大别造山带超高压变质镁铁质-超镁铁质岩的Mg-Cr同位素研究

Previous studies displayed that the sedimentary carbonates have significant lighter magnesium (Mg) isotopic compositions than the mantle, which makes Mg isotopes a potential tracer of recycled carbonate via subducted crust into the mantle. This feature has been widely employed to interpret the observed basic and metamorphic basic rocks with low Mg isotopic compositions. However, so far, direct observation of metasomatic mantle rocks with low Mg isotopic compositions was remain unreported. On the other hand, the researches about the behaviors of chromium (Cr) isotope during many geological processes were still in the infancy. Recent works suggest that the bulk silicate earth (BSE) was homogeneous in their Cr isotopic compositions. With regard to altered oceanic crust, serpentinization could significantly shift the mafic and ultramafic rocks towards to heavier Cr isotopes. In addition, although Cr is compatible element in the mantle, some experiments reveals that the general insolube Cr(III) in aqueous fluids and melts at high H+ and Cl- activity could form soluble complexes. Therefore, whether the dehydration (or partial melting) of subducted altered oceanic crust could shift the Cr isotopic compositions of overlying mantle wedge, we need to find out. So far, very little is known about the behavior of Cr isotope in mantle wedge metasomatism. The previous works suggested that the Maowu mafic-ultramafic body is a ultra-high pressure metamorphic (UHPM) complex from precursor mantle wedge, which experienced metasomatic interaction between refractory harzburgite or peridotites and slab-derived hydrous melt or fluid during oceanic crust subduction and UHP metamorphism during continental subduction. Based on proposer’s previous observations of the Maowu garnet pyroxenites with enriched Th, U and light rare earth element (LREE) and considerably light Mg isotope compositions, this project will aim to obtain the Mg and Cr isotopic compositions of Maowu UHPM mafic and ultramafic rocks. The primary purposes are to investigate the age of mantle metasomatism and the behaviours of Mg and Cr isotopes during this process, exploring the potential ability of Mg and Cr isotopes for tracing the crust material cycle.

前人研究发现沉积碳酸盐的Mg同位素组成显著低于地幔，因此将所观测到的低Mg同位素组成的基性岩和变基性岩解释为碳酸盐随板块俯冲再循环进入地幔的结果。然而到目前为止，直接观测到交代地幔具有低Mg同位素组成的研究尚未见报道。Cr同位素在很多地质过程中研究尚处起步阶段。近期研究表明硅酸盐地球具有均一的Cr同位素组成，而蛇纹石化的超基性岩Cr同位素明显偏重，且Cr能随富Cl流（熔）体发生移动。但蚀变洋壳俯冲交代过程中Cr同位素地球化学行为尚属未知。前人已证明大别山毛屋地区超高压变质镁铁-超镁铁质岩为受到俯冲洋壳交代的地幔楔碎片，本研究拟在前期研究发现了该地石榴辉石岩具有地幔交代成因富集Th、U和大离子亲石元素和低Mg同位素基础上，进一步深入研究地幔交代作用的时代以及该过程中的Mg-Cr同位素地球化学，探索板块俯冲地幔交代过程中Mg-Cr同位素行为及其示踪地壳物质再循环的可能性。