Sm-Nd isotope study of early Archean rocks, Qianan, Hebei Province, China

A Sm-Nd study of mafic amphibolite which occurs as lensoid masses in migmatitic quartz dioriticto-granitic gneiss was undertaken in order to determine if it has an early Archean age. Six amphibolite samples define an isochron yielding an age of 3.50 ± 0.08 b.y. with an initial ɛ_Nd value of +3.3 ± 0.3.. Three samples of quartz dioritic gneiss lie close to this isochron, and have Sm-Nd model ages of ca. 3.5 b.y. referenced to a "epleted mantle" source. A sample of K-feldspar-rich granitic gneiss gives a younger model age of about 2.9 b.y. The non-zero initial ɛ_Nd value of the mafic amphibolites can be compared with chondritic meteorite samples measured in this laboratory, and is, therefore, precisely determined. If the initial ɛ_Nd value can be regarded as representative of the original igneous rocks, then it indicates that the mantle magma source was severely depleted with respect to magmaphile elements. This suggests that the geochemistry of the early Archean mantle may have been similar to that of the modern upper mantle and may require that a large pre-Archean continental crust have been present on the earth.