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

Abstract

We integrate new and existing age data from ophiolite and subophiolite rocks in the Thetford Mines region, Quebec Appalachians, to determine the timing of: (1) oceanic crust formation; (2) oceanic decoupling; (3) ophiolite emplacement onto the continent, and; (4) postemplacement Taconic orogenesis. Few ophiolites possess high-precision temporal data for all these events. This allows us to develop models to explain the origin of emplacement-related granitoids that have intruded the ophiolite. U-Pb zircon ages have been determined for four samples from the Thetford Mines ophiolite. Oceanic plagiogranite from the Lac de l9Est region of the Thetford massif yields an age of 480 ± 2 Ma. This age is identical to that of the adjacent Mont Ham 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 also present in one sample. These granitoids are strongly peraluminous, contain abundant inherited zircon, exhibit variable but high 87 Sr/ 86 Sr initial ratios, and low 208 Pb/ 206 Pb ratios in igneous zircon, which suggest that they were derived by partial melting of Laurentian margin sedimentary rocks. The ages indicate that the ophiolite was thrust over the Laurentian margin within 1–17 m.y. of oceanic crustal crystallization and that these obduction-related granites were generated during a period of Shear heating calculations indicate that shearing can generate the peridotite-hosted granitoid melts and their observed contact-zone mineralogies under specific conditions, although margin sedimentary rock melting in response to west-dipping subduction-related arc magmatism cannot be excluded.