Presentation on theme: "Chapter 12: Layered Mafic Intrusions"— Presentation transcript:
1Chapter 12: Layered Mafic Intrusions Table 12.1. Some Principal Layered Mafic IntrusionsNameAgeLocationArea(km2)BushveldPrecambrianS. Africa66,000DufekJurassicAntarctica50,000DuluthMinnesota, USA4,700StillwaterMontana, USA4,400MuskoxNW Terr. Canada3,500Great DikeZimbabwe3,300KiglapaitLabrador560SkaergårdEoceneEast Greenland100Super Laboratories for the study of fractionationHotRelatively low viscositySlow coolingMafic intrusions come in all sizesfrom thin dikes and sillsup to the huge 66,000 km2 x 9 km thick Bushveld intrusion of South AfricaCan occur in any tectonic environment where basaltic magma is generatedLarge or particularly well-studied LMIs exposed in continents (many in flood basalt provinces)
2The form of a typical LMI Figure From Irvine and Smith (1967), In P. J. Wyllie (ed.), Ultramafic and Related Rocks. Wiley. New York, ppThe Muskox Intrusion
3Layeringlayer: any sheet-like cumulate unit distinguished by its compositional and/or textural featuresuniform mineralogically and texturally homogeneousCrystal accumulation and layering distinguish LMIs
4Uniform LayeringFigure 12.3b. Uniform chromite layers alternate with plagioclase-rich layers, Bushveld Complex, S. Africa. From McBirney and Noyes (1979) J. Petrol., 20,Uniform Layering of magnetite and plagioclase, Bushveld
5Layeringlayer: any sheet-like cumulate unit distinguished by its compositional and/or textural featuresuniform mineralogically and texturally homogeneousnon-uniform vary either along or across the layeringgraded = gradual variation in eithermineralogygrain size - quite rare in gabbroic LMIsCrystal accumulation and layering distinguish LMIs
6Graded LayersFigure Modal and size graded layers. From McBirney and Noyes (1979) J. Petrol., 20,Left: Modal layering of olivine and plagioclase, SkaergaardRight: Size layering Opx and Plag, Duke Island
7Layering (or stratification) Addresses the structure and fabric of sequences of multiple layers1) Modal Layering: characterized by variation in the relative proportions of constituent mineralsmay contain uniform layers, graded layers, or a combination of both
8Layering (or stratification) 2) Phase layering: the appearance or disappearance of minerals in the crystallization sequence developed in modal layersPhase layering transgresses modal layeringAnalogy with sediment stratigraphy: layers are like sedimentary beds, layering is like bedding sequences, and phase layering is like formationsAs with formations, phase layering is commonly used to delineate subdivisions in the layered sequences of differentiated LMIs
93) Cryptic Layering (not obvious to the eye) Systematic variation in the chemical composition of certain minerals with stratigraphic height in a layered sequenceTransgresses both modal and phase layeringExample: Mg/(Mg+Fe) in mafic phases and Ca/(Ca+Na) in plagioclase typically decrease upward from the floor
10The regularity of layering Rhythmic: layers systematically repeatMacrorhythmic: several meters thickMicrorhythmic: only a few cm thickIntermittent: less regular patternsA common type consists of rhythmic graded layers punctuated by occasional uniform layers
11Rythmic and Intermittent Layering Figure 12.3a. Vertically tilted cm-scale rhythmic layering of plagioclase and pyroxene in the Stillwater Complex, Montana.Left: Plag – pyroxene, StillwaterRight: Intermittent graded and non-graded layers, SkaergaardFigure Intermittent layering showing graded layers separated by non-graded gabbroic layers. Skaergård Intrusion, E. Greenland. From McBirney (1993) Igneous Petrology (2nd ed.), Jones and Bartlett. Boston.
12The Bushveld Complex, South Africa The biggest:km x 9 kmLebowa graniticsintruded 5 MaafterwardSimplified geologic Map and cross section of the Bushveld complex. From The Story of Earth & Life McCarthy and Rubidge
13Marginal Zone: the lowest unit, is a chill zone about 150 m thick Fine-grained norites from the margin correspond to a high-alumina tholeiitic basalt
14Stratigraphy Basal Series Thin uniform dunite cumulates alternating with orthopyroxenite and harzburgite layersThe top defined as the Main Chromite LayerFigure Stratigraphic sequence of layering in the Eastern Lobe of the Bushveld Complex. After Wager and Brown (1968) Layered Igneous Rocks. Freeman. San Francisco.
15Critical Series Plagioclase forms as a cumulate phase (phase layering) Norite, orthopyroxenite, and anorthosite layers etcFine-scale layering is impressively developed. The layers are quite parallel over remarkable distancesStriking rhythmic repetition of thin, homogeneous or graded layersFigure Stratigraphic sequence of layering in the Eastern Lobe of the Bushveld Complex. After Wager and Brown (1968) Layered Igneous Rocks. Freeman. San Francisco.
16The Merensky Reef~ 150 m thick sequence of rhythmic units with cumulus plagioclase, orthopyroxene, olivine, and chromiteThe famous Pt-Pd and sulfide-bearing sub-unit is an orthopyroxenite-olivine-chromite cumulate layer1-5 m thick and can be followed for nearly 200 km in the western lobe and 150 km in the eastern lobe, some 200 km apart (a total of 550 km)!It may mark the horizon at which a major fresh surge of magma was introduced into the chamberThe Pt and Pd derived from the residual liquid of the earlier stage, and the sulfides from an immiscible sulfide liquid associated with the latter stageFigure Stratigraphic sequence of layering in the Eastern Lobe of the Bushveld Complex. After Wager and Brown (1968) Layered Igneous Rocks. Freeman. San Francisco.
17Main Zonethe thickest zone and contains thick monotonous sequences of hypersthene gabbro, norite, and anorthositeCumulus olivine and chromite are absentThe layering is poorly developed compared to the lower units, but still presentFigure Stratigraphic sequence of layering in the Eastern Lobe of the Bushveld Complex. After Wager and Brown (1968) Layered Igneous Rocks. Freeman. San Francisco.
18Upper ZoneAppearance of cumulus magnetite (Fe-rich)Well layered: anorthosite, gabbro, and ferrodioriteNumerous felsic rock types = late differentiates
19Also note: Cryptic layering: systematic change in mineral compositions Reappearance of Fe-rich olivine in the Upper ZoneReappearance of olivine in Upper zone is explained in textA stunning example of the problems we face in interpreting cooling and magmatic differentiation processes in what is probably the optimal natural situation: a large, slow-cooling chamber of hot, low viscosity magmaFigure Stratigraphic sequence of layering in the Eastern Lobe of the Bushveld Complex. After Wager and Brown (1968) Layered Igneous Rocks. Freeman. San Francisco.
20Figure The Fo-Fa-SiO2 portion of the FeO-MgO-SiO2 system, after Bowen and Schairer (1935) Amer. J. Sci., 29,The pyroxene + liquid field is bounded by the 1557oC-1305oC ternary peritectic curve on one side and the 1543oC-1178oC pyroxene-silica cotectic curve on the otherThe field pinches out toward higher Fe contentPerfect fractional crystallization of a cooling liquid of bulk composition a follows the schematic path a-b-c-d-e-. Fe-poor olivine forms at point a, and the liquid evolves toward point b. Because olivine is separated as it forms, the peritectic reaction (Ol + Liq = Opx) is not possible. Rather, Fe-poor orthopyroxene forms, and the liquid progresses directly away from the pyroxene composition to point c, where tridymite forms. Fe-rich olivine reappears with pyroxene and tridymite at point d. Temperatures of invariant points are in degrees Celsius. The immiscible liquid solvus in the silica-rich field is ignored.
21How can we explain the conspicuous development of rhythmic layering of often sharply-defined uniform or graded layers?Experimental (and natural) systems show that a cooling magma will first crystallize one liquidus phase, joined by progressively more phases as the liquid line of descent reaches the cotectic and higher order eutecticsThe repetition requires either some impressively periodic reinjection of fresh magma, or cyclic variation in one or more physical properties if it is to be produced by gravitational crystal settling aloneThe pattern of cryptic layering, however, indicates a progressive differentiation that spans the full vertical height of the intrusion, precluding any model based solely on replenishment
22The Stillwater Complex, Montana Figure After Wager and Brown (1968) Layered Igneous Rocks. Freeman. San Francisco.
23Stratigraphy Basal Series a thin ( m) layer of norites and gabbrosUltramafic Series base = first appearance of copious olivine cumulates (phase layering)Lower Peridotite Zone20 cycles ( m thick) of macrorhythmic layering with a distinctive sequence of lithologiesThe series begins with dunite (plus chromite), followed by harzburgite and then orthopyroxeniteUpper Orthopyroxenite Zoneis a single, thick (up to 1070 m), rather monotonous layer of cumulate orthopyroxenite
24olivine + orthopyroxene orthopyroxene The crystallization sequence within each rhythmic unit (with rare exception) is:olivine + chromite olivine + orthopyroxene orthopyroxene orthopyroxene + plagioclase orthopyroxene + plagioclase + augiteThis is a common basaltic crystallization sequence, which, with the sharpness of the base of the dunite layer, suggests that each sequence is initiated by some major change in the crystallization conditions followed by a period of cooling and crystal accumulationOnce again we are faced with the problem of explaining the repetition of the cycles
25Stratigraphy The Banded Series Sudden cumulus plagioclase ® significant change from ultramafic rock types (phase layering again)The most common lithologies are anorthosite, norite, gabbro, and troctolite (olivine-rich and pyroxene-poor gabbro)Such a sudden and dramatic change suggests the introduction of a second principal magma type into the Stillwater magma chamber
26Figure 12-9. Composite stratigraphic column of the Stillwater Complex Figure Composite stratigraphic column of the Stillwater Complex. After McCallum et al., (1980) Amer. J. Sci., 280-A, 59-87, and Raedeke and McCallum (1984) J. Petrol., 25,
27The Skaergård Intrusion E. Greenland The “type locality” for LMIs. It is now perhaps the most intensely studied igneous body in the worldFigure After Stewart and DePaolo (1990) Contrib. Mineral. Petrol., 104,
28Fine-grained chill margin Magma intruded in a single surge (premier natural example of the crystallization of a mafic pluton in a single-stage process)Fine-grained chill marginFine-grained chill margin, about a meter thick, but, like most LMI margins, it has been contaminated, and is no longer representative of the original magma
29Stratigraphy Skaergård subdivided into three major units: Layered SeriesUpper Border SeriesMarginal Border SeriesUpper Border Series and the Layered Series meet at the Sandwich Horizon (most differentiated liquids)It is generally agreed that the Layered Series crystallized from the floor upward, the Upper Border Series from the roof downward, and the Marginal Border Series from the walls inward
30Cross section looking down dip. Figure After After Hoover (1978) Carnegie Inst. Wash., Yearb., 77,
31Upper Border Series: thinner, but mirrors the 2500 m Layered Series in many respects Cooled from the top down, so the top of the Upper Border Series crystallized firstThe most Mg-rich olivines and Ca- rich plagioclases occur at the top, and grade to more Fe-rich and Na- rich compositions downwardMajor element trends also reverse in the Upper Border Series as compared to the LBSThe gravity-driven crystal settling model faces further problems when we attempt to explain the downward increase in differentiation if the first-formed crystals are to have settled in that direction. In this case the higher temperature crystals are near the roofGraded layers with heavy mafics concentrated toward the top and plagioclase toward the bottom do not correspond to the gravity settling hypothesis
32Sandwich Horizon, where the latest, most differentiated liquids crystallized Ferrogabbros with sodic plagioclase (An30), plus Fe-rich olivine and OpxGranophyric segregations of quartz and feldsparF & G = immiscible liquids that evolve in the late stages of differentiation?
33Stratigraphy, Modal, and Cryptic Layering (cryptic determined for intercumulus phases) Figure After Wager and Brown (1968) Layered Igneous Rocks. Freeman. and Naslund (1983) J. Petrol., 25,
34Chemistry of the Skaergård Figure After McBirney (1973) Igneous Petrology. Jones and Bartlett.SiO2, MgO and FeO for both whole-rock analyses and estimated liquid compositions for the Layered SeriesSiO2 as a differentiation index? Now looks dubious for tholeiitesCompatible (Cr and Ni) and incompatible (Rb and Zr) trace elements for the complete sectionTrends are compatible with differentiation of a single surge of magmaNo evidence for any cyclic variations suggestive of repeated injections of fresh magmaComplicated by the presence of layering on a finer scale
35The Processes of Crystallization, Differentiation, and Layering in LMIs LMIs are the simplest possible caseMore complex than anticipatedStill incompletely understood after a half century of intensive studyFractional crystallization a la Bowen and crystal settling & accumulation at the bottom of the chamber explains the major subdivisions, many textural features, and the mineral-cryptic layering in many LMIsBut it fails to explain several other features, particularly the rhythmic layering and the upside-down layering in the Skaergård
36Rhythmic modal layering most easily explained by crystal settling interrupted by periodic large-scale convective overturn of the entire cooling unitReinjection of more primitive magma may explain major compositional shifts and cases of irregular cryptic variationsRhythmic modal layering most easily explained by crystal settling interrupted by periodic large-scale convective overturn of the entire cooling unitA sequence is deposited: settling of denser crystals beneath lighter ones, and expulsion of the late differentiated liquidOverturn removes the late liquid, rehomogenizes the system, and the process would then be repeatedEach cycle would be more evolved due to the removal of the phases in the rhythmic unit, resulting in the phase and cryptic patterns
37Problems with the crystal settling process. Many minerals found at a particular horizon are not hydraulically equivalentSize is more important than density in Stokes’ Law, but size grading is rare in most LMIsDense olivine in the Upper Border Series of the SkaergårdPlagioclase is in the lower layers of the SkaergårdMany of the minerals found at a particular horizon in LMIs are not hydraulically equivalent (they would not be expected to settle at the same rate via Stokes’ Law)Size is more important than density in Stokes’ Law (eq. 11-1) since the radius term is squared, and thus size-graded layers should be more common than modal layering. Size grading is rare in most LMIsWhy is dense olivine in the Upper Border Series of the Skaergård when it should have sunk to the bottom?Plagioclase is in the lower layers of the Skaergård, but is less dense that the liquid from which it crystallized. Why didn’t plagioclase float?
38The Marginal Border Series shows vertical layering Inverted cryptic variations in the Upper Border Series suggests that the early-formed minerals settled upwardThe Marginal Border Series shows vertical layeringBasaltic magmas develop a high yield strength, slightly below liquidus temperaturesCryptic variations in the Upper Border Series are inverted with the most An- rich plagioclase and Mg-rich mafics at the top, suggesting that the early- formed minerals, including heavy mafics, settled upward if they settled at allThe Marginal Border Series shows vertical layering, with mafic minerals concentrated toward the margin in graded layersBasaltic magmas develop a high yield strength, slightly below liquidus temperatures. Crystal settling should not occur soon after crystallization began, particularly if there is some convective motion to keep the crystals suspended
39In-Situ ProcessesNucleation and growth of minerals in a thin stagnant boundary layer along the margins of the chamberDifferential motion of crystals and liquid is still required for fractionationDominant motion = migration of depleted liquid from the growing crystalsCrystals settle (or float) a short distance within the boundary layer as the melt migrates awayBoundary layer interface inhibits material motionDifferential motion of crystals and liquid is still required for fractionation by any in situ processDominant motion = migration of depleted liquid from the growing crystals by diffusive and/or convective processesThe crystals only settle (or float) a short distance within the boundary layer as the melt migrates from the crystals in accordance with its densityThe boundary layer interface inhibits material motionPlagioclase will not easily rise out of a layer at the bottom of a chamber, and olivine will not easily sink from one at the top
40Compositional Convection Systems with gradients in two or more properties (chemical or thermal) with different rates of diffusionEspecially if have opposing effects on density in a vertical directionCompositional ConvectionOf particular interest for layering are systems in which there are gradients in two or more properties (chemical or thermal) with different rates of diffusionIf these gradients have opposing effects on the density of the fluid in a vertical direction, a wide range of novel and complex convective phenomena may occur, collectively known as compositional convectionConsider a container of pure water heated from belowThe result is a convecting system that extends the full vertical length of the container, passing heat upwardA dumb and boring experiment!Next try a container with an aqueous NaCl solution that has a salinity gradient in which the concentration (and density) of the solution increases toward the base
41One gradient (in this case rtemp) is destabilizing (although the total density gradient is stable) The diffusivity of the destabilizing component (heat) is faster than the diffusivity of the saltFigure After Turner and Campbell (1986) Earth-Sci. Rev., 23,
42Double-diffusive convection situation A series of convecting layers Figure After Turner and Campbell (1986) Earth-Sci. Rev., 23,The result: A classic double-diffusive convection situationOpposed gradients with different diffusivities -> series of convecting layers rather than a single large overturning convection cellHeat convection only in a thin bottom layer, which homogenizes both the salinity and thermal density gradientsHeat diffuses faster than the salt, so it is transferred across the diffusive interfaces between the convecting layers and causes a thermal density gradient to develop in the next layer upwardThis creates convective overturn in that layer which homogenizes the salinity and thermal gradients within it and overcomes the thermal density instability while transferring heat upward, where it will diffuse through the interface to the next layer etc.-> a series of layers, each with a different composition and temperature, and the gradual upward transfer of heat from the bottom of the container to the top by convection within layers and diffusion across the interfaces between themThe layers operate in the same fashion as the two-layer mantle model
43Dense crystals held in suspension by agitation Density currentsCooler, heavy-element-enriched, and/or crystal-laden liquid descends and moves across the floor of a magma chamberDense crystals held in suspension by agitationLight crystals like plagioclase also trapped and carried downwardAnother model- Density currentscooler, heavy-element-enriched, and/or crystal-laden liquid may descend (perhaps along the cool walls) and move across the floor of a magma chamber and deposit layersIrvine (1980) proposed gravity-driven currents in several LMIs, supported by such current features as scour-and-fill channels and cross-beddingDense crystals are held in suspension by the agitation of the flowLight crystals like plagioclase can also be trapped and carried downward in the currents
44Figure 12. 15a. Cross-bedding in cumulate layers. Duke Island, Alaska Figure 12.15a. Cross-bedding in cumulate layers. Duke Island, Alaska. Note also the layering caused by different size and proportion of olivine and pyroxene. From McBirney (1993) Igneous Petrology. Jones and BartlettFigure 12.15b. Cross-bedding in cumulate layers. Skaergård Intrusion, E. Greenland. Layering caused by different proportions of mafics and plagioclase. From McBirney and Noyes (1979) J. Petrol., 20,
45Neil Irving’s Vortex model Figure After Irvine et al. (1998) Geol. Soc. Amer. Bull., 110,Cross section of a vortex cell in a density surge current along the boundary layer between floor cumulates (shaded) and magma. Streamlines and arrows portray the instantaneous flow relations relative to points V, R, and S. The floor is actually stationary, of course, and points V, R, and S move as shown with the arrow at V. Flow within the surge moves faster than the vortex (as in a tractor tread), so that the flow material separates from the floor at S and reattaches at R. Forward motion of the vortex results in deposition between it and the chamber floor.Black flow lines and arrows indicate motion relative to the cell
46Figure 12-17. After Irvine et al. (1998) Geol. Soc. Amer. Bull Density currents initiate at cool roof and descend along walls to cross floorCreate layering along whole path- roof, walls, floorPeriodic slumps and drops of accumulated material (autoliths) -> scour/fill and craters
47Figure Cold plumes descending from a cooled upper boundary layer in a tank of silicone oil. Photo courtesy Claude Jaupart.
48Figure Schematic illustration of the density variation in tholeiitic and calc-alkaline magma series (after Sparks et al., 1984) Phil. Trans. R. Soc. Lond., A310,
49Figure Schematic illustration of a model for the development of a cyclic unit in the Ultramafic Zone of the Stillwater Complex by influx of hot primitive magma into cooler, more evolved magma. From Raedeke and McCallum (1984) J. Petrol., 25,