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.

ADK. HIGHLANDSADK. LO

WLA

NDS

FRONTENAC

MSZR

LS

Z SHARBOT LAKE

MAZINAW ccsz

ELZ

EVIR

CMBBZ

BANCR

OFT

BSZ

CENTRALGNEISS BELT

Adapted from Davidson (1998)

40km

Terranes and shear zones

MSZRLSZ CCSZ

LL HL

Adirondack Highlands

Sharbot Lake-Frontenac/Adirondack lowlands

Mazinaw

ElzevirBancroft

Anorthosite (AMCG Suite)

1300-1200Ma

Igneous Events

1180-1120 Ma

MSZ

RLS

Z

AMCG etc.

1120-1095 Ma

HAWKEYE

1090-1050 Ma

CMBBTZ

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: )

OTTAWANTHERMAL EVENT

1090-1050 Ma

ADK H

ADK

LL

F

SL

MAZEL

B

CGB

1050-1020 Ma

1010-980 Ma

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.

ccsz

Dana Hillmetagabbro

Diana Syenite Body

ADK HL

ADK LL

Diana Syenite (ADK lowlands)

Dana Hill Metagabbro (ADK highlands)

1020 Ma

1041.3+/-1.7Ma

After (Streepey et al. 2001)

Sphene U/Pb

Sphene U/Pb

Hbl 39Ar/40Ar

989 +/- 1.7Ma

Hbl 39Ar/40Ar

979+/-8.6Ma

CC

SZ

• 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.

• 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.

aLamb (1993) (Streepey et al. 2001: Johnson et.al (2004)).

CC

SZ

• 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 Thermal Profile

CC

SZ

100

300

500

700

-3 -2 -1 0 1 2 3 4 5

distance (km)

Tem

per

atu

re o

C

CCSZ~1040Ma~1020 Ma

~990Ma

Fluid Inclusion data (syn-post orogenic)_LYON MOUNTAIN GRANITE

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 100 200 300 400 500 600 700 800

Temperature (C)

P (b

ars)

3-phase

Carbonic

Late aqueous

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.

900oC

MOHO700oC

TEMPERATURE

CCSZ

800oC

Channel Flow Thermal Gradients

100 km

A’

TECTONIC CROSS-SECTIONS

MSZRLSZ CCSZ

LL

FLINTON GROUP

Crustal detachment

HL

Old Mt Holly Arc

Trench roll back

~ 1160-1110 Ma

AMCG main phase

Or delamination

1110-1100 Ma

MSZRLSZ CCSZ

LLCrustal detachment

HL

Old Mt Holly Arc

1155-1112 Ma arc

FLINTON GROUP

HAWKEYE GRANITOIDS

AMCG late phase-Hawkeye intrusive suite

FLINTON GROUP

HAWKEYE GRANITOIDS

LYON MOUNTAIN GRANITOIDS

1090-1040 Ma

OTTAWAN PHASE

MSZRLSZ CCSZ

LL HL

Amazonia?

Extrusion of the low viscositygranulite core of the orogen

1080-1030Ma

ADK HLF.T.SLTMAZ ADK LL

Granulite core

Zone of active footwall deformation

Lyon Mtn A-typeGranitoid

ADK HLF.T.SLTMAZ ADK LL

Granulite core

Zone of active footwall deformation

Lyon Mtn A-typeGranitoid

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 (syn-post 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).