Chapter 12 Spring 2010 - Chapter 12 Layered Mafic...

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Unformatted text preview: Chapter 12: Layered Mafic Intrusions Table 12-1. Some Principal Layered Mafic Intrusions Name Bushveld Dufek Duluth Stillwater Muskox Great Dike Kiglapait Skaergrd Age Precambrian Jurassic Precambrian Precambrian Precambrian Precambrian Precambrian Eocene Location S. Africa Antarctica Minnesota, USA Montana, USA NW Terr. Canada Zimbabwe Labrador East Greenland Area (km ) 66,000 50,000 4,700 4,400 3,500 3,300 560 100 2 Large or particularly well-studied LMIs exposed in continents (many in flood basalt provinces) The form of a typical LMI The Muskox Intrusion Figure 12-1. From Irvine and Smith (1967), In P. J. Wyllie (ed.), Ultramafic and Related Rocks. Wiley. New York, pp. 38-49. layer: any sheet-like cumulate unit distinguished by its compositional and/or textural features 3 Layers may be uniform mineralogically and texturally homogeneous Layering Figure 12-3b. Uniform chromite layers alternate with plagioclase-rich layers, Bushveld Complex, S. Africa. From McBirney and Noyes (1979) J. Petrol., 20, 487-554. Layering Layers may be non-uniform vary either along or across the layering v graded = gradual variation in either h mineralogy h grain size - quite rare in gabbroic LMIs q q q q Types of layering Modal Layering: characterized by variation in the relative proportions of constituent minerals 3 may contain uniform layers, graded layers, or a combination of both Phase layering: the appearance or disappearance of minerals in the crystallization sequence developed in modal layers 3 Phase layering transgresses modal layering Cryptic Layering (not obvious to the eye) 3 Systematic variation in the chemical composition of certain minerals with stratigraphic height in a layered sequence Layering (or stratification) q Rhythmic: layers systematically repeat 3 3 The regularity of layering Macrorhythmic: several meters thick Microrhythmic: only a few cm thick A common type consists of rhythmic graded layers punctuated by occasional uniform layers Figure 12-4. Intermittent layering showing graded layers separated by nongraded gabbroic layers. Skaergrd Intrusion, E. Greenland. q Intermittent: less regular patterns 3 Figure 12-3a. Vertically tilted cm-scale rhythmic layering of plagioclase and pyroxene in the Stillwater Complex, Montana. The Bushveld Complex, South Africa The biggest: 300-400 km x 9 km The Red Granite intruded 50-100 Ma afterwards Figure 12-5. Simplified geologic Map and cross section of the Bushveld complex. After Willemse (1964), Wager and Brown (1968), and Irvine et al. (1983). Stratigraphy Marginal Zone: the lowest unit, is a chill zone about 150 m thick Basal Series Thin uniform dunite cumulates alternating with orthopyroxenite and harzburgite layers The top defined as the Main Chromite Layer Critical Series Plagioclase forms as a cumulate phase (phase layering) Norite, orthopyroxenite, and anorthosite layers etc The Merensky Reef ~ 150 m thick sequence of rhythmic units with cumulus plagioclase, orthopyroxene, olivine, and chromite Main Zone The thickest zone and contains thick monotonous sequences of hypersthene gabbro, norite, and anorthosite Upper Zone Appearance of cumulus magnetite (Fe-rich) Well layered: anorthosite, gabbro, and ferrodiorite Numerous felsic rock types = late differentiates Also note: Cryptic layering: systematic change in mineral compositions Reappearance of Fe-rich olivine in the Upper Zone The Stillwater Complex, Montana Figure 12-8. After Wager and Brown (1968) Layered Igneous Rocks. Freeman. San Francisco. q Stillwater Complex Stratigraphy Basal Series Basal Series 3 a thin (50-150 m) layer of norites and gabbros q Ultramafic Series base = first appearance of copious olivine cumulates (phase layering) 3 Lower Peridotite Zone v v 20 cycles (20-150 m thick) of macrorhythmic layering with a distinctive sequence of lithologies The series begins with dunite (plus chromite), followed by harzburgite and then orthopyroxenite is a single, thick (up to 1070 m), rather monotonous layer of cumulate orthopyroxenite 3 Upper Orthopyroxenite Zone v Stillwater Complex Stratigraphy The crystallization sequence within each rhythmic unit (with rare exception) is: v olivine + chromite v olivine + orthopyroxene v orthopyroxene v orthopyroxene + plagioclase v orthopyroxene + plagioclase + augite Stillwater Complex Stratigraphy The Banded Series 3 Sudden cumulus plagioclase significant change from ultramafic rock types (phase layering again) 3 The most common lithologies are anorthosite, norite, gabbro, and troctolite (olivine-rich and pyroxenepoor gabbro) The Skaergrd Intrusion E. Greenland Figure 12-10. After Stewart and DePaolo (1990) Contrib. Mineral. Petrol., 104, 125141. The Skaergrd Intrusion E. Greenland Magma intruded in a single surge (premier natural example of the crystallization of a mafic pluton in a single-stage process) 3 Fine-grained chill margin 3 Stratigraphy Skaergrd subdivided into three major units: 3 Layered Series 3 Upper Border Series 3 Marginal Border Series Upper Border Series and the Layered Series meet at the Sandwich Horizon (most differentiated liquids) Stratigraphy, Modal, and Cryptic Layering (cryptic determined for intercumulus phases) Figure 12-12. After Wager and Brown (1968) Layered Igneous Rocks. Freeman. and Naslund (1983) J. Petrol., 25, 185-212. Cross section looking down dip. Figure 12-11. After After Hoover (1978) Carnegie Inst. Wash., Yearb., 77, 732-739. The Skaergrd Intrusion E. Greenland Upper Border Series: thinner, but mirrors the 2500 m Layered Series in many respects 3 Cooled from the top down, so the top of the Upper Border Series crystallized first v The most Mg-rich olivines and Carich plagioclases occur at the top, and grade to more Fe-rich and Narich compositions downward v Major element trends also reverse in the Upper Border Series as compared to the LBS The Skaergrd Intrusion E. Greenland Sandwich Horizon, where the latest, most differentiated liquids crystallized 3 Ferrogabbros with sodic plagioclase (An30), plus Fe-rich olivine and Opx 3 Granophyric segregations of quartz and feldspar 3 F & G = immiscible liquids that evolve in the late stages of differentiation? Figure 12-17. After Irvine et al. (1998) Geol. Soc. Amer. Bull., 110, 1398-1447. q Density currents within the Skaergrd Intrusion Cooler, heavy-element-enriched, and/or crystal-laden liquid descends and moves across the floor of a magma chamber 3 Dense crystals held in suspension by agitation 3 Light crystals like plagioclase also trapped and carried downward 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 Bartlett Figure 12-15b. Cross-bedding in cumulate layers. Skaergrd Intrusion, E. Greenland. Layering caused by different proportions of mafics and plagioclase. From McBirney and Noyes (1979) J. Petrol., 20, 487-554. Figure 12-17. After Irvine et al. (1998) Geol. Soc. Amer. Bull., 110, 1398-1447. Figure 12-18. Cold plumes descending from a cooled upper boundary layer in a tank of silicone oil. Photo courtesy Claude Jaupart. The Processes of Crystallization, Differentiation, and Layering in LMIs q q q LMIs are the simplest possible case More complex than anticipated Still incompletely understood after a half century of intensive study 3 3 Not a reflection of Bowen's Reaction Series Not simple crystal fractionation The Processes of Crystallization, Differentiation, and Layering in LMIs q 1) Rhythmic modal layering most easily explained by crystal settling interrupted by periodic largescale convective overturn of the entire cooling unit 3 3 3 3 3 Settling of dense liquids Expulsion of liquid by compaction Overturn would homogenize the system Process repeats But..... Problems with the crystal settling process. q q q q q q Many minerals found at a particular horizon are not `hydraulically' equivalent Size is more important than density in Stokes' Law, but size grading is rare in most LMIs Dense olivine in the Upper Border Series of the Skaergrd Plagioclase is in the lower layers of the Skaergrd Inverted cryptic variations in the Upper Border Series suggests that the early-formed minerals settled upward The Marginal Border Series shows vertical layering The Processes of Crystallization, Differentiation, and Layering in LMIs q 2) Reinjection of more primitive magma may explain major compositional shifts and cases of irregular cryptic variations 3 However, does not explain all, evidence lacking for spikes in composition Combined Processes New Model: In-situ Crystallization and Convection q Double diffusive convection..... ...
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This note was uploaded on 05/02/2010 for the course ESCI 322 taught by Professor Evans during the Spring '10 term at Central Connecticut State University.

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