【摘要】 本文通过对青藏高原拉萨地块中西部米巴勒及麦嘎地区的8个超钾质岩样品进行钙、镁同位素测定,在排除了风化作用、地壳混染、分离结晶及部分熔融等后期作用对超钾质岩钙、镁同位素组成的影响后,可能有来自俯冲新特提斯洋壳的碳酸盐沉积物交代超钾质岩的地幔源区。本文研究表明,米巴勒及麦嘎地区超钾质岩的δ⁴⁴Ca=0.59⁰.75（平均值为0.68±0.04）,明显低于上地幔（1.05±0.04）、硅酸盐地球值（0.94±0.05）及已发表的岩浆岩值（0.80±0.10）,指示出超钾质岩源区含有低δ⁴⁴Ca组成的物质加入;Mg同位素组成δ²⁶Mg=-0.33^-0.24（平均值为-0.29±0.03）,比上地幔值（-0.25±0.07）略低,但在误差范围内一致。超钾质岩的钙、镁同位素之间还存在一定的正相关性,进一步指示其源区有同时具有低钙、低镁同位素组成的物质加入。通过对拉萨地块构造演化过程的识别,笔者认为这些具有低钙、低镁同位素组成的物质极有可能是来自俯冲新特提斯洋壳的碳酸盐沉积物。更多还原

【Abstract】 To better understand the compositions of enriched mantle source of ultra-potassic rocks in southern Tibet,here we report Ca and Mg isotopic compositions of eight ultra-potassic rock samples from the Mibale and Maiga areas in the Lhasa Terrane.Excluding the effect of weathering,crustal contamination,fractional crystallization and partial melting on ultra-potassic rocks’ Ca and Mg isotopic compositions,it is determined that some carbonate sediments from subducted Neo-Thethy oceanic crust may join into the mantle source of ultra-potassic rocks.The ultra-potassic rocks show significantly lower δ⁴⁴Ca values ranging from 0.59 to 0.75（average=0.68±0.04）,which are distinctly lower than the upper mantle（1.05±0.04）,bulk silicate earth（0.94±0.05）and igneous rocks（0.80±0.10）.All these features indicate that the mantle source of ultra-potassic rocks might contain materials with low δ⁴⁴Ca value.The δ²⁶Mg values of ultra-potassic rocks display limited varication from-0.33 to-0.24（mean=-0.29±0.03）,which are similar to the upper mantle(δ²⁶Mg=-0.25±0.07)within the analytical uncertainty.The CaMg isotopic compositions of ultra-potassic rocks are positively correlated,indicating their source might be metasomatized by the materials with low δ⁴⁴Ca and δ²⁶Mg value.Considering the tectonic evolution of Lhasa Terrane,we suggest that the low Ca-Mg isotopic matericals are very likely marine carbonate sediments derived from subducted Neo-Tethyan oceanic crust.更多还原