J. Brian Mahoney

High‐precision isotopic characterization of USGS reference materials by TIMS and MC‐ICP‐MS

The Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia has undertaken a systematic analysis of the isotopic (Sr, Nd, and Pb) compositions and concentrations of a broad compositional range of U.S. Geological Survey (USGS) reference materials, including basalt (BCR-1, 2; BHVO-1, 2), andesite (AGV-1, 2), rhyolite (RGM-1, 2), syenite (STM-1, 2), granodiorite (GSP-2), and granite (G-2, 3). USGS rock reference materials are geochemically well characterized, but there is neither a systematic methodology nor a database for radiogenic isotopic compositions, even for the widely used BCR-1. This investigation represents the first comprehensive, systematic analysis of the isotopic composition and concentration of USGS reference materials and provides an important database for the isotopic community. In addition, the range of equipment at the PCIGR, including a Nu Instruments Plasma MC-ICP-MS, a Thermo Finnigan Triton TIMS, and a Thermo Finnigan Element2 HR-ICP-MS, permits an assessment and comparison of the precision and accuracy of isotopic analyses determined by both the TIMS and MC-ICP-MS methods (e.g., Nd isotopic compositions). For each of the reference materials, 5 to 10 complete replicate analyses provide coherent isotopic results, all with external precision below 30 ppm (2 SD) for Sr and Nd isotopic compositions (27 and 24 ppm for TIMS and MC-ICP-MS, respectively). Our results also show that the first- and second-generation USGS reference materials have homogeneous Sr and Nd isotopic compositions. Nd isotopic compositions by MC-ICP-MS and TIMS agree to within 15 ppm for all reference materials. Interlaboratory MC-ICP-MS comparisons show excellent agreement for Pb isotopic compositions; however, the reproducibility is not as good as for Sr and Nd. A careful, sequential leaching experiment of three first- and second-generation reference materials (BCR, BHVO, AGV) indicates that the heterogeneity in Pb isotopic compositions, and concentrations, could be directly related to contamination by the steel (mortar/pestle) used to process the materials. Contamination also accounts for the high concentrations of certain other trace elements (e.g., Li, Mo, Cd, Sn, Sb, W) in various USGS reference materials.

Lithofacies control in detrital zircon provenance studies: Insights from the Cretaceous Methow basin, southern Canadian Cordillera

High-frequency sampling for detrital zircon analysis can provide a detailed record of fine-scale basin evolution by revealing the temporal and spatial variability of detrital zircon ages within clastic sedimentary successions. This investigation employed detailed sampling of two sedimentary successions in the Methow/Methow-Tyaughton basin of the southern Canadian Cordillera to characterize the heterogeneity of detrital zircon signatures within single lithofacies and assess the applicability of detrital zircon analysis in distinguishing fine-scale provenance changes not apparent in lithologic analysis of the strata. The Methow/ Methow-Tyaughton basin contains two distinct stratigraphic sequences of middle Albian to Santonian clastic sedimentary rocks: submarine-fan deposits of the Harts Pass Formation/Jackass Mountain Group and fluvial deposits of the Winthrop Formation. Although both stratigraphic sequences displayed consistent ranges in detrital zircon ages on a broad scale, detailed sampling within each succession revealed heterogeneity in the detrital zircon age distributions that was systematic and predictable in the turbidite succession but unpredictable in the fluvial succession. These results suggest that a high-density sampling approach permits interpretation of finescale changes within a lithologically uniform turbiditic sedimentary succession, but heterogeneity within fluvial systems may be too large and unpredictable to permit accurate fine-scale characterization of the evolution of source regions. The robust composite detrital zircon age signature developed for these two successions permits comparison of the Methow/MethowTyaughton basin age signature with known plutonic source-rock ages from major plutonic belts throughout the Cretaceous North American margin. The Methow/ Methow-Tyaughton basin detrital zircon age signature matches best with source regions in the southern Canadian Cordillera, requiring that the basin developed in close proximity to the southern Canadian Cordillera and providing evidence against large-scale dextral translation of the Methow terrane.

Forearc-basin sedimentary response to rapid Late Cretaceous batholith emplacement in the Peninsular Ranges of southern and Baja California

The eastern Peninsular Ranges batholith is dominated by voluminous La Posta–type tonalite-granodiorite intrusions that compose half of the magmatic arc at present erosion level. Zircon U-Pb and hornblende 40 Ar/ 39 Ar results from these intrusions indicate that they were emplaced in a remarkably narrow interval (99–92 Ma) that closely followed cessation of west-directed compression of the arc system. Emplacement of the La Posta suite coincided with a major pulse of coarse-grained sediment into the adjacent forearc basin in early Cenomanian to middle Turonian time. Paleontologic control, and plutonic age and detrital zircon U-Pb data demonstrate the virtual absence of a time lag between magma emplacement and sedimentary response. The tight linkage between magmatism, arc exhumation, and sediment delivery to the forearc indicates that development of major erosional topography in the arc was driven by thermal and mechanical effects associated with large-volume batholith emplacement.

Archean zircons in Cretaceous strata of the western Canadian Cordillera: The “Baja B.C.” hypothesis fails a “crucial test”

The magnitude of late Mesozoic terrane translation along the western North American continental margin is controversial. Paleomagnetic data suggest more than 3000 km of dextral displacement of the Insular superterrane and associated assemblages between ca. 74 and 50 Ma. Conversely, geologic evidence is more compatible with less than 500 km of offset during this time. Precambrian cratonic rocks older than 2.5 Ga are restricted to northern latitudes (above 40°N) in North America, and the presence or absence of Archean zircons within easterly derived Cretaceous strata has been proposed as a “crucial test” of the major translation hypothesis. There is no plausible source of Archean and Early Proterozoic zircons found in the Queen Charlotte, Nanaimo, and Methow basins other than the Canadian Shield or closely associated Proterozoic strata. The coexistence of Archean–Early Proterozoic, Middle Proterozoic (1.5–1.6 Ga) and Mississippian detrital zircon in these basins indicates deposition in northern latitudes, incompatible with the Baja B.C. hypothesis.

Magmatic evolution of the eastern coast Plutonic Complex, Bella Coola region, west-central British Columbia

The eastern Coast Plutonic Complex in west-central British Columbia (51–54°N) consists of a broad belt of Jurassic to Eocene granitic to dioritic intrusive rocks that collectively represent more than 140 m.y. of nearly continuous, subduction-related magmatism. Seven distinct intrusive suites have been identified in the Bella Coola region, including, from oldest to youngest, the Howe Lake, Stick Pass, Firvale, Desire, Fougner, Big Snow, and Four Mile suites. Pre–Late Cretaceous rocks were produced by episodic subduction-related magmatism characterized by partial melting of preexisting lower-arc crust, with minimal incorporation of evolved continental material. The eastern Coast Plutonic Complex experienced a magmatic lull between ca. 110 and 95 Ma, coincident with sinistral transpression potentially linked to translational displacement of the Wrangellia-Alexander composite terranes. Crustal thickening in Late Cretaceous time (ca. 100–90 Ma), possibly due to underplating of the Wrangellia-Alexander terrane, produced a deep crustal root (>40 km) that likely extended into the eclogite transition zone. Adakitic magmas (ca. 95–67 Ma) were produced by partial melting of an amphibolitic protolith in this thickened lower-arc crust. Adakitic plutons are pre-, syn-, and postkinematic with respect to Late Cretaceous high-angle shear zones, and they are coeval and spatially associated with postkinematic two-mica granites (ca. 70–75 Ma) having a hybrid A-type geochemistry. We suggest that delamination of this dense crustal root led to voluminous magmatism, extension, and crustal exhumation in Paleocene-Eocene time. The successive intrusive events in the eastern Coast Plutonic Complex may reflect a predictable evolutionary progression common to all continental arc systems.

Oriented Clastic Dike Swarms as Indicators of Paleoslope? An Example from the Upper Cretaceous Nanaimo Group, Canada

ABSTRACT An impressive oriented clastic dike swarm (> 165 dikes) occurs within 3.25 m of strata of a submarine-fan turbidite succession of the Upper Cretaceous Nanaimo Group, British Columbia. The dikes are remarkably parallel in orientation, are consistently subvertical, and strike southwest. Slump-fold vergence, paleocurrent indicators, and other evidence indicate that local paleoslope strike was northwest, with a downslope direction to the southwest. Thus the clastic dikes strike downslope, indicating that extension occurred perpendicular to the regional downslope direction, not parallel as generally expected. Two possible models are proposed: (1) Rapid downslope compression caused by sudden loading, perhaps due to slumping events or deposition of thick turbidity current beds. This downslope compression caused minor extension perpendicular to the downslope direction, resulting in small extensional joints that accommodated clastic injection from buried, overpressured sands. (2) Chaotic sedimentary breccias laterally adjacent to the dike swarm formed as part of a large slumping event, which created a local slump channel. Minor slumping into the main channel from the sides caused extension parallel to the channel-margin slope and formation of the clastic dikes parallel to the channel margin but perpendicular to the regional slope. Oriented clastic dike swarms may represent a new type of paleoslope indicator, but they may be created by a number of processes and should therefore be used only in conjunction with supporting evidence.