北祁连中段毛藏碱性斑岩体地球化学特征及稀土成矿作用

GEOCHEMICAL CHARACTERISTICS AND RARE EARTH MINERALIZATION OF MAOZANG ALKALINE PORPHYRY IN MIDDLE SECTION OF NORTH QILIAN

毛藏碱性斑岩体侵入于加里东期毛藏寺中酸性岩基的中心部位,呈小岩株状产出。通过岩石地球化学研究,结果表明碱性斑岩体的主量元素SiO_(2)含量介于57.55%~55.98%之间,REE含量为433×10^(-6)~9824×10^(-6),平均高达5218×10^(-6)。LREE/HREE比值介于24~98之间,(La/Yb)N比值为54~5699,表现为轻重稀土强烈分异,具有富轻稀土、高钾富碱、富铝贫钛、富锶和钡等特征,岩石还相对富集Pb和Zn,含量均超过200×10^(-6),Cu含量也较高(>100×10^(-6)),最高可达1710×10^(-6)。负铕异常中等,δEu值为0.40~0.87,暗示有斜长石的分离结晶作用,具A型花岗岩岩石地球化学特征。结合构造演化认为碱性岩浆的产生与俯冲的北祁连洋壳板片的断离作用有关,区域应力场由挤压转为伸展,在地壳岩浆房中地幔源区部分熔融并发生分离结晶作用形成碱性斑岩体,伴随后期的岩浆—热液作用,稀土元素高度富集形成稀土矿。

Maozang alkaline porphyry body intrudes into the center of the Caledonian Maozangsi intermediate acid batholith and occurs as a small rock stock.Via geochemical method,the results show that contents of major element oxide SiO_(2) in alkaline porphyry is between 57.55% and 55.98%,and REE content is 433×10^(-6)~9824×10^(-6),whose average is up to 5218×10^(-6).LREE/HREE ranges from 24 to 98,(La/Yb)n less 5699 over 54,showing strong differentiation of light and heavy rare earths.The rocks are rich in light rare earths,high in potassium,alkali,aluminum,strontium and barium,but shirt of titanium.Alkaline porphyries are also relatively rich in Pb and Zn,both of which contain more than 200×10^(-6),Cu content is higher than 100×10^(-6),up to 1710×10^(-6)。Europium anomaly is negative,Eu is 0.40~0.87,suggesting fractional crystallization of plagioclase,which has geochemical characteristics of A-type granite.In combination with tectonic evolution,it is believed that generation of alkaline magma is related to detachment of subducted North Qilian Ocean crust plates.Stress field in this region translates from compression into extension,and alkaline porphyry can be formed by partial melting and fractional crystallization in mantle source area of crustal magma chamber.With later magmatic hydrothermal process,rare earth elements are highly enriched to form rare earth ores.