the context of Paleo-Asian oceanic closure and continental amalgamation. These constraints, in conjunction with other geological evidence, indicate that following consumption of the ocean, collision-related calc-alkaline granitoid and mafic magmatism occurred from 255 ± 2 Ma to 251 ± 2 Ma along the Solonker-Xar Moron suture zone. The linear or belt distribution of end-Permian magmatism is interpreted to have taken place in a setting of final orogenic contraction and weak crustal thickening, probably as a result of slab break-off. Crustal anatexis slightly post-dated the early phase of collision, producing adakite-like granitoids with some S-type granites during the Early-Middle Triassic (ca. 251-245 Ma). Between 235 and 220 Ma, the local tectonic regime switched from compression to extension, most likely caused by regional lithospheric extension and orogenic collapse. Collision-related magmatism from the southern CAOB is thus a prime example of the minor, yet tell-tale linking of magmatism with orogenic contraction and collision in an archipelago-type accretionary orogen