PRELIMINARY PETROGRAPHIC AND GEOCHEMICAL APPRAISAL OF A CAMBRIAN WEATHERING PROFILE DEVELOPED ON PROTEROZOIC POTASSIC GRANITE, MARBLE MOUNTAINS, SOUTHEASTERN, CALIFORNIA

A paleosol developed on Proterozoic granite in the southern Marble Mountains, Mojave Desert, California documents extensive chemical weathering and exposure prior to deposition of Cambrian strata on the bedrock. We present preliminary documentation of the paleosol using petrography and whole-rock, major- and trace-element geochemical analysis. Seventeen samples were collected from the paleosol in order of increasing alteration beginning with the protolith and culminating in highly altered rocks at the nonconformity. Petrographic results from fresh granite show abundant orthoclase, plagioclase, quartz and biotite. The uppermost paleosol horizon displays quartz, altered K-feldspar, and complete loss of plagioclase feldspar converted to phyllosilicates. Additionally, muscovite interlayered with metal-oxides is present and interpreted as alteration of original biotite. Major-element abundances from base to top of the profile indicate a decrease in SiO₂, CaO, Na₂O with increasing Al₂O₃. Chemical Index of Alteration (CIA) values peak at ~65, suggesting modest weathering and conflicts with petrographic observations. The data indicate the paleosol has undergone K-metasomatism, which significantly lowered CIA values. In A-CN-K space, data show two groups, one near the protolith composition, and another adjacent to the A-K join where present sampling density has missed the transition. Corresponding Plagioclase Index of Alteration (PIA) values are > 90, indicating that the profile has undergone significant chemical weathering. Chondrite normalized Rare earth elements (REE) of all samples, regardless of weathering intensity, show negative Eu anomalies, fractionated LREE, and flat HREE. All samples excluding the top of the profile have similar chondrite normalized LREE abundances but HREE concentrations are variable. REE normalized to protolith measured from the uppermost horizon are highly depleted in LREE. Progressively deeper samples are variably enriched in HREE up to 7.2 m depth. Trace element abundances of the upper profile are enriched in (Ti, V, Cr, Co, Ni, Y, Cs, Ta) and depleted in (Pb, Th, U) relative to protolith. Trace element patterns deviate significantly from protolith at 8.1 m depth which corresponds to depletion of major-element oxides at that same interval.