柴北缘锡铁山榴辉岩的地球化学特征

Geochemical characteristics of eclogites in Xitieshan area, North Qaidam of northwestern China.

柴北缘锡铁山的榴辉岩呈透镜状分布在片麻岩中,榴辉岩的SiO_2为45.0％～49.83％,A1_2O_3=11.31～16.52,与火成岩中的拉斑玄武质岩石类似。其稀土配分模式有3种类型:(1)轻稀土富集型,(La/Yb)_N=1.74～2.81,δ_(Eu)=0.86～1.07,与E-MORB类似.(2)平坦型,(La/Yb)_N=0.80～1.22,δ_(Eu)=0.96～1.20,与T-MORB相似;(3)轻稀土亏损型,(La/Yb)_N=0.66。δ_(Eu)=1.03,与N-MORB类似。微量元素中Rb、Ba、Th略富集,可能与地壳物质的混染有关。ε_(Nd)(0)=2.52～10.98,ε_(Nd)(800Ma)=3.05～9.20,显示大洋亏损地幔的特征。初步认为榴辉岩是洋壳俯冲的产物,与大别-苏鲁地区榴辉岩的成因明显不同。

Eclogites occur as lenses in gneisses in Xitieshan area, Northern Qaidam. They indicate a whole-rock chemical composition similar to tholeiite with SiO2 = 45.0% similar to 49. 83% and Al2O3 = 11. 3% - 16. 52%. REE patterns of eclogites include three types: (1) REE-enriched pattern with (La/Yb)(N) = 1. 74 similar to 2. 81 and delta(Eu) = 0. 86 similar to 1. 07, indicating the characteristics similar to E-MORB; (2) flat pattern with (La/Yb)(N) = 0. 80 similar to 1. 22 and delta(Eu) = 0. 96 similar to 1. 20, similar to T-MORB; (3) LREE depletion pattern with ( La/Yb)(N) = 0. 66 and delta(Eu) = 1. 03, similar to N-MORB. Eclogites show light enrichment in Rb, Ba, Th, and can be related to crustal contamination. Nd isotopic analyses gave epsilon(Nd) (0) = 2. 52 similar to 10. 98, epsilon(Nd) (800Ma) = 3. 05 similar to 9. 20., indicating characteristics of an oceanic depleted mantle reservoir. Therefore, we tentatively suggest that the eclogites probably resulted from subduction of oceanic crust. This is obviously different from eclogite of Dabie-Sulu area.