云南金平白马寨含矿镁铁-超镁铁岩体岩石地球化学

LITHOGEOCHEMISTRY OF THE SULFIDE-BEARING MAFIC-ULTRAMAFIC ROCK AT BAIMAZHAI, JINPING, SOUTHERN YUNNAN

云南金平白马寨铜镍硫化物矿床含矿岩体为侵位在奥陶系砂页岩的镁铁-超镁铁环状杂岩体。从岩体核心到边缘依次出现橄榄岩—橄榄辉石岩—辉石岩—辉长岩的岩相分带。该岩体的主量元素显示拉斑玄武岩岩浆分异演化的趋势。REE显示富集LREE的配分模式,具明显的Eu异常。微量元素蛛网图显示明显的Ta、Nb、Ti及P的负异常。具有高(87Sr/86Sr)i(0·710974~0·722667)和低εNd(t)(-13·17^-12·09)的特征。微量元素和同位素地球化学证据表明,岩浆源于俯冲地壳物质改造的富集地幔,岩浆上升过程中遭受了一定的地壳物质混染。岩体可能是在弧后拉张的构造环境中形成。

The sulfide-bearing complex at the Baimazhai Cu-Ni deposit was a circular mafic-uhramafic one, which, intruded in the Ordovician sandstones and shales, yielded the ages of 249 ＋ 32 Ma and 262 ＋ 2 Ma,simultaneously with the Emeishan basalts erupting. The complex consisted of, from the core to the edge, peridotite, troctolite, pyroxenite and gabbro. The major ele- ments showed a trend of tholeiitic fractional evolution. The REE distribution pattern showed a type of LREE-enrichment with obvious Eu anomaly. The spider diagram of trace elements showed a marked negative anomaly of elements of Ta, Nb, Ti and P. The isotopic data revealed a character of high （^87 Sr/^86 Sr）,（0. 710 974 -0. 722 667） and low εNd（t）（ -13. 17 ~ -12. 09）. The evidence from the trace elements and isotopic geochemistry indicated that the magma might be derived from enriched mantle modified by subducted crustal material and contaminated by crustal material during its ascending, and the tectonic setting of the mafic-ultra- mafic rocks might be a post-arc extensional environment.