Nb/Ta variations of mafic volcanics on the Archean-Proterozoic boundary: Implications for the Nb/Ta imbalance

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 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.

作 者： LIU Yongsheng GAO Shan WANG Xuance HU Shenghong WANG Jianqi   作者單位： LIU Yongsheng,GAO Shan(State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074,China;Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China)

WANG Xuance,HU Shenghong(State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074,China)

WANG Jianqi(Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China) 

刊 名： 中國科學D輯（英文版）  SCI 英文刊名： SCIENCE IN CHINA SERIES D(EARTH SCIENCES)  年，卷(期)： 2005 48(8)  分類號： P5  關鍵詞： Precambrian mafic volcanics   Nb/Ta deficit   Archean-Proterozoic boundary  

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