| Popis: |
Precise U-Pb isotopic analysis of seventeen selected high-quality laboratory abraded zircon single grains, seven multigrain fractions of similar material and four paramagnetic titanite fractions record the ages of the various detrital grain types from two turbidite samples collected from the top and about 5 km below the top of the Goldenville Formation of the MEguma Group. The results show that the source of both sediments contained ~ 600 Ma and 2000 Ma old volcanic-plutonic components, whereas a third, 3000 Ma old component occurs exclusively in the upper sample and the youngest zircons and titanite are found in the lower sample. Lead-lead ages of bulk fractions at 2300 Ma and two nearly concordant single grains at 2040 Ma and 2140 Ma (upper sample), together with 2157 Ma (bulk fraction) and 2060 Ma and 2080 Ma single grain fractions (lower sample), indicate that the ~ 2000 Ma component dominates the pink-purple, euhedral grain types. Detrital titanite gives ages of 605 Ma (upper sample) and 552 Ma (lower sample), with the latter sample containing a minor ~ 1975 Ma component. Colourless zircons record this same age difference. The lower sample contained the youngest, concordant, single zircon grain (566 Ma) and 580–605 Ma bulk grain fraction, even though two concordant single grains have ages of 632 and 643 Ma. Estimated bulk fraction ages for certain zircon grain types from the upper sample fall in the range of 605 to 675 Ma. These data suggest that while the youngest rocks in the source region were removed by erosion during the early stages of deposition of the Goldenville Formation, the oldest source rocks were not unroofed until later in the depositional history. The 566 ± 8 Ma age of the youngest concordant zircon from the stratigraphically lower sample provides a maximum age for the deposition of this grain. With the base of the Cambrian placed between 545 and 530 Ma, the Goldenville Formation may straddle the Precambrian-Cambrian boundary. Graptolites from the immediately overlying Halifax Formation are of Early Ordovician Tremadocian) age. The lower sample contains spherical zircons that may have been derived from mafic granulite with an estimated age of 2000–2200 Ma. With this one possible exception, all of the zircons analyzed appear to be igneous in origin and all are relatively euhedral, indicating that rocks of the ages referred to above probably outcrop in the source area. Anomalous discordance patterns imply that part of the 2960 Ma area was metamorphosed by the 2000 Ma event and that both the 2960 and 2000 Ma regions were metamorphosed during the 600 Ma event. The complete lack of detritus with ages between 680 and 2040 Ma virtually eliminates a northern North American or Baltic-Shield provenance. However, the available data are not sufficient to distinguish between a West African or Guyana Shield source. |
