PB-ND-SR ISOTOPIC CONSTRAINTS ON THE ORIGIN OF THE CALEDONIAN BINDAL BATHOLITH, CENTRAL NORWAY

The Bindal Batholith is the largest granitoid batholith in the Scandinavian Caledonides, emplaced prior to or during the Scandian collision in a complex scenario of Ordovician to Middle Silurian nappe assembly. The Bindal Batholith ranges in compositon from mafic gabbro to leucogranite, but granites and granodiorites are by far the most abundant rock types. Pb-Pb, Sm-Nd and Rb-Sr isotopic results from plutons of the batholith constrain the origin of the Bindal Batholith magmas. The isotope results suggest the presence of several source reservoirs, giving rise to the granitoid magmas. Both a source relatively depleted in U, Th and Rb and enriched in Sm, a source enriched in U and Rb and depleted in Sm, a source enriched in Th and Rb, but depleted in Sm, and, finally, a source enriched in Th and Sm, but depleted in Rb, is indicated by the initial compositions of the radiogenic isotope ratios. It is suggested that the depleted source reservoirs were contemporaneous depleted mantle and mantle derived rocks in the nappe sequences, that the enriched source reservoir was sediments derived from Proterozoic upper crust of Baltic Shield affinity and that the Th-enriched source reservoir was various Proterozoic rocks, in a lower crustal position, of either Baltic or Laurentian affinity.