ABSTRACT

The Peninsular Ranges of southern California and Baja California Norte, Mexico, represent an uplifted and variably dissected Mesozoic magmatic arc. They are composed mostly of Cretaceous plutonic rocks and Mesozoic greenschist to amphibolite facies metasedimentary and metavolcanic rocks. In order to evaluate the reliability of the diamond-shaped Q_nuQ_uQ_p provenance-discrimination diagram developed by Basu et al. (1975) and the QFL provenance-discrimination diagram developed by Dickinson et al. (1983), we collected 62 samples of Holocene sand from four drainage basins along the west side of the Peninsular Ranges. The QFL composition of each specimen was determined by the Gazzi-Dickinson point-counting method; the relative percentages of med um-grained fragments of Q_nu, Q_u, and Q_p were determined by counting 100 grains in each specimen. The results of our study indicate that Holocene sands derived from the Peninsular Ranges have the following average petrological characteristics:

1) Sands derived mostly or exclusively from plutonic rocks are composed of 16-46 percent Q, 54-83 percent F, and 0.2-0.5 percent L. The medium-grained quartz population produced by erosion of mostly or exclusively plutonic rocks consists of 61-77 percent Q_nu, 4-17 percent Q_u, and 13-27 percent Q_p.

2) Sands originating from the erosion of metasedimentary rocks are composed of 63 percent Q, 13 percent F, and 24 percent L. The medium-grained quartz population derived from a metasedimentary source is made up of 25 percent Q_nu, 9 percent Q_u, and 66 percent Q_p.

3) Sands produced by the erosion of mostly or exclusively metavolcanic rocks are composed of 19 percent Q, 39-44 percent F, and 38-43 percent L. The medium-grained quartz population originating from a mostly or exclusively metavolcanic source consists of 48-74 percent Q_nu, 6 percent Q_u, and 20-46 percent Q_p.

Petrological data obtained during this study indicate that the QFL provenance-discrimination model generally yields a correct interpretation of tectonic setting when applied to Holocene sand from the Peninsular Ranges. Data, however, also show that local source areas produce sands that, when interpreted on the basis of their position on the QFL provenance-discrimination diagram, yield an erroneous interpretation of tectonic setting.

Data obtained from counting varieties of medium-grained quartz show that detrital quartz populations produced by plutonic rocks are discriminated from those derived from metasedimentary rocks by the Q_nuQ_uQ_p provenance-discrimination diagram. In contrast, quartz populations originating from a mostly or exclusively metavolcanic source are difficult to interpret on the Q_nuQ_uQ_p diagram. Therefore, we recommend that the QFL provenance-discrimination model be used with caution when sand or sandstone samples are from a limited geographic area and that the Q_nuQ_uQ_p provenance-discrimination diagram be used with prudence when metavolcanics are suspected in the provenance.