The melt-filled pore structure in the final stages of solidification of cumulates must lie somewhere between the two end-members of impingement (in which pore topology is controlled entirely by the juxtaposition of growth faces of adjacent grains) and textural equilibrium (in which pore topology is controlled by the minimization of internal energies). The exact position between these two end-members is controlled by the relative rates of crystal growth and textural equilibration. For samples in which growth has stopped, or is very slow, textural equilibrium will prevail. A close examination of dihedral angles in natural examples demonstrates that these two end-member textures can be distinguished. The impingement end-member results in a population of apparent solid–melt dihedral angles with a median of ∼60° and a standard deviation of ∼25–30°, whereas the texturally equilibrated end-member population has a median of ∼28° and a standard deviation of ∼14°. For the specific case of cumulates in the Rum Layered Intrusion, residual porosity in troctolitic cumulates was close to the impingement end-member, whereas that in peridotites was close to melt-bearing textural equilibrium. Suites of glass-bearing samples from small, or frequently disturbed, magma systems show modification of initial impingement textures. These modifications may be a consequence of textural equilibration or of diffusion-limited growth during quenching. Distinction can be made between these two processes by a consideration of grain shape. The geometry of interstitial phases in suites of fully solidified cumulates from the Rum Layered Intrusion shows variable approach to sub-solidus textural equilibrium from an initial state inherited by pseudmorphing of the last melt. Textural equilibration at pore corners occurs as a continuous process, with a gradual movement of the entire dihedral angle population towards the equilibrium final state. If the initial, pseudomorphed state is one of disequilibrium (i.e. a melt-present impingement texture) this change is accompanied by a reduction in the spread of the population. If it is one of equilibrium, the change is accompanied by an initial increase in the spread of the population, followed by a decrease. These observations demonstrate that previously published models of dihedral angle change involving the instantaneous establishment of the equilibrium angle in the immediate vicinity of the pore corner are incorrect.
Abundant glass is present along grain boundaries in coarse-grained, glass-bearing, crystalline gabbroic and peridotitic nodules entrained and erupted in lavas from Iceland, Santorini and Mauna Loa (Hawaii), even when the total porosity is less than a few volume per cent. The glass films vary from a few microns to a few tens of microns thick, and are associated with strings of small lensoid grain boundary pockets formed by impingement during crystal growth. Additional porosity occurs as extensive liquid-filled pockets adjacent to included grains within oikocrysts and as large triangular pockets formed by impingement of planar-sided grains. Interstitial material within glass films, and the irregularity of film thickness along a single grain boundary, suggest that the present pore structure is representative of the pore structure before entrainment and eruption. Pore geometry is consistent with a dominant control by crystal growth during solidification, with little or no evidence for control by minimization of internal energies driven by textural equilibration. Similarities between liquid distribution in the crystalline nodules and that of late-stage, interstitial phases in fully solidified mafic cumulates from the Rum and Skaergaard intrusions demonstrate that the crystalline nodules provide information about the latest stages of solidification in slowly cooled mafic plutons. The highly permeable network of intersecting liquid films, lenses and pockets may promote in situ crystallization in the solidifying mush, explaining the common presence of adcumulates in such intrusions.
Abstract The Rum Igneous Centre comprises two early marginal felsic complexes (the Northern Marginal Zone and the Southern Mountains Zone), along with the later central ultrabasic–basic layered intrusions. These marginal complexes represent the remnants of near-surface to eruptive felsic magmatism associated with caldera collapse, examples of which are rare in the North Atlantic Igneous Province. Rock units include intra-caldera collapse breccias, rhyolitic ignimbrite deposits and shallow-level felsic intrusions, as well the enigmatic ‘Am Màm intrusion breccia’. The latter comprises a dacitic matrix enclosing lobate basaltic inclusions (~1–15 cm) and a variety of clasts, ranging from millimetres to tens of metres in diameter. These clasts comprise Lewisian gneiss, Torridonian sandstone and coarse gabbro. Detailed re-mapping of the Am Màm intrusion breccia has shown its timing of emplacement as syn-caldera, rather than pre-caldera as previously thought. Textural analysis of entrained clasts and adjacent, uplifted country rocks has revealed their thermal metamorphism by early mafic intrusions at greater depth than their present structural position. These findings provide a window into the evolution of the early mafic magmas responsible for driving felsic magmatism on Rum. Our data help constrain some of the physical parameters of this early magma–crust interaction and place it within the geochemical evolution of the Rum Centre.
Chert nodules in dolostones in the aureole of the Tertiary Beinn an Dubhaich granite, Skye, have developed mm‐scale concentric monomineralic bands of alternating calcite and olivine during breakdown of a diopside core by the reaction 3 Dol+Di=4 Cal+2Fo+2 CO 2 . A simple textural progression from thin (<1 cm) homogeneous olivine‐calcite rims close to the olivine‐in isograd, to total replacement of the diopside core by a ≤10 cm thick well‐banded rim close to the carbonate‐granite contact demonstrates the developmental stages of the patterning. Correlation of olivine grain size with band spacing, and a greater olivine grain size in banded compared with homogeneous rims support pattern development by post‐nucleation geochemical self‐organization.
Research Article| December 01, 2017 The Skaergaard Intrusion of East Greenland: Paradigms, Problems and New Perspectives Marian B. Holness; Marian B. Holness 1 Department of Earth Sciences University of Cambridge Downing Street, Cambridge, CB2 3EQ, UK E-mail: marian@esc.cam.ac.uk Search for other works by this author on: GSW Google Scholar Troels F.D. Nielsen; Troels F.D. Nielsen 2 Geological Survey of Denmark and Greenland Øter Voldgade 10 DK-1350 Copenhagen K, Denmark E-mail: tfn@geus.dk Search for other works by this author on: GSW Google Scholar Christian Tegner Christian Tegner 3 Centre of Earth System Petrology Department of Geoscience, Aarhus University Høegh-Guldbergs Gade 2 DK-8000 Aarhus C, Denmark E-mail: christian.tegner@geo.au.dk Search for other works by this author on: GSW Google Scholar Elements (2017) 13 (6): 391–396. https://doi.org/10.2138/gselements.13.6.391 Article history first online: 14 Dec 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Marian B. Holness, Troels F.D. Nielsen, Christian Tegner; The Skaergaard Intrusion of East Greenland: Paradigms, Problems and New Perspectives. Elements 2017;; 13 (6): 391–396. doi: https://doi.org/10.2138/gselements.13.6.391 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyElements Search Advanced Search The Skaergaard Intrusion of East Greenland is the quintessential example of low-pressure closed-system fractionation of basaltic magma. Field evidence of extensive layering and associated quasi-sedimentary structures, and the resultant 'cumulate' paradigm of crystal settling in magma chambers, has led to many long-standing controversies. Of particular significance is the lack of consensus about the microstructural record and the mechanisms by which interstitial liquid is expelled from solidifying crystal mushy zones. Skaergaard remains a cradle for new insights into igneous processes, with recent work highlighting the importance of separation of immiscible liquids on magma evolution. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Abstract Replacive symplectites (vermicular intergrowths of two or more minerals) are an important feature of layered igneous intrusions, recording evidence of late-stage reactions between interstitial liquid and crystals. They are common throughout the Layered Series of the 564 Ma Sept Iles layered intrusion in Quebec, Canada, and fall into three types: oxy-symplectites, ‘Type I’ symplectites, and ‘Type II’ symplectites. Oxy-symplectites are comprised of magnetite and orthopyroxene, nucleate on olivine primocrysts, and form via the reaction Olivine + O2 → Orthopyroxene + Magnetite; Type I symplectites (of which there are 3 distinct categories) are comprised of anorthitic plagioclase with pyroxene, amphibole, or olivine vermicules, grow from primocryst oxide grains, and replace primocryst plagioclase; and Type II symplectites (of which there are 2 distinct categories) are comprised of anorthitic plagioclase with orthopyroxene ± amphibole vermicules, grow from primocryst olivine grains, and replace primocryst plagioclase. Rare symplectites composed of biotite and plagioclase are also present. Symplectite growth occurred at 700–1030°C with pressure constraints of 1–2 kbar. We propose that Type I symplectites, and some Type II symplectites, formed from the interaction of primocrysts with residual Fe-rich liquid as a consequence of differential loss of an immiscible Si-rich liquid conjugate from the crystal mush. However, redistribution and concentration of hydrous fluids in incompletely solidified rock, or an increase in water activity of the interstitial melt, may be more plausible processes responsible for the formation of replacive symplectites comprising abundant hydrous mineral assemblages.
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