SOURCES OF BASALTIC MELTS FOR DEVONIAN RIFT BIMODAL IGNEOUS ASSOCIATIONS OF CENTRAL-ASIA - EVIDENCE FROM TRACE-ELEMENT AND STRONTIUM ISOTOPIC DATA ON BASIC ROCKS FROM NORTHEASTERN MONGOLIA

Contents of major and trace (Rb, U, Th, Ba, Sr, Ta, Zr, Hf, REE, and Y ) elements were determined in Devonian highly alkaline rock associatio ns of northwestern Mongolia, whose origin was related to rifting in th e rear part of the Devonian active continental margin of the Middle-Pa leozoic Northern Asian continent. Typical features of the associations are bimodal compositions, correlation between alkalinity of the basic and acid rocks, and major- and minor-element lateral zonation of the volcanic area: a landward increase in the alkalinity and contents of s ome trace lithophile elements. As strontium isotopic data on basic ass ociations indicate, the variation pattern of isotopic composition with in the rift area cuts across the general petro-geochemical zonation. T o explain the observed regularities, we proposed a model suggesting th at the Devonian bimodal associations of northwestern Mongolia were gen erated from two types of sources of basic magmas (subduction-related a nd OIB-like intraplate). These sources were produced by the interactio n between a subducted oceanic slab and isotopically heterogeneous asth enospheric mantle (mantle diapir).