海岸山脈秀姑巒溪剖面八里灣層之鋯石及磷灰石核飛跡定年研究：探討源區山脈的剝蝕演化

中期中新世以來，海岸山脈從板塊隱沒形成的火山弧，演變為弧陸碰撞的構造環境。前人多針對山脈岩層進行熱定年分析來了解造山帶的時空演化，然而隨著山脈不斷抬升剝蝕，早期出露的岩層已成為碎屑沉積物堆積於盆地中。為探討源區山脈（中段脊樑山脈）較早期的的剝蝕演化，本研究針對秀姑巒溪剖面八里灣層（沉積年代1.7–0.8 Ma）的砂岩及變質砂岩礫石，進行鋯石及磷灰石核飛跡定年，輔以砂岩岩象分析與礫石鋯石鈾鉛定年，期望能從碎屑沉積岩中觀察到現今山脈未能觀察到的紀錄。本研究共分析九個變質砂岩礫石，由ZFT及AFT年齡繪製的時間-溫度圖可以明顯辨識出兩種不同的冷卻路徑：較近期抬升的快速冷卻變質砂岩（I型礫石），與較早期抬升的緩慢冷卻變質砂岩（II型礫石）。本研究提出四種模式以解釋兩者的年代差異及來源意義：（1）來自不同高程，封存深度不相同；（2）來自來造山帶南北向不同區域，反映了造山帶由北往南發育的現象；（3）來自脊樑山脈複背斜的軸部與翼部；（4）來自太魯閣帶與玉里帶兩構造單元的上覆岩層。由於無法得知2–1 Ma流域範圍及古地形，未能進一步驗證前兩種模式可能造成的年代差異；而II型礫石較符合前人建立的脊樑山脈玉里帶冷卻歷史，故本研究認為I、II型礫石分別來自太魯閣帶與玉里帶的可能性較高。相較於礫石記錄特定岩層的抬升冷卻歷史，砂岩年代頻譜則是包含源區流域內所有岩層的年代訊息。對砂岩同時分析ZFT、AFT、岩象分析，能大致描繪出山脈岩層的出露情形，顯示1.6–0.8 Ma期間源區已出露大量的硬頁岩、板岩層以及少量的片岩，且低變質度或未變質來源隨時間逐漸減少。砂岩的完全癒合峰值（最年輕峰值P1）來自造山帶變質岩體，其年代與含量變化理應反映山脈的反剝蝕現象；然而，AFT的P1年代卻隨著層序向上變年老，甚至出現老於同樣本ZFT的P1峰值，本研究認為此現象和年輕且低鈾濃度的磷灰石有關，由於單顆粒年代的誤差大，導致BINOMFIT擬合過程中將多種不同來源的顆粒年代混合成年代偏老的單一峰值。另一方面，從礫石的定年結果（I、II型礫石）與砂岩岩象分析（硬頁岩、板岩、片岩）可以得知源區至少含有兩種不同來源，但在砂岩的ZFT及AFT頻譜均只能觀察到單一個完全癒合峰值，代表受到某種程度的年代混合；而現今廣泛使用的核飛跡統計似乎仍無法解決核飛跡年代低精確度的先天限制，為避免對碎屑沉積岩的年代頻譜作出錯誤解釋，同時分析砂岩及礫石樣本便能檢驗砂岩峰值是否受到混合作用。遲滯時間變化能反映了源區山脈的剝蝕速率演化，但受限於礫石數據的不足及核飛跡年代的低精確度，兩種礫石的遲滯時間無法呈現各自的變化趨勢。若由砂岩與礫石整體的遲滯時間變化來看，源區山脈的剝蝕速率自~6 Ma以來即不斷加快，並在~1.5 Ma達到穩定而持續至今（3–4 km/Ma）。從礫石冷卻速率、盆地沉積速率的轉變時間點，以及山脈岩層記錄的剝蝕速率變化來看，則是顯示~2 Ma存在一個明顯的加速現象。

關鍵字

核飛跡定年；鋯石；磷灰石；八里灣層；脊樑山脈；剝蝕歷史

並列摘要

Taiwan orogen has been recognized as an active arc-continent collision belt with extremely high exhumation rates, indicated by bedrock thermochronometric studies. However, bedrock analysis often misses the early, now substantially eroded records of mountain denudation. In order to unravel the exhumation history of the Eastern Backbone Range（EBR）, zircon and apatite fission track thermochronometers（ZFT and AFT）were utilized on metasandstone cobbles and sandstones from the Pleistocene Paliwan Formation in the Coastal Range. By analyzing ZFT and AFT age spectrums at the same time, we can dig deeper into the information about the EBR. Each sandstone yields a dominant total-annealing component（the youngest peak age P1）with a small proportion of partial- or non-annealing component, suggesting an overwhelming distribution of argillite to slate formation in the source region during 1.7–0.8 Ma¬. Sandstone is usually derived from a broad source region where exhumation rates and lithologies vary, while cobble provides in-situ cooling age from a specific source terrain. Two chronofacies（Facies I II） are identified from the T-t paths of nine cobbles, implying that two different sources with fast- and slow-cooling history existed on the EBR then. Among the four models we propose to clarify the cobble provenance, model 4 is supported by the bedrock cooling history, indicating that fast-（Facies I）and slow-cooling（Facies II）cobbles came from the metasandstone overlying Tailuko Belt and Yuli Belt, respectively. Surprisingly, ZFT total-annealing peaks show a typical younging-upward trend from the sandstone samples while those of AFT present a reverse trend. In addition, total-annealing peak ages of the sandstones are not in line with those of the cobbles, suggesting potentially hidden sources of younger components. Discrepancies presented in our AFT data imply a great influence from inherent properties of the low-U apatites, and further suggest that the recognition of clustered peak ages can be a great challenge, especially in the cases of young orogens with rapid exhumation rates like Taiwan. To better inpret the mixed peaks in sandstone, it is of crucial importance to analyze the cobble and sandstone samples from the same stratum simultaneously. The moving direction of lag time trend usually indicates the exhumational evolution of the source region. Despite that the two facies of cobbles can’t reveal their own lag-time trends due to the scarcity of cobble data, we can still observe an accelerating exhumation signal in general. By combining the ZFT and AFT lag-time trend and cobble cooling paths, we believe that the source region, Central EBR, had experienced a long-term accelerating exhumation since ~6 Ma and an intense acceleration at ~2 Ma. Then, the region has achieved the exhumational steady state since ~1.5 Ma.