俄羅斯錫霍特-阿蘭地區埃達克岩的地球化學特性與岩石成因

Abstract

亞洲大陸東緣，包括俄羅斯遠東、日本、菲律賓以及印尼等西南太平洋群島，為亞洲最年輕的「西太平洋造山帶」的範圍。大量新生大陸物質於此增積造山帶形成，因而西太平洋造山帶之研究對探討大陸地殼增生的機制具有重大意義。本研究著重於研究俄羅斯遠東Sikhote-Alin地區東南側的岩石成因。 Sikhote-Alin地區大致分為兩個地質單元：(1)西側的新元古代布瑞亞–佳木斯–興凱地塊(Bureya–Jiamusi–Khanka terrane)，和(2)東側的中生代Sikhote-Alin Orogenic Belt。白堊紀至新生代早期的火成岩帶—East Sikhote-Alin volcano-plutonic belt (ESAVPB)的火山岩與深成岩廣泛覆蓋、侵入在這兩個地質單元中。近幾年，ESAVPB的鋯石鈾鉛定年與全岩微量元素、同位素的報導陸續發表，對Sikhote-Alin地區的晚中生代、新生代的地體構造演化有進一步的認識。本研究在Sikhote-Alin地區南部發現兩期埃達克岩，年代分別為早白堊紀(130–100 Ma)與始新世(~45Ma)。埃達克岩是高壓基性岩發生部分熔融而產生的火山岩，其岩石成因常與特殊的地體構造環境有關。本研究針對兩期埃達克岩進行全岩主量、微量與鍶釹同位素分析，與鋯石鈾鉛定年的結果，討論其岩石成因、以及對東北亞地體構造演化的意義。 早白堊紀與始新世埃達克岩大多分布於興凱地塊(Khanka terrane)中。鋯石定年結果顯示這些岩體在132–98 Ma與46–39 Ma形成，地球化學分析表明有埃達克岩性質： SiO2= 57–73%；Al2O3= 15–18%；Na2O= 3.5–5.9%；K2O= 0.7–3.7%； Na2O/K2O= 1.3–3.9；Sr/Y= 30–140；(La/Yb)N= 11–53，重稀土與高場力鍵結元素皆有虧損情形。早白堊紀埃達克岩的銣、鍶同位素成分為：εNd(T)= -1.0–3.2；ISr= 0.705227–0.708967；始新世埃達克岩則是εNd(T)= -1.9–2.1；ISr= 0.704318–0.705758。 本研究認為，兩期埃達克岩的岩石成因為隱沒海洋地殼與沉積物的熔融，並標誌著Sikhote-Alin地區隱沒帶岩漿作用的開始與結束。早白堊紀埃達克岩於東北亞已有許多前人報導，噴發時間長達三千萬年，空間、時間分布皆十分廣泛。同時期的島弧岩漿ESAVPB也開始噴發。隱沒初始以及高溫的年輕海洋地殼隱沒，可能是產生這一期埃達克質岩漿作用的機制。始新世埃達克岩伴隨基性岩與酸性岩一同出現，顯示同期有數個岩漿源區。同時期的島弧岩漿活動，出現於東側的庫頁島、日本北海道。一次海洋板塊的回捲(Slab roll-back)可能在此時發生，增強的地函熱對流使海洋地殼發生部分熔融，也令島弧岩漿的噴發向東移至庫頁島、北海道。 此外，日本本州Kitakami地區的早白堊紀與始新世TTG、埃達克岩，具有與Sikhote-Alin埃達克岩相似的年代與地球化學特性。分布於兩個地區的埃達克岩可能來自同一個岩漿系統，並且可作為一對比證據，支持在日本海張裂以前，Sikhote-Alin與Kitakami地區為東亞大陸邊緣相鄰區域的古地理重建模式。

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.