Mantle evolution recorded in Cr-spinel from the Bay of Island Ophiolitic Complex, northern

Appalachians.

1.University of Ottawa, Ottawa.2.Geological Survey of Canada, Québec.

Fabio Stern1, Keiko Hattori1 and Jean Bédard2

2

Introduction

Location and Geological Settings

Rock Types

Results

Conclusions

3

Cr-spinel (FeCr2O4) is a useful petrogenic indicator of mafic-ultramafic rocks since it is not easily altered.

Therefore we examined Cr-spinels and olivines from the ultramafic complex to evaluate the mantle evolution of the BOIC.

Introduction

4

The area is located on the west coast of Newfoundland.

It is part the western ultramafic belt of the northern

Appalachians.V

The Bay of Islands Ophiolite Complex is

exposed in four massifs:

• Tablelands

• North Arm

• Blow Me Down

• Lewis Hills

Location

Geological Settings

5

6

Harzburgite

1 m

opxnite

harzburgite

50cmdunite

7

0.5mm

Harzburgite

Ol

Ol

Opx

3mm

Chr

0.5mm

Chr

Chr

8

Dunite contains lenses and veins of coarse grained wehrlites.

Dunite

wehrlite

50 cm

9

Dunite

Ol

Chr

0.5mm

Ol Chr

Chr

Ol

10

Introduction

Location and Geological Settings

Rock Types

Results

Conclusions

11

• Harzburgites have high Ir-type PGEs (Ir, Os, Ru) typical of residual mantle rocks;

• Dunites have higher concentration of Pt and Pd, suggesting a cumulate origin.

Results

12

Chr

0.5mm

All harzburgite samples plot in the Ol-Sp mantle array (OSMA) defined by Shoji Arai (1994).

Fosterite component of olivine vs Cr# of spinel. Olivine-spinel mantle array. After Arai (1994).

spinelHarzburgite

Results

13

0.5mm

Dunites with Ol (Fo>90, ) plot in the OSMA;

Dunites outside the OSMA were products from fractionated melt. Not considered for the evaluation of the mantle.

Fosterite component of olivine vs Cr# of spinel. Olivine-spinel mantle array. After Arai (1994).

spinelDunite

Results

14

Chr

0.5mm

Large variation in the Cr#;

Change in mantle conditions from fertile to more refractory (Abyssal field to Forearc peridotite).

Fosterite component of olivine vs Cr# of spinel. Olivine-spinel mantle array. After Arai (1994).

Hz and Dun

Results

Max AbyssalPer.

15

Chr

0.5mm

Wide range from MORB to Boninitic fields

Results

Modified from Dare et al. (2008). Boninites data from Barnes & Roeder (2001).

Modified from Kamenetsky et al. (2001).

16

Chr

fO2 calc. based on Ballhaus (1991) Ol-Sp exchange thermometry.

Results:

•Hz shows low fO2;

•Dun shows higher fO2, suggesting the influence of subduction.

Asthenosphericmantle

Results

Modified from Wang et al. (2008).

17

Subduction initiation

18

Introduction

Location and Geological Settings

Rock Types

Results

Conclusions

19

Conclusions

• PGE abundance is effective in identifying its origin.

• Dunite in BOIC is a cumulate based on low Ir-type PGEs

• The mantle source had low S;

• The mantle changed gradually from abyssal peridotite-like to more

refractory conditions in subduction setting;

20

References

Arai, S., (1994). Characterization of spinel peridotites by olivine-spinel compositional relationships: review and interpretation. Chemical Geology 111: 191–204.

Ballhaus, C., Berry, R.F., Green, D.H., 1991. High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer: implications for the oxidation state of the upper mantle. Contributions to Mineralogy and Petrology 107: 27–40.

Barnes, S.J., Roeder, P.L., (2001). The range of spinel compositions in terrestrial mafic and ultramafic rocks. Journal of Petrology 42: 2279–2302.

Dare, S., Pearce, J., McDonald, I., Styles, M., (2008). Tectonic discrimination of peridotites using fO2–Cr# and Ga–Ti–FeIII systematics in chrome–spinel. Chemical Geology, 261: 199-216.

Hattori, K., Wallis, S., Enami., Mizukami, T., (2009). Subduction of mantle wedge peridotites: Evidence from the Hgashi-akaiashi ultramafic body in the Sanbagawa metamorphic belt. Island Arc. 1-16.

Kamenetsky, V.S., Crawford, A.J., Meffre, S., (2001). Factors controlling chemistry of magmatic spinel: an empirical study of associated olivine, Cr-spinel and melt inclusions from primitive rocks. Journal of Petrology 42: 655–671.

McDonough, W.F., Sun, S.S. (1995): The composition of the earth. Chem. Geol.120: 223-253.

Nikolaeva, K., Gerya, T.V., Marques, F.O. (2010). Subduction initiation at passive margins: Numerical modeling. Journal of

Geophysical Research, 116: 1-19

Suhr, G. (1992): Upper mantle peridotites in the Bay of Islands Ophiolite, Newfoundland: formation during the final stages of a

spreading centre? Tectonophysics, 206: 31-53.

Wang, J., Hattori, K., Li, J., Stern., C. R., (2008). Oxidation state of Paleozoic subcontinental lithospheric mantle below the Pali Aike volcanic field in southernmost Patagonia. Lithos, 105: 98–110