國立臺灣大學地質科學系暨研究所陳正宏2006-07-252018-06-282006-07-252018-06-282003http://ntur.lib.ntu.edu.tw//handle/246246/11821華南早燕山花崗岩長久以來皆被認為 由侏儸紀隱沒相關的造山事件而產生 (Jahn, 1990; Chen and Jahn, 1996, Zhou et al., 2000)。其中黑雲母花崗岩為主要的岩性 且以大型深成岩基的方式侵入；輝長岩與 正長岩類少量出露且為小型火成岩體。利 用氬氬定年法得到花崗岩中角閃石年齡集 中在~160Ma，黑雲母年齡集中在160-155 Ma，這兩種礦物獲得的年紀十分相近，指 示著這些花崗岩體經歷相當快速的冷卻歷 史，也因此氬氬定年得到的年齡即可代表 侵位年齡。同樣地利用相同方法也可得到 正長岩類的侵位年齡約為135-130 Ma。 在地球化學特徵方面，黑雲母花崗岩 大致落在高鉀鈣鹼系列中，而正長岩則全 屬於鉀玄岩質。至於鍶釹同位素組成方 面，較虧損的正長岩類有著與輝長岩相似 的組成，表示兩者可能來自於相同的岩漿 源；相反的，黑雲母花崗岩的鍶釹同位素 組成表現出富集的現象，同位素呈現雙曲 線分布，暗示著可能是兩個端元混合所組 成：虧損地函與古老的地殼基盤。我們認 為黑雲母花崗岩是經歷印支期造山運動改 造的下地殼重新部分融熔且與底貼的玄武 岩質岩漿混合而來；正長岩類基本上則是 岩漿底貼造成的玄武岩部分融熔而來，又 或多或少在上升的路途中受花崗岩圍岩所 混染。根據定年結果可知，165-155 Ma 如 此短的岩漿生成時間，在隱沒系統中很難 提供如此大量的熱源，而必須在一個板內 岩石圈拉張的環境生成。再由於缺乏早燕 山期同造山的證據，支持了華南早燕山岩 漿運動並不能反映侏儸紀時的造山事件。The Early Yanshanian granitoids in S China have long been suggested to represent a subduction-related orogeny in Jurassic. Biotite granite is the predominant lithology particularly for large plutons, and gabbroic and syenitic rocks are present scarcely as small igneous bodies. 40Ar/39Ar hornblende ages yield ~160 Ma and biotites yield 160-155 Ma for these granites. Similarities of ages between these two minerals indicate rapid cooled thermal histories of the plutons, thus these ages can be used to approximate the emplacing time. Using the same methods, ages obtained are 135-130 Ma for the syenitic rocks. Geochemically, biotite granites belong to high-K calc-alkaline affinity and syenites are shoshonitic rocks. In terms of Sr-Nd isotopic compositions, more depleted syenites are similar to gabbros, suggesting that they could originate from the same source. On the other hand, biotite granites exhibit much more enriched signatures with Sri values and εNd(T). The εNd(T)–Sri isotopic arrays constructed from these Early Yanshanian intrusives could be derived from two-component magma mixing of a primitive component (depleted mantle) with an evolved component (S China old crustal basement). We suggest that biotite granites are the partial melts of the lower crust that has been modified in the Indosinian orogeny, and mixed with the underplated basalts; whereas syenitic rocks are basically the 2 partial melts of the preceded underplated basalts. With the short timespan of the Early Yanshanian magmatism peaked at 165-155 Ma, extraordinary heat supplies other than the subduction system are necessary to account for the regional distribution of granitoids in S China. Alternatively, this may need a tectonic process that is related to the large-scale intraplate lithospheric extension. As for the presence of later syenitic rocks, further local extension may be an explanation. Therefore, the lack of syn-orogenic characteristics on the Early Yanshanian magmatism is hard to support the Jurassic orogenesis in S China.application/pdf125302 bytesapplication/pdfzh-TW國立臺灣大學地質科學系暨研究所reporthttp://ntur.lib.ntu.edu.tw/bitstream/246246/11821/1/912116M002032.pdf