Search

U–Pb zircon single grain dating using the LA-MC-ICP-MS technique was utilized to determine the age of the host rocks of the Juomasuo Au–Co–Cu deposit located in the late Archean Kuusamo supracrustal belt. Even though the dated samples have diverse geochemical signatures that imply felsic volcanic and sedimentary precursors, the U–Pb data revealed heterogeneous detrital zircon populations for all samples. The host rocks are thus considered to belong to reworked sedimentary/volcano-sedimentary sequences. The maximun depositional ages of the samples fall in the 2.75–2.65 Ga age window, and no Paleoproterozoic ages were recovered in the examined zircon grains (151 analytical spots in total). In addition, a younger population (2.65–2.60 Ga) of internally featureless, BSE-pale/CL-dark zircon and zircon domains, was found in the mineralized sequences. These homogenized zircon grains resemble zircon formed in postmagmatic solid–state processes, in which zircon is recrystallized in metamorphic–hydrothermal conditions. This metamorphic-hydrothermal event most probably occurred in the original provenance area of the metasedimentary rocks. [ABSTRACT FROM AUTHOR]

Multi-grain isotope dilution and secondary ion microprobe zircon U-Pb as well as whole-rock Rb-Sr isotope dilution data on the late Paleoproterozoic Suomenniemi rapakivi granite complex (exposed on the northern flank of the Wiborg batholith in southeastern Finland) are discussed in the light of point-specific errors on Pb/U and proposed new values of the decay constant of 87Rb, λ87. U-Pb zircon data on hornblende granite and biotite granite of the main metaluminous-marginally peraluminous granite fractionation series of the Suomenniemi batholith indicate crystallization in the 1644-1640 Ma range, with a preferred age at 1644±4 Ma. A cross-cutting hornblende-clinopyroxene-fayalite granite is probably slightly younger, as are quartz-feldspar porphyry dikes (1634±4 Ma) that cut both the main granite series and the metamorphic Svecofennian country rocks of the Suomenniemi batholith. Recalculation of whole-rock Rb-Sr data published on the main granite series of the batholith by Rämö (1999) implies errorchron ages of 1635±10 Ma and 1630±10 Ma and a magmatic 87Sr/86Sri of 0.7062±0.0024. This relatively high initial ratio is indicative of a major Proterozoic crustal source component in the granites of the batholith. The main granite series of the batholith probably cooled relatively rapidly to and below the closure temperature of the Rb-Sr isotope system, with little subsequent subsolidus adjustment. The three discrete silicic magmatic phases of the batholith (the main granite series, the hornblende-clinopyroxene-fayalite granite, and the quartz-feldspar porphyry dikes) were all probably emplaced before the main volume of rapakivi granite (the Wiborg batholith proper) in southeastern Finland. The Suomenniemi batholith thus represents an early magmatic precursor to the classic Wiborg batholith and was emplaced clearly before the massive rise of isotherms associated with the ascent and crystallization of the magmas that formed the bulk of the Wiborg batholith system. [ABSTRACT FROM AUTHOR]