长江中下游晚中生代中基性岩的铅同位素特征:富集地幔的证据

Pb Isotopic Characteristics of Late Mesozoic Mafic Rocks from the Lower Yangtze Region:Evidence for Enriched Mantle

长江中下游东段庐枞、怀宁、繁昌、铜陵和宁芜地区的中基性岩属于碱性系列 ,具有高的U ,Th含量和Th/Pb ,U/Pb比值 ,分别平均为 2 .82× 10 -6,9.5 6× 10 -6和 0 .6 35 ,0 .184。样品的初始铅同位素 (130Ma)组成为 :(2 0 6Pb/ 2 0 4Pb) i=17.6 5～ 18.6 0 ,(2 0 7Pb/ 2 0 4Pb) i=15 .4 2～ 15 .5 0 ,(2 0 8Pb/ 2 0 4Pb) i=37.6 7～ 38.0 4。形成中基性岩的原始岩浆来源于富集的岩石圈地幔 ,具有EMⅠ和EMⅡ ,且以EMⅡ为主的特征。和长江中下游西段黄石地区以及大别地块西南部玄武岩的比较表明 ,长江中下游地区岩石圈地幔高的Th/Pb和U/Pb比值可能和俯冲板片析出流体的交代有关。晚中生代时期 ,华北板块岩石圈地幔以EMⅠ特征为主 ,华南板块岩石圈地幔以EMⅡ特征为主 ,岩石圈地幔性质的区域性分布与印支期扬子板块深俯冲事件密切相关。虽然中国东部新生代玄武岩因岩石圈大规模减薄表现了亏损特征 。

Mafic rocks from the eastern Lower Yangtze region, including Luzong, Huaining, Fanchang, Tongling and Ningwu areas, belong to alkaline series and are characterised by high U and Th concentrations (averageing 2.90 and 10.01×10 -6 ) and with average Th/Pb and U/Pb ratios of 0.635 and 0.184, respectively. The least contaminated samples show initial Pb isotopic ratios of 206 Pb/ 204 Pb=17.65～18.60, 207 Pb/ 204 Pb=15.42～15.50 and 208 Pb/ 204 Pb=37.67～38.04. The initial basaltic magma came from the enriched lithospheric mantle, which is dominated by EMⅡcomponent, and subordinated by EMⅠcomponent. Since the Pb isotopic ratios of the studied rocks are lower than those of basalts from southwestern Dabie and western part of the Lower Yangtze region, but the U/Pb and Th/Pb ratios of the studied mafic rocks are higher than those of the latter region, it is inferred that the Mesozoic lithoshperic mantle beneath the eastern Lower Yangtze region suffered metasomatism by fluid from subducted slab. EMⅠ type enriched mantle is commonly observed in the North China block, and EMⅡ type isotope signature appears to dominate the South China block in the late Mesozoic Era. This was closely related to deep subduction of Yangtze block in the Triassic time. Although the Cenozoic basalts from eastern China show depleted characteristics because of large scale lithospheric thinning, the residue of Mesozoic lithospheric mantle is still kept in the Cenozoic mantle, shown by their Pb isotope signature.