Rapakivi granites in the geological history of the earth. Part 1, magmatic associations with rapakivi granites: Age, geochemistry, and tectonic setting |
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| Authors: | A M Larin |
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| Institution: | (1) Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. Petersburg, Russia |
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| Abstract: | Rapakivi granites characteristic practically of all old platforms are greatly variable in age and irregularly distributed over the globe. Four types of magmatic associations, which include rapakivi granites, are represented by anorthosite-mangerite-charnockite-rapakivi granite, anorthosite-mangerite-rapakivi-peralkaline granite, gabbro-rapakivi granite-foidite, and rapakivi granite-shoshonite rock series. Granitoids of these associations used to be divided into the following three groups: (1) classical rapakivi granites from magmatic associations of the first three types, which correspond to subalkaline high-K and high-Fe reduced A2-type granites exemplifying the plumasitic trend of evolution; (2) peralkaline granites of the second magmatic association representing the highly differentiated A1-type reduced granites of Na-series, which are extremely enriched in incompatible elements and show the agpaitic trend of evolution; and (3) subalkaline oxidized granites of the fourth magmatic association ranging in composition from potassic A2-type granites to S-granites. Magmatic complexes including rapakivi granites originated during the geochronological interval that spanned three supercontinental cycles 2.7?1.8, 1.8?1.0 and 1.0?0.55 Ga ago. The onset and end of each cycle constrained the assembly periods of supercontinents and the formation epochs of predominantly anorthosite-charnockite complexes of the anorthosite-mangerite-charnockite-rapakivi granite magmatic association. Peak of the respective magmatism at the time of Grenvillian Orogeny signified the transition from the tectonics of small lithospheric plates to the subsequent plate tectonics of the current type. The outburst of rapakivi granite magmatism was typical of the second cycle exclusively. The anorthosite-mangerite-charnockite-rapakivi granite magmatic series associated with this magmatism originated in back-arc settings, if we consider the latter in a broad sense as corresponding to the rear parts of peripheral orogens whose evolution lasted from ～1.9 to 1.0 Ga. Magmatism of this kind was most active 1.8?1.3 Ga ago and represented the distal effect of subduction or collisional events along the convergent boundaries of lithospheric plates. An important factor that favored the emplacement of rapakivi granites and anorthosites in a huge volume was the thermal and rheologic state of the lithosphere inherited from antedating orogenic events, first of all from the event ～1.9 Ga ago, which was unique in terms of heat capacity transferred into the lithosphere. Anorthosite-mangerite-rapakivi granite-peralkaline granite magmatism is connected with activity of the mantle plums only. Degradation of the rapakivi granite magmatism toward the terminal Proterozoic was controlled by the general cooling of the Earth in the course of the steady dissipation of its endogenic energy, as these processes became accelerated since the Late Riphean |
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| Keywords: | rapakivi granites charnockites anorthosites Proterozoic Archean continental crust mantle magma source geochemistry geodynamic settings |
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