粤东花岗岩类地球化学特征及成因探讨

粤东花岗岩类的SiO_2、K_2O及ALK值偏高,Al_2O_3、Fe_2_O_3、FeO及MgO偏低,ANKC值、Na/K比及CaO、Na_2O含量介于华南改造型花岗岩和同熔型花岗岩之间;负铕异常明显,轻稀土显著富集,中稀土(Tb—Tm)略有亏损;铅同位素高于地幔铅和下地壳铅同位素组成,略低于华南中生代上地壳铅同位素组成;锶、氧同位素具正相关关系,不相容元素比与其他元素间无相关关系。其岩浆起源于上、下地壳间的过渡带,该过渡带由上地壳物质及侵入其中的幔源物质组成,岩浆上升定位过程中,未发生过明显的分异结晶一同化作用。

STUDIES ON GEOCHEMICAL FEATURES AND GENESIS OF THE GRANITOIDS IN EAST GUANGDONG

The granitoids in East Guangdong district include mainly biotite granite and monzogranite. These rocks contain higher SiO2,K2O,and K2O + Na2O and lower Al2O3,Fe2O3,FeO and MgO than that of Syntexis and Transformation type granitoids in Southeast China. The values of The rocks are more or less similar to those of A - type granitoids defined by Liu ( 1 989 ) and Chapell et al. (1982). The characteristic values,ANKC,Na/K as well as CaO and Na2O contents of the granitoids are between those of Syntexis and Transformation type granitoids. These rocks show the enrichment in LREE(Ce/Y = 2-2. 7), the depletion in Eu(Eu/Eu* = 0. 2-0. 4) and the slight depletion in Tb -Tm compared with Yb and Lu. The Pb isotopic compositions of feldspar from the granitoids show small variations and are slightly lower than those of Mesozoic upper crust-derived granitoids in Southeast China and higher than those of mantle -derived volcanic rocks. The features indicate that the granitoid magmas are the mixture of mantle - and upper crust - derived materials. The relationships between Sr and O2 isotopes as well as between incompatible element ratios and SiO2 imply that the mixing occurred during the melting process of source rocks or before the process. The features that silica and alkali are slightly high,iron and magnesium are low,LREE enrich and Tb-Tm slightly deplete possibly relate to residual hornblende in source.