SAPROLITE-CORESTONE DEVELOPMENT AT SANTA MARGARITA ECOLOGICAL RESERVE (SMER), PENINSULAR RANGES, CALIFORNIA: CLUES TO THE ONSET OF CHEMICAL WEATHERING IN A MEDITERRANEAN CLIMATE

Quartz diorite, tonalite, and granite underlie large areas of SMER, Temecula, southern California. SMER lies within a Mediterranean climate with an average precipitation of ~39.4 cm/yr and T of ~16.6˚C. U-Pb zircon laser ablation ICP-MS yields ~123.1 ± 1.2 Ma for the tonalite, ~124.6 ± 1.4 Ma for the granite, and ~107.5 ± 1.3 Ma for the quartz diorite. The tonalite and included mafic enclaves, and granite are foliated while the quartz diorite is not. Due to the nearby Elsinore fault and exhumation, these 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. To measure the degree of weathering we determined the chemical index of alteration (CIA) and transport function (τ) for a diorite enclave within the tonalite and for characteristic outcrops of the tonalite and quartz diorite. At each site we sampled corestone-saprolite pairs along ~1 m traverses. Plots of the major elements against the average of the least weathered corestone samples at each site revealed that the mass of Al was conserved during development of saprolite. Using Al as a framework element revealed that the mass of K is significantly depleted within the saprolite at all sites. On A-CN-K diagrams weathering trends extend from the K apex toward the A-CN join. On A-CNK-FM diagrams samples from the saprolite cluster in the field defined by corestone. XRD analyses of biotite and Mg hornblende hand picked from crushed samples of corestone and saprolite showed that (1) biotite is converted to mixed-layer expandable clay as the corestone is converted to saprolite, while (2) Mg hornblende is not generally affected by this process. Thin section analysis supports XRD work as it showed that biotite within the saprolite takes on a pale yellow color as a result of conversion to expandable clays while Mg hornblende retains its optical properties from corestone to saprolite. In saprolite, CIA values in the tonalite ranged from 52.9 to 57.5, in the mafic enclave from 57.0 to 57.5, and in the quartz diorite from 59.1 to 61.6. We interpret the above data to indicate that the onset of weathering in a Mediterranean climate like that at SMER is characterized by the removal of K mass as it is altered to expandable mixed-layer clay.