臺灣西部麓山帶晚中新世南庄與鹿谷玄武岩之定年學與地球化學研究

Geochronology and Geochemistry of Late Miocene Nanjuang and Lugu Basalts of Western Foothills, Taiwan

何恭算(Kung-Suan Ho)

《國立臺灣博物館學刊》 69卷2期 (2016/06) Pp. 43-74

https://doi.org/10.6532/JNTM.2016.69(2).04

摘要

根據前人研究顯示，早中新世公館玄武岩比晚中新世關西－竹東玄武岩有較高放射成因鉛組成。為了探討西部麓山帶伴隨地球動力作用相關之岩漿演化，制約其噴發年代並檢驗岩漿源區之時空變化，我們分析了南庄和鹿谷玄武岩的鉀－氬年齡、元素組成和鍶－釹－鉛同位素比值。經由鉀－氬定年結果，南庄和鹿谷玄武岩屬於晚中新世，噴發年齡分別為11.09 ± 0.41 Ma和 7.69 ± 0.19 Ma。已分析12件岩樣具有典型鈉質玄武岩成分，Na_2O/K_2O 比值從 1.21到 3.63，稀土元素經球粒隕石標準化後之圖形及不共容元素含量均與洋島玄武岩相類似。這些岩樣有相當均質的鍶－釹同位素組成 (^(87)Sr/^(86)Sr ＝ 0.703654-0.704286 ; ^(143)Nd/^(144)Nd =0.512845-0.512891)，顯示源區具輕微虧損特性，然而，鹿谷玄武岩比南庄玄武岩有更高放射成因鉛組成。在鍶－釹－鉛同位素資料顯示，鹿谷玄武岩的地函源具有虧損地函端成分與富化地函第二型端成分混合之特徵，而南庄玄武岩則具有虧損地函端成分與富化地函第二型端成分混合後，再加入富化地函第一型端成分的組成。兩地玄武岩之鉛同位素比值的差異，最可能是地球動力作用和岩漿源區發生變化的關係。我們認為鹿谷玄武質岩漿主要是源自類似東台灣蛇綠岩套正常型玄武岩之虧損軟流圈和一些具富化地函第二型端成分之次大陸岩石圈地函下部的貢獻。反觀，南庄玄武岩除了類似鹿谷玄武岩的形式混合外，因受菲律賓海板塊與歐亞板塊碰撞之地球動力作用下，大陸岩石圈地函淺層發生熱力再活化而使一些具富化地函第一型端成分加入的結果。

關鍵字

鉀－氬年齡 ； 地球化學 ； 晚中新世 ； 南庄和鹿谷玄武岩 ； 西部麓山帶 ； 臺灣

並列摘要

According to previous research, the early Miocene Kungkuan basalts generally have more radiogenic Pb compositions compared to those from late Miocene Kuanhsi-Chutung. In order to constrain the eruptive age and examine the spatial and temporal changes in magma source regions accompanying the evolution of geodynamic-process-related magmatism in the western Foothills, we have analysed K-Ar ages, elemental compositions and Sr-Nd-Pb isotopic ratios for the Nanjuang and Lugu basalts. Based on K-Ar dating, the Nanjuang and Lugu basalts belong to the late Miocene, with ages of 11.09 ± 0.41 Ma and 7.69 ± 0.19 Ma, respectively. Twelve samples analyzed in this study have typical sodic basaltic compositions, with Na_2O/ K_2O ratios ranging from 1.21 to 3.63 as well as exhibit chondrite-normalized REE patterns and incompatible elements contents similar to oceanic island basalts (OIBs). In general, they have relatively homogenous Sr-Nd isotopic compositions (^(87)Sr/^(86)Sr ranging from 0.703654 to 0.704286 and ^(143)Nd/^(144)Nd ranging from 0.512845 to 0.512891), indicating that these lavas have a slightly depleted source. However, the Lugu basalts have more radiogenic Pb compositions compared to those from Nanjuang area. The Sr-Nd-Pb isotopic data indicated that the mantle sources for the basalts have a DMM-EM2 array for Lugu area and a (DMM+EM2)-EM1 array for Nanjuang area. The different Pb isotopic ratios between the basaltic rocks of these two volcanic areas are most likely due to changes in geodynamic processes and magma sources. We suggest that the Lugu basaltic magmas were dominantly derived from an East Taiwan Ophiolite N-type basalt-like depleted asthenospheric source with some contributions from an enriched mantle component (EM2) from the lower part of the subcontinental lithospheric mantle. Except the mixing of EM2 and DMM components, Nanjuang basalts possibly represent the addition of the subcontinental lithosphere mantle with some EM1-type magma as a result of the thermal reactivation of the shallow-level continental lithosphere mantle following the geodynamic interaction of the Philippine Sea plate-Eurasia plate collision.

並列關鍵字

K-Ar age ； geochemistry ； late Miocene ； Nanjuang and Lugu basalts ； western Foothills ； Taiwan

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