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 Paleoproterozo ic mafic amphibolites from SW Zhejiang and NW Fujian, parts of the Cathaysi a Block of SE China. Two suites of contemporaneous amphibolites are distinc t in their geochemical characteristics. Group 1 samples, from NW Fujian, ha ve chemical compositions of transitional and alkali basalts, show LREE-enri ched patterns and plot mainly in the held of within-plate basalt on a numbe r of trace element discrimination diagrams. Group 2 rocks, from SW Zhejiang , have tholeiitic compositions and are characterized by flat to LREE-deplet ed 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 m antle-derived magma and a crustal component, with the least-contaminated sa mples 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 thei r magmatic precursors were likely formed in an environment similar to an en sialic rift developing into a proto-oceanic basin (e.g. the Gulf of Tadjour a). The exceptionally high epsilon Nd(T) values of up to +8.5 for some of t he amphibolites suggest the presence of a time-integrated extremely deplete d mantle source beneath Cathaysia during the Paleoproterozic. (C) 2000 Else vier Science B.V. All rights reserved.