Eocene granites in South Sakhalin, Russian Far East: geochemical and Sr-Nd-Hf isotopic characteristics, petrogenesis and tectonic implications
| Autor: | Jia-Ping Liao, 廖家平 |
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| Rok vydání: | 2017 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 105 The Sakhalin Island in Russian Far East represents a geological component of the northern Western Pacific Orogenic Belt. The island comprises several exotic terranes that have recorded multiple episodes of accretion to the paleo-Asain continental marigin with strike-slip displacements from the Cretaceous to Tertiary. The stratigraphic and structural features of terranes of South Sakhalin can be correlated with those of the Hokkaido Island in northern Japan. However, the origin and correlation of some terranes remain controversial. South Sakhalin is an important link between the tectonic units of the Sakhalin and Hokkaido Islands. Different kinds of tectonic activities including subduction, strike-slip movement, and block rotation have been documented in South Sakhalin in the Cenozoic. Eocene granitic rocks that occurred in South Sakhalin provide clues to examine the crustal formation and terrane correlation between the Sakhalin and Hokkaido Island in specific, and the crustal evolution of the northern Western Pacific orogenic belt in general. This study reports new whole-rock geochemical and Sr-Nd isotope data as well as zircon U-Pb ages and Hf isotope data of granitic rocks from the Okhotsk and Aniva plutons, and associated volcanic dikes and sedimentary rocks in South Sakhalin. Based on these data, I discuss petrogenesis of the granitic and associated volcanic rocks, and its implications on the Cenozoic tectonics and crustal compositions of South Sakhalin. Two mid-Eocene granitic plutons in South Sakhalin show markedly different geochemical characteristics: (1) the Okhotsk granites that intruded into the Ozersk terrane from ca. 44 to 42 Ma are slightly peraluminous (A/CNK = 1.02 - 1.08), ferroan and alkali-calcic, and have transitional I- and A-type features, and (2) the Aniva granites that intruded into the Tonin-Aniva terrane from ca. 41 to 40 Ma are peraluminous (A/CNK = 1.08 - 1.21), magnesian and calc-alkaline, typical of S-type features. The Sr-Nd-Hf isotopic ratios of both plutons suggest a common origin from partial melting of mixed sources with a dominant juvenile mantle component and a subordinate crustal component. The Okhotsk granites have εNd (T) values of +3.1 to +3.7 and initial 87Sr/86Sr ratios of 0.7047 to 0.7048. The Aniva granites have εNd (T) values of +0.5 to +0.9 and initial 87Sr/86Sr ratios of 0.7052 to 0.7055. I argue that the I-/A-type Okhotsk and S-type Aniva granites can be correlated to the mid-Eocene granites in the western and eastern part of the Hidaka terrane in Hokkaido with similar isotopic signatures respectively. The Okhotsk and Aniva granites were presumably generated by partial melting of accretionary complexes, a result of underplating of mantle-derived basaltic magmas during local extension and lithospheric thinning due to the retreat of the Pacific plate. The Aniva granites (Nd (T) = +0.5 +0.9; Hf (T) = +5 +12) were produced with more recycled crustal components than the Okhotsk granites (Nd (T) = +3.1 +3.7; Hf (T) = +11 +16). It indicates that more recycled crustal materials were involved in the Tonin-Aniva terrane. Although the Okhotsk and Aniva granites were generated in a similar period and tectonic setting, the source nature of their intruded terranes (i.e. the Ozersk and Tonin-Aniva terrane) is different, deduced from their distinctive geochemical and petrographic features. During 45 and 37 Ma, the Hokkaido Island may have been in a supra-subduction setting that led to the emplacement of I-type granitoids. The broadly coeval Okhotsk granites in South Sakhalin, however, show different geochemical features with I-/A-type signatures. I attribute the granitic magmatism (44-40 Ma) in South Sakhalin to a transition from supra-subduction to strike-slip tectonics in response to the retreat of the Pacific plate at the junction of the Asian continent, the Pacific plate, and the Okhotsk Sea plate. |
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