Detrital Zircon Geochronology of Cambro-Ordovician Siliciclastic Units of the Humber Arm Allochthon, Newfoundland, Canada

Paleogeographic reconstruction of the Newfoundland area suggests that various arc terranes developed during the opening of the Iapetus Ocean and development of the Cambro-Ordovician Laurentian stable continental margin. These terranes were then accereted to the continental passive-margin upon initial closure of the ocean basin in the Ordovician. The origin and age of these terranes are still questionable as they may contain older Precambrian basement according to some workers, and if so, they could be interpreted as originating from Laurentia or any other bordering landmass to the Iapetus Ocean at that time. Newfoundland is the northernmost extent of the Appalachian Orogenic Belt which first developed during the Taconian Orogeny, a product of Iapetus Ocean closure in the Ordovician. The sedimentary packages of the Humber Arm allocthon were thrust upon the autochthonous sediment of the Laurentian passive-margin and deformed during Taconic Orogeny. Measurement of ages based on U-Pb isotope ratios of detrital zircons found in the syn- and post-rift siliciclastic strata coupled with correlation to known source region ages has provided the following determinations: (1) there is a predominantly Laurentian derived contribution to detrital sediment deposited along the passive margin and foreland basin units during the early Cambrian to middle Ordovician, with relative age contributions ranging from the Archean (3070 Ma) to the Paleozoic (551 Ma) which agrees with previous interpretations and provides a better constraint on the second phase of rifting (591 – 551 Ma); (2) further constraint on the age and nature of the Taconic Arc, based on the method, used remains insufficient, as only two grains yielded ages consistent with an arc age (482 Ma and 480 Ma); (3) a biased analysis of young zircons yielded an arc-age range of 464 – 510 Ma and a peak age of 466 Ma (3 grains), better constraining the detritus from the arc system; (4) the North Atlantic craton and adjacent terranes in the Labrador region (Makkovik Province, Torngat Arc Province, and New Quebec Province) may possibly provide a more local source for sediments derivation; and (5) development and aggradation of an accretionary wedge infront of the advancing arc may have acted as a sediment barrier to arc-derived sediment (e.g. the modern-day Sunda-Banda and Barbados Arcs).