东北和华北地区上地幔成分和热状态对比及对中国东部岩石圈减薄的启示

Comparison of composition and thermal stale of the upper mantle beneath northeast and north China: Implications for lithospheric thinning in eastern China

地幔成分与其上覆地壳年龄存在相关关系,年龄越老,地幔越亏损玄武质组分。本文对产于东北和华北地区的尖晶石相橄榄岩包体的成分进行了统计分析,结果显示东北地区橄榄岩包体比华北地区包体更亏损玄武质组分。这说明东北岩石圈地幔比华北上地幔更难熔,但其上覆地壳年龄却远小于华北地区地壳的年龄。这种地壳年龄和地幔组分之间的解耦暗示东北和华北地区的岩石圈地幔形成之后发生了大规模的改造。华北地区的壳幔解耦与中生代岩石圈减薄和增生有关,而东北地区的壳幔解耦则是该区地壳的多期改造和中生代岩石圈减薄和增生等过程综合作用的结果。两地区地幔成分的差异显然与部分熔融程度的不同有关,但影响部分熔融程度的因素很多,目前尚不能确定。包体的平衡温度统计和地温线对比显示东北岩石圈的地温梯度低于华北的地温梯度,可能是东北地区岩石圈减薄的时间要早于华北地区,或者华北岩石圈减薄程度可能大于东北地区的结果,因此中国东部岩石圈减薄存在时空不均一性。

Mantle composition and its overlying crust age are considered having a positive correlation; more refractory mantle underneath the crust with older ages. Comparison has been carried out on the composition and thermo-barometric condition of spinel peridotite xenoliths from NE China and N China. It is revealed that peridotites from Northeast China are more depleted in basaltic compositions than those from North China. However, the crust in NE China has a younger age than that in North China. This decoupling between crust age and mantle composition indicates that the lithosphere mantle in N and NE China may have been modified after its formation. Specifically, the crust-mantle decoupling in North China is due to the Mesozoic lithospheric thinning and accretion; whereas that in NE China resulted from the combined effect of intense crust movement and the Mesozoic lithospheric thinning processes. The compositional difference of the upper mantle between two areas is certainly related to different degrees of partial melting. The extent of partial melting is dependent upon many factors whose relative contribution is difficult to assess at this stage. Comparison of equilibrium temperature of peridotites and thermal state indicates that the lithosphere mantle beneath NE China is cooler than that under North China. This suggests that either lithospheric thinning took place earlier in NE China than in N China, or the extent of lithospheric thinning in N China is greater than in NE China, highlighting the temporal and spatial variation of the lithospheric thinning process in eastern China.