Comportement du système U-Pb dans la monazite et chronologie de la déformation et du métamorphisme des metasédiments du domaine de Kisseynew, orogène trans-hudsonien (Manitoba, Canada)

Abstract

The Kisseynew domain is the central unit of the Reindeer Zone of the Paleoproterozoic Trans-Hudson Orogen, in Manitoba and Saskatchewan. The southern flank of the domain is a transition zone between the greenschist facies of the volcano-plutonic assemblage of the Flin Flon - Snow Lake belt and the upper amphibolite facies of Kisseynew paragneisses. The Jungle Lake area, in the southern flank of the Kisseynew Domain, comprises mainly quartzofeldspathic gneisses representing continental clastic units of the Missi suite and migmatitic metagraywackes of the Burntwood suite. The area was affected by several phases of deformation, metamorphism, and migmatisation. Detailed mapping and U-Pb geochronology were carried out in order to establish the timing of the deformational and metamorphic phases. The oldest leucosome contains sillimanite formed during peak metamorphism and is associated with F2 folding and S2 fabric. Five single monazites from this leucosome yield ages between 1809 and 1803 Ma taken as the best estimate for the duration of peak metamorphism. Biotite schlieren in diatexites in the Burntwood suite show a S2 fabric folded by F3. Zircon from one of these diatexites yield a crystallization age of 1798+3-2 Ma, considered as the lower limit for the F2 event. Single monazites from the same rock yield ages between 1812 and 1789 Ma, the oldest of which are considered to be inherited. The youngest mobilisate is a pegmatite crosscutting F2 and F3 fabrics and yielded single monazite ages between 1875 and 1788 Ma. The youngest age is taken as the age of the pegmatite and is a minimum age for F3 fabrics. These results, together with those from other areas of the southern Kisseynew Domain, indicate a ca. 30 million year period (1818 and 1785 Ma) of continuous deformation and metamorphism. The data also show the presence of monazite crystals of different ages in the same rock illustrating the need to analyse single grains to obtain geologically meaningful ages.