Development of Archaean lithosphere deduced from chronology and isotope chemistry of Scourie Dykes

Abstract

The chronology and isotope geochemistry of a selection of Proterozoic Scourie dykes has been investigated in order to specify both their time of emplacement within the thermal history of the Archaean crust of N.W. Scotland, and to attempt to characterise the evolution of continental lithosphere. Sm Nd, Rb Sr and U Pb isotope analyses are presented. Primary, major igneous minerals separated from four well preserved dykes yield Sm Nd ages of 2.031 ± 0.062Ga, 2.015 ± 0.042Ga, 1.982 ± 0.044Ga and 2.101 ± 0.078Ga, which are interpreted as crystallisation ages. The initial Nd isotope compositions in the dykes at their emplacement age of ∼ 2.0 Ga, range from +3.4 to −6.8, indicating the presence of an older lithospheric component. Sm Nd whole-rock isotope data for fifteen dykes, if interpreted to have age significance, yield an “age” of 3.05 ± 0.27 Ga. Sm Nd crustal residence ages for the same dykes average 2.95 Ga, which is interpreted as the time that small melts were added to the Lewisian lithosphere. The possibility that correlated 147Sm/ 144Nd and 143Nd/ 144Nd ratios are an artifact of mixing between depleted mantle melts generated at 2.0 Ga, and an older enriched lithospheric component is not eliminated by the data, but the relationship between 1/Nd and 143Nd/ 144Nd ratios rules out any simple mixing. U Pb isotope data for plagioclase feldspars and whole-rock samples of dykes provide useful estimates of initial Pb-isotope composition of the dykes at the time of their emplacement. Initial 206Pb/ 204Pb and 207Pb/ 204Pb ratios vary considerably and range from 13.98 to 15.78, and 14.72 to 15.56 respectively, and suggest that the U Pb fractionation responsible must have occurred at least 2.5 Ga ago. The Scourie dykes have inherited a trace element enriched component from the Lewisian lithosphere, which has resided there since ca. 3 Ga ago. Whether the dykes inherited this material from the crust or the mantle portions of the lithosphere or both, it seems likely that small basaltic melts derived from asthenospheric mantle were ultimately responsible for the enrichment. The simplest view is that these small melt fractions had been resident in the mantle part of the Lewisian lithosphere. In this case the Archaean trace-element enrichment and element fractionation in the Lewisian lithospheric mantle sampled by the dykes was closely associated in time with the generation of the ∼ 2.9 Ga old crustal portion of the lithosphere [36,37].