COUNTRY-ROCK CONTAMINATION OF GRANITES - AN EXAMPLE FROM SOUTHEAST SWEDEN

The chemical composition of granites is influenced by the source rocks , degree of melting, crystal fractionation and various mechanisms of c ontamination. This investigation is based on major element data taken from literature to which new major and trace element data of granitoid s are added. All data come from the Paleoproterozoic Transscandinavian Igneous Belt. Rocks vary from quartz monzonite and quartz syenite to dominating syenogranite. Most rocks of the Belt are suggested to have formed by volatile absent melting of older igneous rocks, and most of them represent near-minimum melting compositions. The literature data cover most of the Belt whereas the new data come from a restricted are a. This new sampling was designed to study possible contamination effe cts. In the rocks analyzed especially for this work, the major element s except Na and K and most trace elements form one coherent group with strong intercorrelations, whereas Na, K, Ba, Sr, Rb, Th, Cs, and Cr s catter. Most of the granites have rather small negative Eu-anomalies; three samples show no or very weak positive anomalies. The scattering of elements and especially an antithetic behaviour of K and Na is typi cal only for a small area where metasediment contamination is probable . Crystal differentiation and varying degrees of melting of a reasonab le homogeneous source rock cannot produce this scattering. Source rock heterogeneity, hydrothermal overprinting and magma mixing are conside red less likely than wall-rock contamination of meta-greywacke and met a-argillite to explain the observed variation.