Geochemical and U-Pb and Sm-Nd isotopic constraints on the evolution of the Paleoproterozoic Ylitornio nappe complex, northern Fennoscandia

Abstract

The Ylitornio area in northern Finland is a typical example of a poorly outcropping, multiply deformed, complex Precambrian terrain. U-Pb, Sm-Nd and geochemical data from metavolcanic, metasedimentary and plutonic rocks combined with structural data define the lithodemic units and tectonostratigraphy of the area. The Mellajoki, Martimo, Hosiojoki, Kierovaara and Uusivirka suites form the Ylitornio nappe complex. Although main thrust stacking had occurred between 1.92-1.89 Ga, later reverse faulting probably continued until 1.78 Ga. The Väystäjä bimodal volcanic rocks (2.09-2.05 Ga) from the Martimo suite are plume-related EMORB-OIB rocks formed during a continent breakup. The Kaskimaa greywacke of the Martimo suite is inferred to represent sedimentation in a deepening rift basin in the continental margin. The coeval Hosiojoki felsic rocks with A-type affinity and the Kierovaara suite granites (2.00-1.98 Ga) represent dry melting of a dominantly 2.1-2.05 Ga contaminated mafic underplate, and hydrous melting of both the mafic underplate and a refractory lower crust of Archean age, respectively. The older rocks of the Uusivirka suite are early-collisional (1.92-1.91 Ga) and derived from poorly mixed local sources, whereas younger psammite-pelite associations (Ristivuoma) are syn-collisional (1.91-1.89 Ga) and derive from multiply recycled and thoroughly mixed sources. These rocks were deformed during continued collision with the development of foreland fold-and thrust belt.