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題名 | 40Ar/39Ar Dating of Plutonic/Metamorphic Rocks from Chinmen Island off Southeast China and Its Tectonic Implications=金門花崗岩與花崗片麻岩之氬-氬定年學研究及其大地構造演化之意義 |
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作者 | 羅清華; 王執明; Onstott, Tullis C.; |
期刊 | 中國地質學會會刊 |
出版日期 | 19930100 |
卷期 | 36:1 1993.01[民82.01] |
頁次 | 頁35-55 |
分類號 | 353.17 |
語文 | eng |
關鍵詞 | 花崗片麻岩; 金門花崗岩; 氬-氬定年學; |
中文摘要 | 本研究工作利用氬-氬定年學與電子微探方法,分析金門島北太武山花崗岩與花崗片麻岩中之角閃石、黑雲母及鉀長石之礦物化學與氬同位素成分,求得中生代晚期燕山運動在此一地區所產生之熱事件歷史與可能之環境,進而討論其區域構造運動之意義。 氬-氬定年之結果,顯示金門島北太武山之花崗岩與花崗片麻岩曾同時一起接受同一熱事件之作用,並因區域構造抬升作用,依循同一溫度-時間路徑冷卻,其冷卻速率早期之∼80度╱百萬年變為晚期之∼8度╱百萬年。在冷卻過程中,氬同位素系統封存於各礦物相之時間分別為:角閃石(100.2 ± 0.9百萬年),黑雲母(97.4 ± 0.9百萬年,97.2 ± 0.9百萬年),鉀長石(83.7 ± 0.3百萬年,82.3 ± 0.3百萬年)。角閃石地質壓力計指示變質作用發生於7-8千粑之壓力狀況下,亦即相當於地殼內部約28-30公里之深度。依此深度及礦物氬同位素封存年代,估計區域抬升速率在早期為∼2.7毫米╱年、晚期為∼0.27毫米╱年。一般相信,此一深成╱變質岩體之快速冷卻及抬升作用,與區域板塊構造運動及花崗岩之侵入有著密不可分的關係。 |
英文摘要 | To reconstruct the thermal history of the Yanshanian Orogeny in SE China, a combination of �渙r/�觠r thermochronological and thermobarometrical techniques have been performed on hornblende, biotite and K-feldspar separates from a granite and a gneiss sample from Chinmen Island. The �渙r/�� Ar results yield a plateau date of 100.2 ± 0.9 Ma for hornblende, and identical plateau dates of 97.4 ± 0.9 and 97.2 ± 0.9 Ma for biotites, and 83.7 ± 0.3 and 82.3 ± 0.3 Ma for K-feldspars. All of the �渙r/�觠r age spectra exhibit flat profiles, suggesting that no significant thermal events occurred after early Cretaceous. The Al-hornblende geobarometer from the gneiss indicates that a metamorphic event occurred at 7-8 Kb corresponding to a depth of at least 28-30 km. This pressure information and the feldspar, biotite and hornblende plateau dates from the granite and gneiss suggest that the granitic plutonism associated with metamorphism may have occurred in the deep part of the crust. Based on closure temperature calculations and the isotopic dates of the minerals, the thermal history of the area is derived as follows: the region experienced plutonism/metamorphism prior to 101 Ma, cooled rapidly in the initial stages at a rate of ∼80 ℃ /Ma between 101 and 97 Ma, and then cooled slowly to 180 ℃ at a rate of ∼ 8 ℃ / Ma until ∼82 Ma. The fast cooling may reflect rapid tectonic uplifting changing from ∼2.7 mm/yr in the early stages to ∼0.27 mm/yr in the later stages during the Yanshanian Orogeny. However, the plutonic/metamorphic complex was not exposed at the surface until the Miocene. A paleo-geothermal gradient of about 30 ℃ /km has also been inferred. |
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