THE GEOCHEMICAL EVIDENCES AND TECTONIC SIGNIFICANCE OF NEOPROTEROZOIC OPHIOLITE IN YUNKAI AREA, WESTERN GUANGDONG PROVINCE, CHINA

Abstract

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), εNd(t) = 1.4~1.8. The Guizi, metabasic rocks T1O2,= 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), ε Nd(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.