The post-Cambrian and pre-Upper Devonian Shoo Fly Complex represents the remnants of an ancient subduction system. The Quartz Hill chert, a thrust-fault-bounded, chlorite-grade greenschist facies unit, is included in the Shoo Fly Complex, and consists of about 25 m of interstratified radiolarite and argillite. Rare earth element (REE) data derived from 15 samples indicate that the Quartz Hill chert contains two chemically distinct groups of rock. Six Group 1 specimens display relatively flat REE/PAAS (post-Archean average Australian shale) distribution patterns, no Ce anomaly, and a variable positive Eu anomaly. In contrast, nine Group 2 specimens exhibit no Ce anomalies, a variable positive Eu anomaly, and display REE/PAAS values that increase from La to Eu, and then decrease from Eu to Lu. The REE, Th, and Sc characteristics of Group 1 samples are like those in Cretaceous, Tertiary, and Quaternary marine sediments containing particles derived from magmatic arcs. In contrast, the REE, Th, and Sc characteristics of Group 2 specimens are suggestive of a mixture of magmatic arc material and alkaline basaltic particulate matter derived from a seamount or ocean island. Data presented here are consistent with the results from previous petrological and geochemical studies of rocks in the Shoo Fly Complex, and indicate that the Quartz Hill chert was deposited on the margin of an oceanic plate adjacent to a magmatic arc and a seamount(s) or ocean island(s). Thus, the data here and in the literature suggest that the REE, Th, and Sc characteristics of chert/argillite sequences deposited in or adjacent to active subduction systems are controlled primarily by source rocks in adjacent magmatic arcs, and in seamounts or ocean islands located within subducting plates.

The Peninsular Ranges of southern and Baja California are divided into a western, predominantly magnetite-bearing plutonic subprovince and an eastern, predominantly magnetite-free plutonic subprovince. The boundary that separates the two subprovinces corresponds roughly to the southwestern margin of the La Posta superunit, but in some places extends into the La Posta granitic province. Neither the pre–La Posta foliated granitic rocks nor the garnet- or muscovite-bearing rocks of the eastern Peninsular Ranges contain magnetite. The magnetite/ilmenite distinction occurs on three scales: regional variations that appear to be independent of host rock or individual plutons, variations paralleling modal facies within zoned plutons, and contact loss of magnetite in the outer margin of a pluton (from meters to more than a kilometer in width). Observations to date indicate that the regional distribution of magnetite- and ilmenite-series granitic rocks may result from generation of parental magma within the dehydration zone of a subduction plane. The gradation within zoned plutons probably results from a lowering of oxygen fugacity in the magma during progressive crystallization. The contact effect appears to be a consequence of reactions between the cooling pluton, the host rocks, and water-rich fluids from a variety of sources.