湘南多金属矿集区燕山期成矿花岗岩的稀土地球化学特征和成岩成矿作用探讨

REE geochemistry and petrogenesis and mineralization of the Yanshanian mineralized granites in the southern Hunan polymetallic deposit concentration region

湘南燕山期成矿花岗岩可划分为3种类型。3种类型成矿花岗岩具有不同的稀土地球化学特征、配分型式和成分变异特征,反映出成矿花岗岩的成岩成矿作用有明显的差别。①MC型花岗岩的∑REE最低,平均为225×10-6;La/Yb比值平均为17、LREE/HREE比值平均为5.4和δEu为1.67,都为最高,稀土配分曲线呈右倾斜的近直线。C型花岗岩的ΣREE最高,平均为353×10-6;La/Yb比值平均为4.3,δEu平均为0.14,LREE/HREE比值平均为1.5,都为最小,稀土配分曲线呈海鸥型。CM型花岗岩总体上介于上述两类花岗岩之间,∑REE变化较大,早期次单元配分曲线呈右倾斜的近直线,晚期次单元配分曲线呈海鸥型。②3种类型成矿花岗岩从MC型-CM型-C型花岗岩以及从同一类型花岗岩的早期次单元至晚期次单元,随岩石酸性程度增高,稀土总量和稀土元素分量总体增高,但轻稀土相对亏损,重稀土相对富集,δEu、La/Yb比值、LREE/HREE比值与SiO2的含量存在较明显的负消长演化关系,重稀土元素则随岩浆演化具逆向演化分异性。③CM型花岗岩晚期次单元与C型花岗岩的稀土元素分馏明显,配分曲线铕谷深,显示经历了较强的分离结晶作用;并导致重稀土元素相对富集,常与Sn、W、Nb、Pb富集成矿。④成矿花岗岩显示富含F、C1等挥发组分的壳幔源熔体混合型花岗岩类的特点,其形成与壳幔岩浆混合作用有关,而CM型花岗岩晚期次的花岗岩类和C型花岗岩类的岩浆演化可能还存在分离结晶作用。

Yanshanian mineralized granites in southern Hunan may be divided into three types, which have different REE geochemical features, distribution patterns and characteristics of compositional variations, showing distinct differences in petrogenesis and mineralization. （1） The total REE of MC-type granites is lowest, 225×10^-6 on average; the average La/Yb ratio is 17, the average LREE/HREE ratio is 5.4, and δEu is 1.67, all of which are highest. Chondrite-normalized REE patterns are nearly straight lines inclined to the right. The total REE of C-type granites is highest, 353×10^-6 on average; the average La/Yb ratio is 4.3, the average LREE/HREE ratio is 1.5, and δEu is 1.67, all of which are lowest. The REE distribution patterns are seagull wing-shaped. CM-type granites are intermediate between MC- and CM-type granites. The distribution patterns of the early-stage CM- type granites incline to the right, while those of the late-stage ones are seagull wing-shaped. （2）With increasing acidity, the ∑ REE and REE concentrations increase from MC-type through CM-type to C-type mineralized granites and from the east to late stage of the same type of granite. However, LREE show somewhat depleted shghtly and HREE enriched; the δ Eu, La/Yb and LREE/HREE show significantly antithetic relationships with SiO2;and HREE display a reversed trend of evolution and differentiation with magma evolution. （3）The REE of the late-stage CM-type and C-type granites are remarkably differentiated. The deep Eu troughs of the REE patterns indicate a significant fractional crystallization of CM- and C-type granitic magma, which caused HREE to be enriched and Sn, W, Nb and Pb to be concentrated to form ore deposits. （4）Mineralized granites show features of crust- and mantle-derived melt mixing granites rich in volatiles such as F and Cl, and their formation was related to crust-mantle magma mixing; whereas the magmatic evolution of the late-stage CM-type granites and C-type granites might also involve fractional crystallization.