海岸山脈馬達吉達溪剖面之碎屑鋯石及磷灰石核飛跡定年研究─探討源區山脈之剝蝕歷史

Abstract

中期中新世以來，歐亞大陸隱沒至菲律賓海板塊之下形成一系列火山島弧，隨著大陸與島弧碰撞，山脈也逐漸抬升剝蝕，大量沉積物堆積於造山帶周邊的盆地中。從現今山脈岩層的熱定年結果可以推估近期山脈的剝蝕歷史，但是欲了解早期造山山脈早期的剝蝕歷史，則必須從盆地中的碎屑沉積物的定年資料，才可以探究此結果。本研究於南段海岸山脈泰源盆地中的馬達吉達溪剖面採集12個砂岩與1個礫石樣本，利用磷灰石（AFT）、鋯石核飛跡定年（ZFT）和砂岩岩象分析探討源區山脈與盆地間的源-匯關係，以及脊樑山脈早期的造山演化史。 砂岩的AFT與ZFT定年結果顯示，源區山脈出露岩層的最高變質溫度大多介於200～260℃之間，而分別在蕃薯寮層與八里灣層頂部出現變質溫度高於260℃的變質岩岩屑，則指示山脈逐漸出露埋藏較深的岩層。其中，最底部的砂岩樣本含大量火成岩岩屑，因此本研究將磷灰石細分為島弧及造山帶來源，並分別計算AFT年代，結果指出火山碎屑可能來自成廣澳火山，而變質岩屑可能來自玉里帶上覆硬頁岩層。唯一的礫石樣本採集自崩積礫岩層，在沉積於泰源盆地前曾短暫堆積於北段海岸山脈的水璉礫岩層，因此本研究比對此礫石與鹽寮坑溪礫石的AFT與ZFT年代，發現兩者定年結果相近，均來自變質溫度200～260℃的岩層。 根據砂岩的AFT遲滯時間變化可知，2 Ma時遲滯時間突然由0～1 m.y.增加至2～4 m.y.，指示源區可能有新的來源開始剝蝕，且該岩層的冷卻速率較慢。本研究提出三種可能的新來源岩層，分別是複背斜翼部的板岩層、中段脊樑山脈玉里帶西側的太魯閣帶，以及山脈較南側的岩體。綜合前人及本研究結果推測後前淵盆地的碎屑物主要來自玉里帶上覆岩層，且AFT遲滯時間顯示玉里帶剝蝕歷史從初始抬升（～12 Ma）、加速剝蝕階段，到～3 Ma達到與現今相同的冷卻速率，並於2～1.2 Ma之後趨於穩定剝蝕，冷卻速率自15 ℃/m.y.增加至200～400 ℃/m.y.並持續至今。

Taiwan orogen has been developing since middle Miocene due to the subduction of South China Sea Plate and the arc-continent collision of the Eurasian Plate and the Philippine Sea Plate, which is called the Penglai Orogeny. As the orogen exhumed, those detritus filled into the basins around the orogenic belt. In order to reconstruct the early stage of the orogeny and the source-to-sink relationship of the orogenic belt and basins, we collected 12 sandstones and 1 boulder from Madajidachi-section in retro-foredeep basin, then analyzed with Apatite (AFT), Zircon Fission Track (ZFT) Dating, and also the petrography of sandstones. The dating results of AFT and ZFT show that most of the source rocks experienced the highest metamorphic temperature between 200～260℃, while part of them experienced even higher than 260℃, which indicates the deeper formation has been gradually exhumed. The petrography analysis reveals a source change from arc to orogen between 3.5~4 Ma. Compiling the AFT and ZFT ages of the sandstone samples at the base of the profile, our results suggest that the arc-derived detritus may be derived from the Chengkuang’ao volcano, while the orogen-derived detritus may come from the argillite formation on the Yuli Belt. The only boulder sample between Fanshuliao and Paliwan Formation is recognized as the colluvium from the Shuelien Conglomerate Formation in the northern Coastal Range, and the AFT and ZFT dating results indicate that the protolith experienced the highest metamorphic temperature between 200~260℃. Based on the trend of AFT lag time, there exists an increase from 0～1 m.y. to 2～4 m.y. around 2 Ma, indicating a new source in the provenance which presented lower cooling rate. We suggest that the new source might be the slate formation on the limb of the multi-anticline, the Tailuko Belt on the west side of Yuli Belt, or the southern terrain on the orogenic belt. In conjunction with previous studies, we argue that the detritus in retro-foredeep basin are mainly derived from the cover of the Yuli Belt on the middle Backbone Range, and the change of AFT lag time suggests that Yuli Belt has experienced acceleration stages since the initial uplift ~12 Ma, and gradually approached the highest cooling rate at ～3 Ma, then finally reached the steady state between 2～1.2 Ma. The cooling rate of the Yuli Belt increased from 15 ℃/m.y. to 200～400 ℃/m.y., and remained as the highest cooling rate until now.