High-level Intrusions Research Articles

Discussion on rifting and transgressions in the Zambian Copperbelt

D r P. L. B inda writes: Annels (1984) suggests that the Zambian Copperbelt was the site of rifting during the deposition of at least part of the Katanga Sequence and that the mineral deposits are Red-Sea type.Intracratonic depositional settings may be fault-bounded or slowly subsiding basins, but rifting does not necessarily imply hydrothermal systems capable of brininging about mineralizations on the scale of the Zambian–Zaïrean Copperbelt. Schneiderhöhn (1937) psroposed that copper was derived from the erosion of a mineralized hinterland and carried—partly as detritus, partly dissolved─to be deposited, syngenetically and diagenetically, in rocks of the Katanga Sequence. Pre-Katangan rocks in Zambia are rich in copper, and there is no difficulty in deriving enough copper from weathering of the basement to account for the Katangan deposits. Some of the Zambian deposits contain economic concentrations of cobalt, but data on cobalt in the basement are lacking. Annels proposes that cobalt and copper were derived from basic magmas at depth, and interprets the various bodies of amphibolite shown in his fig. 5 as high-level intrusives, and possibly submarine extrusives Most of the Copperbelt amphibolites are probably metamorphosed gabbros, but there is no evidence that any are volcanic rocks. The only volcanics recorded in the Katanga Sequence in Zambia are in one drill hole (NE6B) in the Ndola area. The spatial relationship between arnphibolites and cobalt distribution is not as clear as Annels suggests, and in his fig. 4 the Upper Roan cobalt occurrence of Mufulira is ignored. Furthermore, the Série des Mines

Hydrothermal alteration and mineralization of the alkaline anorogenic ring complexes of Nigeria

The Jurassic alkaline anorogenic granitic ring complexes in central Nigeria are discordant high level intrusions emplaced by piecemeal stoping through a collapsed central block. Biotite granites predominate over fayalite or amphibole bearing variants and are important for their ore deposits. A series of alteration processes, with related mineralization, can be recognized. Early high temperature sodic metasomatism may introduce niobium mineralization, can be recognized. Early high temperature sodic metasomatism may introduce beginning with potash metasomatism affect only the biotite granites. Accessory monazite, zircon, cassiterite, rutile, molybdenite and occasionally wolframite are associated with mica distribution. Subsequent acid metasomatism results in greisenization. The ore assemblage deposited at this stage is dominantly of oxides with early monazite, zircon and ilmenite followed by cassiterite—the major ore, wolframine, sometimes columbite or siderite and always rutile. During silica metasomatism early cassiterite is followed by abundant sphalerite, chalcopyrite, galena and occasionally arsenopyrite or pyrite. Chloritization and argillization are important but more restricted alteration processes. Fluid inclusion studies indicate that there was an initial separation of dense saline brine from the residual melt followed by vapour phase separation, to evolve a low density fluid. Fluids responsible for early metasomatic alteration and pegmatide development are highly saline and may contain glassy fluorosilicate phases. Homogenization temperatures are between 380 and 550°C. Later fissure-filling veins form in the range 300–380°C at salinites up to 15 eq. wt% NaCl, with the latest mineralized quartz veins in the range 200–300°C and salinities <10 eq. wt% NaCl. In addition, low temperature, monophase inclusions occur which are probably associated with late stage argillic alteration. Superimposed on this general pattern of decreasing temperature and salinity are important stages of immiscible phase separation related to CO 2 loss and local increase of salinity due to boiling as pressure is released. Mineralization may take the form of: (i) late magmatic pegmatites, (ii) pervasive metasomatic disseminations, (iii) pre- and postjoint pegmatitic lenses, (iv) quartz rafts, stockworks, sheeted veins and altered wall rock, (v) fissure-filling veins, (vi) irregularly shaped replacement bodies, (vii) quart veins, (viii) ring-dykes, (ix) alluvial and eluvial deposits. The different processes of alteration and associated mineralization characterize different parts of a granite pluton: (i) the roof zone, (ii) marginal zone, (iii) contact zone, (iv) the country rock, (v) the ring-dyke zone.

On the role of diapirism in the, segregation, ascent and final emplacement of granitoid magmas

Only bodies of magma with a high crystal content and partially molten (crustal) country rocks can ascend as diapirs; once such an envelope is pierced, the diapiric ascent of the pluton is arrested by the high viscosity of a solid aureole. Deformation by shortening of the carapace of these bodies may lead to the expulsion of a magma with a relatively low crystal content, which may then continue ascent via fractures and dykes. The details of the mechanisms of granitoid magma segregation are still unknown, but it appears that many magmas hegin their ascent through the crust as mushes with at least 50% melt, and that such magmas are rheologically able to ascend through a thickness of crust. This ascent mechanism explains the dearth of structures attributable to the ascent of granitoids, in contrast to the abundance of structures that developed during their final emplacement. When a magma becomes too crystalline (melt < 25%) to continue its ascent via dykes, it is immobilised. At approximately this stage, a hydrous magma may become saturated with water and release fluids into the aureole, making it particularly susceptible to deformation. Magma that continues to arrive at this level is also immobilised, and the pluton grows as a ballooning diapir. These characteristically deform themselves and their aureoles by bulk shortening. Magmas that are able to ascend to shallow depths, largely by virtue of lower water contents and higher initial temperatures, tend to become finally accommodated by such brittle processes as stoping and cauldron subsidence. High level intrusions lend to be tabular, are also fed by dykes or conduits, and assemble in tabular batholiths.

Quartz-topaz-loellingite rocks near Eldorado, Victoria

Two small dykes consisting of a quartz-topaz-loellingite rock type have recently been discovered within the aplitic phase of the Pilot Range granite, near Eldorado, in NE Victoria. Minor biotite, muscovite, chlorite, kaolinite, anatase and pharmacosiderite are associated. Apart from the loellingite, the dykes are similar mineralogically to the ‘topazites’ from New England, NSW. These were considered to be magmatic in origin, based on field relationships and high homogenization temperatures for fluid inclusions in topaz (Eadington & Nashar 1978, Contrib. Mineral. Petrol. 67, 433–438). Although experimental evidence on F-enriched ‘granitic’ systems is inconclusive, the emplacement of the Eldorado topazite dykes most likely involved both magmatic and hydrothermal components operating essentially simultaneously. The topazite melt represented a F-rich residual granitic magma, from which aqueous alkali halide-rich solutions separated during high level intrusion. Separation of these aqueous solutions was responsible for miarolitic cavities into which topaz crystals grew. F-OH equilibration calculations for coexisting topaz-biotite pairs suggest the minerals equilibrated in the presence of hydrothermal solutions of variable composition (in terms of the HF/H2O fugacity ratio), at temperatures around 550°C. Alteration of topaz to muscovite, the precipitation of loellingite and the formation of clay and fluorite in the cavities occurred at progressively lower temperatures. The widespread alluvial topaz in the Beechworth-Eldorado area may be derived from similar quartz-topaz dykes.

Geology and geochemistry of a F-Sn-W skarn system—The Hole 16 deposit, Mt Garnet, North Queensland, Australia

The Hole 16 deposit is a small unexposed F-Sn-W exoskarn deposit with underlying associated endoskarn, greisenized granite and largely ungreisenized Carboniferous ‘Elizabeth Creek’ granite. Sphalerite geobarometry indicates that this was a high level intrusion. The skarn formed above a granite cusp and is mantled by pure marble. Assemblages and textures representing successive stages of skarn genesis are: (1) massive andradite, (2) massive Sn-rich garnet + magnetite ± clinopyroxene ± fluorite, (3) ‘wrigglite’ which refers to a characteristic fine-grained contorted rhythmically layered texture, consisting of alternating layers of magnetite and fluoro-vesuvianite + fluorite, or cuspidine ± fluorite and magnetite ± Zn-Fe spinel, (4) massive fluoro-vesuvianite + fluorite, (5) Fe-F-amphibole + Fe-F-biotite + Fe-F-phyllosilicate + calcite ± scheelite, (6) chlorite ± hematite ± clay minerals. Assemblages (5) and (6) are found in very minor amounts throughout the skarn. Stages (1)–(4) also correspond to the skarn...

Quartz-topaz-loellingite rocks near Eldorado, Victoria

Two small dykes consisting of a quartz-topaz-loellingite rock type have recently been discovered within the aplitic phase of the Pilot Range granite, near Eldorado, in NE Victoria. Minor biotite, muscovite, chlorite, kaolinite, anatase and pharmacosiderite are associated. Apart from the loellingite, the dykes are similar mineralogically to the ‘topazites’ from New England, NSW. These were considered to be magmatic in origin, based on field relationships and high homogenization temperatures for fluid inclusions in topaz (Eadington & Nashar 1978, Contrib. Mineral. Petrol. 67, 433–438). Although experimental evidence on F-enriched ‘granitic’ systems is inconclusive, the emplacement of the Eldorado topazite dykes most likely involved both magmatic and hydrothermal components operating essentially simultaneously. The topazite melt represented a F-rich residual granitic magma, from which aqueous alkali halide-rich solutions separated during high level intrusion. Separation of these aqueous solutions was respons...

Petrology of the Campbell Island Volcanics, Southwest Pacific Ocean

Abstract The disposition and petrology of a fractionated alkali olivine basalt—peralkaline rhyolite suite from subantarctic Campbell Island are discussed. These rocks (Campbell Island Volcanics: new name) are flows and high-level intrusions derived from two centres of igneous activity. Their age is Upper Miocene and they evolved over a period of 5 Ma. A gabbro intrusion pre-dates volcanism by 5 Ma. The ages of the flows and high-level intrusions cannot be separated, although the intrusions are chemically distinct as they contain all the intermediate members of the suite (mugearite and benmoreite). Similar La/Ce and Zr/Nb ratios for flows and high-level intrusions suggest a co-magmatic origin. Chemical variations indicate that the suite resulted from low-pressure mafic then felsic-dominated fractional crystallisation, which is substantiated for intermediate members of the suite by least-squares and Rayleigh fractionation modelling. One flow of alkali olivine basalt clearly pre-dates other volcanic rocks, and is thus regarded as being genetically unrelated. Although chemically similar to alkali olivine basalt and hawaiite, variations in the mineral chemistry and modal mineralogy of gabbro indicates a prolonged period of in-situ fractionation and re-equilibration.

Alkaline peridotite, pyroxenite, and gabbroic intrusions in the Oman Mountains, Arabia

High-level intrusions of highly undersaturated alkalic ultrabasic and gabbroic rocks occur in four areas of the Oman Mountains. They all intrude either the Haybi volcanic – Oman Exotic limestone (Permo-Triassic) thrust slice immediately beneath the Semail Ophiolite (Cenomanian) or the uppermost thrust slice of the underlying Hawasina (Permian to Cenomanian) Tethyan sediments. Detailed structural mapping indicates that the sills were all emplaced prior to the Late Cretaceous thrusting of the Oman allochthon onto the Mesozoic continental margin of Arabia, and therefore in an oceanic setting. These differentiated sills consist of biotite wehrlites at the base and kaersutite-bearing jacupirangites above, with kaersutite gabbros at the top. Olivine occurs only at the base. Titanaugite, kaersutite, titanium phlogopite, apatite, and opaque iron–titanium oxides are common mineral phases.Fractional crystallization and gravity differentiation processes and a rapid increase in volatile components at decreasing pressures all played a part in the petrogenesis of these uncommon intrusive rocks. K–Ar ages on biotites span the mid-Jurassic to Cenomanian, and in the northern Oman Mountains kaersutite jacupirangites are incorporated into the Cenomanian–Turonian amphibolite facies metamorphic sheet beneath the Semail Ophiolite. Alkaline magmas were present at depth along the passive continental margin, right up until Cenomanian times when northeast subduction was initiated and compressional tectonics began. Alkaline volcanism of Cenomanian age in the Dibba Zone indicates that tensional rifting processes were operative along the continental margin at the same time as compressional thrusting was occurring outboard. The alkaline rocks are unrelated to the ophiolite but are artifacts of Mesozoic rifting events in Tethys now preserved in footwall thrust slices beneath the ophiolite.

Felsic mineral crystallization trends in differentiating alkaline basic magmas

Mineral compositional relationships have been studied in a variety of alkaline basic rocks from small, highlevel intrusions. These small intrusions must have cooled quickly as the phases are all zoned, especially those forming the matrix of some of the rocks which seem to have formed under conditions of almost perfect fractional crystallization.

Magma genesis and crustal spreading in the northern neovolcanic zone of Iceland: telluric-magnetotelluric constraints

Regional telluric-magnetotelluric measurements performed as a cooperative research project between Brown University and the National Energy Authority of Iceland are interpreted in terms of physical processes in the crust and upper mantle associated with magma genesis and crustal spreading. First, these results confirm, over a larger region than was previously studied, the presence of a relatively conducting crust (25–30 ohm-m) at shallow depth (d ≦ 4.5 km) reflecting electrolytic conduction in hydrothermal pore fluids on a regional scale. Secondly, and perhaps of greatest interest to current work on processes of accretion along plate boundaries, is the presence of an anomalous conducting layer (T ≦ 4 km, ρ ≦ 10 ohm-m) at the base of the crust (8–15km) beneath the neovolcanic zone. This layer we feel represents an accumulation of molten magma. There is some suggestion that it is at shallower depth beneath the most active portion of the neovolcanic zone, and that its depth increases with age of the crust. The presence of this feature suggests that the crust is being underplated by the accretion of molten material to its base which, as it solidifies, contributes a significant component of crustal thickening. We feel that the molten phase residing in this basal zone represents an available source of magma for high level intrusions. Finally, our results confirm that, beneath the zone of magma accumulation, the resistivity changes little with depth in the mantle. Although there may be an indication that the upper few tens of kilometres of the mantle is somewhat more conducting, reflecting the presence of a minor amount of melt, this is not a firmly resolved feature of our analysis. Nevertheless, the mantle has almost a constant resistivity to a depth exceeding 100 km. Assuming a homogenous composition for the mantle beneath Iceland, the vertical gradient of temperature is of the order of only a few °Ckm‒ or less.

Geology and seismic structure of the northern section of the Oman ophiolite

In the north Oman mountains, a continuous ophiolite succession is exposed, from tectonized harzburgites and dunites at the base, through layered gabbros and peridotites, high‐level gabbros and plagiogranite, to a dike swarm and pillowed volcanics overlain by pelagic shales. The upper part of this sequence possesses a static metamorphic overprint, which passes downward from greenschist facies in the lowermost volcanics and most of the dike swarm to amphibolite facies in the lowermost dike swarm and in the high‐level intrusives. These pervasively altered rocks are underlain by layered gabbros and peridotites, where hydration is restricted to fractures. Compressional and shear wave velocities have been measured to confining pressures of 6 kbar for 139 water‐saturated cores from this sequence. The samples were field‐oriented and collected from known stratigraphic levels, which allows the construction of velocity‐depth profiles and an examination of anisotropy within the ophiolite. Compressional wave velocities measured for the basalt section are quite variable, ranging from 4.5 to 6.0 km/s at in situ pressures. Within the sheeted dike section, Compressional and shear wave velocities vary from 5.4 to 6.3 km/s and from 2.9 to 3.6 km/s, respectively. From the basalt‐sediment contact through the dike section, velocities increase rapidly with depth. This gradient is related to increasing metamorphic grade and a decrease in grain boundary porosity. The top of seismic layer 3 is located near the lower boundary of the sheeted dike swarm and marks the transition from greenschist facies to amphibolite facies metamorphics. The dikes decrease in abundance downward into the high‐level intrusives: coarse‐grained gabbroic and granitoid rocks, which often contain abundant hornblende, with isotropic textures and lacking compositional layering. Mean compressional and shear wave velocities increase uniformly from 6.7 and 3.6 km/s, respectively, in the highlevel intrusives, to 7.5 and 3.9 km/s, respectively, at the base of the layered sequence. Measured velocities of the ultramafic tectonites are highly variable because of the anisotropy and serpentinization. Petrofabrics of relic olivine in the Wadi Ragmi region show strong preferred fabrics for the tectonites, and calculated compressional wave velocities for the serpentine‐free rocks range from 7.8 to 8.5 km/s, with the fast direction parallel to the direction of spreading inferred from dike orientations, in excellent agreement with observed upper oceanic mantle seismic anisotropy. The crust‐mantle boundary, which is defined by the abrupt increase in seismic velocities, coincides with the petrological contact between layered gabbros and peridotites and tectonized harzburgite and dunites.

The Topsails igneous complex: Silurian–Devonian peralkaline magmatism in western Newfoundland

The Topsails igneous complex of western Newfoundland is a large (&gt; 6000 km2) intrusive complex of two contrasting geochemical suites and at least four distinct granitoid facies: (i) a fine-to medium-grained biotite-bearing peraluminous granite; (ii) a medium- to coarse-grained metaluminous biotite–hastingsite granite; (iii) a coarse-grained peralkaline aegirine-arfvedsonite granite; and (iv) fine- to medium-grained biotite-hornblende granites and syenites. Peralkaline microgranite dykes cut the other granitoid phases and the country rocks. Peralkaline granites occur within the central parts of the complex, the other granitoid phases being marginal. Volcanic and subvolcanic roof pendants are divisible into peralkaline and non-peralkaline groups. The former are clearly related to the peralkaline granite, and the latter appear to represent the other intrusive phases of the complex. Contemporaneous basalt magmatism is indicated by dykes which cut the biotite granites and are in turn cut by the peralkaline granite. Available Rb–Sr data show ages of 386 ± 9 and 419 ± 5 Ma for the peraluminous and peralkaline granites respectively.The main difference in major elements between the peralkaline and non-peralkaline granites is in their agpaiitic indices, although there is some overlap between them. There are, however, marked dissimilarities in their respective trace element characteristics, e.g., K/Rb, K/Zr, and Rb/Sr, with high Zn concentrations being especially characteristic of the peralkaline rocks.The origin of these rocks is interpreted in terms of a model involving crustal melting due to the interaction of crustal extension, basaltic intrusion, and volatile fluxing, with subsequent high-level intrusion and fractionation, along with metasomatism by magmatically-derived fluids.

The Mafic and Ultramafic Lavas of the Belingwe Greenstone Belt, Rhodesia

The Belingwe Greenstone Belt (2.8 × 109 yrs old) contains a 7 km succession of mafic and ultramafic lavas and high-level intrusions which overlie a thin sedimentary formation, itself unconformable on a granitic basement. The lavas range in composition from andesites (4 per cent MgO) to peridotitic komatiites (32 per cent MgO). The mineralogy and textures of the most magnesian lavas demonstrate that they were extruded in a completely liquid state. If the source mantle had an MgO content around 40 per cent, then partial melts in the range 35 per cent to 55 per cent would be required to produce the most magnesian liquids observed. Chemical constraints on the petrogenesis of the ultramafic lavas allow estimates of source mantle composition. In particular, if the source had an MgO content around 40 per cent, then the overall source composition would be similar to that of garnet Iherzolite nodules in kimberlites. The calculated REE contents of the source are close to chondritic. If all the ultramafic lavas were derived from the same source then the variation in liquid composition may have been controlled by orthopyroxene as well as olivine during partial melting at depth. The evolution of the less magnesian komatiites, basalts, and andesites can be explained by lower degrees of partial melting of a common source, and by high-level fractionation of parent liquids similar to those extruded as ultramafic lavas. Physical constraints on the origin of the lavas imply derivation from a depth of 150 km or more, at temperatures of 1600–2000 °C.

The Carbonatite of Hogenakal, Tamil Nadu, South India

Abstract The carbonatite complex (pyroxenite-syenite-carbonatite) of Hogenakal area of Tamil Nadu and adjoining parts of Karnataka were emplaced along NNE - ssw fracture zones within the Precambrian gneissic complex. The carbonatite is Sovite in composition and its characteristics are (1) occurrence as fracture filling dykes, (2) presence of agglomeratic and flow structures and (3) absence of dolomitic carbonatite or beforsite, true fenite and major rare earth mineralization. The carbonatite of Hogenakal belongs to the high temperature and deep level intrusion of sub-volcanic origin. It has certain similarity with that of Strangways Range area of Central Australia and differs in certain respects from other carbonatite occurrences of Tamil Nadu.

The mineral chemistry and origin of xenoliths from the lavas of Anjouan, Comores Archipelago, Western Indian Ocean

Mineralogical data for xenoliths occurring as inclusions in the fissure erupted alkali basalts and the basanitic tuffs of Anjouan reveal three xenolith suites: 1) the lherzolites, 2) the dunites and wehrlites, 3) the gabbros and syenites. The “ dunite-wehrlite ” suite and the gabbro suite are shown to represent high-level cumulate sequences resulting from ankaramitic fractionation of the hy-normative shield-building lavas and cotecictic fractionation of the alkali basalt lavas respectively, whilst the syenitic xenoliths represent evolved high-level intrusions. Mineralogical and rare earth element (REE) data indicate that the most likely origin for the spinel lherzolite xenoliths is by extraction of a basaltic phase from spinel peridotite, leaving a light REE-poor spinel lherzolite residuum. REE models, constructed using model peridotite assemblages, imply that the hy-normative basalt lavas may be derived by partial melting of spinel peridotite at pressures of 20–25 kb. The spinel lherzolite source for the hy-normative basalts has been accidentally sampled during explosive eruption of the alkali basalt and basanite magmas.

Geology and geochemistry of the St. Lawrence peralkaline granite and associated fluorite deposits, southeast Newfoundland

The southern part of the Burin Peninsula of southeast Newfoundland is marked by extensive Carboniferous granite magmatism, intruding Late Precambrian to Middle Cambrian volcanic and sedimentary rocks. The St. Lawrence pluton, representative of this Carboniferous magmatism, ranges from equigranular to porphyritic in texture, displays much evidence of gas brecciation, and contains no pegmatites or schlieren, all features of high-level intrusion. The bulk chemistry corresponds to cotectic compositions in the synthetic Q–Ab–Or–H2O system for 500 bar [Formula: see text], likewise suggesting shallow levels of intrusion and crystallization. The granite is alkaline to peralkaline and of commenditic affinity, with agpaitic indices around 1 and modal aegirine and riebeckite. Its chemical affinity with less silicic alkaline granites of the same age in southeast Newfoundland suggests an origin by differentiation of such granites. A high initial Sr87/Sr86 ratio of 0.722 suggests substantial interaction with or derivation from continental crust. The St. Lawrence fluorite deposits occur as veins confined to and derived from the granite during the final stages of cooling.

A simple-shear model for the ductile deformation of high-level intrusions in South Greenland

Major ring-intrusions in the Gardar Province are frequently elliptical in cross-section and are located at the intersections of regional dyke swarms with sinistral wrench-fault systems. Fault movement was contemporaneous with igneous intrusion and the ellipticity may be explained by ductile deformation of hot intrusions undergoing large-scale simple-shear. Geometrical measurements on four of the intrusions give shear-strain values ( γ ) ranging from 0.6–1.2. Away from the intrusions, deformation is entirely by brittle fracture and the amount of fault movement correlates well with the sum of brittle and ductile movement within the intrusions.

The Camsell River–Conjuror Bay Area, Great Bear Lake, N.W.T.

Roof pendants of a late Aphebian (~1800 m.y.) volcano–sedimentary complex, within Hudsonian granites, outcrop on the eastern shore of Great Bear Lake. The volcano–sedimentary complex is similar to, and may be correlated with, the Echo Bay Group which outcrops farther to the north.The volcanic rocks are interbedded with immature volcanoclastic sedimentary rocks and with thinner units of calcargillite and dolomite. A unit of conglomerate, tuff, and siltstone, previously correlated with the Cameron Bay Group, is assigned to the 'Balachey Unit', until its relationships with other groups in the area are more clearly understood.Hudsonian granitic stocks and batholiths (~ 1750 m.y.) intruded the complex. Sharp contacts and narrow aureoles characterize these as high level intrusions. Zones of sulfide replacement are common in the aureoles. Porphyry dikes and stocks of similar age to the granites and rare, pegmatitic magnetite–apatite–actinolite bodies, also intrude the volcano–sedimentary complex.Three sets of diabase intrusions are identified, and related to three distinct fault directions. Giant quartz veins and mineralization of the U–Ag–Ni, Co arsenide – Bi type are found in zones of northeast-striking dextral faults.The volcano–sedimentary complex is interpreted as the molasse phase of the rising orogen of the Coronation Geosyncline, and is related to earlier deposits (the Snare and Epworth Groups) closer to the craton in the eastern part of the geosyncline.

Archaean mafic and ultramafic rock associations in the Eastern Goldfields region, Western Australia

Recent studies in Archaean volcanic belts of the Eastern Goldfields region of Western Australia have led to the development of a general picture of Archaean volcanic activity in this area. Three volcanic associations are recognized: a felsic volcanic-porphyry-clastic and volcanogenic sediment association, a pillow basalt-dolerite association and an ultramafic association. The ultramafic association consists of high-Mg basalts and zoned ultramafic lenses, both with textures indicative of high level intrusion or extrusion, and numerous small layered sills. The association commonly forms the lower parts of individual volcanic sequences, and is overlain by basalt. In some areas felsic volcanic rocks terminate this sequence. It is believed that the mafic and ultramafic associations represent two distinct magma sources. Elements of similarity between these stratiform belts and more recent oceanic crust are discussed.

Volcanism and its relationship to recent crustal movements in the Canadian Cordillera

"Compressive" folding, thrusting, and transcurrent fault movement have occurred during late Tertiary and Recent times in parts of southeastern Alaska and southern Yukon west of a line joining the Denali, Queen Charlotte, and San Andreas fault systems. East of this line "compressive" deformation and transcurrent faulting ceased in the Canadian Cordillera by late Eocene time or earlier, and was followed by a period of crustal relaxation that has continued to the present. The onset of relaxation was accompanied by acid volcanism, block faulting, and high-level intrusion of granitic plutons and dike swarms. Explosive eruptions that characterized early Tertiary volcanism were followed in late Miocene and Pliocene times by quiet outpouring of plateau basalt in central British Columbia and later, during Pleistocene and Recent times, by construction of nearly 150 cinder cones and strata volcanoes. Most of these young volcanic centers are confined to two north-south trending belts and an east–west belt offsetting them. The two north–south belts parallel the direction of post Eocene dike swarms and normal faults, and are considered to have formed in response to east–west regional extension that has persisted in the western Canadian Cordillera since late Eocene time.