Abstract Subduction zones are major sites of magmatism on the Earth. Dehydration processes and associated element transport, which take place in both the subducting lithosphere and the down-dragged hydrated peridotite layer at the base of the mantle wedge, are largely responsible for the following characteristics common to most subduction zones: (1) the presence of dual volcanic chains within a single volcanic arc; (2) the negative correlation between the volcanic arc width and the subduction angle; (3) selective enrichment of particular incompatible trace elements; and (4) systematic across-arc variations in incompatible trace element concentrations. The occurrence of two types of andesites, calcalkalic and tholeiitic, typifies magmatism in subduction zones. Examination of geochemical characteristics of those andesites in the NE Japan arc and bulk continental crust reveals marked compositional similarity between calc-alkalic andesites and continental crust. One of the principal mechanisms of generation of calc-alkalic andesites, at least those on the NE Japan arc, is the mixing of two magmas, having basaltic and felsic compositions and being derived from partial melting of the mantle and the overriding basaltic crust, respectively. It may be thus suggested that this process would also have contributed greatly to continental crust formation. If this is the case, then the melting residue after extraction of felsic melts should be removed and delaminated from the initial crust into the mantle in order to form ‘andesitic’ crust compositions. These processes cause accumulation in the deep mantle of residual materials, such as delaminated crust materials and dehydrated, compositionally modified subducted oceanic crusts and sediments. Geochemical modelling suggests that such residual components have evolved to form enriched mantle reservoirs.

Boron contents were measured in predominantly mafic Quaternary lavas from the Central American Volcanic Arc to evaluate along-strike variations in subduction processes. Despite the significant range in B concentrations (∼2 to 46 ppm), B/La ratios vary in a systematic fashion along the arc: Higher values (>1) are typical between Guatemala and northern Costa Rica, whereas low values (most <0.5) typify central Costa Rica and western Panama. Because B/La is highly correlated with 10 Be/Be (r 2 = 0.94, excluding one sample), B/La may be a useful indicator of subduction contributions to the magma sources. If enrichments of both B and 10 Be are proportional to the flux of subducted sediment, along-strike variations in B/La suggest at least a twofold variation in this flux with maximum values below western Nicaragua and minimum values below Costa Rica and western Panama where the Cocos Ridge is being subducted. These data may also reflect significant differences in thermal state of the downgoing slab, which in turn differentially affect release patterns of fluids and fluid-mobile trace elements and possibly melting processes beneath different parts of the arc. The following scenario is suggested to explain the geochemical results. Steep subduction of cold slab beneath the northwestern arc favors more efficient subduction of fluid components to depths beneath the volcanic front. Their release provides fluid-mobile elements to the overlying mantle, which upon melting produces calcalkalic magmas. Shallow subduction of warmer slab beneath the southeastern arc favors shallow release of fluids and limits fluid-related metasomatism of sub-arc mantle beneath the volcanic front. Under such conditions, B/La and Ba/La ratios in the sub-arc mantle vary little from values seen in oceanic island basalts. Magmas in this part of the arc nevertheless display the highest La/Yb and lowest Ba/La and B/La ratios, which are consistent with prior light rare-earth-element enrichment in the source, significantly lower degrees of melting, or a combination thereof. Because some of the largest volcanoes occur in Costa Rica, and magma flux there is nearly an order of magnitude higher than elsewhere in the arc, source enrichment is considered to be the more plausible explanation. It is proposed that Quaternary magma production below Costa Rica involved lithospheric sources containing trapped or “stored” melt components, but this enrichment process is unlikely to have involved arc magmas or subduction fluids because we see no B-enrichment.

Abstract Continental Alaska has been the site of widespread magmatism throughout much of the late Mesozoic and Cenozoic, but until recently, most of this magmatism was unrecognized due to the lack of modern geologic maps or isotopic age data for large tracts of Alaska. Although parts remain unmapped, progress in reconnaissance mapping and dating have enabled workers to identify major late Mesozoic and Cenozoic magmatic provinces outside the well-known Aleutian arc and to speculate as to their tectonic implications and origin (Wallace and Engebretson, 1984). This chapter defines major Late Cretaceous and Cenozoic magmatic provinces in Alaska outside the Aleutian arc (Kay and Kay, this volume; Vallier and others, this volume; Miller and Richer, this volume) and southeast Alaska (Brew, this volume), and discusses their distribution, age, petrology, and tectonic implications. The available data suggest that Late Cretaceous and Cenozoic magmatism in continental Alaska can be roughly divided into three periods: (1) latest Cretaceous and early Tertiary (76 to 50 Ma), (2) middle Tertiary (43 to 37 Ma), and (3) late Tertiary and Quaternary (6 Ma to the present). Late Cretaceous and early Tertiary calc-alkalic volcanism and plutonism were widespread over much of western, central, and southern Alaska and on the Bering Sea shelf. Middle Tertiary magmatism was characterized by the eruption of small volumes of calcalkalic rocks in interior Alaska, contemporaneous with the inception of a major pulse of magmatism in the Aleutian arc. Late Tertiary and Quaternary volcanism has been characterized by the eruption of voluminous basaltic magma at

Abstract The Rockies concerned are those of Montana, Wyoming, Colorado, New Mexico, and the Colorado Plateau portions of Utah and Arizona, east of the Paleozoic miogeosyncline. Evidence is presented that suggests that all the Rocky Mountain features of this region are the result primarily of Laramide vertical uplifts of oval or irregularly broad shape. They generally lack linear, narrow, or sinuous aspect. Some are conspicuously asymmetrical; others are fairly symmetrical. The structural relief ranges from 500 feet (Bowdoin dome) to 40,000 feet (Wind River uplift). Later Tertiary faulting has modified these Laramide uplifts considerably in places, and sediments and volcanic fields have partly covered some of them. When the thrust faults of the province are charted, they prove to be, for the most part, marginal to the uplifts. The uplifts of low and intermediate amplitude generally do not have associated border thrusts, but those in which Precambrian rock is exposed in the core commonly are bordered on one side or both by outwardly displaced thrusts. A firmer tie of uplift to border thrust is found in those where a structural relief of 20,000 feet or more exists. The above-noted relations suggest that vertical uplift was the primary deformation and that thrusting was a secondary lateral deformation caused by gravity sliding and flowing. Inasmuch as the basins were filled with sediments as the uplifts rose, it would appear that thrusting is likely not to be related directly to the structural relief of uplift over adjacent basins, but either to absolute relief at any one time as uplift exceeded sedimentation, or to density differences between basin sediments and the rocks of the uplift. Anticlines suitable for oil and gas accumulation seem to be related to the marginal gravity creep from the uplifts. The locale is one of interplay of thrusting and folding of the surficial strata, and of sedimentation. The Rocky Mountain region of uplifts is essentially the igneous province of alkalic and calcalkalic rocks. Consideration of the origin of these rocks, of the nature of the uplifts, and of geophysical data bearing on deep-seated crustal structure, leads the writer to postulate that the uplifts are due to megasills or megalaccoliths deep in the silicic (granitic) layer, perhaps near the boundary of the silicic and basaltic layers. It is expected that model experiments will indicate size, shape, and depth of intrusion necessary to produce the various surface structures, and the nature of the border faults.