非洲中部剪切帶岩石學分析與構造史之研究

Petrographic Analysis and Structural History of Central African Shear Zone

吳詩敏

指導教授 ： 李通藝 ； 葉孟宛

國立臺灣師範大學/理學院/地球科學系/碩士(2012年)

摘要

非洲中部剪切帶 (Central African Shear Zone，CASZ) 為一東北-西南走向從喀麥隆至查德向東北延伸至蘇丹之剪切帶，於喀麥隆地區分為北偏東70 度之中喀麥隆剪切帶 (Central Cameroon Shear Zone，CCSZ) 與北偏東50 度之 Tcholliré-Banyo 剪切帶(Tcholliré-Banyo Shear Zone，TBSZ)，且在大陸漂移前延伸至巴西東北部。而板塊構造背景可追溯至泛非洲時期，剛果地塊、西非地塊及東撒哈拉地塊之拼合。部分學者提出CASZ 有同構造之花岡岩侵入，並以鈾-鉛鋯石定年640 至557 Ma 之結果為花崗岩與剪切帶形成之時間；另一部分研究則以沿CASZ 有裂谷盆地分布與其打開的時間為大西洋張裂之時，進而探討130 Ma 至84Ma 之板塊構造演化，認為CASZ 之形成與大西洋張裂相關，而同剪切花崗岩中常含有高溫變質岩，因此本研究於TBSZ 系統以顯微岩石學之分析來了解其礦物相，並以指標礦物
推測其變質事件發生之溫度條件；並根據野外觀察、顯微構造及應變分析釐清整個構造演化歷史及應力分布情況，歸納此地區的變形史可分為至少三期：早期的為Dn-1，形成一東北－西南走向，低角度向東南傾之偃臥褶皺，葉理Sn-1 組成礦物為角閃石+黑雲母+石榴子石+鈉長石，此礦物相為角閃岩相之變質條件，而Dn-1 應為泛非洲時期東撒哈拉地塊由北向南碰撞變形造成之逆衝nappe；Dn 形成東北－西南走向，近垂直傾向西北走向平行剪切帶之直立褶皺，顯微影像與電子影像分析顯示Sn 葉理礦物相為黑雲母+綠泥石+鉀長石+榍石，此礦物成分指出溫壓條件為綠色片岩相，而此變形期為CASZ 左移剪切的時期，由重建應變橢球顯示主應力方向在北北西-南南東至西北-東南向，應
變模式為左移伸張；Dn+1 為脆性變形事件，發生於接近地表的低溫條件。

關鍵字

非洲中部剪切帶 ； 泛非洲 ； 顯微構造 ； 應變分析 ； 岩石學 ； Rf/φ分析法 ； 應變橢球 ； 福林氏圖

並列摘要

The N70˚E trending Central Cameroon shear zone (CCSZ) and the N50˚E trending Tcholliré-Banyo Shear zone (TBSZ) are the southwest extension of the Central African Shear Zone (CASZ). The evolution of CASZ is interpreted in two different models. The activation of the CASZ was considered to relate to the evolution of the Western Gondwana. Many researchers proposed the emplacement of these plutons are syn-kinematic, with U-Pb zircon ages of 640-580 Ma for various shear zone parallel batholith. The activation of the CASZ is due to amalgamation of the Congo craton to the West Africa craton and the Eastern Saharan Block. However, there are many rift basins along CASZ. The formations of these rift systems are associated with the opening of Atlantic Ocean. CASZ can also be activated or reactivated due to the opening of the southern Atlantic Ocean during 130-84 Ma. There are existences of high-temperature metamorphic rocks due to shearing heating in syn-shearing granite. In order to understand the metamorphic conditions, micropetrography helps to indicate the mineral assemblage and, which helps to delineate the temperature comdition of metamorphism. With new field observation, microstructural data and strain analysis, the tectonic history of TBSZ can be devided into at least three events. The earlier event Dn-1 formed recumbent folds with NE-SW striking, SE dipping fold axial planes. The mineral assemblage of Sn-1 is hornblend ± garnet ± biotite ± Albite suggested that P-T conditions in under middle Amphibolite facies. Dn-1 is result from the amalgamation of the Congo craton and the Eastern Saharan Block in N-S direction by the analysis of structure and strain. Dn event formed tight upright folds with NE-SW striking subvertical axial planes and parallel to CASZ. Sn with mineral assemblage Biotite ± Chlorite ± Sphene ± K-feldspar form under greenschist facies and left-lateral shearing. By reconstruction of strain ellipsoids, the main stress direction is NNW-SSE to NW-SE during Dn with transtension strain pattern . Dn+1 is brittle deformation close to surface by low temperature.

並列關鍵字

Central African Shear Zone ； Pan-African ； microstructure ； strain analysis ； petrography ； Rf/φ method ； strain ellipsoid ； Flinn diagram

參考文獻

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