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The Caledonian Adakite-Like Granodiorites in Banshanpu Area, Eastern Hunan Province, South China: Petrogenesis and Geological Significance

XU De-ru,Chen Guang-hao1,Xia Bin1,Li Peng-chun1,2,He Zhuan-li1,2   

  1. 1. Key Lab of Marginal Sea Geology, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640,China; 2. Graduate College of Chinese Academy of Sciences, Beijing 100049, China
  • Received:2006-12-20 Revised:2006-12-20 Online:2006-12-20 Published:2006-12-20

Abstract: The Caledonian granitoid rocks in Banshanpu area, eastern Hunan Province, South China are attributable to granodiorites with the affinities to typical adakites and Archean high Al-TTG suites. These rocks are characterized by high SiO2 (~70%) and Al2O3 (~16.5%), contents, intermediately high ratio of Na2O/K2O (generally >1.0 and up to 1.4) and low but variable Mg# value (37-59). The relatively high Sr (average: 520 ppm) and Ba (1052 - 1502 ppm) concentrations, and extremely low Y (< 12 ppm) and Yb (< 1 ppm) concentrations led to low Rb/Sr ratio (ranging from 0.17 to 0.42) and high Sr/Y ratio (ranging from 34 to 64). The REE distribution patterns show strongly LREE-enriched [(La/Yb)N= ~27-71] with weakly negative Eu-anomaly (Eu/Eu*=0.76 ~ 0.91). Thus source rocks for our studied rocks probably contain garnet-bearing amphibolites and/or transitional garnet-bearing amphibolites to amphibole-bearing ecolgites. The low value of ?Nd(400 Ma) (~-6.7 to -8.4) and high ratio of (87Sr/86Sr)i (0.7078 ~ 0.7140) suggest the Banshanpu granitoid rocks were impossible to have sourced from partial melting of subducted ocean crust or depleted mantle. However, the high ratiogenic Pb [(206Pb/204Pb)i= 18.22~18.78] and the high ratios of Ba/La (~15-28) and Th/Ce (0.29-0.36) imply deep-sea sediments and recycled terrestrial materials had involved into the provenance for our studied rocks. Combined with regional geological data, we suggest that the early Paleozoic continent-arc-continent collision in South China was a possible mechanism responsible for the generation of the Banshanpu adakite-like granodiorites. Underplating of basaltic magma, which had been derived from previously enriched-mantle lithosphere, led to partial melting of the thickened, intermediate-acidic lower crust beneath an arc system. Thus assimilation and fractional crystallization (AFC) processes to produce the magma for our studied rocks were allowed to take place. The confirmation of the adakite-like granodiorites in Banshanpu area implies not only a new crustal growth of South China continent during early Paleozoic, but also a synchronous occurrence of gold and gold-polymetallic mineralization in South China, particularly in southeastern margin of the Yangtze Block.