喀麥隆喬立雷-班約剪切帶之板塊演化史
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2014
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位於喀麥隆境內的班約-喬立雷剪切帶 (Tcholliré-Banyo Shear Zone, TBSZ)是一個東北-西南走向的剪切帶，在喀麥隆屬於非洲中部剪切帶 (Central African Shear Zone, CASZ) 的分支。位處非洲三大地塊：西非古陸塊 (West African Craton)、剛果古陸塊 (Congo Craton)、與東撒拉古陸塊 (Eastern Sahara Block) 之交界處，其構造演化史可以完整記錄此區域相關板塊聚合與分離等板塊演化史。根據本研究於喀麥隆所採集之兩個片麻岩 (10CC02C、10CC05)、兩個糜稜岩(10CC03A、10CC03C) 與花崗岩 (10CC04) 之鋯石鈾-鉛定年結果顯示：主要年代集中於 1.8 ~ 2.1 Ga 與 696 ~ 514 Ma之間。而其鈾釷比值數據顯示：片麻岩 (10CC02C、10CC05)、糜稜岩 (10CC03A、10CC03C) 之核心鋯石 (1.85 ~ 2.14 Ga) 為火成鋯石 (Th/ U＝0.12 ~ 1.23 > 0.1)，但其外圍環帶 (696 ~ 514 Ma) 則為變質鋯石 (Th/ U＝0.0013 ~ 0.0098< 0.01)，且其協和線之下交點約位於六億年前處。雖然花崗岩 (10CC04) 之鋯石鈾-鉛定年結果亦顯示相同之年代區間，但只有非常少的鋯石顯示較老的核心年代，大多數的鋯石顯示核心與周圍環帶記錄了相同之年代區間 (658 ~ 501 Ma)。除此之外，其鈾釷比值皆大於0.1 (Th/ U = 0.12 ~ 1.17) 顯示此花崗岩成岩於618 Ma ，而其較老之核心為繼承鋯石。
為釐清不同火成事件之板塊架構與其岩漿來源，本研究亦進行了全岩主要及微量元素分析與鋯石鉿同位素分析。除花崗岩之外，其他岩石樣本之微量元素皆遠低於正常火成岩之含量，這指出班約-喬立雷剪切帶內之片麻岩 (10CC02C、10CC05)、糜稜岩 (10CC03A、10CC03C) 之原岩為沈積岩。其沈積物質可能源自於伴隨古元古宙時期之埃伯尼造山運動 (Eburnean orogeny) 所形成之火成岩。雖其確切沈積事件之時間還無法確定，但可能與中元古宙時期形成的雅溫得統 (Yaoundé series) 及隆盆地 (Lom basins) 之年代相當。花崗岩樣本之鋯石鉿同位素 (εHf (T) values: －18.5 ~－11.2; +1.0 ~ +10.5) 顯示其岩漿包含地殼與地幔來源之混合訊號，因此可推測此伴隨東撒哈拉古陸塊 (Eastern Sahara Block)、西非古陸塊 (West African Craton) 與剛果古陸塊 (Congo Craton) 碰撞之泛非洲造山運動所形成之火成事件處於一弧陸碰撞之環境，且同時伴隨著角閃岩相之區域/接觸變質作用。根據岩石學之分析顯示此區域之變質沈積岩在泛非洲造山運動之後主要經歷了從角閃岩相變成綠片岩相之褪變質作用。鉀長石 (10CC02C) 之氬-氬定年結果顯示此區域之鉀長石由589 ~ 519 Ma持續受到班約-喬立雷剪切帶變形作用之影響。
關鍵字: 鋯石鈾-鉛定年、喬立雷-班約剪切帶、板塊演化、泛非洲
Tcholliré-Banyo Shear Zone (TBSZ), north Cameroon, is a NE-SW trending branch of the Central African Shear Zone (CASZ). The TBSZ situated within the boundaries of three major cratons: the Western African Craton, the Congo Craton, and the Eastern Sahara Block. By conducting geochronology, geochemistry and petrology analyses on ten samples coming from TBSZ and CASZ, this study attempt to decipher the tectonic evolution of this region. The U-Pb zircon ages of gneiss (10CC02C、10CC05), mylonite (10CC03A、10CC03C), and granite (10CC04) revealed two major age groups of (1.85 ~ 2.14 Ga) and (696 ~ 514 Ma). Except for the granite sample, the Th/ U (0.12 ~ 1.23) of the older core of all other samples are all larger than 0.1, which indicate a magmatic origin. The younger rim, however, showed Th/ U (0.0013 ~ 0.0098)< 0.01 indicating a metamorphic origin. The Th/ U are all larger than 0.1 for the granite sample indicating the granite crystallized during the 618 Ma (concordant age) Pan-African Orogeny, and the few older cores are inherited zircons. In order to decipher the tectonic setting of the magmatic sources, whole rock XRF analysis of major and trace element with Hf isotope analysis of zircons are also conducted. Although the zircon U-Pb geochronology data of the Paleoproterozic core suggested a magmatic origin, the whole rock trace element distributions are far below the normal magmatic rocks. Which pointed out that the gneiss and mylonite samples are metasedimentary rocks, and the age in the core of zircons are inherited. Since the age in the core of zircons corresponds to the Paleoproterozoic magmatic event formed during the Eburnean orogeny by the collision between the South America and West Africa. This could be the source of sediments for these metasedimentary rocks. Although the sedimentary age was uncertain, it is possible that they have the same age as the Mesoproterozoic Yaoundé series in the Lom basins. The wide range of εHf (T) values (－18.5 ~ －11.2; +1.0 ~ +10.5) of the 618 Ma granite sample suggested the formation of this magmatic event is probably due to the Pan-African orogeny with collision between the Eastern Sahara Block with the Congo Craton and West African Craton under an arc environment. Other than magmatic event, the Pan-African orogeny also generated theregional metamorphism of surrounding metasedimentary rocks. Petrological analysis of the metasedimentary rocks showed retrograde metamorphism from amphibolite facies to greenschist facies after the Pan-African event. 40Ar/39Ar dating of metamorphosed K-feldspar mineral separates of 10CC02C sample showed age ranges from 589 ~ 519 Ma indicating the deformation event of the TBSZ had occurred under the lower greenschist facies during this time. Key Words:U-Pb Zircon Dating, Tcholliré-Banyo Shear Zone (TBSZ), Tectonic evolution, Pan-African
Tcholliré-Banyo Shear Zone (TBSZ), north Cameroon, is a NE-SW trending branch of the Central African Shear Zone (CASZ). The TBSZ situated within the boundaries of three major cratons: the Western African Craton, the Congo Craton, and the Eastern Sahara Block. By conducting geochronology, geochemistry and petrology analyses on ten samples coming from TBSZ and CASZ, this study attempt to decipher the tectonic evolution of this region. The U-Pb zircon ages of gneiss (10CC02C、10CC05), mylonite (10CC03A、10CC03C), and granite (10CC04) revealed two major age groups of (1.85 ~ 2.14 Ga) and (696 ~ 514 Ma). Except for the granite sample, the Th/ U (0.12 ~ 1.23) of the older core of all other samples are all larger than 0.1, which indicate a magmatic origin. The younger rim, however, showed Th/ U (0.0013 ~ 0.0098)< 0.01 indicating a metamorphic origin. The Th/ U are all larger than 0.1 for the granite sample indicating the granite crystallized during the 618 Ma (concordant age) Pan-African Orogeny, and the few older cores are inherited zircons. In order to decipher the tectonic setting of the magmatic sources, whole rock XRF analysis of major and trace element with Hf isotope analysis of zircons are also conducted. Although the zircon U-Pb geochronology data of the Paleoproterozic core suggested a magmatic origin, the whole rock trace element distributions are far below the normal magmatic rocks. Which pointed out that the gneiss and mylonite samples are metasedimentary rocks, and the age in the core of zircons are inherited. Since the age in the core of zircons corresponds to the Paleoproterozoic magmatic event formed during the Eburnean orogeny by the collision between the South America and West Africa. This could be the source of sediments for these metasedimentary rocks. Although the sedimentary age was uncertain, it is possible that they have the same age as the Mesoproterozoic Yaoundé series in the Lom basins. The wide range of εHf (T) values (－18.5 ~ －11.2; +1.0 ~ +10.5) of the 618 Ma granite sample suggested the formation of this magmatic event is probably due to the Pan-African orogeny with collision between the Eastern Sahara Block with the Congo Craton and West African Craton under an arc environment. Other than magmatic event, the Pan-African orogeny also generated theregional metamorphism of surrounding metasedimentary rocks. Petrological analysis of the metasedimentary rocks showed retrograde metamorphism from amphibolite facies to greenschist facies after the Pan-African event. 40Ar/39Ar dating of metamorphosed K-feldspar mineral separates of 10CC02C sample showed age ranges from 589 ~ 519 Ma indicating the deformation event of the TBSZ had occurred under the lower greenschist facies during this time. Key Words:U-Pb Zircon Dating, Tcholliré-Banyo Shear Zone (TBSZ), Tectonic evolution, Pan-African
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鋯石鈾-鉛定年, 喬立雷-班約剪切帶, 板塊演化, 泛非洲, U-Pb Zircon Dating, Tcholliré-Banyo Shear Zone (TBSZ), Tectonic evolution, Pan-African