湖南金鸡岭铝质A型花岗岩的厘定及构造环境分析

Ascertainment of the Jinjiling aluminous A-type granite, Hunan Province and its tectonic settings

湘南九嶷山中生代金鸡岭复式花岗岩体出露面积约390km2,由螃蟹木和金鸡岭岩体组成。该岩体以富Si(SiO275.00%～76.86%)、富碱(ALK6.60%～8.88%)、贫Mg(MgO0.01%～0.19%)和Ca(CaO0.30%～0.93%)以及高FeO/MgO比值(7～86,平均39)为特征。其K2O/Na2O>1、NK/A=0.70～0.92(平均0.86),A/CNK=1.00～1.20,属偏铝—过铝质钙碱—弱碱性岩石。在微量元素和同位素组成上,岩石富Ga、Th、Y、Zr、U和Nb等高场强元素及亏损Ni、Cr、Eu、Ti、V、P和Sr等元素。10000×Ga/Al比值(2.9～4.9,平均3.3)较高,ISr值(0.71301～2.95741)变化大。在Zr、Nb、Ce和Y对Ga/Al以及FeO/MgO和(Na2O+K2O)/CaO对(Zr+Nb+Ce+Y)等A型花岗岩多种判别图上,投影点主要落在A型花岗岩区,而与高分异的I、S型花岗岩明显不同。上述特征表明,金鸡岭复式花岗岩与国内外铝质A型花岗岩(如广东南昆山、江苏苏州和澳大利亚Lachlan褶皱带铝质A型花岗岩)十分相似。与一般A型花岗岩相比,金鸡岭复式花岗岩的εNd(t)(-6.7～-7.5)较低,Nd模式年龄(1486～1556Ma)小于区域上变质基底和中国东南部中生代花岗岩类的平均Nd模式年龄,表明其主要来源于地壳物质的熔融,但可能有少量新生地幔物质加入。区域岩石地球化学和岩石组合特点显示,研究区铝质A型花岗岩形成于大陆边缘裂谷?

The Mesozoic Jinjiling composite granite, composed of the Pangxiemu and the Jinjiling plutons, occurs in the Jiuyishan region of southern Hunan Province, with an outcrop area of about 390 km2. The granite is of subaluminous to peraluminous and calc-alkaline to weakly alkaline and rich in SiO2 (75.00%～76.86%) and Na2O+K2O (6.60%～8.88%), poor in CaO (0.30%～0.93%) and MgO (0.01%～0.19%), high in FeO/ MgO (7.32～86.50, with an average of 39.14). Its K2O/Na2O > 1, NK/A=0.70～0.92 (0.86, on average), A/CNK=1.00～1.20. In trace elements, it is enriched in Th, Y, Zr, U, Nb and Ga with high 10 000×Ga/Al ratios (3.34, on average), and depleted in Ni, Cr, Ti, V, P, Sr and Eu (δEu=0.00～0.20). On Zr, Nb, Ce, Y vs. 10 000×Ga/Al diagrams and FeO/MgO, (Na2O+K2O)/CaO vs. Zr+Nb+Ce+Y diagrams, most of samples are plotted in the position of the A-type granite, which is different from the high fractionated I-type and S-type granites. These characteristics mentioned above show that it belongs to aluminous A-type granite and is not typical S-type granite as is considered previously. Isotopically, the granite has high initial Sr ratios (ISr=0.713 01～2.957 41) and low εNd(t) (-6.7～-7.5) than the typical A-type granite, and its t2DM (1 486～1 556 Ma) is a little lower than the metamorphic basement and the Mesozoic granitoids in the southeastern China, which shows that the Jinjiling composite granite was mainly derived from crustal material, but had a little influence of new mantle-derived materials, and its petrogenesis mechanism needs further investigation. Based on the regional geochemistry and rock assemblage, it is suggested that the Jinjiling composite granite was formed in the continental marginal rift setting.