Volcanic rocks and pseudotachylytes from sources of crust-mantle complementary layers: Insight into geodynamics of the Baikal Rift System, Southern Siberia

We present results of detail geochemical study of 18–12 Ma volcanic rocks from the Kamar-Stanovoy Zone of Hot Transtension (KSZHT), located in the central Baikal Rift System (BRS), and older pseudotachylytes from the Main Sayan Fault (MSF). These rocks designate geochemically distinguished from the OIB sources that are referred to the Slyudyanka zone of paleocollision occurred between the Khamardaban terrane and Siberian paleocontinent about 488 Ma ago. We define crustal and mantle signatures (with and without garnet, respectively) for the KSZHT volcanic rocks and crustal ones for the MSF basic pseudotachylytes. The signatures are indicative for tracing complementary relations between layers of the crust–mantle transition (CMT). We infer that the KSZHT volcanic activities accompanied rifting of a paleocontinental margin but got quiescent after a structural separation of the South Baikal Basin from the Tunka Valley, when the former had been sufficiently extended and subsided in contrast to the latter, which had been notably compressed and uplifted. From geological evidence and a detail seismic tomography model, we suggest that the KSZHT crust–mantle magmatic processes were due to delamination of a thickened root part of the South Baikal Orogen that preceded rifting in the South Baikal Basin area in the Late Cretaceous and Paleogene. Volcanic rocks of the past 17 Ma from the southwestern BRS denoted similar CMT sources and delamination processes beneath the East Hangay orogen and adjacent Orkhon-Selenga saddle. In the central and southwestern BRS, the CMT sources marked mutually overlapping deformational fields related to Indo-Asian convergence and pool-to-axis forces of the Japan-Baikal geodynamic corridor.

This work is supported by the RSF grant 18-77-10027.