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題 名 | The Nature and Origin of Penghu Basalts: A Review=綜觀澎湖玄武岩的特性與成因 |
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作 者 | 莊文星; 陳汝勤; | 書刊名 | 經濟部中央地質調查所彙刊 |
卷 期 | 12 1999.06[民88.06] |
頁 次 | 頁147-200 |
分類號 | 358.38 |
關鍵詞 | 澎湖; 玄武岩; |
語 文 | 英文(English) |
中文摘要 | 澎湖位處臺灣與福建間之臺灣海峽中,由六十四個大小島嶼所構成。這群島嶼除 花嶼外,都由玄武岩與沉積岩交替組成。 初期之玄武岩多為海底火山噴出岩, 為具水平板 狀節理之多孔質玄武岩,夾有數層含海相化石之沉積岩層,後因地盤上升,露出空中,其後 陸續有熔岩噴出,其特徵為具有顯著的柱狀節理。熔岩流大都似由縱橫交錯排列之孔洞噴出 而非以裂隙噴出,加上侵蝕作用之進行,使得各島嶼分割支離破碎。 鉀 - 氨法定年的結果 顯示火山活動的年代由 16.2 至 8.2 百萬年,而以一千二百萬至一千萬年為鼎盛時期, 相 當臺灣西部麓山帶角板山期的火山活動。澎湖玄武岩主要由矽質與鹼性玄武岩所組成。鹼性 玄武岩比矽質玄武岩具有較高的不共容元素 (銣、鋇、釷、鈮、鉀、輕稀土元素等 )。矽質 與鹼性玄武岩雖然具有截然不同的主要與微量元素成分, 但其不共容親岩漿元素間的比值 (K/Ba、K/Rb 與 Zr/Nb) 近乎定值且鍶、釹和鉛同位素比相仿,推測可能代表源自地球化學 特性相近的來源地函,且來源地函具有輕稀土元素富化現象。由微量元素的系統性變化可以 看出澎湖玄武岩乃是歷經固 - 液相平衡,結晶分化在岩漿的演化過程中可能曾經發生。 鹼 性玄武岩以橄欖石與輝石之結晶分化為主,而矽質玄武岩則以橄欖石、輝石和斜長石為要。 稀土元素與其他方面的證據推測,大部分的澎湖鹼性玄武岩可由富化的上部地函物質,經由 3 - 9 %部分熔融而獲得。而由較淺處未受換質之石榴兩輝橄欖岩,經由 5 - 10 %部分熔 融,可產生澎湖矽質玄武岩。澎湖玄武岩鍶、釹和鉛同位素比,相對於其他大陸裂谷型玄武 岩而言, 相當侷限一小範圍內。 87Sr/86Sr=0.70345-0.70440 (ε sr =-14.9 - -1.4 ); 143Nd/144Nd=0.513037 - 0.512854 (ε Nd =+7.8 - +4.2); 206Pb/204Pb=18.46 - 18.79; 207Pb/204Pb=15.45 - 15.72; 208Pb/204Pb=38.44 - 38.97( △ 8/4Pb=38 - 83)。 總體而言,釹 - 鍶 - 鉛同位素與南海海盆和海底山玄武岩有相類似的區域性變化, 為具有顯著的杜巴異常現象。 由 87Sr/86Sr 對 143Nd/144Nd、206Pb/204Pb 對 87Sr/86Sr 或 143Nd/144Nd 之系統變化趨勢, 推測澎湖玄武岩乃源自由 EM1、EM2 與 N-MORB 三個端 成分源的混合而得。 |
英文摘要 | The basaltic lavas in Penghu Islands erupted from many vents rather than from fissures. K-Ar dating of the basalts reveals that the volcanic activitywas from 16.2 to 8.2 Ma and reached a climax between 12 to 10 Ma,equivalent to the Miocene Chiaopanshan stage of basaltic volcanism in theWestern Foothills of Taiwan. The basaltic rocks of the Penghu Islands arecomposed mainly of quartz tholeiite, olivine tholeiite, olivine alkali basalt,and basanite and can be tentatively divided into tholeiitic (quartz th oleiite),transitional (olivine tholeiite), and alkalic basalt (olivine alkali basalt andbasanite) . The alkali basalt is characterized by its higher incompatibleelement (Rb, Ba, Th, Nb, K, LREE, etc.) contents as compared with thetholeiite. However, the similar incompatible element ratio (K/Ba, K/Rb and Zr/Nb) and similar Sr, Nd and Pb isotopic ratio in the alkali and tholeiiticbasalts from the Penghu basalts are consistent with a petrogenetic modelinvolving mixing of mantle plume-derived melt and lithosphe ric mantle-derived component. The systematic compatible trace element variations suggest that thepossibility of fractional crystallization can not be excluded in the evolutionaryhistory of these magmas. The fractionation of olivine and pyroxene waspredominant in the alkali basalt, while the fractionation of olivine, pyroxeneand plagioclase prevailed in the tholeiite. The Penghu basalts have a relatively limited range in isotopic composition compared with other continental rifting basalta: 87Sr/86Sr=0.70345-0.70440 (ε sr =-14.9 - -1.4 );143Nd/144Nd=0.513037 - 0.512854( ε Nd =+7.8 - +4.2);206Pb/204Pb=18.46 - 18.79; 207Pb/204Pb=15.45 - 15.72; 208Pb/204Pb=38.44 - 38.97( △ 8/4Pb=38 - 83). Overall, the basalts have Sr-Nd-Pb isotopic systematic similar to those of seamount basalts in the South China Sea which are characterized by a Dupal-type Pb isotopic anomaly. Some of the apparent mixing arrays identified by 87Sr/86Sr-143Nd/144Nd are also clearly marked by congenial changes in 206Pb/204Pb to 87Sr/86Sr or 143Nd/144Nd ratios. This reflects hybrids of 206Pb/204Pb-poor EM1, N-MORB and 206Pb/204Pb-rich EM2 three end-members mixing. |
本系統中英文摘要資訊取自各篇刊載內容。