On the Origin of Ultramafic Rocks
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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 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 24 K. McTAGGART—ORIGIN OF ULTRAMAFIC ROCKS 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, 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 ALPINE ULTRAMAFIC BODIES 25 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,