An unaltered norite cumulate dyke from the central part of the Isua area, West Greenland, has been studied in an attempt to determine the isotopic composition of early Archaean mantle. A mineral (plagioclase Á/orthopyroxene) and whole rock Sm Á/Nd isochron yields an age of 34139/57 Ma, which is slightly younger than a previously published U Á/Pb zircon SHRIMP age of 35129/6 Ma (Nutman et al., 1997). While the mineralogy (opx'/plag'/ol'/cr-sp) and mineral chemistry of the dyke (opx: En 90 Á 70 , plag: An 73 Á 54 , ol: Fo 90 Á 87 , and Cr-sp: Cr 66 Á 56) are consistent with crystallization from a near primary melt, a strong crustal signature characterizes the trace element and isotopic composition of the dyke i.e. (Nb/La) PM 0/0.20 Á/0.25; o Nd 0/(/1.39/1; (La/Yb) N 0/4.8 Á/4.1. Hydrothermally altered samples up to 5 cm distant from a cross-cutting quartz-vein exhibit systematic changes in trace element and isotopic composition approaching the quartz-vein (a total variation of Â/2.5 o Nd -units; (La/Yb) N 0/5.5 Á/4.1). This illustrates one danger of simple interpretations of isotope compositions of early Archaean whole rocks. Modeling the chemical and isotopic signature of the dyke either requires unrealistically large amounts (!/20%) of crustal assimilation or introduction of smaller amounts (Â/5%) of crustal material directly into a mantle reservoir prior to melting. Therefore, we suggest a petrogenetic model involving early Archaean crustal recycling processes. # 2002 Elsevier Science B.V. All rights reserved.