云开地区新元古代蛇绿岩的地球化学证据及其构造意义

The Geochemical Evidences and Tectonic Significance of Neoproterozoic Ophiolite in Yunkai Area,Western Guangdong Province,China

粤西云开隆起区西北缘信宜贵子、茶山一带浅变质云开群底部层状、似层状变基性、超基性岩的研究表明,变基性岩以变玄武岩、变辉长岩、斜长角闪岩为主,变超基性岩则主要为蛇纹石化橄榄岩、橄辉岩和角闪辉石岩,它们是遭受后期构造变形变质作用强烈改造的蛇绿混杂岩。变基性岩的岩石地球化学特征可分为略有差异的两类:茶山的TiO2=0．71％-2．16％(平均1．38％),(La／Yb)N=0．67-1．91,轻稀土略亏损-平坦,La／Nb、Ce／ Zr、Zr／Nb、Zr／Y、Ti／Y平均值分别为1．19、0．31、15．75、2．08和434．98,Nb／Th平均值为4．0,εNd(t)=1．4-1．8;贵子的TiO2=1．55％-1．89％(平均1．74％),(La／Yb)N=2．13-2．80,轻稀土略富集,La／Nb、Ce／Zr、Zr／Nb、Zr／Y、 Ti／Y平均值分别为0．84、0．22、13．04、4．15和351．93,Nb／Th平均值为5．8,εNd(t)=2．4-5．0,均显示俯冲带上 (SSZ)和洋中脊(E-MORB)构造环境的玄武岩,并来自富集地幔源区。变超基性岩蛇纹石化橄榄岩的稀土元素配分具典型的U形特征,显示为具地幔交代作用的LREE富集地幔残余。茶山和贵子变基性岩的Sm-Nd、Rb-Sr全岩同位素等时线年龄分别为824±77 Ma和667±43 Ma、663±1 7 Ma。这为华南存在新元古代-早古生代的洋盆提供了重要证据,而且在洋盆初始阶段有地幔柱或OIB物质组分的加入。

There are stratified and stratoid metabasic-ultrabasic rocks in the bottom of epimetamorphic Yunkai Group. They distribute along the northestern edge of Yunkai upwelling area, western Guangdong province, China. The metabasic rocks are mainly metabasalt, metagabbro, and amphibolite. The ultrabasic rocks are mainly serpentinized peridotite, olivine pyroxenite and amphibolite pyroxenite. They constitute the tectonic ophiolite melange which was intensively transformed by structural deformation and metamorphism. The metabasic rocks can be geochemically divided into two slightly different types. The Chashan, metabasic rocks TiO2 =0. 71% ～2.16% （average 1.38% ） （La/Yb）N= 0. 67～ 1.91 ,LREE slightly depletion-flat, La/Nb, Ce/ Zr, Zr/Nb, Zr/Y and Ti/Y are 1.19, 0. 31, 15.75, 2.08 and 434.98 （average）, respectively, Nb/Th=4.0 （average）, ENd （t） = 1. 4 ～ 1. 8. The Guizi, metabasic rocks TiO2 = 1.55 % ～ 1.89 % （average 1.74 %）, （La/ Yb）N=2.13～2.80, LREE slightly enriched, La/Nb, Ce/Zr, Zr/Nb, Zr/Y, Ti/Y are 0. 84, 0. 22, 13.04, 4.15 and 351.93 （average）, respectively, Nb/Th = 5.8 （average）, ENd （t） = 2. 4 ～ 5. 0. The both indicate that the rocks were formed in the tectonic environment of oceanic ridge （E-MORB） and supra-subduction zone （SSZ）, and in enriched mantle resources region. The REE distribution of serpentinized peridotite in ultrabasic rocks is the characteristic of U-type, showing that it was the LREE enriched residual mantle experienced in mantle metasomatism. The Sm-Nd ,Rb-Sr isotope whole-rock isochrone ages of metabasic rocks in the Chashan and Guizi are 824±77Ma and 667±43Ma.663±17Ma. It provides the important evidence for the existence of Neoproterozoic-early Palaeozoic oceanic basin in South China, and mantle plume or OIB material were mixed in the early oceanic basin stage.