DIFFERENTIATION AND SOURCE OF THE NIPISSING DIABASE INTRUSIONS, ONTARIO, CANADA

Major and trace element data are presented for 2.2 Ga Proterozoic diab ase sills from across the Nipissing magmatic province of Ontario. In s itu differentiation of the magma coupled with assimilation of Huronian Supergroup roof sediments is responsible for the variation in composi tion between quartz diabase and granophyric diabase seen within many o f the differentiated intrusions. Uniform trace element and isotope rat io signatures, such as La/Sm (2.8-3.7) and epsilon(Nd)CHUR (-2.7 to -5 .9) characterize chilled margins and undifferentiated quartz diabases. These chemical signatures suggest the existence of a single magma sou rce that was parental to intrusions throughout the magmatic province; this magma has higher La/Sm and lower Ti/Y than primitive mantle and i s displaced towards the composition of shales. Most chilled diabases a nd quartz diabases have a similar Mg# (0.64 and 0.60) and Ni content ( 98 and 127 ppm), and it is argued that the magma differentiated at dep th and was emplaced as a uniform low-Mg magma. The Wanapitei intrusion and Kukagami Lake sill are an exception in that although the quartz d iabase has La/Sm similar to the Nipissing magma type, which suggests t hat they came from the same source, the Mg#(0.68-0.71) and Ni content (130-141 ppm) are higher, which may suggest that they are either sligh tly more primitive examples of the normal Nipissing magma or that cumu lus hypersthene has been resorbed. The light rare earth element enrich ed signature of the Nipissing magmas was perhaps introduced from the c ontinental crust as the magma migrated from the mantle to the surface, but a remarkably constant and large amount (>20%) of crustal contamin ation would be required. An addition of 1-3% shale to the source of a transitional mid-ocean ridge basalt type magma can broadly reproduce t he compositional features of the Nipissing magma type. The source char acteristics were perhaps imparted during subduction accompanying the t erminal Kenoran orogeny.