林口台地鋯石核飛跡定年

Abstract

本研究利用觀音火山西南方林口台地的一口鑽井，採取林口礫石層上部約~100公尺內的十個沉積物岩芯，分離出鋯石測定核飛跡年代，以探討林口礫石層的沉積年代及台灣北部的古剝蝕/冷卻速率。本研究測得鋯石顆粒年代的範圍介於0.9 ± 0.6 ~ 245.6 ± 201.7 (1σ) Ma.。根據前人的鋯石核飛跡年代資料，可區分出林口礫石層鋯石顆粒年代分別代表三個來源區的冷卻或生成年代：小於4 Ma者來自北台灣火山區；4 ~ 90 Ma的顆粒來自雪山山脈輕度變質岩區；老於90 Ma的顆粒來自西部麓山帶沉積岩區。 屬於在火成岩來源區的鋯石顆粒核飛跡年代介於2.6 ± 0.4 ~ 0.9 ± 0.6 (1σ) Ma。其中深約100公尺處的岩芯樣品，可得最年輕的鋯石顆粒年代1.2 ± 1.8 (1σ) Ma，故林口礫石層在高度~130公尺處最可能的沉積年代為≦1.2 Ma。最近地表的岩芯樣品，最年輕的鋯石顆粒年代為0.9 ± 0.6 (1σ) Ma。顯示林口礫石層最上部約100公尺厚的地層沉積年代極可能為1.2 ± 1.8 ~ 0.9 ± 0.6 (1σ) Ma，進而推論台北盆地因山腳正斷層活動而陷落，以致於古新店溪不再提供沉積物堆積至林口沖積扇的年代約為~0.9 ± 0.6 (1σ) Ma。 核飛跡年代介於4.1 ± 0.9 ~ 90.8 ± 18.4 (1σ) Ma的鋯石顆粒研判來自雪山山脈，核飛跡年代反應鋯石經歷不同程度的熱作用而部分癒合或全部癒合。其中≦~6 Ma的鋯石顆粒應係受蓬萊造山運動熱事件而全部癒合後的冷卻年代。此年代扣減林口台地的沉積年代（1.2 ± 1.8 Ma），可估算含該鋯石顆粒的岩石自通過封存溫度後至暴露地表的時間介於5 ~ 3 m.y.。以鋯石核飛跡的封存溫度~240℃，地溫梯度約為30℃/km，則雪山山脈北段在6 ~ 1 Ma以來的平均冷卻速率為69.0 ℃/m.y.，而平均剝蝕速率為2.3 mm/yr。 來自西部麓山帶的鋯石，核飛跡年代介於91.5 ± 1.1 ~ 245.6 ± 201.7 (1σ) Ma之間，其中又以110 ~ 160 Ma為主，與前人研究結果大致符合，進一步證實西部麓山帶沉積物源自於大陸東南白堊紀燕山晚期的岩石為主。假設鋯石核飛跡定年的封存溫度為~240℃，地溫梯度為30℃/km，沉積年代約介於7 ~ 33 Ma，則中國東南區域從~160 Ma至西部麓山帶沉積以來，冷卻速率與剝蝕速率分別介於2.7 ~ 1.7 ℃/m.y.與0.09 ~ 0.06 mm/yr之間，與前人對本區露頭採樣定年研究結果比較，顯示中國東南地區曾經歷了高溫階段快速冷卻而低溫階段緩慢冷卻的過程。 若不計極少量的北台灣火山來源的鋯石顆粒，則本研究所採樣品中來自西部麓山帶與雪山山脈的鋯石顆粒比例約為1：3，與現今新店溪流域內的西部麓山帶與雪山山脈集水區面積比例大致相同。這表示自~1.2 Ma以來，新店溪流域出露的地層與其分佈面積的比例，變化並不顯著。

In order to understand the depositional ages and the provenance of the sedimentary rocks of the Linkou Tableland in northern Taiwan, a total of 10 core samples were collected from the borehole drilled on the Linkou Tableland for fission-track dating (FTD) of zircon grains. The grain ages obtained range from 0.9 ± 0.6 to 245.6 ± 201.7 (1σ) Ma. Based on previous FT studies on northern Taiwan, the author showed that those ages clearly reflect the FT ages characteristics of the provenance of the Linkou Formation, i.e., volcanic terrain (less than 4 Ma), northern Hsuehshan Range (4 ~ 90 Ma) and Western Foothills (＞90 Ma). The precision for the young ages is not good due to the scarcity of fission tracks. The youngest grain age of the sample collected from the deepest part of the borehole is 1.2 ± 1.8 (1σ) Ma, implying that the depositional age for the level of 130m below the ground surface is very likely ≦ 1.2 Ma. On the other hand, the zircon FT age from near the ground surface is 0.9 ± 0.6 (1σ) Ma. Therefore, the maximum depositional age of the uppermost 100m-thick gravel strata can be estimated at 1.2 ± 1.8 ~ 0.9 ± 0.6 (1σ) Ma. It is also inferred that the onset of the subsidence of the Taipei Basin due to the Shantzechiao normal fault is very probably around 0.9 ± 0.6 Ma, when the ancient Hsintein Stream and the Keelung River stopped to feed sediments to the Linkou fan delta. The zircon grains which have FT ages between 4.1 ± 0.9 and 90.8 ± 18.4 (1σ) Ma are considered to be derived from the northern Hsuehshan Range. The grains which have ages of ＜~6 Ma are regarded as totally reset due to the Penglai Orogeny. Lag time was defined as the difference between the time of closure in the source region and time of deposition in the basin which is represented the processes of the exhumation. By subtracting the maximum depositional age of the Linkou Formation (~1.2 ± 1.8 (1σ) Ma) from each totally reset age, the lag time of the northern Hsuehshan Range was estimated at 5~3 m.y. With the assumption of a thermal gradient 30 ℃/km and a closure temperature of 240℃ for zircon FT system, the cooling and exhumation rates during the period of 6 ~ 1 Ma are calculated as 81.0 ~ 46.2 ℃/m.y. and 2.7 ~ 1.5 mm/yr, respectively. This result is basically consistent with that obtained from previous apatite FT study. The oldest component of the zircon FT ages ranges from 91.5 ± 1.1 to 245.6 ± 201.7 (1σ) Ma with three best-fit peak ages between 110 and 160 Ma, obviously implying that unreset zircon grains of the Western Foothills were most likely derived from the Jurassic－Cretaceous rocks in Southeast China. According to the lag time concept, we calculate the cooling and exhumation rate during the period of ~160 to ~7 Ma as 2.8 ~ 2.2 ℃/m.y. and 0.10 ~ 0.07 mm/yr, respectively. It also implies that the source rocks have experienced rapid cooling during the early higher temperature stage and slow cooling during lower temperature period. The ratio of the number of zircon gains from the Western Foothills versus that of the Hsuehshan Range is about 1:3, which is similar to the proportion of the drainage area of the present Hsintein Stream flowing on the two different geological belts. This is also implies that the drainage area and exposured strata of the ancient (~1.2 Ma) and present Hsintein Stream do not change significantly.