Gifford_Lessons learned from the Great Falls tectonic zone_Formatted.pptx
1. Lessons learned from the Great Falls
tectonic zone, Medicine Hat-
Wyoming suture: crustal xenoliths
and the Little Rocky Mountains
Jennifer N. Gifford
Paul Mueller, Dave Foster, Dave Mogk
2. • Great Falls Tectonic Zone (GFTZ)
located between Medicine Hat
Block (MHB) and Wyoming Craton
• Trans-Hudson Orogeny (THO) East
of WY
(Gifford et al., 2018)
(Mueller et al., 2005)
U-Pb zr. & Nd LBM volcanic arc
~1.86 Ga subduction of oceanic crust
(Mueller et al., 2002)
North-directed subduction under the
MHB during the Paleoproterozoic
3. Magnetic anomaly map (Lemieux et al., 2000)
No distinctive coherent pattern makes the MHB
distinction inconclusive
4. Samples
Xenoliths and basement exposures
• Grassrange & Missouri Breaks xenoliths
– Generally small: Igneous and Metamorphic
• Little Rocky Mountains
– Metamorphic:
• Gneissic
• Amphibolites
Zircon Geochronology
Isotope Geochemistry
(Gifford et al., 2014)
5. 1) Xenoliths
Both Archean and Proterozoic granitic xenoliths show
HFSE depletions and LIL enrichments characteristic of
formation in a subduction modified environment.
• The limited enrichment of HREE relative to
primitive mantle values reflects the source
mineralogy, likely lower crustal materials where
garnet and/or amphibole was residual after
melt extraction.
(Gifford et al., 2014)
6. • Grassrange (Gifford et al., 2014):
– 2 leucogranitoids
• Xstal. age: ~1.73 & 1.74 Ga
– 4 meta-igneous
• Xstal. age: ~2.45 – 2.53 Ga
• Meta. age: ~1.77 – 1.84 Ga
• Missouri Breaks (MS in prep.):
– 7 meta-igneous
• Xstal. age: ~1.77 – 1.89 Ga
– 5 meta-igneous
• Xstal. age: ~2.44 – 2.68 Ga
• Meta. age: ~1.72 – 1.85 Ga
Results - Xenoliths
7. Conclusions - Xenoliths
• Between 1.89 and 1.79 Ga subduction was
ongoing
– Arc magmatism active in LBM & xenolith
crystallization
• 1.78 to 1.77 Ga terminal phase of subduction
– Crustal thickening, deformation, wide-scale
metamorphism
• 1.77 to 1.75 Ga
– Melt mixing, metamorphism, xenolith
crystallization
• 1.75 to 1.73 Ga Post-orogenic collapse and
extension after subduction stopped driving
crustal thickening
– Upwelling of mantle, crustal heating and
partial melting of lower crustal material:
~1.73 Ga Grassrange granitoid xenoliths
(Gifford et al., 2014)
(Mueller et al., 2005)
9. Results – Little Rockies
• Little Rocky Mtns. (Gifford et al.,
2018)
– 8 orthogneisses
• 2 Xstal. ages: ~2.42 Ga
• 5 Xstal. ages: ~2.77 – 2.83 Ga
• 1 Xstal. ages: ~3.20 Ga
– 1 amphibolite
• Xstal. age: ~3.01 Ga
– Metamorphic ages
• (2 grains total)
• ~1.78 Ga & 1.86 Ga
LRM
10. Conclusions - LRM
• U-Pb ages range from 2.42 - 3.21 Ga,
– Numerous crust-forming episodes in LRM
• 2 metamorphic ages are correlative
with tectonic and magmatic activity
throughout the GFTZ during
Paleoproterozoic
• Unlike the LBM, LRM show some
similarities in age to the BBMZ in
Archean, but not Proterozoic
• Transpressional model of Mueller et al.
(2005) most likely scenario for
juxtaposition of WP to Laurentian
supercontinent in Paleoproterozoic
(Gifford et al., 2018)
11. Some Still Open Questions…
• How did the GFTZ and THO interact during Laurentian
formation?
• Do the ~2.4-2.5 Ga ages in the GFTZ and MMT record a
previous collision of the WP with the MHB?
• Are the LRM part of WP, MHB, or neither?
• Gifford, J.N., Mueller, P.A., Foster, D.A., Mogk, D.W., 2014. Precambrian Crustal Evolution in the Great Falls tectonic zone: Insights
from Xenoliths from the Montana Alkali Province. Journal of Geology, v. 122, n. 5, p. 531-548.
• Gifford, J.N., Mueller, P.A., Foster, D.A., Mogk, D.W., 2018. Extending the realm of Archean crust in the Great Falls tectonic zone:
Evidence from the Little Rocky Mountains, Montana. Precambrian Research, v. 315, p. 264-281.
• Mueller, P.A., Heatherington, A.L., Kelly, D.M., Wooden, J.L., Mogk, D.W., 2002, Paleoproterozoic crust within the Great Falls tectonic
zone: Implications for the assembly of southern Laurentia: Geology, v. 30, n. 2, p. 127-130.
• Mueller, P., Burger, H., Wooden, J., Brady, J., Cheney, J., Harms, T., Heatherington, A., and Mogk, D., 2005, Paleoproterozoic
metamorphism in the northern Wyoming Province: Implications for the assembly of Laurentia: Journal of Geology, v. 113, p. 169-
179.
Editor's Notes
The Wyoming province is on of the oldest cratons and is surrounded on all sides by suture zones, orogenic belts, and proterozoic terranes. The GFTZ is located between the MHB and the WY craton, and also covers a great deal of the Montana alkali province.
U-Pb zircon data recording the timing of high temperature tectonic events in samples from reworked Archean and Paleoproterozoic igneous and metamorphic rocks from the GFTZ suggest simultaneous activity with the THO.
These observations support the proposal that the GFTZ formed as a result of collision between the Hearne and Wyoming provinces.
Based on isotopic and geochemical data, Mueller et al. (2002) showed that volcanic arc rocks exposed in the Little Belt Mountains record the generation of new crust in this region at ~1.86 Ga and proposed that this resulted from subduction of oceanic crust beneath the region.
This supports the notion of north directed subduction under the MHB during the Paleoproterozoic.
GFTZ represents a critical target and may have shaped the subsequent history of this part of ancestral North America
Medicine Hat Block: Completely buried by sediment, no reported exposures, Nature of Archean crust is poorly defined
Short wavelength filtered magnetic anomaly map (Lemieux et al., 2000): linear northwest and northern trending fabrics, no coherent pattern across GFTZ
Previous Work:
Sweetgrass Hills Xenoliths: (3): 2.60 – 2.84 Ga (Davis et al., 1995; Gorman et al., 2002)
Boreholes: (5): 2.62 – 2.72 Ga, 3.28 Ga (Villeneuve et al., 1993)
Montana Alkali Province: partial melting initiated by flat-slab subduction of Farallon Slab, Enriching lithosphereic mantle
Diatreme eruptions: Alnoitic and lamproitic magmas (high in K and Mg), Possible deep magma root, Opportunities to sample material ranging from the mantle to the surface of the crust
Various diatremes within the Montana Alkali Province have yielded mantle xenoliths as well as more common crustal xenoliths.
Crustal xenoliths came from two main suites.
The Grassrange diatremes <points> and a wider-spread sampling labeled under the name of the Missouri Breaks diatremes.
The last sampling location is a mountain range called the LRM.
We investigated this mountain range for a number of different reasons.
Other than the LBM, the LRM are the only basement exposure within the GFTZ.
The LRM had to potential to be similar to the LBM and record the ocean closure between WY and MHB, or there was the possibility that the LRM exposed Archean basement, whether it is WY province, MHB, or neither.
Zircon U-Pb and Lu-Hf geochronology
Whole Rock Isotope Geochemistry
Within the xenoliths from the GR, 2 leucogranitoids yielded ages of 1.73 and 1.74 Ga. 4 orthogneiss samples yielded ages ranging from 2.45 to 2.53 with metamorphic ages of 1.7 to 1.84.
Within the xenoliths from the MB, 7 meta-igneous xenoliths yielded ages of 1.77 to 1.89 Ga,
overlapping with the ages of the LBM.
5 meta-igneous xenoliths further yielded crystallization ages ranging from 2.44 to 2.68 with metamorphic ages of 1.72 to 1.85.
These ages overlap with those from the GR, and similar to the Grassrange xenoliths.
When all of the igneous and meta-igneous samples are plotted eHf vs. age, a number of things become clear.
The Archean and earliest Paleoproterozoic xenoliths with one exception <Show> show much greater interaction with a juvenile source (DM).
The Paleoproterozoic xenoliths that range in age from 1.89 to 1.77 Ga show patterns similar to that seen in the LBM, indicating that the DM played a large part in the xenolith petrogenesis, which could mean that the xenoliths formed in an ocean-subduction like environment similar to the LBM.
Note, however, that the ~1.7 Ga granitoid xenoliths from the GR and MB plot with lower eHf values, indicating more interaction with evolved sources, and are likely not the result of ocean subduction.
Similar to the Hf, Sm-Nd isotopes indicate patterns of varying involvement with juvenile and crustal sources.
Also note, that the Paleoproterozoic xenoliths that show more –eNd values are still less negative than they would be in they were partial melts of WY province crust, meaning they are likely anatectic melts of MHB crust at depth.
Geochronology and geochemistry of crustal xenoliths from the Grassrange provide new insight into the complex history of the crust in the GFTZ, including:
(1) Both Archean and Proterozoic granitic xenoliths show HFSE depletions and LIL enrichments characteristic of formation in a subduction modified environment.
The limited enrichment of HREE relative to primitive mantle values reflects the source mineralogy, likely lower crustal materials where garnet and/or amphibole was residual after melt extraction.
(2) U-Pb data show distinct intervals (∼1.7 and ∼2.5 Ga) that are coincident with documented tectonothermal events in the MHB and LBM, suggesting that the buried crust sampled by the Grassrange diatremes represents reworked Medicine Hat block crust in addition to juvenile Paleoproterozoic arc material similar to that in the LBM.
The possibility that the ∼1.7 Ga granitoids at least partially represent melts of new arc (lower) crust produced during Little Belt Mountain magmatism cannot be ruled out because isotopically it is difficult to distinguish contributions from recently formed, juvenile lower crust (indirect mantle contribution) from direct depleted mantle involvement.
(3) Zircon Hf and whole-rock Nd isotopic data for the ∼2.5 Ga granitic samples indicate that juvenile and reworked crustal material mixed to varying degrees during crustal formation at ∼2.5 Ga.
These data, however, provide no direct evidence for involvement of the Meso- to Eoarchean crust that characterizes the northern Wyoming Province (2.8–3.5 Ga).
(4) The Hf and Nd isotopic data for the ∼1.7 Ga granitic samples indicate a mixture of older crust (e.g., the ∼2.5 Ga crust in the older xenoliths) and a more juvenile source.
This is in contrast to 1.8–1.9 Ga igneous material exposed in the LBM, which involve far higher proportions of juvenile material and are hypothesized to represent the arc formed during closure of the Little Belt ocean (Mueller et al. 2002).
(5) The GFTZ was initiated prior to ∼1.9 Ga as a convergent boundary related to the closing of an ocean basin between the Wyoming craton and the Hearne province/Medicine Hat block that also produced the igneous suite of the Little Belt arc.
The GFTZ then evolved into a more transpressional boundary by ∼1.77 Ga, as Wyoming moved east (present coordinates) toward final collision with the Superior and Hearne provinces as well as the MHB (Dahl et al. 1999; Mueller et al. 2000, 2002, 2005).
9 metamorphic samples yielded zircon data. 8 orthogneiss samples yielded 3 disctinct age groups. ~2.4 Ga, ~2.8 Ga, & ~3.2 Ga. One amphibolite sample yielded an age of ~3.2 Ga.
Minimal metamorphic ages of 1.78 Ga and 1.86 Ga came from 2 grains
When all of the igneous and meta-igneous samples are plotted eHf vs. age, a number of things become clear.
The LRM samples show different mixtures of evolved and crustal sources, with the ~2.8Ga samples plotting with much more –eHf, indicating that the samples might be crustal melts.
Similar to the Hf, Sm-Nd isotopes indicate patterns of varying involvement with juvenile and crustal sources.
It is interesting to note, that the LRM eNd values do not correlate well with published eN values from the northern Wy Craton.
The U-Pb ages of zircons from meta-plutonic samples from the LRM range from 2.42 to 3.21 Ga, documenting numerous crust-forming episodes in the LRM.
Two zircon ages interpreted as metamorphic at∼1.78 and∼1.86 Ga are correlative with tectonic and magmatic activity throughout the GFTZ during the Paleoproterozoic, recording what is likely a period of active crustal growth, ocean closure, and continental collision not recorded in the BBMZ of the Wyoming craton.
Geochemical data from the Little Rocky Mountains show HFSE depletions and LIL enrichments characteristic of subduction zone magmatism for rocks that formed over a span of∼500 m.y.
The limited enrichment of HREE relative to primitive mantle values reflects the source mineralogy, likely lower crust where garnet and/or amphibole-pyroxene is residual after melt extraction.
This pattern suggests partial melting of sufficiently thick crust for garnet stability beginning before 3.2 Ga and continuing into the Proterozoic.
Unlike the LBM, the exposed rocks in the LRM show some similarities in age to the BBMZ of the Wyoming craton in the Archean, but not the Proterozoic.
The transpressional model of Mueller et al. (2005) remains the most likely scenario for juxtaposition of the Wyoming craton to a rapidly forming Laurentian supercontinent in the Paleoproterozoic.
