1. Neoarchean and Paleoproterozoic basement rocks of the Clearwater
complex: Constraints on the formation of western Laurentia
Jeff Vervoort, Da Wang, Chris Fisher,
Reed Lewis, Andy Buddington
2. Introduction
Modified after Foster et al., 2006; Vervoort et al., 2016
• Newly recognized basement in
NW Laurentia – largest belt of
basement exposures outside the
Wyoming Province.
• 2.65 Ga and 1.86 Ga basement
rocks in Clearwater complex.
• Collision between the Wyoming
Province and the Medicine Hat
block may not adequately explain
the formation of CW and PR.
19. One important Mesoproterozoic unit
Augen gneiss, northern PR
“La Clede gneiss” (Doughty and Price, 1998)
1KE001, Evans and Fischer, 1986 LASS U-Pb result, this study
Mean = 1573 ± 6
IKE001
North American Magmatic Gap
20. Laser Ablation Split Stream (LASS) analysis:
simultaneous U-Pb age and Hf isotope
measurements in zircon
U-Pb age
Hf isotopes
Yuan et al. 2008; Xie et al., 2008; Tollstrop et al., 2012; Fisher et al., 2014
21. Laser Ablation Split-Stream
Analysis (LASS)
Laser
U-Pb
Hf isotopes
L
New Approach – Laser Ablation Split Stream
Simultaneous U-Pb and Hf isotope analyses
of the same zircon ablation volume
26. Crustal evolution (176 Lu/177 Hf = 0.015)
-10
-5
0
5
10
Depleted Mantle
CHUR
-15
Age (Ma)
18001600 240022002000 2600 2800
Samples from PR
Samples from CW
27.
28. Basement of NW Laurentia
1) The Clearwater complex consist of a bimodal population of
basement rocks with well-defined peaks at 2.66 and 1.86 Ga.
2) The Neoarchean gneisses are fundamentally juvenile—derived
from the depleted mantle at 2.66. The Paleoproterozoic gneisses
appear to be mixtures of mantle-derived magmas and (likely)
Neoarchean crust
3) In as much as these rocks lie near the former rifted margin of
Rodinia, they should provide important piercing points for tectonic
reconstructions.
34. 1.86 Magmatism
1) The 1.86 Ga magmatism is not limited to GFTZ and the Trans-
Challis Fault System; There are other volumetrically important
exposures in the region.
2) Oldest samples have inheritance and negative epsilon Hf,
indicative of interaction with Neoarchean crust
3) Younger rocks have larger mantle contributions and less evidence
of interaction with older crust.
4) The time 1.9-1.7 Ga is an important period of crustal
production/preservation worldwide; it is important to consider the
1.86 Laurentian magmatism in this context
38. Kröner et al., 2016
Badplaas Field Workshop on Archaean Geodynamics
39.
40. Evaluating zircon Hf isotope data: Criteria
1. Need to establish an unambiguous crytallization age
--the age that corresponds to the age of the Hf in the zircon
2. Filter the Hf isotope analysis for quality, ideally with internal precision
better than 1 epsilon Hf unit (2SD)
--higher uncertainties signal problems with the analysis or multiple
age/isotope components in the zircon
3. Take pooled analysis of a minimum of 6 zircons; the total analyses
should be within ± 2 epsilon Hf units
--a larger range indicates problems with the analyses or multiple
components in the zircon population
41.
42. ~ 2.7 -3.0 Ga Lewisian Gneiss (W Scotland)
T. Kemp, M. Whitehouse, unpubl. Data; Whitehouse and Kemp 2010
43. Laser ablation Lu-Hf isotopes, SIMS U-Pb
Range of <1 epsilon unit
207Pb/206Pb Age (Ma)
0.2806
0.2807
0.2808
0.2809
0.2810
0.2811
0.2812
0.2813
2400 2500 2600 2700 2800 2900 3000 3100 3200 3300
176Hf/177Hf
CHUR
Lewisian granulites
99-FI, 99-04
0.280989 ± 16 (2 SD)
0.280947 ± 20 (2 SD)
T. Kemp, M. Whitehouse, unpubl. Data; Whitehouse and Kemp 2010
44. -10
-8
-6
-4
-2
0
2
4
6
8
10
2400 2500 2600 2700 2800 2900 3000 3100 3200 3300
MORB-DM
CHUR
207Pb/206Pb Age (Ma)
eHf(t)
Range of 18 epsilon units
‘Age’ spread of 700 Ma
Pb gain
Pb loss
Laser ablation Lu-Hf isotopes, SIMS U-Pb
Ultra-depleted
Mantle??
Protracted
Reworking?
T. Kemp, M. Whitehouse, unpubl. Data; Whitehouse and Kemp 2010
45.
46. Zircon U-Pb Results (Archean)
• PR/CW orthogneiss & amphibolite both at ~2.65 Ga
• No inheritance or overgrowth
47. Zircon U-Pb Results (Paleoproterozoic)
• ~1.87 Ga Paleoproterozoic orthogneiss in PR
• Inheritance of ~2.6 Ga
48. Zircon U-Pb Results (Paleoproterozoic)
• ~1.87 Ga Paleoproterozoic orthogneiss in CW
• Inheritance of ~2.6 Ga
49. Summary of ages
A. Late Archean ~ 2.65 Ga
B. Paleoproterozoic ~ 1.86 Ga
C. No evidence of events > 2.65 Ga
D. No significant events between
2.65 and 1.86 Ga
E. 1.58 Ga Laclede gneiss
50. Zircon Hf Results
Archean ~2.65 Ga
• eHf(t) = 0 - +4
• Mantle source
• Well-preserved and unaffected
Paleoproterozoic ~1.86 Ga
• eHf(t) – wide range
• Mixed sources
• Juvenile source
• Pre-existing crust
(possibly 2.65 Ga)
Pb Loss
Inheritance
• Broadly consistent with 2.65 Ga
• Possible lead loss
younger apparent age
56. Conclusion
• Bimodal magmatism at 2.65 Ga and 1.86 Ga.
• Hf study indicates the source of the Archean rocks to be juvenile, mantle-
derived; the Paleoproterozoic rocks represent mixed source of juvenile
material and pre-existing crust, possibly the 2.65 Ga crystalline basement.
• Identical ages and Hf isotopic compositions of PR and CW suggest they are
likely derived from the same basement crust.
• Clearwater block – maybe distinct from the surrounding Archean provinces.
• Hf studies suggest it may correlate to the Gawler craton and eastern Antarctica.
