Obduction of Tethyan-type ophiolites--A case-study from the Thetford-Mines ophiolitic Complex, Quebec Appalachians, Canada
Résumé
Oceanic plagiogranites and peridotite-hosted granites (PHG) of the Thetford-Mines ophiolite (TMO) yield mean U-Pb zircon ages of 479.2±1.6 Ma and 469.5±2.8 Ma, and are interpreted to be the products of fractionation of basaltic magma and melting of the Laurentian margin, respectively. The existing 40Ar/39Ar age for the ophiolitic metamorphic sole of the TMO is a hornblende isochron age of 477±5 Ma for amphibolite facies rocks. Sites previously sampled for U-Pb and 39Ar-40Ar analyses were resampled for more accurate 39Ar-40Ar dating, in order to better constrain the thermo-tectonic evolution of the ophiolite and underlying Laurentian margin rocks on which they were overthrust, and quantify the time required for obduction of Tethyan-type ophiolites in the Canadian Appalachians. Amphiboles from TMO gabbro and plagiogranite yield 39Ar-40Ar plateau ages of 477-475 Ma. In contrast, muscovites from PHG yield cooling ages of 466-465 Ma, and show evidence for thermal resetting at ca. 460 Ma for both a granitic mylonite and a crosscutting pegmatite facies. The amphibolitic metamorphic sole yields 39Ar-40Ar ages of ca. 471 Ma in amphibole, and high-temperature ages of ca. 466 Ma in muscovite, with evidence for recrystallization to ages as young as 457 Ma. Muscovites from the sub-ophiolitic Laurentian metasedimentary basement, and micaschist clasts in supraophiolitic debris flows of the TMO, yield similar pseudo-plateau and high-temperature ages of ca. 463- 464 Ma. The U-Pb and 39Ar-40Ar data suggest that the obduction of peri-Laurentian oceanic crust lasted approximately 5 to 10 m.y. Since both facies formed approximately m.y. after the ophiolitic crust, this supports models whereby the still-hot ophiolitic mantle provided the heat needed to generate the PHG and the metamorphic sole between ca. 475 and 470 Ma. Muscovite ages of PHG record cooling below ca. 450- 500 °C by ca. 465 Ma, but 39Ar-40Ar ages from both a mylonitized PHG facies and micaschist clasts suggest that the emplacement of the oceanic lithosphere on the Laurentia margin had nearly ended by ca. 460 Ma, likely due to foreland-directed downward thrust propagation and related exhumation of the collisional wedge. The new age data imply that the southern Quebec ophiolites were thrust 100s of km over the Laurentian margin, which has major implications for the location of the subduction suture and the early stages of the Taconic orogeny in the Northern Appalachians.