Petrogenesis and age of the felsic volcanic rocks from the North Baikal volcanoplutonic belt,Siberian craton

Detailed geochemical, isotopic, and geochronological studies were carried out on felsic volcanic rocks from the southern part of the North Baikal volcanoplutonic belt. U-Pb zircon dating showed that the rocks previously ascribed to a single stratigraphic unit (Khibelen Formation of the Akitkan Group or the Khibelen Complex) have significant age differences. The Khibelen Formation was found out to include both the oldest dated rocks (1877.7 ± 3.8 Ma) of the North Baikal belt and the younger volcanic rocks (1849 ± 11 Ma). Two other dated volcanic rocks have intermediate ages (1875 ± 14 and 1870.7 ± 4.2 Ma). It was established that the volcanic rocks from various areas in the southern part of the North Baikal belt not only have different ages but also differ in geochemical and isotopic signatures. In particular, the felsic volcanic rocks from various sites show the following variations in trace-element composition: from 220–280 to 650–717 ppm Zr, from 8–12 to 54–64 ppm Nb, and from 924–986 to 1576–2398 Ba. The ?_Nd obtained for felsic volcanic rocks and comagmatic granitoids from various areas in the southern part of the North Baikal belt vary, respectively, from ?1.7 to ?2.8 and from ?8.0 to ?9.2. Based on geochemical and isotopic signatures, the felsic volcanic rocks in various areas of the southern part of the North Baikal volcanoplutonic belt were formed via the melting of a Mesoarchean crustal source of tonalite composition with contribution of variable amounts of juvenile mantle material at different magma generation conditions. Isotopic data indicate that the contribution of juvenile mantle material to their sources varied from ～33–40 to 77–86%. The maximal calculated temperatures of the parent melts for felsic volcanic rocks were 908–951°C, and the lowest temperatures were 800–833°C. The geochemical signatures of dacites with an age of 1877.7 ± 3.8 Ma such as high Th (46–51 ppm) and La (148–178 ppm) contents indicate that these rocks, along with Mesoarchean granitoid and juvenile mantle material, contain an upper crustal component with high Th and LREE contents. Extremely low Y and Yb contents in these dacites implies their formation at pressures of ～ 12–15 kbar in equilibrium with garnet-bearing residue. These rocks were presumably formed in the collisional-thickened crust at the earliest stages of its collapse, possibly during syncollisional collapse, with additional hear input to the lower crust. Other felsic rocks are geochemical analogues of A-type granites and were formed during the subsequent stages of collapse (post-collisional collapse).