- 學位論文
利用鋯石鈾鉛定年與鉿同位素組成探討台灣東部火成岩之岩石成因
Zircon U-Pb and Hf isotope constraints on the petrogenesis of igneous rocks in eastern Taiwan
摘要
本研究利用原位鋯石鈾鉛定年與鉿同位素組成探討台灣東部火成岩的岩石成因，包括北呂宋火山島弧海岸山脈、綠島及蘭嶼火山岩、東台灣蛇綠岩以及瑞穗藍片岩。實驗結果顯示海岸山脈火山岩的岩漿鋯石206Pb/238U年代介於9.2 ~ 4.2 Ma、蘭嶼及綠島火山岩的主要噴發年代分別為~2.6 Ma與~1.3 Ma、東台灣蛇綠岩的岩漿年代介於17.5 ~ 14.1 Ma、瑞穗藍片岩的原岩岩漿年代介於16.0 ~ 15.4 Ma。本研究推測南中國海板塊隱沒到菲律賓海板塊的起始時間大約為20 Ma之後，奇美火成雜岩中~14.2 Ma的鋯石族群可能為南中國海板塊隱沒到菲律賓海板塊下方最早的岩漿產物。整體而言，海岸山脈的岩漿活動大約在蓬萊運動(6 ~ 5 Ma)後不久即結束，且海岸山脈最後一期的岩漿活動大約在4.2 Ma，而蘭嶼與綠島的岩漿活動則一直持續到上新世-更新世。此外，本研究在奇美火成雜岩、月眉火山岩、奇美火山岩、以及蘭嶼火山岩中皆發現到比例很高的繼承鋯石，且其與華夏陸塊有強烈的相似性，因此認為北呂宋火山島弧系統下方極可能增積著隨著南中國海板塊擴張而分裂自華夏陸塊的大陸地殼殘塊，島弧岩漿上升過程中通過這些華夏陸塊殘塊而造成地殼混染，海岸山脈、綠島、蘭嶼的火山岩皆有不同程度的地殼混染現象。根據岩漿鋯石漸增的鈾濃度、遞減的εHf(T)值，本研究推測隨著菲律賓海板塊持續向華夏陸塊移動，導致陸源沉積物隱沒而使岩漿源區漸漸混染富化。 東台灣蛇綠岩中，深成岩的岩漿生成年代分別為：(1) 輝長岩17.5 ~ 17.4 Ma，(2) 閃長岩14.3 Ma，(3) 斜長花崗岩14.3 ~ 14.1 Ma。鋯石鈾鉛定年結果顯示~17.5 Ma輝長岩樣本中含有大量的繼承鋯石，表示著地殼混染的現象，且鋯石鉿同位素分析結果顯示隨著年代越年輕，岩漿來源有越來越虧損的趨勢，本研究認為東台灣蛇綠岩極可能為南中國海板塊隱沒至菲律賓海板塊下方初期弧前擴張的高程度虧損岩漿產物，在晚中新世-早上新世的蓬萊運動夾雜於利吉混同層中。 瑞穗藍片岩的原岩岩漿年代為16.0 ~ 15.4 Ma，而非前人研究一般認為的白堊紀外來/構造岩塊或更老的海床碎塊，因此本研究認為玉里帶的形成年代應晚於中中新世。根據瑞穗藍片岩鋯石鉿同位素變化(高達~32 ε-units)、全岩地球化學(高MnO含量)，推測藍片岩的原岩岩漿極有可能為虧損地函來源岩漿、錳含量高的深海沉積物與陸源富集物質混合下的隱沒作用相關岩漿產物，在後期的隱沒環境中發生局部高壓低溫之藍片岩相變質，變質時間可能介於晚中新世時期，且在隨之的蓬萊運動過程中快速隆升到地表。簡言之，台灣大南澳變質雜岩中高溫低壓的太魯閣帶與低溫高壓的玉里帶並非所謂的成雙變質帶。
並列摘要
This thesis reports new zircon U-Pb and Lu-Hf isotopic data to better understand the petrogenesis of igneous rocks from eastern Taiwan. These include (1) volcanics from the Coastal Range, Lutao and Lanyu, (2) ophiolitic rocks from the Lichi melange, and (3) glaucophane-bearing rocks from Juisui area, the Yuli belt. Magmatic zircons yielded mean 206Pb/238U ages of 9.2 ~ 4.2 Ma for Coastal Range volcanics, ~2.6 Ma for Lanyu volcanics, ~1.3 Ma for Lutao volcanics, 17.5 ~ 14.1 Ma for east Taiwan ophiolitic rocks, and 16.0 ~ 15.4 Ma for the protolith of Juisui blueschists, respectively. We speculate that the subduction of South China Sea plate beneath the Philippine Sea plate has been initialized since ~20 Ma, and the mean inherited zircon 206Pb/238U ages of ~14.2 Ma from the Chimei igneous complex might be the earliest product of north Luzon arc magmatism. In general, magmatism in the Coastal Range had been terminated soon after the Penglai orogeny (6 ~ 5 Ma) and the final magmatism occurred at ~4.2 Ma. However, the arc magmatism of Lanyu and Lutao still spanned from Pliocene to Pliestocene. Besides, abundant inherited zircons have been obtained from the Chimei igneous complex, Yuemei volcanics, Chimei volcanics, and Lanyu volcanics, providing significant evidence for the existence of Cathaysia-affinity continental fragments rifted by the opening of the South China Sea were underneath the Luzon volcanic arc. The presence of abundant inherited zircons in volcanics suggests the magmas ascent though the old continental crust, feeding the Miocene arc, picked up the ancient zircons through assimilation or partial melting. Based on the increasing zircon uranium concentrations and the decreasing zircon εHf(T) values, we speculate that the continent-ward approach of the Manila trench allowing much more enriched continental material to involve into the mantle wedge. The magmatism of the east Taiwan ophiolite occurred during mid-Miocene, including gabbros of 17.5 ~ 17.4 Ma, diorite of 14.3 Ma and plagiogranites of 14.3 ~ 14.1 Ma. Based on the abundant inherited zircons from the ~17.5 Ma gabbro, we suggest a process of crustal contamination in ETO’s magmas during ~17.5 Ma. Moreover, the zircon Hf isotopic data indicated that the magmas were more depleted through time. In this study, we consider that the ETO originated from the highly depleted forearc magmas due to the subduction initiation of the South China Sea plate and incorporated into the Lichi melange as allochthonous blocks during the late Miocene-early Pliocene Penglai orogeny. The magmatic ages of protolith of Juisui blueschists can be well-constrained at 16.0 ~ 15.4 Ma by the zircon U-Pb ages, instead of exotic/tectonic blocks that were thought to be mid-Cretaceous or older ocean-floor fragments. Thus, we speculate that the Yuli metamorphic belt should have taken place after mid-Miocene. Based on the zircon εHf(T) variations (~32 ε-units) and whole-rock geochemistry (high MnO content), we suggest that the protolith of Juisui blueschists was product which mixed from depleted magmas (mantle-derived), enriched continental material, and Mn-rich deep-sea sediments. The protolith had undergone a high-pressure blueschist facies metamorphism and eventually rapidly exhumed and accreted to the eastern slope of the Central Range during the late Cenozoic. Briefly, the consensus of “paired metamorphic belts” of Taiwan should be thus falsified.