Abstract
In the northwestern Pontiac Subprovince, metavolcanic rocks are exposed
within a metagraywacke sequence that is intruded by metamorphosed mafic
dykes. The metavolcanics are Al-undepleted komatiites ([La/Sm]N = 0.3, [Tb/Yb]N = 0.9) and tholeiitic Fe-basalts ([La/Sm]N = 0.8 and [Tb/Yb]N = 0.8).
The nearly flat chondrite-normalized distributions of high field
strength elements (HFSE), Ti and P, the constant Zr/Y, Nb/Th, Ti/Zr, and
Ti/P ratios, and the lack of depletion of HFSE relative to rare-earth
elements (REE) in both ultramafic and mafic metavolcanics, imply that
crustal assimilation and magma mixing with crustal melts were not
significant during differentiation and argue against the presence of
subduction-related magmatic components. Contemporaneous volcanism and
sedimentation in the northwestern Pontiac Subprovince are unlikely. The
metavolcanics do not show any evidence of crustal contamination and
likely represent a structurally emplaced, disrupted assemblage,
chemically similar to early volcanics of the adjacent southern Abitibi
Subprovince.Metamorphosed mafic dykes intruding the metagraywackes are
not genetically related to the metavolcanics. The dykes have high CaO, P2O5, K2O, Ba, Rb, and Sr, intermediate Cr and Ni contents, and strongly fractionated REE patterns ([La/Yb]N = 10.8).
Normalized to the primitive mantle, they display pronounced negative
Nb, Ta, Ti, Zr, and Hf anomalies. These amphibolites are metamorphosed
equivalents of Mg-rich calc-alkaline lamprophyre dykes, most likely
derived from a hybridized mantle source. Mantle metasomatism was
probably related to a subduction event prior to the peak of
compressional Kenoran deformation in the Pontiac Subprovince.
Original language | English |
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Pages (from-to) | 1110-1122 |
Number of pages | 13 |
Journal | Canadian Journal of Earth Sciences |
Volume | 30 |
Issue number | 6 |
DOIs | |
Publication status | Published - Jun 1993 |
ASJC Scopus subject areas
- General Earth and Planetary Sciences