臺灣中部魚藤坪砂岩至頭嵙山層磷灰石及鋯石核飛跡定年：探討北部雪山山脈與前陸盆地的剝蝕歷史

Abstract

晚中新世以來，菲律賓板塊（呂宋島弧）與歐亞板塊發生碰撞，造成一系列北東走向的造山帶，同時在造山帶前緣形成前陸盆地。造山山脈歷經長時間抬升剝蝕，其碎屑沉積物會大量堆積於盆地中，透過分析盆地地層的沉積物得以揭露過去已剝蝕的造山帶之演育。本研究針對草湖溪剖面及大甲溪剖面的魚藤坪砂岩至頭嵙山層，進行磷灰石及鋯石核飛跡定年分析，並綜合前人岩象研究成果，探討中部造山帶與前陸盆地的剝蝕歷史。 本研究從魚藤坪砂岩至頭嵙山層依序採集10個砂岩樣本，砂岩中磷灰石及鋯石核飛跡年代頻譜呈現多峰值，代表源區變質度特性，所以可反映當時盆地接收的來源。當砂岩樣本曾經受抬升作用通過磷灰石及鋯石核飛跡封存溫度的地溫時，便開始累積核飛跡，因此，透過計算礦物中核飛跡數量，可得到冷卻年代頻譜。其中，若源區岩層埋藏溫度與礦物封存溫度相近，核飛跡年代則可以記錄岩層開始抬升的相近時間，並透過樣本的遲滯時間隨層序變化的趨勢來了解造山帶的冷卻速率及剝蝕速率演化。 本研究分析各層序中砂岩樣本的磷灰石及鋯石核飛跡年代頻譜結果，得知臺灣中部造山帶的剝蝕歷史：穩定剝蝕階段（從6.4 Ma至2.6-1.4 Ma），此時造山帶的冷卻速率約 25.7±12.7-42.9±18.4 ℃⁄Ma，剝蝕速率約 0.5±0.3-0.9±0.4 km⁄Ma。約4.2 Ma出露磷灰石完全癒合的沉積岩層，約2.6 Ma出露磷灰石完全癒合、鋯石部分癒合的硬頁岩層；加速剝蝕階段（從2.6-1.4 Ma至 0.9 Ma），造山帶的冷卻速率從 64.3±31.4 ℃⁄Ma 加速到 300.0±250.5 ℃⁄Ma，剝蝕速率從 1.3±0.7 km⁄Ma 加快到 6.0±5.3 km⁄Ma。約1.2 Ma出露磷灰石完全癒合、鋯石部分癒合的低度變質岩層。

Since late Miocene, the Philippine Plate (Luzon Arc) has collided with the Eurasian Plate, resulting in a series of north-east trending orogenic belts, while forming foreland basins at the front edge of the orogenic belts. The detrital sediments from the basin strata can be analyzed to reveal the evolution of the eroded orogenic belts in the past. In this study, we used apatite and zircon fission-track thermochronologies to investigate the exhumation history of the central orogenic belt and the foreland basin. We collected ten sandstone samples from the Yutengping Sandstone to Tukoshan Formation in the Tsaohuchi section and the Tachia section. We find the spectrum of apatite and zircon fission-track dating ages in the sandstone showed multiple peaks, representing the metamorphic characteristics of the source area and the accumulation of orogenic detritus in foreland basins at that time. The cooling age spectrum can be obtained by counting the number of fission tracks in the mineral when the sandstone sample was subjected to uplift through the mineral closure temperature and started to accumulate fission tracks. The thermal history of each studied grain bears burial information of own sourced terrain and provides the constraints on thermal status of the exposed outcrops at that time, which reflects the emerging stages of the orogen. The result indicates that exhumation history of the central orogenic belt is divided into two stages: First, the stable exhumation stage ( from 6.4 Ma to 2.6-1.4 Ma ): the cooling rate of the orogenic belt is 25.7±12.7-42.9±18.4 ℃⁄Ma, and the exhumation rate is 0.5±0.3-0.9±0.4 km⁄Ma. Timeline wise, sedimentary formations with fully reset apatite were exposed about 4.2 Ma while argillite formations with fully reset apatite and partially reset zircon were not exposed until 2.6 Ma. Second, the accelerated exhumation stage ( from 2.6-1.4 Ma to 0.9 Ma ): the cooling rate of the orogenic belt is 64.3±31.4-300.0±250.5 ℃/Ma, and the exhumation rate is 1.3±0.7-6.0±5.3 km/Ma. During this period, low grade metamorphic formations with fully reset apatite and partially reset zircon were exposed about 1.2 Ma.