| 其他摘要 | Mafic dikes are of importance and potential in geodynamics studies, four international dike conferences launched in 1985, 1990,1995,2002 energized the global study of mafic dikes, A number of geological and geochemical and geophysical investigations including distribution and emplacement and geometry and plaeomagnetism and petrologic and geochemical and isotopic dating and tectonic evolution have undertaken on mafic dikes from earth and venues and Mars in the last two decades, the geodynamics of maifc dikes have been received serious attention by a number of geologist. In general, they are in area too little to easily neglect their scientific implications, Mesozoic and Cenozoic mafic dikes in east China holding on geodynamics setting have not been invited attention by some geologists utile last five years, even some geologists point out the recognition for them is important in developing a better understanding of distribution characteristics from global mafic dikes. Petrologic and geochemical investigations of mafic dikes in detail were concentrated-in north Gangdong from southeast China so far, a few half-backed study on mafic dikes of south Hainan and coastal belt of Fujian and southeast Hunan and ore region of some hydrothermal deposits, so systemically and large area investigations of mafic dikes from southeast China is still few and need to undertake in the future. In addition, previous geological works revealed that some late Mesozoic basic igneous bodies were related to crust extension in southeast China, but this conclusion are mainly draw from, preliminary investigations of basic igneous bodies from coastal :;helt in southeast China. This paper is to choose late Mesozoic and Cenozoic mafic dikes (bodies) relating to crust extension from Jiangxi, use new mineral composition, major element, trace element, Sr-Nd isotopic and K Ar ages data of them to systematically investigate, their geological and-geochemical characteristics and to preliminary discuss crust extension episodes and mantle characteristics through emplacement time and source characteristics of mafic dikes (bodies).The following conclusions have been drawn: (1) According their field and petrologic characteristics and building upon previous works, Late Mesozoic and Cenozoic mafic dikes (bodies) form Jiangxi province are complex in rock associations which are compose of lamprophyre and gabbro-diabase and.diabase and diabase-porphyrite, are mainly distributed into three north-north-eastern strike belt including Xingzi-Shanggao- Pingxiang and Dexing-Yujiang-Xiangshan-Jian-Shangyou and Caotaobei- Dajishan. (2) According to K-Ar isotopic ages of maifc dikes (bodies), They are mainly formed on range of 140~50Ma.Combining with emplacement age of alkaline-rich igneous rock from previous works, There are six episodes of crust extension since late Mesozoic in southeast China, are named as: 50-60 Ma (Paleocene),90 + Ma(late Cretaceous), 100-110Ma(late epoch of early Cretaceous), 125 ±Ma(middle epoch of early Cretaceous), 140 ± Ma(early epoch of early Cretaceous) and 165-180 Ma(middle and late Jurassic), respectively. (3) A number of geological and mineral composition and geochemical investigations indicate that the deep geodynamics process and mantle characteristics of middle Jurassic gabbros suits from south Jiangxi province are distinguish from Cretaceous gabbros suits from coastal belt of Fijian province, the former are related to mantle upwelling and lithosphere extension, the later are related to subduction and lie on back-arc basin in tectonic environment. (4) 140±Ma(early epoch of early Cretaceous) mafic dikes from south and. North Jiangxi province, respectively, are available to comparative geological arid geochemical discuss reveal that they are related to curst extension event and distinguish between rock series and trace element and isotopic characteristics of mafic dikes from south and north Jiangxi. In view of regional geological setting, the author thinks that source characteristics of them result from difference in emplacement depth and /or mantle composition. (5)In this paper, 125±Ma (middle epoch of early Cretaceous) of mafic dikes (bodies) are found in Jiangxi province, have probably confirmed that there are 125 ±Ma (middle epoch of early Cretaceous) important crust extension events in southeast China indeed. The source of mafic dikes (bodies) are mixed with depleted mantle and enrichment mantle (including EM I and EM II )which are probably subdction-modified heterogeneous lithosphere mantle in the earlier stages. This paper reveals that 100-llOMa and 90±Ma mafic dikes(bodies) have occurred in Jiangxi province, being similar to south Gangdong province, representing two important episodes of crust extension in southeast China. The source of 100-11 OMa and 90 ± Ma mafic dikes (bodies)is similar to that ofl25±Ma mafic dikes, and the source of 100-11 OMa mafic dikes (bodies) include more depleted mantle content. 50-60Ma(Paleocene) of mafic dikes (bodies) are found in Jiangxi province, have probably confirmed that there are 50-60Ma(Paleocene) important crust extension events in southeast China indeed. The source of mafic dikes (bodies) are mixed with depleted mantle and enrichment mantle, but the contents of the former is more than that of the later. (8) Combining with Cenozoic basalts and mantle xenoliths from southeast China, Source characteristics of late Mesozoic and Paleocene mafic dikes (bodies) indicate that there are the shifting from lithosphere-derived (late Mesozoic) to asthenosphere-derived (Cenozoic) in southeast China, which is related to asthenosphere upwelling and lithosphere extension, resulting from thinning of the lithosphere, of course probably from other mechanism. |
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