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Peter Sorjonen-Ward, Geological Survey of Finland (Finland)
Olli Äikäs, Geological Survey of Finland (Finland)
Asko Kontinen, Geological Survey of Finland (Finland)
Heikki Lukkarinen, Geological Survey of Finland (Finland)
Jorma Paavola, Geological Survey of Finland (Finland)
Tapio Ruotoistenmäki, Geological Survey of Finland (Finland)
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The Archean - Proterozoic boundary zone in central Finland is an ideal area for investigating interactions between deformation and granitoid emplacement. Bedrock mapping and isotopic dating, combined with airborne geophysical data provide a framework for strain patterns and structural evolution at the present erosion level, while information on deep crustal architecture and composition is available from seismic reflection and refraction surveys, and also from kimberlite xenolith suites.
The FIRE 1 and 3A reflection data image major listric structures, which may represent high strain zones transferring magma from source regions to sites of pluton construction in the middle and upper crust. Nearer the surface, there is good correlation between reflectivity and concordant sheet-like bodies of granite. These features also integrate well with regional structural evolution, which records a transition from early thrusting to a system of NW-trending ductile shear zones and bivergent thrusting and folding, subparallel to the former craton margin; this kinematic framework applied from 1.89 to at least 1.84 Ga. Compositional differences between successively emplaced plutons, and deformation into spectacular sigmoid patterns allow estimates of cumulative displacement across shear zones. Pluton geometry is consistent with a vertical transition in deformation style and architecture, and a combination of magma flow through steep shear zones and lateral accumulation as more gently dipping bodies in adjacent terrain; some migmatitic, enclave-rich zones may represent pluton bases, above which more homogenous, discrete and concentrically zoned plutons developed. Extensive swarms of fine-grained tonalite dykes display spectacular mingling textures and entrain large angular blocks of country rock. Cross-cutting relations militate against these dykes being feeders to plutons at the present erosion level. They may reflect volatile-rich melting and overpressuring in the deep crust but if so, fluid source and origin is as yet unconstrained. This area is underlain by thick crust, with a high velocity lower crustal layer which could be attributed to postorogenic delamination and underplating, lithospheric subcretion during continental breakup, or less likely, subducted oceanic lithosphere. Rb-Sr and Sm-Nd data indicate that some1.88-1.86 Ga granites are sourced mainly from Archean lower crust, while other intrusions are more mafic, with magmatic hornblende and adakitic and appinitic affinities. This poses an intriguing dilemma, for a brief episode of late orogenic subduction beneath Archean lithosphere is difficult to reconcile with orogenic polarity. Plume impingement or delamination may be permissible, but are they necessary? Is it possible instead that sustained crustal thickening could perturb the underlying mantle geotherm, promoting small degrees of partial melting, thus explaining both the mingling phenomena in the tonalite dyke swarms and more mafic plutons?
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