This field guide describes a three-day trip from Vancouver, British Columbia, to the Wells Gray–Clearwater volcanic field (WGCVF) in east-central British Columbia. The WGCVF is the site of transitional to alkali olivine basaltic volcanism erupted over the last three million years. The small volume magmas (<1 km3) erupted along preexisting normal faults related to the late stages of Cordilleran terrane amalgamation, along the boundary between the miogeoclinal and pericratonic rocks of the Kootenay terrane and the allochthonous Slide Mountain and Quesnellia terranes west of ancestral North America. The magmas are highly enriched in incompatible elements, especially large-ion lithophile elements, and are interpreted as the result of low degrees of partial melting of a heterogeneous, metasomatized mantle. Upon ascent through the crust, they carried up both crustal and mantle xenoliths. During the eruptive period of the WGCVF, at least four glacial periods have occurred. The interplay between volcanism and glaciation is captured in the wide range of volcanic features found in the region. Field trip participants will view numerous diverse volcanic landforms and deposits: from tuyas to ice-marginal valley-edge deposits, volcanoclastic-lacustrine deposits, and associated pillow lavas and hyaloclastites.

In this review we summarize the major lithological and geochemical characteristics of the Mesoarchean (ca. 3075 Ma) Ivisaartoq greenstone belt, Nuuk region, southern West Greenland. In addition, the geological characteristics of the Ivisaartoq greenstone belt are compared with those of other Archean greenstone belts in the area. The Ivisaartoq greenstone belt is the largest Mesoarchean supracrustal lithotectonic assemblage in the Nuuk region. The belt contains well-preserved primary magmatic structures including pillow lavas, volcanic breccias, and cumulate (picrite) layers. It also includes variably deformed gabbroic to dioritic dikes, actinolite schists, serpentinites, siliciclastic sediments, and minor cherts. The Ivisaartoq rocks underwent at least two stages of postmagmatic metamorphic alteration, including seafloor hydrothermal alteration and syn- to post-tectonic calc-silicate metasomatism, between 3075 and 2961 Ma. The trace element systematics of the least altered rocks are consistent with a subduction zone geodynamic setting. On the basis of lithological similarities between the Ivisaartoq greenstone belt and Phanerozoic forearc/backarc ophiolites, and intra-oceanic island arcs, we suggest that the Ivisaartoq greenstone belt represents a relic of dismembered Mesoarchean suprasubduction zone oceanic crust. The Sm-Nd isotope system appears to have remained relatively undisturbed in picrites, tholeiitic pillow lavas, gabbros, and diorites. As a group, picrites have more depleted initial Nd isotopic signatures (ɛNd = +4.2 to +5.0) than gabbros, diorites, and tholeiitic basalts (ɛNd = +0.3 to +3.1), consistent with a strongly depleted mantle source. In some areas gabbros include up to 15 cm long white inclusions (xenoliths). These inclusions are composed primarily (>90%) of Ca-plagioclase and are interpreted as anorthositic cumulates of the lower oceanic crust brought to the surface by upwelling gabbroic magmas. Alternatively, the inclusions may represent the xenoliths from older (>3075 Ma) anorthositic crust onto which the Ivisaartoq magmas were emplaced as an autochthonous sequence. However, no geological evidence has been found for such older anorthositic crust in the region. The anorthositic cumulates have significantly higher initial ɛ Nd values (+4.8 to +6.0) than the surrounding gabbroic matrix (+2.3 to +2.8), suggesting two different mantle sources for these rocks.

Cores recovered on Deep Sea Drilling Program leg 43 and on Bermuda itself, together with geophysical data (anomalies in basement depth, geoid, and heatflow) and modeling have long suggested that the uplift forming the Bermuda Rise, as well as the initial igneous activity that produced the Bermuda volcanoes, began ca. 47–40 Ma, during the early to middle part of the Middle Eocene. Some authors attribute 65 Ma igneous activity in Mississippi and 115 Ma activity in Kansas to a putative “Bermuda hotspot” or plume fixed in the mantle below a moving North America plate. While this is more or less consistent with hotspot traces computed from “absolute motion” models, the hotspot or plume must resemble a blob in a lava lamp that is turned off for up to 25 million years at a time, and/or be heavily influenced by lithosphere structure. Moreover, Cretaceous igneous activity in Texas and Eocene intrusions in Virginia then require separate mantle “blobs.” The pillow lavas forming the original Bermuda shield volcano have not been reliably dated, and the three associated smaller edifices have not been drilled or dated. A well-dated (ca. 33–34 Ma) episode of unusually titaniferous sheet intrusion in the Bermuda edifice was either triggered by platewide stress changes or reflects local volcanogenic events deep in the mantle source region. The high Ti and Fe of the Bermuda intrusive sheets probably relate to the very high-amplitude magnetic anomalies discovered on the islands. Numerical models constrained by available geophysical data attribute the Bermuda Rise to some combination of lithospheric reheating and dynamic uplift. While the relative contributions of these two processes cannot yet be wholly separated, three features of the rise clearly distinguish it from the Hawaiian swell: (1) the Bermuda Rise is elongated at right angles to the direction of plate motion; (2) there has been little or no subsidence of the rise and the volcanic edifice since its formation—in fact, rise uplift continued at the same site from the late Middle Eocene into the Miocene; and (3) the Bermuda Rise lacks a clear, age-progressive chain. We infer that the Bermuda Rise and other Atlantic midplate rises are supported by anomalous asthenosphere, upwelling or not, that penetrates the thermal boundary layer and travels with the overlying plate. The elongation along crustal isochrons of both the Bermuda volcanoes and the Bermuda Rise and rise development mostly within a belt of rougher, thinner crust and seismically “slower” upper mantle—implying retention of gabbroic melts at the ancient Mid-Atlantic Ridge axis—suggest that the mantle lithosphere may have helped localize rise development, in contradiction to plume models. The Bermuda Rise area is seismically more active than its oceanic surroundings, preferentially along old transform traces, possibly reflecting a weaker upper mantle lithosphere. We attribute the “Bermuda event” to a global plate kinematic reorganization triggered by the closing of the Tethys and/or the associated gravitational collapse into the lower mantle of subducted slabs that had been temporarily stagnant near the 660 km mantle discontinuity. The widespread onset of sinking slabs required simultaneous up-welling for mass balance. In addition, the global plate kinematic reorganization was accompanied by increased stress in some plate interiors, favoring magma ascent along fractures at structurally weak sites. We suggest that the Bermuda event and concomitant igneous activity in Virginia, West Antarctica, Africa, and other regions were among such upwellings, but structurally influenced by the lithosphere, and probably originated in the upper mantle. Drilling a transect of boreholes across and along the Bermuda Rise to elucidate turbidite offlap during rise formation might discriminate between a widely distributed mantle source (such as a previously subducted slab) and a narrow plume whose head (or melt root) spreads out quasi-radially over time, generating an upward and outward expanding swell.

New data from an Upper Triassic to Lower Cretaceous deep marine succession—the herein reinstated and restricted Gemuk Group—provide a vital piece of the puzzle for unraveling southwestern Alaska's tectonic history. First defined by Cady et al. in 1955, the Gemuk Group soon became a regional catchall unit that ended up as part of at least four different terranes. In this paper we provide the first new data in nearly half a century from the Gemuk Group in the original type area in Taylor Mountains quadrangle and from contiguous rocks to the north in Sleetmute quadrangle. Discontinuous exposure, hints of complex structure, the reconnaissance level of our mapping, and spotty age constraints together permit definition of only a rough stratigraphy. The restricted Gemuk Group is at least 2250 m thick, and could easily be at least twice as thick. The age range of the restricted Gemuk Group is tightened on the basis of ten radiolarian ages, two new bivalve ages, one conodont age, two U-Pb zircon ages on tuff, and U-Pb ages of 110 detrital zircons from two sandstones. The Triassic part of the restricted Gemuk Group, which consists of intermediate pillow lavas interbedded with siltstone, chert, and rare limestone, produced radiolarians, bivalves, and conodonts of Carnian and Norian ages. The Jurassic part appears to be mostly siltstone and chert, and yielded radiolarians of Hettangian-Sinemurian, Pliensbachian-Toarcian, and Oxfordian ages. Two tuffs near the Jurassic-Cretaceous boundary record nearby arc volcanism: one at 146 Ma is interbedded with red and green siltstone, and a second at ca. 137 Ma is interbedded with graywacke turbidites. Graywacke appears to be the dominant rock type in the Lower Cretaceous part of the restricted Gemuk Group. Detrital zircon analyses were performed on two sandstone samples using SHRIMP. One sandstone yielded a dominant age cluster of 133–180 Ma; the oldest grain is only 316 Ma. The second sample is dominated by zircons of 130–154 Ma; the oldest grain is 292 Ma. The youngest zircons are probably not much older than the sandstone itself. Point counts of restricted Gemuk Group sandstones yield average ratios of 24/29/47 for Q/F/L, 15/83/2 for Ls/Lv/Lm, and 41/48/11 for Qm/P/K. In the field, sandstones of the restricted Gemuk Group are not easily distinguished from sandstones of the overlying Upper Cretaceous turbidite-dominated Kuskokwim Group. Petrographically, however, the restricted Gemuk Group has modal K-feldspar, whereas the Kuskokwim Group generally does not (average Qm/P/K of 64/36/0). Some K-feldspar-bearing graywacke that was previously mapped as Kuskokwim Group (Cady et al., 1955) is here reassigned to the restricted Gemuk Group. Major- and trace-element geochemistry of shales from the restricted Gemuk Group and the Kuskokwim Group show distinct differences. The chemical index of alteration (CIA) is distinctly higherforshales of the Kuskokwim Group than forthose of the restricted Gemuk Group, suggesting more intense weathering during deposition of the Kuskokwim Group. The restricted Gemuk Group represents an estimated 90–100 m.y. of deep-water sedimentation, first accompanied by submarine volcanism and later by nearby explosive arc activity. Two hypotheses are presented for the tectonic setting. One model that needs additional testing is that the restricted Gemuk Group consists of imbricated oceanic plate stratigraphy. Based on available information, our preferred model is that it was deposited in a back-arc, intra-arc, or forearc basin that was subsequently deformed. The terrane affinity of the restricted Gemuk Group is uncertain. The rocks of this area were formerly assigned to the Hagemeister subterrane of the Togiak terrane—a Late Triassic to Early Cretaceous arc—but our data show this to be a poor match. None of the other possibilities (e.g., Nukluk and Tikchik subterranes of the Goodnews terrane) is viable; hence, the terrane subdivision and distribution in southwestern Alaska may need to be revisited. The geologic history revealed by our study of the restricted Gemuk Group gives us a solid toehold in unraveling the Mesozoic paleogeography of this part of the northern Cordillera.

Pumpellyite and prehnite are associated closely with epidosite in two well-exposed sections of the Josephine ophiolite and are interpreted to have formed during hydrothermal metamorphism beneath a spreading axis. In the upper 75 m of the extrusive sequence, epidosite grades upward into “pumpellyosite” (granoblastic pumpellyite + quartz + chlorite ± epidote rock) and, in interpillow hyaloclastite, into “prehnitite” (granoblastic prehnite + quartz + epidote ± chlorite rock). Probable hydrothermal pumpellyite also occurs in the lower hematitic pillow lavas as amygdules that contain pumpellyite + chlorite ± epidote ± chalcopyrite. The second occurrence of pumpellyosite and prehnitite is in the basal sheeted dike complex, where these minerals formed during the late stages of retrograde hydrothermal metamorphism. The bulk-rock composition of pumpellyosite and prehnitite shows extensive Ca metasomatism very similar to that of epidosite. Like epidosites, these rocks are inferred to have formed by interaction with large volumes of upwelling, highly reacted hydrothermal fluids, similar to those at modern high-temperature hot springs on mid-ocean ridges. The change in the upper 75 m of the pillow lavas from epidosite to pumpellyosite and prehnitite may reflect cooling of upwelling fluids to less than ~315 °C. The presence of interpillow prehnitite immediately below sediments overlying the ophiolite implies that these fluids leaked directly onto the sea floor. The Josephine pumpellyosites and prehnitites formed at temperatures between 200 and 315 °C, within the overlapping stability fields of epidote, prehnite, and pumpellyite. The influence of fluid composition on mineral equilibria is evaluated at 250 °C and 500 bar using an a Ca 2 + / a H + 2 versus a Fe 3 + / a H + 3 diagram modified from Rose and Bird (1987). The topology of the pumpellyite-epidote and pumpellyite-prehnite phase boundaries was derived using compositions of coexisting Ca-Al silicates in the Josephine samples. The pumpellyite and prehnite stability fields are generally at lower a Fe 3 + / a H + 3 than epidote, whereas the pumpellyite stability field is generally at lower a Ca 2 + / a H + 3 than prehnite.