ASSIMILATION OF COUNTRY-ROCK ILMENITE AND RUTILE IN THE SOUTH MOUNTAIN BATHOLITH, NOVA SCOTIA, CANADA

Abstract

The South Mountain Batholith (SMB) of southwestern Nova Scotia is a peraluminous granitoid complex showing physical and chemical evidence of contamination by metasedimentary country-rocks of the Meguma Group. Some ilmenite grains in the granitic rocks are texturally and chemically similar to ilmenite grains in the country rocks, whereas other ilmenite grains in the granitic rocks appear to be texturally (more euhedral) and chemically (lower and less variable Mn contents) unique. Some rutile grains in the granitic rocks are texturally and chemically similar to rutile grains in the country rocks, whereas other rutile grains in the granitic rocks appear to be texturally (more euhedral, possible cavity-fill) and chemically (higher Nb + Ta contents, oscillatory compositional zoning) unique. In the least evolved rocks of the SMB, ilmenite is magmatic and xenocrystic in origin, with cation exchange the probable reaction causing the assimilation of the xenocrysts, whereas rutile appears to be exclusively xenocrystic, remaining as the product of incomplete dissolution. In rocks from intermediate stages of evolution of the SMB, magmatic ilmenite and rutile are more abundant than xenocrystic ilmenite and rutile. In the most evolved rocks of the SMB, magmatic and xenocrystic ilmenite are absent from the mineral assemblage, probably through a combination of a peritectic reaction and dissolution, respectively, whereas most grains of rutile appear to be largely of magmatic origin, with some xenocrystic grains remaining after incomplete cation-exchange reactions.