Sm-Nd Isotopic System Of The Bixiling Eclogites During Retrogression And Its Effect On Sm-Nd Isochron Dating

Sm-Nd isochron ages of 210-218 Ma,reported in previous studies for coesite-bearing eclogites from the Bixiling massif,located at Central Dabie Zone(CDZ),were obviously younger than 226 Ma,which is the generally accepted peak metamorphic time of the CDZ.The cause of the younger ages is an important scientific issue to be well studied.In this thesis,a comprehensive study of the petrography,mineral chemistry,garnet water content,Fe,Mg and Sm-Nd isotopic composition of eclogites from the Bixiling were taken.Petrographic analysis reveals heterogeneity in colour within single fractured garnet grains.There are light-pink garnet(Grt-P)and red garnet(Grt-R)types that possess distinct chemical compositions.Compared to Grt-P,Grt-R has higher FeOT and andradrite contents,but lower Al2O3 and grossular.Grt-P also has lower water contents(15-35 ppm)than those of Grt-R(34-65 ppm),which together with Grt-R being linked to fractures suggests that the colour change is related to fluid alteration.Grt-P is a UHP mineral,and Grt-R is the product of the reaction between Grt-P and fluid during retrogression.Moreover,omphacite in the same sample is partly retrogressed to aegirine-augite with increasing of iron content after retrogression.The increase of Fe,especially Fe3+,in both retrogressed garnet and pyroxene,indicates that the fluid was rich in iron and oxidizing.The element exchanges between Ca2+ and Mg2+ or Fe2+ induced coupled REE content and distribution pattern variations of garnet.Grt-R shows lower Sm and Nd contents,but higher Sm/Nd ratios than Grt-P.Sm-Nd isochrons defined by UHP minerals(Grt-P + Omp + Rt or Grt-P + Cpx+WR)of three eclogite samples yield consistent ages of 226.0±3.8 Ma,225.0±3.9 Ma and 226.2±6.9 Ma,respectively,which are identical to the peak metamorphic age of 226±3 Ma for the CDZ.Retrogressed garnet Grt-R,together with omphacite and rutile,define pseudoisochrons with younger ages of 218.9±5.9-202.8±4.8 Ma,which are geologically meaningless.The increase of Sm/Nd ratio with constant or lower 143Nd/144Nd ratio when Grt-P transformed to Grt-R,was probably the cause of these younger ages.Though the retrogressive fluid exchanged Fe with Grt-P,the product Grt-R has a similar Fe and Mg isotopic compositions with Grt-P.The reason for nearly indentical Fe isotope between Grt-P and Grt-R may be identical Fe isotope between retrogressive fluid and Grt-P or small portion of Fe increase after mineral-fluid interaction.A further study is needed to investigate this issue.The retrogressive assemblages of Grt-R+aegirine-augire without amphibolite + sodic plagioclase or symplectite in the Bixiling eclogite,indicates that the fluid-mineral interaction took place prior to the amphibolite facies retrogression,and closely following the peak metamorphic time.