湘东北地区连云山花岗岩的成因及地球动力学暗示

Petrogenesis of the Lianyunshan granites in northeastern Hunan Province, South China, and its geodynamic implications

江南古陆是华南地区一个重要的花岗岩和金及金多金属矿床集中区。位于该区中段的连云山晚中生代花岗岩具有高的SiO_2(69.41%～75.14%)和Al_2O_3(13.61%～17.46%)、相对偏低的铁镁质(0.92%～3.14%)以及变化范围较大的Na_2O(2.04%～3.83%)、k_2O(1.94%～4.94%)和CaO(0.82%～3.24%)等氧化物含量;在微量元素和稀土元素组成上,Y(5.2×10^(-6)～18.0×10^(-6))和Yb(大多数在0.29×10^(-6)～0.70×10^(-6))丰度普遍偏低、而Sr/Y(多数在55与82之间)和(La/Yb)_N(大多数在31与111之间)比值较高,且普遍表现LREE强烈富集的REE配分特征。结合Sr(^(87)Sr/^(86)Sr_i=0.71008～0.73852)-Nd(ε_(Nd)(t)=-9.95～-12.37)-Pb(^(206)Pb/^(204)Pb_i=17.972～19.959)同位素组成,以及野外地质和岩相学特征,连云山岩体为典型的强过铝质花岗岩,并显示埃达克质岩地球化学亲和性,是玄武质岩浆底侵作用下由加厚的下地壳部分熔融产生的岩浆经同化混染和分异结晶而形成,其源岩可能主要为变杂砂岩、长英质片麻岩和变英云闪长岩。结合区域构造演化,我们推测晚中生代连云山花岗岩是三叠纪时期华北板块和扬子板块碰撞导致地壳加厚及随后的太平洋板块向华南大陆俯冲导致华南大陆伸展、减薄的联合作用构造背景下,主要由古元古代连云山群(?)物质部分融熔而产生。连云山花岗岩的成因及地球动力学背景的研究将有助于深刻揭示湘东北地区金和金多金属矿产形成的地球动力学机制。

The Jiangnan Oldland, an important region in South China, is characterized by multistage granitic magmatisms and large-scale gold and gold-polymetallic mineralization. The Lianyunshan Late Mesozoic granites, which are sited in central section of the Oldland, contain not only high contents of SiO(2) (69.41% similar to 75.14%) and Al(2)O(3) (13.61% similar to 17.46%), but also relatively low mafic contents (0.92% similar to 3.14%) and broad ranges of Na(2)O from 2.04% to 3.83%, K(2)O from 1.94% to 4.94%, and CaO from 0.82% to 3.24%, etc. These rocks, which mainly present strongly LREE-enriched REE patterns, also show low concentrations of Y (5.2 x 10(-6) similar to 18.0 x 10(-6)) and Yb (mostly between 0.29 x 10(-6) and 0.70 x 10(-6)) but high ratios of Sr/Y (largely between 55 and 82) and (La/Yb)(N) (mostly between 31 and 111). In concert with their relatively high Sr ((87)Sr/(86)Sr(i) = 0.71008 similar to 0.73852), low Nd (epsilon(Nd)(t) = -9.95 similar to -12.37) and radioactive Pb ((206)Pb/(204)Pb(i) = 17.972 similar to 19.959), as well as their geological and petrographic data, the Lianyushan granites can be ascribed to strongly permaluminous type with geochemical affinities to typical adakitic rocks. We infer that the source rocks for our studied granites mainly are composed of metagreywackes, felsic gneisses and meta-tonalites, and that the parental magma to the granites which resulted from partial melting of over-thickened lower continental crust due to basaltic underplating had undergone assimilation and fractional crystallization. Considering the regionally tectonic development of South China, we further postulate that the Lianyunshan granites are majorly from partial melting of the Paleoproterozoic 'Lianyunshan Group (?)' due to a common result of over-thickened crust by the Triassic collision of the Yangtze with the North China Blocks, and the subsequent extension and lithospheric thinning of the South China continent owing to subduction of the Pacific plate under the South China. Detailed study on the petrogenesis and geodynamic background of the Lianyunshan granites is useful to open out the geodynamic mechanism for Au and Au-polymetallic mineralization in northeastern Hunan Province, China.