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

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

一产于大别山北部铙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 2 2 1± 5～ 2 2 8± 3Ma和 2 10± 6～ 2 14± 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14 3 Nd/ 14 4 Nd降低所造成的。对该样品石榴子石、绿辉石和石英的δ18O测定表明 ,这些矿物之间的实测分馏值小于它们在 5 0 0～ 90 0℃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。

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 isochron 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 143 Nd/ 144 Nd ratio of the retrograde rim in garnet. Oxygen isotope analysis 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.