Transform margin Maastrichtian-Paleogene magmatism in East Asia: The problem of “belts” in the Koryak-Western Kamchatka region

Abstract

The paper presents data on the specifics of continental-margin magmatism in pull-apart structures at the Cretaceous-Paleogene boundary and discusses the sources of this magmatism, which were produced when the Asian-Pacific transform boundary was formed in place of the Cretaceous convergent boundary. The continental margin was thereby broken by strike-slip and down-dip-strike-slip faults, which formed the East Asian Rift System (EARS). The system induced transform magmatism of two complexes: (i) late Campanian-Eocene weakly differentiated basaltoids (transform boundary basalts, TBB) and (ii) Eocene-Oligocene complexes of acidic rocks (transform boundary acidic rocks, TBA). Similar to plateau basalts, the former complex was produced by fissure eruptions and occurs as extensive areas of volcanic rocks and shield volcanoes, while the latter complex comprises central volcanic edifices crowned with calderas. The specifics of the TBB volcanics is their multicomponent composition: along with dominant OIB-WPB features, they display MORB and IAB signatures (in the absence of a synchronous subduction zone). Isotopic-geochemical and seismic-tomography evidence testifies that the rocks were produced by melts of three types: lower mantle upwelling material that interacted with the asthenosphere and/or fragments of depleted oceanic mantle and with lithospheric domains of the Asian continental margin that had suffered multistage reworking in suprasubduction environments. The isotopic-geochemical features of the acidic TBA rocks, which were formed after (or locally, simultaneously with) the TBB basaltoids, suggest their crustal genesis. The anatexis of the continental margin was triggered by the transition from an extensional to compressional environment (because of mid-Eocene subduction from the ocean) and lag of the mantle magmatic front, which maintained a higher temperature regime. The geodynamic conditions in which the plateau basalts were generated in the extensional zones on continental margins (in East Asia) and in the within-plate environment itself (in the central Arctic) were different.