长江中下游晚中生代橄榄玄粗岩系列火山岩:年代学格架、地球化学特征及成因讨论

长江中下游晚中生代橄榄玄粗岩系列火山岩发育在宁芜、庐枞、溧水和怀宁四个火山岩盆地内，该系列的火山岩地球化学性质上以相对富碱、高K、明显富集Rb、Th、U、K等强不相容元素和轻稀土元素，亏损高场强元素Nb、Ta和Ti为特征。Ba和Sr的特征在不同盆地内随岩性的不同表现各异，表明斜长石的分离结晶可能在这些盆地内岩浆演化过程中起着一定的作用。这些盆地内的橄榄玄粗岩系列火山岩的部分地球化学性质(如Ce/Yb比值)与大多数大陆环境下的橄榄玄粗岩系列岩石不同，而类似于大洋岛弧内的，可能意味着区内由于岩石圈的减薄，软流圈地幔上涌到了相对较浅的部位，控制源区部分熔融的主要是尖晶石相地幔岩。这些火山岩的Sr、Nd同位素组成总体处于扬子克拉通岩石圈地幔附近，指示这些橄榄玄粗质母岩浆主要是由富集的岩石圈地幔部分熔融形成的。但盆地所处的构造位置对岩浆的性质也有显著的影响，指示地壳基底物质的混染也不同程度存在。区内榄玄粗岩系列火山活动持续的时间很短，主要集中在约128134 Ma左右，峰值约130 Ma，其成因的动力学机制可能与晚中生代发生在中国东部的岩石圈减薄事件有关。

Late Mesozoic Shoshonotic Volcanic Rocks in the Middle and Lower Yangtze River Reaches: Ages, Geochemical and Genesis

The Mesozoic shoshonotic series volcanic rocks in the Lower Yangtze River reaches restricted in the Ningwu, Luzong, Lishui and Huaining four basins. The series are characterized by high-K and alkali enrichment. In geochemistry, these rocks enriched in strong incompatible elements such as Rb, Th, U and K as well as light rare earth elements (LREE), depleted in high-field-strong-elements (HFSE) such as Nb and Ta. The Sr and Ba characteristics of the rocks vary with lithology, indicating that the fractional crystallization of plagioclase may play an important role during magma evolution in these basins. Some geochemistry characteristics(such as Ce/Yb) of the shoshonotic series volcanic rocks in the basins are similarly with that of intra-oceanic island arcs, possibly because the lithospheric thinning allowed the asthenospheric mantle upwelled to relatively shallow depth where a greater proportion of spinel-facies melting can contribute to magma generation. The Nd and Sr isotope compositions of the rocks are roughly similar with the enriched subcontinental lithospheric mantle of the Yangtze craton, imply that their parental magma were mainly come from partial melting of the enriched subcontinental lithospheric mantle. But the tectonic setting of the basins are also play an obvious role on the magma characteristics, indicating the existing of crustal contamination. The whole shoshonotic series volcanic rocks in the Lower Yangtze region formed in a very short period from 128 to 134 Ma, with a peak about 130 Ma. The genesis of the shoshonotic magma may be related with the lithospheric thinning of the eastern China in the late Mesozoic.