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| 題 名 | Fission-Track Dating of Lutao Volcanics--Implications of Partial Annealing and Eruption History=由綠島火山岩之核飛跡定年看部份癒合之意義及其火山噴發史 |
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| 作 者 | 楊燦堯; 陳正宏; 李太楓; | 書刊名 | 中國地質學會會刊 |
| 卷 期 | 35:1 1992.01[民81.01] |
| 頁 次 | 頁19-43 |
| 分類號 | 354.1 |
| 關鍵詞 | 火山; 火山岩; 核飛跡定年; 部分癒合; 綠島; 噴發史; |
| 語 文 | 英文(English) |
| 中文摘要 | 本研究選擇綠島代表性之火山岩，分離出其中晶形良好之鋯石，從事核飛跡定年分析，共計得到22個標本之鋯石核飛跡年代。標本之鋯石核飛跡年代。各標本之總和平均年齡有相當大之分怖範圍（49.9百萬年至0.75百萬年），由年代機率分佈曲線圖可以發現，許多標本有雙高峰分佈的傾向，顯示部份標本之鋯石有些仍保有前期熱事件之殘餘紀錄，故這些總和平均年齡並無法全部代表其火山噴發年代。但經過卡方測試分離修正後（Brandon and Vance，1991），所得之年齡（2.0∼0.54 Ma）與各標本之年代機率分佈曲線中之高峰年齡相當一致，故可用以代表其噴發年代。 將本結果與前人利用其他定年法所得結果相比較，發現利用氬--氬法經過校正後之高原年齡（Lo et al., 1991）與本研究修正後之年齡有相近的結果，但與鉀--氬法所得之結果（Richard et al.，1986；Juang，1988；Juang and Chen，1990）差異甚大。以往利用銣-鍶法分析結果（Lan et al.，1986）顯示，本地區火山岩有明顯放射性同位素不平衡的現象，表示綠島地區火山岩與前期火山物質（或其他物質）之混染作用並不十分均勻。故而利用鉀-氬法所得之年齡也要比核飛跡定年法所得噴發年齡老，當我們要利用這些年齡時要特別小心。 由於大部分標本仍保有前期熱事件之殘餘記錄，顯示綠島地區在二百萬年或更早以前已經有火山活動；由本研究中異常老的部份癒合年代，支持綠島地區岩漿活動時，在上部地殼處有沉積物混染的說法。配合各火山噴發中心及其主要噴發產物之岩性、分佈位置，我們可以將本地區之火山活動至少分為五期。 |
| 英文摘要 | Representative volcanic rocks of Lutao were collected for fission-track dating (FTD) of their contained zircons. Twenty-two out of thirty-eight samples yielded zircons for FTD. The pooled ages range widely from 49.9 Ma to 0.75 Ma. Some of the samples produce bi-modal or even poly-peaked composite probability density plots, indicating partial annealing effects, so the pooled ages can not represent their eruption ages. Most of the X�� ages (brandon and Vance, 1991) are in good accord with the youngest peak date in the composite probability density plots. They range from 2.0 Ma to 0.54 Ma, and can be interpreted as the ages of volcanic eruptions. When compared with "ages" derived from other radiometric dating methods (Ar-Ar, Rb-Sr and K-ar methods) applied to rock samples from the same localities, the FTD results always give the youngest age among them, but are close to the Ar-Ar plateau ages (Lo et al., 1991). Coexisting phenocrysts in Lutao volcanics do not define concordant Rb-Sr isochrons (Lan et al., 1986), implying that the magmas had not been homogenized, but rather retained unequilibrated older volcanic products. Similarly, the K-ar dates (Richard et al., 1986; Juang, 1988; Juang and Chen, 1990), which are always older than the eruption ages determined by the FTD technique, may reflect isotopic disequilibrium. The partial annealing of zircon populations in most samples recorded thermal activiites prior to the volcanic eruptions. This suggests that sediment contamination may have occurred, thereby introducing some xenocrystic zircons into the Lutao volcanics. Based on the eruption ages derived by FTD, the distributions of various rock types and possible eruption centers, we propose an eruption history for Lutao volcanics consisting of five stages of magmatic activities. |
本系統中英文摘要資訊取自各篇刊載內容。