EVIDENCE OF EXTENSIVE WEATHERING PROFILE DEVELOPMENT AND SUBSURFACE ALTERATION OF GRANITE BEDROCK BELOW THE GREAT UNCONFORMITY, MARBLE MOUNTAINS, SOUTHEASTERN CALIFORNIA, USA

The “Great Unconformity,” as exposed in the Marble Mountains, is a nonconformity with virtually no local relief that separates Proterozoic granite from Cambrian sedimentary strata. A weathering profile developed on the granite during the Cambrian records evidence for advanced chemical weathering that has been overprinted by widespread and localized diagenetic and hydrothermal processes. We present initial interpretations of petrographic, major-element oxide, trace element, and rare earth element (REE) data from two weathering profile transects, B and C, sampled perpendicular to the nonconformity. Fresh bedrock is comprised of orthoclase, plagioclase, quartz, biotite, and accessory phases, and three distinct alteration zones (1-3) developed therein can be identified. Above fresh bedrock, zone 1 shows partial saussuritization and hydrolysis of plagioclase as well as some chloritization of biotite. Zone 2 is characterized by pervasive biotite oxidation and Fe-mineralization in fractures. Zone 3, the most severely altered, shows complete conversion of plagioclase to clays, partial conversion of orthoclase and biotite to clays, and predominant biotite alteration to interlayers of optical muscovite and opaque Fe-oxides. Evidence of chemical weathering and K-metasomatism is depth-consistent between transects, yet evidence for other diagenetic and hydrothermal processes is seen at varied depths. During chemical weathering, plagioclase hydrolysis decreased CaO, Na₂O, SiO₂, Sr, light REEs, and Eu/Eu* values while increasing Al₂O₃, whereas biotite oxidation increased Fe₂O₃ and decreased MnO. Peak chemical index of alteration (CIA) values of 64-65, while elevated from fresh bedrock, imply only minimal chemical weathering. However, plagioclase index of alteration values > 90 indicate severe chemical weathering overprinted by K-metasomatism, thus lowering CIA and increasing K₂O, Rb, and Ba. At varied depths in each transect, occurrence of Na-metasomatism is evidenced by trends in (Al₂O₃-K₂O)-CaO*-Na₂O ternary space and increased Na₂O relative to CaO* (i.e., silicate-hosted CaO) as compared to Na₂O relative to CaO* in fresh bedrock. Additional evidence for subsurface alteration includes saussuritization of plagioclase, chloritization of biotite, and abundant Fe-oxide filled fractures.