湘东北早中生代花岗闪长岩地球化学特征及其构造意义

GEOCHEMICAL CHARACTERISTICS OF THE EARLY MESOZOIC GRANODIORITES AND THEIR TECTONIC IMPLICATIONS

湘东北早中生代花岗闪长岩 (约 16 5Ma)SiO2 =6 5 .4 %～ 6 9.6 5 % ,K2 O +Na2 O =6 .0 %～ 7.17% ,K2 O≈Na2 O ,A/NKC =0 .96～ 1.13,为弱过铝质钙碱性岩石。该花岗闪长岩 (La/Yb) CN=12 .75～ 37.5 0 ,(Gd/Yb) CN =1.2 8～ 2 .90 ,Eu/Eu =0 .6 2～ 0 .90 ,以富集LILE和亏损Nb Ta、Ti、P为特征。初始87Sr/ 86Sr =0 .7114 5 8～ 0 .7174 6 1,εNd=- 9.0～ - 12 .3,其同位素组成明显有别于被认为其成因与地幔派生岩浆有关的湘东南高钾花岗闪长质岩石。暗示湘东北早中生代花岗闪长岩可能是在华南早中生代伸展构造背景下中 /下地壳物质深熔作用的产物。湘东北早中生代代表性花岗闪长质岩石不能与湘东南同期花岗闪长质岩石一起构成一条连续的高εNd、低TDM钾质岩石带。

The early Mesozoic granodiorites (ca.165 Ma) in the northeastern Hunan Province have SiO_2=65.4%～(69.65%), K_2O/Na_2O=0.95～1.3 and K_2O+Na_2O>6%, and belong to high-K calc-alkaline series. They are characterized by LREE and LILEs enrichment, and HFSE depletion with slightly negative Eu anomalies. The initial()^(87)Sr/^(86)Sr ratios range from 0.711458 to 0.717461, and εNd values from -9.4 to -12.3, distinct from those of the contemporaneous granodiorites from the southeastern Hunan Province (^(87)Sr/^(86)Sr(i)=0.707962～0.710396, ε_(Nd)(t)=-6.98～(-2.30)). By contrast, such signatures are roughly similar to those of the other neighboring Mesozoic granitic plutons (Eu/Eu~*=0.30～0.70;()^(87)Sr/^(86)Sr>0.710; ε_(Nd)=-12～-16) in South China, which have been interpreted as the remelting products of Precambrian basement. The Proterozoic lower-middle crust is an important contributor to the petrogenesis of these early Mesozoic granodiorites in the northeastern Hunan Province. An intracontinental extension setting is present in the northeastern Hunan Province at that time due to the demand of enough thermal transfer.