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題 名 | Mafic Granulite Xenoliths From Penghu Islands--Evidence for Basic Lower Crust in SE China Continental Margin=澎湖群島的基性粒變岩捕獲岩:中國東南大陸邊緣具基性下部地殼之證據 |
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作 者 | 李寄嵎; 鍾孫霖; 陳正宏; 謝瑩玲; | 書刊名 | 中國地質學會會刊 |
卷 期 | 36:4 1993.10[民82.10] |
頁 次 | 頁351-379 |
分類號 | 358.38 |
關鍵詞 | 大陸邊緣; 中國; 東南; 捕獲岩; 基性下部地殼; 基性粒變岩; 澎湖群島; |
語 文 | 英文(English) |
中文摘要 | 澎湖群島的中新世鹼性玄武岩中,有基性捕獲岩的出露。它 們主要由斜長石、斜輝石和正輝石以及少量的鉀長石、橄欖石及 鈦鐵礦所組成。岩象學觀察顯示,時有長石與輝石富集帶交替出 現所形成之帶狀構造,且粒狀變晶組織相當普遍,並偶有葉理產 生。此外,尚可見殘留的火成微組構,表示這些捕獲岩應由火成 岩變質而成。所分析的標本全屬基性(SiO��=48-50wt%),卻具有 相當不均勻的全岩化學成份(例如Al�烙 ��=8.18-19.88wt%,MgO= 3.27-12.40wt%,La��=8-40),此係因各岩樣中之礦物組成比例不 同所致。利用兩輝石地溫計所計算之捕獲岩的平衡溫度為810- 970℃,而其平衡壓力可根據鍾(1990)利手相伴之地函捕獲岩所 建立的中新世古地溫梯度間接求得,約為0.6-0.9GPa。因此,它 們代表了源自深度約為20-35公里的基性粒變岩相岩石。此一深 度之估算與玄武岩類之相變研究結果吻合,並指示了在澎湖地區 有基性下部地殼的存在。臺灣海峽附近地區之地球物理資料也支 持此一論點。這些捕獲岩均具有輕微的輕稀土元素富集現象,並 經常可見小量卻明顯的銪元素正異常。此一現象最易由富含斜長 石的火成沉聚作用來解釋。由於未見高場力鍵結元素(HFSE)之虧 損,其成因應不與隱沒作用有直接關聯。我們認為其生成與早新 生代以來之張裂作用密切相關;後者所導致之板塊內部岩漿活動 ,在地殼/地函交界附近,可能造成廣泛的玄武岩漿底附。此外 ,同時結晶沉聚與隨後的粒變豈相變質作用,亦應為中國東南 大陸邊緣基性下部地殼形成之重要機制。 |
英文摘要 | Mafic xenoliths occur in some Miocene alkali basalts in Penghu Islands, Taiwan Strait. They comprise plagioclase, clinopyroxene and orthopyroxene with K-feldspar, olivine and ilmenite in trace amount. Petrographically, banded structure containing feldspar-rich and pyroxene-rich Iayers in alternation can be observed. Granuloblastic texture, occasionally with foliation, is very common. In addition, the presence of relic igneous microfabrics suggests a meta-igneous origin of these rocks. The samples are overall basic (SiO �� =48-52 wt.%) but heterogeneous in bulk chemical compositions (e.g., Al �� O �� =8.18-19.88 wt.%, MgO=3.27-12.40 wt,%, Las �� =8-40), resulting from the variable proportions of the mineral phases. By using two-pyroxene thermometry, the equilibrium temperature of the xenoliths is estimated to be 810-970 ℃. Coupled equilibrium pressure should thus be ca. 0.6-0.9 GPa following the extension of a Miocene paleogeotherm derived from the associated mantle xenoliths (Chung, 1990). Therefore, they are considered to represent the mafic granulite facies rocks from ca. 20-35 km depth. The depth estimation is consistent with phase transformation studies on basaltic lithologies and argues for an occurrence of basic lower crust in Penghu area. This proposition is supported by geophysical data available around the Taiwan Strait. All xenoliths show mild LREE enrichment, commonly with small but distinct En(+) anomalies. They could be best interpreted as signatures of igneous cumulates rich in plagioclase. The absence of apparent HFSE depletions does not favor an origin via subduction-related processes. Extensive basaltic underplating accompanied by crystal accumulation with subsequent cooling under granulite facies condition aroung the crust/mantle boundary, most possibly related to the early Cenozoic intraplate magmatism, is suggested as a key mechanism for the formation of the basic lower crust in SE China continental margin. |
本系統中英文摘要資訊取自各篇刊載內容。