Abstract
The Svecofennian orogeny in Fennoscandia is an example of a long-lived orogeny characterised by low pressure, high temperature (LP-HT) metamorphism. In this paper, we present new LA-ICP-MS age data on monazite from fifteen paragneiss samples and two granitoid samples from the southeastern part of the Svecofennian orogen, including the Sulkava granulite complex. The high-grade rocks in the study area were metamorphosed at ca. 750-800 °C and 5-6 kbar, followed by near-isothermal decompression down to 3-4 kbar. The analysed monazites yielded ~(207)Pb/~(206)Pb dates from 1953 to 1737 Ma. The youngest dates are typically yielded by rims and overgrowths of monazites and by younger domains in patchy monazite grains. Psammitic and pelitic layers from the same outcrop display differences in monazite date distributions, with younger dates being more common in pelitic layers. Both detrital and metamorphic origins are possible for the =1.91 Ga monazite grains found in some samples. For the younger (<1.91 Ga) monazite grains, we found three main age peaks. The peak at 1.89-1.86 Ga fits well with the high-grade metamorphic event at 1.89-1.88 Ga observed elsewhere in southern and central Finland. The 1.87-1.86 Ga interval dates a near-isothermal decompression stage. The age peak at 1.83-1.82 Ga records the age of younger high-grade metamorphism in the study area. The age peak at 1.80-1.77 Ga is interpreted to represent isotopic resetting due to fluid-induced alteration during a shield-wide exhumation stage, when waning magmatism, leucosome crystallisation and a change towards brittle-ductile deformation led to localised fluid flow before cratonisation. The dates between the age peaks could be explained in part by mixed isotopic ages in patchy grains without clear growth zones.
NSTL