台灣大濁水片麻岩磷灰石核飛跡年代與其地質意義

Abstract

台灣位於呂宋島弧與歐亞板塊之間的碰撞聚合帶，形成連續且年輕的地體構造活動。為了瞭解大南澳片岩帶北端自蓬萊造山運動以來的熱歷史，本研究沿大南澳片岩區的和平林道採集九個不同高度的大濁水片麻岩體之岩樣，測定磷灰石核飛跡年代，討論海拔高度與磷灰石核飛跡年代的關係。 根據磷灰石核飛跡年代分佈，顯示本研究區的磷灰石顆粒均在蓬萊運動期間受熱作用而完全癒合，核飛跡年代介於0.6±0.1Ma與0.3±0.1Ma之間，表示在0.6Ma之後，和平林道至中橫公路一帶的熱作用已經低於磷灰石核飛跡的封存溫度。 本研究岩樣的磷灰石核飛跡年代與海拔高度呈正相關。但於前人磷灰石核飛跡年代中有些樣本之年代與海拔正相關情況不明顯，推測這些樣本因該地的等地溫線以及地下熱源分佈不均、剝蝕速率各地不一致等原因，造成核飛跡年代的差異。 本研究比較兩種不同的方法計算剝蝕速率，其一是使用兩岩樣的高度差除以年代差估計的速率為2.2mm/yr（0.6Ma以來）；其二是以假設地溫梯度與封存溫度來估算，速率介於4-9mm/yr（0.6Ma以來）。此二數據以後者可信度較高。 綜合前人之鋯石與磷灰石核飛跡年代研究，大南澳片岩帶北端之冷卻速率自蓬萊造山運動以來不斷地加快，然而因古地溫梯度、冷卻初始溫度、初始深度等資料不足，無法直接將冷卻速率轉換為剝蝕速率。往後若能取得範圍更大的海拔高度對應核飛跡年代之資料，對於解決此問題會有所助益。 關鍵字：大南澳片岩、大濁水片麻岩、磷灰石、核飛跡、剝蝕速率

Taiwan is located in a remarkable tectonic setting at the convergence between the Luzon arc and Eurasian plate. In order to understand the thermal history of the northern Tananao Schist of East. Taiwan, since the Penglai Orogeny, a total of nine bedrock samples were collected from the Tachosui Gneiss along the Hopin forest trail as a vertical transect within an area of few km2. The samples were analyzed by fission-track dating technique. The results and the localities of the sub-vertical section allowed us to discuss the relationship between elevation and apatite fission-track ages. The apatite fission-track ages of the nine samples range from 0.6±0.1Ma to 0.3±0.1Ma and display unimodal distributions, implying these bedrock samples were totally annealed during the Penglai Orogny. It suggests that the northern Tananao Schist had been subjected to a higher temperature and then cooled down below the closure temperature of the apatite fission-track system which probably initiated at ~0.6 Ma. The positive correlation between apatite fission-track ages and sampling elevations implied that the bedrock of the studied area was close to exhume steady state. In this study, two different uplift rates of the studied area based on different approaches can be obtained. The denudation rate of 2.2mm/yr was estimated by the difference in elevation divided by the difference in apatite fission-track ages between two samples; the other estimation involves assumption of geothermal gradient and closure temperature which gives uplift rates in the range of 4-9 mm/yr. Combined with the previous zircon and apatite fission-track studies, these data demonstrate that the cooling rates have continuously increased since the Penglai Orogeny. Key word: Tananao schist, Tachosui gneiss, fission track, apatite, denudation rate