The Deformation History of the Finlayson Lake Greenstone Belt,...

The Deformation History of the Finlayson Lake Greenstone Belt, Atikokan, OntarioThe Deformation History of the Finlayson Lake Greenstone Belt, Atikokan, OntarioNils R. Backeberg*, Christie D. Rowe and Eric Bellefroid, McGill University, Montreal, Canada

*contact: nils.backeberg@mail.mcgill.ca

Acknowledgements:Our thanks go to Osisko Mining Corporation, speci�cally Anne Charland, Robert Wares and Zoran Madon for hosting us during our �eld season and ongoing support for this study. Special thanks to Denver Stone for sharing his insights and expertise with us in the �eld once again.

[1] Stone, D. and Kamineni, D.C. (1989). Geology, Atikokan Area, Ontario; Geological Survey of Canada, Map 1666A, scale 1:50 000

[2] Stone, D. (2008). Precambrian geology, Atikokan area; Ontario Geological Survey, Preliminary Map P.3349-Revised, scale 1:50 000

[3] Stone, D. (2010). Precambrian geology of the central Wabigoon Subprovince area, northwest Ontario; Geological Survey of Canada, Open File Report 5422

References:

Conclusionsthe structural history is common throughout the Finlayson belt

data does not require allochtonous sub-belts

the structures can be explained by closure of a single volcanic �lled basin with an extended active eastern margin

structures and palaeo-stratal way-up indicators are not consistent with a synformal (or keel) geometry

sinistral transpression kinematics and steep fold axes suggest ~horizontal shortening axis orientations throughout the structural history

WTWT

Kw Keweenawan

KwKw

MTWRT

WRTWRT

WRT

PROTEROZOIC

PHANEROZOIC

ARCHEAN

WWTWWT

EWTEWTERT

NCS

QTQT

Th Trans-Hudson orogen

Ph Platform sediments

KUKU

AT

PT

OcS

OnSLG

Ph

Th

Th

BS

AC

AT

PT

OcS

OnSLG

Ph

Th

Th

BS

AC

I II IVV VII II IVV VI

VIIVIIV

BRBR

WT Wawa terrane

AT Abitibi terranePT Pontiac terraneOcS Opatica subprovinceAC Ashuanipi complex

OnS Opinaca subprovinceLG La Grande subprovinceBS Bienville subprovinceI Inukjuak domainII Tikkerutuk domainIV L. Minto domainV Goudalie domain

VII Douglas Harbour domainVI Utsalik domain

Lake Superior

OSD

NSS

ERT

NCS

OSDMLD

ILDILDMLD

NSS

WRT Winnipeg R. TerraneWWT W. Wabigoon terraneEWT E. Wabigoon terraneQT Quetico terrane

KU Kapuskasing uplift

ERT English R. terraneNCS North Caribou superterraneOSD Oxford-Stull domainNSS Northern Superior superterraneMT Marmion terrane

BR Bird R. subprovinceMLD Molson L domain

ILD Island L domain

11 22

344

567

910

8

567

910

8

3

Finlayson Lakegreenstone belt

(approx.)

200 km

Percival 2007

Superior Province, Canada

brittle slip reactivation of foliation surfaces and pre-existing structures

ep

ti

am

appl

pl

plpl pl pl

pl

pl

titi

ti

am

am

am

am

amam

amhornblende + epidotematrix

am

ep

ep

ep

pl

pl

ti

ap

qtzchl chl

chlchl

chl

chl qtz

Quartz (qtz)Epidote (ep)Titanite (ti)Amphibole (am)Chlorite (chl)Plagioclase (pl)Apatite (ap)

100 µm

schematic plan of way-up pillow reversals

Trace of folded surface

folded pillow

axial trace

axial trace

lower Amphibolite facies sinistral transpression - D1

10 m

60

zone of p

illow re

versal

West Finlayson

N

spaced foliation fabric

pillow basalts

antiform

fold closure

Masuba

Bay

Masuba

Bay

Mineralogy

10m scale ccw folds result in zones of way-up reversals

D1 coeval with peak metamorphism - hbl+epchlorite fabric is retrogressive

sinistral amphibolite facies kinematicsconsistent across belt

steep fold axis folds provideexplanation for local pillowreversals

10 cm

fold

ed b

oud

inag

ed q

uart

z ve

in

62º

10 cm

cryptic folds, only seen in hinge zones, where intersection angles increase

two sub-parallel cross-cutting structural events

intensity of overprinting chlorite foliation increases eastward

62º 5 cm

4 cm

Cryptic D1 folds with strong chlorite foliation overprintN

Hardtack

Gneiss

western Finlayson

Marmion Gneiss

DiversionStock

decreasing chlorite foliation intensity

faul

t

fault and �uid conduit

eastern Finlaysoncentral Finlayson

felsic intrusives

cryptic foldschlorite schists

N N

N N

Pure Shear chlorite foliation (D2)- greenschist facies- quartz ductile

shallow reactivation- quartz brittle

sinistral transpression (D1)- amphibolite facies

felsic dykes and local contactmetamorphism

1 2

43

σ1

σ3 σ1

Marmion fa

ult

σ1

Marmion fa

ult

felsic intrusive withmetamorphic halo

damage zone

ccw folds with steep fold axis

Legend

gradationalchlorite foliation

low intensity

high intensity

Finlayson Lakegreenstone belt

dyke

chl

am

qtz+alb

50 µm

500 µm

chloriteschist

a

a

c

c

b

b

western Finlayson central Finlayson eastern Finlayson

N

N = 83for 33 locations 228/85N

N

238/84N

N

N = 63for 25 locations

N = 98for 39 locations

30 cm30 cm30 cm

5 % Area ContoursEqual AreaLower Hemisphere

Rock colour: light green (pale, weak pleochroic chlorite)dark green (amphibole) dark green (dark, strong pleochroic chlorite)

Rock texture: moderate to strong chlorite foliation fabricabsent to weak chlorite foliation fabric strong chlorite foliation fabric

Pillow Basalts withpreserved margins

Pillow Basalts withretrogressed margins retrogressed chlorite schists

1cm

pillow basaltscontact

felsic dyke

folded quartz vein

chlo

rite

folia

tion

East

coarse grainedpillow basalts

boudinaged quartz veins

folded quartz vein

shortening rotation

fold axis quartz vein (e)trend of quartz vein (d, e)

local chlorite foliation (d, e)

Equal AreaLower Hemisphere

local poles to foliationregional chlorite foliation

poles to boudinaged veins (a)

calculated fold axis

Folded quartz vein (a)

measured fold axeslimb surface

folded quartz vein (b)

21

Equal AreaLower HemisphereN = 35

N

ankerite veining

faulted vein

qtz +- ank vein

chloriteschist

foliation(slip)

surface

chloriteschist

calciteveins

105 cm

~040º

Slip lineations on exposed foliation surfaces

eastern Finlayson - retrogressed chlorite schist

6

0.5cm

late calcite veins cut foliation

calcite veins faulted by late stage slipalong foliation surfaces

strong foliation intensity

chlorite schist in eastern Finlayson

N

dext

ral f

ault

orie

ntai

ons

sinistral faultorientaions

N

228/85N

N = 98for 39 locations

2 % Area Contours

N

σ1

D2

average chlorite foliationpoles to foliation

Sinistral slipDextral slipRegional foliation

Equal AreaLower Hemisphere

Hornblende Tonalite to Granite

Hornblende-Biotite Tonalite

Biotite Tonalite to Granodiorite

Ma�c to ultrama�c metavolcanics

Ma�c intrusives

Intermediate to felsic metavolcanics

Metasediments

Neoarchean intrusives

LEGEND

Eye-Dashwa gneiss(2.93 Ga)

Hardtack gneiss(2.94 Ga)

Finlayson gsb(2.93 - 3.00 Ga)

Lumby Lake gsb(2.96 - 3.10 Ga)

Steep Rock gsb(2.73 - 2.93 Ga)

Marmion gneiss(3.00 Ga)

Marmion gneiss(3.00 Ga)

Diversi

on Stock

N

Fig 1,2

Red Paint Lake area

Pillow basalt way-upStone et. al. 1989this study

altered fault zone

dyke1 km

1 km

5 km

Masuba Bay

Map by Stone 2008

2

1

no synformal geometry across beltdykes intrude along preexistingstructural fabric

pillow basalts have two consistent facingorientation separated a by belt-scale fault

Stone and Kamineni 1989

cross section

easternweste

rnce

ntral

centra

l2.7 G

a

eastern

3.00 Ga

western

2.93 Ga

?

? - age from Witch Bay unit,Steep Rock greenstone belt

Stone 2008, pers. comm.

2

thin skin tectonics

map view(not to scale)

1

Structural geometry of greenstone beltsThe process of Archean tectonics has been widely debated and is mainly focused around “when did subduction begin?” As a consequence, greenstone belts globally have been classi�ed as either:1 - synformal keels developed between rising TTG (gneiss) diapirs, or2 - juxtaposed terranes resulting from plate tectonicsThe Finlayson Lake greenstone belt is a well-studied greenstone belt [1,2,3] and has been interpreted under both regimes: as either a synformal keel (1) or allochtonous sub-belts juxtaposed by major faults (2). The aim of this study is to produce a structural and kinematic database with which to test these hypotheses.

South-central Wabigoon Subprovince(Marmion Terrane)

20 cm

sheared pillow basalts

074/81N

ccw

81074

D1σ1

20 cm Nschematic of sheared pillow basalts

Local sinistral shear zone in central Finlayson

overprinting greenschist facies retrogression (chlorite foliation) - D2

contact metamorphism with local pillow preservation within chlorite schists

pure shear chlorite retrogression

Regional structure - gradational chlorite regtrogression

comparison to chlorite foliation stress �eld

would resolve dextralslip on structure oriented074 degrees

σ1

chlorite foliation

048/85N

N

Field map of Masuba Bay area- zone of pillow reversal

counterclockwise fold and folded pillow basalts Plot of fold axes data from Finlayson belt

dominant regional overprinting fabric intensity decreases westward parallel to Marmion gneiss boundary(poorly constrained to the west)

Summary of results - deformation histroy

tonalitedyke

coarse grained pillow basalts

2 cm

pillow m

argin

chlorite

amphibole

coarse amphibole mineralogy with margins retrogressedto chlorite

coarse grained pillow basalts have same aspect ratio as western Finlayson

contact metamorphism predates chloriteretrogressive foliation

intrusive felsic dykes spacially associated withcoarse grained pillow basalts

coarse grained pillow basalts

coarse grained pillow basalts

quartz veins parallel to chlorite foliation: boudinaged (1) quartz veins oblique to chlorite foliation: asymmetrically folded and rotated (2)

quartz veins in felsic dykes record same shorteningquartz veins perpendicular to chlorite foliation: symmetrically folded (1)

shortening kinematically consistent with chlorite foliation

brittle fractures predominantlyreactivated surfaces

dextral vs sinistral slip related toorientation at time of reactivation

shortening orientation similar to D2quartz-ductile chlorite foliation (1)

local conjugate set (3) and compiledbrittle fault data (2) indicate southeastshortening axis

reactivation post dates chlorite foliation

intensity of brittle fault reactivation event increases towards eastern boundary

damage zone related to brittle reactivation of eastern boundary?

schematic cross section of Finlayson belt showing all structural features

pillow facing direction

Existing hypotheses for the structure of the Finlayson belt

synformal keel

allochtonous sub-belts

1 2 3