Syn-orogenic High Fe-Ti Tholeiitic Magmatism in the Hinterland of Central Grenville Province: Implications for the Evolution of Large Hot Long Duration Orogens.

Barun Maity, Memorial University of Newfoundland, Earth Sciences, St. John's, NF, Canada, Aphrodite Daphnee Indares, Memorial University of Newfoundland, Earth Sciences, St John's, NL, Canada and Greg Dunning, Memorial University of Newfoundland, Earth Sciences, St John's, NF, Canada

Contact First Author: Barun Maity; barun.maity@mun.ca

Abstract ID#: 35180

 

English Abstract:
Late syn-orogenic (~1000 Ma) tholeiitic (high Fe-Ti) mafic rocks with granulite-facies assemblages and textures are documented for the first time in the hinterland of the central Grenville Province (Manicouagan area) where they form concordant or slightly discordant layers interleaved with felsic and mafic gneisses. Geochemically they are characterized by high FeOt, TiO2, P2O5 and HFSE ratios and low SiO2, MgO, #Mg, Ni and Cr. Based on petrography, geochemistry and age they can be separated into two groups. The older group consists of granoblastic hornblende+ plagioclase+ clinopyroxene+ garnet+ ilmenite± orthopyroxene, and is characterized by (La/Yb)n= 2.3-3.3, (Nb/Th)n= 0.6-1.3, eNd(1.0)= 1.5-2.8 and TDM1 = 1487-1523 Ma. One sample yielded a crystallization age of 1006±4 Ma and a metamorphic age of 1005.5±2.7 Ma (zircon U-Pb by CA-TIMS). The younger group consists of porphyroblastic garnet+ plagioclase+ orthopyroxene+ biotite+ ilmenite, large partially resorbed quartz grains, and minor K-feldspar, allanite, barite, thorite, monazite and apatite. A sample from this group has (La/Yb)n= 5.3, (Nb/Th)n= 2.4, eNd(1.0)= 4.6 and TDM1 = 1131 Ma, and yielded a crystallization age of 997±2.6 Ma.

All samples plot in the within-plate or ocean floor basalt fields in tectonic discrimination diagrams, and the geochemical and isotopic characters of both groups indicate partial melting of a mantle source followed by fractional crystallization or assimilation-fractional crystallization along tholeiitic trend. The textural and age data suggest the rocks underwent medium-P granulite-facies metamorphism shortly after their emplacement. This record of late syn-orogenic magmatism and high-T metamorphism is compatible with delamination or convective thinning of the overthickened lithosphere beneath a large hot orogen (LHO) formed during the Ottawan phase of Grenvillian orogeny, followed by decompression melting of the rising asthenosphere and conductive heating of the thinned crust from below.

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