About 30 percent of the 175,000-km 2 area of southeastern Alaska is underlain by intrusive igneous rocks. Compilation of available information on the distribution, composition, and ages of these rocks indicates the presence of six major and six minor plutonic belts. From west to east, the major belts are: the Fairweather-Baranof belt of early to mid-Tertiary granodiorite; the Muir-Chichagof belt of mid-Cretaceous tonalite and granodiorite; the Admiralty-Revillagigedo belt of porphyritic granodiorite, quartz diorite, and diorite of probable Cretaceous age; the Klukwan-Duke belt of concentrically zoned or Alaskan-type ultramafic-mafic plutons of mid-Cretaceous age within the Admiralty-Revillagigedo belt; the Coast Plutonic Complex sill belt of tonalite of unknown, but perhaps mid-Cretaceous, age; and the Coast Plutonic Complex belt I of early to mid-Tertiary granodiorite and quartz monzonite. The minor belts are distributed as follows: the Glacier Bay belt of Cretaceous and(or) Tertiary granodiorite, tonalite, and quartz diorite lies within the Fair-weather-Baranof belt; layered gabbro complexes of inferred mid-Tertiary age lie within and are probably related to the Fairweather-Baranof belt; the Chilkat-Chichagof belt of Jurassic granodiorite and tonalite lies within the Muir-Chichagof belt; the Sitkoh Bay alkaline, the Kendrick Bay pyroxenite to quartz monzonite, and the Annette and Cape Fox trondhjemite plutons, all interpreted to be of Ordovician(?) age, together form the crude southern southeastern Alaska belt within the Muir-Chichagof belt; the Kuiu-Etolin mid-Tertiary belt of volcanic and plutonic rocks extends from the Muir-Chichagof belt eastward into the Admiralty-Revillagigedo belt; and the Behm Canal belt of mid- to late Tertiary granite lies within and next to Coast Plutonic Complex belt II. In addition, scattered mafic-ultramafic bodies occur within the Fairweather-Baranof, Muir-Chichagof, and Coast Plutonic Complex belts I and II. Palinspastic reconstruction of 200 km of right-lateral movement on the Chatham Strait fault does not significantly change the pattern of the major belts but does bring parts of the minor mid-Tertiary and Ordovician(?) belts closer together. The major belts are related to the stratigraphic-tectonic terranes of Berg, Jones, and Coney (1978) as follows: the Fairweather-Baranof belt is largely in the Chugach, Wrangell (Wrangellia), and Alexander terranes; the Muir-Chichagof belt is in the Alexander and Wrangell terranes; the Admiralty-Revillagigedo belt is in the Gravina and Taku terranes; the Klukwan-Duke belt is in the Gravina, Taku, and Alexander terranes; the Coast Plutonic Complex sill belt is probably between the Taku and Tracy Arm terranes; and the Coast Plutonic Complex belts I and II are in the Tracy Arm and Stikine terranes. Significant metallic-mineral deposits are spatially related to certain of these belts, and some deposits may be genetically related. Gold, copper, and molybdenum occurrences may be related to granodiorites of the Fairweather-Baranof belt. Magmatic copper-nickel deposits occur in the layered gabbro within that belt. The Juneau gold belt, which contains gold, silver, copper, lead, and zinc occurrences, parallels and lies close to the Coast Plutonic Complex sill belt; iron deposits occur in the Klukwan-Duke belt; and porphyry molybdenum deposits occur in the Behm Canal belt. The Muir-Chichagof belt of mid-Cretaceous age and the Admiralty-Revillagigedo belt of probable Cretaceous age are currently interpreted as possible magmatic arcs associated with subduction events. In general, the other belts of intrusive rocks are spatially related to structural discontinuities, but genetic relations, if any, are not yet known. The Coast Plutonic Complex sill belt is probably related to a post-Triassic, pre-early Tertiary suture zone that nearly corresponds to the boundary between the Tracy Arm and Taku terranes. The boundary between the Admiralty-Revillagigedo and Muir-Chichagof belts coincides nearly with the Seymour Canal-Clarence Strait lineament and also is probably a major post-Triassic suture.

Abstract Kennecott-type deposits, among the highest grade copper deposits in the world, are located in the Wrangell Mountains of Alaska. They are strata bound within the lowermost 100 m of the Upper Triassic Chitistone Limestone, which overlies the Triassic Nikolai Greenstone, a sequence of largely subaerial basalt flows more than 1,800 m thick. The deposits are mainly fissure fillings of massive Cu 2 S-CuS minerals and lesser amounts of copper carbonates. Between 1911 and 1938, the Kennecott mines produced 4 million metric tons (Mt) of ore with a grade of 13 percent copper. The deposits are within the fault-bounded Wrangellia terrane, which is made up of Pemsylvanian and Permian marine volcanic and sedimentary rocks, the Triassic Nikolai Greenstone, the Upper Triassic Chitistone and Nizina Limestones, and overlying Upper Triassic and Jurassie marine sedimentary rocks. The Upper Triassic and Jurassic rocks are thickest and best developed in the area around the Kennecott deposits. According to paleomagnetic evidence, the terrane formed near the paleoequator and shifted north to its present position in the Early Cretaceous. Thrust faults, vertical faults, and gentle folds record a Late Jurassic to Early Cretaceous orogeny believed to be associated with the docking of Wrangellia. The lower member of the Chitistone Limestone is composed of three upward-shoaling lime mud cyclic sequences. Each sequence grades upward into intertidal dolostone and stromatolite, marked by textures that inclicate the presence of bacterial mats and replacement of gypsum and anhydrite by ealcite. The uppermost zone contains remnants of caliche and represents a paraconformity of regional extent Oxygen isotope data are consistent with upward-increasing evaporation and salinity in the lower member. We believe that brines developed in this environment mayy have descended into, and leached copper from, the Nikolai Greenstone. Alternatively, brines may have originated from connate fluids in the Permian seclimentary rocks that underlie the Nikolai Greenstone. In either case, brines that acquired a high copper content during migration through the Nikolai Greenstone are considered to be the source of copper in the deposits. The major copper deposits are controlled by steeply dippiling fissures surrounded by zones of intense, welldeveloped breccia and transgressive hydrothermal dolomite and by thin, gouge-filled faults that are roughly parallel to bedding. Most of the steep fissures do not displace the underlying contact between the Chitistone Limestone and Nikolai Greenstone and probably formed shortly alter deposition of the lower member of the Chitistone Limestone in the Late Triassic. Numerous steep, widespread faults that displace the Chitistone-Nikolai contact are believed to be related to the Late Jurassic-Early Cretaceous orogeny. Some of these faults contain small orebodies, but most of them are weakly mineralized or barren. Locally, the ore-hosting fissures are enlarged at their bases and contain massive chalcocite-rich orebodies many tens of meters in length and up to 20 m wide that taper upward into veins. This unusual deposit form is believed to have resulted from a widening of fissures by solution during subaerial exposure of the lower member of the Chitistone Limestone in the Late Triassic. The breccia surrounding the fissures is considered to be formed by partial collapse of the open fissures during loading that resulted from deposition of 3,600 m of overlying Mesozoic rocks. Oxygen isotope data indicate that dolomitization of the breccia was caused by reaction of limestone with formation waters from within the lower member at temperatures similar to those that prevailed during formation of the dolostone and stromatolites. The deposits are composed mainly of massive chalcocite and djurleite, but they also contain clots of earlystage minerals from which a mineral parugenesis can be defined. Early pyrite, now found only as traces, was replaced by chalcopyrite which was in turn replaced by bornite and minor covellite. Temperatures of sulfide deposition fell during these stages from near 200° to 150°C. The main-stage are minerals, chalcocite and djurleite, made up 95 percent of the are and were deposited at tempemtures of 90° ± 10°C. Later, as oxidized are fluids overwhelmed the reductants in the host rock, chalcocite was party replaced by anilite and covellite, and finally by malachite and azurite. Ore deposition took place by reaction between oxidized copper-rich brines, which moved out of the Nikolai Greenstone, and sulfur-rich fluids derived from thermal reduction of gypsum in the presenee of organic matter within the lower Chitistone Limestone. The hydrologic head required to mix these fluids resulted from folding and faulting related to the Late Jurassic-Early Cretaceous orogeny. Folding caused parts of the Nikolai Greenston to be uplifted to levels well above the Chitistone Limeston host rocks, and vertical faulting opened passageways through which brines could flow from the Nikolai Greenstone into permeable parts of the Chitistone. Thus, the copper source in the Nikolai Greenstone, the sulfur source in the Chitistone Limestone, and the permeability related to solution widening, brecciation, and dolomitization were all established in the Late Triassic, but the ore-limning process of fluid mixing occurred nearly 100 m.y. later. One significant finding from our study of Kennecott-type, strata-bound, sediment-hosted copper deposits is the importance of structure and its influenc on the hydrology of the fluid mixing event.

Approximately 3,000 Ar, Sr, and Pb isotopic age determinations for Canadian Cordilleran rocks have been cataloged, categorized as to reliability and significance, and plotted on histograms, distribution maps for different time intervals, and space-time plots to show the magmatic evolution in this 2,300-km portion of the Circum-Pacific Mobile Belt. The history revealed is episodic, with stable distribution patterns within episodes and distinct lulls and changes in distribution between the episodes. From 230 to 214 Ma (during Late Triassic time), extensive mafic volcanism occurred in the Wrangell, Quesnel, and Stikine terranes. Volcanic-related ultramafic complexes are found scattered through the two latter terranes. Large calc-alkaline granitic plutons are known only in a belt crossing Stikinia in northern British Columbia. At the same time, blueschists formed in the Cache Creek accretion wedge. From 214 to 200 Ma (end of Triassic and part of Early Jurassic time), Early to Middle Jurassic arc magmatism began in Wrangellia and in the northern Quesnel, Stikine, and Yukon terranes. A distinct magmatic event is recognizable only in southern Quesnellia. Magmatism was absent on the North American craton. The Cache Creek and Quesnel terranes were definitely linked, Stikine and Cache Creek terranes were probably linked, and a regional metamorphic episode was completed in the Yukon Terrane by this time. From 200 to 155 Ma (late Early to early Late Jurassic time), magmatism was extensive in the Wrangell, Quesnel, Stikine, and Yukon terranes. Magmatism over-lapped into North America only east of southern Quesnellia after about 180 Ma. By the middle of this time interval, the southern Quesnel-Slide Mountain-North America linkage was complete, and major deformation and metamorphism had affected the Omineca Belt in British Columbia. Early to Middle Jurassic magmatism in southern Wrangellia (Vancouver Island) is distinctly older than the Middle to Late Jurassic magmatism that occurred in central Wrangellia (Queen Charlotte Islands). From 155 to 140 Ma (during Late Jurassic time), a few last-gasp plutons of the late Early to early Late Jurassic episode and other rocks with partially reset 155- to 145-Ma dates occur in the Wrangell, Quesnel, and Stikine terranes. Late Jurassic magmatism (160 to 140 Ma) occurred in the Alexander Terrane (Saint Elias region). From 145 to 138 Ma (latest Jurassic and beginning of Early Cretaceous time), plutonism occurred in the Endako area of central British Columbia (Francois Lake suite) but is virtually unknown elsewhere. From 135 to 125 Ma (during Early Cretaceous time), there was a magmatic lull of major significance present throughout western North America. From 110 to 90 Ma (middle Cretaceous time), widespread plutonism occurred across all terranes. Dual culminations are evident: the Coast Plutonic and Ominica belts. Before this time all sutures except those outboard of Wrangellia had been closed. From 80 to 70 Ma (during Late Cretaceous time), a narrow, sinuous belt of magmatism persisted, mostly in the southeastern Coast Plutonic Belt, southwestern Yukon Territory, and scattered across the Skeena and Stikine arches. From 70 to 60 Ma (latest Cretaceous to Paleocene time), a distinct lull in magmatism occurred. Rare plutons of this time interval are known in the Coast Plutonic Belt, on the Skeena Arch, and in the southern Intermontane Belt. From 55 to 45 Ma (latest Paleocene to Middle Eocene time), widespread and voluminous magmatism occurred in all terranes. The early Cenozoic volcanic front crossed the Coast Plutonic Complex from its east side in the south to its west side in the north. Associated thermal and tectonic effects were strong even into the Omineca Belt, producing large reset metamorphic areas in the Coast and Omineca belts. This was a short-lived event, synchronous from southern British Columbia through the Yukon Territory. West of the volcanic front, offshore of Wrangellia, Metchosin volcano growth was underway at this time. Late in this time interval, the 50?–45–36-Ma Catface–Leech River event(s) of southern Wrangellia occurred. There is also time overlap with a diffuse Massett magmatic event in the Queen Charlotte Islands, and with Baranoff Island and Yakutat–Saint Elias region magmatism. Initial 87 Sr/ 86 Sr ratios and petrographic characteristics of Canadian Cordilleran igneous rocks are reviewed in the time frame just described. These reflect the nature of underlying crust, contemporaneous lithosphere thickness, and distance from the subduction zone. Comparisons with other parts of the Circum-Pacific Magmatic Belt shows both out-of-phase magmatism (Japan and southwestern Alaska) and perfect matching of some episodes (Sierra Nevada). Major magmatic episodes correspond to times of increased westward motion of North America with respect to hot spots or to times of increased convergence between western North America and the Farallon Plate.

Abstract The pre-Cenozoic kinematic and tectonic history of the Arctic Alaska Chukotka (AAC) terrane is not well known. The difficulties in assessing the history of the AAC terrane are predominantly due to a lack of comprehensive knowledge about the composition and age of its basement. During the Mesozoic, the AAC terrane was involved in crustal shortening, followed by magmatism and extension with localized high-grade metamorphism and partial melting, all of which obscured its pre-orogenic geological relationships. New zircon geochronology and isotope geochemistry results from Wrangel Island and western Chukotka basement rocks establish and strengthen intra- and inter-terrane lithological and tectonic correlations of the AAC terrane. Zircon U–Pb ages of five granitic and one volcanic sample from greenschist facies rocks on Wrangel Island range between 620 ± 6 and 711 ± 4 Ma, whereas two samples from the migmatitic basement of the Velitkenay massif near the Arctic coast of Chukotka yield 612 ± 7 and 661 ± 11 Ma ages. The age spectrum (0.95–2.0 Ga with a peak at 1.1 Ga and minor 2.5–2.7 Ga) and trace element geochemistry of inherited detrital zircons in a 703 ± 5 Ma granodiorite on Wrangel Island suggests a Grenville–Sveconorwegian provenance for metasedimentary strata in the Wrangel Complex basement and correlates with the detrital zircon spectra of strata from Arctic Alaska and Pearya. Temporal patterns of zircon inheritance and O–Hf isotopes are consistent with Cryogenian–Ediacaran AAC magmatism in a peripheral/external orogenic setting (i.e. a fringing arc on rifted continental margin crust). Supplementary material: Secondary ion mass spectrometry (SIMS) U–Pb zircon geochronology data, SIMS zircon 18 O/ 16 O isotopic data, laser ablation inductively coupled mass spectrometry zircon Lu–Hf isotopic data and zircon cathodoluminescence images are available at https://doi.org/10.6084/m9.figshare.c.3741314

Study of Late Triassic biofacies and associated paleoecology reveals new silicified shallow-water corals and other fossils from new and previously known localities within the Alexander terrane (Keku Strait and Gravina Island, southeast Alaska) and Wrangellia (Wrangell Mountains, Alaska, and Vancouver Island, British Columbia). Twenty-five species of coral are identified from eight localities within the Alexander terrane and 34 species are identified from four localities in Wrangellia. Distributions of silicified shallow-water marine fossils contribute to Late Triassic (Norian–Rhaetian) paleoecology, biotic diversity, and terrane paleogeography. Depositional environments establish the conditions in which these organisms lived as well as provide evidence for lithological correlation between tectonically separate fragments. This study also confirms the presence of biostrome reef buildups in the southern Alexander terrane (Gravina Island), indicating warm, clear, and nutrient-free water with lots of sunlight; this differs from the central Alexander terrane (Keku Strait) and northern Wrangellia (Wrangell Mountains), where corals grow as individual colonies, not in a structured, reef-like buildup, and are accompanied by filter- and detritus-feeding organisms indicating warm, cloudy and nutrient-rich water in a back-reef environment. Paleobiogeographic results from silicified Upper Triassic corals show faunal similarity between Gravina Island and Keku Strait (Alexander terrane) and no similarity between northern and southern Wrangellia. Likewise, no similarity was found between the Alexander terrane and either northern or southern Wrangellia.