榴辉岩中石榴子石退变质环带对高压变质矿物之间Nd，O同位素平衡的影响

榴辉岩中石榴子石退变质环带对高压变质矿物之间Nd，O同位素平衡的影响

李曙光1, 刘贻灿1 2, 徐树桐2, 郑永飞1, 龚冰1

1. 中国科学技术大学地球和空间科学学院，合肥 230026; 2. 安徽省地质科学研究所，合肥 230001

收稿日期:2003-06-20 修回日期:2003-06-20 出版日期:2003-06-20 发布日期:2003-06-20

Effects of Retrograde-Zoning of Garnet in Eclogite on Nd and O Isotopic Equilibrium between High-Pressure Minerals

LI Shu-guang1, LlU Yi-can1 2, XU Shu-tong2, ZHENG Yong-fei1, GONG Bin1

1. School of Earth and Space Sciences, University of Science and Technology of China, Hefei, Anhui 230001, China; 2. Anhui Institute of Geology, Hefei, Anhui 230001, China

Received:2003-06-20 Revised:2003-06-20 Online:2003-06-20 Published:2003-06-20

摘要/Abstract

摘要： 产于大别山北部饶钹寨面理化橄榄岩中的强变形榴辉岩的石榴子石具有退变质环带，表现为边部CaO含量下降和MnO含量升高，指示Ca和Mn在石榴子石具有较快的扩散速率。该榴辉岩中石榴子石+绿辉石连线给出的Sm-Nd年龄为187±5Ma，显著小于大别山南部超高压榴辉岩及北部榴辉岩的Sm—Nd年龄(分别为221±5—228±3Ma和210±6—214±6Ma)。这一偏低的Sm—Nd年龄可能是由于石榴子石退变质环带引起^143Nd／^144Nd降低所造成的。对该样品石榴子石、绿辉石和石英的δ^18O测定表明，这些矿物之间的实测分馏值小于它们在500-900℃条件下的平衡分馏值，从而证明它们之间存在氧同位素不平衡。榴辉岩中石榴子石的退变质环带也可造成绿辉石与石榴子石之间显著的Nd同位素和氧同位素不平衡现象。

Abstract: Compositional zoning of garnet was observed in a strongly deformed eclogite associated with a foliated peridotite at Raobazhai in North Dabie． CaO concentration of garnet decreases from core to rim ，whereas its MnO concentration increases，suggesting a retrograde origin of such Ca-Mn zoning and a fast rate of Ca and Mn diffusion in garnet．The tie line of garnet + omphacite from this eclogite gave a Sm-Nd age of 187±5 Ma，which is significantly less than Sm-Nd isoehron ages of 221±5 Ma to 228±3 Ma and 210±6 Ma to 214±6 Ma for ultrahigh-pressure eclogites in South Dabie and North Dabie，respectively．This younger Sm-Nd age is interpreted to be resulted from a decrease in 143Nd/144Nd ratio of the retrograde rim in garnet．Oxygen isotope an alysis of omphacite and garnet shows less fractionation values than equilibrium fractionation values between them at temperature of 500 to 900℃ ．suggesting a oxygen isotope disequilibrium． This study demonstrates that the retrograde zoning of garnet in eclogite could result in significant disequilibrium of Nd and O isotopes between omphacite and garnet．

李曙光, 刘贻灿 , 徐树桐, 郑永飞, 龚冰. 榴辉岩中石榴子石退变质环带对高压变质矿物之间Nd，O同位素平衡的影响[J]. J4.

LI Shu-guang1, LlU Yi-can1 2, XU Shu-tong2, ZHENG Yong-fei1, GONG Bin1. Effects of Retrograde-Zoning of Garnet in Eclogite on Nd and O Isotopic Equilibrium between High-Pressure Minerals[J]. J4.