摘要
本文研究了明溪地区尖晶石二辉橄榄岩、尖晶石-石榴石二辉橄榄岩和金云母-石榴石二辉橄榄岩包体的REE及Pb,Sr,Nd同位素。它们的Pb,Nd同位素成分表明该区上地幔在67—82km深度上是,MORB型亏损地幔。标本中放射性Sr同位素成分高于“地幔系列”可能解释为岛弧区俯冲大洋板块物质的加入。包体的sm-Nd、Rb-Sr年龄可能与十亿年前地幔亏损事件、早古生代H_2O,K_2O等地幔交代作用以及新生代的火山作用过程有关。
The REE and Pb, Sr, Nd isotopes in three xenoliths from limburgite and scoria-breccias, including spinel-lherzolite, spinel-garnet-lherzolite and phlogopite-garnet-lherzolite, were analysed. The REE contents of the xenoliths are 1.3 to 3.3 times those of the chondrites with their REE patterns characterized by weak LREE depletion. The 143Nd/144Nd values of whole rocks and minerals range from 0.51306 to 0.51345 with εNd = +8.2-+ 15.8, 206Pb/204Pb < 18.673, and 207Pb/204Pb < 15.574. All this goes to show that the upper mantle in Mingxi at the depth of 67-82km is a depleted mantle of MORB type, with 87Sr/86Sr ratios 0.70237-0.70390. In Nd-Sr diagram the data points of whole rocks are all out of the mantle array, implying that the xenoliths from Mingxi have more radiogenic Sr isotopes than those of the mantle array.The isochron (and reference isochron) ages of xenoliths may be connected with ancient geological events in this area. The Sm-Nd isochron built up by the three data points of whole rocks dated at 1.016±0.019Ga seems to be related with a depletion event resulting from partial melting during the Late Proterozoic. The Rb-Sr reference isochron age of phlogopite-garnet-lherzolite is t = 491± 108Ma, probably due to the collision of two terrenes during the Early Palaeozoic, which resu_ lted in the incorporation of sea water into the upper mantle, followed by H2O metasomatism. The Sm-Nd reference isochron age of spinel-garnet-lherzolite is t= 31.8±9Ma, which is likely to be related with intensive volcanism during the Ce-nozoic.
出处
《地球化学》
CAS
CSCD
北大核心
1992年第2期101-113,共13页
Geochimica
