作　　者： ; ; ; ;

机构地区： 中国科学院广州地球化学研究所

出　　处： 《岩石学报》 2008年第10期2301-2312,共12页

摘　　要： 铁钛磷灰岩仅由磷灰石和铁钛氧化物组成,常赋存于岩体型斜长岩中,成因上有不混溶和分异堆晶两种不同的认识。本文从磷灰石角度讨论河北大庙铁钛磷灰岩的形成机制。大庙铁钛磷灰岩常产出于浸染状Fe-P矿体内部,有时与块状铁矿石交互出现形成韵律条带状矿石,为岩浆结晶分异的产物。铁钛磷灰岩中磷灰石呈浑圆状,含量变化于15%～34%。铁钛磷灰岩的全岩和磷灰石微量元素分析显示,磷灰石比全岩相对富集稀土元素达2.96～6.93倍,但两者的配分型式基本平行。质量平衡计算(R∝1/F)的结果表明,铁钛磷灰岩中几乎100%的稀土元素赋存于磷灰石中。综合上述特征,反映磷灰石为结晶分离的堆晶矿物,铁钛磷灰岩应为堆晶成因。因为如果磷灰石结晶于铁钛磷灰岩不混溶熔体,它的稀土元素分配系数也不会变化达2.3倍(变化于2.96～6.93)。计算出该磷灰石的母岩浆稀土元素组成,与浸染状Fe-P矿石最为相似,结合它与铁钛磷灰岩之间紧密共生的野外特征以及相似的全岩及磷灰石稀土元素配分型式,认为磷灰石最可能是在浸染状Fe-P矿浆中,经结晶分离作用形成铁钛磷灰岩。 Nelsonites, which consist essentially of Fe-Ti oxides and apatite, are spatially and temporally associated with Proterozoic massif-type anorthosites, and have been interpreted as either cumulate rocks or the products of immiscible melts. The Damiao anorthosite massif in Hebei Province in China, contains numerous disseminated and vein-type Fe-Ti oxide and apatite orebodies. The disseminated ores are hosted in homogeneous silicate rocks, whereas the vein-like bodies occur as veins, lenses and rhythmic layers of massive Fe-Ti oxides ± apatite that are in sharp contact with the host anorthosite or norite. Here we provide new trace element data on individual apatite crystals and whole-rock samples of the host nelsonites of the Damiao anorthosite massif and use these data to investigate the mechanism of apatite accumulation in these rocks. Apatites in all analyzed nelsonites from Damiao are enriched in REE relative to the whole-rock samples by factors of 2.96 to 6.93, but both have parallel chondrite-normalized REE patterns. Mass-balance calculations （theoretical calculated curve R ∝ 1/F） confirms that the whole-rock REE concentrations of nelsonites are nearly 100% controlled by the apatite. This finding strongly suggests that the apatites are cumulus minerals, because if the nelsonites were derived from an immiscible liquid, the apatite DREE value （ratio of REE in apatite and whole-rock ） would not show such large variations. Calculated melts in equilibrium with the cumulus apatite, have fight-dip chrondrite-normalized REE patterns with small negative Eu and Ce anomalies and （Yb/Lu） N 〈 1. These melts are similar in composition to the disseminated Fe-P ores in Damiao, suggesting that the disseminated ores may represent the parental magma. This interpretation is supported by the fact that the nelsonites are usually associated with disseminated Fe-P ores, and by the similarity of the apatite and whole-rock REE patterns.

关 键 词： 岩体型斜长岩 铁钛磷灰岩 大庙斜长岩体 矿床 不混溶 分异结晶 堆晶

领　　域： [天文地球] [天文地球] [天文地球] [天文地球]