ORIGIN OF THE PLUTONIC MAFIC ROCKS OF SOUTHERN NOVA-SCOTIA

Mafic rocks of the Appalachian erogenic belt of southern Nova Scotia were analyzed for major elements, Ba, Sr, Rb, Zr, Nb, Y, and rare-earth elements. These plutonic rocks comprise plagioclase peridotite, norite, high-K diorite, and tonalite. Comparisons with volcanic rocks and other plutonic calc-alkalic rocks indicate that mafic rocks of orogenic belts are very similar in composition to continental tholeiitic basalts, whereas the plagioclase peridotite is chemically comparable to picrites. The high-K diorites have distinctive geochemical characteristics and show only a few analogies with volcanic rocks. It is suggested, on the basis of the modeling of the data, that the noritic magma could have been originated by partial melting of upper-mantle material with light rare-earth element abundances five to ten times those of chondrites and heavy rare-earth element abundances two to five times the chondritic abundances. The plagioclase peridotite can be derived from a contaminated magma, although the contamination appears to be of mantle (or lower crust) origin. An origin by hybridization of mafic magma with a granitic liquid for the high-K diorites is consistent with the data available. © 1979 Geological Society of America.