Geochemistry of pelitic and psammopelitic Precambrian metasediments from southwestern Sri Lanka: implications for two contrasting source-terrains and tectonic settings

Abstract

Most of the pelitic and psammopelitic metasediments of southwestern Sri Lanka are compositionally transitional between shales and greywackes and are of two isotopically distinct types. Metasediments of the northern part have Nd-model ages of ∼1.5 Ga and belong to the relatively younger Nd-model age province of Wanni Complex (WC). Model ages of two, southerly located samples are ∼2.8 Ga, being compatible with those of the Highland Complex (HC). In accordance with mostly Mesoproterozoic to Late Archaean crustal residence ages, Cr, Ni, La, Th and REE data also suggest that the sources of these metasediments are plagioclase fractionated post-Archaean upper crustal material. Crustal differentiation of source terrains corresponding to the HC and the WC, probably occurred at distinctly different time intervals. Significant differences exist between the major and trace element chemistry of the metapelites from the WC (north) and the HC (south), suggesting derivation from two different source terrains. Metasediments of the south are slightlymmore siliceous, low in Sc and V, and highly enriched in LREE (220 ppm). Their sources have been chemically evolved felsic igneous rocks and/or recycled sediments. Relatively high V, Sc, (FeO+MgO+TiO 2 ) and slightly low SiO 2 in the rocks of the WC indicate contribution of more basic sources. CIA (chemical index of alteration) estimates and comparative alkali-alkaline earth element patterns suggest relatively fresh source material. The O-isotope data indicate that the pre-metamorphic isotope signatures are preserved, and the clay component of the original sediments mainly consisted of illite, typifying continental weathering conditions. Sediments of the south (HC) were probably deposited in a passive continental margin./continental rift environment, while those of the northern part (WC) were probably formed at an Andean-type active continental margin. The two units were tectonically juxtaposed prior to Pan-African granulite metamorphism.