GEOCHEMISTRY OF SIMILAR-TO-1.9 GA SEDIMENTARY-ROCKS FROM NORTHEASTERNLABRADOR, CANADA

Fifty-eight rock chips from fifteen samples of sedimentary rocks from the Ramah Group (similar to 1.9 Ga) in northeastern Labrador, Canada, were analyzed for major and minor elements, including C and S, to eluc idate weathering processes on the Earth's surface about 1.9 Ga ago. Th e samples come from the Rowsell Harbour, Reddick Eight, and Nullatakto k Formations. Two rock series, graywacke-gray shales of the Rowsell Ha rbour, Reddick Eight and Nullataktok Formations, and black shales of t he Nullataktok Formation, are distinguishable on the basis of litholog y, mineralogy, and major and trace element chemistry. The black shales show lower concentrations than the graywackes-gray shales in TiO2 (0. 3-0.7 wt% vs. 0.7-1.8 wt%), Al2O3 (9.5-20.1 wt% vs. 13.0-25.0 wt%), an d Sigma Fe (<1 wt% vs. 3.8-13.9 wt% as FeO). Contents of Zr, Th, U, Nb , Ce, Y, Rb, Y, Co, and Ni are also lower in the black shales. The sou rce rocks for the Ramah Group sediments were probably Archean gneisses with compositions similar to those in Labrador and western Greenland. The major element chemistry of source rocks for the Ramah Group sedim entary rocks was estimated from the Al2O3/TiO2 ratios of the sedimenta ry rocks and the relationship between the major element contents (e.g. , SiO2 wt%) and Al2O3/TiO2 ratios of the Archean gneisses. This approa ch is justified, because the Al/Ti ratios of shales generally retain t heir source rock values; however, the Zr/Al, Zr/Ti, and Cr/Ni ratios f ractionate during the transport of sediments. The measured SiO2 conten ts of shales in the Ramah Group are generally higher than the estimate d SiO2 contents of source rocks by similar to 5 wt%. This correction m ay also have to be applied when estimating average crustal composition s from shales. Two provenances were recognized for the Ramah Group sed iments. Provenance I was comprised mostly of rocks of bimodal composit ions, one with SiO2 contents similar to 45 wt% and the other similar t o 65 wt%, and was the source for most sedimentary rocks of the Ramah G roup, except for black shales of the Nullataktok Formation. The black shales were apparently derived from Provenance II that was comprised m ostly of felsic rocks with SiO2 contents similar to 65 wt%.Comparing t he compositions of the Ramah Group sedimentary rocks and their source rocks, we have recognized that several major elements, especially Ca a nd Mg, were lost almost entirely from the source rocks during weatheri ng and sedimentation. Sodium and potassium were also leached almost en tirely during the weathering of the source rocks. However, significant amounts of Na were added to the black shales and K to all the rock ty pes during diagenesis and/or regional metamorphism. The intensity of w eathering of source rocks for the Ramah Group sediments was much highe r than that of typical Phanerozoic sediments, possibly because of a hi gher P-CO2 in the Proteruzoic atmosphere. Compared to the source rock values, the Fe3+/Ti ratios of many of the graywackes and gray shales o f the Ramah Group are higher, the Fe2+/Ti ratios are lower, and the Si gma Fe/Ti ratios are the same. Such characteristics of the Fe geochemi stry indicate that these sedimentary rocks are comprised of soils form ed by weathering of source rocks under an oxygen-rich atmosphere. The atmosphere about 1.9 Ga was, therefore, oxygen rich. Typical black sha les of Phanerozoic age exhibit positive correlations between the organ ic C contents and the concentrations of S, U, and Mo, because these el ements are enriched in oxygenated seawater and are removed from seawat er by organic matter in sediments. However, such correlations are not found in the Ramah Group sediments. Black shales of the Ramah Group co ntain 1.7-2.8 wt% organic C, but are extremely depleted in Sigma Fe (< 1 wt% as FeO), S (<0.3 wt%), U (similar to 1 ppm), Mo (<5 ppm), Ni (<2 ppm), and Co (similar to 0 ppm). This lack of correlation, however, d oes not imply that the similar to 1.9 Ga atmosphere-ocean system was a noxic. Depletion of these elements from the Ramah Group sediments may have occurred during diagenesis. Copyright (C) 1997 Elsevier Science L td.