台灣蛇綠岩同位素定年及其大地構造意義

Abstract

蛇綠岩是在板塊聚合帶構造活動中被擠壓到大陸邊緣出露的海洋地殼與上部地函的碎塊，是板塊縫合帶在陸地上的標誌，海洋地殼能夠進入造山帶的主要方式是通過發生在聚合性板塊邊界的隱沒與地殼增生造山作用。本研究針對台灣造山帶中的蛇綠岩系礦物樣本進行同位素定年分析，利用鋯石的鈾鉛定年以及變質礦物的鉀氬衰變系統進行氬氬年代測定，鋯石的鈾鉛年代可以提供較為明確的證據來釐清蛇紋岩的來源，變質礦物的氬氬年代則可作為進一步變質時間的約束。以現代主流的板塊運動重建以及前進型增生造山作用的設定，解釋台灣造山帶中蛇綠岩塊在台灣約6.5百萬年前的弧陸碰撞造山作用發生前的生成與變質紀錄，了解板塊聚合帶從海洋岩石圈隱沒到被動大陸邊緣隱沒之間的轉換。 通過玉里帶的高壓基性地塊、利吉混同岩體中的東台灣蛇綠岩塊以及恆春半島的基性礫石的採樣分析，同位素定年結果顯示其中的蛇綠岩塊均來自23-15百萬年前生成的南中國海岩石圈。玉里帶的基性地塊在9百萬年之前發生了變質作用，並從隱沒通道折返到增積岩體底部；東台灣蛇綠岩塊則在進入增積岩體後並未深埋就被從增積後楔向東傾瀉到菲律賓板塊的弧前盆地。恆春半島的基性礫石則在約13百萬年前發生動態變質後沉積，基性沉積物來自大陸邊緣上的一個由海洋地殼孤立隆起形成的高地，很可能是由於歐亞大陸邊緣對馬尼拉海溝斜向隱沒造成的應力加上應變重組引起的板塊邊緣構造重組。綜合而言，台灣造山帶中的蛇綠岩紀錄著南中國海板塊開始向菲律賓板塊隱沒後的13-9百萬年間發生的一系列構造事件，有時間與空間上的關聯，揭示了洋-陸板塊隱沒與弧陸碰撞過程中所隱藏的構造演化過程。

Ophiolite serves as a signature of suture zone, derived from oceanic lithosphere. The main mechanism which induces oceanic crust entering the orogenic belt is through subduction and crustal accretion at convergent plate boundaries. This study conducts a study of isotopic geochronology on the ophiolites cropping out around the Taiwan mountain belt. The obtained zircon U-Pb ages provide evidences helping to clarify the origin of ophiolites, while the 40Ar/39Ar ages of metamorphic minerals further constrain the timing of metamorphic events associated with subduction and accretion processes before the arc-continent collision at ~ 6.5 Ma, and therefore provide information to better understand the transition of convergence boundary from subduction of oceanic lithosphere to passive continental margin subduction around the Taiwan Mountain belt. Geochronological data from the high-pressure mafic blocks in the Yuli belt, the East Taiwan Ophiolite (ETO) blocks in the Lichi mélange, and the mafic conglomerates in the Hengchun Peninsula show that these mafic blocks were all originated from the South China Sea plate formed at 23-15 million years ago. The mafic blocks in the Yuli belt experienced metamorphism before 9 Ma and exhumated from the subduction channel to the bottom of the accretionary complex. The ETO blocks were not deeply buried after entering the accretionary complex, instead transported eastward with retrowedge to the forearc basin of the Philippine Sea Plate. New geochronological constraints further indicate that a piece of Early Miocene South China Sea crust was dynamically metamorphosed at ~13 Ma, forming an isolated high-relief subaerial mountain range along the Chinese continental margin, and then cropped out as mafic conglomerates in the Hengchun Peninsula. Therefore, these ophiolitic blocks in the Taiwan orogenic belt record a series of tectonic events occurring between 13 and 9 million years ago. These events exhibit temporal and spatial correlations, revealing the hidden structural complexity during the transition from oceanic subduction to arc-continental collision.