LOESS GEOCHEMISTRY AND ITS IMPLICATIONS FOR PARTICLE ORIGIN AND COMPOSITION OF THE UPPER CONTINENTAL-CRUST

Chemical and Nd-Sr isotopic compositions of loess samples from Argenti na, Europe and Spitsbergen were analyzed to examine the nature of sour ce terrains, the origin of silt-size particles and the suitability of using loess as starting material for estimating the average chemical c omposition of the upper continental crust. From the relations between Na2O/Al2O3 and K2O/Al2O3 ratios and CIA values (chemical index of alte ration), the loess protoliths must have undergone previous sedimentary differentiation and subjected to moderate chemical weathering. REE pa tterns are remarkably uniform with (La/Yb)(N) approximate to 10, which is characteristic of the upper continental crust (UCC). Negative Eu a nomalies, expressed in Eu/Eu ratios, vary from 0.65 for European loes s to 0.8 for Pampean loess from Argentina. All loess deposits have nea rly constant La/Th or Th/U ratios, which are very similar to those of the average UCC or post-Archean shales. These ratios are not fractiona ted in size-fractions relative to the whole-rock values. Nd and Sr iso topic compositions clearly distinguish Argentinean loess (Sr-87/Sr-86 = 0.706-0.709, epsilon(Nd)(0) = -6 to -1.5) from all other loess depos its (Sr-87/Sr-86 = 0.712-0.730, epsilon(Nd)(0) = -13 to -8). The REE a nd isotopic results clearly indicate a significant contribution of you ng Andean volcanics to the Pampean loess deposits, whereas multi-recyc led and well-mixed ancient sediments are principal sources for the oth er deposits. The present results reinforce the earlier conclusion reac hed by S.R. Taylor, S.M. McLennan and M.T. McCulloch [Geochemistry of loess, continental crustal composition and crustal model ages, Geochim . Cosmochim. Acta 47 (1983) 1597-1905] that the average chemical compo sition of UCC can be obtained from eolian deposits as well as from fin e-grained elastic sediments. (C) 1998 Elsevier Science B.V. All rights reserved.