scholarly journals Sr isotopes indicate millennial-scale formation of metal-rich layers by reactive melt percolation in an open-system layered intrusion

2020 ◽  
Author(s):  
J. Stephen Daly ◽  
Luke Hepworth ◽  
Brian O'Driscoll ◽  
Chris Johnson ◽  
Ralf Gertisser ◽  
...  
Keyword(s):  
Open Systems ◽  
Layered Intrusion ◽  
Chemical Zoning ◽  
Mafic Intrusions ◽  
Ionisation Mass Spectrometry ◽  
Pge Mineralization ◽  
Isotopic Analyses ◽  
Melt Percolation ◽  
Mineral Scale ◽  
Reactive Melt

<p>In order to test whether the crystal mushes that form layered mafic intrusions can behave as open systems, we investigated mineral-scale textural, chemical and Sr isotopic heterogeneity in the c. 60 Ma Rum intrusion, Scotland. Within Unit 10 of the Rum intrusion, intercumulus plagioclase and clinopyroxene crystals in peridotite 1-2 cm above and below millimetric Cr-spinel seams exhibit complex optical and chemical zoning (Hepworth et al. 2017). These Cr-spinel seams are closely associated with sulphide and platinum-group element (PGE) mineralization. High precision Sr isotopic analyses (undertaken by thermal ionisation mass spectrometry) of individual intracrystal zones (sampled by micromilling) in intercumulus plagioclase and clinopyroxene from within the PGE-enriched Cr-spinel seams have revealed significant intra-crystalline heterogeneity. <sup>87</sup>Sr/<sup>86</sup>Sr heterogeneity is present between plagioclase crystals, between clinopyroxene and plagioclase, and within plagioclase crystals, throughout the studied section. The preservation of Sr isotope heterogeneities at 10-100 µm length-scales implies cooling of the melts that formed the precious metal-rich layers at rates >1 °C per year, and cooling to diffusive closure within 10s-100s of years. The combined textural observations and intra-crystal plagioclase <sup>87</sup>Sr/<sup>86</sup>Sr data also highlight the importance of repeated cycles of dissolution and recrystallization within the crystal mush, and together with recent documentation of ‘out-of-sequence’ layers in other layered intrusions (Mungall et al. 2016; Wall et al. 2018), raise the prospect that basaltic magmatic systems may undergo repeated self-intrusion during solidification.</p><p>Hepworth, L.N., O’Driscoll, B., Gertisser, R., Daly, J.S. and Emeleus, H.C. 2017. Journal of Petrology 58, 137-166; Mungall, J. E., Kamo, S. L. & McQuade, S. 2016. Nature Communications 7, 13385; Wall, C. J., Scoates, J. S., Weis, D., Friedman, R. M., Amini, M. & Meurer, W. P. 2018. Journal of Petrology 59, 153–190.</p>

Petrology and Ni-Cu-Cr-PGE Mineralization of the Largest Mafic Pluton in Europe: The Early Proterozoic Burakovsky Layered Intrusion, Karelia, Russia

1995 ◽  
Vol 37 (6) ◽  
pp. 509-525 ◽  
Author(s):  
Evgenii V. Sharkov ◽  
O. A. Bogatikov ◽  
Tanya L. Grokhovskaya ◽  
A. V. Chistyakov ◽  
Vyacheslav A. Ganin ◽  
...  
Keyword(s):  
Layered Intrusion ◽  
Early Proterozoic ◽  
Pge Mineralization

Age and Geochemical Studies of the Snyder Breccia, Coastal Labrador

1975 ◽  
Vol 12 (3) ◽  
pp. 361-370 ◽  
Author(s):  
Jackson M. Barton Jr. ◽  
Erika S. Barton
Keyword(s):  
Trace Element ◽  
North Atlantic ◽  
Layered Intrusion ◽  
Basement Complex ◽  
Fine Grained ◽  
The North ◽  
Isotopic Analyses ◽  
The Matrix ◽  
The North Atlantic ◽  
Geochemical Studies

The Snyder breccia is composed of angular to subrounded xenoliths of migmatites and amphibolites in a very fine grained matrix. It is apparently intrusive into the metasediments of the Snyder Group exposed at Snyder Bay, Labrador. The Snyder Group unconformably overlies a migmatitic and amphibolitic basement complex and is intruded by the Kiglapait layered intrusion. K–Ar ages indicate that the basement complex is Archean in age (> 2600 m.y. old) and that the Kiglapait layered intrusion was emplaced prior to 1280 m.y. ago. Major and trace element analyses of the matrix of the Snyder breccia indicate that while it was originally of tonalitic composition, later it locally underwent alteration characterized by loss of sodium and strontium and gain of potassium, rubidium and barium. Rb–Sr isotopic analyses show that this alteration occurred about 1842 m.y. ago, most probably contemporaneously with emplacement of the breccia. The Snyder Group thus was deposited sometime between 2600 and 1842 m.y. ago and may be correlative with other Aphebian successions preserved on the North Atlantic Archean craton.

Petrochemistry, tectonic history, and Sr–Nd systematics of the Liscomb Complex, Meguma Lithotectonic Zone, Nova Scotia

1993 ◽  
Vol 30 (3) ◽  
pp. 449-464 ◽  
Author(s):  
D. B. Clarke ◽  
A. K. Chatterjee ◽  
P. S. Giles
Keyword(s):  
Crustal Contamination ◽  
Granitic Rocks ◽  
Low Grade ◽  
South Mountain Batholith ◽  
Mafic Intrusions ◽  
Depleted Mantle ◽  
Isotopic Analyses ◽  
Wide Range ◽  
Granitoid Rocks ◽  
Lithotectonic Zone

The Liscomb Complex (area ca. 240 km2), located in the Meguma Lithotectonic Zone of the Canadian Appalachians, consists of three main lithological components: high-grade gneisses, mafic plutons, and peraluminous granitoid bodies. Field relations and 40Ar/39Ar dating (369–377 Ma) embracing all three lithological groups show that the complex is post-Acadian. The gneisses occur as a domal uplift and represent a mixed volcano-sedimentary package that is structurally, metamorphically, and chemically distinct from the surrounding low-grade metawackes and metapelites of the Meguma Group. The mafic intrusions (quartz gabbro to quartz diorite) have major and trace element compositions (e.g., Ti–Zr–Y, Nb–Zr–Y, Th/Yb – Ta/Yb, rare earth elements) typical of within-plate or volcanic arc materials. The peraluminous granitoid rocks range from two-mica granodiorites to leucomonzogranites, and are mineralogically and chemically very similar to granitic rocks elsewhere in the Meguma Zone. Neodymium and strontium isotopic analyses show that (i) the gneisses have a wide range of εNd and initial Sr isotopic ratios, with Nd model ages that are generally younger than those of the Meguma Group; (ii) the mafic intrusive rocks represent magmas derived from slightly depleted mantle sources (εNd +3.3 to +1.4), in part modified by crustal contamination (εNd +0.5 to −5.0); and (iii) the granitoid rocks isotopically overlap both the South Mountain Batholith and the intermediate gneisses of the Liscomb Complex. The combined field, petrological, and chemical evidence suggests that underplating by mafic magmas, followed by thermal doming of the gneisses, diapirism through the Meguma Group, anatexis, and multiple intrusion of both mafic and felsic magmas best explain the observed relationships in the Liscomb Complex. This mechanical model may also apply to granite generation throughout the Meguma Zone.

scholarly journals Microstructures and Late-Stage Magmatic Processes in Layered Mafic Intrusions: Symplectites from the Sept Iles Intrusion, Quebec, Canada

2020 ◽  
Vol 61 (7) ◽  
Author(s):  
Halley A Keevil ◽  
Olivier Namur ◽  
Marian B Holness
Keyword(s):  
Late Stage ◽  
Layered Intrusion ◽  
Type I ◽  
Type Ii ◽  
Hydrous Mineral ◽  
Mafic Intrusions ◽  
Igneous Intrusions ◽  
Mineral Assemblages ◽  
Rich Liquid ◽  
Layered Mafic Intrusions

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.

Effects of melt-percolation, refertilization, and deformation on upper mantle seismic anisotropy: constraints from peridotite xenoliths, Marie Byrd Land, West Antarctica

2021 ◽  
pp. M56-2020-16
Author(s):  
V. Chatzaras ◽  
S. C. Kruckenberg
Keyword(s):  
Upper Mantle ◽  
Mantle Xenoliths ◽  
P Wave ◽  
West Antarctica ◽  
Seismic Properties ◽  
West Antarctic ◽  
Peridotite Xenoliths ◽  
Melt Percolation ◽  
Supplementary Material ◽  
Reactive Melt

AbstractWe report on the petrology, microstructure, and seismic properties of 44 peridotite xenoliths extracted from the upper mantle beneath Marie Byrd Land (MBL), West Antarctica. The aim of this work is to understand how melt-rock reaction, refertilization, and deformation affected the seismic properties (velocities, anisotropy) of the West Antarctic upper mantle, in the context of MBL tectonic evolution and West Antarctic Rift System formation. Modal compositions, mineral major element compositions, microstructures, and crystallographic preferred orientations (CPOs) provide evidence for diachronous reactive melt percolation and refertilization. Olivine shows three main CPO patterns, the A-type, axial-[010], and axial-[100] texture types. Average seismic properties of the MBL mantle lithosphere are mainly controlled by the strength of olivine crystallographic texture. Reactive melt percolation and refertilization likely modified seismic velocities and anisotropy, as is suggested by a systematic decrease in maximum P-wave and S-wave anisotropies with increasing modal abundance of pyroxene. At larger spatial scales, the seismic properties of the MBL mantle xenoliths are dominated by the anisotropy resulting from the A-type olivine CPO. Variations between individual volcanic centers, however, attest to spatial variations in the mantle structure, potentially related to 3-D deformation and the prolonged tectonic history of MBL.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5315261

Low-sulfide PGE mineralization in layered intrusion ring, Vestfold Hills, Antarctica

2015 ◽  
Vol 57 (8) ◽  
pp. 657-673
Author(s):  
M. S. Egorov ◽  
V. S. Semenov ◽  
N. L. Alekseev
Keyword(s):  
Layered Intrusion ◽  
Pge Mineralization ◽  
Vestfold Hills
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