苏鲁皖新生代玄武岩中巨晶组合的稀土元素特征

我国东部苏鲁皖地区新生代碱性玄武岩中,除了含有大量地幔橄榄岩类捕虏体以外,尚含有一定数量的石榴石、普通辉石和歪长石巨晶。这些巨晶是在地幔不同深度上从玄武岩中晶出的。巨晶组合的分离结晶作用对熔体稀土元素含量有很大影响。赋存巨晶的碱性玄武岩所具有的LREE富集、HREE亏损的稀土元素分配型式是由地幔橄榄岩类部分熔融程度、石榴石巨晶和普通辉石巨晶的早期高压熔离和玄武岩的结晶分异作用等综合因素造成的。

REE Characteristics of Megacryst Assemblages in Alkali Basalts of Jiangsu-Shandong-Anhui Area

The megacrysts discussed in this paper occur in Cenozoic alkali basalts of Jiangsu-Shandong-Anhui area. Garnet megacrysts belong to pyrope-almandine series, while monoclinic pyroxene megacrysts are of aluminous augites. Both megacrysts were formed through early crystallization of basaltic magma at high pressure. In contrast, the anorthoclase megacrysts seem to be of a relatively low pressure phase.Seven megacryst samples were analyzed for REE. The garnet megacrysts are characteristically rich in HREE, and the chondrite-normalized patterns show that (La/Yb)_N ratio and (Ce/Yb)_N are respectively 0.01-0.02 and 0.03-0.08. Fractional crystallization of the garnets caused HREE depletion in the melt. The augite megacrysts contain higher MREE, lower LREE and lowest HREE, with (La/Yb)_N ratio and (Ce/Yb)_N being respectively 3-16 and 4-27. The augites led to MREE depletion and LREE enrichment in the basaltic magma. The anorthoclase megacrysts show relatively high positive Eu anomalies, Eu/Eu ratio being 4-7. Their crystallization gave rise to slight, negative Eu anomalies in the melt. The patterns of megacryst/melt REE partition coefficients are consistent with chondrite-normalized REE patterns of megacrysts.It can be concluded from the above discussion that REE distribution patterns of LREE enrichment and HREE depletion in alkali basalts are caused jointly by degree of partial melting, fractional crystallization of megacrysts and differentiation of basaltic magma.