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 Stern 1, Keiko Hattori 1 and Jean Bédard 2

2 Introduction Location and Geological Settings Rock Types Results Conclusions

3 Cr-spinel (FeCr 2 O 4 ) 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 50cm dunite

7 0.5mm Harzburgite Ol Opx 3mm Chr 0.5mm Chr

8 Dunite contains lenses and veins of coarse grained wehrlites. Dunite wehrlite 50 cm

9 Dunite Ol Chr 0.5mm Ol 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). spinel Harzburgite 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). spinel Dunite 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 Abyssal Per.

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 fO 2 calc. based on Ballhaus (1991) Ol-Sp exchange thermometry. Results: Hz shows low fO 2 ; Dun shows higher fO 2, suggesting the influence of subduction. Asthenospheric mantle 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., 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: 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 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: 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: 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