[1] In this study we used LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) to determine U-Pb ages of 5 zircon samples of known age ($1800 Ma to $50 Ma) in order to determine the reproducibility, precision, and accuracy of this geochronologic technique. This work was performed using a ThermoFinnigan Element2 magnetic sector double-focusing ICP-MS coupled with a New Wave Research UP-213 laser system. The laser ablation pit sizes ranged from 30 to 40 mm in diameter. Laserinduced time-dependent fractionation is corrected by normalizing measured ratios in both standards and samples to the beginning of the analysis using the intercept method. Static fractionation, including those caused during laser ablation and due to instrumental discrimination, is corrected using external zircon standards. Total uncertainty for each laser analysis of an unknown is combined quadratically from the uncertainty in the measured isotope ratios of the unknown and the uncertainty in the fractionation factors calculated from the measurement of standards. For individual analyses we estimate that the accuracy and precision are better than 4% at the 2 sigma level, with the largest contribution in uncertainty from the measurement of the standards. Accuracy of age determinations in this study is on the order of 1% on the basis of comparing the weighted average of the LA-ICP-MS determinations to the TIMS ages. Due to unresolved contributions to uncertainty from the lack of a common Pb correction and from potential matrix effects between standards and unknowns, however, this estimate cannot be universally applied to all unknowns. Nevertheless, the results of this study provide an example of the type of precision and accuracy that may be possible with this technique under ideal conditions. In summary, the laser ablation technique, using a magnetic sector ICP-MS, can be used for the U-Pb dating of zircons with a wide range of ages and is a useful complement to the established TIMS and SHRIMP techniques. This technique is especially well suited to reconnaissance geochronologic and detrital zircon studies.

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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.

The post-Cambrian and pre-Upper Devonian Shoo Fly Complex represents the remnants of an ancient subduction system. The Quartz Hill chert, a thrustfault-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.

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