Chemical and Sr-Nd compositions and ^(40)Ar/^(39)Ar ages of NW-trending dolerite dikes of Burkina Faso: Evidence for a Mesoproterozoic magmatism in the West African Craton  

作　　者：Boukare Tapsoba Ching-Hua Lo Urbain Wenmenga Yoshiyuki Iizuka Sun-Lin Chung Gregory Shellnutt 

机构地区：[1]Department of Geosciences, National Taiwan University [2]UFR-SVT, Département de Géologie, Université Pr.Joseph Ki-Zerbo de Ouagadougou [3]Institute of Earth Sciences, Academia Sinica [4]Department of Earth Sciences, National Taiwan Normal University

出　　处：《Geoscience Frontiers》2018年第6期1957-1980,共24页地学前缘（英文版）

基　　金：financially supported by research grant from the National Science Council(NSC 102-2811-M-002-200);the Ministry of Science and Technology grant(MOST 106-2116-M-002-017-MY3)

摘　　要：The Paleoproterozoic basement of the northeastern part of the Leo-Man craton is intruded by generally NW-trending dikes. These regional scale dikes extend over 1000 km in Burkina Faso, Mali and Niger. We present chemical and Sr-Nd isotope compositions, as well as 40 Ar/39 Ar ages of these dikes with the following strikes N98°-N112°, and N114°-N124° in NE Burkina Faso. Field relationships show that the dikes are posterior to all other rock types dated between 2.26 Ga and 2.0 Ga. Chemical data indicate that the dikes are continental flood basalts and composed of low-Ti(TiO2 ≤ 2 wt.%) sub-alkaline basalts and andesites. They exhibit a minor negative Europium anomaly(0.86-0.99) and slightly fractionated REE patterns((La/Yb)_N= 2.5-9.1; Yb_N = 9.5-19.9). The ratios of Th/Ta(1.3-11.4) and Ce/Pb(5.2-58.5)suggest a varying crustal assimilation of the dike magmas during ascent in the continental crust for all studied samples. Calculated P-T conditions indicate that the magma reached temperatures of 1285 ℃(calculated from olivine compositions) and pressures of 6.9 kbar(calculated for pyroxene minerals).Calculated initial 87 Sr/86 Sr(0.70040-0.70260) and ε_(Nd)(t)=+2.1 to-3.5 at 1575 Ma,also point to a crustal contamination with the most primitive samples showing T_(DM) values of 1946 Ma and 2154 Ma. The low values of La/Ba(<0.2) and Nb/La(<1.0), contrast with the low Th/Nb(<0.9), and suggest a lithospheric mantle or subduction-modified mantle as possible source for the dikes. Sr-Nd data, Mg# and NbTa-Zr-Y-Th-Tb-Yb compositions further suggest that the most primitive samples were emplaced in a none orogenic setting and their magmas were subjected to variable crustal contaminations. Literature and the present whole rock 40 Ar/39 Ar age determinations show that the dikes were emplaced during a widespread Mesoproterozoic magmatism between 1.6 Ga and 1.2 Ga, and were affected by a thermal event causing the argon systematics resetting, best constrained by the date of sample KK1(1236 ± 20 Ma,^(40)Ar/^(36)Ar = 294 ±13, The Paleoproterozoic basement of the northeastern part of the Leo-Man craton is intruded by generally NW-trending dikes. These regional scale dikes extend over 1000 km in Burkina Faso, Mali and Niger. We present chemical and Sr-Nd isotope compositions, as well as 40 Ar/39 Ar ages of these dikes with the following strikes N98°-N112°, and N114°-N124° in NE Burkina Faso. Field relationships show that the dikes are posterior to all other rock types dated between 2.26 Ga and 2.0 Ga. Chemical data indicate that the dikes are continental flood basalts and composed of low-Ti(TiO2 ≤ 2 wt.%) sub-alkaline basalts and andesites. They exhibit a minor negative Europium anomaly(0.86-0.99) and slightly fractionated REE patterns((La/Yb)_N= 2.5-9.1; Yb_N = 9.5-19.9). The ratios of Th/Ta(1.3-11.4) and Ce/Pb(5.2-58.5)suggest a varying crustal assimilation of the dike magmas during ascent in the continental crust for all studied samples. Calculated P-T conditions indicate that the magma reached temperatures of 1285 ℃(calculated from olivine compositions) and pressures of 6.9 kbar(calculated for pyroxene minerals).Calculated initial 87 Sr/86 Sr(0.70040-0.70260) and ε_(Nd)(t)=+2.1 to-3.5 at 1575 Ma,also point to a crustal contamination with the most primitive samples showing T_(DM) values of 1946 Ma and 2154 Ma. The low values of La/Ba(<0.2) and Nb/La(<1.0), contrast with the low Th/Nb(<0.9), and suggest a lithospheric mantle or subduction-modified mantle as possible source for the dikes. Sr-Nd data, Mg# and NbTa-Zr-Y-Th-Tb-Yb compositions further suggest that the most primitive samples were emplaced in a none orogenic setting and their magmas were subjected to variable crustal contaminations. Literature and the present whole rock 40 Ar/39 Ar age determinations show that the dikes were emplaced during a widespread Mesoproterozoic magmatism between 1.6 Ga and 1.2 Ga, and were affected by a thermal event causing the argon systematics resetting, best constrained by the date of sample KK1(1236 ± 20 Ma,^(40)Ar/^(36)Ar = 294 ±13,

关 键 词：MAFIC dike swarm Depleted mantle MESOPROTEROZOIC Continental THOLEIITES Nuna West African Craton 

分 类 号：P[天文地球]