GEOLOGY AND MAJOR AND TRACE-ELEMENT CHEMISTRY OF LATE ARCHEAN WEATHERING PROFILES IN THE FORTESCUE GROUP, WESTERN-AUSTRALIA - IMPLICATIONS FOR ATMOSPHERIC P-O2

We have identified two thick weathering profiles between flows of the Mt. Roe Basalt unit of the Fortescue Group, in the Pilbara area of Wes tern Australia. The age of weathering is constrained by U-Pb zircon da ting to lie between 2765 and 2715 Ma. The profiles, herein referred to as the Mt. Roe #1 and #2 paleosols, were mapped in detail for 1500 m and 760 m along strike, respectively. Samples were collected along tra verses across both profiles where they were well-developed and well-ex posed. The profiles, which may be structurally offset parts of the sam e weathering horizon, have well-developed upper sericite zones (1-15 m thick); these grade downward into chlorite-rich zones which are chemi cally and physically heterogeneous. The chlorite zones grade downward into unweathered basalts. The Mt. Roe #1 profile is overlain by 2-6 m of bedded, probably lacustrine sediments; these in turn are covered by succeeding lava flows. Evidence of a surficial origin for the paleoso ls includes mud cracks, ripple marks, and rip-ups of the paleosol mate rial in the post-paleosol sediments. Soft-sediment deformation and pro bable dewatering structures are present along the paleosol-sediment co ntact. The Mt. Roe #2 profile is overlain directly by the succeeding l ava Bow with little or no intervening sediments. Some brecciation and pillowing is observed along the base of the overlying basalt flow in b oth cases, probably due to emplacement of lava into a shallow lake or onto wet soil. In the sericite zones of both profiles nearly all of th e Fe, Mn, Mg and Zn and a substantial fraction of the Ca, Pb, Na, Ni a nd Si have been leached. The chlorite zones have elevated Fe, Pb, Ni a nd Zn contents and appear to be repositories of elements lost from the sericite zone. Al, Ti, Zr and Th were immobile in the Mt. Roe #1 pale osol, and are concentrated by factors of 2.5-2.6 over the unweathered basalt. V, U and Cr were also nearly immobile. Major element and trace element data indicate that weathering took place under conditions of low atmospheric oxygen. K is strongly enriched toward the upper contac t of the sericite zone, and was introduced during post-weathering meta somatism together with Rb and Sr. Na and Ca are also enriched near the top of each profile, and small amounts of each may have been introduc ed during regional, low-grade metamorphism of the Hamersley Basin at 2 168 +/- 10 Ma.