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Taltson
Queen
Maud
block
Hearne
Great Slave Lake
Slave
Thelon orogen
Thelon
basin
Athabasca basin
NU
MB
SK
NT
0
60 N
SK
AB
Snowbird tectonic zone
orogen
Rae
Rae
Yellowknife
Hudson
Bay
0
120 W
200 km
<2.45 Ga basins
<1.85 Ga basins
Paleoproterozoic
orogens
Archean –
Paleoproterozoic
cratons
Arrowsmith
8
overprint
Study area
206 238
Pb/ U
206 238
Pb/ U
207 235
Pb/ U 207 235
Pb/ U
2516 +/- 6 Ma
MSWD = 0.88
n = 29
2495 +/- 6 Ma
MSWD = 1.3
n = 21
2382 +/- 6 Ma
MSWD = 2.6
n = 17
Inherited
2313 +/- 6 Ma
MSWD = 1.4
n = 17
ca. 2.30 Ga biotite monzogranite
ca. 2.38 Ga biotite leucogranite ca. 2.53 Ga foliated–gneissic
biotite hornblende granodiorite
ca. 2.50 Ga K-feldspar
porphyroclastic biotite granodiorite
8. Findings
Three granitoid suites have been identified:
Ÿ2.34–2.30 Ga granites are alkali-calcic and contain 68–75 wt.% SiO . These granites may
2
12
be part of a larger bimodal magmatic suite in the Rae craton , which is perhaps related to
post-orogenic extension. Their REE profiles are consistent with relatively shallow magmatic
processes and their variable initial εNd (-5 to -11) values could reflect heterogneous crustal
sources, or mixing of mantle and crustal melts.
Ÿca. 2.38 Ga leucogranites are strongly HREE depleted. They contain abundant ca. 2.5 Ga
inherited zircon and have initial εNd values that are consistent with derivation from local ca.
2.5 Ga crust. These rocks may reflect partial melting of deeper-seated, garnet-bearing,
equivalents to the 2.53–2.48 Ga suite, during the collisional phase of the Arrowsmith
1
orogeny .
5
The Arrowsmith orogeny is evidence against a plate tectonic shutdown from 2.45–2.2 Ga ,
but our field area, and the Rae craton as a whole, yields little-to-no evidence for
volumetrically significant juvenile crust formation (i.e. arc magmatism) in that time period.
Ÿ2.53–2.48 Ga granitoids are juvenile crustal additions. They are broadly calc-alkaline and
have wide-ranging but dominantly intermediate (55–65 wt.% SiO ) compositions. In this
2
respect, they are similar to Phanerozoic continental arc rocks. However, their strictly
chondritic Nd isotope compositions are inconsistent with such a setting.
9. Next steps
Examples of U-Pb zircon age data are shown below. Preferred crystallization ages are based on
207 206
weighted mean Pb/ Pb dates. Age uncertainties are 95% confidence and include long-term
external reproducibility. Data point ellipses are 2σ and laser spots are 25μm.
0.7
0.8
wt.% SiO2
Magnesian
t t
FeO /(FeO +MgO)
Ferroan
50 55 60 65 70 75
calcic
calc-alkaline
alkali-calcic
alkalic
wt.% SiO2
Na O+K O-CaO
2 2
50 60 70
-8
-4
4
8
55 65 75
0
0.6
10
Frost et al. (2001) 10
Frost et al. (2001)
9
Modified after Ashton et al. (2013)
2.5–2.3 Ga Granitoids of the southwestern Rae craton, NT:
Implications for the Arrowsmith orogeny
1 1 1 2
Benjamin Neil , Tom Chacko , Larry Heaman ,Edith Martel
[1] University of Alberta, [2] Northwest Territories Geological Survey
2
-2
-6
-10
2300 2400 2500
>2.5 Ga basement (Neil, unpublished)
Age (Ma)
εNd
Chondritic uniform reservoir
11
Depleted arc mantle
Typical 2σ
uncertainty
Chondritic uniform resevoir
2.34–2.30 Ga
2.38 Ga
2.53–2.48 Ga
Assumed
Dated
2.34–2.30 Ga
2.38 Ga
2.53–2.48 Ga
Assumed
Dated
ca. 2.5 Ga crust
2534 + 76 Ma
initial εNd = 0.3 + 2.2
MSWD = 14, n = 23
147 144
Sm/ Nd
143 144
Nd/ Nd
2534 +/- 76 Ma
initial εNd = 0.3 +/- 2.2
MSWD = 5, n = 23
1. Introduction
2. Samples and Methods
4. Geochronology
3. Field Relationships
5. Major elements
6. Rare Earth elements
7. Sm-Nd isotopes
2.53–2.48 Ga suite ca. 2.38 Ga suite 2.34–2.30 Ga suite
1000
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Rock/ Chondrite
2.53–2.48 Ga Suite
source to the 2.53–2.48 Ga granitoids.
This errorchron provides evidence for
closed system Sm-Nd isotope behaviour
and a homogeneous chondritic
ŸFurther geochronology of the 2.34–2.30 Ga suite, to better constrain its age and to confirm
the sample correlations presented here.
ŸZircon Hf +/- O isotope data to better understand the Nd isotopic homogeneity and
heterogeneity of the 2.53–2.48 Ga and 2.35–2.30 Ga suites, respectively.
2.5–2.3 Ga magmatism, metamorphism and deformation in the
western and central Rae craton is commonly attributed to a
1
convergent margin, termed the Arrowsmith orogeny . 2.5 Ga
2 3
magmatism is ascribed to extensional and/or arc settings,
whereas 2.4–2.3 Ga metamorphism and granitic plutonism is
1,4
interpreted to reflect syn- to post-collisional processes .
The putative 2.5–2.3 Ga Arrowsmith orogeny is in apparent
5,6
conflict with the so called ‘quiet period’ , a proposed global
shutdown of plate tectonics and juvenile crust formation from
7
2.45–2.2 Ga . The nature and geodynamic setting of ‘Arrowsmith
aged’ magmatism therefore bears on global tectonic models for
the early Paleoproterozoic.
Here, we document widespread 2.5–2.3 Ga granitoids in a ca.
100 x 50 km area of the southwestern Rae craton. To understand
the role of these rocks in the Arrowsmith orogeny and their
implications for the ‘quiet period’, we undertook a detailed field,
geochronological and geochemical study.
ŸU-Pb zircon geochronology: LA-MC-ICPMS at the University of
Alberta.
ŸWhole-rock Sm-Nd isotope data: isotope dilution MC-ICPMS at
the University of Alberta.
Twelve weeks of field work were conducted over the course of
three summers. Sampling and analytical work focused on the
least altered orthogneiss and granitoid rocks. Data from 38
samples known or inferred to be 2.5–2.3 Ga in age are presented
here. Undated samples are correlated on the bases of field
appearance, petrography and geochemistry.
ŸWhole-rock major- and trace-element data: ICP-OES and ICP-
MS at Activation Laboratories.
10
1) Berman et al. (2013). Precambrian Research, 232, 44-69. 2) Schultz et al. (2007). Geology, 35, 707-710. 3) Cloutier et al. (2021). Precambrian Research, 362, 106303. 4) Hartlaub
et al. (2007). Journal of Geology, 115, 181-195. 5) Pehrsson et al. (2014), 26, 803-815. 6) Partin et al., (2014). Earth and Planetary Science Letters, 388, 123-133. 7) Condie et al.
(2009). Earth and Planetary Science Letters, 282, 294-298. 8) Berman (2010). Geological Survey of Canada, Open File 5279. 9) Ashton et al. (2013). Precambrian Research, 232, 70-
88. 10) Frost et al. (2001). Journal of Petrology. 42, 2033-2048. 11) DePaolo (1981). Nature, 291, 193-196. 12) Tersmette (2013). MSc thesis, University of Alberta.
2.53–2.48 Ga suite 2.53–2.48 Ga suite
ca. 2.38 Ga suite 2.34–2.30 Ga suite
207Pb/206Pb date (Ma)
238U cps
2500
2400
4E+6
2E+6