William A. Crawford

U-Th-Pb geochronology of the Coast Mountains batholith in north-coastal British Columbia: Constraints on age and tectonic evolution

Previously published and new U-Pb geochronologic analyses provide 313 zircon and 59 titanite ages that constrain the igneous and cooling history of the Coast Mountains batholith in north-coastal British Columbia. First-order findings are as follows: (1) This segment of the batholith consists of three portions: a western magmatic belt (emplaced into the outboard Alexander and Wrangellia terranes) that was active 177–162 Ma, 157–142 Ma, and 118–100 Ma; an eastern belt (emplaced into the inboard Stikine and Yukon-Tanana terranes) that was active ca. 180–110 Ma; and a 100–50 Ma belt that was emplaced across much of the orogen during and following mid-Cretaceous juxtaposition of outboard and inboard terranes. (2) Magmatism migrated eastward from 120 to 80 (or 60) Ma at a rate of 2.0–2.7 km/Ma, a rate similar to that recorded by the Sierra Nevada batholith. (3) Magmatic flux was quite variable through time, with high (>35–50 km 3 /Ma per km strike length) flux at 160–140 Ma, 120–78 Ma, and 55–48 Ma, and magmatic lulls at 140–120 Ma and 78–55 Ma. (4) High U/Th values record widespread growth (and/or recrystallization) of metamorphic zircon at 88–76 Ma and 62–52 Ma. (5) U-Pb ages of titanite record rapid cooling of axial portions of the batholith at ca. 55–48 Ma in response to east-side-down motion on regional extensional structures. (6) The magmatic history of this portion of the Coast Mountains batholith is consistent with a tectonic model involving formation of a Late Jurassic–earliest Cretaceous magmatic arc along the northern Cordilleran margin; duplication of this arc system in Early Cretaceous time by >800 km (perhaps 1000–1200 km) of sinistral motion (bringing the northern portion outboard of the southern portion); high-flux magmatism prior to and during orthogonal mid-Cretaceous terrane accretion; low-flux magmatism during Late Cretaceous–Paleocene dextral transpressional motion; and high-flux Eocene magmatism during rapid exhumation in a regime of regional crustal extension.

Terrane assembly and structural relationships in the eastern Prince Rupert quadrangle, British Columbia

The eastern part of the Prince Rupert quadrangle, British Columbia, is subdivided into two regions: the western metamorphic belt and the central belt. The western metamorphic belt is underlain by five distinct mappable rock sequences. From west to east these are the Digby, Venn, Delusion Bay, Kaien, and Tsimpsean sequences. Rocks of the Digby sequence seem to correlate with Triassic and upper Paleozoic rocks of the Alexander terrane; the overlying Venn sequence contains rocks of the same age as parts of the Gravina belt. The Tsimpsean sequence is considered to be part of the Yukon-Tanana terrane, a continental margin assemblage. Correlation of the two intervening sequences, Kaien and Delusion Bay, is uncertain; they may belong either to the Taku terrane or to the Yukon-Tanana terrane. We tend to favor the latter interpretation. North- to northwest-striking thrust faults approximately bisect the belt and separate the Digby and Venn sequences on the west from the structurally overlying units to the east. Near Prince Rupert, west-directed, probably mid-Cretaceous, thrusting accompanied emplacement of tonalite and basalt dikes and sills. These thrusts and others to the north juxtapose upper amphibolite facies rocks over greenschist facies schist. The northern thrusts appear to postdate the regional metamorphism. Above and below the thrusts the rocks show intense ductile deformation. The Prince Rupert shear zone, east of the thrusts, records a strong flattening deformation, possibly due to overthrusting by the 91 Ma Ecstall pluton and associated high-grade gneiss. This tectonic emplacement of hot rocks and the associated igneous activity represented by syntectonic tonalite sills emplaced in the shear zone are inferred to be the cause of the 90 Ma amphibolite facies metamorphism that underlies the Prince Rupert shear zone. The western metamorphic belt is bounded on the east by the Coast shear zone, which separates the western belt from the central belt. This shear zone evolved during emplacement of 65-52 Ma plutons of the Paleogene Coast Mountains batholith that underlie much of the central belt. As the Coast Mountains batholith was emplaced, strain within the arc changed from dominantly contractional normal to the batholith and to the orogen, to extensional parallel to the length of the batholith. The latter stages of batholith emplacement apparently accompanied batholith exhumation. The youngest part of the Coast shear zone is a 1-2-km-wide zone of vertical fabric along the western side of the shear zone and is at the westernmost extent of 85-50 Ma plutons. This Work-Behm shear zone formed during the late stages of uplift of the western metamorphic belt by westward tilting about a northwest-trending hinge. After a hiatus with no identified tectonic activity other than gradual exhumation, Miocene and younger felsic and mafic dikes and young brittle faults cut the rocks throughout the eastern part of the quadrangle.

Magmatism and deformation, southern Revillagigedo Island, southeastern Alaska

The mid-Cretaceous, cale-alkaline, LREE-enriched plutons on southern Revillagigedo Island, southeastern Alaska, form two distinct groups: (1) biotite-epidote-hornblende tonalite of the Moth Bay pluton and (2) biotite-bearing, hornblende-absent leucotonalite. Higher total REE and lower Na 2 O, Al 2 O 3 , and Sr distinguish the Moth Bay tonalite from the leucotonalite. The plutons are surrounded by narrow contact metamorphic aureoles containing kyanite and staurolite. Pressure estimates for emplacement based on metamorphic conditions of contact rocks and from igneous amphibole compositions are ∼8-9 kbar. Detailed analyses of pluton/host rock relationships indicate that pluton emplacement was synchronous with orogen deformation. Pre-emplacement thrusting placed the metavolcanic and metasedimentary rocks of the Taku terrane structurally above the younger metasedimentary rocks of the Gravina belt. The contact between these units was subsequently cut by the Moth Bay pluton. The southern margin of the Moth Bay pluton has been transformed to blastomylonite. Contact metamorphic textures and undeformed dikes which cut the blastomylonite indicate that shearing did not outlast crystallization of the pluton. A still later phase of deformation transposed the pluton-related structures and generated a northeast-dipping, ductile thrust fault (Southern Revillagigedo shear zone) along the southern shore of Revillagigedo Island. The shear zone and earlier structures all trend approximately northwest. Fission-track age determinations and track-length measurements in apatite were used to determine the cooling histories of the plutonic units. Fission track ages from the western part of the study area are 51.3 ± 4.0 Ma (apatite), 57.8 ± 4.4 Ma (zircon), and 69.8 ± 4.2 Ma (sphene); eastward, toward the Coast batholith, fission track ages are ∼10 m.y. younger. These younger ages may result from a tectonic/thermal event associated with batholith emplacement. This same event may have generated north-trending structures that overprint older northwest-trending structures on the east side of the study area.

Glauconitic greensand: A possible filter of heavy metal cations from polluted waters

Experiments using distilled water, spiked with heavy metal cations and passed through a filtration system composed of greensand containing 80% glauconite at a rate of 2–4 ml/min, showed: (1) starting fluids containing Cd, Co, Cu, Pb, Mn, Ni, Ag, or Zn had an average of 90% of the contaminating cation removed from acidic solutions and an average of 84% removed from the basic solutions; and, (2) filtration through greensand tended to neutralize both acidic and basic solutions. The removal of the contaminant cation from starting fluids containing K, Na, Ca, Mg, Fe, Si, or Al (the principal constituents of glauconite plus Ca from shell material) is not as consistently effective as for the first named metals.

Removal of contaminants from landfill leachates by filtration through glauconitic greensands

Passing landfill leachate through glauconitic greensand filters reduces the heavy metal cation content, lessens the unpleasant odor, and diminishes the murkiness of the leachate. The capability of the greensand to trap metal cations is increased by prolonging the contact time between the leachate and the greensand. Flushing the charged greensand filter with water does not cause significant release of cations back into solution, suggesting that polluted greensand might be disposed of at landfill sites without endangering the quality of either ground or surface water.