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The 2.5 Ga Guyang greenstone belt is a major lithological unit in the northwestern part of the North China Craton.Komatiites have recently been identified to occur within the lower part of the meta-volcanic sedimentary sequence of the greenstone belt.The biggest komatiite near Guyang has been mapped out,which is 500 m long and 50 m wide lenticular block.Re-Os isotopic compositions of 9 samples were analyzed to date the komatiites.The Os contents are from 0.88 to 2.63 ppb,identical with typical komatiites,and slightly lower than the normal mantle.The 187Os/188Os ratios are from 0.1115 to 0.1197,which are lower enough to exclude the origin of Phanerozoic magma as widely developed in adjacent areas.The calculated Re depletion model ages(TRD) are from 1346 to 2454 Ma,among them the oldest age of 2454 Ma gives the minimum evaluation for the formation age of the Guyang komatiite.Therefore,the komatiites are a part of the Guyang greenstone belt,indicating high degree melting of mantle during～2.5 Ga crustal growth event.
The 2.5 Ga Guyang greenstone belt is a major lithological unit in the northwestern part of the North China Craton. Komatiites have recently been identified to occur within the lower part of the meta-volcanic sedimentary sequence of the greenstone belt. The biggest komatiite near Guyang has was mapped out, which is 500 m long and 50 m wide lenticular block. Re-Os isotopic compositions of 9 samples were analyzed to date the komatiites. The Os contents were from 0.88 to 2.63 ppb, identical with typical komatiites, and slightly lower than than the normal mantle.The 187Os / 188Os ratios are from 0.1115 to 0.1197, which are lower enough to exclude the origin of Phanerozoic magma as widely developed in adjacent areas. The calculated Re depletion model ages (TRD) are from 1346 to 2454 Ma, among them the oldest age of 2454 Ma gives the minimum evaluation for the formation age of the Guyang komatiite.Therefore, the komatiites are a part of the Guyang greenstone belt, indicating high degree melting of mantle during 2.5 Ga crustal growth event.