摘要：
以往将位于湘南、桂东北的都庞岭花岗岩基分为西体、中体和东体三部分.野外观察和岩相学研究表明,都庞岭中体和东体主要由黑云母正长花岗岩、黑云母二长花岗岩和二云母二长花岗岩组成,岩石具斑状结构,部分钾长石斑晶呈椭球状至球状,具斜长石环边,构成环斑结构.采用锆石SHRIMP U-Pb法获得都庞岭中体和东体中环斑花岗岩的侵位年龄分别为226.6±6.9 Ma和209.7±3.1 Ma,均属于晚三叠世,相当于印支晚期.都庞岭环斑花岗岩富硅、碱,贫钛、磷、镁和钙,其Rb、Cs、Th、U、REE、Pb、Y含量和Rb/Sr、Rb/Ba比值较高,而Sr、Ba含量和Zr/Hf比值(8.16～25.01)较低,具强烈的Eu负异常(δEu=0.02～0.13),10000×Ga/Al比值(2.64～4.38,平均3.15)高,显示A型花岗岩的地球化学特征.与华南印支早期S型花岗岩相比,都庞岭环斑花岗岩的εNd(t)值(-8.0～-8.3)明显偏高(前者低于-10),而tDM2值(1624～1645 Ma)则明显偏低(前者＞1800 Ma),表明它们可能直接源于地壳物质的部分熔融,但成岩过程中有地幔物质的参与.都庞岭环斑花岗岩的发现及其时代的确定,揭示了晚三叠世华南东部处于大陆裂解或造山后伸展的构造环境.结合华南东部沉积/岩石大地构造分析,认为华南早中生代构造体制的转换发生在中、晚三叠世,而非前人所认为的发生在中、晚侏罗世;同时,环斑花岗岩的出现,指示了华南中生代大规模成矿作用的来临,晚三叠世是华南中生代大规模成矿的第一个高峰期.%The Dupangling granitic batholith comprises three separate intrusive units,namely the eastern pluton,the middle pluton and the western pluton.Detailed field observations and petrographic studies show that the middle and eastern plutons mainly consist of biotite syenogranite,biotite monzogranite,and two-mica monzogranite with porphyritic texture.Part of the ellipsoidal to spherical K-feldspar phenocrysts are enclosed by the plagioclase rims (rapakivi texture).Zircon SHRIMP U-Pb dating for rapakivi granite of the middle and eastern plutons yielded ages of 226.6±6.9 Ma and 209.7±3.1 Ma,respectively,indicating that both granite plutons were emplaced during the Late Triassic (Indosinian).The Dupangling rapakivi granites are characterized by high silicon and alkaline but depleted in titanium,phosphorus,calcium and magnesium.They have high contents of Rb,Cs,Th,U,REE,Pb,Y,high Rb/Sr and Rb/Ba ratios,low contents of Sr,Ba,low Zr/Hf ratio (8.16-25.01),strongly negative Eu anomalies (δEu=0.02-0.13) and high 10000×Ga/A1 ratio (2.64-4.38,mean 3.15),showing an affinity to A-type granite.In comparison with the early Indosinian S-type granites (εNd(t)＜--10 and tDM2 ＞1800 Ma) in South China,the Dupangling rapakivi granites have higher εNd(t) (--8.0---8.3) and lower tDM2 (1624--1645 Ma).These demonstrate that the granites originated from partial melting of the crustal material and have undergone insignificant mantle material mingling.The discovery of this A-type granite (210-226 Ma) reveals that the eastern South China was involved in a post-collision extension or continental rift setting during the Late Triassic.In combination with sedimentary/tectonic information,we propose that the change of tectonic system during the Early Mesozoic in South China took place in the Middle-Late Triassic rather than the Middle-Late Jurassic proposed previously.Moreover,this study also suggests that the A-type granites may be an indication of the forthcoming Mesozoic large-scale mineralization in South China,first peak of which was the Late Triassic.
