Relationship between petrological core-mantle transition and the seismic Moho discontinuty below granulite terranes: evidence on transformation of a root part beneath the Eastern Tunka block in deep-seated nodules from late ceniozoic volcanic rocks

Abstract

From compositions of garnet-bearing and garnet-free assemblages of deep-seated nodules brought to a surface by Cenozoic and older (Phanerozoic) magmatic melts, petrological zones of crust-mantle transition (PZСMT) are defined under granulite terranes of craton and non-craton regions. Present-day Moho discontinuities only partially coincide with petrological estimates of a change from felsic-basic rocks (that belong to the continental crust) by predominantly ultramafic rocks (that represent the continental mantle lithosphere) and often lie much deeper than the PZСMT. Depths of such zones change over time. For a garnet-free assemblage of deep-seated nodules ejected by basaltic melts about 13 Ma ago from the root of a granulite terrane exposed in the eastern part of the Tunka Valley, two PT trends were obtained, one of which corresponds to a high (up to 120 mW/m2) rift conductive geotherm, another one crosses low conductive geotherms (drops below 60 mW/m2 one). The PZСMT shows here a temperature approximately 200°C lower than the PZСMT of granulite terranes in Eastern Australia, China, and Svalbard. Deep-seated nodules characterize the development of hot transtension under the rift valley in the cold root part of the East Tunka block with the accumulation and release of elastic stresses accompanied by significant synkinematic (metasomatic and magmatic) processes in the time interval 18–12 Ma ago. The transtension was followed by a crustal transpression with inversional uplift of an area and a probable relative increase in the depth of the Moho discontinuity, determined from the both P- and S-wave velocities for the modern crust and lithospheric part of the mantle.