Origin and evolution of overlapping calc-alkaline and alkaline magmas: The Late Palaeozoic post-collisional igneous province of Transbaikalia (Russia)

The Late Palaeozoic voluminous magmatism in Transbaikalia, Russia (a territory of >600,000km² to the east of Lake Baikal) is highly diverse and complex. Of special interest are (1) the significant overlap in time between magmatic suites commonly ascribed to post-collisional and within-plate settings and (2) the provenance of the coeval, but distinct, granitoid magmas that are closely spaced within a large region. Magmatic activity lasted almost continuously from ~330Ma to ~275Ma and included five igneous suites occupying a total area of ~200,000km²: (1) the Barguzin suite of high-K calc-alkaline granite (330-310Ma); (2 and 3) the coeval Chivyrkui suite of low-silica calc-alkaline granitoids and the Zaza suite of high-K calc-alkaline to alkaline granite and quartz syenite which were emplaced between 305 and 285Ma; and (4 and 5) the partially overlapped in time Lower-Selenga monzonite-syenite suite (285-278Ma) and the Early-Kunalei suite of alkali-feldspar and peralkaline quartz syenite and granite (281-275Ma). The overall increase in alkalinity of the granitoids with time reflects the progress from post-collisional to within-plate settings. However, a ~20m.y. long transitional period during which both calc-alkaline and alkaline granitoids were emplaced indicates the coexistence of thickened (batholiths) and thinned (rift) crustal tracts.Sr-Nd-O isotope and elemental geochemical data suggest that the relative contribution of mantle-derived components to the generation of silicic magmas progressively increased with time. The high-K calc-alkaline granite magmas that formed the Angara-Vitim batholith were generated by high degree melting of supracrustal metamorphic rocks [εNd(t)=-5.7 to -7.7; δ¹⁸O(Qtz)=12‰], with minor contribution of H₂O and K from the underplated mafic magma (the convective diffusion model). The coeval calc-alkaline Chivyrkui suite and the transitional to alkaline Zaza suite formed as a result of mixing of crustal silicic and mantle-derived basic melts in roughly equal proportions. In so doing, the former crystallized immediately from the hybrid magmas, whereas the latter (Zaza suite) formed by fractional crystallization of the hybrid melts following magma mixing. Finally the partly overlapping in time monzonite-syenite (Lower-Selenga) suite and highly alkaline syenite-granite (Early-Kunalei) suite were produced from the similar K-rich basalt source. For the former suite, the magma generation process was dominated by fractional crystallization of K-rich basalt magma. In contrast, the highly alkaline granitoid magmas were likely generated by partial melting (~20%) of K-rich mafic rocks in the lower crust.