乍得西南部中新生代火山作用与盆地演化

Located in the Central and West African, Chad, which is not well geological explored, is characterized by Mesozoic- Cenozoic intra-continental rift basins. The boreholes exposed that, during Mesozoic-Cenozoic times, volcanic activities were intense in these basins, but study on volcanic rocks is very weak, especially on those embedded in rift basins, and so far systematic and detailed work has still no carried out. Based on the project of China National Oil and Gas Exploration and Development Corporation, “The analysis of reservoir condition and the evaluation of exploration targets of seven basins in block H in Chad”, and the cooperative project between Institute of Geology and Geophysics, CAS and CNPC International (Chad) Co. Ltd., “Chronology and geochemistry studies on Mesozoic-Cenozoic volcanic rocks from southwestern Chad Basins”, systematic geochronology, geochemistry and Sr-Nd-Pb isotopic geochemistry studies on volcanic rocks from southwestern Chad basins have been done in the thesis for the first time. Detailed geochronological study using whole-rock K-Ar and Ar-Ar methods shows the mainly eruption ages of these volcanic rocks are Late Cretaceous- Paleogene. Volcanic rocks in the well Nere-1 and Figuier-1 from Doba basin are products of the Late Cretaceous which majority of the K-Ar (Ar-Ar) ages fall in the interval 95-75 Ma, whereas volcanic rocks in the well Ronier-1 from Bongor Basin and the Well Acacia-1 from Lake Chad Basin formed in the Paleogene which the ages concentrated in 66-52Ma. Two main periods of volcanic activity can be recognized in the study area, namely, the Late Cretaceous period and the Paleogene period. Volcanic activities have a general trend of south to north migration, but this may be only a local expression, and farther future studies should be carried on. Petrology study exhibits these volcanic rocks from southwestern Chad basins are mainly tholeiitic basalt. Major- and trace elements as well as Sr-Nd-Pb isotopic geochemistry studies show that the late Cretaceous and the Paleogene basalts have a definitely genetic relationship, and magmas which the basalts in southwestern Chad basins derived from were produced by fractional crystallization of olivine and clinopyroxene and had not do suffered from crustal contamination. These basalts are prominently enriched light rare earth elements (LREE), large-ion lithophile elements (LILE) and high field strength elements (HFSE) and depleted compatible elements. They have positive Ba, Pb, Sr, Nb, Ta, Zr, Hf anomalies and negative Th, U, P,Y anomalies. It is possible that the basalts from southwestern Chad basins mainly formed by mixing of depleted mantle (DM) and enriched mantle (EMⅡ) sources. The late Cretaceous basalts have higher (87Sr/86Sr)i ratios than the Paleogene basalts’, whereas have lower (143Nd/144Nd)i ratios than the latter, showing a significant temporal evolution. The mantle sources of the Late Cretaceous basalts may have more enriched mantle(EMⅡ) compositions, whereas those of the Paleogene basalts are relatively more asthenospheric mantle (DM) components. The mantle components with temporal change observed in basalts from Chad basins were probably correlated with the asthenospheric mantle upwelling and lithospheric thinning in Central and Western Africa since Mesozoic. Mesozoic- Cenozoic Volcanism in Chad basins probably is a product of intra- plate extensional stress regime, corresponded to the tectonic setting of the whole West and Central African during Cretaceous. Volcanism is closely correlated with rifting. As time passed from early period to late, the basaltic magma of Chad basins, characterized with shallower genetic depth, higher density and smaller viscosity, probably indicates the gradual strengthening evolution of the rifting. In the initial rife stage, volcanic activities are absent in the study area. Volcanic activities are basiccally corresponded with the strong extensional period of Chad basins, and the eruption of basalts was slightly lagged behind the extensional period. In the post-rift stage (30-0Ma), these basins shifted to the thermal sag phase, volcanic activities in the study area significantly decreased and then terminated.