Description: Little is known about the benthic communities of the Arctic Ocean's slope and abyssal plains. Here we report on benthic data collected from box cores along a transect from Alaska to the Barents Abyssal Plain during the Arctic Ocean Section of 1994. We determined: (1) density and biomass of the polychaetes, foraminifera and total infauna; (2) concentrations of potential sources of food (pigment concentration and percent organic carbon) in the sediments; (3) surficial particle mixing depths and rates using downcore 210Pb profiles; and (4) surficial porewater irrigation using NaBr as an inert tracer. Metazoan density and biomass vary by almost three orders of magnitude from the shelf to the deep basins (e.g. 47 403 individuals m**-2 on the Chukchi Shelf to 95 individuals m**-2 in the Barents Abyssal Plain). Water depth is the primary determinant of infaunal density, explaining 39% of the total variability. Potential food concentration varies by almost two orders of magnitude during the late summer season (e.g. the phaeopigment concentration integrated to 10 cm varies from 36.16 mg m**-2 on the Chukchi Shelf to 0.94 mg m**-2 in the Siberia Abyssal Plain) but is not significantly correlated with density or biomass of the metazoa. Most stations show evidence of particle mixing, with mixing limited to 〈=3 cm below the sediment-water interface, and enhanced pore water irrigation occurs at seven of the nine stations examined. Particle mixing depths may be related to metazoan biomass, while enhanced pore water irrigation (beyond what is expected from diffusion alone) appears to be related to total phaeopigment concentration. The data presented here indicate that Arctic benthic ecosystems are quite variable, but all stations sampled contained infauna and most stations had indications of active processing of the sediment by the associated infauna.

Description: The tilted Wooley Creek batholith (Klamath Mountains, California, USA) consists of three main zones. Field and textural relationships in the older lower zone suggest batch-wise emplacement. However, compositions of augite from individual samples plot along individually distinct fractionation trends, confirming emplacement as magma batches that did not interact extensively. The younger upper zone is upwardly zoned from tonalite to granite. Major and trace element compositions of hornblende show similar variations from sample to sample, indicating growth from a single magma batch that was homogenized by convection and then evolved via upward percolation of interstitial melt. Highly porphyritic dacitic roof dikes, the hornblende compositions of which match those of upper zone rocks, demonstrate that the upper zone mush was eruptible. The central zone contains rocks of both lower and upper zone age, although in most samples hornblende compositions match those of the upper zone. The zone is rich in synplutonic dikes and mafic magmatic enclaves. These features indicate that the central zone was a broad transition zone between upper and lower parts of the batholith and preserves part of the feeder system to the upper zone. Homogenization of the upper zone was probably triggered by the arrival of mafic magma in the central zone. Continued emplacement of mafic magmas may have provided heat that permitted differentiation of the upper zone magma by upward melt percolation. This study illustrates the potential for use of trace element compositions and variation in rock-forming minerals to identify individual magma batches, assess interactions between them, and characterize magmatic processes.

Description: The Wooley Creek batholith is a tilted, zoned, calc-alkaline plutonic complex in the Klamath Mountains, northern California, USA. It consists of three main compositional-temporal zones. The lower zone consists of gabbro through tonalite. Textural heterogeneities on the scale of tens to hundreds of meters combined with bulk-rock data suggest that it was assembled from numerous magma batches that did not interact extensively with one another despite the lack of sharp contacts and identical ages of two lower zone samples (U-Pb [zircon] chemical abrasion–isotope dilution–thermal ionization mass spectrometry ages of 158.99 ± 0.17 and 159.22 ± 0.10 Ma). The upper zone is slightly younger, with 3 samples yielding ages from 158.25 ± 0.46 to 158.21 ± 0.17 Ma, and is upwardly zoned from tonalite to granite. This zoning can be explained by crystal-liquid separation and is related to upward increases in the proportions of interstitial K-feldspar and quartz. Porphyritic dacitic to rhyodacitic roof dikes have compositions coincident with evolved samples of the upper zone. These data indicate that the upper zone was an eruptible mush that crystallized from a nearly homogeneous parental magma that evolved primarily by upward percolation of interstitial melt. The central zone is a recharge area with variably disrupted synplutonic dikes and swarms of mafic enclaves. Central zone ages (159.01 ± 0.20 to 158.30 ± 0.16 Ma) are similar to both lower and upper zones crystallization ages. In the main part of the Wooley Creek batholith, age data constrain magmatism to a short period of time (〈1.3 m.y.). However, age data cannot be used to identify distinct magma chambers within the batholith; such information must be extracted from a combination of field observations and the chemical compositions of the rocks and their constituent minerals.

Description: The presence of ca. 1.63 Ga monzogranite (the "white quartz monzonite") in the southern Sierra Madre, southeastern Wyoming, is anomalous given its distance from the nearest documented plutons of similar age (central Colorado) and the nearest contemporaneous tectonic margin (New Mexico). It is located immediately south of the Cheyenne belt—a ca. 1.75 Ga Archean-Proterozoic tectonic suture. New geochronological, isotopic, and geochemical data suggest that emplacement of the white quartz monzonite occurred between ca. 1645 and 1628 Ma (main pulse ca. 1628 Ma) and that the white quartz monzonite originated primarily by partial melting of the Big Creek Gneiss, a modified arc complex. There is no evidence that mafic magmas were involved. Open folds of the ca. 1750 Ma regional foliation are cut by undeformed white quartz monzonite. On a regional scale, rocks intruded by the white quartz monzonite have experienced higher pressure and temperature conditions and are migmatitic as compared to the surrounding rocks, suggesting a genetic relationship between the white quartz monzonite and tectonic exhumation. We propose that regional shortening imbricated the Big Creek Gneiss, uplifting the now-exposed high-grade rocks of the Big Creek Gneiss (hanging wall of the thrust and wall rock to the white quartz monzonite) and burying correlative rocks, which partially melted to form the white quartz monzonite. This tectonism is attributed to the ca. 1.65 Ga Mazatzal orogeny, as foreland shortening spread progressively into the Yavapai Province. Mazatzal foreland effects have also been described in the Great Lakes region and have been inferred in the Black Hills of South Dakota. We suggest that the crustal-scale rheologic contrast across the Archean-Proterozoic suture, originally developed along the southern margin of Laurentia, and including the Cheyenne belt, facilitated widespread reactivation of that boundary during the Mazatzal orogeny. This finding emphasizes the degree to which crustal heterogeneities can localize subsequent deformation in accretionary orogens, producing significant crustal melting in the distal foreland—a region not typically associated with orogenic magmatism.

Description: In the central Klamath Mountains, the English Peak plutonic complex (EPC) invaded the faulted contact between the outboard Eastern Hayfork and inboard North Fork terranes of the Western Paleozoic and Triassic Belt (WTrPz). This calc-alkaline igneous complex is composed of two small, ~1–2-km-diameter, relatively mafic satellitic plutons peripheral to the younger, much larger, ~10–15-km-diameter English Peak zoned granitic pluton. The EPC magmas were mantle derived and reflect temporary residence and mixing at various depths in the overlying crust, with initial storage and modification near the Moho, and uppermost crustal emplacement at 5–10 km depths. Phase assemblages suggest pre-emplacement magma storage at a depth of ~20–25 km for the early satellitic plutons, versus ~15–20 km for samples from the larger zoned granitic pluton. We obtained zircon U-Pb geochronologic results (reported as internal and external weighted-mean 207 Pb-corrected 206 Pb/ 238 U ages, 95% confidence level) from seven samples in the complex via laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). The 172.3 ± 2.0 [3.7] Ma Uncles Creek and 166.9 ± 1.6 [3.4] Ma Heiney Bar satellitic plutons range from gabbro–quartz diorite to granodiorite in bulk-rock composition. The main English Peak pluton consists of an early stage of gabbro-tonalite (three samples: 160.4 ± 1.1 [3.1] Ma, 158.1 ± 1.1 [3.1] Ma, and 158.0 ± 1.2 [3.1] Ma) and a late stage (two samples: 156.3 ± 1.3 [3.1] Ma and 155.3 ± 1.2 [3.0] Ma) passing inward from tonalite through granodiorite to a central zone of granite. The 172 Ma age of the Uncles Creek pluton makes it coeval with Middle Jurassic Western Hayfork arc magmatism. In contrast, Heiney Bar and the main English Peak igneous ages overlap some of the oldest and youngest components, respectively, of the Middle to Late Jurassic Wooley Creek plutonic suite. Study of this multiple-intrusion complex provides an illuminating example of the gradual intermediate-to-felsic modification of the upper crust in the central Klamath Mountains. Inherited zircon ages of ca. 172 Ma in two other EPC samples indicate potential Middle Jurassic crustal sources or contaminants. Geochronologic correlation of the EPC with geologic histories of other Klamath terranes provides fresh insights for understanding spatial and temporal elements of Middle to Late Jurassic arc magmatism in the Klamath Mountains sector of the Cordilleran margin. This igneous activity illuminates some petrotectonic processes whereby accreted ophiolitic basement terranes were modified and incorporated into the evolving Jurassic continental crust. It took place prior to the earliest Cretaceous onset of westward transport of the stack of Klamath allochthons relative to the active Jura-Cretaceous Sierran calc-alkaline arc.