Paleocene A-type igneous suites in the Sikhote-Alin (the East Asian continental margin): Petrological, geochronological, isotopic, and geodynamic constraints

Global geodynamic reconstruction models of the Pacific margin of Asia refer to materials collected throughout Japan, Korea and northeastern China, but they lack data on southern Sikhote-Alin. Therewith, the Sikhote-Alin orogenic belt (NE Asia) constituted a single eastern margin of the paleo-Asian continent with the abovementioned territories in the Paleocene. New isotopic, geochemical, and geochronological data show that Paleocene igneous activity (~61–55 Ma) is widely developed in southern Sikhote-Alin. Bulk rock compositions of the igneous rocks of this period yield ferroan, peraluminous, calc-alkaline to alkaline affinities, highly abundant LILE and HFSE (with pronounced Ba, Sr, Eu, and Ti negative anomalies) and depleted HREE. The initial melts, displaying Zr + Y + Ce + Nb > 350 ppm and 10,000 × Ga/Al > 2.6, derived from an OIB-like mantle source crystallised under fairly reducing conditions (below FMQ buffer), and yield high temperature of zircon saturation (>850 ℃), indicating typical A-type granite and related volcanic rock features. It is logical to associate variations in A-type rock geochemical composition with an enrichment of the upper part of the magma chamber with fluid-mobile components involving a redistribution of major and trace elements through fluid-magmatic differentiation. Strong depletion in Ca, Mg, Ba, Sr, Eu in the A-type rocks is caused by an outflow of these elements by an oxidizing, initially reduced, acidic fluid beyond the zone of magma generation. Whole-rock Sr-Nd isotope data argue for the generation of the A-type rocks by melting of dominantly a juvenile mantle component with a subordinate component of the ancient crust. Isotope variations of igneous rocks of the reference area: ⁸⁷Sr/⁸⁶Sr(t) (0.7024–0.7118), ε_Nd(t) (−0.9 to −5.1) and T_DM2 (934–1277 Ma), result from the mixing of the OIB-like mantle source with selective melts or from the metapelite contamination of the Samarka terrane accretionary prism and of the Zhuravlevka-Amur turbidite basin, later followed by fluid-magmatic differentiation that led to the formation of anatectic or hybrid melts. We further suggest that the origin of the A-type granites and related volcanic rocks is the result of the oblique interaction of oceanic and continental plates. This interaction accounts for the simultaneous formation of tears in the slab, enabling sub-slab asthenospheric upwelling, and strike-slip fault-related extensional structures in the overriding continental plate.