Metamorphism of the MacLean Lake and Central Metavolcanic belts, La Ronge domain, Trans-Hudson Orogen: pressure-temperature variations and tectonic implications

Metamorphic assemblages differ between the metasedimentary MacLean Lake bel t and the adjacent Central Metavolcanic belt in the La Ronge domain, Trans- Hudson Orogen. The former consists of meta-arkoses, psammitic gneisses, met aconglomerates, and calc-silicate gneisses of upper amphibolite facies (600 -740 degrees C, 440-660 MPa) with local migmatization, whereas the latter i s comprised mainly of metavolcanic and plutonic rocks, with minor metasedim entary schists of greenschist to lower amphibolite facies (480-630 degrees C, 520-560 MPa). Petrographic evidence indicates that peak metamorphic cond itions were reached towards the end of D-1 deformation during which the Cen tral Metavolcanic belt was thrust onto the MacLean Lake belt along the McLe nnan Lake tectonic zone, which separates the two belts. Peak metamorphic as semblages did not undergo retrograde alteration during D-2 deformation, ind icating that high temperature was maintained during D-2 deformation. Differ ences in pressure (P) and temperature (T) between the northeastern and sout hwestern parts of the Central Metavolcanic belt may have resulted from tilt ing along strike after peak metamorphism. Peak temperatures increase gradua lly from the Central Metavolcanic belt to MacLean Lake belt across the McLe nnan Lake tectonic zone. Peak pressures in the two belts are similar, imply ing that the Central Metavolcanic belt thrust sheet was probably thin. The P-T data for the MacLean Lake belt indicate a relatively high thermal gradi ent (40-50 degrees C/km), similar to that in the metasedimentary Kisseynew domain in the orogen.