钠长石花岗岩中雪球结构形成机理的研究

在某些富锂、氟含稀有金属花岗岩的石英和钾长石斑晶中常见 (半 )雪球结构。雪球结构的产出特征、雪球体中钠长石的电子探针分析结果以及其他间接证据都说明 ,雪球结构是在岩浆结晶分异过程中形成的。对矿物结晶顺序、石英和钠长石的生长速率以及固相线温度的研究表明 ,富锂、氟、钠的花岗质残余岩浆完全具备形成雪球结构的条件。岩浆熔体中较高的Na2 O/K2 O比值和F、H2 O含量在雪球结构的形成过程中起着重要的作用 :F的高含量使岩浆固相线温度降低 ,岩浆得以充分分异演化 ,形成接近端员组分的钾长石和钠长石 ;Na2 O/K2 O比值较大使钠长石首先结晶 ;较高的F和H2 O含量使岩浆粘度降低 ,石英的生长速率相对加快并逐渐包裹钠长石形成雪球结构。

Formation Mechanism of Snowball Texture in Albite Granite

The snowball texture in quartz and K_feldspar phenocrysts is commonly observed in some Li_F enriched and raremetal_bearing granites. The characteristics of its occurrence, the electron microprobe analyses of albite in snowball texture and other evidence indicate its genesis of magma crystallization differentiation. Based on the study of mineral crystallization sequence, solidus temperature and growth rates of quartz and albite, it suggests that Li_F_Na enriched granitic residual magma is qualified for the formation of snowball texture, and that high_ratio of Na 2O/K 2O and contents of F and H 2O in the granitic melt are important to the formation of snowball texture. The increase of fluorine content might result in the drop of solidus temperature, thereby magma might be sufficiently differentiated, and K_feldspar and albite of near end_member compositions are formed. The high_ratio of Na 2O/K 2O leads albite to be crystallized firstly. The higher F and H 2O contents might decrease viscosity of magma and increase the growth rate of quartz. Consequently, the albite might be wrapped by quartz phenocrysts and finally the snowball texture formed.