MESOZOIC TO EARLY CENOZOIC GRANITOIDS OF THE SIKHOTE-ALIN OROGENIC BELT, RUSSIAN FAR EAST: PETROGENESIS, CRUSTAL GROWTH AND REGIONAL TECTONIC EVOLUTION

The Sikhote-Alin Range of the Russian Far East is an important accretionary orogen of the Western Pacific Orogenic Belt. In order to study the formation and tectonic evolution of the orogen, we performed zircon U-Pb dating, as well as geochemical and Sr-Nd-Hf isotopic analyses on 24 granitoid samples from various massifs in the Primorye and Khabarovsk regions. The granitoids were dated at 131 to 56 Ma (Cretaceous to Paleogene). Together with the age information from the vast expanse of silicic volcanic rocks, the most important tectonothermal events in the Sikhote-Alin orogen are known to take place in the Cretaceous.

Most samples are I-type granitoids and they have initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7040 to 0.7083 and ε_Nd(t) values of +3.0 to -6.0 (mostly 0 to -5). Geochemical and isotopic data suggest that the granitoid magmas were produced by partial melting of sources with mixed lithologies, including subducted accretionary complex and probably some hidden Paleozoic-Proterozoic basement rocks. Nd isotopic data allowed us to estimate variable proportions of mantle component (30-77%) in the generation of the granitic magmas. Furthermore, positive zircon Hf isotopic data (ε_Hf(t) = 0 to +15) also suggest an important contribution of the mantle component. However, the large range of ε_Hf(t) values indicate that the zircon grains crystallized from melts of mixed sources and that crustal assimilation occurred during magmatic differentiation.

The quasi-continuous magmatism in the Sikhote-Alin orogen suggests that the Paleo-Pacific plate subduction was very active in the Late Cretaceous. The age pattern seems to indicate that in the Late Cretaceous the Paleo-Pacific plate motion changed from parallel or sub-parallel to oblique relative to the continental margin of the Sikhote-Alin, leading to the change of magmatic source region and geochemical characteristics of the derived igneous rocks. Late Cretaceous rapid sea-floor spreading at ca. 100 Ma induced highly active subduction and led to voluminous magmatism in the entire Circum-Pacific realm. Finally, the present age and isotopic study lends support to the hypothesis of geologic and tectonic correlation between Sikhote-Alin and SW Japan.