云南个旧碱性杂岩体的岩石成因及稀土元素富集机制

云南个旧碱性杂岩体由边缘相碱长正长岩和中心相霞石正长岩组成。全岩地球化学分析表明,该碱性杂岩体具有高碱、富钾、富铁、低镁、高分异的碱性-过碱性岩石特征,晚期更富集碱金属元素; LREE/HREE值为20～59,(La/Sm)N=8～50,(Sm/Yb)N=1.2～5.0,富集轻稀土元素,轻稀土元素较重稀土元素分馏程度高,具Eu负异常,亏损Ti、Nb、P、K、Sr等元素,富集Zr、Hf、Th、La、Ce、Nd、U、Rb等元素,岩浆来源与幔源物质有关;碱长正长岩和霞石正长岩具有相似的微量元素和稀土元素特征,具有同源岩浆分异演化的特点; Rb/Sr、Nb/Ta、Zr/Hf等比值均高于或接近于原始地幔的相应值; CIPW标准矿物计算结果表明边缘相碱长正长岩中出现紫苏辉石、锥辉石、橄榄石,中心相霞石正长岩中出现橄榄石。结合(Th/Nb)N和Nb/La值特征以及前人Sr-Nd同位素研究成果,认为个旧碱性杂岩体的岩浆来源于遭受交代作用的富集地幔部分熔融,同时受有限的地壳混染作用而成,形成于后碰撞的伸展环境。碱性岩浆演化晚期更加富碱、经历了更高程度的结晶分异作用是稀土元素、Nb、Ga和Zr元素超常富集的重要原因。

Petrogenesis and REE enrichment mechanism of Gejiu alkaline complex in Yunnan Province

The Gejiu alkaline complex is composed of alkaline feldspar syenite in border facies and nepheline syenite in central facies. The whole-rock geochemical analysis shows that the alkaline complex has the characteristics of high alkalinity, rich potassium, rich iron, low magnesium, and highly differentiated alkaline-peralkaline rocks, with the late stage more enriched in alkali metal elements. LREE/HREE=20~59, (La/Sm)_N=8~50, and (Sm/Yb)_N=1.2~5.0. It is rich in light rare earth elements. The fractionation degree of the light rare earth elements is higher than that of the heavier rare earth elements. It has Eu negative anomaly. The Gejiu alkaline complex is depleted in such elements as Ti, Nb, P, K, and Sr, and rich in such elements as Zr, Hf, Th, La, Ce, Nd, U and Rb. All these characteristics indicate that the magma source is related to mantle derived materials. Alkali syenite and nepheline syenite have similar characteristics of trace elements and rare earth elements, and have the characteristics of homologous magmatic differentiation and evolution. The ratios of Rb/Sr, Nb/Ta, and Zr/Hf are all higher than or close to the corresponding values of the original mantle. CIPW standard mineral calculations show that perilla pyroxene, cone pyroxene, and olivine appear in the alkali syenite in marginal facies. Olivine appears in the nepheline syenite in central facies. Combined with the characteristics of the ratios of (Th/Nb)_N and Nb/La and the research results of previous Sr-Nd isotope, it is considered that the magma of Gejiu alkaline complex was derived from partial melting of enriched mantle subjected to metasomatism and limited crustal contamination, which was formed in the extensional environment of the post-collision. In the late period of alkaline magama evolution, alkali was richer, and the higher degree of crystallization differentiation constituted an important factor for the abnormal enrichment of REE, Nb, Ga and Zr.