Principles of Igneous and Metamorphic Petrology John D. Winter Second Edition

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1 Principles of Igneous and Metamorphic Petrology John D. Winter Second Edition

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3 The previous analysis indicates that only mafic intrusions are susceptible to the effects of differentiation by gravity settling of crystals. There is evidence, however, that many silicic bodies have evolved along a liquid line of descent. Several investigators have used the approach to the ternary eutectic in the systems albite orthoclase silica (Figure 3) to indicate the evolution of late granitic liquids toward the thermal minimum, or eutectic, composition. Others have used the same clustering of analyses around the minimum to indicate eutectic partial melts of sialic material in the continental crust. Certainly, a eutectic magma could result from either process. Included in Figure 3 are the cation norm components of the successive intrusive phases of the Tuolumne Intrusive Series. Note the progressive approach of these magmas to the eutectic composition along a trend that follows the decompression eutectic path. Whatever the origin of the parental magma in this case, this series appears to have evolved toward the low-pressure thermal minimum. Harker-type bivariate variation diagrams of intrusive sequences also indicate evolutionary trends. Bateman and Chappell (1979) interpreted the trends for the Tuolumne Intrusive Series to be the result of fractional crystallization. Similar fractional crystallization-based interpretations for Sierran zoned granitoids have been proposed by Bateman and Nokelberg (1978) and Noyes et al. (1983). Although the compositional trends may be compatible with fractional crystallization, crystal settling has been considered very problematic in such viscous silicic magmas (Brandeis and Jaupart, 1986; Sparks et al., 1984). Harper et al. (2005), however, cited viscosity and field criteria supporting crystal settling in hydrous granites. Later in this chapter, we shall explore methods by which fractional crystallization can occur without crystal settling. As we shall see in Section 4, mixing of silicic (crustal melts) and mafic (mantle melts) is a popular alternative interpretation to the evolutionary trends in some of these systems. In addition to gravity settling, three other mechanisms may facilitate the separation of crystals and liquid. Filter pressing (compaction), mentioned earlier in reference to partial melting, is also possible in crystal mushes that form as cumulates or crystal suspensions. The amount of trapped intercumulus liquid between cumulate minerals may be as high as 60 vol. % (Irvine, 1980b). With the added weight of further accumulation, the crystal mush may be compacted (McKenzie, 1984), squeezing much of the liquid out into the main magma body. Another method of filter pressing involves the movement of a phenocryst-laden crystal mush. Any constriction in the conduit causes the crystals to interfere and slow with respect to the liquid. Another similar mechanism by which crystals may be segregated from the liquid occurs when crystal-rich magmas flow in a laminar fashion near the walls of the magma body. The process is known as flow segregation (or flow[age] separation, or flow[age] differentiation). The motion of the magma past the stationary walls of country rock (Figure 4) creates shear in the viscous liquid as a result of the velocity gradient near the walls. The resulting differential motion forces the magma to flow around phenocrysts, Relative shear Qtz P total = P H2 O Country Rock Direction of flow 1 atm 1 GPa 3 GPa Grain dispersive pressure Ab Or Magma FIGURE 3 Position of the H 2 O-saturated ternary eutectic (minimum melt composition) in the albite orthoclase silica system at various pressures. The shaded portion represents the composition of most granites. Included are the compositions of the Tuolumne Intrusive Series, with the arrow showing the direction of the trend from early to late magma batches. Experimental data from Wyllie et al. (1976). FIGURE 4 Flow of magma adjacent to a wall of country rock results in differential motion and shear in the magma. Where such shear is constricted, as between adjacent phenocrysts or between phenocrysts and the contact, a force (called grain-dispersive pressure) is generated and pushes the phenocrysts apart and away from the contact. 217

4 thereby exerting pressure on them at constrictions where phenocrysts are near one another or near the contact itself. The pressure, called grain dispersive pressure (Komar, 1972), forces the grains apart and away from the contact. This effect is greatest near the walls, and it drops off quickly toward the magma interior, where the flow becomes uniform. Phenocrysts thus concentrate away from the walls to mitigate the pressure buildup. This concentration is most apparent in dikes and sills, where the volume affected by the contact comprises a substantial proportion of the body, resulting in a distinct concentration of coarse phenocrysts toward the center (Figure 5). Flow segregation is an interesting, though localized, phenomenon and cannot be responsible for the evolution of more than a small proportion of igneous rocks. A third mechanism involves the separation and rise of buoyant liquids from boundary layers in which crystals form without themselves moving. This relatively new model has become popular recently and will be introduced in Section 5. The majority of fractional crystallization models assume that fractionation has taken place in a stationary magma chamber at constant pressure. The rise of basaltic magmas, as pointed out by O Hara (1968b), may involve fairly continuous fractional crystallization as it rises, which must obviously be a polybaric fractionation process. One result is that the fractionating minerals vary as their stability fields are crossed (e.g., garnet to spinel to plagioclase). Another is that the shift in the eutectic point with pressure also causes the quantity of the liquidus phases that crystallize to vary. In particular, the increase in the size of the field FIGURE 5 Increase in size and concentration of olivine phenocrysts toward the center of small dikes by flow differentiation. Isle of Skye, Scotland. After Drever and Johnston (1958). Reproduced by permission of the Royal Society of Edinburgh. a b c d e for olivine with decreasing pressure requires that a lot of olivine must form as the melt composition follows the liquidus away from the olivine side of the diagram in a rising basaltic melt (see Problem 1). Thus, the relative amount of olivine that crystallizes with a rising basaltic magma will be far greater than the amount that forms during isobaric crystallization. The broad acceptance of Bowen s (1928) contention that fractional crystallization is the predominant mechanism of magmatic differentiation is now being questioned. This one process cannot account for all of the diversity in the broad spectrum of natural igneous rocks, even if we allow for variations due to the influence of changes in pressure or associated fluids. Some observed chemical trends simply cannot be accomplished by fractional crystallization. Other classical examples of fractional crystallization have not withstood more critical analysis and the test of time. For example, the 300-m thick Triassic diabase Palisades Sill, on the eastern banks of the Hudson River, is commonly cited as an example of a vertically differentiated sill with layers formed by gravity settling. The overall composition of the sill is tholeiitic basalt, as demonstrated by the upper and lower chill zones. The 10- to 20-m thick olivine-rich layer at the base is commonly attributed to differentiation by settling and accumulation of early-forming dense olivine crystals. Although vertical chemical trends in the sill are compatible with fractional crystallization of pyroxene and pyroxene accumulation zones occur near the bottom of the sill, the striking olivine layer is not compatible with the trends, and olivine is far too rare elsewhere in the sill to be consistent with the concentration in the layer. The layer has recently been reinterpreted as one of several late intrusions of magma into the crystallizing tholeiitic liquid of the sill. This injected pulse was olivine rich and dense, so it accumulated near the base (Husch, 1990). Some magma series, such as the calcalkaline series associated with subduction zones, may involve mixing of components to a greater extent than fractional crystallization trends. Other cases against fractional crystallization were based on proportionality arguments. The great granite batholith belts, for example, are thought to be too extensive to have been created by fractional crystallization from a basaltic parent. It would require approximately 20 parts of original basalt to create 1 part late granitic liquid by fractional crystallization. We need not walk long in places like the Sierra Nevada, with so many square kilometers of granitic rocks, to wonder where all of the basalt went! Modern theories that consider granite batholiths to be much thinner than originally thought (Hamilton and Myers, 1967, see Chapter 4) reduce the magnitude of the problem, so we could still follow Bowen (1948) and postulate that the lower levels of the crust are composed of the denser fractionated gabbros. Seismic and gravity surveys, however, argue against this possibility, making the origin of granite batholiths via fractionation from a basaltic parent untenable (Presnall, 1979). These arguments serve only to demonstrate that fractional crystallization cannot lead to all of the magmatic rocks now exposed at the surface of the Earth. It is still a 218

5 common and important process, particularly in the early crystallization of mafic liquids, but there are other important differentiation processes and other primary magmas. 2.2 Volatile Transport Chemical differentiation can also be accomplished when a separate vapor phase coexists with a magma and liquid vapor fractionation takes place. A vapor phase may be introduced in any of three principal ways. First, a fluid may be released by heating of hydrated or carbonated wall rocks. We shall discuss some ramifications of this process in later sections of this chapter. Second, as a volatile-bearing but undersaturated magma rises and pressure is reduced, the magma may eventually become saturated in the vapor, and a free vapor phase is released. Because the vapor phase has a lower density than the melt, it rises, diffusing through the magma, and concentrates near the top of the magma chamber. Such concentrated fluid may even permeate into the roof rocks. This process usually involves an H 2 O-rich fluid, and it produces a variety of hydrothermal alteration effects. For example, the alkali metasomatism known as fenitization above nephelinite carbonatite bodies has been attributed to alkali-rich fluids derived from the highly alkaline intrusives. A third mechanism for generating a separate fluid phase is a result of late-stage fractional crystallization. Most early-formed igneous minerals are anhydrous (even hydrous minerals are less so than associated melts), so their segregation from a hydrous melt enriches the melt in H 2 O and other volatile phases. Eventually the magma reaches the saturation point, and a hydrous vapor phase is produced. This somewhat paradoxical boiling off of water as a magma cools has been called retrograde (or resurgent) boiling. Of course, the three processes by which a vapor can be produced need not be entirely separate, and all three may contribute to saturation and volatile release from a magma, depending upon the composition of the original magma, the rates of cooling and rise, the initial volatile content, the extent of fractional crystallization, the temperature, the nature of the wall rocks, etc. As a separate vapor is produced, the chemical constituents in the system partition themselves between the liquid and vapor phases in appropriate equilibrium proportions, some remaining preferentially in the melt and others becoming enriched in the vapor phase. The result is a silicate-saturated vapor phase in association with a vapor-saturated silicate liquid phase. The cation sites in minerals are much more constrained and selective than in melts, so the chemical constituents in minerals are generally much simpler. As a result, the process of fractional crystallization tends to remove only a few elements from the liquid in significant quantities, and a number of incompatible, LIL, and non-lithophile elements become concentrated in the latest liquid fraction. Many of these will further concentrate in the vapor, once formed. This is particularly true in the case of resurgent boiling because the melt already is evolved by the time the vapor phase is released. The vapor phase may contain unusually high concentrations of volatile constituents such as H 2 O, CO 2, S, Cl, F, B, and P, as well as a wide range of incompatible and chalcophile elements. The volatile release and concentration associated with pluton rise or resurgent boiling may momentarily increase the pressure at the top of the intrusion and fracture the roof rocks in some shallow intrusions (it may also initiate volcanic eruptions). Both the vapor phase and some of the late silicate melt are likely to escape along a network of these fractures as dikes of various sizes. The silicate melt commonly crystallizes to a mixture of quartz and feldspar. It is typically found in small dikes with a sugar-like texture, which is informally called aplite. The vapor phase is typically concentrated as dikes or pods in, or adjacent to, the parental granitic pluton, where it crystallizes to form a characteristically magmagenic form of pegmatite. Although pegmatite is used as a textural classification term for very coarse grain size, and there are other methods of creating large crystals, the type of pegmatite described above is the most common. The large grain size in magmagenic pegmatites is not due to a slow cooling rate but is a result of poor nucleation and very high diffusivity in the H 2 O-rich phase, which permits chemical species to migrate readily and add to rapidly growing minerals. The size of crystals in pegmatites can occasionally be impressive, such as spodumene, microcline, or mica crystals 6 to 10 m across. Most pegmatites are simple, essentially very coarse granites. Others are more complex, with a tremendous con centration of incompatible elements and a highly varied mineral ogy, commonly displaying a concentric zonation (Jahns and Burnham, 1969; C erný, 1991; Simmons et al., 2003), as shown in Figure 6. Because the late fluid segregation concentrates several unusual elements, pegmatites are important economic resources and are mined for Li, Be, the rare earths, W, Zr, and a host of others elements that are rarely concentrated in other environments. They are also a major source of gems. Vapors that completely escape the magma and move to higher levels may cool further and precipitate low-temperature minerals, such as sulfides in a hydrothermal system (commonly mixed in part with meteoric water). Miarolitic pods, or cavities, are smaller fluid segregations trapped in the plutonic host. When finally exposed at the surface, they are coarse mineral clusters (usually a few centimeters across), the centers of which are typically hollow voids from which the fluid subsequently escaped. The hollow cavities have euhedral crystals (of the same minerals comprising the pluton) that extend inward, where they grew into the fluid, unimpeded by other minerals. Like complex pegmatites, some miarolitic cavities or pods have a concentric structure consisting of layers of different mineralogy (Jahns and Burnham, 1969; McMillan, Because the addition of H 2 O lowers the melting point of magmas, the release of hydrous fluid into the country rocks causes the liquidus temperature in the main magma body to rise suddenly, resulting in rapid crystallization of much of the liquid remaining with the previously formed 219

6 FIGURE 6 Schematic sections of three zoned fluid-phase deposits (not at the same scale). (a) Miarolitic pod in granite (several centimeters across). (b) Asymmetric zoned pegmatite dike with aplitic base (several tens of centimeters across). (c) Asymmetric zoned pegmatite with granitoid outer portion (several meters across). From Jahns and Burnham (1969). minerals. This is an alternative way of generating porphyritic texture and is common in many silicic plutons. 2.3 Liquid Immiscibility Two liquids that don t mix seems an unlikely occurrence, and it is. Yet most of us are familiar with salad oil and oil slicks, so we have some concept of the phenomenon. Many oils do not mix with water, and, because they are less dense, the oil floats to the top of the water, forming a distinct layer. Most immiscible phases, whether liquids or solids, homogenize at elevated temperatures due to the increased entropy and molecular vibrational energy, although for oil water at atmospheric pressure, the homogenizing temperature is above the boiling point of water. The solvus, representing liquid or solid immiscibility on a phase diagram, is therefore convex upward on a temperature composition diagram. We have already encountered immiscible liquids in the forsterite silica system, where, on the high-silica side of the diagram, a highly silica-rich liquid separates from a less silica-rich one. Throughout the 20th century, geologists appealed to liquid immiscibility as a mechanism for magmatic differentiation, thinking that it might be responsible for the separation of a granitic liquid from an evolving system (presumably from an initial basaltic parent). Such a separation into contrasting liquid systems was also used to explain enigmatic cases of bimodal volcanism, such as the basalt rhyolite occurrences of the Snake River Yellowstone area, or the Basin-and-Range of the southwestern United States. There are two problems with applying the forsterite silica liquid immiscibility gap to natural magmas. First, the temperature of liquid immiscibility is far too high (over 1700 C) to represent a reasonable crustal process. Of course, the Mg-Si-O system is rather restricted, leading one to ask whether the addition of other components, required to create more natural magmas, would lower the temperature of the solvus. The effect, however, of adding alkalis, alumina, etc. is to eliminate the solvus completely. When this was experimentally demonstrated, liquid immiscibility was relegated to the compost pile of magmatic processes. Interest was renewed when Roedder (1951) discovered a low-temperature immiscibility gap in the central portion of the fayalite leucite silica system (Figure 7) at temperatures and compositions that are conceivable for some Fe-rich natural magmas. Roedder (1979) provided a review of liquid immiscibility in silicate magmas, citing dozens of references in which natural occurrences of immiscible liquids were described, including a significant proportion of the lunar samples returned by the Apollo program. Three natural magmatic systems are widely recognized as having immiscible liquids in some portion of their compositional range. The first is the system mentioned above, which most commonly translates to natural Fe-rich tholeiitic basalts, which experience an initial trend toward iron enrichment. In the later stages of fractionation, a granitic melt ( 775% SiO 2 ) separates from a basaltic melt (' 40% SiO2 ). Once separated, the silicic liquid must have a much lower density than the Fe-rich mafic liquid, and we would expect it to rise and collect near the top of the magma 220

7 SiO Fa Fa Two liquids Lc Kfs SiO 2 SiO 2 FIGURE 7 Two immiscibility gaps in the system fayalite leucite silica. The central one is of a composition, and at a low enough temperature (see the section in the upper left) to be attainable in some Fe-rich natural magmas (after Roedder, 1979, copyright the Mineralogical Society of America). Projected into the simplified system are the compositions of natural immiscible silicate pair droplets from interstitial Fe-rich tholeiitic glasses (Philpotts, 1982). chamber. Crystallization of the magma must be advanced by the time liquid separation occurs, however, and both liquids are likely to become trapped in the already-formed crystal network. Philpotts (1982) described the textures of some Ferich Hawaiian basalts in which small droplets of the two immiscible liquids are mingled in the interstitial glass trapped between plagioclase and augite crystals. The separate droplet compositions may be determined by microprobe and are projected into the Fa-Lc-silica system in Figure 7, along with the liquid immiscibility gap of Roedder (1951). The actual liquid compositions plot slightly outside the experimental gap, probably because of the effects of Fe 2 O 3, TiO 2, and P 2 O 5, which expand the immiscible field. The low oxygen fugacity of the lunar basalts is the probable reason that immiscible liquids are so common in them. Observing immiscible droplets is clear evidence of the process, but evidence is far less obvious that immiscible granitic liquids have separated and formed substantial segregations from Fe-rich tholeiites that are over 70% crystallized. Perhaps filter pressing may aid the process, and the granophyric layers and lenses at the top of many mafic intrusions, including the Palisades Sill and the Skaergård intrusion (McBirney, 1975) may be the products of immiscible liquids. In such cases, liquid immiscibility is a late-stage addition to a more extensive process of fractional crystallization in these mafic intrusions. Granitic bodies and other large-scale evolved liquids, however, are unlikely products of immiscible liquids. A second system displaying immiscible liquid behavior is the separation of a sulfide-rich liquid from a sulfidesaturated silicate magma. Less than one-tenth of a percent of sulfur is sufficient to saturate a silicate magma and release an iron sulfide melt that is also rich in Cu, Ni, and other chalcophile elements. Small, round, immiscible sulfide droplets in a silicate glass matrix, similar to Philpotts (1982) granitic tholeiitic examples above, have been observed in a number of quenched ocean basalt glasses. Economically important massive sulfide segregations in large, layered mafic complexes have formed by separation and accumulation of immiscible sulfide melts. A third liquid immiscibility gap occurs in highly alkaline magmas that are rich in CO 2. These liquids separate into two fractions, one enriched in silica and alkalis and the other in carbonate. These give rise to the nephelinite carbonatite association. Although these are the three generally recognized occurrences of immiscible liquids, other magmas might separate into two liquid phases under certain circumstances. These possibilities include lamprophyres (Philpotts, 1976; Eby, 1980), komatiites, lunar mare, and various other volcanics (see Roedder, 1979, for a summary). The close spatial and temporal association of contrasting liquids may result from a number of processes in addition to liquid immiscibility. We can apply three tests to juxtaposed rocks to evaluate them as products of immiscible liquids. First, the magmas must be immiscible when heated experimentally, or they must plot on the boundaries of a known immiscibility gap, as in Figure 7. Second, immiscible liquids are in equilibrium with each other, and thus they must also be in equilibrium with the same minerals. If the two associated liquids crystallized different minerals or the same mineral with different compositions, they cannot be an immiscible pair. Finally, we may be able to use the pattern of trace element fractionation between the two liquids to evaluate them as immiscible. Partitioning of minor and trace elements between Fe-rich mafic liquids and granitic liquids, for example, can be distinctive when compared to the more common mafic magmas with less Fe. Some incompatible elements (P, for example) are preferentially incorporated into an Fe-rich mafic liquid over the complimentary silicic one. A granitic rock relatively depleted in these incompatible trace elements may be a product of liquid immiscibility. Of course, a low concentration in a particular trace element can also result if the liquid was derived from a similarly depleted source. It is far more reliable, then, if rocks representing both of the immiscible liquids can be evaluated. This has been accomplished for some mixed dike rocks (Vogel and Wilband, 1978), but no one has yet succeeded in identifying a mediumsized or larger granite as derived from an immiscible liquid. Although liquid immiscibility is now widely accepted as a phenomenon in natural magmas, the extent of the process is still in question, and its importance in generating large bodies or a significant proportion of evolved magmatic rocks is doubtful. 3 MAGMA MIXING Magma mixing is a bit like liquid immiscibility in reverse, and so was some of the reasoning behind its historical origins. The reigning paradigm of fractional crystallization implies 221