大陆地幔交代作用:地台活化的先驱事件?

地幔交代作用已得到幔源岩石地球化学研究的证实,并已广泛地用于解释上地幔的不均一性、地幔地球化学演化和幔源碱性岩浆的成因问题。本文通过分析国内外某些典型地洼区(中国滇西、华北;西德莱茵地堑;东非裂谷;澳大利亚东部;法国中央地块)幔源岩石的微量元素和Sr、Nd、Pb同位素资料,发现这些地洼区在地台阶段向地洼阶段转化过程中,上地幔化学结构由亏损状态向富集状态转化。上地幔的化学结构的这种转化主要由地幔交代作用所造成。同位素体系计时结果表明地幔交代作用是导致地台活化的先驱事件。地幔交代作用不仅改变了上地幔的化学结构,而且导致交代地幔热流升高、密度减小、体积膨大、固相线下降。所有这些效应加剧了地幔的蠕动和向地壳的热量释放,并产生地台的活化。

CONTINENTAL MANTLE METASOMATISM:A PRECURSORY EVENT OF PLATFORM ACTIVATION?

Mantle metasomatism has been verified by geochemical studies on mantle-derived rocks and used widely to explain heteroge- neity, geochemical evolution of the mantle and origin of mantle_derived alkaline magmas. The author, analyzing the trace element and Sr-. Nd-, and Pb_isotopic data of mantle_derived rocks in some typicaldiwa regions worldwide (western Ynnnan and North China, Rhine Graben, East Africa Rift, eastern Australia, and Central Massif of France), notes that chemical texture of the upper mantle changed from depleted to enriched state during the evolution from the platform to diwa stage in these diwa regions. This transition of the chemical texture of the upper mantle was caused by mantle metasomatism. The timing result of isotopic systematies reveals that mantle metasomatism was a precursory event resulting in platform activation. Mantle mctasomatism not only changes chemical texture of the upper mantle, but also results in increase of heat flux, decrease of density, expansion of volume and lowering of solidus of metasomatic mantle. All these effects strengthen creep of the mantle and heat release toward the crust and result in platform activation.