| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 104 === The Mesozoic-to-Cenozoic Sikhote-Alin orogenic belt and late Precambrian Khanka block are two major tectonic units in the southmost Russian Far East. The Sikhote-Alin belt comprises several tectonostratigraphic terranes, including late Precambrian nappes, and Mesozoic accretionary prisms and turbidite basins. These terranes are overlain by Cretaceous to Paleocene felsic to intermediate volcanic rocks and intruded by granitoids. The magmatic rocks are collectively known as “the East Sikhote-Alin volcano-plutonic belt” (ESAVPB), and mainly characterized by acid-to-intermediate compositions. This work concerns a petrogenetic study of adakitic rocks from Sikhote-Alin, and discusses its tectonic implications.
Adakitic rocks of Sikhote-Alin were emplaced in two main periods: Early Cretaceous (132–98 Ma) and Eocene (46–39 Ma). They mainly occur in the Khanka block, with a subordinate amount in the ESAVPB. The adakites show a large range of chemical composition: SiO2 = 57–74%, Al2O3 = 15–18%, Na2O = 3.5–6.1%, K2O = 0.7–3.2%, Na2O/K2O = 1.1–3.9, Sr/Y = 33–145, and (La/Yb)N = 11–53. HREE and HFSE are remarkably depleted. The Early Cretaceous adakites show Nd(T) = -1.0 to +3.2；ISr = 0.7040 – 0.7090, and the Eocene adakites have Nd(T) = -2.0 to +2.2；ISr = 0.7042 – 0.7058. Thus, the Sr-Nd isotopic compositions of Cretaceous and Eocene adakites are not readily distinguishable.
Adakites may have different modes of generation. However, according to a modal calculation, partial melting of meta-basic rocks in a subduction zone is considered as the most likely mode for the present case. The two periods of adakites have probably formed in the following scenario. The early Cretaceous (130–100 Ma) emplacement time for the adakites and the oldest granitoids of the ESAVPB, is considered as the time of initiation of the Paleo-Pacific subduction in NE Asia. Furthermore, to explain a slab melting process with such long period of time, the generation of the adakites was probably connected with subduction of a young slab. The Eocene (~45 Ma) adakites were also generated in subduction zone, and during the generation, a small amount of andesite and rhyolite was also produced. Contem- poraneous granitoids were emplaced 200-400 km to the east of the study area in Sakhalin as well as in Hokkaido (Japan). With this scenario, we may speculate a roll-back of subducting Pacific plate during the Eocene, and a shifting of arc magmatism from the ESAVPB to Sakhalin Island and Hokkaido.
Note that abundant adakitic rocks of early Cretaceous and Eocene ages also occur in the Kitakami and Abukuma Mountains of NE Japan. Consequently, geological correlation between Sikhote-Alin and Kitakami-Abukuma is highly probable, particularly before the opening of the Japan Sea that took place in late Cenozoic.
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