U-Pb-Hf characterization of the central Coast Mountains batholith: Implications for petrogenesis and crustal architecture

We present U-Pb geochronologic and Hf isotopic data from 29 plutonic samples within the Coast Mountain batholith, 
north-coastal British Columbia and southeast Alaska. Hf isotopic values do not correlate with age or variation in magmatic flux, but rather they increase systematically from west (e_(Hf)[t] = +2 to +5) to east (e_(Hf)[t] = +10 to +13) in response to changing country rock assemblages. By comparing our pluton Hf data with previously reported Nd-Sr and detrital zircon characteristics of associated country rocks, we identify three crustal domains in an area where crustal affinity is largely obscured by metamorphism and voluminous pluton intrusion: (1) a western domain, emplaced into continental-margin strata of the Banks Island assemblage; (2) a central domain, emplaced into the Alexander terrane; and (3) an eastern domain, underlain by the Stikine terrane and its inferred metamorphic equivalents. Between the interpreted Alexander and Stikine terranes, there is a zone of variable e_(Hf)(t) 
(+2 to +13) that coincides with the suture zone separating inboard (Stikine and Yukon-Tanana) from outboard (Alexander and associated) terranes. This variation in e_(Hf)(t) values apparently results from the structural imbrication of juvenile (Alexander and Stikine) and evolved (Yukon-Tanana) terranes along mid-Cretaceous thrust faults and the latest Cretaceous–early Tertiary Coast shear zone. Shifts in the Hf values of plutons across inferred terranes imply that they are separated at lower- to midcrustal levels by steep boundaries. Correlation between these Hf values and the isotopic character of exposed country rocks further implies the presence of those or similar rocks at magma-generation depths.

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title: Tectonic accretion and the origin of the two major metamorphic and plutonic welts in the Canadian Cordillera
abstract: The Omineca Crystalline Belt and Coast Plutonic Complex are the two major regional tectonic welts in the Canadian Cordillera in which were concentrated intense deformation, regional metamorphism, granitic magmatism, uplift, and erosion. The welts, which formerly were thought to result from subduction of Pacific Ocean lithosphere beneath the western edge of North America, can now be viewed partly as the result of tectonic overlap and/or compressional thickening of crustal rocks during collisions between North America and two large, composite, allochthonous terranes that were accreted to its ancient western margin. The inner composite terrane, Terrane I, includes four smaller terranes that apparently were together by the end of Triassic time. The outer composite terrane, Terrane II, comprises two terranes, amalgamated by Late Jurassic time. The Omineca Crystalline Belt formed mainly from mid-Jurassic time onward, during and following the collision of Terrane I with North America. This belt straddles the zone of overlap of autochthonous and allochthonous terranes, and its characteristic metamorphism and structure are superimposed on both. The Coast Plutonic Complex formed mainly in Cretaceous to early Tertiary time during and following the attachment of Terrane II to the new, Jurassic, continental margin. It lies along the boundary of Terrane I and Terrane II and involves elements of both terranes. The collisions took place within the overall setting of the North American plate moving relatively westward into various Pacific plates from Jurassic time onward and in conjunction with subduction of Pacific Ocean lithosphere.

title: Nd isotopic characterization of metamorphic rocks in the Coast Mountains, Alaskan and Canadian Cordi
abstract: Nd isotopic data are reported for 52 samples from the crustal region between the Alexander-Wrangellia terrane and the Stikine terrane of the Alaskan and Canadian Cordillera. This region is composed of the Gravina belt, a Jurassic-Cretaceous assemblage of volcanic and clastic sedimentary rocks, the Taku terrane, a terrane of probable Early Permian to Late Triassic age, and four assemblages of metamorphic rocks that occur to the west of and within the Coast Mountains batholith. The Gravina belt has eNd(T) values that range from −1.1 to +8.3, similar to values of the underlying Alexander terrane, and consistent with the interpretation that it is a juvenile belt that formed in a back-arc or intra-arc basin within the Alexander terrane. Mid-Cretaceous plutons that were emplaced into the Gravina belt have eNd(T) values of +4.4 to +5.7 and were probably produced by mantle-derived melts that incorporated some Alexander terrane crust. The Taku terrane has eNd(0) values that range from −5.5 to +3.3, with corresponding depleted-mantle model (TDM) ages of 440 to 1430 Ma. A mid-Cretaceous pluton intruding the Taku terrane has an eNd(T) value of +5.1, a value indistinguishable from those determined for Cretaceous plutons intruding the Gravina belt. Metamorphic rocks east of and structurally overlying the Taku terrane are divided into the Tracy Arm assemblage, eNd(0)=−26 to 0, TDM=800–2450 Ma; the Endicott Arm assemblage, eNd(0)=−10 to −1.3, TDM=950–1500 Ma; the Port Houghton assemblage, eNd(0)=−9.4 to +1.1, TDM = 550–1500 Ma; and the Ruth assemblage, eNd(0) = −9.4 to +2.0, TDM=650–1300 Ma. These isotopic signatures indicate that a substantial component of each metamorphic assemblage was derived from Precambrian continental crust. The metamorphic rocks from these assemblages are lithologically very similar to rocks of the Yukon-Tanana (YTT) terrane of eastern Alaska and Yukon Territory and have such similar U-Pb detrital zircon ages and Nd isotopic compositions to YTT rocks that they are considered part of that terrane. Possible tectonic scenarios that can explain the present geometry of the YTT with respect to the Alexander-Wrangellia and Stikine terranes include: (1) The YTT is the upturned stratigraphic basement of the Stikine terrane, (2) part of the YTT was structurally emplaced beside the Stikine terrane in a transpressive tectonic regime, (3) the Stikine terrane and other inboard terranes are huge sheets that were thrust over the margin of the YTT before the final accretion of the Alexander-Wrangellia terrane.

title: Yukon-Tanana Terrane in the Scotia-Quaal Belt, Coast Plutonic Complex, central-western British Columbia
abstract: The Scotia-Quaal belt of amphibolite facies metamorphic rocks is in the core of the Coast Plutonic Complex southeast of Prince Rupert, inboard of the Alexander terrane and outboard of the westernmost documented extension of the Stikine terrane. Correlation of the Scotia-Quaal assemblage with continental margin strata of the Yukon-Tanana terrane is based primarily on lithologic grounds, including the presence of abundant quartz-rich sedimentary rocks and Middle Devonian orthogneiss associated with metavolcanic rocks hosting massive sulfide deposits. Evolved Nd and Sr isotopic signatures in the Scotia-Quaal belt also support the correlation. Dissimilarities in age, lithology, and isotopic signatures differentiate the Scotia-Quaal units from those of the Wrangellia, Alexander, and Taku terranes. The Scotia-Quaal belt shows some similarities with the Stikine terrane, mainly the presence of Devonian igneous rocks, suggesting that part of the Scotia-Quaal belt is correlative with the Stikine terrane. However, the isotopic and lithological differences make it unlikely that all of the Scotia-Quaal rocks should be included in the Stikine terrane. The Scotia-Quaal belt is part of a discontinuous belt of Proterozoic to lower Paleozoic continental margin strata overlain and intruded by middle to upper Paleozoic arc-type rocks; the belt extends longitudinally through the Coast Plutonic Complex from the Yukon Territory to at least as far south as lat 52°N. If the Scotia-Quaal belt contains some Stikine terrane rocks, then accretion of continental margin rocks to Stikine terrane occurred prior to Early Jurassic time. The possible presence of Stikine strata in the Scotia-Quaal belt or of continental-margin assemblages in the Central Gneiss Complex also suggests that the Work Channel structural zone may not mark the boundary between the Insular superterrane and the Stikine terrane. Even if the Stikine terrane strata are absent from the Scotia-Quaal area, the presence of Early Jurassic plutons hints at an Early Jurassic or older link between the Scotia-Quaal and the Yukon-Tanana assemblage and the Stikine terrane.

title: The Central Gneiss Complex, Coast Mountains, British Columbia
abstract: The Central Gneiss Complex of British Columbia, between lat 54° and 55°N, consists mainly of orthogneiss, rusty-weathering migmatite, leucogneiss, amphibolite, and minor calc-silicate. It forms the country rock for the large intrusions of tonalite and granodiorite that constitute the Coast Mountains batholith. With the exception of calc-silicates, all lithologies intruded by the Paleogene plutons have been partially melted to varying degrees. Metamorphic grade within the complex varies from upper amphibolite facies to granulite facies. The granulite facies rocks are in close proximity to the margins of the large tonalite-granodiorite plutons, suggesting that the plutons locally heated gneisses to the granulite facies. The geologic structure of the Central Gneiss Complex is the result of sequential regional deformations related to the emplacement of voluminous tonalite-granodiorite plutons. Much of the structure can be interpreted to be due to progressive deformation during dextral transpression, with a final imprint produced by flattening during extension. Kilometer-scale tight to isoclinal folds with steeply east-dipping axial planes and shallow to moderately north-plunging hinges were formed during east-side-up shearing with a dextral component along the eastern side of the complex. Transfer of strain from the eastern side of the complex to the western side was accommodated by top-to-the-west shearing that produced reclined isoclinal folds with east-plunging fold axes and shallowly north-dipping axial planes that constitute a flat between two ramps. A domain of steeply dipping foliation dominates the western side of the complex and is cored by the 58.6 Ma Quottoon pluton. Dextral strike-slip shearing occurred in the country rock along the eastern side of Quottoon pluton, and east-side up shearing occurred within the Quottoon pluton. The Quottoon pluton intruded late during transpressive deformation. Intrusion of the Kasiks sill (53 Ma) resulted in flattening of the underlying gneisses, and the interference pattern of this late flattening with foliations produced by the earlier transpressive deformation produced a foliation triple point to the south of the sill. The occurrence of a distinctive lithologic package crossing the Central Gneiss Complex restricts any major zones of strike-slip displacement to be located along its eastern side or along its western side; dextral strike slip strains are recorded along the western side. These features suggest that if the Baja British Columbia fault system passes through the Central Gneiss Complex, it is likely to be where the Quottoon pluton is currently located.

title: Nd and Sr Isotopic Characterization of the Wrangellia Terrane and Implications for Crustal Growth of the Canadian Cordillera
abstract: Nd and Sr isotopic data are reported from samples of most of the major lithologic units of the Wrangellia terrane exposed on Vancouver Island and the Queen Charlotte Islands. \epsilon_{Nd}$$ values range from +1.0 to + 7.3 and initial $$^{87}Sr/^{86}Sr$$ ratios range from 0.70323 to 0.70481. These ratios are similar to those of modern island arcs and demonstrate the unevolved, juvenile nature of this terrane. These data indicate that Wrangellia probably resided in an intra-oceanic environment until its accretion to North America. Nd crustal residence ages for clastic sedimentary rocks of Wrangellia (mean $$T_{CR} = 0.77 \pm 0.33 Ga, T_{DM} = 0.57 \pm 0.35 Ga$$) are much closer to their stratigraphic ages than is the case for the ages of many other Phanerozoic sedimentary rocks, thus providing strong evidence for Phanerozoic crustal addition to North America. Early Proterozoic crustal components cannot comprise more than about 6% of Wrangellia; therefore the terrane consists of -94% new, mantle-derived crust. The amount of new crust in Wrangellia is very similar to the amount determined for two other large Cordilleran terranes, Alexander and Stikine. The accretion of juvenile terranes to a continent appears to be an important mechanism of crustal growth.

title: Regionally extensive mid-Cretaceous west-vergent thrust system in the northwestern Cordillera: Implications for continent-margin tectonism
abstract: The Intermontane-Insular superterrane boundary zone represents a fundamental crustal boundary separating the two largest allochthonous crustal fragments in the North American Cordillera. Structural, stratigraphic, and geochronologic relations along this boundary indicate that substantial west-vergent compression and concomitant crustal thickening occurred there in mid-Cretaceous time. This orogenic zone extends for more than 1200 km along strike length, between southern southeast Alaska and northern Washington. In southern southeast Alaska and northwest British Columbia, rocks of the Insular superterrane were imbricated along a series of west- to southwest-vergent thrust faults. In northern Washington and southwestern British Columbia, a wide zone encompassing the margins of the two superterranes, as well as numerous intervening smaller fragments, was shortened principally along west-vergent thrusts. Known geologic relations do not discriminate among existing tectonic models that explain the origin of the mid-Cretaceous thrust system.

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

title: Evidence from neodymium isotopes for mantle contributions to Phanerozoic crustal genesis in the Canadian Cordillera
abstract: SmNd and RbSr isotopic studies of several Phanerozoic orogenic belts have shown that much of the crust in these belts is composed of reworked, pre-existing continental crust. In contrast, isotopic data collected from two of the largest terranes in the Canadian Cordilleran orogenic belt, the Alexander and Stikine terranes, show that these tectonic fragments are composed of mantle-derived continental crust. In many respects the style and rate of crustal accretion of these and related terranes appear similar to those of some Proterozoic crustal regions

title: Reconnaissance geology and U-Pb geochronology of the west flank of the Coast Mountains between Bella Coola and Prince Rupert, coastal British Columbia
abstract: Metamorphic pendants within and west of the Coast Mountains between Bella Coola and Prince Rupert British Columbia belong to three northwest-trending belts of metasedimentary and metavolcanic rocks. These belts separate Mesozoic and older strata of the Stikine terrane on the northeast from Wrangellia terrane strata on the southwest. The northeastern belt, herein referred to as the Burke Channel assemblage, consists primarily of interlayered quartzite, marble, metapelite, and metavolcanic rocks that are interpreted to have accumulated along a continental margin with one or more phases of arc-type(?) volcanism. Similarities in protoliths and detrital zircon ages indicate that this assemblage correlates with components of the Yukon-Tanana, Nisling, and/or Stikine terranes of northern British Columbia, Yukon, and eastern Alaska. The southwestern belt, referred to as the Banks Island assemblage, consists mainly of quartzite, marble, and metapelite that may also have accumulated in a continental margin setting. The tectonic affinity of these rocks is uncertain. Separating these assemblages is a belt of metasedimentary and metavolcanic rocks of the Alexander terrane and possibly the Gravina belt. Structures bounding the three sets of pendants are not well known, but are interpreted to include: (1) an early Tertiary reverse fault (part of the Coast shear zone) that juxtaposes Stikine strata westward over the Burke Channel assemblage in the Bella Coola area; (2) a mid-Cretaceous thrust fault that juxtaposes the Burke Channel assemblage over Alexander and possibly Gravina strata; and (3) the sinistral Kitkatla fault, which separates the Banks Island assemblage from the Alexander terrane.

title: Paleomagnetism of the Quottoon plutonic complex in the Coast Mountains of British Columbia and southeastern Alaska: evidence for tilting during uplift
abstract: The Quottoon plutonic complex (part of the great tonalite sill) was emplaced into eastern parts of the Coast shear zone along the west flank of the Coast Mountains. U-Pb crystallization ages range from 72.3 to 55.5 Ma. A regional compilation of K-Ar hornblende dates from the tonalite sill indicates a west to east decrease from ~60 Ma to ~54 Ma across the intrusive suite. Paleomagnetic samples were collected at 160 sites (♢ 8 samples per site) along six transects across the Quottoon complex between the Skeena River and Willard Inlet. Directions of characteristic remanent magnetization (ChRM) were successfully determined by principal component analysis of thermal demagneti- zation data for about half of the samples collected. Site-mean ChRM directions were determined for 56 sites from the Skeena River, Quottoon Inlet, Steamer Passage, and Wales Island transects, using criteria of ♢ 4 sample ChRM direc- tions per site and 95% confidence limit (α95) ≤ 25°. For Filmore Island and Willard Inlet, few sites provided adequate determinations of site-mean ChRM directions, but many sample ChRM directions were well determined and consistent. The mean direction from the Skeena River - Quottoon Inlet collection is nearly concordant with the expected Eocene direction. Mean directions from other transects have clockwise-rotated declinations and inclinations that are shallower than the expected direction by up to 23°. Consistent with expectations for east-side-up tilting of crustal panels, sites from the western part of each transect have mean directions farther from the Eocene expected direction than do sites from the eastern part of the transect. The magnitude of east-side-up tilt varies along strike: the Skeena River to Quottoon Inlet segment experienced little or no tilt; Steamer Passage south of Portland Inlet records 15° tilt; north of Portland Inlet, tilts are 40° for Wales Island and 35° for Filmore Island; farther north, tilt decreases to 12° for Willard Inlet. Tilting was driven by Eocene extension of the Coast Mountains, with tilted crustal panels bounded by northwest- striking, east-side-down normal faults and northeast-striking transfer faults or shear zones. The ACCRETE seismic transect imaged east-dipping reflectors down to ~15 km depth, which likely correspond to the east-side-down normal faults. The most significant of the northeast-striking transfer structures must underlie Portland Inlet. Based on a tilting domino model, about 30% extension is required to produce the 40° tilt of crustal blocks at Wales and Filmore islands.

title: Detrital zircon geochronology of the Alexander terrane, southeastern Alaska
abstract: U-Pb analyses of 101 detrital zircon grains from Paleozoic and Triassic clastic strata of the Alexander terrane indicate that most detritus in the terrane was derived from intraterrane igneous rocks, although the occurrence of 1.0–3.0 Ga grains in a Lower Devonian sandstone indicates proximity to a continental region during the Silurian–Devonian Klakas orogeny. Because these Precambrian grains are not the same age as grains that were accumulating along the western margin of North America, it appears unlikely that the terrane was in proximity to the Cordilleran margin during Early Devonian time. Continental regions that could have shed grains of the appropriate ages, and that also record Silurian–Devonian orogenic activity, include the paleo-Pacific margin of Australia and the Scandinavian portion of Baltica. Existing paleomagnetic data are consistent with either paleoposition, whereas previous biogeographic studies suggest closer ties with Baltica.

title: Structural and geochronologic relations along the western flank of the Coast Mountains batholith : Stikine River to Cape Fanshaw, central southeastern Alaska
abstract: Geologic and U-Pb geochronologic studies in central southeastern Alaska provide constraints on the mid-Cretaceous to mid-Tertiary deformation west of the Coast Mountains batholith. The NE-dipping Sumdum and Fanshaw faults record the W-directed emplacement of the Ruth assemblage (Yukon-Tanana terrane) and Taku terrane over the subjacent Gravina belt and Alexander terrane. Ductile fabrics of the Sumdum-Fanshaw fault system truncate early-formed foliation and thrust faults in Albian and older Gravina belt strata. U-Pb age data from syn- and post-tectonic plutons suggest that deformation was ongoing at 92.9 ± 3.0 Ma but had mostly ceased by 90 Ma. The Sumdum-Fanshaw fault system marks the tectonic boundary between the Alexander-Wrangellia terrane and inboard Yukon-Tanana and Stikine terranes and lies within a thrust belt that extends from southern Alaska to northern Washington. Mid-Cretaceous structures are truncated to the east by NE-dipping ductile fabrics of the Coast shear zone. Sheet-like Paleocene tonalites were emplaced into and deformed within the shear zone. Undeformed mid-Eocene dikes cross-cut the tonalites. Kinematic relations in the Coast shear zone suggest a complex displacement history that includes both east-side-up (reverse) and west-side-up (normal) shear. The shear zone probably accommodated the collapse of overthickened crust developed during mid-Cretaceous shortening.

title: Geochronologic investigations along the Alexander-Taku terrane boundary, southern Revillagigedo Island to Cape Fox areas, southeast Alaska
abstract: The southern Revillagigedo Island to Cape Fox area, southeast Alaska, is critical in 
understanding the regional structural development of the Alexander-Taku terrane 
boundary in southeast Alaska and its implications for the tectonic history of the Insular 
suture zone, which extends from southern Alaska to northern Washington. The study 
area offers what currently appears to be the greatest structural relief through the 
Alexander-Taku terrane boundary. This study presents U-Pb zircon and supporting 
Sr-Nd isotopic data as a structural and stratigraphic tool to augment detailed field studies 
that are nearing completion. The principal relations established are as follows. (1) 
Ordovician-Silurian and apparently Late Proterozoic-Cambrian crystalline basement 
rocks and cogenetic volcanic arc rocks of the Alexander terrane tectonically underlie 
nearly the entire area west of the Coast batholith. Many of the more highly tectonized 
and metamorphosed rocks of this structural basement were originally included in the 
Taku terrane. (2) The Taku terrane consists of two distinct sequences that include a Silurian- 
Devonian and possibly older North American continental margin sedimentary 
sequence with interbedded and intruded arc volcanic and plutonic members (Kah 
Shakes sequence), and an upper Paleozoic-lower Mesozoic island arc-marginal basin 
sequence (Alava). Data presented in this chapter, along with detailed field relations, pose 
the possibility of a depositional link between these two sequences. (3) The Taku terrane 
occurs as an isolated folded nappe sequence above the Alexander terrane. (4) The 
Alexander terrane was thrust eastward beneath the Taku terrane prior to Late Jurassic 
time, at which point dikes and sills crosscut the original thrust boundary. (5) The Late 
Jurassic-Early Cretaceous Gravina basinal arc sequence lapped unconformably across 
the terrane boundary thrust complex. (6) The Alexander-Taku terrane thrust complex 
underwent high-magnitude horizontal flattening and stretching that may represent 
deep crustal flow within the suture zone or, alternatively, a distinct postsuturing extensional 
event commensurate with Gravina basin formation. (7) Mid-Cretaceous superposed 
deformation within a fold-thrust belt resulted in the reimbrication of the 
suture-zone assemblage and the destruction of the Gravina basin.  Fold-thrust deformation was west-directed along the western domain of 
the belt, upright along its axis, and east directed along its eastern domain.  Alexander 
terrane rocks from the lower plate of the original terrane boundary were thrust back up against the Taku terrane along the axis of the fold-thrust belt and, as a result, 
constitute a major gneiss complex to the east of the Taku terrane along the western wall 
of the Coast batholith. (11) Latest Cretaceous to Paleocene, mainly northnorthwest- 
trending elongate tonalitic plutons of the Coast batholith intruded the eastern 
margin of the fold-thrust belt and, at least locally, assimilated substantial quantities 
of Alexander terrane crustal materials. (12) The Alexander-Taku terrane boundary in 
the study area does not map as a linear contact, but has considerable topologic complexity, 
including large domains that have low dips. The complexity of the map pattern 
is a result of prolonged suture-zone deformation, a finding that contrasts sharply with 
commonly held ideas of the boundary having formed abruptly at mid-Cretaceous time 
in conjunction with fold-thrust belt deformation.

title: Large‐scale transpressive shear zone patterns and displacements within magmatic arcs: The Coast Plutonic Complex, British Columbia
abstract: The Coast Plutonic Complex is the largest magmatic arc of the North American Cordillera, extending from northwestern Washington State to eastern Alaska. It forms the transition between two tectonic domains that are suspected to have undergone several phases of large (several thousands of kilometers) orogen-parallel displacement during the Mesozoic and early Cenozoic. A compilation of fabric data, published isotopic ages, and new structural observations shows that the western Coast Plutonic Complex was affected by subvertical, orogen-parallel, crustal-scale shear zones. These shear zones mainly reflect sinistral transpression and were sequentially active from ∼110 to 87 Ma during the intrusion of voluminous batholiths. Sinistral shearing was roughly coeval with the development of the thrust belts flanking the Coast Plutonic Complex (between ∼101 and ∼85 Ma), suggesting plate-scale transpression was a first-order process in the construction of the Coast Mountains orogen. These shear zones separate panels with distinct plutonic and cooling histories, suggesting the sinistral displacements between crustal blocks were large (greater than tens to hundreds of kilometers). This transpressive shear system likely reflects the Jurassic to early Late Cretaceous migration of outboard Cordilleran terranes to the south suggested by paleomagnetic evidence and plate reconstruction models. This example from the Coast orogen shows how transpression is partitioned between a thermally weakened magmatic arc and outwardly vergent fold-and-thrust belts. Our analysis further shows that the ∼2000-km-long Late Cretaceous to early Tertiary Coast shear zone has a minimum extent toward the south to at least 51°30′N.

title: Reconnaissance geology and U-Pb geochronology of the western flank of the Coast Mountains between Juneau and Skagway, southeastern Alaska
abstract: Reconnaissance-scale mapping and related structural, stratigraphic, and U-Pb geochronologic studies suggest that the western flank of the Coast Mountains between Juneau and Skagway, Alaska, preserves structural and stratigraphic relations that have not been recognized along other segments of the Coast Mountains orogen. In much of the area southeast of Juneau, the western Coast Mountains area is underlain by moderate- to high-grade metamorphic rocks that are imbricated along mid-Cretaceous thrust faults and intruded by deep-level mid-Cretaceous plutons. Near and just north of Juneau, the metamorphic rocks and associated thrusts are truncated by the latest Cretaceous-early Tertiary Coast shear zone and associated tonalite sills, and mid-Cretaceous plutons have not been recognized. North of this zone of truncation, the western flank is underlain by relatively low grade and only moderately deformed Triassic strata of the Wrangellia terrane and overlying Jurassic-Cretaceous strata of the Gravina belt. Stratigraphic relations preserved in these rocks allow for the recognition of a new unit within the Gravina belt, the Treadwell Formation, and provide evidence that Triassic strata of the Taku terrane are lateral facies equivalents of Triassic basalts of the Wrangellia terrane. The northern Coast Mountains also preserve important relations between the Coast shear zone and the Denali and Chatham Strait faults. All of these structures are known or suspected to have moved during early Tertiary time, with ∼370 km of dextral offset on the Denali fault, -150 km of dextral motion on the Chatham Strait fault, and predominantly dip-slip displacement on the Coast shear zone. Where the Denali fault swings abruptly westward into the northeast-striking Chatham Strait fault, the nearby segment of the Coast shear zone records oblique dextral and/or reverse motion. The apparent linkage between these structures supports Lanphere's (1978) suggestion that the difference in offset of the Denali and Chatham Strait faults (∼220 km) is accommodated by dextral slip on the Coast shear zone and related structures along the west flank of the Coast Mountains. This provides an explanation for dextral offset within and adjacent to the Coast shear zone that does not involve large-scale (1500-2000 km) transform motion.

title: Southern continuation of the Coast shear zone and Paleocene strain partitioning in British Columbia–southeast Alaska
abstract: This paper documents the newly recognized southern continuation of the early Tertiary Coast shear zone, extending its known length by ∼350 km to more than 1200 km. Three sites along the shear zone in British Columbia, Douglas Channel, Bella Coola, and Machmell River, have similar histories during the period ca. 65–55 Ma. The shear zone is 2–11 km thick and is defined by well-developed mylonite zones that strike northwest and dip steeply northeast. Motion on the shear zone was predominantly reverse, with the northeast side up. Synkinematic plutons are common in the shear zone. Lower plate rocks, high-grade gneiss derived from an ancient continental margin assemblage, show little evidence of the extensive deformation and plutonism in the shear zone. North of Bella Coola, high- grade gneiss forms the upper plate, but to the south weakly metamorphosed rocks of Stikinia compose the upper plate. Geochronologic data show that the shear zone was active between ca. 60 and 55 Ma at Douglas Channel, ca. 62 and 56 Ma at Bella Coola, and after 66 and before 56 Ma near the Machmell River. These features match those of the Coast shear zone in southeast Alaska and adjacent British Columbia. Together the shear zones formed a continental-scale reverse ductile fault in Paleocene time. Plate reconstructions show an oblique component to the dominantly dextral transcurrent Paleocene margin. Partitioning of this motion across the continental margin produced a regional system of strike-slip faults and contraction on the Coast shear zone.

title: Metamorphic history of the Coast Mountains orogen, western British Columbia and southeastern Alaska
abstract: The northern Coast Mountains are underlain by the Coast Plutonic Complex, which separates the Intermontane and Insular superterranes. Syndeformational contact and regional metamorphism associated with compressional and transpressional tectonic events occurred from about 101 to 50 Ma, followed by contact metamorphism adjacent to late Oligocene and Miocene plutons and stocks. Metamorphic events comprise pre-101 Ma regional metamorphism (M R 1 ), 101-90 Ma contact metamorphism (M C 2 ) and related regional metamorphism (M R 2 ), 90-85 Ma regional metamorphism (M R 3 ) that formed during thrusting that inverted metamorphic isograds, and 72-50 Ma contact and regional metamorphism (M C - R 4 ) in the central part of the orogen. Middle to Late Cretaceous moderate- to high-pressure metamorphic assemblages in the western metamorphic belt record tectonic burial and subsequent exhumation. Late Cretaceous to Tertiary sillimanite-zone metamorphic assemblages in the central pluton gneiss and the adjacent edge of the western metamorphic belt record emplacement and subsequent exhumation of plutons in the Paleogene Coast Mountains batholith, including Coast Plutonic Complex sill plutons. The final metamorphic event was ca. 25-20 Ma contact metamorphism (M C 5 ) associated with gabbroic and granitic plutons.

title: Concurrent Pb–Hf isotope analysis of zircon by laser ablation multi-collector ICP-MS, with implications for the crustal evolution of Greenland and the Himalayas
abstract: The in situ Lu–Hf isotope analysis of zircon by laser ablation has emerged as a high-calibre tool for tackling magmatic and crustal evolution. The strength of the approach lies with the ability to target specific zircon growth domains identified by imaging, and thus to unravel polyphase crystallisation histories. However, due to the volume of material being sampled during analysis there remains the possibility of ablation-induced mixing between Hf from domains of different age. Inaccurate Hf isotope ratios and spurious geological interpretations could result. One approach to this problem involves dating the same volume of material analysed for Hf isotopes by concurrently measuring 207Pb/206Pb ratios during ablation [Woodhead, J.D., Hergt, J.M., Shelley, M., Eggins, S., Kemp, R. 2004. Zircon Hf-isotope analysis with an excimer laser, depth profiling, ablation of complex geometries, and concomitant age estimation. Chemical Geology 209, 121–135.]. This paper explores the viability of this dual analysis by investigating complex zircons from three different geological contexts, detrital zircons in sedimentary rocks, inherited zircons in granites, and zircons in metamorphosed Eo-Archaean TTG gneisses from Greenland. The implications of the Greenland data for Archaean crustal evolution are discussed in the light of published solution zircon Hf isotope datasets from these gneisses. A case study of detrital zircons from modern river sands in the Himalayas highlights the potential of the technique for providing a rapid, cost-effective picture of crustal evolution that should complement regional bulk rock studies.

title: Zircon formation versus zircon alteration — New insights from combined U–Pb and Lu–Hf in-situ LA-ICP-MS analyses, and consequences for the interpretation of Archean zircon from the Central Zone of the Limpopo Belt
abstract: In this study we demonstrate that the combination of U–Pb and Lu–Hf isotope data obtained by laser ablation-inductive coupled plasma-mass spectrometry provides a powerful tool for distinguishing zircon domains grown during several metamorphic or magmatic events from altered domains affected by complete single or multiple Pb-loss. This discrimination is possible because the U–Pb and Lu–Hf isotope systems are decoupled during zircon alteration. Initial 176Hf/177Hf once “incorporated” into the zircon lattice remains nearly unaffected during later alteration processes which cause Pb-loss. Zircon overgrowths always have higher initial 176Hf/177Hf than previously grown domains, due to the incorporation of additional radiogenic 176Hf formed by 176Lu decay in the rock's matrix between successive zircon growth events. Incorporation of such 176Hf into zircon domains affected by post-growth alteration is negligible. By applying the U–Pb and Lu–Hf systems, in combination with cathodoluminescence imaging, we show that zircon in the Sand River orthogneiss of the Limpopo Belt grew during magma crystallization at 3.28 Ga and anatectic events at 2.64 Ga and 2.02 Ga. Our data demonstrate that abundant zircon domains underwent multiple Pb-loss, and that many of the 2.02 Ga domains result from complete Pb-loss from zircon overgrowth domains already formed at 2.64 Ga. In contrast, all domains of complex zoned zircon grains from the Zanzibar granodiorite gneiss were formed during a single magmatic event at 2.61 Ga, as indicated by their uniform Hf isotope composition, but most subsequently suffered multiple Pb-loss.

title: Hf isotope ratio analysis using multi-collector inductively coupled plasma mass spectrometry: an evaluation of isobaric interference corrections
abstract: From measurements of Hf–Yb mixtures, we have found that the correction of isobaric interferences involving accepted Yb isotope ratios and reasonable estimates of mass bias result in a significantly under-corrected 176Hf, which is proportional to the amount of Yb added. This can be explained by (1) a significant difference in the instrumental mass bias between Hf and Yb, and (2) that the accepted values for isotopic ratios within the Yb and/or Hf systems are incorrect. We have evaluated these possibilities by measuring mixed solutions of Yb and Hf on two MC-ICP-MS instruments and undertaking a series of REE fractionation experiments using a thermal ionisation mass spectrometer (TIMS). Our results indicate that the presently accepted abundances of the Yb isotopes are not appropriate. We present new values for Yb isotopic abundances based on the TIMS and MC-ICP-MS results. Using the newly defined Yb values, we demonstrate that Yb and Hf have similar levels of mass bias in plasma ionisation instruments, and that Hf isotope ratios can be used to correct Yb mass bias before subsequent correction of isobaric interference. A laser ablation comparison of Yb and Hf indicates that similar relationships exist, and can be applied to micro-analytical techniques where chemical separation is not possible.

title: Magmatic evolution of the southern Coast Belt: constraints from Nd–Sr isotopic systematics and geochronology of the southern Coast Plutonic Complex
abstract: Igneous rocks of the southern Coast Belt (SCB) and adjacent Insular Belt developed within a Jurassic–Quaternary magmatic arc built across accreted juvenile-arc and oceanic terranes. SCB plutons are mostly of intermediate composition, with I-type characteristics and major element, trace element, and rare earth element geochemistry consistent with genesis in a subduction-related magmatic arc. Ubiquitous xenoliths and migmatitic zones at pluton–county rock contacts indicate that assimilation of crustal rock was an important magmatic process. U–Pb zircon crystallization ages for SCB and Insular Belt igneous rocks indicate an overall eastward migration of the magmatic axis from Middle Jurassic through Late Cretaceous time. Although absent in most rocks, traces of old inherited zircon are present in several Middle Jurassic–Upper Cretaceous plutons in the southeastern Coast Belt. The primitive character and restricted range of Nd–Sr isotopic data for Middle Jurassic to Quaternary igneous rocks of the SCB (eNd = ...

title: Cyclicity in Cordilleran orogenic systems
abstract: Cordilleran orogenic systems, such as the modern Andes, are long belts of deformation and magmatism that are associated with the subduction of oceanic plates beneath continental ones. Although the oceanic plates have been thought to control the evolution of such systems, a number of processes operating in the upper continental plates have not been fully accounted for. The western American Cordilleras, for example, display a 25–50 million year (Myr) cycle of linked upper-plate processes. In a typical cycle, as the two plates converge and a magmatic arc forms, most of the continental crust shortens by thrusting behind the arc, whereas the lowermost continental lithosphere is shoved beneath the arc — a process that fuels episodic high-flux magmatism in the arc and simultaneously generates dense melt residues. On reaching a critical mass, these residues sink into the mantle, creating space beneath the arc and setting the stage for renewal of the cycle. This alternative model explains key features of Cordilleran systems, such as cyclical trends in the flux and composition of magma supplied to the upper plate, and the foundering of arc roots. Cordilleran orogenic systems are long belts of deformation and magmatism that form when oceanic plates subduct beneath continental ones. Links between processes in the upper continental plate explain key features of Cordilleran systems, such as cyclical trends in the flux and composition of magma supplied to the upper plate.