Geochemical and Sm-Nd isotopic study of amphibolites in the Cathaysia Block, southeastern China: evidence for an extremely depleted mantle in the Paleoproterozoic

Geochemical and Sm-Nd isotopic results are reported for late Paleoproterozoic mafic amphibolites from SW Zhejiang and NW Fujian, parts of the Cathaysia Block of SE China. Two suites of contemporaneous amphibolites are distinct in their geochemical characteristics. Group 1 samples, from NW Fujian, have chemical compositions of transitional and alkali basalts, show LREE-enriched patterns and plot mainly in the held of within-plate basalt on a number of trace element discrimination diagrams. Group 2 rocks, from SW Zhejiang, have tholeiitic compositions and are characterized by flat to LREE-depleted patterns and fall, into the MORB held. All the amphibolite samples have high epsilon Nd(T) values of +5.6 to +8.5 (T = 1766 +/- 19 Ma). A positive correlation between epsilon Nd(T) and Nb/Th suggests possible mixing of a mantle-derived magma and a crustal component, with the least-contaminated samples having very high epsilon Nd(T) values (+8 similar to +8.5) and Nb/Th ratios of similar to 13. The geochemical and isotopic characters and close temporal relationship of these two suites of amphibolites suggest that their magmatic precursors were likely formed in an environment similar to an ensialic rift developing into a proto-oceanic basin (e.g. the Gulf of Tadjoura). The exceptionally high epsilon Nd(T) values of up to +8.5 for some of the amphibolites suggest the presence of a time-integrated extremely depleted mantle source beneath Cathaysia during the Paleoproterozic. (C) 2000 Elsevier Science B.V. All rights reserved.