Paleogeographic development of the east Laurentian margin;Constraints from U-Pb dating of detrital zircons in the Newfoundland Appalachians

摘要

An upper Neoproterozoic to Ordovician Laurentian margin sedimentary sequence in the Newfoundland Appalachians records a cycle of ocean opening and closing. U-Pb dating by ion microprobe of 341 detrital-zircon grains from six samples from this se-quence yielded ages from 3592 +- 5 Ma to 572 +- 14 Ma. Three of the analyzed sam-ples are from rift-related formations, two are from drift-related formations, and one is from a foreland-basin unit. The U-Pb data allow grouping of the zircons into four major age components: (1) Archean grains with a maximum age frequency between 2850 and 2600 Ma, (2) Paleoproterozoic grains ranging from 1950 to 1750 Ma, (3) Mesoproterozoic to early Neoproterozoic grains ranging in age between 1450 and 950 Ma, and (4) Neoproterozoic grains ranging between 760 and 570 Ma. The relative pro-portions of these four age components vary among samples. Mesoproterozoic detritus is present in all samples, whereas Neoproter- ozoic detritus is restricted to two of the three rift-related samples. Two samples-one from a rift-related formation and one from a drift-related formation, but both ly-ing at the base of their stratigraphic col-umn-lack Paleoproterozoic and Archean detrital zircons.Potential source regions for the detrital zircons occur within the Laurentian hinter-land: zircons of Archean age correspond to the age of rock units formed during major magmatic and tectonothermal pulses in the Superior craton, zircons with Paleo-proterozoic ages correspond to the ages of orogenic belts marginal to the craton (e.g., Ungava, New Quebec, and Torngat), Me-soproterozoic zircons correspond to the ages of lithologies from the Grenville oro-gen, and Neoproterozoic zircons are the age of rift-related igneous activity along the Laurentian margin that is now preserved within the Appalachian orogen. The youn-gest grains in the rift-related sedimentary rocks yield ages of 580-570 Ma and provide a maximum age for accumulation of the Newfoundland sedimentary sequence.