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The HFSE and REE of the Precambrian mafic volcanics from the North China craton demonstrate obvious A(Archean)-P(Proterozoic) boundary. The Neoarchean mafic vol- canics show weak correlation between HFSE and TiO2. Their superchondritic Nb/Ta ratio (18.8±1.2) could be attributed to partial melting of mantle peridotite in the presence of garnet. Compared with Neoarchean mafic volcanics, the Paleoproterozoic ones have higher HFSE contents and lower Nb/Ta ratio (15.6±2.9). The significantly elevated HFSE and REE contents of Paleoproterozoic mafic volcanics imply metasomatic enrichment of mantle source, in which Ti-rich silicates could be present as suggested by significant positive correlations between TiO2 and HFSE. The global database of Precambrian mafic volcanics shows a similar A-P boundary. 23 Archean mafic volcanic suites yield an average Nb/Ta ratio of 17.8±1.9 higher than or close to the PM value; Proterozoic mafic volcanics from 28 suites yield an average Nb/Ta ratio of 14.7±4.1 close to the bulk continental crust (BCC) value (11―17.5). Thus, we suggest that the Nb/Ta deficit could be mainly formed in post-Archean time. Archean mafic volcanics could be one of the geochemical reservoirs complementing the low Nb/Ta of the post-Archean continental crust and DM.
The HFSE and REE of the Precambrian mafic volcanics from the North China craton demonstrate obvious A (Archean) -P (Proterozoic) boundary. The Neoarchean mafic vol- canics show weak correlation between HFSE and TiO2. Their superchondritic Nb / Ta ratio (18.8 ± 1.2) could be attributed partially melting of mantle peridotite in the presence of garnet. Compared with Neoarchean mafic volcanics, the Paleoproterozoic ones have higher HFSE contents and lower Nb / Ta ratio (15.6 ± 2.9). The significant elevated HFSE and REE contents of Paleoproterozoic mafic volcanics imply metasomatic enrichment of mantle source, in which Ti-rich silicates could be present as suggested by significant positive correlations between TiO2 and HFSE. The global database of Precambrian mafic volcanics shows a similar AP boundary. 23 Archean mafic volcanic suites yield an average Nb / Ta ratio of 17.8 ± 1.9 higher than or close to the PM value; Proterozoic mafic volcanics from 28 suites yield an average Nb / Ta ratio of 14.7 ± 4.1 close to the bulk continental crust (BCC) value (11-17.5). Thus, we suggest that the Nb / Ta deficit could be mainly formed in post-Archean time. Archean mafic volcanics could be one of the geochemical reservoirs complementing the low Nb / Ta of the post-Archean continental crust and DM.