俄羅斯遠東地區庫頁島南部始新世花崗岩之地球化學特徵、岩石成因與大地構造含義

Abstract

庫頁島(Sakhalin Island)地處俄羅斯遠東地區，是西太平洋造山帶(Western Pacific Orogenic Belt)北部的一環，由數個自白堊紀至第三紀增積至古東亞大陸邊緣的地塊(terrane)構成。地層學以及構造帶的對比指出庫頁島的各地塊與東北日本北海道島的各地塊有地質上的連續性。然而，部分地塊的起源與對比仍具爭議。 為了探討庫頁島及北海道島的地質帶對比，地處兩島交界處的南庫頁島之研究顯得更為重要。庫頁島南部的地塊記錄了新生代以來的隱沒與地塊增積事件，以及走向滑移與塊體旋轉運動。南庫頁島始新世花崗岩可提供此區地殼組成以及地塊對比的重要線索，並且能進一步納入西太平洋造山帶的地殼演化的討論之中。 鄂霍次克(Okhotsk)與阿尼瓦(Aniva)岩體為庫頁島南部的兩大花崗岩體。此研究將報導在此兩岩體中花崗岩、火山岩脈與沉積圍岩的鋯石鈾鉛定年與Hf同位素分析以及全岩元素與Sr-Nd同位素分析結果，進一步討論花崗岩以及伴生火山岩的成因，並提供對新生代南庫頁島大地構造解釋以及與北海道地塊對比的制約。 研究結果表明此兩花崗岩體具有顯著不同的地球化學特徵。鄂霍次克花崗岩於44 ~ 42 Ma侵入Ozersk地塊，屬輕度過鋁質(A/CNK = 1.02-1.08)、鐵質、鹼鈣系列，有著過渡I型與A型花崗岩的地球化學特徵。阿尼瓦花崗岩於41 ~ 40 Ma侵入Tonin-Aniva地塊，屬過鋁質、鎂質、鈣鹼系列，有著典型的S型花崗岩特徵。 同位素特徵顯示此兩岩體的岩漿來自於新生的地函岩漿來源組分(juvenile mantle component)為主的混合源岩，而此源岩的地殼組分(增積岩體)比例相對較少。鄂霍次克花崗岩的 Nd (T)值介於+3.1 ~ +3.7，初始Sr同位素值介於0.7047 ~ 0.7048之間。阿尼瓦花崗岩的 Nd (T)值介於+0.5 ~ +0.9，初始Sr同位素值介於0.7052 ~ 0.7055之間。北海道中始新世(45~37 Ma)的花崗岩體有著分別與此兩岩體相對應的同位素特徵。因此，此中始新世期南庫頁島的花崗岩可與同期北海道的花崗岩作為對比。 本研究提出鄂霍次克與阿尼瓦花崗岩生成於在因太平洋板塊向東後退(retreat)而造成局部張裂與岩石圈減薄的環境。來自地函的玄武岩質岩漿底侵造成下地殼年輕增積岩體的部分融熔，並造成較基性的玄武岩質岩漿與較酸性的地殼源融體混合而形成花崗岩質岩漿。阿尼瓦花崗岩(Nd (T) = +0.5  +0.9; Hf (T) = +5  +12)相較於鄂霍次克花崗岩(Nd (T) = +3.1  +3.7; Hf (T) = +11  +16)有更富集的同位素特徵，顯示Tonin-Aniva地塊有著更古老的地殼沉積物組成。雖然此兩花崗岩體岩漿形成年代以及構造環境相似，但藉由地球化學與岩相學特徵的差異推斷它們各自繼承的侵入地塊源岩性質並不相似。 本研究推論庫頁島南部的中始新世花崗岩成因與當時該區處於太平洋板塊邊界向東後退(retreat)的大地構造環境轉換有關，此岩漿期也對應到南庫頁島由隱沒轉換為轉型板塊邊界的變換期。同期的北海道島可能仍為隱沒帶之上的岩漿活動造成的I型花崗岩所主導。南庫頁島則出現具有不同地球化學特徵的I-/A-型花崗岩。當時南庫頁島位於歐亞大陸板塊、鄂霍次克海板塊與太平洋板塊的交界，由隱沒到轉型板塊邊界的轉變造成末期隱沒帶之上(supra-subduction)的岩漿作用轉變為走向滑移斷層邊界上局部張裂環境的花崗岩漿作用。

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.