U-pb Ages from the Selkirk Allochthon, Seymour Arm Map

During the COUIX of I :50 000 scale regional mapping in the Northern Selkirk Mountains (NTS 082M) various granitic rocks were sampled for U-Pb geochronology. This report presents new U-Pb data, interpreted ages, and the implications of these results for hvo such samples from the Selkirk Allochthon: the Downie Creek otihogneiss, and the Goldstream Pluto”. The Downie Creek orthogneiss consists of a series of foliated granix and granodiorite sheets which intrude a quartz-rich sequence of garnet-muscovite-biotite and chlorite bearing paragneiss and schist within the Selkirk Allochthon. These orthogneisses were affected by regional ductile polrphase contractional deformation and related metamorphism and later overprinted by brittle deformation associated with the Columbia River fault. They are lithologically and structurally similar to, and likely currelative with, Devono-Mississippian gneisses of the Clachnacudainn suite exposed near Revelstoke (Parrish, 1992). U-Pb geochmnology of the Downie Creek orthogneiss was undertaken to strengthen or refute this correlation. The Goldstream Pluto” is an elongate, east-trending plutonic complex consisting of granitic to monzodioritic sills intimately mixed with meta!;edimentaq pendants, septa and xenolithr. These granitoids commonly have foliated margir.s and massive interior zones. The Goldstream Pluton was previously thought to have been emplaced prior to, or during regional deformation, based largely on its regional structural concordance with surrounding foliated counhy rocks, (Htiy, 1979). However, the presence of a chiefly retrograded contact metamorphic aureole, which overprints penetrative foliations in the country rocks, indicates that this body post-dates most of the ductile deformation in the area. This conclusion is further supported by the presence of foliations in xenoliths and pendants, which are interpreted to b’: correlative with regional deformation fabrics, and zre cut by tie Goldstream pluton. Hornblende and biotite 40Ar/3!‘Ar plateau ages of 114+4.5 Ma ac,d 100&l Ma, respectively, have been previously reported for the Goldstream pluton, indicating a relatively slow cooling rate for this body (Logan and Cclpron, 1995). U-Pb geochronology of this body was carried out to ldetermine its crystallizx:ion age, and to complement these 4oAr/39Ar c soling datr..