臺灣東部海岸山脈泥岩中黏土礦物所隱示之地質意義

The Clay Minerals of Mudstones in the Plio-Pleistocene Formations of Coastal Range, Eastern Taiwan , and Their Implications

丁禕(I Ting)

指導教授 ： 宋聖榮

國立臺灣大學/理學院/地質科學研究所/碩士(2008年)

https://doi.org/10.6342/NTU.2008.01111

摘要

伊萊石結晶度可以作為良好的界定變質帶的指標。透過研究地層中泥岩的伊萊石結晶度，可以回推沉積物來源區所受的變質程度。前人研究海岸山脈馬達吉達溪與秀姑巒溪剖面中伊萊石結晶度，發現具有結晶度向上變佳的現象，且結晶度最佳可達到淺變質帶，因而認為，此現象可用以解釋中央山脈在造山抬升時，表面沉積岩逐漸剝蝕至變質岩的過程。本研究探討上新世到更新世時期，中央山脈剝蝕與弧前盆地沉積兩者之間的相互關係，因此採集海岸山脈由北到南數個剖面泥岩標本，以X光繞射法分析伊萊石結晶度，並整理馬達吉達溪剖面前人研究伊萊石結晶度數據資料，以了解海岸山脈地層中伊萊石結晶度變化的時空關係。結果顯示，海岸山脈地層中伊萊石結晶度與年代關係良好，越年輕的地層中伊萊石結晶度越好。
岩體的侵蝕，從侵蝕初期，一直到侵蝕顛峰，通常會有一段時間間隔，讓侵蝕營力能夠醞釀與累積，而這段時間間隔，可從地層中泥岩與砂岩記錄來推得。本文整理海岸山脈地層砂岩岩象中板岩成分比例變化，與伊萊石結晶度的變化作比較，當伊萊石結晶度開始進入近變質帶(anchizone)，表示此時中央山脈的剝蝕程度，相當於板岩開始出露到地表的階段。而當砂岩岩象中，板岩含量比例開始大量增加時，表示進入了侵蝕作用劇烈的時期。研究結果顯示，從侵蝕初期的碎屑沉積到侵蝕顛峰的碎屑沉積，存在一段時間間隔，這段時間間隔約0.5-1Ma左右。
海岸山脈北段在沉積時，粗顆粒尺度與細顆粒尺度的沉積物來源時間較為一致，在弧陸碰撞早期，都來自於未變質或是輕度變質的源岩。而海岸山脈南段在沉積時，粗顆粒尺度與細顆粒尺度的沉積物來源時間差異較大，導致會有細顆粒尺度的沉積物比粗顆粒尺度的沉積物變質度要來得高之現象。海岸山脈南段在弧陸碰撞早期，粗顆粒尺度沉積物來自未變質源岩，細顆粒尺度沉積物來自低度變質源岩，弧陸碰撞後期，粗顆粒尺度沉積物來自於低度變質岩，細顆粒尺度沉積物來自變質度更高的源岩。
本文同時將西部麓山帶地層中伊萊石結晶度資料做比較，相較於海岸山脈地層，西部麓山帶地層中伊萊石結晶度向上變佳的趨勢並不明顯，且最頂部均未達到淺變質帶(epizone)。西部麓山帶南部與北部麓山帶地層變化差異很大，原因為上新世到更新世時，西部前陸盆地的構造演育史相當複雜所致。

關鍵字

海岸山脈 ； 伊萊石結晶度 ； 黏土礦物

並列摘要

Illite crystallinity is usually a good indicator to define low grade metamorphic zone for pelitic rocks. In previous studies, the sedimentary rocks in the Madagida-chi and Hsiukuluan-chi sequences of the Coastal Range, eastern Taiwan showed increasing the illite crystallinity from the bottom to the top. It was interpreted to record progressive unroofing of the nearby prograde metamorphic terrane during arc-continent collision. In this study, we collected and analyzed clay mineralogy and illite crystallinity of mudstones from several sequences in the northern and middle sedimentary basins of Coastal Range, and also combined the data having published in southern one. The aims are to understand the spatial and temperal variations of illite crystallinity which recorded the unroofing history of source area, the Central Range of Taiwan.. The results show that the younger formation age was, the better illite crystallinity was in all sedimentary basins of the coastal Range, eastern Taiwan.
The anchizone illite crystallinity of sediments suggests that the slate belt of Central Range has been exposed to the surface for erosion. However, the abrupt increase of slate ratio in sandy components is to infer that the slate belt have been uplifted and eroded quickly. From the beginning of slate exposed to the erosion peak, it has time lag in general. The result from illite crystallinity and prtrography in each sections of the Coastal Range showed that the time lag was approximately to 0.5-1.0Ma.
The deposited timeof coarse and fine particles in the northern part of Coastal Range, east Taiwan seems to more consistent than southern one due to the oblique arc-continent collision which the coarse and fine sediments eroded and deposited in the forearc basin of the north almost simultaneously.. However, there was a time lag for the fine and coarseparticles which the former eroded and deposited in forearc basin of southern one was earlier than the latter during the early stage of collision, Furthermore, the coarse particles abruptly increased when the slate terrane uplift to the peak. .
We also compiled the data from the Western Foothills and found that the illite crystallinity increasing upward were not significant comparing with the Coastal Range, eastern Taiwan. This result suggests that it may due to the complicated foreland basin tectonic processes in the Western Foothills of Taiwan.

並列關鍵字

the Coastal Range ； illite crystallinity ； clay minerals

參考文獻

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