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研究生:王國龍
研究生(外文):Kuo-Lung Wang
論文名稱:台灣北部及外海晚上新世─第四紀火山岩的地球化學特性與岩石成因
論文名稱(外文):Geochemistry and Petrogenesis of Late Pliocene-Quaternary Volcanic Rocks from the Northern Taiwan Volcanic Zone
指導教授:鍾孫霖鍾孫霖引用關係陳中華陳中華引用關係
指導教授(外文):Sun-Lin ChungChang-Hwa Chen
學位類別:博士
校院名稱:國立臺灣大學
系所名稱:地質學研究所
學門:自然科學學門
學類:地球科學學類
論文種類:學術論文
論文出版年:2000
畢業學年度:88
語文別:中文
論文頁數:169
中文關鍵詞:台灣北部火山活動帶後碰撞拉張作用岩漿活動地球化學岩石成因
外文關鍵詞:Northern Taiwan Volcanic Zonepost-collisional extensionmagmatismgeochemistrypetrogenesis
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“台灣北部火山活動帶”代表晚上新世至第四紀期間在台灣北部及外海所形成的一系列火山活動,傳統上被認為是由琉球隱沒系統隨著菲律賓海板塊的西向遷移所造成。本研究藉由較為廣泛的岩樣蒐集和詳盡的地球化學分析結果,重新探討這些火山活動的地球化學特性、岩石成因及其和台灣造山作用、琉球隱沒系統演化之關聯,認為此一火山活動帶並非琉球火山島弧的延伸,而是北台灣造山帶拉張崩毀的產物。
台灣北部火山活動帶的火山岩於晚上新世(~2.8~2.6Ma)時開始形成於赤尾嶼、棉花嶼和大屯火山群等地,其後陸續在活動帶中各處噴發,直到大約十餘萬年前停歇。整體而言,其岩性以鈣—鹼質的安山岩和玄武岩為主(大屯和基隆火山群),但亦涵蓋矽質(赤尾嶼和棉花嶼)和鉀玄質(觀音山和黃尾嶼)組成,另草嶺山更展現高鉀質(K2O/Na2O2)的特性。若以其中基性岩漿(SiO252wt.%)分析,整個活動帶由東北向西南似呈矽含量遞減而鉀含量遞增的趨勢;在不相容微量元素標準化作圖中,火山岩性則由東北向西南表現出大離子岩石圈元素和輕稀土元素逐漸富集的趨勢。除棉花嶼外,活動帶的火山岩均具有高場力元素相對虧損以及大離子岩石圈元素和鉛元素相對富集的現象,顯示地函源區普遍受到隱沒帶富集作用的影響。就微量元素和鍶—釹同位素組成綜合解析,除了此一“隱沒組分”端元之外,棉花嶼和草嶺山的火山岩還分別界定了另兩個重要的地函源。棉花嶼火山岩屬高鎂玄武質安山岩(SiO252.4~54.1wt.%; MgO5.9~8.1wt.%),不但高場力元素未見虧損,還具有活動帶內火山岩最高的釹同位素比值(Nd+5.1~+7.2),反映其地函源區為相對虧損的軟流圈地函,其組成類似於富集型中洋脊玄武岩(E-MORB)者。另一方面,草嶺山火山岩呈現高鎂和高鉀的特性(MgO14.2~16.3wt.%; K2O2.9~5.6wt.%),其整體地球化學特徵與所謂的造山型高鉀岩漿相當,並具有活動帶中最低的釹和最高的鍶同位素比值(Nd-1~+0.8; 87Sr/86Sr0.705402~0.705509),推測應源自含金雲母的耐熔岩石圈地函,且此地函源曾受到與隱沒帶脫水作用有關的流體交代變質。就鍶—釹同位素組成而言,台灣北部火山活動帶內的其他火山岩皆落於棉花嶼與草嶺山者的分布範圍之間,似乎反映活動帶的岩漿乃由軟流圈和岩石圈地函的交互作用而形成。本研究以上湧的軟流圈和富集的岩石圈地函作為端元成分,可模擬計算上述的台灣北部火山活動帶隨空間分布而展現的地球化學變化趨勢。
大地應力和地震震源機制研究顯示,台灣北部已從早期的碰撞壓縮轉變為現今的伸張地體構造環境;綜合外海地質與震測資料看來,北台灣造山帶在晚上新世已進入拉張崩毀的階段,並有廣泛的岩漿活動伴隨。台灣北部火山活動帶大致沿著此造山帶的變質基盤呈東北—西南向分布。不論在時空關係或岩石地球化學特徵上,都無法和琉球火山島弧做合理的對比。因此,作者認為台灣北部火山活動帶乃肇因於北台灣造山帶的拉張崩毀而非琉球島弧的西延。由於棉花嶼火山岩的地球化學特性必須藉由軟流圈地函的上湧加以解釋,暗示造山帶的崩毀可能和岩石圈底部的脫落(如層脫作用)有關。上湧的軟流圈不僅造成地溫梯度的驟昇,並與受到近期琉球隱沒作用影響的岩石圈地函發生交互作用而導致部份熔融形成不同成分的岩漿。這些岩漿所展現隨空間變化的地球化學特徵,可能反映台灣北部火山活動帶在東北側發生的熔融程度較大,而在西南側者較小,應和現今台灣中南部仍處於碰撞擠壓的構造環境有關。隨著北台灣造山帶的拉張崩毀,原本受台灣弧陸碰撞阻擋的沖繩海槽張裂以及琉球弧溝系統得以向西南發育延伸,以致在初生的西南沖繩海槽內形成極為年輕的海底火山,構成現今西南琉球島弧的火山前緣。因此,台灣造山帶演化和琉球弧溝系統、沖繩海槽發育的關係應是一極為獨特的先碰撞、後拉張,而後隱沒的時空演化架構。
Being part of an active mountain belt formed by oblique collision of the Luzon arc with Asia, northern Taiwan and offshore islets mark with a series of late Pliocene-Quaternary volcanoes, named Northern Taiwan Volcanic Zone (NTVZ), whose eruptions have been conventionally ascribed to westward propagation of the Ryukyu volcanic arc. Based on comprehensive sample collection and high-quality chemical analysis, this study presents overall geochemical characteristics of the NTVZ volcanic rocks for better understanding their petrogenesis and relation with the tectonic evolution of the Taiwan mountain belt and nearby Ryukyu subduction system. Consequently, it is proposed that this young volcanic zone resulted from post-collisional lithospheric extension associated with collapse of the northern Taiwan mountain belt.
The NTVZ began around late Pliocene (~2.8~2.6 Ma) in the Mienhuayu, Chiweiyu and Tatun volcano group and was active in many localities in the Quaternary. The youngest eruption has been dated as ~0.11 Ma in the Tatun Volcano Group. The NTVZ is composed dominantly of calc-alkaline andesites and basalts, with subordinate low-K tholeiitic, high-K shoshonitic, and ultrapotassic rocks in several outcrops. From the Chiweiyu in the northeast to the Tsaolingshan in the southwest, the NTVZ mafic rocks display a spatial geochemical variation characterized by a southwestward increase in the potash content coupled with silica undersaturation. Additionally, a southwestward increase in enrichment of the large ion lithophile elements (LILE) and light rare earth elements (LREE) is also observed. Except for the Mienhuayu magmas, all NTVZ volcanic rocks show "arc-like" geochemical features represented by significant enrichments of LILE and Pb enrichment and depletion of the high field strength elements (HFSE). This indicates that their mantle source has been affected by subduction-related processes. In addition to such a 烞ubduction component, trace element and Nd-Sr isotope systematics of the NTVZ magmas suggest involvement of two mantle source components, represented by the Mienhuayu and Tsaolingshan lavas, respectively, in magma generation. The Mienhuayu rocks are high-Mg basaltic andesites (SiO252.4~54.1wt.%; MgO5.9~8.1wt.%) which have a rather flat spidergram pattern and high Nd isotope ratios (Nd+5.1~+7.2). Thus, they are most likely derived from decompression melting of an ascended asthenospheric mantle, which is similar to the mantle source of E-MORB. On the other hand, the Tsaolingshan lavas are high-Mg potassic absarokites (MgO14.2~16.3wt.%; K2O2.9~5.6wt.%), marking with extremely enriched LILE and Pb, moderately enriched LREE and relatively depleted HFSE and HREE. These geochemical features are similar to certain post-collisional potassic magmas from the Tethyan orogenic belt. Showing the highest Sr and lowest Nd isotope ratios (Nd-1~+0.8; 87Sr/86Sr0.705402~0.705509) among the NTVZ volcanics, the Tsaolingshan magmas are therefore interpreted as products of small degree melting from a phlogopite-bearing harzburgite lithospheric mantle source metasomatized recently by hydrous fluids from the Ryukyu subduction zone. In the Nd-Sr isotope correlation diagram, the Mienhuayu and Tsaolingshan magmas define the "end-members" so that other NTVZ volcanics plot between a binary mixing trend. This suggests that the NTVZ magmas may be explained as derived from interaction between the depleted asthenospheric and enriched lithospheric mantle sources. Accordingly, a two component partial melting calculation is performed for modeling the incompatible trace element variation patterns of the NTVZ volcanic rocks in the hope to explain the observed spatial geochemical variation.
Whereas the collisional tectonism is still undergoing in central and southern Taiwan, structural and seismological data demonstrate that the northern part of the Taiwan mountain belt has been subjected to an extensional deformation in the Quaternary. Thus, the northern Taiwan orogen has virtually gone through mountain''s build-up and is in the process of collapsing. Geological and seismic data from offshore regions, furthermore, show that the northern Taiwan mountain belt has stepped into extensional collapse since the Plio-Pleistocene boundary accompanying widespread magmatic activities. Therefore, it is proposed that the NTVZ resulted from post-collisional lithospheric extension owing to collapse of the northern Taiwan mountain belt, but does not represent part of the western Ryukyu volcanic arc. Derived from the decompression melting of an ascended astheosphere, emplacement of the Mienhuayu magmas suggests that extensional collapse of the northern Taiwan mountain belt may have been related to removal or delamination of the basal lithospheric mantle. As a result, the asthenospheric upwelling caused not only decompression melting but also perturbation of the geotherm that hence induced partial melting of the metasomatized lithospheric mantle to produce the NTVZ magmas. The spatial geochemical variation observed in NTVZ, which might be explained by southwestward decrease in degrees of partial melting in the mantle source region, reflects southwestward weakening of extension due to ongoing collision in the central-southern Taiwan mountain belt. The post-collisional extension in northern Taiwan, furthermore, might have played a role in reactivation of opening in the middle Okinawa Trough, and gave way to its rapid southwestward propagation with associated development of the Ryukyu subduction zone west of ~124?. This nascent subduction, in turn, resulted in abundant submarine volcanoes which delineate an embryo volcanic front along the southern margin of the southernmost Okinawa Trough. In this case, the southernmost part of Okinawa Trough is not a "fore-arc" basin as previously alleged, but represents an "atypical" back-arc basin which developed broadly synchronously or prior to its arc-trench system in this particular collision/extension/subduction tectonic environment.
封面
中文摘要
英文摘要
目錄
圖目
表目
圖版目錄
第一章 緒論
1.1前言
1.2前人研究
1.3研究動機與目的
1.3.1研究動機
1.3.2研究目的
第二章 研究方法
2.1研究區域
2.2標本蒐集與野外工作
2.2.1標本蒐集
2.2.2野外工作
2.3顯微鏡岩象觀察
2.4礦物化學成分分析
2.5全岩主量元素含量分析
2.6全岩微量元素含量分析
2.7鍶一釹同位素組成分析
2.7.1鍶同位素分析
2.7.2釹同位素分析
2.8氬一氬定年分析
第三章 觀察與分析結果
3.1棉花嶼野外觀察結果
3.2岩象觀察與礦物化學分析結果
3.2.1棉花嶼
3.2.2草嶺山
3.3全岩主量元素
3.4全岩微量元素
3.5鍶一釹同位素組成
3.6氬一氬定年結果
第四章 地球化學特性與岩石成因
4.1地球化學特性
4.1.1空間上之變化
4.1.2時間上之變化
4.2地函源區成份性質
4.2.1地函端元成份特性
4.2.1.1棉花嶼火山岩
4.2.1.2草嶺山火山岩
4.2.1.3隱沒組分及其影響
4.2.1.3-1隱沒組分
4.2.1.3-2支持流體作用的證據-高鋁玄武岩之成因
4.2.2部份熔融程度之變化
4.2.3地函端元成份混合模式
4.3岩漿序列與岩石成因
第五章 綜合討論
5.1岩漿生成機制與台灣造山帶演化和沖繩海槽張裂之關係
5.2後碰撞岩漿特性-與其他產區之對比
第六章 結論
誌謝
參考文獻
圖版
附錄
中文部份:
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