Two-stage Weathering Model for K-feldspar-absent Granitoids in a Mediterranean (hot summer) Climate: Evidence From Corestone-saprolite Pairs, Santa Margarita Ecological Reserve (SMER), Temecula, California

Aaron K. Hebeler, Sarah L. Johnson, Chris R. Martinez, Melanie A. Biggs, Richard W. Berry, and Gary H. Girty
Geological Sciences, San Diego State University, 5500 Campanile Drive, San Diego, CA 92182

W. Nesbitt and colleagues proposed that weathered plutonic material will spread linearly from parental fields toward the A apex subparallel to the A-CN join on A-CN-K diagrams. Such linear trends are common because soil solutions are typically supersaturated with respect to K, but not CN. At SMER, our studies reveal a weathering trend that deviates from that proposed by W. Nesbitt and colleagues. SMER lies within a Mediterranean climate with an average precipitation of ~39.4 cm/yr and average T of ~16.6˚C. The sampled intrusive bodies are highly jointed containing spherical to ellipsoidal corestones with 1 – 2 m diameters. Corestones are surrounded by a 6 – 10 cm thick concentrically fractured rind that gives way laterally to saprolite. We sampled corestone-saprolite pairs in Cretaceous tonalite (123.1 ± 1.2 Ma), a dioritic enclave within the tonalite, and quartz diorite (107.5 ± 1.3 Ma) for thin section study and chemical analyses. We used the chemical index of alteration (CIA) to determine the degree of weathering and used the transport function (τ) for determining changes in elemental mass. Each type of sampled corestone generally lacks modal K-feldspar but contains 7.25% to 13.9% biotite. On A-CN-K diagrams samples spread linearly from the parental material away from the K apex toward the A-CN join. XRD and microscope analyses indicate that this trend is due to the conversion of biotite to mixed-layer expandable clay. This process is accommodated by a loss of ~50% of the K elemental mass. We interpret these results to imply that the initiation of chemical weathering in granitoids lacking K-feldspar, in a Mediterranean climate like that at SMER begins with the alteration of biotite to mixed-layer expandable clay and the loss of K mass. At SMER, only the most severely weathered tonalitic material displays the weathering trend identified by Nesbitt and colleagues.

AAPG Search and Discovery Article #90076©2008 AAPG Pacific Section, Bakersfield, California