海岸山脈南段泰源盆地凝灰岩之初步研究

Abstract

泰源盆地為北呂宋火山島弧的弧前盆地，保存來自島弧火山的噴發物，是研究火山演化的良好題材。但是這些分佈於盆地中的凝灰岩卻鮮少被探討與研究，鑒此，本研究針對盆地內的凝灰岩進行產狀觀察與岩象分析，並依凝灰岩隨著時序演進所發生的變化，推敲火山的噴發演化歷史。 凝灰岩依據產狀特徵的差異共可區分為//sT、lensT、bT、sLT四種不同的岩相，各岩相特徵如下:(1)//sT岩相具有上下層面平行的層形，以及近似包馬序列Ta、Tb層的沉積構造。部分//sT岩相中有雙正級層(Doubly graded)的沉積構造特徵，此構造指示水下火山噴發作用。(2)lensT岩相具有透鏡狀的層理，正級層或反級層的沉積特徵，層中夾有泥質岩塊。(3)bT岩相具有上下層面不規則的層理以及不明顯的正級層特徵。(4)sLT岩相具有上下不規則的層理，層內具有火山礫(Lapilli)反粒級層(Inverse graded)和侵蝕水道的沉積構造。最下部的都巒山層於東側剖面有角礫岩、凝灰角礫岩與//sT和sLT凝灰岩互層，西側剖面則為//sT岩相的凝灰岩與角礫岩互層。其上覆的蕃薯寮層為//sT和bT互層，八里灣層下段的凝灰岩於東側剖面具有lensT岩相特徵，西側則轉變為bT岩相特徵。八里灣層上段的凝灰岩於兩側剖面皆為//sT的岩相特徵。岩象分析結果顯示凝灰岩屬於晶屑型，組成以晶屑為主、火成岩岩屑次之，含有少量的火山玻璃。晶屑組成以斜長石為主、鐵鎂礦物次之，鐵鎂礦物的組成會隨時間具特徵性的變化:(1)都巒山層以輝石為主，角閃石次之，無黑雲母；(2)都巒山層上段則含有黑雲母；(3)蕃薯寮層以輝石與角閃石為主，黑雲母次之；(4)八里灣層以角閃石與黑雲母為主，含少量輝石。礦物外型特徵以破碎的角礫狀至次角礫狀為主，大多數無明顯風化跡象，僅少數受到風化作用影響。 這些來自於盆地東方的凝灰岩沿著火山斜坡衝入，以碎屑流與濁流的方式搬運至盆地，主要為噴發後直接沉積，僅少部分為多次搬運後再沉積，形成今日所見之凝灰岩層。由於都巒山層上段黑雲母始現並與角閃石隨時間逐漸增加，指示源區火山的岩漿由中基性朝酸性演化，其含水量增大，噴發性亦增強。這些凝灰岩指示泰源盆地周圍於3.8Ma後仍有火山活動的發生，1.5Ma後有水下火山噴發事件。

The Tai-Yuan Basin is the fore-arc basin of northern Luzon Arc. The well-preserved tephra sequence in the basin is a very good record of eruption events and the evolutionary history of the arc volcanoes. However, not much is known about these tuffs. This study discusses the eruption and evolutionary history of the volcanoes using geological observations and petrographic analysis of tuff beds. Based on occurrence characteristics, these tuff beds can be divided into four lithofacies: //sT、lensT、bT、sLT. that each have a different sedimentary structure, such as: graded、inverse graded、doubly graded、flame structure, etc., which indicate the features of turbidites or underwater fallout deposits. Petrographic analysis reveals that the tuff in the basin is crystal-rich, mainly composed of feldspar and mafic mineral. The relative composition of mafic minerals present a decrease in pyroxene but an increase in amphibole and biotite with time. Lithic fragments in the tuff are mainly of volcanic composition, rarely are metamorphic lithic seen, and a small amount of volcanic glass can be found. The crystal component is dominated by broken angular to sub-angular occurences, most of which have no obvious signs of weathering. By integrating the above mentioned, this study suggests that these tuffs from the eastern part of the basin are transported along the volcanic slopes into the basin by debris flow and turbidite or fallout, and are mainly directly deposited after an eruption. Only a few parts have been reworked. The magma of the source volcano evolves from andesitic to rhyolitic: its water content increases, causing eruption intensity to also increase., Volcanic activities around Tai-Yuan Basin continued after 3.8Ma, and underwater eruptions began after 1.5Ma.