Timing of accretion, intra-orogenic sedimentation and basin inversion in the Paleoproterozoic Svecofennian orogen: The Pyhäntaka area, southern Finland

Abstract

The Pyhäntaka area in southern Finland represents the accretionary Svecofennian orogen, presently exhumed to upper-mid crustal level. The post-1.87 Ga volcanic-sedimentary Pyhäntaka formation is separated from underlying cordierite paragneiss (metagraywacke) by an unconformity, marked by an ancient weathering surface on top of the paragneiss and ultramature quartzite at the base of the Pyhäntaka formation. Structural studies and U–Pb datings carried out in the paragneisses, metavolcanic rock interlayer and crosscutting granitoid rocks revealed four tectonic units, separated from each other by shear zones and representing different depositional ages (pre-1.87 Ga and post-1.87 Ga). Yet unidentified post-1.87 Ga rock sequences are anticipated in the Svecofennian orogen. The model of tectonothermal evolution in the Pyhäntaka area includes contraction at 1.89 Ga causing rapid burial of graywackes (Härkälä graywackes) to mid-crustal levels where they were metamorphosed into cordierite gneisses and intruded by syntectonic granite at 1875 Ma. Deposition of exhumation-related graywackes (Himattu graywackes) after 1.87 Ga, followed by development of a weathering surface and deposition of quartz sands, indicate gradual stabilization of the upper crust. A sequence of quartz sands, immature sands, volcaniclastic sands and basaltic rocks (the Pyhäntaka formation), and graywackes on top (Kesua graywackes) were deposited in an intra-orogenic rift basin during crustal extension that may have initiated 1.85 Ga ago. Contraction at 1825 Ma caused basin inversion with thrusting and folding of the post-1.87 Ga sequence. All rocks in the Pyhäntaka area were subject to a new high-grade metamorphic event that culminated at 1.82 Ga. Subsequent uplift of a pre-1.87 Ga rock sequence relative to the post-1.87 Ga sequence resulted in the present pattern of tectonic units. The inferred evolution at Pyhäntaka conforms to several tectonic models presented for the Svecofennian orogen. However, the occurrence of an ancient weathering surface and ultramature quartzite, an evidence for a 10–20 Ma stable period, is more compatible with continental interior setting with outboard subduction than active continental margin setting during deposition of the Pyhäntaka formation.