Sources of stream sediments in the granulite terrain of the Walawe Ganga Basin, Sri Lanka, indicated by rare earth element geochemistry

Thirty-eight samples of stream sediments draining high-grade metamorphic ro cks in the Walawe Granga (river) Basin, Sri Lanka, were analysed for their REE contents, together with samples of metamorphic suites from the source r egion. The metamorphic rocks are enriched in light REE (LREE) compared to h eavy REE (HREE) and are characterised by high La/Lu ratios and negative Eu anomalies. The chondrite-normalised patterns for these granulite-grade rock s are similar to that of the average post-Archaean upper crust, but they ar e slightly enriched with La and Ce. The REE contents of the <63-mu m fracti on of the stream sediments are similar to the probable source rocks, but th e other grain size fractions show more enriched patterns. The <63-mu m stre am sediments fraction contains lower total REE, more pronouned negative Eu anomalies, higher Eu-N/Sm-N and lower La (N)/Ln(N) ratios relative to other fractions. The lower La (N)/Lu-N ratio is related to the depletion of heav y minerals in the <63-mu m Fraction. The 63-125-mu m and 175-177-mu m grain size fractions of sediments are particularly enriched in LREE (average Sig ma LREE=2990 mu g/g and 3410 mu g/g, respectively). The total HREE contents are surprisingly uniform in all size fractions. However, the REE contents in the Walawe Ganga sediments are not comparable with those of the granulit e-grade rocks from the source region of the sediments. The enrichment of RE E is accounted for by the presence of REE containing accessory mineral phas es such as zircon, monazite, apatite and garnet. These minerals are derived from an unknown source, presumably from scattered bodies of granitic pegma tites. (C) 2000 Elsevier Science Ltd. All rights reserved.