地幔富硅交代与大陆岩石圈的演化

富硅交代是弧下地幔中熔体岩石相互作用的主要表现形式 ,是造成古老克拉通陆下岩石圈地幔富硅的主要机制。在弧下地幔捕掳体中 ,橄榄岩被来自俯冲洋壳物质部分熔融生成的含水富硅熔体交代后 ,斜方辉石含量的增加使全岩富集SiO2 ,斜方辉石显示异常低的Al2 O3和Cr2 O3,微量元素上表现为强烈富集LILE ,强烈亏损Nb ,Ta和Ti。在古老克拉通地幔岩样品中 ,方辉橄榄岩具过剩的斜方辉石 ,橄榄石的Ni含量与斜方辉石的组成含量成正比 ,而和橄榄石的x(Mg) /x(Mg +Fe2 +)值没有正比关系 ,被解释为亏损的地幔橄榄岩和来自俯冲板片的富硅熔体相互作用的结果。熔体岩石相互作用最终导致了陆下岩石圈地幔富集SiO2 ,这种被含水富硅熔体改造后的地幔岩石的部分熔融可能是造成陆壳富硅富镁的主要原因。含水富硅熔体对岩石圈地幔的影响程度也可能是大陆岩石圈增生或裂解、增厚或减薄的关键因素之一。

SILICA ENRICHMENT OF MANTLE AND THE EVOLUTION OF CONTINENTAL LITHOSPHERE

Silica enrichment,one of the main melt-rock interaction processes,is popular at mantle wedge. Mantle xenoliths erupted from sub-arc mantle show the same chemical characteristics with island arc basalts, which are extremely enriched LILE and depleted HFSE. Mantle samples from craton have too much Opx and show a positive correlation between Ni contents of Ol and modal proportions of Opx, which are not the results of partial melting but of metasomatism by slab-derived silicic fluids. The reaction is olivine+SiO 2 -rich melt=Opx (+SiO 2 -depleted melt). This kind of melt-rock interaction makes the uppermost mantle enriched in Si, which might be the key to know the compositional characteristics of continental crust. Detailed research on melt-rock interaction will help us to know better about the breakup and growth of supercontinent which were related with global significant tectono-thermal events.