摘要
大别山北缘中生代火山 -侵入岩的 14个全岩铅同位素组成表现为高的非放射性成因2 0 4 Pb ,较低的2 0 6 Pb/ 2 0 4 Pb比值和低且相对一致的2 0 7Pb/ 2 0 4 Pb比值 ,属于明显的低U/Pb体系 ,并暗示它们是由具低 μ值的源岩衍生的。同时其2 0 8Pb/ 2 0 4 Pb比值变化较大 ,显示源区具有不均一的Th/Pb比值。与区域铅同位素组成的比较研究表明 ,它们在很大程度上与大别 -胶南亚省中大别地块中生代花岗岩长石铅同位素组成相似 ,总体上具有华北板块的铅同位素特征。从岩石系列的角度来看 ,橄榄安粗岩系列岩石具有最高的2 0 4 Pb含量和最低的铅同位素比值 ,碱性系列岩石具有最低的2 0 4 Pb含量和最高的铅同位素比值 ,而高钾钙碱性系列岩石则介于两者之间 ,表明不同系列岩石的源区在形成时代和性质 (或成熟度 )上具有较明显的区别。河南商城的玄武安山岩具有低的2 0 4 Pb含量和偏高的2 0 8Pb/ 2 0 4 Pb比值 ,说明其源区具有明显富Th的特征。
This paper presents 14 whole rock lead-lead isotopic compositions, contents of U, Th and Pb and hence the calibrated Pb isotopic compositions of Mesozoic volcano-intrusive rocks from the northern margin of Dabieshan Mt These rocks have relatively high 204 Pb (1 371-1 439%, averaging 1 411%), low 206 Pb/ 204 Pb ratios (15 975-17 363, averaging 16 713), slightly varied 207 Pb/ 204 Pb ratios (15 305-15 537, averaging 15 422) and varied 208 Pb/ 204 Pb ratios (36 856-38 816, averaging 37 500) It suggests that these rocks were derived from a source of low μ values which was mainly related to the basement metamorphic rocks with obvious U loss during intermediate to high grade metamorphism Comparative studies with published regional lead isotopic data show that they are to a great extent similar to the feldspar-lead isotopic compositions of the Mesozoic granitoids from Dabie Block within Dabie-Jiaonan subprovince and generally have the lead isotopic characteristics of North China plate In view of rock series of the research region, we found that, except basaltic andesite from Shangcheng, the shoshonitic series have the highest content of 204 Pb and the lowest ratios of Pb isotopes ( 204 Pb=1 438-1 439, 206 Pb/ 204 Pb=15 975-16 010, 207 Pb/ 204 Pb=15 349-15 442 and 208 Pb/ 204 Pb=36 873-36 931); the alkaline series have the lowest content of 204 Pb and the highest ratios of Pb isotopes ( 204 Pb=1 385-1 387, 206 Pb/ 204 Pb=17 308-17 363, 207 Pb/ 204 Pb=15 494-15 537 and 208 Pb/ 204 Pb=37 984-38 088); while the high potassium calc-alkaline series have the intermediate content and ratios ( 204 Pb=1 415-1 427, 206 Pb/ 204 Pb=16 385-16 621, 207 Pb/ 204 Pb=15 305-15 435 and 208 Pb/ 204 Pb=36 965-37 487) These indicate that the sources of these three different rock series were quite different in forming age and nature Compared with those of the other samples, the basaltic andesite samples from Shangcheng, Henan Province were of abnormal lead isotopic compositions ( 204 Pb=1 371, 206 Pb/ 204 Pb=17 307-17 315, 207 Pb/ 204 Pb=15 405-15 419 and 208 Pb/ 204 Pb=38 738-38 816). It can be inferred that the source was of distinct Th-enrichment character
出处
《高校地质学报》
CAS
CSCD
1999年第4期384-389,共6页
Geological Journal of China Universities
基金
原地质矿产部地质调查局地调专项!(地科专 96 3 2 )
教育部博士点基金资助项目
关键词
铅同位素
火山-侵入岩
中生代
大别山北缘
lead isotope, volcano-intrusive rocks, Mesozoic, northern margin of Dabieshan Mt.
