U-Pb geochronology and origin of granitoid rocks in the Thetford Mines ophiolite, Canadian Appalachians

We integrate new and existing age data from ophiolite and suhophiolite rock s in the Thetford Mines region, Quebec Appalachians, to determine the timin g of: (1) oceanic crust formation; (2) oceanic decoupling; (3) ophiolite em placement onto the continent, and; (3) postemplacement Taconic orogenesis. Few ophiolites possess high-precision temporal data for all these events. T his allows us to develop models to explain the origin of emplacement-relate d granitoids that have intruded the ophiolite. U-Pb zircon ages have been determined for four samples from the Thetford Mi nes ophiolite. Oceanic plagiogranite from the Lac de l'Est region of the Th etford massif yields an age of 480 +/- 2 Ma. This age is identical to that of the adjacent Mont Nam massif thrust sheet (478 (+3)/(-2) Ma), suggesting a close temporal and spatial association of these two units in the source oceanic domain. Two peridotite-hosted granitoid samples yield crystallization ages of 470 ( +5)/(-3) Ma and 469 +/- 4 Ma. A Precambrian inherited zircon component is a lso present in one sample. These granitoids are strongly peraluminous, cont ain abundant inherited zircon, exhibit variable hut high Sr-87/Sr-86 initia l ratios, and low Pb-208/Pb-206 ratios in igneous zircon, which suggest tha t they were derived by partial melting of Laurentian margin sedimentary roc ks. The ages indicate that the ophiolite was thrust over the Laurentian mar gin within 1-17 m.y. of oceanic crustal crystallization and that these obdu ction-related granites wc re generated during a period of <10 m.y. Shear he ating calculations indicate that shearing can generate the peridotite-hoste d granitoid melts and their observed contact-zone mineralogies under specif ic conditions, although margin sedimentary rock melting in response to west -dipping subduction-related are magmatism cannot be excluded.