PETROGENESIS OF 1.9 GA LIMESTONES AND DOLOSTONES AND THEIR RECORD OF PALEOPROTEROZOIC ENVIRONMENTS

Fibrous calcite cements from limestones of the Paleoproterozoic Pethei Group in northwest Canada have relatively constant delta(13)C values, near 1 parts per thousand, that indicate the C exogenic cycle of ca 1 .9 Ga ago had similar balances between oxidized and reduced reservoirs of carbon as has existed throughout most of the Phanerozoic. Sr-87/Sr -86 ratios of these cements are near 0.7051, and support a progressive ly increasing influx of radiogenic Sr from continental weathering into the Paleoproterozoic oceans. delta(18)O values of the cements vary fr om -14 to -8.5 parts per thousand, although the original values of the cements were probably higher because the cements have been subsequent ly altered, as evidenced by their variable trace element contents. Pro posed higher temperatures or substantially lower delta(18)O values of ancient seawater based on such values may be in error. Fabric retentiv e dolomites, generally regarded as syndepositional, also have variable delta(18)O values, the highest of which are ca 30 greater than the hi ghest values of the fibrous calcite cements. The dolomites have more r adiogenic Sr isotopic compositions indicating that they formed from fl uids that interacted with rocks such as those of the elastic sediments which underlie the limestones. These fluids may have partially altere d the delta(18)O values and chemical compositions of the fibrous calci te cements. Late-stage calcitic and dolomitic cements that formed from late diagenetic fluids have similar delta(18)O values and radiogenic Sr isotopic ratios as the fabric retentive dolomites. If these potenti ally syndepositional dolomites have had their primary isotopic composi tions reset through interaction with diagenetic fluids, this would hav e significant implications to other studies using early dolomite to co nstrain ancient environmental conditions. (C) 1998 Elsevier Science B. V.