Multiphase volcanism of the Archean Tipasjärvi greenstone belt (Eastern Finland) revealed by single-grain U-Pb dating of zircons

Thursday, 18 December 2014
Elina Lehtonen, University of Helsinki, Helsinki, Finland and Asko Käpyaho, Geological Survey of Finland, Espoo, Finland
The Tipasjärvi greenstone belt (TGB) represents the southern tip of the largest Archean greenstone belt complex in Finland. Despite intensive research, the relationship of the belt with surrounding Archean granitoid-dominated bedrock has remained ambiguous. To approach this problem and to characterize the volcano-sedimentary evolution of the TGB we have performed new U–Pb single-grain zircon dating of felsic-intermediate volcanic rocks and sedimentary rocks by applying SIMS, as well as produced new geochemical analyses from the selected key locations.

The new results show that there are at least three different age groups of felsic-intermediate volcanic rocks in the TGB: 2.84 Ga, 2.82 Ga, and 2.80 Ga. The oldest volcanic rocks predate the oldest dated unmigmatised granitoidic rocks in the immediate vicinity of the TGB, and thus the depositional basement for these volcanic rocks is still unknown. The second volcanic event coevals with some of the plutonic rocks, which are thus likely to be related to this volcanic phase. The 2.80 Ga Koivumäki Formation, containing Ag-Zn-Pb-Au mineralization, is the youngest of these volcanic episodes, contrary to what has previously been suggested. The results of detrital zircons from the sedimentary rocks from the TGB indicate that they were deposited at least 40 Myr after the youngest known volcanic rocks. The sedimentary rocks have heterogeneous sources and contain >3.0 Ga crustal material.

The volcanic evolution of the TGB resembles, e.g., the evolution of the Archean ca. 2897–2744 Ma Island Lake greenstone belt (within the Superior Province in Canada) that is also characterized by multiphase volcanism and the presence of contemporaneous plutonic and volcanic rock units. Unlike in the case of the TGB, however, the depositional basement for the oldest volcanic rocks of the Island Lake greenstone belt has been recognized.