Integrated geophysical studies of Precambrian mobile belts to constrain evolutionary and mineragenic crustal models (experience from Fennoscandian Shieldтезисы доклада
Дата последнего поиска статьи во внешних источниках: 21 мая 2019 г.
Место издания:Indian School of Mines Alumni Association Kolkata Chapter Kolkata
Первая страница:166
Последняя страница:166
Аннотация:Mobile belts surrounding Archean cores of the ancient cratons have imprinted and preserved distinct marks of structural evolution events and mineragenic processes reached one of their bright peaks in Proterozoic time. Modern integrated geophysical investigations effectively help to develop our knowledge of the earlier history of the planet and to find optimal directions for intensification of mineral resources exploration. Seismic, electromagnetic and potential fields’ methods put constraints on the modeling of the crustal structure and material properties, with two first methods producing maximal resolution allowing meaningful geological interpretation.
The paper is focused on the results of modern world-class crustal experiments in important mineragenic provinces: CMP seismic reflection profiling (FIRE) and broad-band/long-period MT soundings (FIRE-MT and LADOGA) across Raahe-Ladoga pericratonic zone at the border of the Arhean Karelian craton and Palaeoproterozoic Svecofennian accretionary Orogen (both at Russian and Finnish territories).
Original geotectonic model of the Central Svecofennian Orogen, constructed on the seismic data and constrained by surface geology, presents a sequence of tectonic sheets submerged in NE direction under the Karelian Craton and tracing at distances up to 200km from the surface border and to upper mantle depths. The model is successfully verified by geoelectric data, which confirm structural pattern and help to resolve ambiguity in understanding of its filling with material properties.
Despite the absence of appropriate seismic data at the SE segment of the AR-PR border of the Fennoscandian shield the crustal architecture here is resolved in much more details than previously due to recent MT soundings performed across prominent Lake Ladoga conductivity anomaly. Relevant correspondences of geoelectric, density and magnetic images of crustal structures in cross-section of profile line are demonstrated to support ideas of regional predominance of the collisional tectonic over extensional one in the Paleo- and Mesoproterozoic times. It is suggested that long period MT Lake Ladoga anomaly is caused by deeply metamorphosed complexes of South-Finland Granulite-Gneiss Belt, which include crystal graphite and could be similar to exhumed formations of Lapland Granulite Belt. While at NE, in pericratonic zone, enhanced upper-crustal conductivity is connected with frequently exposed graphite- and/or sulphide-bearing sedimentary-volcanic schists, fabrics of lower metamorphic stages.
New integrated geophysical models of SE Fennoscandia highlight the ancient or modern boundaries of the crustal units, which are favorable for increased heat and mass transfer and participate in the formation of geodynamic activity and mineral deposits. They give grounds for more deep understanding of mineragenic and ore formation processes and directs prospecting activity. Thus, in particular, they explain the deep roots and origin of the famous Ikhal'sky deposit of the coarse- and medium-flaky graphite, located in the granulite nappe of the anomalously conductive South-Finland Belt. On this background and on the experience of near-surface prospecting works at the Kola Peninsula the effective set of appropriate geophysical methods is suggested for further detailed exploration of this field.
The authors are grateful to the Russian Foundation for Basic Research (RFBR) for supporting projects No. 15-05-01214, 16-05-00543, 16-05-00975. The work was supported also by the FASO of Russia (project 0135-2016-0014).
