On the Origin of Ultramafic Rocks

KENNETH C. McTAGGART Department of Geology, The University of British Columbia, Vancouver 8, Canada

On the Origin of Ultramafic Rocks

ABSTRACT Pipes of the Bushveld Complex may have formed entirely by replacement along and It is argued that most alpine ultramafic adjacent to "gas chimneys." bodies originated as cumulates in basic magma chambers high in the crust. From some of these INTRODUCTION chambers, magmas were intruded upward or Three principal types of ultramafic bodies are extruded, leaving sill-like ultramafites behind. now recognized: (1) stratified differentiates of These were subsequently folded or dismem- basic magmas, exemplified by those of the bered by faulting, and, because of their high Bushveld, Stillwater, and Muskox complexes; density, subsided during tectonism, to form (2) alpine ultramafic bodies, such as those of the cold, fault-enclosed intrusions. In other places, Appalachians, California, British Columbia, tectonism interrupted crystallization of the and New Zealand; and (3) zoned ultramafic magmas, magma chambers were deformed, bodies, for example, those of southeastern loose cumulates and hot coherent cumulate Alaska and the Ural Mountains. rocks slumped and slid, and hot masses, as There is little difficulty in distinguishing crystal mushes and hot lenses, worked down typical members of the 3 groups. Criteria for fault zones. These hot masses were subject to them have been given by Hess (1955), Thayer high temperature recrystallization and meta- (1960), and Taylor and Noble (I960). Never- somatism, as water from adjacent rocks theless, many bodies are difficult to fit into this permeated them, forming veins of pyroxenite threefold classification. and dunite, and intercumulus plagioclase was Smith (1958) has pointed out that the Bay of destroyed. Contact aureoles were formed, but Islands complex of Newfoundland seems to some were abandoned as the cooling ultra- represent a type intermediate to the stratified mafites descended. differentiates and the alpine type. Watkinson The Bay of Islands complex and the Great and Irvine (1964) described bodies of horn- Dyke are considered to be examples of bodies blende peridotite, hornblendite, and feld- whose origins as stratiform differentiates is spathic hornblendite from western Ontario clear but which show some of the features of that in form and structural relations are like alpine ultramafic masses. It is argued that alpine ultramafite (second type), but which are Franciscan peridotites are differentiates that believed to have been formed by fractional have been folded or have descended along crystallization of tholeiitic magma and thus in faults during tectonism. It is hypothesized that origin resemble the first group. Some of these ultramafites of British Columbia are com- (Watkinson and Irvine, 1964, p. 71), with plementary to late Paleozoic or Triassic vol- central parts of peridotite and marginal parts of canic rocks. hornblendite, resemble members of the class of Zoned ultramafic complexes of Alaska, zoned bodies, the third group mentioned British Columbia, and the Urals, currently above. Challis (1965) proposed that the alpine believed by many to have formed by crystal- peridotites of New Zealand were originally lization of ultramafic magmas, are re-examined. stratiform ultramafic differentiates comple- It is concluded that the central peridotites of mentary to Permian volcanics. Raleigh (1965, these consist of cumulates from basic magmas p. 739) suggested that certain alpine ultra- and have been dropped as crude cylinders along mafites in northwestern Washington were ring fractures or are downward intrusions of cumulates and that parent magma could have ultramafic crystal mushes. Their zoning is due been removed along an active fault. Gresens to subsequent metasomatism by hydrous (1970) stated that ultramafic rocks of the fluids that were guided by marginal fractures. Franciscan may be differentiates of basaltic

Geological Society of America Bulletin, v. 82, p. 23-42, 2 figs., January 1971 23

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magma high in the crust. Irvine (1967) sug- p. 401), "tectonic emplacement may be of two gested that, at Duke Island, Alaska, zoned types: (1) by solid flow much as salt flows—this bodies, representatives of the third class, are seems limited to olivine rocks at considerable possibly differentiates of basic magma, the depth, and (2) as fault-bounded blocks or balance of which was intruded upward or per- slices without internal flow—this is char- haps extruded as lava. acteristic of serpentines at shallow depths. . . . Thus, there is a body of opinion that many Thus solid serpentine bodies may move into examples of the three types are differentiates overlying sediments in much the same way that from basic magmas and are, therefore, only a water-melon seed moves when squeezed be- variations of a single type. The writer concurs, tween one's fingers." Green (1964, p. 179; and the purpose of this paper is to bring to- 1970, p. 2163) argued that a relatively hot gether evidence in support of this thesis and to peridotite, subjected to horizontal stress, account for the differences between the three would be intruded upward as a diapir. He cited types that seem at first to refute the idea. the Lizard ultramafite as an example. It is generally agreed that the ultramafk Many have argued for the intrusion of layers of the stratiform complexes, like the crystal mushes (Bowen, 1928; Smith, 1958; Bushveld Complex, are cumulates from basic Auken, 1959; Mackenzie, 1960; Thayer, 1963; magmas. Wager and Brown (1968) have Lipman, 1964), possibly lubricated by in- provided a comprehensive modern account of terstitial magma or watery fluid. There are few this type, and there is no need to discuss it recent suggestions that alpine peridotites are here. emplaced as ultramafic magma but (see below) such magmas are in favor for the zoned com- ALPINE ULTRAMAFIC BODIES plexes of the third type. The writer does not believe that solid General Nature peridotite or crystal mush would rise along Alpine ultramafic bodies (Hess, 1955; faults from the mantle to high levels of the Thayer, 1960) are found in folded erogenic crust, impelled by random squeezing. Crustal belts where they occur as lenticular, sill-like or squeezing ought, other things being equal, to irregular masses, a few feet to a few miles thick push the peridotite down as often as up. In- and up to scores of miles long, commonly lying deed, since the density of peridotite is about in faults parallel to the tectonic trend. Most are 3.3 and that of upper crustal material about of peridotite, contain chromite, and are devoid 2.7, or perhaps less, peridotite should sink of plagioclase. Olivine and pyroxene are re- rather than rise. What is needed is closely con- ported to be nearly constant in composition in trolled squeezing—a kind of peristalsis—but most bodies but in others to show a narrow it is difficult to see how this might be provided. range, generally unrelated to structure. In The writer proposes that many alpine many bodies, olivine is very close to Fogj, and peridotites have descended along faults or associated orthopyroxene is En90 (Table 1). fractures from differentiating or differentiated Some bodies have layers of pyroxene, olivine, gabbroic bodies rather than rising from the or chromite. Some contain dikes or veins of mantle. The behavior of these can be con- orthopyroxenite, clinopyroxenite, peridotite, trasted with that of salt intrusions. Moderately or dunite—these do not extend into adjacent thick salt beds interbedded with average sedi- country rock. Most are moderately to strongly mentary rocks are gravitationally unstable (the serpentinized. Although adjacent rocks may density of salt is 2.17) and subject to con- show thermal effects, most appear to be un- vective overturn. Such salt tends to flow affected. Bodies of gabbro are associated with toward and rise at a high point in the salt bed many of them, but these are generally small to form an intrusion into and through the compared to the ultramafic masses. overlying beds. It is suggested here that many alpine peridotites behave as negative salt in- Origin trusions and, because of their high density, Many consider that alpine ultramafites are tend to subside along faults, especially, and slices of the mantle that have moved along perhaps only, during tectonism. faults or have been intruded as diapirs into the In the case of the stratiform ultramafic dif- upper parts of the crust (Taliaferro, 1943; ferentiates (type 1, above), magmas are em- Bowen and Tuttle, 1949; Ragan, 1963; Green, placed in a variety of magma chambers 1964; Ernst, 1965). According to Hess (1955, ranging from compact to lopolithic or sill-like,

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TABLE 1, COMPOSITIONS OF OLI VINES AND ORTHOPYROXENES

Olivine Orthopyroxene Fo En

Alpine Ultramafics Addie, North Carolina 92-93.5 87.5-94.5 Miller, 1953 Burro Mt., California 91 90 Burch, 1968 Klamath Mts., California 91-93 84-93 Lipman, 1964 Twin Sisters, Washington 90 90 Ragan, 1963 Cypress Island, Washington 89-93 90 Raleigh, 1965 Shulaps Range, British Columbia 87-92 89.5-92.5 Leech, 1953 Aiken Lake, British Columbia 87-96 100? Roots, 1954 McDame Area, British Columbia 87-95 88-96 Gabrielse, 1962 South Island, New Zealand 89-94 88.5-93 Challis, 1965 Tinaquillo, Venezuela 90 90-92 MacKenzie, 1960 Lizard, Cornwall 89.7-91 89.0-91.1 Green, 1964 Ultramafics of the Stratified Complexes Stillwater, Montana 80-88 77-87 Hess, 1960 81-90 83-88 Wager and Brown, 1968 Bushveld, South Africa 86-88? 75?-83 Wager and Brown, 1968 Great Dyke, Southern Rhodesia, 86-92 86-93 Worst, 1958 lower 6000 ft Muskox, Canada, lower 4000 ft 80-85 Wager and Brown, 1968 Intermediate Type Bay of Islands, Canada 92 93-97 Smith, 1958

in nonorogenic regions where they differ- and Thompson, 1967) may represent feld- entiate under quiet conditions and remain rel- spathic differentiates dragged down by the atively intact and undisturbed until exposed sinking ultramafites, or may represent only a by erosion. The alpine peridotites, on the thin layer of gabbroic differentiates that ac- other hand, are formed in erogenic regions cumulated before the bulk of the magma was (Smith, 1958; Raleigh, 1965; Gresens, 1970) drained off. Finally, if orogeny intervenes where gabbroic magmas differentiate high in during crystallization of the basic magma, the crust. One of several lines of development ultramafic differentiates are jostled, loose may then be followed. After accumulation of crystal accumulations slump and slide. Hot thick ultramafic layers, the lightened overlying coherent cumulates are faulted and dismem- magma may rise along developing faults and bered. Because of their high density, these fractures to form shallow intrusions or ex- differentiates, still hot, may intrude downward trusions. Subsequent folding of residual sill-like along faults or unconformities, as crystal masses of peridotite may form domed sheets, mushes or solid bodies, producing thermal like the Webster-Addie ultramafic ring (Miller, effects in adjacent rocks (Gresens, 1970). At 1953), the synclinal Red Mountain, California, later stages, especially during subsequent pluton (Maddock, 1964), or tightly folded tectonism, the ultramafites may descend to tabular bodies like those of the Klamath greater depths, leaving behind any earlier Mountains (Lipman, 1964). With intense formed contact metamorphic aureole. folding and faulting, sill-like bodies may be The advantages of the proposed mechanism sliced into lenses, and some of them work down of downward intrusion are several. The high faults, as cold intrusions. Alternatively, some density of the ultramafites becomes an asset magmas crystallized entirely within the magma rather than a liability to their emplacement in chambers, to form complete differentiated the crust. Certain of the cumulates will arrive bodies like the Stillwater Complex. If these at their present sites relatively hot, producing were later involved in orogeny, ultramafic thermal aureoles; others will arrive cool. It is masses would tend to become detached from difficult to believe that mantle peridotites less dense overlying gabbros and, if opportunity would arrive in the upper part of the crust by a offered, to plunge to greater depths along process of slow creep along faults or diapiric rise faults. Relatively small layered gabbros, some while still retaining their original heat. Their with cumulate textures, commonly associated location along faults of all types, gently dipping with ultramafites (Leech, 1953; McTaggart thrusts, steep normal faults, and strike-slip

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faults—in which a great variety of stress con- acteristic of stable regions. Brown (1967, p. ditions must obtain—is easily explained. 137) stated that there is not enough informa- Are there criteria for distinguishing ultra- tion to allow generalizations on the com- mantes that crystallized at low pressure from positions of olivines separating from various those that crystallized at high pressure? The types of basalt magma. Rothstein (1957) criterion of relatively high A^Os in pyroxenes described cryptic layering (and rhythmic (Green, 1964) has been questioned by Ito and layering) in the Dawros alpine ultramafite. Kennedy (1967). Xenoliths of eclogite are not Lipman (1964, p. 206), in his careful study of diagnostic because these could be brought up the Trinity pluton in northern California, by basic magma and could settle among the reported compositional variation in olivine cumulates. Experiment supports a high-pres- (cryptic layering?), which he correlates with sure origin for garnet peridotites, but these are height above the base of the folded tabular rare and the criterion not generally useful. alpine ultramafic pluton. That cryptic layering Green (1964, 1967, 1970) stated that the as- has rarely been reported from alpine ultra- semblage: olivine-aluminous orthopyroxene- mafites may be due to its narrow range even in aluminous clinopyroxene-aluminous spinel is stratiform complexes (Table 1), to its dis- stable at high pressure, whereas olivine- ruption during tectonism, to its destruction plagioclase-pyroxenes-chromite forms a chem- during recrystallization and metasomatism ically identical low-pressure assemblage. This (see below), or to a kind of crystallization conclusion is supported by the experimental peculiar to syntectonic magma chambers. For work of Kushiro and Yoder (1966) who example, if crystallization in the magma studied the reaction: chambers from which the alpine bodies were orthopyroxene + clinopyroxene + spinel derived was such that olivine nucleated and —> anorthite + forsterite grew uniformly throughout the magma cham- ber, was held is suspension for a long time, and They determined that the univariant line for finally accumulated relatively quickly, there the reaction lay nearly parallel to the tempera- would be little compositional variation even ture axis and that the reaction proceeded to the though these crystals would accumulate in right at about 8 kbars and 1200°C with falling layers. Crystallization during ascent of the pressure or increasing temperature. The posi- magma or during continued deformation of the tion and slope of the univariant line for the magma chamber would inhibit crystal accumu- reaction, however, have not been closely deter- lation until a late stage when a great crop of mined and, moreover, the effects of Cr, Fe, Na suspended crystals of uniform composition etc., present in natural ultramafites, were not would collect on the floor of the magma taken into account. It is conceivable, therefore, chamber. Sudden loss of volatiles, due to that in natural rocks the supposedly high pres- fracturing of the chamber roof, leading to sure assemblage would crystallize at low or supercooling, might cause rapid and pervasive moderate pressure. Burch (1968), summarizing crystallization before convection could set in. evidence for mantle origin of certain Cali- The shape of the magma chamber, controlling fornia peridotites, considered the strongest distribution of heat loss, is probably an im- evidence to be lack of an alternative source. portant factor in controlling crystallization and What are the objections to the hypothesis? differentiation (Hess, 1960; Bartlett, 1969). It has been pointed out that olivine and A second group of features not accounted for pyroxene in these bodies are more magnesian by the hypothesis outlined above includes the than those of the stratified complexes (Table 1). abundant dikes, sills, veins, flow layers, and The latter also show small but regular differ- irregular bodies of dunite, peridotite, orthopy- ences (cryptic layering) with level of accumu- roxenite, and clinopyroxenite that cut the lation. The generally more magnesian com- alpine bodies. These have been described by position of the mafic minerals of alpine ultra- many (Leech, 1953; Smith, 1958; Thayer, mafites as compared with those of stratiform 1960, 1963; Gabrielse, 1963; Aitken, 1959), intrusions may be due to variations in the and detailed accounts are given by Little original differentiating magma—possibly the (1949), Lipman (1964), Challis (1965), and parent magmas of the alpine type, evolved Burch (1968). That these bodies originated by during early tectonism, are more magnesian metasomatism, recrystallization, and high- than those of the tholeiitic magmas char- temperature hydrothermal vein formation, as

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suggested by several of these authors, rather (Little, 1949) may be metasomatic, or may be than by injection of ultramafic magmas, is in- due to slumping and auto-intrusion by plunging dicated by lack of dilation offset by diagonally cumulates, later to be modified by recrystalliza- cross-cutting veins (Burch, 1968, but see tion and metasomatism. Moores, 1969, p. 27), their coarseness of grain The writer would compare these processes (commonly several inches), concentration of with the somewhat similar process that goes on chromite in wall rocks adjacent to the veins, at lower temperatures. At temperatures below zoning of veins, and orientation of elongated about 500°C, large alpine ultramafic bodies can crystals at high angles to the vein walls. In be converted partly or completely to ser- many alpine bodies (Thayer, 1963, 1967; pentinite. This change involves complete and Moores, 1969) layering in peridotite can be pervasive recrystallization and metasomatism traced across contacts into adjacent gabbro or in the presence of water, and, if serpentiniza- into adjacent pyroxenite, and so it appears that tion is volume-for-volume as is believed by either the layering or the contact is of second- some (Thayer, 1966; but see also Hosteller and ary origin. The correspondence in composition others, 1966), it involves also considerable of olivine and pyroxene of the veins with those redistribution of MgO or SiO2 (or both). of the host rock, commonly peridotite (one Veins of chrysotile in massive serpentine are exception noted by Challis, 1965, p. 334), is analogous to the anhydrous veins formed at strong evidence that large volumes of the ultra- higher temperatures. It is possible that some mafic masses were in equilibrium with per- serpentinization is simply an extension to low meating solutions, and led quite commonly to temperatures of the metasomatic processes recrystallization and re-equilibration of olivine postulated above, but it is probable that many and pyroxene through large volumes of rock ultramafic masses cool under dry conditions within the plutons (Himmelberg and Loney, and are later serpentinized. 1969). Compelling evidence of widespread Another difficulty with the hypothesis that metasomatic replacement of clinopyroxenite alpine ultramafites are differentiates of basic by dunite in a zoned ultramafic body (Irvine, magma is that the former rarely contain any 1967) is summarized in a later section of this feldspar, whereas the latter almost invariably report. do. The ultramafic part of the Stillwater Com- The evidence cited and also other evidence plex is estimated (Hess, 1960) to contain 7.5 to brought forward in connection with the origin 12 percent of intercumulus calcic plagioclase. of zoned ultramafics (below) indicate that Challis (1965) suggested that filter-pressing and peridotites and dunites are highly susceptible to diffusion (Hess, 1960) could account for the metasomatism (Bowen and Tuttle, 1949; Hess, complete absence of feldspar. It is clear that 1960). It is suggested that the process goes for- these processes or some other mechanisms are ward as water (absorbed from adjacent sedi- highly effective in removing intercumulus mentary and metamorphic rocks) gains access magma because the lower 6000 ft of the ultra- to hot cumulates after disruption of the magma mafites of the Great Dyke contain only about chamber or as the hot cumulates descend from 0.5 percent plagioclase (Worst, 1958) and a their original sites, and takes place at tempera- section more than 10,000 ft thick of dunite and tures well above those of the formation of talc peridotite of the lower part of the Bay of or serpentine. Especially strong or prolonged Islands complex is devoid of feldspar (Smith, metasomatism and mineralization along frac- 1958). tures allow formation of veins or irregular On the assumption that alpine ultramafites bodies of pyroxenite or dunite. As these are are cumulates from basic magmas, how can the sealed, continuing tectonism opens new ones, to absence of feldspar be explained? There are at allow fresh solutions to produce a new suite of least three possibilities: (1) filter-pressing and veins. diffusion during the magmatic stage, the com- During this suggested recrystallization, orig- ponents of plagioclase remaining in the liquid inal cumulate textures are blurred or de- fraction later removed; (2) destruction of stroyed. Early cataclastic textures developed plagioclase, its components going into other during movement are obliterated, but if minerals; and (3) solution and transport of cataclasis outlasts recrystallization, late textures plagioclase to layers or replacement bodies will survive. Certain very large bodies of within the ultramafite or to adjacent wall rocks. dunite apparently cutting across peridotite For those ultramafites that show no signs of

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recrystallization or metasomatism the writer and an overlying gabbro part, partly layered, 2 favors filter-pressing and diffusion. For other or 3 mi thick. In its general aspect, it resembles bodies, it is more reasonable to suppose that a differentiated gabbroic layered complex. The plagioclase was destroyed. If this occurred, the ultramafic rocks, however, in their lack of Na of the original might remain in the ultra- cryptic layering (Table 1), in the complete mafite. Analyses by Steuber and Goles (1967) absence of intercumulus plagioclase in the main show that alpine dunites contain about 100 part of the ultramafic zone, in the presence of ppm Na, alpine peridotites about 1000 ppm cross-cutting dunite bodies, and in the absence Na, and that alpine intrusions of all types of obvious textural evidence of crystal sedi- average about 500 ppm Na. These amounts of mentation from magma, resemble typical Na would be contained by about 0.12 percent members of the alpine class. Smith suggested albite, 1.2 percent albite, and 0.60 percent (1958, p. 81) that, in general, stratified ultra- albite, or 0.6 percent Ango, 6 percent Ango, and mafites and alpine ultramafites were derived 3 percent Ango respectively. These analyses are from similar parent magmas and that the Bay of compatible with the suggestion that plagioclase Islands complex is intermediate to the two reacted to form other minerals. For example, in types. the Lizard complex, Green (1964) shows that The Great Dyke of Southern Rhodesia at low temperatures, plagioclase, pyroxene, (Worst, 1958; Wager and Brown, 1968, p. 460) olivine, and water reacted to form pargasite. extends for about 330 mi and has an average Smith (1958) described the alteration of in- width of about 3.5 mi. It shows rhythmic and terstitial plagioclase to hydrogarnet in the up- cryptic layers that dip gently toward the mid- per part of the ultramafic zone of the Bay of dle of the dike. About 10,000 ft of layers have Islands complex. Green (1964), however, con- been studied, of which the lower 7000 ft are cluded that plagioclase in marginal parts of the ultramafic and the upper part is composed of Lizard complex was the product of a reaction various types of gabbro. The ultramafites are between pyroxenes and spinel with falling "similar in scale and mineralogy to the units pressure. The writer would suggest the alterna- in the Basal Zone of the Bushveld and in the tive—that plagioclase was destroyed in the Ultramafic Zone of the Stillwater intrusion" central part of the complex with falling tem- (Wager and Brown, 1968, p. 463). Worst perature. Why is not the plagioclase of the (1958) concluded that the "dyke" is not a stratified complexes destroyed in one of these dike but a graben in which are preserved the ways? Such reactions may be facilitated by the downfaulted lower parts of several coalescing presence throughout the hot mass of water, differentiated lopoliths. The range of com- which, at lower temperatures, causes serpentin- positions of olivines and pyroxenes is com- ization. parable to those of the Shulaps alpine body The third possibility, that plagioclase is not (Table 1), and the scarcity of plagioclase has only destroyed but also transported, is sug- been mentioned above. The resemblance in gested by calcic plagioclase in secondary layers gross shape and mineralogy to the alpine ultra- in peridotite (Mackenzie, 1960) and in feld- mafic bodies is striking. One can speculate that spathic pegmatites (Lipman, 1964) within the had the Great Dyke been involved in active ultramafites, and by the presence of albite in tectonism rather than passive rifting and contact rocks (Challis, 1965). graben formation, with disruption or modifica- If the general hypothesis outlined above is tion of primary layering and with recrystal- true, there ought to exist bodies that have lization and metamorphism, it would be in- been only partly modified, forming types inter- distinguishable from the classic alpine bodies. mediate to alpine and stratiform ultramafites; the Bay of Islands complex and the Great Dyke Alpine Ultramafites in California appear to be examples of these. Dietz (1963) related the emplacement of The Bay of Islands complex of western New- serpentine and other ultramafic rocks to under- foundland (Smith, 1958; Wager and Brown, thrusting of the continental margins by oceanic 1968, p. 451) is made up of four principal layers. He (p. 949) suggested that during un- masses, forming a discontinuous belt some 60 derthrusting, the "continental rise is converted mi long and 20 mi wide. The complex consists into a eugeosyncline. . . . Bodies of spilite, of an ultramafic lower part, mainly of layered serpentine, and ultramafic rock become incor- peridotite and dunite, with a chromite zone porated as fragments of the sea floor." He cites near the top, totalling 2.5 to 4 mi in thickness, the Franciscan formation, with its abundant

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ultramafites, as illustrating the hypothesis. described in reports by the Geological Survey Ernst (1965), Burch (1968), and others have of Canada and in many of these, based only on suggested that these ultramafites were em- reconnaissance, ultramafites are given brief con- placed from the mantle. sideration. The writer will attempt to sum- Tectonic emplacement of serpentinite (S.G. marize the nature of these ultramafites and 2.3) upward along faults seems possible, but recent views on their origin and will then con- many of the ultramafites of the Franciscan are sider how they fit the hypothesis of differentia- only slightly serpentinized, and others seem to tion of basic magma, and subsiding cumulates. have been serpentinized after emplacement and Alpine ultramafites range in size from bodies before folding (Saad, 1969) The difficulty has a few tens of feet across to the more than been stated by Burch (1968, p. 542) who 70-sq-mi bodies of the Shulaps area (Fig. 1) and wrote of the unserpentinized Burro Mountain of the Fort St. James area and the even larger body of California: "we are faced with the bodies associated with the Nahlin fault. In problem of carrying an isolated, dense tectonic shape, they include bodies that are sill-like, block upward in a lighter matrix. During any thinly lenticular (Coquihalla belt), thickly len- shift in stress or differential movement, the ticular (Shulaps), irregularly lenticular or body would tend to move down, not up." He merely elongate (Nahlin), or equidimensional concluded that "... the body was simply and batholithic (Fort St. James area). Most are rafted upward in a matrix of sheared Franciscan, of peridotite and dunite. Many are reported to the Franciscan actually moving up faster than show layering, but details are rarely given and ultramafic rock [moved down]" [!]. Dietz's it is generally impossible to decide whether hypothesis, mentioned above, that has led to layers are cumulates, the result of shearing, the suggestion that ultramafic-rich belts rep- metasomatism, or injection. Some bodies such resent fossil "Benioff zones," seems to this as the Polaris (Roots, 1954; Irvine, 1968) and a writer to be defective in that transport is body northwest of Fort St. James (Armstrong, downslope and would not be effective in in- 1949) resemble the zoned ultramafic complexes serting dense bodies upward into the crust. It discussed in a later section. Dikes, sills, veins, seems more reasonable to propose that basic and irregular bodies of pyroxenite, dunite, and magma was emplaced high in the Franciscan so on, cut the ultramafic bodies. Many have (Gresens, 1970). Ernst (1965) suggested that 20 pockets, lenses, or stringers of chromite. to 25 km of strata lay above the serpentinites Serpentinization ranges from incipient or and blueschists of the Panoche Pass area, and so partial in many of the large bodies to almost there appears to have been ample vertical ac- complete in small ones. commodation there for thick sill-like magma Many of the ultramafites are enclosed by chambers. In these, magmas differentiated, rocks of the late Paleozoic Cache Creek and, depleted of olivine and pyroxene, were Group. Some were exposed to erosion by Upper intruded toward the surface. Parts of the ultra- Triassic time (Armstrong, 1949). Several of the mafic differentiates remained as sill-like bodies largest bodies lie along faults that separate in moderately folded Franciscan beds—an Cache Creek strata and younger rocks (Coqui- example is the Red Mountain body (Maddock, halla, Shulaps, Nahlin). In a few places in 1964)—and produced mild metamorphism ap- central British Columbia (Lord, 1948) and in propriate to basic intrusions into wet sand- Yukon (Wheeler, 1961; Mulligan, 1963), they stones (Turner, 1968). To the north, in the lie in Upper Triassic or Jurassic rocks. Klamath Mountains, sill-like ultramafites were Hypotheses of emplacement include in- tightly folded (Lipman, 1964). Where folding trusion of ultrabasic magma (Armstrong, 1949; and faulting were intense, dismembered bodies Little, 1949; Muller, 1967), injection of crystal worked downward along faults or uncon- mush (Gabrielse, 1963), and cold intrusion formities. (Wheeler, 1961; Souther and Armstrong, 1966). Several authors are noncommittal and Alpine Ultramafites of British Columbia consider two or three of these hypotheses, and Southern Yukon reaching no firm conclusion (Leech, 1953; Scores of alpine ultramafites lie along a 1000- Aitken, 1959). mi belt extending from east of Vancouver in How do the ultramafites fit the hypothesis the south, northwestward through central and under consideration? Aitken (1959) has called northern British Columbia into southern attention to the spatial relationship between Yukon (Fig. 1). Most of these have been Cache Creek basalts and andesites and ultra-

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mafites of the Atlin Area. J.W.H. Monger has already presented arguments against this sort suggested (1970, personal commun.) that the of transport. In the Fort St. James area ultramafites are cumulates from Cache Creek (Armstrong, 1949), certain large ultramafic basalts. That the ultramafites are comple- bodies lie in the lower part of the Cache Creek mentary to Cache Creek volcanics (compare Group, and there, basic volcanics predominate Challis, 1965; Gresens, 1970) implies that at the in the upper part of the Group. It seems time of differentiation, the ultramafites ac- possible that ultramafites found there could be cumulated at a low level and volcanic dif- complementary to the basalts of the Cache ferentiates were extruded at a high level, and Creek. It is possible, on the other hand, that thus the close association at Atlin (Aitken, the association of ultramafites with Cache above) seems incompatible with the hypothesis. Creek strata is not a close genetic one, and that The difficulty could be resolved if the ultra- the predominantly volcanic Takla Group of mafites followed the basalts toward the surface Upper Triassic age represents magmas com- (compare Smith, 1958), but the author has plementary to the ultramafites.

WHITEHORSE O v v

YUKON TERRITORY

;vxRolaris complex v"

Pinchi Lake fault

.Coquihalla belt Zoned Ultramafic Complex Tulameen complex

\\ | Alpine Ultramafic Bodies I Upper Triassic (and Lower Jurassic) mainly basalt and andesite I Late Paleozoic Sedimentary and Volcanic Rocks

Figure 1. Sketch map showing ultramafic belts in British Columbia, Yukon, and southeastern Alaska.

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In southern British Columbia, many of the and that these pluglike bodies have been em- ultramafites are enclosed entirely in late placed as "negative salt domes," or alterna- Paleozoic strata. Some of the large bodies lie tively, as down-dropped cylinders bounded by against faults that separate late Paleozoic ring fractures. He contends that the zoning of rocks from Jurassic, Cretaceous, or Eocene these bodies is the result of metasomatism beds. At most such places, the ultramafites after emplacement of the central zones. occur not only at the fault but also as isolated In order to make clear the discussion of the bodies within the adjacent late Paleozoic rocks origin of the zoned complexes, it is necessary to suggesting that younger rocks have been describe briefly some of the best known ex- downfaulted against the older rocks and their amples. For illustrations of Alaskan and Uralian enclosed or attached ultramafites (Cairnes, ones, the reader is referred to Taylor, 1967. 1930, 1943; Duffcll and McTaggart, 1952; Trettin, 1961; McTaggart and Thompson, Alaska 1967). There seem to be no examples of ultra- According to Taylor and Noble (1960), 35 mantes enclosed in rocks younger than Triassic ultramafic bodies lie in a belt extending along except in the Shulaps Range (Leech, 1953) the Alaskan Panhandle (Fig. 1). Most are small where a subsidiary sliver is enclosed in late and consist largely of hornblende pyroxenite, Lower(?) Jurassic strata. The writer suggests but eight are large and partly of peridotite. that the alpine ultramafites are complementary Nearly all bodies are in contact with or are sur- to the basalts to the Triassic Nicola Group rounded by gabbro. The large ultramafic com- (Schau, 1970) that cover hundreds of square plexes (Figs. 2a, 2b), many of them equant in miles along the east flank of the ultramafite- plan, are zoned (Taylor, 1967, p. 101 and 106), rich belt. and zoned in a consistent way. Hornblendite and hornblende pyroxenite are marginal in the ZONED ULTRAMAFIC COMPLEXES zoned bodies in which they appear. Olivine pyroxenite forms a zone inside these horn- Introduction blendic rocks or forms a marginal zone if these Zoned ultramafic bodies, the third type of are not present. Dunite or peridotite forms a ultramafic pluton, have been singled out as a central body in nearly all zoned examples. distinctive type and recognized in many parts Zones may be continuous around the body, or of the world (Taylor and Noble, 1960, p. 188). discontinuous. A recent volume (Wyllie, 1967) contains ac- The Union Bay body (Ruckmick and Noble, counts of zoned ultramafites of southeast 1959) consists of continuous zones, successively Alaska and of the Urals. Detailed accounts of inward, of hornblende pyroxenite, pyroxenite, two of the Alaskan bodies, Union Bay and olivine pyroxenite, peridotite, and dunite. Duke Island, have been provided by Ruckmick Olivine and pyroxene become more iron-rich and Noble (1959) and by Irvine (1967), toward the external contacts. Cumulate layer- respectively. A zoned complex near Tulameen, ing and stringers of chromite seem to be re- British Columbia, has recently been described stricted to the olivine-rich core. Veins of by Findlay (1969). Aho (1956) has given an pyroxenite and sills of dunite cut across account of a pluton near Hope, British peridotite. The complex is almost completely Columbia, at a nickel mine formerly called enclosed by gabbro which is extensively Pacific Nickel, now Giant Mascot—this will be saussuritized at the contact. referred to here as the Hope body. Hall (1932), The Duke Island complex (Irvine, 1967) Hoffman (1931), Heckroodt (1959), and Wil- consists of two main ultramafic bodies separated lemse (1964) provide information on ultra- by highly altered gabbro. These consist largely mafic pipes of the Bushveldt complex that of olivine-rich rocks showing cumulate layer- seem to belong to the group of zoned ultra- ing, with remarkable graded bedding, uncon- mafic plutons under consideration. formities, slump structures, and fragmental In much of the recent literature (Wyllie, layers, most of which dip steeply and many of 1967; Findlay, 1969), it is argued that the which can be traced for great distances. Horn- zoned plutons are formed by crystallization blende clinopyroxenite and hornblendite occur from ultrabasic magma. This writer suggests irregularly at the margins, in contact with that olivine-rich central zones of the zoned gabbro that has been altered to hornblende and complexes are cumulates from basic magmas calcic plagioclase. Where adjacent gabbro is

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/1/23/3428534/i0016-7606-82-1-23.pdf by guest on 28 September 2021 Kane Peak Blashke Is. '^] Dunite IT] Peridotite A$S+ *++'+ ^Olivine pyroxenite t- 4- +- i:-. Hornblende p'xenite -f + •+-1-'- & pyroxenite Mile Gabbro Fig. 2a Fig. 2b r/TTTn Metamorphic ^^J country rock

Kovdozero

Fig. 2c Fig. 2d

Carbonatite A_fJ Granite, fenitized granite x x Olivinite x | Syenitic fenite Peridotite ^\ Ijolite etc.

Pyroxenite Ijolitic chilled zone Turjaite (TfT| Serpentinite Ijolite etc. Carbonatite

Figure 2. a and b: zoned ultramafics, southeastern and Gittens, 1966, p. 505); d. Shawa complex (after Alaska (after Walton, 1951, and Taylor, 1967); c. Johnson, 1966). carbonite complex, Kovdozero, U.S.S.R. (after Tuttle

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unaltered, hornblende clinopyroxenite and belt, running parallel to the first, lies about 30 hornblendite are absent, and olivine-ric'h mi to the west. There, a nearly continuous belt ultramafics are in contact with gabbro. of gabbro more than 400 mi long encloses about This note on the Blashke Island body (Fig. 15 ultramafic plutons, most of them zoned. 2b) (Walton, 1951) is taken from Taylor (1967, They resemble the Alaskan zoned complexes in p. 109): "It consists of almost concentric, the sequence of zones from hornblendic and vertical, cylindrical zones of different ultra- pyroxenic margins to olivine-rich cores. Both mafic types. A circular core of dunite ... is belts are dated on stratigraphic evidence as encircled by a continuous ring of olivine Silurian and were emplaced during the pyroxenite . . . outside of which is a nearly Caledonian orogeny. Thick assemblages of complete ring of gabbro . . . hornblendite and spilite and basalt are of the same age. gabbroic pegmatite occur in the pyroxenite zone . . . where the gabbro zone is wide, the South Africa rock changes from olivine-augite gabbro near According to Hall (1932), some 60 ultrabasic its inner contacts with olivine pyroxenite into a pipes cut the lower part of the Bushveld Com- hornblende gabbro at the outer margins." plex. They occur in clusters (Willemse, 1964, According to Kennedy (1955), the composition p. 109), and nearly all occur in the lower part of olivine changes from Fo9o at the center to of the complex, in the ultrabasic zone or in the Foso at the margin. immediately overlying norites. Their long axes are nearly vertical and about at right angles to British Columbia igneous layering of the complex. A few of the The ultramafic body north of Hope, about pipes contain platinum ore and these have been 80 mi east of Vancouver, British Columbia, explored underground. lies within but near the eastern edge of a late The platinum pipe at Onverwacht (Hall, Cretaceous tonalite batholith, part of the Coast 1932, p. 323) is about 80 ft in diameter and has Range batholithic complex. Within a few been mined to a depth of 800 ft and, like many hundred yards of the ultramafite, tonalite of the pipes, is carrotlike, tapering with depth. grades into diorite and locally into norite. The It cuts across bronzitite that contains a prom- ultramafic body (Aho, 1956), about 2 mi by 1 inent chromite layer. An outer cylinder of mi, is irregularly zoned, showing in most places dunite (Fogo) and wehrlite surrounds an inner a rim of hornblendite succeeded inward by pipe of hortonolite dunite (Fojo) and hortono- pyroxenite, through which are scattered several lite wehrlite about 20 ft in diameter which cores of peridotite up to 1000 ft across, some of contains platinum ore. The contact between which contain pipelike nickel-ore bodies. the core and the surrounding cylinder is very Orthopyroxene is more common than clinopy- irregular. Veins of hortonolite dunite and a roxene and there are some bronzitites. vein of diallage-hornblende-hortonolite rock lie The Tulameen ultramafic body (Findlay, close to the central pipe. Wagner's sketch (Hall, 1969) is an ultramafite-gabbro complex 110 mi p. 323) shows "large masses of chromitite northeast of Vancouver, British Columbia. The scattered through the pipe at about the level of ultramafic part consists of a central dunite, a intersection with the chromite layer in the en- marginal zone of hornblende pyroxenite, and closing bronzitite." an intermediate zone of olivine pyroxenite. The Mooihoek pipe, about 700 ft in diameter Zoning is neither as symmetrical nor as con- and explored to a depth of 500 ft, cuts through tinuous as in the Alaskan bodies. The ultra- anorthositic norite. An outer cylinder of coarse mafic rocks are separated by and overlain by diallagite, feldspathic pyroxenite, and olivine syenogabbro and syenodiorite. norite surrounds an inner one of dunite and wehrlite. A core, about 45 ft in diameter, Ural Mountains consists of hortonolite dunite and is platinif- Two belts of ultramafic rocks are distin- erous. guished in the Urals (Beliaevsky and others, The Dreikop pipe (Heckroodt, 1959), about 1960; Noble and Taylor, 1960). A belt of .25 mi across, differs from the first two pipes alpine ultramafics, composed largely of harz- described in that there is no obvious concentric burgite, less dunite and rare pyroxenite, partly structure. It consists of dunite (Fos4_7i) with serpentinized, containing important chromite a pipelike central mineralized zone. The pipe deposits, extends for 1000 mi along the moun- cuts through layered norites. tain chain. In the northern Urals, a second Other bodies, distinguished (Willemse, 1964,

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p. 109) as olivine-diallagite pegmatoids are es- tinuous hornblende and pyroxene-rich belts sentially pipelike and contain brown amphi- mainly near the gabbro; those enclosed by bole, biotite, ilmenite, and magnetite. Dunite large gabbro bodies are well zoned and have forms the central parts of some of these. wide continuous belts of hornblende- or pyroxene-rich rocks. In the Urals (Taylor, Relation of Gabbro to the Zoned 1967), only two small plutons are shown to be Ultramafic Plutons enclosed by metamorphic country rock, and Most of the zoned ultramafic bodies lie sur- these are unzoned, consisting only of dunite. rounded by or in contact with gabbroic rocks This relationship apparently does not hold at that include gabbro, hypersthene gabbro, Hope or at Tulameen. norite, hornblende gabbro, and diorite. The At Duke Island, rhythmic layering in gabbro gabbroic rocks are considered to be older lies nearly parallel to layering in adjacent (Taylor, 1967) than the zoned ultramafics. olivine clinopyroxenite. At Union Bay "most Irvine (1967) concludes that gabbros are older outcrops (of gabbro) exhibit subparallel ori- than ultramafics at Duke Island, cites radio- entation of pyroxene and plagioclase crystals" metric ages in support of this conclusion, and (Ruckmick and Noble, 1959, p. 986). This argues that they belong to different magma foliation is seen on their map to be concordant types and are unrelated. with the layers of the zoned pluton. Contacts and age relations between the At Union Bay, gabbro grades southward to ultramafics and the gabbros, however, are in diorite at a contact with schist. Inspection of most places obscured by metasomatic effects. Brew's (1966) compilation of Alaskan geology Ruckmick and Noble (1959, p. 1005) cite in- suggests that at many of the ultramafic bodies, clusions of gabbro in hornblende pyroxenite adjacent gabbro is succeeded in a short distance and dikes of hornblendite cutting gabbro as by more acidic rocks (Kane Peak, Snettisham, evidence that the gabbro at Union Bay is older Woronofski Island, and possibly at Percy than the ultramafic complex. The present Islands) and that most of the bodies of "gabbro author contends that these hornblendic rocks to diorite" shown by Taylor (1967, Fig. 4.9) are metasomatic and younger than central are "acidic to intermediate." At Hope, noritic dunites, that inclusions are therefore unreplaced rocks grade into tonalite, and at Tulameen, relics, that "dikes" of hornblendite are veins of syenogabbro seems to grade to syenodiorite. skarn, and that these relations do not prove The writer suggests, because of the general that gabbros are older than the central dunite. association of gabbroic rocks with zoned ultra- At Hope, British Columbia, Aho (1956) found mafic plutons, the restriction of hornblendic contact relations ambiguous. He concluded zones to the neighborhood of altered gabbro, that the gabbroic rocks were probably older the absence or scarcity of zones where gabbro but that they had a strong genetic connection is absent, and the structural conformity be- with the ultramafites. At Tulameen, as at Hope, tween certain of the gabbros and adjacent structural evidence is contradictory. Findlay ultramafites, that the zoned complexes and (1969) considered basic and ultrabasic rocks to gabbro are closely related in origin. Further- belong to the same magma type and to be more, the proposition must be considered that genetically but distantly related. It is hard to at some of these complexes, the gabbros are find detailed information on the relation of younger than the central dunites and peri- gabbro to ultramafic rocks in the Urals. Noble dotites. Evidence of intrusion of ultramafites by and Taylor (1960) state that the ultramafics diorite or gabbro is cited at Hope (Aho, 1956, intrude the gabbro. Beliaevsky and others p. 449) and at Tulameen (Findlay, 1969, p. (1960) refer to the ultramafites as intrusive 402) and remobilized gabbro dikes are said to complexes but consider the gabbro and ultra- cut the ultramafics in the Urals. The common mafites to be of nearly the same age. gradation from somewhat acidic rocks to gabbro There seems to be a rough correlation be- could in some places be due to contamination tween the abundance of gabbro and the degree and basification near the margins of large ultra- of development of zoning. In Alaska, those mafic rafts. ultramafic plutons not in contact with gabbro show little zoning and consist of dunite or Magmatic Hypotheses of Origin peridotite; those with narrow and discontin- At this time, magmatic hypotheses for zoned uous bodies of ga'ubro at their margins are ultramafic bodies seem to be widely accepted mainly dunite or peridotite but show discon- (Wyllie, 1967). It is instructive to consider

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what magmatic mechanisms have been pro- Urals. Where the Union Bay body is in contact posed to explain concentrically zoned feldspar- with phyllites, thermal effects seem to be no rich basic, intermediate, and acidic plutons: more than those produced at gabbro contacts cauldron subsidence and injection of ring dikes; in other parts of the world. flow differentiation (Bhattacharji, 1967); mar- To this author, an overwhelming objection ginal contamination (Compton, 1955); in- to the development of the zoned ultramafites trusion of a heterogeneous migma-magma by injection of successive magmas lies in the (Larsen and Poldervaart, 1961); and diffusion- constancy of the succession inward of the con- convection (Hess, 1960, p. 188; Wager and centric zones and, for example, in the Union Brown, 1968). Few if any feldspar-bearing Bay body, the remarkable continuity, sym- igneous complexes show the narrow but dis- metry, and parallelism of the zones with respect tinctive and continuous concentric zones that to the external contacts. It seems highly un- are found in many of the zoned ultramafic likely that serial injection of magmas would complexes. invariably give rise to the same sequence of Various schemes of magmatic emplacement zones. Furthermore, one would expect rep- have been proposed for the zoned complexes. resentatives of late ultramafic magmas to cut in The body at Union Bay is said to have been a few places across earlier ones or to cut the emplaced as serial intrusions of ultramafic mag- enclosing gabbro in haphazard fashion. mas generated in the mantle (Ruckmick and Findlay (1969) suggested that the zoned Noble, 1959, p. 1014). Findlay (1969) argued complex at Tulameen, originally stratified by that the Tulameen body was formed by dif- differentiation of an ultrabasic magma, has been ferentiation in place of a single hornblende- deformed, producing the present succession of pyroxenite magma. Irvine (1967) offered steeply dipping zones. The succession of zones textural evidence that the layered ultramafics at Tulameen, however, is the usual one, and it at Duke Island are magmatic, involving the would be remarkable if fractional crystalliza- emplacement of two different magmas, but did tion at Tulameen could lead to the sequence of not specify whether the ultramafics are cumu- zones produced at Union Bay by injection of lates from basic magmas (later removed) or successive magmas. It should be noted that the from ultramafic magmas. All stated that there sequence of magmas produced by partial fusion is no direct genetic connection between gabbro in the mantle is unlikely to be duplicated in and ultramafics at these places. Kennedy (1955) reverse by fractional crystallization high in the suggested for the body at Blashke Island a crust (Presnall, 1969). mechanism that depends on early crystalliza- Both Irvine at Duke Island (1967, p. 95) and tion of relatively dry melt in the central part, Findlay at Tulameen (1969, p. 418) found with crystallization delayed by high volatile certain of the marginal hornblende pyroxenite content at the margin, thus accounting for awkwardly placed to fit their schemes as late high-temperature olivine (Fo90) at the core and differentiates. Irvine argued that some horn- lower-temperature olivine (Fog0) at the rim. blende pyroxenite is a late differentiate but What are the objections to the ultramafic that other hornblende pyroxenite, apparently magma hypotheses? Ruckmick and Noble did underlying his early ultramafic differentiates, not consider the high temperatures of dunite is a product of metasomatic reaction at the and peridotite magmas to be fatal to their main ultramafic-gabbro contacts. Findlay, on hypothesis—in most places, the body at Union the other hand, construed some hornblende Bay intrudes only refractory gabbro. They pyroxenites as late differentiates but another as suggested also that during emplacement, water an early phase representing his original ultra- was absorbed by the ultrabasic magma, and mafic magma. heat was not conducted outward very far. Recent experimental work (Presnall, 1966) Metasomatic Hypotheses of Origin showed that magmas corresponding in com- Various workers have suggested that con- position to certain of the ultramafic zones exist centric zoning of ultramafic plutons is meta- at temperatures slightly below 1300°C. The somatic. obstacle to dunite magmas, fusion temperature The zoned bodies of the Urals were ex- of up to 1800°C, still seems to hold for bodies plained by Wyssotsky and Zavaritsky (in at Kane Peak (Fig. 2a), Blashke Island (Fig. Taylor, 1967, p. 118) "by intrusion of dunite 2b), Annette Island and Red Bluff Bay, magma into the earlier gabbro. The pyroxenites Alaska, and in certain examples from the which surround the dunite were envisioned as

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gigantic reaction rims formed by the action of Onverwacht pipe (see above), where chromite dunite magma on the gabbro, and the gabbro layers are only slightly displaced as they cross was thought to be partly melted by the dunite the body from the layered walls, replacement magma," origin for the whole body seems probable. For certain of the platiniferous pipes of the Veins of hortonolite dunite and of diallage- Bushfeld Complex, Lipovsky (cited in Hall, hornblende-hortonolite rock can be construed 1932, p. 106) suggested "penetration along the as replacement bodies—metasomatic pegma- actual or potential fissures of gases and magma tites or skarns. rich in gases, which changed olivine-dunite into Noble and Taylor (1960, p. 195), remarking hortonolite-dunite and deposited sulfides and on the similarity of the platiniferous pipes to platinum." the Alaskan complexes, suggested that the pipes At Blashke Island, Walton (1951) suggested may be of metasomatic origin and invoked the intrusion of H2O-rich peridotite magma, with well-known experiment of Bowen and Tuttle transfer (by both diffusion and migration of (1949) in which enstatite was converted to volatiles) of material along temperature and olivine by action of a vapor phase. This sug- concentration gradients between magma and gestion, involving only desilication by water- the surrounding rocks. Si, Ca, and H2O were rich gases, cannot apply to the pipes in general considered to diffuse outward and Mg inward. because some of them (for example, the Olivine in the hot core was supposed to crys- Dreikop pipe) cut across layered norites. tallize outward while augite crystallized in- Metasomatism there must involve movement ward, replacing early-formed olivine. of Al, Mg, Ca, and Fe as well. At Hope, British Columbia, Aho (1956) of- Evidence for wholesale replacement is fered two hypotheses of origin, magmatic and abundant at Duke Island, southeast Alaska. metasomatic. In the second, he suggested that Irvine (1967, p. 92) described areas of dunite, the mineralized pipes are due to metasomatism up to 300 ft across, that transgress layered by solutions capable of removing or adding clino-pyroxenite and in which traces of the SiOa and depositing sulfides, thus accounting layering are preserved. These bodies, com- for the origin of the pipes and the location of parable to dunite bodies described by Little the ore. He extended the process to the whole (1949) were considered by Irvine to be volume ultramafic mass and suggested that the original for volume replacements and are associated ultramafite was emplaced as some sort of solid with "pegmatoid clinopyroxene veins, and intrusion, and that "the large-scale zoning with zones of coarse recrystallized textures that from peridotite to pyroxenite and to horn- are similarly transgressive to the layering." blendite in the ultrabasic stock as a whole can There can be no question that metasomatic be explained similarly by redistribution of replacement of clino-pyroxenite by dunite on silica, lime, etc.... (p. 479). He found textural this scale involves removal of much calcium evidence supporting this suggestion in that and either removal of silica or addition of hornblende formed largely, if not entirely, by magnesium or both. Swarms of hornblende- replacement of pyroxene and olivine. anorthite pegmatite bodies up to 200 ft wide, with crystals of hornblende commonly 1 to 6 Evidence of Metasomatism in. long, and occasionally up to 4 ft, cut the The writer agrees in a general way with the eastern (Judd Harbour) olivine clino-pyrox- work cited in the immediately preceding sec- enite. Here, the pyroxene of the pyroxenite tion. Further evidence of metasomatism in "commonly shows pegmatoid texture and a zoned complexes and related rocks is given faint preferred orientation normal to the below. stratification. The over-all grain size of the Hoffman (1931, p. 207), writing of the pipe- rock is much greater than that of the graded like nickel deposits of the western Bushveldt layering, and there is some evidence that the Complex, reported that "along with the in- vague stratification is due to pervasive 'dia- troduction of sulfides, there has been some genetic' recrystallization of better developed development of biotite and pyroxene as gangue, layering shortly after its accumulation" in the form of coarse tabular crystals up to one (Irvine, p. 90). It seems probable that the inch in length." At the Dreikop pipe, Heck- pegmatoid rocks are metasomatic and that the roodt (1959, p. 67) found that in the norite wall recrystallized rocks are partly or largely meta- rock, olivine replaces orthopyroxene. At the somatic. The restriction of hornblende pyrox-

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enite and hornblendite to contacts with It is instructive to compare the maps of the altered gabbro suggests strongly that these simple zoned ultrarnafites (Figs. 2a, 2b) with ultramafic rocks are metasomatic. those of two carbonatite complexes (Figs. 2c, Because of the difficulties associated with the 2d), one from Shawa, Rhodesia, and the other magmatic hypothesis and in view of the strong from Kovdozero in the U.S.S.R. These (Tuttle evidence of metasomatism, the writer favors and Gittins, 1966) are selected for comparison the hypothesis that the horn blende-rich and because they show circular plugs of peridotite pyroxene-rich zones peripheral to the gen- or serpentine and are remarkably similar to the erally central dunites and peridotites are zoned ultramafic bodies. The sequence of metasomatic, that metasomatism has occasion- ultramafic rocks at Kovdozero is especially ally produced dunite, pyroxenite, bronzitite, striking. The wide zone of metasomatic fenite peridotite, and so on, and that sills, dikes, and surrounding the Shawa serpentinite is analo- veins of dunite and pyroxenite, and pegmatoid gous to some of the wide zones of hornblendic hornblende-anorthite bodies of the zoned and pyroxenic rocks surrounding the peridotite ultramafics are metasomatically emplaced as, it cores of the ultramafic complexes. Upton is argued, are similar bodies in alpine ultra- (1967) argues that such ultramafic plugs are mafites described above. The central dunites or cumulates from basic alkalic magmas. The peridotites, especially those that show primary chief value of the comparison, however, is in structures and textures (for example, Duke the clue it offers to the origin of the central Island) are not entirely metasomatic, although peridotite. Ring dikes and cone sheets are it seems clear that some of these are partly common in the zoned alkalic complexes (Tut- metasomatic and their textures pseudomorphic. tle and Gittins, 1966). The Shawa example shows a ring intrusion of carbonatite concentric ORIGIN OF THE ZONED with the ultramafic mass. It seems reasonable ULTRAMAFIC COMPLEXES not only to suggest that the pluglike dunite The zoned ultrarnafites from many parts of and peridotite bodies at these alkalic com- the world show striking general similarities in plexes are cumulates that were dropped along composition, in zoning, in relation to gabbroic ring fractures into their present positions but rocks, and in structure that suggest they have to extend the mechanism to the zoned ultra- had similar origins. In detail, however, many mafic complexes. differences in size and shape, in degree of It is suggested for the subgroup of small, development of zoning, presence or absence of structurally simple, pluglike zoned ultrarnafites marginal gabbro, and so on, suggest that cer- of Alaska and for similar bodies in the Urals tain genetic processes are much more important that gabbroic magma was injected at a high in one area than in another. It is argued below level of the crust, perhaps into a subvolcanic that, in general, the zoning of all the complexes is environment, forming magma chambers com- due to metasomatism, but that different ultra- pact but irregular in shape, perhaps partly in- mafic bodies have had quite different histories terconnected, and locally overlapping. Differ- subsequent to their original formation as entation proceeded far enough to form thick cumulates from gabbroic magmas. One of the cumulate masses of olivine or olivine and greatest barriers to better understanding of pyroxene, perhaps partly concentrated by these bodies is uncertainty about the genetic slumping and sliding due to tectonic move- and age relations between the ultrarnafites and ments. As the overlying magma was extruded, adjacent gabbro. or shortly after, the roof of the magma The simplest members of the Alaskan zoned chamber collapsed onto the ultramafic mass. complexes are pluglike, relatively small in Ring fracturing allowed subsidence of a diameter (less than 2 mi), with little or no as- pistonlike mass of cumulates into the under- sociated gabbro, and with little zonal develop- lying complex of metamorphic rocks and ment [Annette Island, Kane Peak (Fig. 2a), crystallizing gabbro. Water from the meta- and Red Bluff Bay of southeast Alaska]. morphic rocks diffused along the marginal Members of this subgroup, somewhat atypical fractures into the still-hot plug, homogenizing, in their simplicity, are included with the zoned recrystallizing, and destroying original textures, complexes, because moderately zoned but removing plagioclase at temperatures above similar bodies (Blashke Island, Fig. 2b) provide those of serpentinization in the manner already a clear connecting link. suggested for the alpine peridotites. Local

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marginal metasomatism, involving CaO, Fe, explanation of the cassiterite pipes of the Red AUOs, SiO2, and so on, produced hornblende- Granite that cuts through the Bushveld Com- and diopside-rich rocks. plex. Wagner (cited in Hall, 1932, p. 489) "re- At Duke Island, olivine-rich ultramafites are gards these puzzling pipe structures as having cumulates from either ultrabasic or basic originated from trains of miarolitic zones in the magma. Evidence of widespread and intense granite comparable with the trains of bubbles metasomatism has been summarized above, and found in glass melts. . . ." Applying this the present author believes that the horn- mechanism to the quite unrelated and older blende pyroxenite and hornblendite zones of Bushveld Complex, the writer proposes that if the complex there are replacement bodies. H2O and other volatiles were fed continuously Metasomatism was apparently guided and from underlying sedimentary rocks to high controlled by contacts between gabbro and points in the base of the crystallizing magma, olivine-rich cumulates. The genetic relation- they could form "gas chimneys" (Waters, ship, however, between gabbro and cumulates 1960, p. 358) that, although small in cross remains obscure. section, might remain open during crystalliza- It is suggested that at Union Bay, cumulates tion of the Bushveld magma and provide a descended as a coherent cylinder bounded by passageway for metasomatizing fluids. Willemse ring fractures or possibly as a glacierlike down- (1964, p. 122) states that carbonaceous shales plunging olivine "salt dome," part of it in- underlying the complex might have provided jecting sideways along a fracture to form a sulfur for the nickel-sulfide-bearing pipes found laccolithic lateral lobe. Metasomatism produced above them. It is suggested that metasoma- the usual sequence of zones. Again, the genetic tizing fluids were able to replace the walls of connection of dunite to adjacent gabbro re- the gas chimneys with olivine, pyroxene, horn- mains unclear. blende, sulfides, and so on, and deposit iron- The writer has no detailed information on rich hortonolite along the axis of the pipe. In the geology of the ultramafic belts of the comparing this mechanism of metasomatism Urals. There, an eastern zone of alpine peri- with that outlined for the Alaskan bodies, it is dotites is paralleled in the north by a western interesting to note that the most iron-rich belt of zoned ultramafites lying in a continuous olivine in each type was deposited closest to belt of gabbro. According to current views on the structures along which the fluids are the origins of alpine and zoned ultramafic believed to have passed—in the Bushveld, the plutons, 50-mi-long lenses of ultramafic rock central axis, in Alaska, the outside contacts of were being squeezed up from the mantle to the pre-existing ultramafic cumulate. form the eastern belt, while during the same orogeny, a succession of ultramafic magmas SUMMARY generated by melting in the mantle was being It is suggested that many alpine ultramafic emplaced in an older belt of gabbroic in- plutons originated in magma chambers high in trusions some 30 mi to the west. To the author, the crust as ultramafic cumulates. From some it seems far more reasonable to suggest that of these chambers, magmas were intruded up- gabbroic magma was intruded high into the ward or extruded, leaving sill-like ultramafic crust during the orogeny. Ultramafic cumu- plutons behind. These were subsequently lates, in great volume, sank along faults in the folded, or dismembered by faulting, and, be- eastern belt to form alpine ultramafics far re- cause of their high density, they subsided moved from their original magma chambers. during tectonism, to form cold, fault-enclosed To the west, cumulates descended only short intrusions. In other places, tectonism inter- distances, were extensively metasomatized, rupted crystallization of the magmas, magma producing hornblendic and pyroxenic margins, chambers were deformed, loose cumulates and and are still associated with a belt of contem- hot coherent cumulate rocks slumped and slid, porary basic plutons. Abundant basic lavas may and hot masses (as crystal mushes and hot represent magmas expelled after formation of lenses) worked down fault zones. These hot cumulates. masses were subject to high temperature re- Evidence has been presented above that the crystallization and metasomatism as water from mineralized pipes of the Bushveld Complex are adjacent rocks permeated them, forming re- metasomatic in origin. If one accepts this view, placement bodies ot pyroxenite and dunite, the problem remains of what localized them. and intercumulate plagioclase was destroyed. A possible answer is provided by P. A. Wagner's Contact aureoles were formed but some were

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