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TitleMantle petrology and mineralogy of the Thetford Mines Ophiolite Complex
AuthorPagé, P; Bédard, J H; Schroetter, J -M; Tremblay, A
SourceLithos vol. 100, 2008 p. 255-292,
Alt SeriesEarth Sciences Sector, Contribution Series 20070254
PublisherElsevier BV
Mediapaper; on-line; digital
File formatpdf
AreaAppalachians; Thetford Mines
Lat/Long WENS-71.5000 -71.0000 46.2500 46.0000
Subjectsigneous and metamorphic petrology; mineralogy; geochemistry; igneous rocks; intrusive rocks; mantle; ophiolites; partial melting; metasomatism; orogenic regions; metamorphism; harzburgites; orthopyroxenites; dunites; chromitites; silicates; textural analyses; tectonic models; Thetford Mines Ophiolite; Thetford Mines Ophiolitic Complex; Humber Zone; Dunnage Zone; Caribou Mountain Block; Duck Lake Block; Paleozoic; Precambrian; Phanerozoic; Cambrian; Ordovician
Illustrationssketch maps; photographs; photomicrographs; plots; tables
ProgramNSERC Natural Sciences and Engineering Research Council of Canada
ProgramDIVEX - Diversification de l'exploration minérale au Québec
AbstractThe Ordovician Thetford Mines ophiolite complex (TMOC) formed by boninite-fed seafloor-spreading, probably in a fore-arc environment. The mantle section is dominated by foliated harzburgite (= 5-6% clinopyroxene), cut by dunitic (± chromitite cores) and orthopyroxenitic veins and dikes. Contrasting structures, textures and mineral compositions allow us to subdivide the mantle. The granular-textured rocks of the Duck Lake Block (DLB) have two steeply-dipping foliations. The older foliation strikes NW, is sub-perpendicular to the Moho, and is interpreted to have resulted from upflow of the asthenosphere beneath the spreading ridge. This fabric is overprinted by a 2nd ductile foliation striking ENE, oriented sub-parallel to the Moho, which we interpreted as having formed by crust-mantle shear as the lithosphere migrated away from the spreading ridge. The DLB mantle has a limited range of spinel Cr# (100Cr/(Cr+Al) = 51-71). Comparison with experimentally determined residual spinel compositions (equilibrium melting) implies a maximum loss of 27-38% melt if the protolith had a fertile MORB mantle composition. However, interstitial-textured clinopyroxene may have high TiO2 (<0.04 wt%) and Na2O (<0.27 wt%), and some interstitial spinel has higher TiO2 (<0.09 wt%), suggesting interaction with (or crystallization from) an “impregnating” melt. Interstitial tremolitic amphibole also indicates the passage of late hydrous fluids. The harzburgite in the Caribou Mountain Block (CMB) has a porphyroclastic texture, with a strong, locally mylonitic foliation striking roughly N-S, parallel to the orientation of seafloor-spreading related paleo-normal faults in the crust. These fabrics and textures imply a colder, lithospheric deformation, possibly related to tectonic denudation (oceanic core complex). This would explain problematic lava/mantle contacts, favour infiltration of seawater, serpentinization, and reduced fO2 conditions. The CMB mantle shows a wider range of mineral compositions than the DLB, with spinel Cr# (28-86) implying = 15-45% of equilibrium melting. Locally higher TiO2 in spinel (<0.05 wt%) and clinopyroxene (<0.11 wt%), a local rimward decrease in spinel Cr#, clinopyroxene Cr#, and olivine Fo-content, and traces of interstitial amphibole, are attributed to the circulation of an evolved hydrous melt during peridotite deformation. This suggests that the lower limit to the extent of melting inferred for the CMB (15%), established on the basis of Al-rich spinel rims and neoblasts, is probably too low. On the other hand, the higher inferred degree of depletion of the CMB is probably unaffected by the metasomatic overprint and is a more robust conclusion.

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