Cross Lake Project (1983-86)

Geologist P.G. Lenton Year 83 STATION 5084
EASTING 571868 NORTHING 6039087 ZONE 14 DATUM NAD83
Date of mapping 06/30/1983

STRUCTURAL DATA

PETROLOGIC DATA

Rock Number: 1

Rock Type: diabase
Descriptors:
Molson dike
layered
Field Description:

Major Molson dyke approximately 135m wide.
See the sketch map in the field notes.
Section:
80cm chill - aphanitic grey-black high density pyroxenite
1.1m 55% mafics; .5mm pyroxenes in .01mm lath plagioclase. Weathers fine speckled grey.
1.4m 60% mafics; pyroxene .75mm in aphanitic plagioclase. First appearance of .5-1.0mm glomerocrysts of sub- to euhedral plagioclase as amoeboid aggregates upto 2.5cm. Rock has a distinctive hackley fracture.
5m 65% mafics; pyroxene 1-1.2mm; plagioclase 0.1mm; 1% plagioclase
glomerocrysts. 3x9cm gabbro pegmatite with hornblende, plagioclase and approx. 3% quartz.
8m 65% mafics; pyroxenes 1.0mm; plagioclase 0.2mm; glomerocrysts 2x3cm.
Pegmatite more abundant- upto 11x30cm with distinct quartz cores - probable good source of zircons for age dating.
9.2m green hornblende gabbro; grain size 1.0-1.2mm; 65% mafics. 24cm angular xenolith of porphyritic granodiorite country rock
10.8m grey green gabbro with 35 cm layer of plag. glomero-aggregates up to 3 to 5 %
Sample 14.7 m.
13 m Distinctly 2 pyroxene gabbro
35 % black hornblende pseudomorphs (of clinopyroxene ?) and 40 % honey brown pseudomorphs (?) of orthopyroxene (?)
plagioclase .2 @ 25 %
grain size 1.0
Sample 20.0 m.
18 m. Last occurance of glomeroporphyritic aggregates of plagioclase.
21.4 m. Transition into coarser more leuco gabbro with only one pyroxene pseudomorphed by olive green hornblende.
CI-55 pyroxene 1x2 mm plagioclase 1X2mm plag .5
transition gradual over 1 - 2 m
Sample 24.3
29 m as above but mafic increase gradually: CI 80
32 m sharp transition into 2 pyroxene leucogabbro CI 50-55
pyroxene 1 to 1.5 mm, honey brown and black in equal proportions
34.4 m 28x32 cm pegmatite pod, 5x9 cm epidotized core
35.6 m sample
37.4 m first rhythmic cumulate layering
45 m strong cumulous layering with tops by grading 217/72 tops upright to west
rhythmic beds average 2.0 to 2.5 cm with 1.7 to 2.2 cm clinopyroxene cumulous, 2 photos
45.6 m photo of upswell in cumulus bands - up is toward bottom of photo.
cumulus layers starting to be very erratic in attitude
47.7m 200/81
49.4m 207/80
48.9m 234/87
49.7 m sample
51.5 m 38x53 cm gabbro pegmatite-sampled cumulus 221/76
cumulus layers associated with homo-equi leuco-gabbro CI=55, grain size=1 mm
56.4 m 206/85
59.4 m 197/84
59.6 m 2 photos of parallel cumulus with bed load structures at top of massive cumulus with thin anorthosite layer capping
61.2 m cumulus 206/73
-looks like clino cumulus witn clino and ortho above cumulus base
SAMPLE 66.9
69.2 m plag float layers becoming thicker (up to 4 cm) and starting to line up to roughly oval pods
73.8 m fairly sharp change into leuco-gabbro
CI=50%
-large platy ampb pseudomorphs probably of orthopyroxene 3 to 7 mm wide
-30 % black hornblende (20%) probably after clinopyroxene
-1st occurance of late shearing
80.2 -sample (?)
-coarse leuco gabbro--platy pyroxene 7 - 10 mm
-some suggestion of directed growth pattern CI=45
-may be remnant orthopyroxene in platy ampb. pseudomorphs plate
85.5 m -fabric becoming quite strong with development of tiger stripe leuco-gabbro
94.3 m -234/81 up to 14 cm pod of pegmatite
-above reappearance of cumulus banding change from coarse leuco-gabbro to more normal cumulus unit
98.8 m 57x54 cm pegmatite
100.8 m Sample
-cumulus beds 209/83 top upright to west.
-photo of cumulus beds with very thick pyroxene cumulates and thin plagioclase float
-pyroxene 34 mm and plagioclase 5 mm
105.3 m 226/78 tops still upright to west
109.7 m 241/80
-photo of 10.1 cm cumulus bed going from 1.2 cm clinopyroxinite at base (on left of late) grading through gabbro to leuco gabbro and capped by 0.4 cm of anorthosite gabbro
125.9 m Sample
129.2 m 208/37 tops upright to west
-cumulus appears to be largely orthopyroxene
-anorthositic float is becoming thicker with appearance of irregular patches of near
leuco-gabbro
129.8 m what appears to be basal sections of cross beds
135.0 m back into leuco-gabbro
- coarse pyroxene 1 cm, CI=45
- weakly foliated appearance
135 to 150 m -uniform coarse leuco-gabbro
@ 150 m coarse leucocratic 2 pyroxene gabbros with trace of serpentized olivine (<<1%)
@150 m just starting to turn corner
150 to 165 m
-still in leuco-gabbro--trace olivine
- magnetite content quite significant
- may be slightly coarser
-olivine - .1 to .2 mm tubby rusty porous serpentine-magnetite pseudomorphs
-chaining around corner
162.7 m -small 1.7 x 3.5 cm
-cognate inclusion of old chill
-angular deep rusty brown shade
165 - 180 m - same but with a distinct 225/61 fabric
-may be tectonic but it does correspond with the cum. layering and may be primary crystal imbrication and flare fabric
-photo (Bruce to scale) of fabric in rock
185.0 m -contact between leuco-gabbro and more mafic gabbro
- contact 237/67 sharp
- leuco-gabbro CI 40(35) with well defined tectonic fabric - sub gneissic
-mafic gabbro CI=64
-more magnetite rich
195.0 m -slow decrease in CI back to 40 % with same indication of primary cumulus habit but m large scale not small
-fabric still quite not strong
-all primary igneous fabric gone-reduced to altered plagioclase and gneissic
hornblende
200.8 sample
195 to 210 m -leuco gabbro with slowly decreasing tectonic fabric
210 to 255 m -transition to lower units
210m -252/53 contact between glomeroporphyritic leucogabbro (above) and 2 pyroxenes meso-gabbro (CI=55) below
-3 samples
214.9 m - 7 to 12 cm gabbro pegmatite dike-irregular small chill margin inclusions
215 to 220 m -not exposed
220 to 225 m -two pyroxene gabbro slowly increasing in orthopyroxene content
225 to 235.6 m -as above
-minor quartz possible
235.6 to 237.5 -chill marging containing olivine
237.5 m -footwall contact 241/75
0 to 255 m -transition to lower units
210m -252/53 contact between glomeroporphyritic leucogabbro (above) and 2 pyroxenes meso-gabbro (CI=55) below
-3 samples
214.9 m - 7 to 12 cm gabbro pegmatite dike. Irrregular small chill margin inclusions
215 - 220 - no exposure
220 - 225 two pyroxene gabbro slowly increasing in orthopyroxene content
225 - 235.6 - as above but with possiblity of minor olivine
235.6 - 237.5 - chill margin containing olivine
237.5 - footwall contact 241/75

PHOTOS

Traverse line at station 5084 going from the footwall to the hangingwall of a large mafic/ultramafic dike.

Glomerocryst aggregates of sub- to euhedral plagioclase in a dense fine grained pyroxenite.

Partially assimilated and hybridized xenolith of granodiorite

Orthocumulate layering with tops upright to left.

Detail of layering showing tops by grading.

Upswell in layering with overlying layering draped over rise and showing some slumping. Top is to bottom of photo.

Cumulates of large pyroxene crystals tightely packed with little intersticial plagioclase and topped by thin anorthositic gabbro bed.

Coarse anorthositic gabbro with lathe plagioclase surrounding centimetre-size pyroxene pseudomorphs.

Remarkable bed-loading and slump features in the orthocumulate layering.

Detail of bed-loading features.

Close-up of orthocumulate bed set showing slump at left edge and density grading upright to top of photo.

View showing the penetrative fabric developed in the 050° direction.

Manitoba Geological Survey, 2017