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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: [email protected]
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
