PETROGENESIS OF CAMBRO-ORDOVICIAN SUBDUCTION-RELATED GRANITIC MAGMAS OF THE QUEBEC APPALACHIANS, CANADA

In the Quebec Appalachians, Cambro-Ordovician subduction-related grani tic rocks are found as intrusive bodies (ACI) in the Ascot Complex vol canic arc and as eroded fragments (MGC) in the adjacent Magog Group se dimentary basin. Major- and trace-element geochemistry indicates that ACI and MGC rocks are typical peraluminous granitoids. Both suites are LREE-enriched with minor negative Eu anomalies; ACI granitoids displa ying higher MREE and HREE abundances than MGC. REE and trace-element d ata indicate that ACI and MGC granitoids are chemically similar, but d ifferences in their respective fractionation trends and trace-element ratios demonstrate a significant petrological decoupling. Nd-143/Nd-14 4 are higher in the MGC granitoids than in the ACI. Both suites displa y negative epsilon(Nd) ratios (- 10 to - 3) implying limited juvenile material input. Sm-Nd isotopes from both suites suggest that they orig inated from the anatectic partial melting of a compositionally-zoned, Grenville-like continental crust.