PETROLOGICAL AND CHEMICAL PROPERTIES OF CORESTONE AND SAPROLITE, SANTA MARGARITA ECOLOGICAL RESERVE (SMER), CALIFORNIA: EVALUATING THEIR ORIGIN

Quartz diorite and tonalite underlie large areas of SMER. 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. Here we report our initial steps into evaluating how saprolite develops from corestone. We collected 8 samples from a quartz diorite corestone-saprolite pair and 18 from a tonalite corestone-saprolite pair along ~1 m traverses. Samples were analyzed for geochemistry, grain and bulk density, and petrology. Assuming that the mass of Ti is conserved, the transport function (t) for samples collected from the tonalite corestone-saprolite pair suggests that masses of Si and Al remain relatively unchanged, but are increased within the rind and a few centimeters into the saprolite. Ca (8-25%), Na (6-25%), K (16-43%), and P (21-52%) are significantly depleted throughout the saprolite relative to the corestone. As might be expected Sr and Ba are depleted following trends established by Ca and Na. However, Rb exhibits episodic variation ranging from -6% to 6% with respect to the corestone. Within saprolite t values for Fe range from 1-3%. In contrast, Mg and Mn are generally depleted with  ranging from 3-8% and 2-6%, respectively. For samples collected from the quartz diorite corestone-saprolite pair t-values indicate that Si, Al, Ca, Na, and Sr are preserved across the traverse. However, within the rind masses of these elements have been increased as indicated by t values of 15%, 14%, 21%, 17%, and 15% respectively. In contrast, masses of K, Rb, and Ba are depleted within the rind and saprolite by as much as 40%, 40%, and 25% respectively. Fe (5%), Mg (8%), Mn (12%), and P(6%) are increased within the rind but are otherwise unchanged between corestone and saprolite. Our data suggest that plagioclase is the most susceptible to chemical weathering during the genesis of saprolite from corestone. It is preferentially attacked by solutions which remove Ca and Na from saprolite. Our preliminary attempts at measuring in situ bulk density suggest that volume strains are significantly increased within the saprolite, a result that is contrary to the commonly assumed isovolumetric development of saprolite.