摘要
阿尔泰造山带喀纳斯地区出露一套钾质富镁英安岩,锆石U-Pb定年显示其形成于早泥盆世(411±4 Ma)。该套英安岩富SiO_2(64.71～67.77 wt%),MgO(3.28～3.53 wt%)和K_2O(2.12～2.75 wt%),贫Na_2O(1.70～1.85wt%),K_2O/Na_2O比值介于(1.20～1.53),具较高的Al_2O_3(14.12～15.33 wt%)和较低的CaO(1.12～1.28 wt%),ASI指数>1.8,显示过铝质钙碱性岩浆系列特征,明显不同于初始的高镁安山质岩浆。样品具有中等的Cr(134×10^(-6)～153×10^(-6))、Co(17.47×10^(-6)～18.65×10^(-6))、Ni(71.64×10^(-6)～80.76×10^(-6))含量,富集大离子亲石元素Rb、K和高场强元素U、Th、Pb及LREE,发育Sr、Ba及"TNT"(Ta、Nb、Ti)负异常,显示俯冲带岩浆岩地球化学特征。其高的Mg~#(51.71～52.47)和低的ε_(Nd)(t)值(-4.6～-4.2)说明该套火山岩可能来自遭受强烈交代作用的地幔楔部分熔融。样品具有较高的Ba/La、Pb/La比值和较低的Th/Yb、Nb/Y比值,显示地幔交代作用以俯冲沉积物熔体的交代作用为主。以上地球化学特征说明,喀纳斯钾质富镁英安岩可能主要是受俯冲沉积物熔体交代的上覆地幔橄榄岩部分熔融的产物,阿尔泰泥盆纪处于俯冲作用相关的活动大陆边缘构造环境。此外,该套火山岩的形成时代制约了其下覆的哈巴河群沉积上限,显示哈巴河群沉积于早泥盆世之前。
Potassic magnesian-rich dacites occur to the south of the Kanasi, Altai, northwestern Chinese. U-Pb dating of zircons from these rocks yields an early Devonian age of 411±4 Ma. The dacites are enriched in SiO2 （64.71 - 67.77 wt% ）, MgO （3.28 - 3.53 wt% ） and K20 （2. 12-2. 75 wt% ）, and depleted in Na20（1. 70- 1.85 wt% ）. The rocks have low CaO content（1. 12- 1.28 wt% ）,high Al2O3 content（ 14. 12 - 15.33 wt% ）and K20/Na20 ratios （ 1.2 - 1.6） , and ASI indexes larger than 1.8, indicating features of peraluminous potassic calc-alkali igneous rocks, which are clearly different to primitive High-Mg andesites. The dacites possess moderate Cr （ 134×10^-6 - 153 ×10^-6 ） , Co （ 17.5×10^-6- 18. 7 ×10^-6） , and Ni （72×10^-6 -81×10^-6） contents. These rocks are enriched in LREEs, large ion lithophile elements （ e. g. Rb, K） and high field strength element （ e. g. U ,Th, Pb） with moderate Sr, B a, and ＂TNT＂ （ Ta, Nb, Ti） negative anomalies, showing geochemical characteristics of igneous rocks in subduction zone. Their high Mg# （ 51.7 - 52.5 ） and low εNd （t） ratios （ -4.6 - -4.2 ） imply partial melting of metasomatic peridotites in the mantle wedge. The relative high Ba/La, Pb/La and low Th/Yb, Nb/Y ratios suggest that the metasomatism of mantle peridotites was predominated by the melts of subducted sediments. Therefore,the above geochemistry indicates that the potassic magnesian-rich dacites were originated from the partial melting of metasomatic mantle peridotites mainly altered by the melts of subducted sediments, showing an active continental margin in the Devonian. In addition, the timing of the potassic magnesian-rich dacites provides a good constrain on the minimum depositional age of the underlying Habahe group and suggests the sediments of the group deposited before the early Devonian.
出处
《地质科学》
CAS
CSCD
北大核心
2010年第1期12-29,共18页
Chinese Journal of Geology(Scientia Geologica Sinica)
基金
国家"973"项目(编号:2007CB411308)
自然科学基金项目(编号:40772130
40803009)
中国科学院知识创新工程领域前沿项目(编号:GIGCX-07-03)资助
