D4 Published development or research report or study
Jyväskylän seudun kallioperä




List of Authors: Mikkola P, Heilimo E, Aatos S, Ahven M, Eskelinen J, Halonen S, Hartikainen A, Kallio V, Kousa J, Luukas J, Makkonen H, Mönkäre K, Niemi S, Nousiainen M, Romu I, Solismaa S
Publisher: Geologian tutkimuskeskus
Publication year: 2016
Title of series: Tutkimusraportti / Geologian tutkimuskeskus
Number in series: 227
eISBN: 978-952-217-371-3
ISSN: 0781-4240

Abstract

The Geological Survey of Finland (GTK) mapped the bedrock in the Jyväskylä area during 2012-2016 in order to update basic knowledge of the area to the level required for ore potential estimation and for identifying the most suitable areas for the production of dimension stone and rock aggregates. Geologically, the area is located in the central part of the Svecofennian orogeny, more precisely in the border zone between the Central Finland Granitoid Complex and the Pirkanmaa migmatite and intrusive suites flanking it to the south. The Makkola suite, mainly consisting of volcanic rocks, forms a discontinuous northeast-trending belt that in most places coincides with the boundary between the Central Finland Granitoid Complex and the Pirkanmaa suites. The Pirkanmaa migmatite suite contains both well-preserved greywackes and paragneisses migmatized to variable extents. Based on detrital zircons, the maximum deposition age for most of these rocks is 1900-1920 Ma. The Pirkanmaa intrusive suite in the area is mainly composed of granodiorites with ages close to 1895 Ma and containing locally abundant enclaves of the Pirkanmaa migmatite suite. Both of the Pirkanmaa suites are intruded by relatively small volume Vammala suite gabbros and ultramafic plutonic rocks, which have potential with respect to magmatic Ni and Cu ores. The Makkola suite (1895-1875 Ma) mainly consists of intermediate to felsic volcanic rocks and is calc-alkaline in character. In addition to the supracrustal units, it also includes plutonic rocks with similar ages to the volcanic units. Locally, the primary volcanic/sedimentary textures are well preserved, but often destroyed by deformation and metamorphism. Dykes correlated with the Makkola suite also crosscut the surrounding plutonic units. The Central Finland Granitoid Complex within the study area mainly consists of two lithodemes: Muurame porphyritic granitoids and coeval Vaajakoski quartz diorites. These rocks yield ages from 1885 to 1880 Ma, often display gradual contacts and have been variably deformed. The bimodal Saarijärvi suite (1885-1875 Ma) consists of granitoids forming the Puula lithodeme, as well as diorites and gabbros of the Istruala lithodeme. These rocks are partly coeval with the Muurame and Vaajakoski lithodemes, but differ from them on a compositional basis, as the Puula granitoids display A-type characteristics contrary to the calc-alkaline I-type character of the Muurame lithodeme. Granitoids of the Oittila suite (~1875 Ma) crosscut all of the above-mentioned units as dykes and small intrusions. These rocks are often more leucocratic than, for example, the Muurame porphyritic granitoids. Rocks of the Oittila suite contain some potential for porphyry-type deposits, although all mineralizations found so far are of limited size. The area is transected by the relatively young sinistral Leivonmäki shear zone trending northeast. Practically all of the gold showings in the region are located in the vicinity of this shear and are likely to be orogenic in character. The youngest fault system in the area consists of southeast-trending dextral faults.


Last updated on 2019-21-08 at 22:18