ND ISOTOPES OF A LIP OF THE 1.9 GA FLIN FLON BELT, TRANS-HUDSON OROGEN AND RATES OF MANTLE PROCESSES

The characteristics of volcanic rocks from Paleoproterozic orogenic belts are important because they shed light on fundamental debates regarding the evolution of Earth at this time such as: rates and intensities of tectonic processes, and the degree of mantle depletion and homogeneity at this time. The Amisk succession (1.9-1.87 Ga), Flin Flon belt, Trans-Hudson Orogen is a 10 km-thick sequence comprising tholeiitic basalts to basaltic andesites, boninitic basalts, high-Fe tholeiites, and calc-alkaline to shoshonitic basalts to basaltic andesites that are part of a large igneous province. These different rock types have unique trace element signatures, and are not co-magmatic. The compositionally diversity of these rock types requires formation from different mantle sources, including partial melting of recycled oceanic crust and of subduction modified sub-arc mantle. However, the Amisk succession is relatively homogeneous with respect to Nd isotopes (eNd = +4 ± 1.3), with little evidence for significant mantle source heterogeneity beyond analytical uncertainty. Comparison with ocean floor and arc assemblages of the central Flin Flon belt indicates that this homogeneity existed during the development of the Amisk succession, and throughout much of the central part of the Flin Flon belt, currently covering an area of about 6000 km2. All the mafic rocks form a well-constrained, pseudo-isochron with an age of 1.872 ± 0.2 Ga, that is virtually identical to the age of other juvenile arc sequences of the pre-accretionary assemblages of the Amisk collage given by U-Pb dating of zircons. This similarity suggests that the regression line has age significance, and that the changes to the source regions that produced the distinct geochemical groups must have taken place immediately prior to melt extraction, so that there was a very short time between the production of the different sources and volcanism (i.e., « 100 Ma), consistent with a major thermal event at this time in the history of Earth.