Contrasting geochemical and Sm-Nd isotopic compositions of Archean metasediments from the Kongling high-grade terrain of the Yangtze craton: Evidencefor cratonic evolution and redistribution of REE during crustal anatexis

Twenty-three clastic metasediments from the Kongling high-grade terrain of the Yangtze craton, South China were analyzed for major, trace and rare ear th elements and Sm-Nd isotopic ratios. Associated dioritic-tonalitic-trondh jemitic (DTT) and ganitic gneisses as well as amphibolites were also analyz ed in order to constrain provenance. The results show that the elastic meta sediments can be classified into 3 distinct groups in terms of mineralogica l, geochemical and Sm-Nd isotopic compositions. Group A is characterized by having no to slight negative Eu anomalies (Eu/Eu* = 0.82-1.07), being high in Cr (191-396 ppm) and Ni (68-137 ppm), and low in Th (3.3-7.8 ppm) and R EE (Sigma REE = 99-156 ppm). These characteristics are similar to those of metasediments from Archean greenstone belts. In addition, the Group A metas ediments have the value of the Chemical Index of Alteration (CIW) close to felsic gneisses. Their Sm-Nd isotopic, REE and trace element compositions c an be interpreted by mixtures of the DTT gneisses and amphibolites. Dating of detrital zircons from 2 Group A samples by SHRIMP reveals a major concor dant age group of 2.87-3.0 Ga, which is identical to the age of the trondhj emitic gneiss. These results strongly suggest that Group A was principally the first-cycle erosion product of the local Kongling DTT gneiss and amphib olite. Moreover, the higher than amphibolite Cr content and slight Eu deple tion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3.0-3.3 Ga in age also played a role . Group B is characterized by the presence of graphite and shows a more evolv ed composition similar to post-Archean shales with a prominent negative Eu anomaly (Eu/Eu* = 0.48 - 0.77) and high CIW. On paired Cr/Th vs La/Co and C o/Th plots, Group B samples conform to a two-end member mixing line of the Kongling granitic gneiss and amphibolite. However, data on Nd model age and CIW suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source. Both Groups A and B exhibit a clear positive correlation between CIW and T- DM and a negative one between CIW and Eu/Eu*. These correlations point to t he crustal evolution of the Yangtze craton towards coupled increasing CIW a nd Eu depletion with decreasing age. This in turn reflects the change of gr anitoid magmatism from local Na-rich dioritic-tonalitic-trondhjemitic rocks to widespread K-feldspar granite. The change led to the intracrustal diffe rentiation, stabilization and growth of the craton. Group C is restite and contains abundant sillimanite and garnet and unusual ly high ilmenite (7-11 vol%), which can be seen to be dehydration melting p roducts of biotite under the microscope. This group shows extremely varied REE distributions from LREE enriched to depleted and from negative to stron g positive (Eu/Eu* = 1.63) Eu anomalies. Compared to Groups A and B, Group C is severely depleted in Na2O, K2O, LREE, Rb and Ba, whereas TiO2, Co, V, Sc and HREE and Y are considerably enriched. This is accompanied by anomalo us high Sm/Nd (0.21-0.28), Sm-147/Nd-144 (0.1361-0.1738) and Nd-143/Nd-144 (0.511589-0.511958) ratios.:T-DM correlates clearly with Sm/Nd ratio and 2 out of 3 samples give significantly older to unrealistic T-DM (3.9-3.9 Ga). The results document-redistribution of REE and an open behavior of the Sm- Nd isotope system during the biotite dehydration melting of metasediments. Copyright (C) 1999 Elsevier Science Ltd.