THE OTTAWAN OROGENY

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THE OTTAWAN OROGENY

• Himalayan style crustal channel?

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GOALS

• To present a Himalayan model for the development
  of the Grenville Orogenic Event.
• To apply this model to the a portion of the
  Canadian (Ontario) and adjacent Adirondack (New
  York) terrane assemblage.
• To examine this model with particular reference
  to the nature and timing of activity along the
  major terrane-bounding shear zones, periods of
  igneous intrusive activity, and timing of
  metamorphic events.

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(No Transcript)
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Terranes and shear zones
Sharbot Lake-Frontenac/Adirondack lowlands
Adirondack Highlands
Mazinaw
Elzevir
Bancroft
CCSZ
RLSZ
MSZ
LL
HL
Anorthosite (AMCG Suite)
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Igneous Events
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1180-1120 Ma
RLSZ
MSZ
AMCG etc.
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1120-1095 Ma
HAWKEYE
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1090-1050 Ma
CMBBTZ
LYON MOUNTAIN
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Observations

• There is a shift in the locus of igneous activity
  through time (generally W to E) reflecting the
  assembly of Grenvillian Terranes along eastern
  Laurentia.
• The anorogenic/rift AMCG event magmas intrude the
  Adirondack (ADK) Highlands, Lowlands, Frontenac,
  Sharbot Lake and Mazinaw Terranes but are
  focused in the ADK Highlands
• Post AMCG pre Ottowan Orogney Hawkeye granitoids
  are primarily found in the ADK Highlands.
• Lyon Mountain Granitoids rim the ADK Highlands
  are common along or adjacent to the CCSZ and
  scattered through the other terranes.
  (Pressure-release A-type magmas )

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OTTAWANTHERMAL EVENT
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1090-1050 Ma
CGB
SL
B
MAZ
EL
F
ADK LL
ADK H
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1050-1020 Ma
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1010-980 Ma
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ObservationsMetamorphic events

• Adirondack Highlands and the Mazinaw terranes
  record the Ottawan Event.
• Adirondack Lowlands record a weak Ottawan Event
  focused along the CCSZ
• Frontenac and Sharbot Lake Terranes do not record
  the Ottawan Event.
• Terranes west of the CMBBZ record pluses of
  metamorphism at a slightly later time and cool
  quickly.
• From Davidson (1998) An overview of the Grenville
  Province Geology, Canadian Shield.

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Dana Hill Metagabbro (ADK highlands)
Sphene U/Pb
989 /- 1.7Ma
Hbl 39Ar/40Ar
1020 Ma
After (Streepey et al. 2001)
Diana Syenite (ADK lowlands)
Sphene U/Pb
1041.3/-1.7Ma
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Lowlands
Highlands

• Early 30m wide shear zones in the DHMG record
  recrystallization temperatures in excess of
  700oC, sphene replacement of Fe-Ti Oxide
  minerals, and a complete lack of scapolite.
• Late (sub-meter wide) shear zones in the DHMG
  record recrystallization temperatures ranging
  from 670-700oC and widespread repalcement of
  plagioclase by scapolite.

• Shear zones (meter and sub meter widths) in the
  Diana Syenite Body span a wide range of
  temperature conditions (sub 550oCa), and most
  show sphene replacement of Fe-Ti oxide minerals
• Shear zones contain widespread scapolite
  replacement of plagioclase.

CCSZ
aLamb (1993) (Streepey et al. 2001 Johnson
et.al (2004)).
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Lowlands
Highlands

• Large early-formed shear zones in Dana Hill
  Metagabbro (high temp) NO SCAPOLITE
• Scapolite replacement of plagioclase in (sub
  meter wide shear zones) Dana Hill Metagabbro Body
• Dana Hill Metagabbro (Highlands) Sphene cooling
  through 650oC at 1020 Ma.
• 39Ar/40Ar cooling through closure for hornblende
  (550oC) 1000-978 Ma

• Scapolite replacement of plagioclase in shear
  zones (DSB)
• Diana Syenite Body (DSB) Sphene-growth in shear
  zones at 1041 Ma.
• Thermal resetting/growth of sphene in DSB at
  1035-1050 Ma age. Zircon records 1150 Ma AMCG
  age (McLelland et al (2003).
• 39Ar/40Ar cooling through closure tor hornblende
  (550oC) 990-978 Ma

CCSZ
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CCSZ Thermal Profile
CCSZ
1040Ma 1020 Ma 990Ma
700
500
Temperature oC
H.L.
L.L.
300
CCSZ
100
-3
-2
-1
0
1
2
3
4
5
distance (km)
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Fluid Inclusion data (syn-post orogenic)_LYON
MOUNTAIN GRANITE
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Channel Flow Thermal Gradients
100 km
Adapted from Beaumont, R.A. Jamieson, M.H. and
Lee B. (2001) Himalayan tectonics explained by
extrusion of a low-viscosity crustal channel
coupled to focused surface denudation Nature,
414, pp. 738-742.
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TECTONIC CROSS-SECTIONS
A
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1160-1110 Ma
Old Mt Holly Arc
MSZ
RLSZ
CCSZ
LL
Crustal detachment
HL
Trench roll back
Or delamination
FLINTON GROUP
AMCG main phase
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1110-1100 Ma
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1090-1040 Ma
LYON MOUNTAIN GRANITOIDS
OTTAWAN PHASE
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Extrusion of the low viscositygranulite core of
the orogen
1080-1030Ma
Lyon Mtn A-type Granitoid
ADK HL
F.T.
MAZ
ADK LL
SLT
Granulite core
Zone of active footwall deformation
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Lyon Mtn A-type Granitoid
ADK HL
F.T.
MAZ
ADK LL
SLT
Granulite core
Zone of active footwall deformation
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Conclusions

• The Shawinigan- AMCG- pre-Ottawan development for
  this part of the Grenville can be explained via
  tectonic switching and or collision along an
  Andean-type margin (first proposed by Hanmer et
  al. (2000)).
• AMCG magmas intrude into an evolving back-arc
  basin at a time when the ADK Highlands are
  structurally beneath the ADK LL/Frontenac/Sharbot
  Lake Terranes. All of these terranes receive
  AMGG magmas. (Flinton Group deposited)
• Continental-Continenal Collision occurs at or
  near to 1090Ma (Ottawan Orogeny)

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Conclusions

• During Ottawan compression, the ADK H.L. Terrane
  is mobilized and extruded along a tectonic
  channel.
• This extrusion is oblique thrust in character and
  focused along the CCSZ. Thermal gradients
  established across the CCSZ with localized
  heating of the adjacent ADK lowlands (resets
  isotopic systems in the ADK lowlands near to the
  CCSZ Dahl (2002)).
• Major un-named shear zones in the eastern
  Adirondack Highlands my represent the base of
  this channel.
• Extrusion is aided by the formation of pressure
  release melts (syn-post Ottawan Lyon Mountain
  Granitoids). Fluid inclusion data from Lyon
  Mountain Granitoids adjacent to the CCSZ record
  nearly isothermal uplift.

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Conclusions

• Uplift results in gravity-driven collapse of
  cover rocks (ADK L.L. Frontenac over Sharbot Lake
  Terranes) to the W- NW. Extrusion of the
  granulite core with deflation of the mid-lower
  crust, rotates the Mazinaw Terrane triggering
  amphibolite facies metamorphism.
• Continued convergence reactivates shear zones to
  the west driving short tectonothermal events in
  terranes to the west (Central Gneiss Belt).