华南四种成因类型花岗岩类岩石化学特征对比

PETROCHEMICAL FEATURES OF FOUR GENETIC TYPES OF GRANITOIDS IN SOUTHEAST CHINA

华南花岗岩分为如下四种成因类型。1.幔源型花岗岩,与晚元古代蛇绿岩套共生,以花岗闪长岩为主,ε_(Sr)通常为负值(-29.8,N=11)。2.改造型花岗岩,以普通花岗岩为主,ε_(Sr)大于120(平均216,N=64)。3.同熔型花岗岩,以二长花岗岩为主,具中等ε(Sr),值(平均为46.9,N=42)。4.A型花岗岩,可进一步分为魁岐亚型和黄梅尖亚型,前者以富硅、富碱的钾长花岗岩为主,后者以石英正长岩为主,其ε_(Sr)值与同熔型的相似。文中给出四种不同成因花岗岩的平均化学成分并讨论其相互区分的某些岩石化学准则。

Based on the studies of Xu Keqin et al. (1984), Wang Dezi et al.(1983,1984), Tu Guangchi et al.(1984) and recent works of present authors, the granitoids exposed in Southeast China can be divided into four genetic types. Some important features of these types are generalized here as follows: 1. Mantle-derived type granitoids are closely associated with the ophiolite suite of Late Proterozoic age in time and space. Most rocks are of tonalite and granodiorite composition and rich in soda(Na_2O=3.21％, N=24), with a very high Na/K ratio, averaging 2.85. They have high abundances of Cr(34-216ppm) and Ni(26-48ppm), and contain chromite as distrinctive accessory mineral. As products of derivation from the mantle, they possess negative ε_(Sr)-Values, varying from-10.9 to-68.9 (-29.8 in average, N=11). 2. Transformation type granites(correlating to the S-type) are widely distributed inland the Southeast China continent. They are mostly more acidic(average SiO_2=72.8％, N=300), peralumineus(average ANKC=1.265) and monzogranites and common granites in composition. The mafic constituent is essentially Fe-biotite antl siderophyllite, with MF value<0.38. Most rocks of this type are rich in potassium,with an average Na/K ratio of 0.98. They have high ε_(Sr), values ranging 102.2-514.7 and averaging 226.7, which evidently indicates that they have been derived from the sedimentary source rocks of the earth crust. 3. Syntexis type granitoids(correlating to the Ⅰ-type) are usually found along the East China coastal zone and some deep fault belts. Compositionally they are granodiorites and monzogranites, usually metaluminous(ANKC=0.95, N=182), with hornblende and Mg-biotite (MF>0.38) as their main mafic constituents. Most rocks contain less silica(average SiO_2=64.85％) and meresoda (average Na/K=1.41). The ε_(Sr), values fall in a range between those for the first two types of granitoicls, varying from 11.5 to 111.5 and averaging 46.9 (N=42). It is suggested that their material source could be a mixture of mantle and crustal components. 4. A-type granitoids can be divided into two subtypes: the Kuiqi subtype and the Huangmeitian subtype. The former one is more silliceous alkali-feldspar granite(average SiO_2=76.78％), and the latter one is less siliceous quartz syenite(average SiO_2=68.07, N=40). The A-type granitoids, as a whole, are fairly rich in alkalies(average 9.87％, N=68) and characterized by Na>K, with an average Na/K ratio of 1.41. Among the fo r different types granitoids, A-type granitoids have the highest(Na+K)/Al values(0.96 in average). They may contain small amount of arfvedsonite and aegirine as their distinctive alkali mafic constituents. Compared with syntexis type, the A-type granitoids usually have a higher ^(87)Rb/^(86)Sr ratio(2.25-73.1), but their S_(Sr) values are close to each other. Therefore, the A-type granitoids are considered to be mixed products of mantle and crustal source materials.