Sr isotopes and trace element geochemistry of the impact melt and target rocks at the Mistastin Lake crater, Labrador

Abstract

The Mistastin Lake meteorite crater lies completely within a batholith composed of mangerite and adamellite with lenses of anorthosite and is located in central Labrador. The multivariate statistical technique of correspondence analysis was used to summarize the relationships between the different rock units for the trace elements Nb, Zr, Y, Sr, Rb, Th, Pb, Zn, Cu, Ni. The samples of impact melt form a linear array of points on a factor plot joining the anorthosite samples to the mangerite and adamellite samples. This indicates that the various melt samples can be formed as a result of the complete fusion of different proportions of anorthosite and granitic rocks. A least-squares mixing model utilizing the average trace element composition of the four rock types indicates that an average melt rock can be formed by mixing 60% anorthosite, 38% mangerite and 2% adamellite. An isochron obtained on the combined mangerite and adamellite units of the batholith gives an age of 1347 ±15 m.y. (1σ) with an initial ratio of 0.7082 ± 0.0003. The anorthosite samples plot below the isochron and the melt rocks fall along a mixing line joining the locus of anorthosite points to an average granite sample on the isochron. This is a further indication that the melt was formed by melting of anorthosite and granitic rocks that form the local geological environment.