牦牛坪稀土矿床碳酸岩Pb同位素地球化学

四川牦牛坪稀土矿床与稀土矿化时空密切共生的碳酸岩一正长岩碱性杂岩体的成岩时代为喜山期，碳酸岩呈脉状沿正长岩岩体中心侵入。两者具有相似的^206Pb／^204Pb和^208Pb／^204Pb比值，但碳酸岩^207Ph／^204Ph比值变化较大，且低于正长岩。这种差异并不能归因于地壳物质的混染作用，而是反映了地幔源区的特征。在Ph、Sr和Nd同位素图解中，矿区碳酸岩和正长岩显示低Ph，高Sr同位素的特征，部份碳酸岩Ph同位素落在MORB内，而Sr和Nd同位素明显不同于MORB，相对接近洋岛玄武岩的Ⅰ型富集地幔(EM1)。喜山期扬子板块呈楔形体插入龙门山地壳之中，受挤压的中下部地壳向前陆深处发生俯冲，并延伸至攀西裂谷顶部富集地幔体中，被交代的富集地幔经不同程度的和不连续的部份熔融作用形成碱性岩浆，整个演化过程导致了源区成份的不均一性。

The Pb isotopic geochemistry of carbonatites from the Maoniuping REE deposit, China

The carbonatite-syenite complexes in the Maoniuping REE deposit, Sichuan province, which are spatially and temporally associated with rare earth mineralization, were emplaced at the time of Himalayan. They show similar ~(206)Pb/~(204)Pb and ~(208)Pb/~(204)Pb ratios, but the carbonatites show large spread in ~(207)Pb/~(204)Pb ratios and lower than those for syenites. This reflects the characteristics of their mantle source, rather than crustal contamination. In Sr-Nd-Pb diagrams, the carbonatites and syenites are characterized by lower Pb and higher Sr isotopic ratios. The Pb isotopes for carbonatites are plotted to the range of the MORB, while the Sr and Nd isotopes are different to the MORB, relatively close to the EM1 of the OIB. The Yangtze plate was penetrated as a wedge into the crust at Longmenshan during the Himalayan period. The extruded middle and lower crust onwards subducted and extended into the enriched mantle in the top of the Panxi rift. This was followed by variable degrees and discrete partal melting of the metasomatized enriched mantle to produce alkali magma. The whole evolutionary process resulted in inhomogeneity of the mantle source.