利用碎屑鋯石鈾鉛定年探討大南澳片岩的地層年代與構造演化

Abstract

大南澳片岩出露於脊樑山脈東翼，為台灣最古老的地層，岩性主要為大理岩、片岩及片麻岩，變質基性岩則零星出露。根據地層與構造的研究，將大南澳片岩由下而上區分為九曲大理岩、谷園片岩、白楊片岩、天長大理岩及碧綠層。九曲大理岩原岩沉積年代利用鍶同位素定年結果為250±20 Ma(晚二疊紀)，片麻岩由鋯石鈾鉛定年及地化分析，顯示為90-86 Ma的I型花崗岩。由於受到南澳運動及蓬萊運動變質事件的影響，使分布最廣泛的片岩受到複雜的變形，難以判斷其與其他岩體的接觸關係，化石保存不佳難以得知地層時代，造成資料解讀的困擾，因此本研究藉由碎屑鋯石鈾鉛定年，分析片岩中的碎屑鋯石來討論地層的沉積年代與沉積物來源，同時利用鈾鉛定年得知變質基性岩的生成年代，綜合上述結果建立大南澳片岩地層層序並探討其大地構造演化。 本研究共分析了31個標本，太魯閣帶西側的白楊片岩與碧綠層最年輕碎屑鋯石群加權平均年齡為180-135 Ma，相較於前人於東側的谷園片岩與白楊片岩分析結果(120-110 Ma)還老。但不論從地層的巨視構造、微視構造分析、變質度差異等皆指示太魯閣帶西側的地層較為年輕，基於源區分析，本研究認為太魯閣帶的沉積物來源主要來自於現今台灣海峽地區底下中生代基盤的火成岩，部分來自於福建地區，而西側地層缺乏年輕的碎屑鋯石是受到源區反剝蝕的影響。 太魯閣帶的變質基性岩，藉由地球化學分析多認為是殘存的海洋地殼，但根據本研究結果，90-80 Ma基性岩侵入至九曲大理岩且具有碎屑鋯石的年齡頻譜等特徵，顯示太魯閣帶中部分變質基性岩可能為大陸地殼的板內玄武岩，之後詳細的地化分析與檢視有其必要性。 綜合上述結果，台灣地區於晚古生代至中生代地體構造演化是由被動大陸邊緣轉為活動大陸邊緣的環境。二疊紀時期，台灣位於大陸邊緣，沉積大量石灰岩；約於早侏羅紀晚期，古太平洋向西低角度隱沒至華夏古陸，使石灰岩受隱沒深埋作用變質為大理岩，此隱沒變質作用稱為太魯閣運動。於早白堊紀晚期，碎屑沉積物不整合沉積在九曲大理岩之上，同時受古太平洋板塊回捲作用，火山弧逐漸朝東遷移，90-86 Ma的I型花崗岩與基性岩侵入台灣的大理岩與沉積岩。之後再受板塊持續隱沒影響，碎屑沉積物變質成為片岩，此變質作用即為南澳運動。

The Tananao Schist, the oldest rock, exposed at the Backbone Range which consists of marble, schist, gneiss, and subordinate scattered metabasites. According to the stratigraphy and structural analysis, the Tananao Schist in the ascending order can be divided into the Chiuchu Marble, the Kuyuan Schist, the Paiyang Schist, the Tienchang marble, and the Pilu Formation. Based on previous study, depositional age of the Chiuchu Marble is closed to 250±20 Ma(Late Permian). Using zircon U-Pb dating method, magmatic age of gneiss (I-type granite) yields 90- 86 Ma. Depositional age of schist formation was difficult to define because fossils were poorly-preserved. In present study, we try to use detrital zircon U-Pb dating to yield the maximum depositional age of schist. In this study, the youngest weight mean ages of the Paiyang Schist and Pilu Formation in western part of Tailuko belt are 180-135 Ma, which are older than the Kuyuan and Paiyang Schists exposed in eastern part of the Tailuko belt. Based on macroscopic structure of straitigraphy, microscopic structure analysis, and metamorphic facies, stratigraphy in western part of the Tailuko belt is younger than that in eastern part. Source of the Tarluko belt mainly derived from ignous rock of the Mesozoic basement in Taiwan Strait. Stratigraphy in western part of the Tailuko belt lacks younger detrital zircons because of reverse exhumation. The chemical composition of the metabasites of the Tailuko belt was suggested that they were relics of oceanic crust. But in our study, 90-80 Ma metabasite intruded into Chiuchu Marble probability represts for fractions of metabasites of the Tailuko belt might form in origin of continental crust. More detail chemical analysis and examinations are necessary in the future. Results as above mentioned, we explain tectonic setting of the Tananao Schist. Taiwan region was located on passive continental margin and deposited carbonates during late Permian period. The paleo-Pacific plate westward subducted to the Cathaysia Block along continental margin, and the carbonates are metamorphosed to marble since early Jurassic. The subduction related metamorphism is called the Tailuko orogeny. Clastic sediments were unconformably deposited on the Chiuchu Marble during early Cretaceous. At the same time, the island arc migrated from southeastern China to Taiwan Strait because of the retreating arc system. And then, I-type granite and mafic igneous rock intruded into marble and clastic sediments of Taiwan during 90-86 Ma. Afterwards, clastic sediments were deeply buried caused by continuing subduction and metamorphosed. The metamorphic event is known as the Nanao orogeny.