Trace-element and Nd isotopes in shales as indexes of provenance and crustal growth: The early Paleozoic from the Brabant Massif (Belgium)

The Cambrian-Ordovician shales from the Brabant Massif show an increase in Th (6.7-17.7 ppm) and La (16.6-71.2 ppm) contents in LREE/HREE ratio ( LaN YbN = 3.8-13.9) and in ε{lunate}tNd-values (-3.3 to -8.9) with younger age of the sedimentary rocks. This evolution is related to a change in the dominant source rock from basic metavolcanics in the early Cambrian to granite-gneiss-schist rocks in the early Ordovician. The Ta content appears to be controlled by the provenance area as well. The modification in the origin of detritus is not reflected in the abundance of Sc, Cr and V, but it is slightly recorded in the Co and Ni concentrations. ε{lunate}tNd responds more sensitively to source changes than Sm/Nd ratios and crustal residence ages and it is confirmed to be a more powerful parameter to detect new addition of mantle material to the crust. The evolution of ε{lunate}tNd with stratigraphic age (ε{lunate}tNd vs. t) for the Brabant shales parallels the trends for coeval clastic rocks from Brittany (France) and southern Britain (Michard et al. 1985; Davies et al. 1985) and is considered to be a characteristic of the Armorica plate. The ε{lunate}tNd vs. t path of sediments deriving from crystalline rocks ("first-cycle" sediments) turns out to be a convenient function to delineate the boundaries of fossil lithospheric plates. On the contrary, such first-cycle sediments are not very useful to investigate the long-term continental growth, because their ε{lunate}tNd vs. t path represents a second-order pattern (episodic evolution) that disturbs the first-order trend of the sedimentary Nd-isotope record (secular evolution). © 1986.SCOPUS: ar.j