Upper crustal recycling in southern Britain: evidence from Nd and Sr isotopes

Abstract

Analyses of representative rocks from southern Britain are used to examine in detail the evolution of this segment of continental crust. The vast majority of Late Precambrian basement rocks have unradiogenic 87Sr/ 86Sr ratios and model t DM Nd ages of ⋍ 1.3 Ga, although locally the Rosslare Complex and Rushton schists have model t DM Nd ages ⋍ 1.7 Ga identifying for the first time, an “old” component in the basement of southern Britain. Following the closure of the Iapetus Ocean sediments exhibit a progressive increase in model t DM Nd ages ⋍ 1.35 Ga in the Lower Palaeozoic to 1.75 Ga in the Jurassic. This variation demonstrates the southerly transport of detritus ( t DM Nd > 2.0 Ga) from the mixed Archaean/Proterozoic terrains of northern Europe and America. The granitoids have significantly higher ε Nd than those of contemporaneous sediments and thus yield a different Nd isotope evolution curve when considered in isolation. The majority of rocks analysed have model t DM Sr ages in the range 1.2 to 0.46 Ga with youngest sediments generally recording the lowest model ages. Therefore, with time, the sediments record a striking increase in the ratio of model Nd to model Sr age. The data suggest that the Rb/Sr ratios of the crustal rocks is increased by a factor of approximately 2 every 900 Ma implying that on average the British upper crust was reworked by erosion and sedimentation every 600 Ma.