Mathematics – Logic
Scientific paper
Aug 1989
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989e%26psl..94...29g&link_type=abstract
Earth and Planetary Science Letters, Volume 94, Issue 1-2, p. 29-44.
Mathematics
Logic
11
Scientific paper
Sedimentary rocks from the southeastern Canadian Cordillera varying in age from Mid-Proterozoic to Triassic have been analysed for their Nd isotopic composition. ɛNd(T) values range from +1.5 to -17.2 and Nd model ages TDM(Nd) vary from 1.00 to 2.32 Ga, much in excess over their stratigraphic ages. The difference between the two ages ranges from 0.43 to 1.93 Ga, decreasing as the sediments become stratigraphically older.
Nd isotopic compositions of sediments suggest that there are two distinct continental sources of sediments in the Cordilleran miogeocline: (a) an older source (ɛNd(0) from -20.3 to -23.8) with apparent crustal residence ages of 2.26-2.32 Ga, and (b) a younger source (average ɛNd(0) ⋍ -14.0) with the crustal residence age of about 1.8 Ga. The older residence ages are interpreted to be due to mixtures of materials from the Archean and Proterozoic crusts. Sediments belonging to the Laib Formation (Lower Cambrian) and lower clastic part of the Milford Group (Namurian) appear to have derived from an Archean continental segment, with some admixture of sediments from the Early Proterozoic source. The younger crustal source, which seems to be the principal supplier of sediments for most of the stratigraphic units (Aldridge Formation (Middle Proterozoic), Horsethief Creek Group (Upper Proterozoic), Hamill and Lardeau Groups (Lower Cambrian), Nelway Formation (Middle Cambrian), Active Formation (Ordovician) and part of Milford Group) is not the Archean Canadian shield immediately to the east. Recent subsurface mapping of the Alberta basement beneath the Phanerozoic sedimentary cover using U/Pb zircon dating of igneous lithologies [1] has established the extensive Archean and Early Proterozoic crustal provinces. It is proposed here that the Early Proterozoic province of west-central Alberta is the most likely source of the easterly derived sedimentary rocks of the Cordilleran miogeocline with TCR ages from 1.94 to 1.50 Ga. On the basis of sedimentologic, tectonic and isotopic evidence, we suggest that the younger sediments could be to some extent recycled older sediments. For the Triassic Slocan and Ymir Groups (eastern part of Quesnel terrane), sedimentologic, lithologic, tectonic and isotopic evidence suggest the source was a western one with about 1.8 Ga crustal residence age.
Comparison of the crustal residence ages with stratigraphic ages shows the best fit Nd evolution curve for the Cordilleran miogeocline approximately coincides with the continental growth curve deduced for the North Atlantic Province and similar to the growth curve for Gondwana during the Phanerozoic. This curve also passes through the North American Composite. The levelling-off of crustal residence ages for the Mid-Proterozoic to Triassic sediments at about 1.8 Ga supports the idea of negligible crustal growth since at least 1.7 Ga, the average crustal residence age of the modern miogeoclinal and cratonic sediments.
Ghosh Dipak K.
Lambert Richard St John
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