摘要
The supracrustal rocks of the Wuyang metamorphic terrain are divided into the Zhao anzhuang,Tieshanmiao and Yangshuwan Formations.These three Formations were dated at 3000-2550Ma,2550-2300Ma and 2300-2200Ma,respectively.∑REE and La/Yb)n of the Zhao anzhuang Formation volcanic rocks are obviously higher than those of the Tiesanmiao Formation equivalents,suggesting a sedimentary gap(2550 Ma boundary)between these two formations,The Zhao'anzhuang Formation is older than the Tieshanmiao Formation.The sediments of these two Formations show no obvious differences in REE and are generally characterized by low ∑REE and positive Eu anomalies.On the contrary,the sediments of the Yangshuwan Formation are characterized by high ∑REE and negative Eu anomalies.Detailed discussions demonstrate that the Yangshuwan Formation was deposited in an oxidizing environment whereas the other two formations were formed in a reducing environment.At the end of the evolution of the Tieshanmiao Formation about 2300 Ma ago,the sedimentary environment was transformed from reducing to oxidizing .On the basis of the SHAB (soft/hard acid and base)theory,an oxidation-reduction model for sedimentary REE evolution has been established .It is proposed that the mantle tends to become gradually depleted in REE.especially in LREE,and the indices ∑REE and La/Yb) n of mantle-dervived volcanic rocks also tend to become lower and lower.
The supracrustal rocks of the Wuyang metamorphic terrain are divided into the Zhao anzhuang, Tieshanmiao and Yangshuwan Formations.These three Formations were dated at 3000-2550 Ma, 2550-2300 Ma and 2300-2200 Ma,respectively.ΣREE and La/Yb)_n of the Zhao'anzhuang For- mation volcanic rocks are obviously higher than those of the Tieshanmiao Formation equivalents,sug- gesting a sedimentary gap(2550 Ma boundary)between these two formations.The Zhao'anzhuang Formation is older than the Tieshanmiao Formation.The sediments of these two Formations show no obvious differences in REE and are generally characterized by low ΣREE and positive Eu anomalies. On the contrary,the sediments of the Yangshuwan Formation are characterized by high ΣREE and negative Eu anomalies.Detailed discussions demonstrate that the Yangshuwan Formation was depos- ited in an oxidizing environment whereas the other two formations were formed in a reducing environ- ment.At the end of the evolution of the Tieshanmiao Formation about 2300 Ma ago,the sedimentary environment was transformed from reducing to oxidizing.On the basis of the SHAB(soft/hard acid and base)theory,an oxidation-reduction model for sedimentary REE evolu- tion has been established.It is proposed that the mantle tends to become gradually depleted in REE,especially in LREE,and the indices ΣREE and La/Yb)_n of mantle-derived volcanic rocks al- so tend to become lower and lower.
