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題 名 | 40Ar/39Ar Dating of High-Pressure Rocks in the Tananao Basement Complex, Taiwan=臺灣東部大南澳片岩區高壓變質岩之氬40-氬39定年學研究 |
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作 者 | 羅清華; 俞震甫; | 書刊名 | 中國地質學會會刊 |
卷 期 | 39:1 1996.01[民85.01] |
頁 次 | 頁13-30 |
分類號 | 358.6 |
關鍵詞 | 定年學; 氬40-氬39定年學; 高壓變質岩; 大南澳片岩; Geochronology; 40Ar/39Ar dating; High-pressure metamorphism; Tananao schist; |
語 文 | 英文(English) |
中文摘要 | 本研究利用□Ar-□Ar定年法分析臺灣東部大南澳片岩基盤區內出露之高壓變質 相礦物,藉以探討其在大地構造上的地質意義。 前人工作結果顯示,大南澳片岩基盤玉里帶所出露之高壓變質岩代表著該地區之岩層曾受 類似於美國西岸Franciscan與日本Sambagawa變質帶之高壓變質作用,並暗示著臺灣地區曾 發生板塊隱沒作用。然而,由於定年資料的不足,此一隱沒作用發生的年代一直有所爭論, 多位學者推測此一高壓變質作用可能發生於中生代晚期;部分學者甚至懷疑此一高壓變質 作用係發生於晚第三紀。 本研究工作結果顯示,玉里帶內出露之高壓礦物相(綠輝石及藍閃石),由於歷經多次變 質作用,多具有複雜之同位素組成。惟這些高壓礦物相然都具有1.0-1.1億年的年代,直接 意味著高壓變質作用係發生於中生代晚期。由於後期變質相礦物的陸續成長,在同一標本 內,亦隱含有年輕的變質年代(4-12百萬年),顯示這些高壓變質礦物相亦曾受後期蓬萊 運動變質作用的冶煉。 另外,同一標本中的雲母類礦物(多矽白雲母及金雲母)都呈現年輕的年代(約4Ma與約 11Ma),且都具有較單純的同位素組成,這些平坦的年代譜指示著此一地區之雲母同位素 系統在歷經最後一次地質熱事件封存後,並未受其他的熱事件影響;其年代可能代表雲母 生成年代或蓬萊運動變質作用後之冷卻年代。若為後者,那麼高壓岩體之冷卻速率在四百 萬年前由25℃╱Ma變為70℃╱Ma。此種冷卻速率的改變,可能代表著臺灣造山運動的加速 進行。 總而言之,本研究不但確認大南澳片岩基盤區高壓變質作用係發生於晚中生代南澳運動時 期,也釐清多次變質作用對這些礦物同位素系統的可能影響。 |
英文摘要 | The appearance of the glaucophane schists and the omphacite-bearing rocks in the Yuli Belt of the Tananao Basement Complex indicates that a high-pressure metamorphic event occurred in Taiwan. The precise message of these high-pressure metamorphic rocks on the regional tectonic evolution history, however, is not yet clear mainly because of the complicated thermal history and the rarity of reliable geochronological data. To delineate the geohistory of these rocks, amphibole, phengite, phlogopite and omphacite separated from one glaucophane schist and two omphacite-bearing rocks were analyzed by using the □Ar/□Ar dating technique. Omphacite and phlogopite separated from the omphacite-bearing rocks yield □Ar/□Ar dates of 100-110 Ma and 4.4±0.1 Ma, respectively. Two phengite fractions of different grain size extracted from the glaucophane schist sample exhibit flat age spectra with consistent plateau dates of 10.4±0.1 Ma and 11.3±01 Ma. However, the age spectrum of amphibole from the glaucophane schist displays a disturbed shape with a geologically meaningless total gas age of 20.1±0.2 Ma. Such a disturbed isotopic system may be interpreted as resulting from mixing of two radiogenic Ar components which exhibit □Ar/□Ar dates of ~110 Ma and 12.2 Ma, respectively. Based on these results, three major conclusions could be made: (1) the high-pressure metamorphic event in Taiwan, as disclosed by the □Ar/□Ar dates of omphacite and amphibole took place during 110-100 Ma; (2) the K-Ar isotopic systems of most of the minerals in these high-pressure rocks have been significantly affected by the thermal event of the late Tertiary Penglai Orogeny; and (3) the thermal event of the Penglai Orogeny may have occurred since as early as ca. 11 Ma ago. |
本系統中英文摘要資訊取自各篇刊載內容。