Model lead and radiometric ages from the Churchill Province, Canadian Shield

Details Model lead and radiometric ages from the Churchill Province, Canadian Shield Model lead and radiometric ages from the Churchill Province, Canadian Shield

Oct 1970

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1970gecoa..34.1089s&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 34, Issue 10, pp.1089-1106

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Scientific paper

The isotopic composition of primary lead has been determined in 11 galenas and 26 feldspars from the Churchill Province, Canada. Samples were collected from a triangular area bounded by La Ronge, Saskatchewan, Lake Wollaston, Saskatchewan and Flin Flon, Manitoba. The galena lead model ages are consistent around 2030 m.y. assuming a closed system isotopio evolution for their lead and 4550 m.y. as the age of the earth. The least radiogenic lead from feldspars gives model ages similar to the galena ages, while others are more radiogenic and give anomalously younger ages even after correction for in situ uranium decay. The Pb 208 / Pb 204 ratios of the galenas and the least radiogenic feldspars are indistinguishable. Rb-Sr ages determined on whole rocks and potassium feldspars from pegmatites and whole rocks give an isochron age of 1846 ± 60 m.y. This is probably the time of the formation of the galena ore bodies also. Feldspars from a belt of metamorphic rocks of possible pre-Hudsonian age contain lead with aberrant model ages, which are mostly much younger than the ages of the galenas and other feldspars. The difference between the radiometric and model ages seen in this area is of the same sign, but lesser magnitude than the difference found in the 2720 m.y. old rocks of the Superior Province in Canada. On the other hand, ores and rocks younger than 1400 m.y. generally have model ages less than radiometric ages. These observations cast doubt on the validity of closed system evolution models for common lead.

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