53 research outputs found
U-Pb, Re-Os, and Ar/Ar Geochronology of Rare Earth Element (REE)-Rich Breccia Pipes and Associated Host Rocks from the Mesoproterozoic Pea Ridge Fe-REE-Au Deposit, St. Francois Mountains, Missouri
Rare earth element (REE)-rich breccia pipes (600,000 t @ 12% REO) are preserved along the margins of the 136 Mt Pea Ridge magnetite-apatite deposit, within Mesoproterozoic (~1.47 Ga) volcanic-plutonic rocks of the St. Francois Mountains terrane in southeastern Missouri, USA. The breccia pipes cut the rhyolite-hosted magnetite deposit, and contain clasts of nearly all local bedrock and mineralized lithologies. Grains of monazite and xenotime were extracted from breccia pipe samples for SHRIMP U-Pb geochronology; both minerals were also dated in one polished thin section. Monazite forms two morphologies: (1) matrix granular grains composed of numerous small (<50 μm) crystallites intergrown with rare xenotime, thorite, apatite, and magnetite; and (2) coarse euhedral, glassy, bright yellow grains similar to typical igneous or metamorphic monazite. Trace element abundances (including REE patterns) were determined on selected grains of monazite (both morphologies) and xenotime. Zircon grains from two samples of host rhyolite and two late felsic dikes collected underground at Pea Ridge were also dated. Additional geochronology done on breccia pipe minerals includes Re-Os on fine-grained molybdenite and 40Ar/39Ar on muscovite, biotite, and Kfeldspar. Ages (± 2-sigma errors) obtained by SHRIMP U-Pb analysis are as follows: (1) zircon from the two host rhyolite samples have ages of 1473.6 ± 8.0 and 1472.7 ± 5.6 Ma; most zircon in late felsic dikes is interpreted as xenocrystic (age range ca. 1522-1455 Ma); a population of rare spongy zircon is likely of igneous origin and yields an age of 1441 ± 9 Ma; (2) pale yellow granular monazite—1464.9 ± 3.3 Ma (no dated xenotime); (3) reddish matrix granular monazite—1462.0 ± 3.5 Ma and associated xenotime—1453 ± 11 Ma; (4) coarse glassy yellow monazite—1464.8 ± 2.1, 1461.7 ± 3.7 Ma, with rims at 1447.2 ± 4.7 Ma; and (5) matrix monazite (in situ) —1464.1 ± 3.6 and 1454.6 ± 9.6 Ma, and matrix xenotime (in situ) —1468.0 ± 8.0 Ma. Two slightly older ages of cores are about 1478 Ma. The young age of rims on the coarse glassy monazite coincides with a Re-Os age of 1440.6 ± 9.2 Ma determined in this study for molybdenite intergrown with quartz and allanite, and with the age of monazite inclusions in apatite from the magnetite ore (Neymark et al., this volume). A 40Ar/39Ar age of 1473 ± 1 Ma was obtained for muscovite from a breccia pipe sample. Geochronology and trace element geochemical data suggest that the granular matrix monazite and xenotime (in polygonal texture), and cores of coarse glassy monazite precipitated from hydrothermal fluids during breccia pipes formation. The second episode of mineral growth at ca. 1443 Ma may be related to faulting and fluid flow that rebrecciated the pipes. The ca. 10 m.y. gap between the ages of host volcanic rocks and breccia pipe monazite and xenotime suggests that breccia pipe mineral formation cannot be related to the felsic magmatism represented by the rhyolitic volcanic rocks, and hence is linked to a different magmatic-hydrothermal system
High-precision U-Pb and 40Ar/39Ar dating of an Alpine ophiolite (Gets nappe, French Alps)
Coarse-grained gabbros from two different localities in the Gets nappe
(Upper Prealps) have been dated by U-Pb and Ar-40/Ar-39 isotopic
analyses. Zircons from both gabbros gave identical concordant U-Pb ages
of 166 +/- 1 Ma (Fig. 4). Amphibole from one of them gave an Ar-40/Ar-39
plateau age of 165.9 +/- 2.2 Ma (Fig. 5). This concordance implies that
166 +/- 1 Ma is the age of magmatic crystallization of these gabbros.
The Gets wildflysch with its mafic and ultramafic lenses is an
ophiolitic melange, that we infer to come from a proximal part of the
accretionary prism at the foot of the active SE margin of the Piemont
ocean. In this position we can expect to find remnants of the oldest
parts of the Piemont oceanic crust. These are the first high-precision
dates using modern techniques from an Alpine ophiolite and are in
excellent agreement with the following:
1) The few, somewhat younger, reliable ages on ophiolites from the
probable continuation of the Piemont basin into the Apennines and
Corsica;
2) Recent data on the age of the first supra-ophiolitic sediments (Late
Bathonian to Early Callovian radiolarites);
3) The structural and stratigraphic evolution of the Brianconnais (s.s.)
domain, the future NW margin of the Piemont ocean. We note a remarkable
coincidence, in Late Bajocian time, between: (A) the end of tensile
fracturing in the Brianconnais continental crust; (B) the beginning of
its subsidence; (C) the age of the Gets ophiolites. This coincidence is
consistent with an ocean opening mechanism based on a combination of
subhorizontal extension and thermally driven vertical movements of the
lithosphere
Heterogeneous 40Ar* distributions in naturally deformed muscovite: in situ UV-laser ablation evidence for micro structurally controlled intragrain diffusion
Micas are commonly used in Ar-40/Ar-39 thermochronological studies of
variably deformed rocks yet the physical basis by which deformation may
affect radiogenic argon retention in mica is poorly constrained. This
study examines the relationship between deformation and
deformation-induced microstructures on radiogenic argon retention in
muscovite, A combination of furnace step-heating and high-spatial
resolution in situ UV-laser ablation Ar-40/Ar-39 analyses are reported
for deformed muscovites sampled from a granitic pegmatite vein within
the Siviez-Mischabel Nappe, western Swiss Alps (Penninic domain,
Brianconnais unit). The pegmatite forms part of the Variscan (similar to
350 Ma) Alpine basement and exhibits a prominent Alpine S-C fabric
including numerous mica `fish' that developed under greenschist facies
metamorphic conditions, during the dominant Tertiary Alpine tectonic
phase of nappe emplacement. Furnace step-heating of milligram quantities
of separated muscovite grains yields an Ar-40/Ar-39 age spectrum with
two distinct staircase segments but without any statistical plateau,
consistent with a previous study from the same area. A single (3 X 5 mm)
muscovite porphyroclast (fish) was investigated by in situ UV-laser
ablation. A histogram plot of 170 individual Ar-40/Ar-39 UV-laser
ablation ages exhibit a range from 115 to 387 Ma with modes at
approximately 340 and 260 Ma. A variogram statistical treatment of the
(40)Ad/Ar-39 results reveals ages correlated with two directions; a
highly correlated direction at 310 degrees and a lesser correlation at 0
degrees relative to the sense of shearing. Using the highly correlated
direction a statistically generated (Kriging method) age contour map of
the Ar-40/Ar-39 data reveals a series of elongated contours subparallel
to the C-surfaces which where formed during Tertiary nappe emplacement.
Similar data distributions and slightly younger apparent ages are
recognized in a smaller mica fish. The observed intragrain age
variations are interpreted to reflect the partial loss of radiogenic
argon during Alpine (similar to 35 Ma) greenschist facies metamorphism.
One-dirnensional diffusion modelling results are consistent with the
idea that the zones of youngest apparent age represent incipient shear
band development within the mica porphyroclasts, thus providing a
network of fast diffusion pathways. During Alpine greenschist facies
metamorphism the incipient shear bands enhanced the intragrain loss of
radiogenic argon. The structurally controlled intragrain age variations
observed in this investigation imply that deformation has a direct
control on the effective length scale for argon diffusion, which is
consistent with the heterogeneous nature of deformation. (C) 2001
Elsevier Science B.V. All rights reserved
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