V53D-01 INVITED
Evolution of Texture and Layering in Layered Intrusions - A Review
It is becoming increasingly apparent form a number of studies that igneous textures in layered intrusions evolve over time. Mechanisms include crystal aging, the effects of which are to produce crystal size distributions to develop characteristic profiles that, unfortunately, mimic those produced by hydraulic sorting. However, crystal aging does allow crystal in the center of large intrusions (where heat loss is slow) to continue to grow at the expense of energetically unfavored crystals. Furthermore, field observations, laboratory experiments and numerical modeling demonstrate that many small scale features (e.g., inch-scale doublets of the Stillwater Complex) can be formed by such processes. In this regard, igneous layering has many parallels with metamorphic banding. Compaction tends to affect both texture and bulk composition and is well documented in many intrusions, but recognizing its development on sloping crystal piles where it may have a shear component is less clear. Future work should more fully understand the transition from ophitic to cumulus textures. Modeling too often focuses either on nucleation and crystal growth leading to an overall increase in the crystal number density, or models of crystal aging that assume an existing crystal assemblage as a starting point. Future modeling should include a fusion of these two approaches to develop a full model of textural development from liquid to solid rock, and encompass textural development in high temperature metamorphic rocks as well.
V53D-02
Understanding the Magmatic Construction of the Dufek Complex, Antarctica
The Jurassic (~180Ma) Dufex Complex in the Pensacola Mountains of Antarctica is arguably one of the largest layered mafic intrusions in the world, with a minimum areal extent of 6600km2. It is mostly buried beneath the Antarctic Icesheet, but is exposed in two parallel mountain ranges; the 45km long Dufek Massif and the 85km long Forrestal Range, which have exposed stratigraphic thicknesses of ~1.8 km and ~1.7 km respectively (Ford, 1976). The two sections appear to be petrologically related, showing a continuous differentiation trend; although some geophysical studies suggest they may represent separate intrusive events (Ferris et al., 1998). The Dufek Massif section consists of the ~230m thick Walker Anothosite unit overlain by the 1550m thick Augenbaugh Gabbro unit. The bottom of the intrusion is not exposed, although geophysical data suggest the presence of an ultramafic basal unit. In the Antarctic summer of 2006/07, we collected and logged 630 oriented rock cores from the lowermost 600m of the section producing a revised and more detailed stratigraphy for this part of the intrusion. In particular, we re-located the boundary between the Walker Anorthosite upwards, so that the Lower Anorthosite of the Augenbaugh Gabbro unit becomes the top of the Walker Anorthosite. We also collected and logged an additional 210 cores from a 100m section higher in the Augenbaugh Gabbro unit. Magnetic susceptibility variation with height was used to correlate between stratigraphic sections. The Walker Anorthosite consists of ortho- and clinopyroxene-bearing spotted anorthosites, interbedded on the meter scale with norites and layered gabbronorites. Modal plagioclase exceeds 65%. Slumped horizons a few meters thick are common, demonstrating a lack of stability of the accumulating mush. The lower part of the Augenbaugh Gabbro unit consists of massive and weakly banded gabbronorites with both cumulus pyroxene and plagioclase, and modal plagioclase ranging from 55- 65%. Rare, thin (<1-2m) anorthosite layers occur as do occasional pyroxenite horizons varying from a few tens of centimeters to several meters thick. The pyroxenite horizons tend to have modally sharp bottoms and gradational tops and may indicate, and constrain the volume of, replenishment events. Large orthopyroxene oikiocrysts (4-20+cm) are common throughout the Augenbaugh gabbro. Horizons of mafic volcanic and calc silicate sedimentary xenoliths ranging in size from 5cm to 5m are present at several levels within the sequence. Pegmatoids and other evidence of volatile enrichment are extremely rare. Our study aims to combine geochemical and textural studies together with magnetic remanence and fabric studies to determine the constructional and thermal history of the lower part of the Dufek Intrusion.
V53D-03 INVITED
Dufek Layered Mafic Intrusion and Basement Sill, Antarctica: Constraints on their Magma Sources Based on PGE Abundance Patterns, Nd-Sr-Pb Isotopic Ratios and Trace Element Modeling
The Dufek layered mafic intrusion in the Pensacola Mts. and Basement Sill in the Dry Valleys of southern McMurdo Sound are part of the Ferrar large igneous province (FLIP), emplaced in a Paleozoic mobile belt developed adjacent to the East Antarctic Craton during the Mid Jurassic. Zircon, baddeleyite and rutile U-Pb dating shows that emplacement occurred in a short amount of time at 184 ± 1 Ma, contemporaneously with fragmentation of the supercontinent Gondwanaland. Understanding the rapid production of copious amounts of magma in a short time is fundamental to understanding LIPs in general. Mantle plumes provide an attractive mechanism for generating short-duration, voluminous magmas in LIPs while at the same time providing a mechanism for the often concurrent break-up of supercontinents. However, plagioclase and pyroxene initial Sr, Nd and Pb isotopic ratios from the Dufek Intrusion and whole-rock samples from the Basement Sill do not resemble the isotopic signatures of lavas associated with South Atlantic hotspots which are closest to this body. Furthermore, PGE abundance patterns for the FLIP appear inconsistent with the plume interpretation. Extreme depletions in Os and Ir compared to the Ru, Pt and Pd abundances – with the Os/Ir ratios in the range of 0.1 to 0.3 for these samples compared to the more commonly observed values close to 1 – are atypical of plume-derived magmas and are more consistent with the alternative view that the FLIP resulted from the decompression of a fossil subduction zone along the Proto-Pacific margin of Gondwanaland, disaggregated by the rifting related to plate rearrangements that occurred during supercontinent break-up.
V53D-04
A Statistical and Wavelet Analysis of Physical Property Data From the 2950 m Deep Bellevue Borehole, Bushveld Complex, South Africa
Susceptibility (n=~110,000) and density (n=~~2500) measurements on core samples have been collected in a stratigraphic context from the Bellevue (BV-1) 2950 m deep borehole in the Northern Lobe of the Bushveld Complex. This drill core starts in the granitoid roof rocks, extends through the entire Upper Zone, and ends approximately in the middle of the Main Zone. These physical property measurements now provide an extensive database useful for geophysical modeling and stratigraphic studies. In an effort to quantify the periodicity of the layering we have applied various statistical and wavelet methods to analyze the susceptibility and density data. The density data have revealed a strong periodic layering with a scale of ~~80 m that extends through the Main and Upper Zones. In the Main Zone the layering is unusual in that the density values increase upwards by as much as 10%. This is due to systematic variation in the modal abundance of mafic silicates and appears to be related to separate pulses during emplacement. The magnetic susceptibility data in the Upper Zone also show a strong cyclicity of similar scale. The discrete wavelet transform, using the real Haar wavelet, has been applied to help discretise the susceptibility data and clarifies the geological boundaries without blurring them, which is a common problem with multipoint moving averages. As expected, the histogram of the entire data set is non-Gaussian, with a long tail for high values. We can roughly fit a power law to the log histogram plot indicating a probable fractal distribution of susceptibilities. However if we window the data in the range 750-1000 m the histogram is very different. This region shows a strong peak and no power law relationship. This dramatic change in statistical properties prompted us to investigate these properties more thoroughly. To complement the wavelet analysis we have calculated various statistical measures (mean, standard deviation, skew, and kurtosis) of a window of data 21 points wide for the length of the drill core. In order to graphically image how the 21 point windowed histogram varies with depth, we have developed a "Windowed Histogram Image" (WHI) which graphically demonstrates how much the histogram for a particular window size varies downward. This highlights the changing statistical pattern in the drill core highlighting pattern breaks that are likely to be indicative of emplacement pulses.
V53D-05
Searching for a better understanding of differentiation in the Skaergaard Intrusion
The Skaergaard Intrusion is probably the most intensely studied body of igneous rocks on Earth, and yet it seems that the more we have learned about it the less confident we are that we understand the basic processes of magmatic differentiations that it so magnificently displays. The elegant model of differentiation by crystal settling that Lawrence Wager and his colleagues left us is in shambles and is yet to be replaced by a coherent alternative. Interpretations based on so-called cumulate textures have been discredited by our recognition that few if any of the rocks preserve their original textures, compositions, or modal proportions; they re-equilibrated during a long period of slow cooling. Many of these rocks have undergone extensive metasomatism that, in some instances, produced nearly mono-mineralic mafic and felsic assemblages. An abrupt change in the volatile components midway through the course of crystallization was accompanied by marked changes in the oxygen fugacity, partitioning of trace elements, and crystal-liquid equilibria. Equally important, our recognition that compositional changes had a greater effect on liquid densities than thermal expansion has forced us to reject the earlier interpretation of the convective regime, and this in turn led to the realization that the Layered Series did not crystallize from the main reservoir of liquid but from liquids that ponded on the floor after evolving during partial crystallization on the steep walls. In an effort to sort out all these new developments and find better explanations for the trends of compositioal evolution, we have abandoned all our previous assumptions in favor of letting the rocks themselves tell us how they reached their final state. To do this, we are constructing a numerical model that incorporates as many known physical-chemical processes as possible in the hope that it may lead to a better understanding of the fundamental principles of magmatic differentiation.
V53D-06 INVITED
The Petrogenesis of the Unit 7/8 and 11/12 Chrome-spinel Seams of the Rum Eastern Layered Intrusion (NW Scotland) Re-evaluated
Laterally extensive (~2 mm thick) chrome-spinel seams in the Rum Layered Suite, NW Scotland, occur at the junctions of several of the coupled peridotite-troctolite macro-rhythmic units that make up the bulk of the eastern part of the intrusion. A detailed petrographic study of the rocks immediately above and below two of these seams suggests that existing models for seam formation involving early crystallisation and gravitational settling of chrome-spinel crystals from a newly emplaced body of picritic magma may be flawed. Instead, the textural relationships between minerals suggest that olivine crystallisation in the peridotite above each of the seams occurred before that of most of the chrome-spinel. Reaction textures between olivine and chrome-spinel crystals are commonly observed, with plagioclase usually occurring as thin rims between both olivine and chrome-spinel where both are in close proximity. The textural evidence suggests a significant degree of olivine crystal-shape change; it seems that many of the olivine crystals immediately above the main seams may initially have had much more complex (harrisitic) crystal shapes before modification to simpler morphologies in a crystal mush. Plagioclase occurs in the peridotite as large oikocrysts up to several cm in size. Additionally, the chrome-spinel seams occur only in those units that display extensive evidence of syn-magmatic deformation of unconsolidated cumulate in the underlying troctolite, and the seams themselves often exhibit small-scale load structures. A model suggesting in-situ crystallisation of the chrome-spinel seams is proposed, whereby mixing of an evolved interstitial liquid with a primitive picritic melt occurred approximately at the crystal mush-magma interface. The former was released from the unconsolidated troctolite mush as a response to re-mobilization and chaotic slumping, possibly triggered by emplacement of some of the hot picrite into the crystal mush pile. Significant undercooling in the picrite due to emplacement-related cooling had already produced a crystal framework comprising complex skeletal olivine crystal morphologies with very fast growth rates. It is envisaged that the significantly modified olivine textures in the peridotite immediately above both seams can be attributed to upward- moving porosity waves of the same ‘mixed' interstitial melt that precipitated the chrome-spinel seams. In addition to formation of the seams at the main unit junctions, ‘necklace' or ‘chain-like' distributions of chrome-spinel crystals around olivine crystals in the peridotite, as well as the large plagioclase oikocrysts, argue for the presence of a mobile interstitial melt with a protracted cooling history.
V53D-07 INVITED
Cooling Rates in the Lower Crust of the Oman Ophiolite: Ca in Olivine, Revisited
Debate over the mechanism of accretion of the layered gabbros of the lower oceanic crust has been centered on the gabbro glacier and the sheeted sills models. In the gabbro glacier model it is proposed that the lower oceanic crust (below the sheeted dikes) crystallizes in a single shallow melt lens and is then transported ductilely downward. Conversely, in the sheeted sills model the lower oceanic crust is formed in several melt lenses and crystallizes in situ. The thermal profile of the crust, specifically the roles of hydrothermal circulation and cooling rate, is a key component in distinguishing between these two models. Results from Ca in olivine in this study show no systematic variation of cooling rate with depth in the lower crust of the Khafifah section in the Wadi Tayin massif of the Oman ophiolite. Additionally, very high cooling rates recorded near the base of the crust suggest that hydrothermal circulation plays an important role in the removal of heat throughout the crust. On the basis of these results it can be concluded that the sheeted sills model is a thermally viable mechanism for accretion of oceanic lower crust. Ca in olivine results for the oceanic lower crust are also compared with other magmatic systems such as layered mafic intrusions and arc lower crust to quantify relative cooling rates among these different systems. Perhaps surprisingly, large layered intrusions, such as the Bushveld Complex, are found to cool at approximately the same rate as oceanic lower crust.
V53D-08
Magmatic Processes and Emplacement Dynamics in Layered Mafic Intrusions: Insights From Magnetic Fabric Studies
This study focuses on documentation of magmatic structures and constraining emplacement dynamics, using new developments in rock magnetism, in two layered mafic intrusions: Insizwa sill (South Africa) and the Sonju Lake intrusion (Minnesota). Magnetic susceptibility tracks subtle variations in magnetic mineral content and is used to distinguish between open and closed magmatic systems, which are expected to yield distinct magnetic patterns. AMS measurements provide petrofabric data, used to constrain the dynamics of magma emplacement and to distinguish between flow and static layering processes. The mafic systems examined include an open system in which multiple pulses of magma most certainly occurred and an almost completely closed system. The Insizwa sill, an open magmatic system, is part of the Mesozoic Karoo Igneous Province in South Africa. Continuous borehole core logging of magnetic susceptibility and the anisotropy of magnetic susceptibility (AMS) confirms that variation in the prominent vertical zonation of magnetic properties generally correspond to petrologically and geochemically determined layers. Magnetic properties also document the lateral continuity of recognizable petrologic layers, and corroborate earlier suggestions that the sill formed by multiple pulses of magma. The Sonju Lake intrusion, an almost completely closed system, is part of the Proterozoic Midcontinent Rift System, NE Minnesota. The observed upward increasing magnetic susceptibility trend suggests construction of a simple sill by a single magma pulse. A petrofabric study investigated the emplacement and flow patterns within the Sonju Lake intrusion. The magnetic fabrics documented in the layered series of the intrusion are consistent with sub-horizontal to inclined emplacement and show evidence that the cumulate layers were deposited in a dynamic environment. By integrating aspects of petrology, geochemistry, and rock magnetism, a better understanding of layered mafic intrusions is achieved. Magnetic properties provide additional insight into magmatic processes and magma transfer, and may be used to distinguish between open and closed system magmatic systems.