Enrichment of high field strength elements in stream sediments of a granulite terrain in Sri Lanka - evidence for a mineralized belt

Size fractions (< 63, 63-125, 125-177 and 177-250 mum) of stream sediments draining high-grade metamorphic sources from the Walawe Ganga Basin located in a high-grade metamorphic terrain of Sri Lanka have been analysed for th eir major and trace element composition. The chemical compositions of sedim ents were compared with those of the probable metamorphic source rocks in t he tributary basins. Rocks in the area are characterized by higher contents of some of the large ion lithophile (LIL) elements such as K, Rb, Ba, Sr, Pb and high field strength elements (HFSE) compared to similar granulitic t errains. Geochemicallyl the terrain is comparable to the post-Archean upper crust. The size fractions of stream sediments indicate that most of the HF SE and related trace elements are more enriched in the 63-125 and 125-177 m um size fractions compared to the < 63 and the 177-250 mum fractions. Studi es of elemental ratios in sediments show that the geochemistry of most elem ents such as Ti, Nb, Zr, Hf, Th, La and Ce in sediments is mainly controlle d by the heavy mineral phases. However, geochemical data of these rocks do not correlate with elemental content in stream sediments. Only alkali, alka line earth and some trace elements such as Cr, Mn, Co, Ni, and Y roughly co rrelate with the: chemical composition of the probable source rocks. Nevert heless, most of the HFSE and related trace elements (e.g. Ti, V, Nh, Zr, Hf , Th, La, Ce) are highly enriched (factor of 10 or more) in sediments compa red to the probable sourer rocks. Compared to the < 63 mum fraction, other selected fractions indicate higher enrichment relative to their probable so urce rocks. This presumably indicates the presence of HFSE-rich mineral sou rces such as granitic pegmatites in the drainage area. (C) 2001 Elsevier Sc ience B.V. All rights reserved.