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  • 學位論文

布吉納法索東北部西非地盾古元古代花崗岩之地球化學及年代學研究

Geochemical and Geochronological study of the Paleoproterozoic granitoids from Burkina Faso, West Africa: Implication for crustal growth and tectonic evolution of the West African Craton

指導教授 : 羅清華

摘要


西非地盾經歷了早元古代由於Eburnean造山作用而形成的大規模的新生地殼物質的增生作用(2.1Ga)。在本研究中,主要研究了布吉納法索東北部似花崗岩的地球化學特徵,同位素組成;鋯石U-Pb年齡;似花崗岩中角閃石,白雲母,黑雲母,鉀長石的40Ar/39Ar年代學測定。這些研究數據可以用來限定西非地盾演化的地球化學特徵,岩體侵位年齡,熱演化歷史,以及地球動力學過程。野外觀察 布吉納法索西北部的早元古代似花崗岩侵入到火山岩以及火山沉積序列所組成的綠岩帶內。研究顯示該區的富鈉似花崗岩是TTG組分,形成於含石榴石角閃岩地殼組分。花崗岩侵入體是部分熔融長英質鈣鹼性基底,並加入了部分變質沉積岩的貢獻。 鋯石U-Pb同位素分析測定富鈉似花崗岩的形成年齡在2122 ± 15 Ma與2181 ± 7 Ma之間,而黑雲母花崗岩IJ10的年齡為2151 ± 10 Ma。 Sr-Nd同位素數據顯示樣品有很低的初始Sr含量(ISr (T) ≤ 0.7007),εNd (T) = +0.7- +2.5,Sm-Nd模式年齡(TDM)為2363 – 2558 Ma。這些數據與Pissila綠岩帶出露的基性岩石的同位素數值相似(ISr (T) = 0.7003 - 0.7015, εNd (T) = +3.2 to +4.1, TDM = 2336- 2552 Ma)。這進一步說明富鈉似花崗岩是後期陸殼增生所形成。 U-Pb年齡以及40Ar/39Ar年齡都顯示似花崗岩在侵位後以6.5°C/Ma的冷卻速度在2190Ma 到2020 Ma年間從750°C 降到315°C。由此通過計算得到該區在岩體侵位後在300°C以上的溫度下以平均0.65 km/Ma上升速率隆升。似花崗岩的侵位與 布吉納法索 東北部的D1構造運動(Tangaean 事件, 2170 - 2130 Ma)有關。白雲母以及角閃石的年齡(2132 ± 13 - 2046 ± 12 Ma)限定了D2 構造運動事件,其平均溫度在360°C以上。白雲母的(1918 ± 24 – 1984 ± 12 Ma)年齡以及前人所做的工作反映了在1918 ± 24 – 1984 ± 12 Ma發生了D3構造事件(Wabo-Tempelse 事件)。鉀長石數據顯示在2020 Ma之後較緩慢的冷卻速度(0.54°C/Ma),通過計算得出該區平均上升速度為0.054 km/Ma。這減緩的冷卻速度可能與WAC區在2.0 Ga發生的大規模地殼增生活動的結束有關。 本次研究以及前人對西非地盾的Birimian火山岩以及變質沉積岩的研究可以推論布吉納法索東北部的岩漿岩活動並未受到太古代的物質的混染作用。早元古代的富鈉似花崗岩的侵位年齡在(2.19 - 2.15 Ga)之間,伴隨著該區大規模的地殼增生活動(2.2 - 2.0 Ga)。大量的新生物質從虧損地幔加入到地殼中。其構造環境應該類似於現今的火山弧地區。

並列摘要


The West African Craton was the site of massive juvenile crustal addition in the early Proterozoic at 2.1 Ga. Chemical and isotopic compositions as well as zircon U-Pb and 40Ar/39Ar amphibole, muscovite, biotite and K-feldspar ages of granitoid samples from northeast Burkina Faso were determined in this study. These data were then used to constrain the chemical characteristics, emplacement ages, thermal history and geodynamic setting of the studied samples in the context of the West African Craton evolution. Paleoproterozoic granitoids of northeast Burkina Faso are associated with volcanic and volcano-sedimentary sequences of the greenstone belts. Analyses indicate that Na-rich granitoids are of TTG (tonalite-trondhjemite-granodiortite) compositions and were generated by partial melting of a garnet-bearing amphibolite crust. Granite intrusions derived from partial re-melting of the deep basement of felsic calc-alkaline nature with contribution of metasediments. Zircon U-Pb isotopic analyses yielded ages between 2122 ± 15 Ma and 2181 ± 7 Ma for the Na-rich granitoids and 2151 ± 10 Ma for IJ10 (biotite granite). Sr-Nd isotopic data show very low initial Sr composition (ISr (T) ≤ 0.7007), positive epsilon neodymium values (εNd (T) = +0.7 to +2.5) and Sm-Nd model ages (TDM) of 2363 - 2558 Ma. These data are similar to those of mafic rocks of the Pissila greenstone belt: (ISr (T) = 0.7003 - 0.7015, εNd (T) = +3.2 to +4.1, TDM = 2336- 2552 Ma), and further suggest that the Na-rich granitoids are of juvenile nature. The U-Pb ages together with the 40Ar/39Ar dating results suggest that after emplacement, the granitoids cooled at a mean rate of 6.5°C/Ma from 750 to 315°C between 2181 and 2020 Ma corresponding to a mean uplift rate of 0.65 km/Ma maintaining temperatures above 300°C. In northeast Burkina Faso, the emplacement of the granitoids was concomitant with or preceded the D1 deformation or Tangaean event at 2130 - 2170 Ma. 40Ar/39Ar micas and amphibole ages (2132 ± 13 - 2046 ± 12 Ma) constrained the D2 deformation event, with mean temperatures above 360°C. Micas ages (1918 ± 24 - 1984 ± 12 Ma) together with those reported in the literature, were used to constrain the D3 deformation event or the Wabo-Tempelse event at 1918 ± 24 - 1984 ± 12 Ma. K-feldspar data revealed slow cooling rates (0.54°C/Ma) after 2020 Ma with a mean uplift rate of 0.054 km/Ma; the decrease of the cooling rates may be subsequent to the end of the major crustal growth in the West African Craton at 2.0 Ga. The present study together with published data on the Birimian volcanics and metasediments of the West African Craton preclude contamination of the juvenile Birimian/Paleoproterozoic rocks of northeast Burkina Faso by reworked Archean materials. The Paleoproterozoic Na-rich granitoids of northeast Burkina Faso were mostly emplaced during the early stage (2.19 - 2.15 Ga) of a large scale crustal growth event (2.2 - 2.0 Ga) where large amount of juvenile materials were added to the crust from the depleted mantle in a tectonic environment comparable to modern volcanic arcs.

參考文獻


Klein, E.L., Moura, C.A.V., 2008. Sao Luis craton and Gurupi belt (Brazil): possible links with the West-African craton and surrounding Pan-African belts. In: Pankhurst, R.J., Trouv, R.A.J., Brito Neves, B.B., de Wit, M.J. (Eds.), West Gondwana: Pre-Cenozoic Correlations across the South Atlantic Region. Special Publications, Geological Society of London 294, 137-151.
Abouchami, W., Boher, M., Michard, A., Albarede F., 1990. A major 2.1 Ga event of mafic magmatism in West-Africa: an early stage of crustal accretion. Journal of Geophysical Research 95 (B11), 17605-17629.
Adam, J., Rushmer, T., O’Neil, J., Francis, D., 2012. Hadean greenstones from the Nuvvuagittuq fold belt and the origin of the Earth’s early continental crust. Geology 40, 363-366.
Ama Salah, I., Liegeois, J.P., Pouclet, A., 1996. Evolution d’un arc insulaire oceanique birimien precoce au Liptako Nigerien (Sirba): geologie, geochronologie et geochimie. Journal of African Earth Sciences 22, 235-254.
Anderson, T., 2002. Correction of common lead in U-Pb analyses that do not report 204Pb. Chemical Geology 192, 59-79.

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