Charles Knaack

U‐Pb dating of zircon by LA‐ICP‐MS

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 μm in diameter. Laser-induced 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.

Assessing Differences in Composition between Low Metamorphic Grade Mudstones and High-Grade Schists Using Logratio Techniques

Based on stratigraphic and petrologic similarities, it has been proposed that rocks exposed in roof pendants of the central Sierra Nevada, California are part of a tectonically displaced fragment of the Cordilleran miogeocline. We have identified significant geochemical similarities between metamudstones making up this miogeoclinal fragment and rocks comprising its proposed parents in the Mojave Desert-southern Great Basin region by geochemical analysis of samples collected from Snow Lake and Boyden Cave roof pendants, and from the Precambrian to Cambrian section of the Cordilleran miogeocline exposed in the Nopah Range, southeastern California. In order to circumvent the constant-sum problem inherent in geochemical data, the data were transformed into a continuous-variable format using logratio techniques. When the differences in means of aluminum-normalized logratioed data derived from the Snow Lake-Boyden Cave data set and the Nopah Range samples were calculated on an element by element basis, ~86% of the differences were not statistically significant at the 95% confidence level. The large number of compositional similarities identified between Snow Lake, Boyden Cave, and Nopah Range samples are consistent with the idea that the metasedimentary rocks of the central Sierra Nevada batholith are composed of material shed from the western North American interior and that roof pendants in the central Sierra Nevada are part of a displaced miogeoclinal fragment. This work demonstrates that rigorous statistical analyses of geochemical data transformed into a continuous-variable format can be a useful tool in evaluating the plausibility of lithostratigraphic correlations between metamorphosed, complexly deformed, and displaced metasedimentary rocks.

Provenance of Paleozoic mudstones in a contact metamorphic aureole determined by rare earth element, Th, and Sc analyses, Sierra Nevada, California

Rare earth element (REE), Th, and Sc analyses of mudstones from the contact metamorphic aureole of the Middle Jurassic Emigrant Gap composite pluton support the idea that the lower Paleozoic Lang, Black Oak Spring, and Zion Hill sequences are composed of detritus derived from a continental landmass. On REE distribution diagrams, our samples exhibited (1) no systematic change as a result of varying metamorphic grade, (2) light-REE enrichment trends, and (3) Eu anomalies that vary from 0.46 to 0.93 and average 0.66. On a La-Th-Sc ternary diagram, our data cluster within the fields of passive-margin sediment and post-Archean Australian shale (PAAS). These new data—in conjunction with the quartz-rich character of sandstones, the absence of volcanic material, and Precambrian detrital zircon ages—suggest that clastic material in the Lang, Black Oak spring, and Zion Hill sequences was derived from a continental landmass that may have been North America or, alternatively, some continental fragment oceanward of early Paleozoic North America.