Transcript 1020 Ma

THE OTTAWAN
OROGENY
Himalayan style crustal
channel?
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.
40km
RLSZ
CENTRAL
GNEISS BELT
SHARBOT LAKE
FRONTENAC
MAZINAW
ADK. HIGHLANDS
Adapted from Davidson (1998)
Terranes and shear zones
Sharbot Lake-Frontenac/Adirondack lowlands
Adirondack Highlands
Mazinaw
Bancroft
Elzevir
RLSZ
MSZ
CCSZ
LL
HL
Anorthosite (AMCG Suite)
Igneous Events
1300-1200Ma
1180-1120 Ma
AMCG etc.
1120-1095 Ma
HAWKEYE
1090-1050 Ma
LYON MOUNTAIN
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: )
OTTAWAN
THERMAL EVENT
1090-1050 Ma
CGB
SL
B
EL
MAZ
F
ADK H
1050-1020 Ma
1010-980 Ma
Observations
Metamorphic 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.
Dana Hill Metagabbro (ADK highlands)
ccsz
989 +/- 1.7Ma
Sphene U/Pb
Dana Hill
metagabbro
Hbl 39Ar/40Ar
ADK HL
Diana Syenite Body
After (Streepey et al. 2001)
Diana Syenite (ADK lowlands)
Hbl
data-point error ellipses are 68.3% conf 
Sphene U/Pb
0.1775
39Ar/40Ar
1048
1044
0.1755
206
Pb/238U
0.1765
979+/-8.6Ma
1020 Ma
1040
0.1745
0.1735
1.765
1036
1041.3+/-1.7Ma
1.775
1.785
207
1.795
235
Pb/ U
1.805
• Shear zones contain
widespread scapolite
replacement of plagioclase.
aLamb
(1993)
(Streepey et al. 2001: Johnson et.al (2004)).
CCSZ
• 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
•
Early 30+m 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.
• Large early-formed shear
zones in Dana Hill
Metagabbro (high temp)
NO SCAPOLITE
• Scapolite replacement of
plagioclase in shear
zones (DSB)
Ma.
• Thermal
resetting/growth of
sphene in DSB at 10351050 Ma age. Zircon
records ~1150 Ma
AMCG age (McLelland et
al (2003).
•
39Ar/40Ar
cooling
through closure tor
hornblende (550oC)
~990-978 Ma
CCSZ
• Diana Syenite Body
(DSB) Sphene-growth in
shear zones at ~ 1041
• 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
~1040Ma
~1020 Ma
700
~990Ma
500
CCSZ
Temperature oC
CCSZ Thermal Profile
300
100
-3
-2
-1
0
1
2
distance (km)
3
4
5
Fluid Inclusion data (syn-post orogenic)_
LYON MOUNTAIN GRANITE
9000
8000
Late aqueous
7000
P (bars)
6000
5000
3-phase
4000
Carbonic
3000
2000
1000
0
0
100
200
300
400
500
Temperature (C)
600
700
800
Channel Flow Thermal Gradients
CCSZ
TEMPERATURE
800oC
900oC
MOHO
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. 738742.
TECTONIC CROSS-SECTIONS
A’
~ 1160-1110 Ma
Old Mt Holly Arc
RLSZ
MSZ
CCSZ
LL
HL
Trench roll back
Or delamination
FLINTON GROUP
AMCG main phase
1110-1100 Ma
Old Mt Holly Arc
RLSZ
MSZ
CCSZ
1155-1112 Ma arc
LL
HL
FLINTON GROUP
HAWKEYE GRANITOIDS
AMCG late phase-Hawkeye intrusive suite
1090-1040 Ma
RLSZ
MSZ
CCSZ
LL
HL
FLINTON GROUP
HAWKEYE GRANITOIDS
LYON MOUNTAIN GRANITOIDS
OTTAWAN PHASE
Extrusion of the low viscosity
granulite core of the orogen
1080-1030Ma
Lyon Mtn A-type
Granitoid
MAZ
SLT
F.T.
ADK LL
ADK HL
Zone of active footwall deformation
Lyon Mtn A-type
Granitoid
MAZ
SLT
F.T.
ADK LL
ADK HL
Zone of active footwall deformation
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)
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 (synpost Ottawan Lyon Mountain Granitoids). Fluid inclusion data from
Lyon Mountain Granitoids adjacent to the CCSZ record nearly
isothermal uplift.
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).