SR-ND-HF ISOTOPIC CHARACTERIZATION OF GRANITOIDS IN ACCRETIONARY OROGENS OF ASIA AND IMPLICATIONS FOR CRUSTAL DEVELOPMENT

Accretionary orogens in Asia are major building blocks of the global continental mass in the Phanerozoic. We address the issues of crustal development in two gigantic orogens: the Central Asian Orogenic Belt (CAOB) and Western Pacific Orogenic Belt (Nipponides). Extensive geochemical and Sr-Nd-zircon Hf isotopic studies have revealed that (a) the generation of granitic rocks in the CAOB involved significant contribution from the upper mantle; that is, substantial amount of juvenile crust has been added to the Asian continent; (b) the juvenile crust was produced by both lateral accretion of arc complexes and vertical accretion of underplated material of mantle derivation. In the Nipponides, available geochemical and isotopic data suggest that a large proportion of Mesozoic and Cenozoic granitoids from SW Japan possess an old crustal signature, thus the bulk crust of SW Japan is probably composed much of “recycled” continental crust of Proterozoic ages. By contrast, the bulk crust of the Pre-Tertiary basement rocks in NE Japan (north of the Tanakura Tectonic Line) and the island of Hokkaido are quite “juvenile”. Sr-Nd and zircon Hf isotopic data of Cretaceous granitoids from the Russian Far East (Sikhote-Alin Range; zircon ages = 110 to 56 Ma) suggested that the granitoids were derived from mixture of juvenile and recycled source rocks in variable proportions.

A comparison between the accretionary orogens of the CAOB and Nipponides leads to the following conclusions: (1) The crustal development of NE Japan is distinguished from that of SW Japan; NE Japan (with Hokkaido) is quite similar to the Junggar Terrane of NW China and the Lake Zone of Mongolia (CAOB), whereas SW Japan is more comparable with the composite Tianshan orogen. (2) Accretionary orogens could be distinguished by the nature of the accreted lithological assemblages. Orogens with dominantly island arc assemblage would lead to generation of granitoids with juvenile characters; by contrast, orogens with accretionary complexes developed in a Precambrian continental margin would have granitic rocks generated with more crustal signature. This is represented by SW Japan, in which the “recycled Precambrian crust” component is significant in the granitoid magma generation.