電漿質譜術在西藏南部火成岩岩石成因上的應用

Application of ICP-MS to the study of Transhimalayan petrogenesis

朱美妃(Mei-Fei Chu)

指導教授 ： 鍾孫霖

國立臺灣大學/理學院/地質科學研究所/博士(2006年)

https://doi.org/10.6342/NTU.2006.02031

摘要

隨著儀器設計以及分析技術的成熟，雷射燒蝕感應耦合電漿質譜術（LA-ICPMS）已被廣泛地應用在地質樣品的微量元素微區（in situ，或原位）分析上；本論文工作即應用此一技術分析藏南拉薩地塊火成岩岩體中的副礦物－－鋯石以及磷灰石，它們的原岩包括（一）、分佈於拉薩地塊南緣岡底斯帶中的I型花岡岩質岩，（二）、位居北部火成岩帶、念青唐古喇山地區的S型花岡岩，以及（三）、侵入岡底斯岩基中的（後）碰撞型埃達克岩，並將藉由這兩礦物的微區微量元素或同位素組成，解析拉薩地塊此三大火成活動事件的岩漿成因與演化。
國立臺灣大學地質科學系擁有New Wave LUV213雷射系統以及安捷倫（Agilent）7500s四極桿式感應耦合電漿質譜儀，本論文工作內容亦涵蓋該實驗室與感應耦合電漿質譜術相關分析程序的建立，包括：（一）、建立岩石樣品（含岩石粉末以及四硼酸鋰玻璃樣品）溶液的製備流程與分析方法，（二）、成功聯結雷射系統與質譜儀，完成磷灰石微量元素的微區分析；而藉此，進一步為今日該實驗室中New Wave UP213新雷射系統與同質譜儀對鋯石微區鈾-鉛定年的穩定表現奠下根基。
鋯石的鉿同位素組成，一如全岩的釹同位素可為岩漿源示蹤，但往往還「暗藏」了岩漿演化的諸多細節。統合鋯石的鈾-鉛定年結果與本研究所得的鉿同位素資料，其間揭櫫的藏南火成岩岩石成因可歸結如下：
一、眾多岩樣中岩漿鋯石的鉿同位素值展現可多達近15 ε單位的顯著變化，闡明了岩漿混合事件在這些藏南火成活動中普遍存在；
二、岡底斯岩漿鋯石可高達+19.8的鉿同位素初始值（εHf(T)）確切辨證出全岩同位素特徵中「隱匿」的端成分－虧損地函（DM）的存在；
三、除卻普遍所認知的白堊紀、古第三紀岡底斯岩漿事件，本研究自岡底斯岩帶內新確認出另一期早侏羅紀的活動，說明新特提斯洋隱沒的發端至少可遠溯至此時；
四、北部火成岩帶內的S型花岡岩富含188- 210 Ma結晶的繼承鋯石，這些鋯石的鉿同位素值（εHf(T)= -3.9 to -13.7）指示其強烈地殼組份的模式年齡（TDMC）約是1.4- 2.1 Ga，暗示原古代是拉薩地塊地殼的重要増生時期， 而該地殼物質其後於早侏羅紀時發生了重熔事件；
五、漸新世、中新世的埃達克岩，其岩漿鋯石的鉿同位素特徵與白堊紀、古第三紀岡底斯岩漿相似，因而為藏南在印亞碰撞以後，地殼增厚的時間與機制提供了關鍵性的制約；
六、綜觀上述藏南火成岩中的鋯石，它們與時變化的鉿同位素初始值（εHf(T)）及其對應的模式年齡（TDMC）記錄了拉薩地塊多階段的造山與地殼形成歷史。
磷灰石的次量與微量元素含量可因其原岩的不同而出現顯著差異。本研究中，電子微探（EPMA）以及雷射燒蝕感應耦合電漿質譜術（LA-ICP-MS）分別被用以量測藏南火成岩中磷灰石的主量與微量元素含量，微區分析的結果顯示，磷灰石的氟（F）、錳（Mn）、鍶（Sr）以及稀土元素（REEs）均與岩漿源成分展現良好的對比關係，揭示了它們應用在岩石成因示蹤上的優秀潛力：其中，磷灰石氟、錳含量的變化與原岩的鋁飽和情形（如：鋁飽和指數ASI）息息相關，因此可以作為岩漿分異程度的指標；而鍶以及稀土元素則在不同岩石類型來源的磷灰石中展現了明顯的豐度變化，因此綜合這些元素將有機會進一步建立岩漿來源的「鑑別圖」，甚而以此討論岩石成因，例如：岩漿混合與源區組成的不均一性。

關鍵字

電漿質譜術 ； 微區 ； 西藏 ； 鋯石 ； 鉿同位素 ； 磷灰石 ； 拉薩地塊

並列摘要

Recent progress on the development of Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) makes precise in-situ measurement of trace element concentrations of accessory minerals in igneous rocks possible and thus the method a powerful tool for studying the complex processes that form and modify the host magmas. In this thesis, focused on the application of LA-ICP-MS to the study of Transhimalayan petrogenesis, in-situ trace element and isotope data of zircon and apatite separates from different types of Transhimalayan rocks in the Lhasa terrane, southern Tibet were carried out. These rocks include: (1) I-type granitoids from the Gangdese batholith cropping out in the southern part of the Lhasa terrane, (2), S-type granitoids from the Nyainqentanglha or northern magmatic belt in the Lhasa terrane, and (3) post-collisional adakites emplaced in the Gangdese belt.
This thesis, furthermore, contributes to the set up of the first LA-ICP-MS system at Department of Geosciences, NTU, composed of a New Wave LUV213 laser and an Agilent 7500s quadruple ICP-MS. The contribution includes: (1) establishment of the routine analytical procedures of rock samples (powder and glass bead) using solution method, (2) successful link the laser with the ICP-MS for in-situ analysis of trace elements in apatites, and (3) collection of knowledge that forms a basis for setting up the new LA-ICP-MS system, attached with a New Wave UP213 laser, at NTU that can perform zircon U-Pb dating as well as in-situ trace element measurements.
ZIRCONS’ Hf isotope ratios can be used in much the same way as whole-rock Nd isotopes. They, furthermore, often record “hidden” information that allows more detailed studies of the magma generation processes. Based on zircon Hf isotope data obtained in this study, together with associated U-Pb ages, the following conclusions regarding Transhimalayan petrogenesis are reached: (1) There are significant variations in Hf isotopes of magmatic zircons, up to ~15 ε-units in some samples, suggesting magma mixing to be a common process; (2) A “hidden” DM (depleted mantle) component, with εHf(T) values up to +19.8, is identified to be prevalent in the Gangdese magmatic zircons. This DM-type component has never been revealed by any whole rock isotope analysis; (3) While the “conventional” Gangdese magmatism has been known as most active in the Cretaceous and Paleogene, this study identifies a new magmatic episode within the Gangdese belt that occurred in Early Jurassic time resulting from the long-lasting Neo-Tethyan subduction; (4) The S-type granitoids of the northern magmatic belt contain abundant inherited zircons aged from ca. 188 to 210 Ma, in which a crustal component that shows εHf(T)= -3.9 to -13.7 and TDMC model ages of ca. 1.4- 2.1 Ga is identified. This implies a major stage of crustal growth in Proterozoic time and remelting of the crustal material in Early Jurassic time; (5) The Oligocene-Miocene adakites contain magmatic zircons that show similar Hf isotope compositions to the Cretaceous-Paleogene Gangdese batholiths, providing a key constraint that allows evaluation of the nature and timing of crustal thickening in southern Tibet owing to the India-Asia collision; and (6) As a whole, the Hf isotope information observed in zircons from the above Transhimalayan rocks demonstrates a temporal variation in εHf(T) values, and thus TDMC ages, that suggests multiple stages of orogenic or crustal formation events.
APATITES from different types of igneous rocks generally reveal significant variations in the abundance level of minor and trace elements. In this study, EPMA and LA-ICP-MS were used to determine the major and trace element concentrations, respectively, of apatites from Transhimalayan granitoids. The results indicate that F, Mn, Sr and REEs in apatites generally show good correlations with compositions of their host magmas and thus have high potential to be utilized as petrogenetic tracers. More specifically, F and Mn contents in apatites are covariant with the aluminosity (or ASI values) of the host rocks so that can be used as an indicator for magma differentiation. Combining with Sr and REE data, which show significant variations in apatites from different rock types, these elements may be furthermore used to construct “discrimination diagrams” for more detailed investigations of complex petrogenetic processes such as magma mixing and compositional heterogeneity.

並列關鍵字

ICP-MS ； in situ ； Tibet ； zircon ； Hf isotope ； apatite ； Lhasa terrane ； Transhimalaya

參考文獻

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