Isolation of early Paleohelikian remanence in Grenville anorthosites of the French River area, Ontario

Abstract

Specimens were collected at 19 sites from Precambrian anorthosites in the French River region of the Grenville Province. AF and thermal step demagnetization analyses indicate the presence of four components: A, B, C, and D. After bulk AF and thermal demagnetization at 550, 650, and 670 °C, conventional two-tiered statistical analysis reveals only the presence of the B component for the collection. Point density contour plots isolate objectively all four components. Vector subtraction shows that the A component survives up to ~40 ± 10 mT or 300 ± 100 °C and resides in magnetite. Its pole position of 45°W, 26°N, δp = 9°, δm = 10° is consistent with it being a Grenvillian orogenic metamorphic overprint acquired 975 Ma ago at the same time as the K–Ar dates were reset. The B component survives up to 100 mT and resides both in magnetite up to 585° C and in hematite up to 670 °C. Its pole position of 26°W, 13°S, δp = 1°, δm = 2° is ~30° off the pre-1000 Ma portions of the APW path, but falls directly on the 1725 Ma portion. This fits the known whole-rock Rb–Sr and zircon U–Pb ages of 1700 ± 100 Ma found for these rocks, which date the Hudsonian Orogeny. The C component is found only at 650 °C or above. Its pole of 57°W, 27°S, δp = 4°, δm = 7° is 60° away from 1000–1400 Ma portion of the APW path but falls directly on the 1800 Ma path. The D component is isolated only after the 670 °C treatment. Its pole of 1°W, 6°N, δp = 4°, δm = 7° falls on the 1250 Ma segment of the APW path. It is attributed to alteration associated with the Sudbury olivine diabase dikes of this age, which are adjacent to some sites. Thus the Grenville Orogeny is essentially only a thermal event in the anorthosites, and the B and C components were acquired during cooling after the more intense Hudsonian Orogeny. Also population level screening provides a more efficient and objective way of isolating multicomponents than conventional tiered statistics after bulk AF or thermal cleaning of a collection.