~(40))Ar- ~(39)Ar ages for detrital white mica in Meguma terrane, Nova Scotia, Canada: Implications for provenance of the Goldenville and Halifax groups

摘要

Single-grain ~(40)Ar/ ~(39)Ar ages are reported for detrital white mica, along with conventional step-heating data for whole rocks, from low-grade metasandstone samples from the Goldenville, Halifax, and Rockville Notch groups in the Meguma terrane of southern Nova Scotia. The majority (166) of single grains from 11 samples yielded ages between ca. 615 and 350 Ma, and the remaining 12 grains yielded ages between ca. 1900 and 870 Ma. The late Neoproterozoic-Paleozoic age distributions are consistent with derivation of sediments from the rapidly uplifted flanks of an active rift, where partial to complete resetting of white mica ages occurred at ca. 520-500 Ma, preceding sediment deposition. The ca. 615 Ma ages may be relics of the original detrital white mica that existed in the source rocks prior to the rifting event. Ages from the Upper Silurian White Rock Formation appear to reflect this same ca. 520-500 Ma event, suggesting that sediments in the White Rock Formation were recycled from the Goldenville and Halifax groups. The older Precambrian ages are inferred to represent white mica in the source region, likely Amazonia. The whole-rock age spectra are discordant, with pronounced age gradients and no well-defined age plateaus. Initial gas released from five of the samples at low laboratory extraction temperatures (ca. 450-500 °C) yielded ages of ca. 260-300 Ma, not seen in the single-grain data, whereas gas released at the highest extraction temperatures yielded ages in the range ca. 510-530 Ma, possibly reflecting the principal result obtained from the single-grain data.