Geochemical and tectono-magmatic evolution of the volcano-sedimentary rocks of Pechenga and other greenstone fragments within the Kola Greenstone Belt, Russia

The early Proterozoic volcano-sedimentary Pechenga and Imandra–Varzuga regions form the two larger fragments within the Kola Greenstone Belt, a prominent feature of the north-eastern Baltic Shield. This belt also contains three smaller fragments at Polmak, Pasvik and Ust’Ponoy. The geology of the larger fragments consists of an older northern and a younger southern part, separated by prominent conformable deformation zones. Accordingly, the 2350–1850 Ma lithological succession of the Pechenga Greenstone Fragment is subdivided into the Northern (2550–1905 Ma) and Southern Pechenga Complexes (1905–1700 Ma), likewise the Imandra–Varzuga Fragment comprises the Northern- and Southern Imandra–Varzuga Complexes. latively better-known Northern Pechenga Complex is subdivided into four units, which in turn are subdivided into 11 formations. In contrast, Southern Pechenga Complex comprises only one unit, subdivided into four volcano-sedimentary and two volcanic formations. Both the volcanic and volcaniclastic components of these complexes are characterised by magmatic compositions ranging from basaltic-andesite, to trachybasalt-trachyandesite, basalt, ferropicrite-basalt, picrite-basalt-andesite, picrite-basalt and andesite-dacite-rhyolite. The gabbro-wehrlite intrusions hosting the Pechenga Ni–Cu sulphide deposits, lower down in the stratigraphy, are petrogenetically related to ferropicritic volcanics. Volcanic rocks of the Southern Pechenga Complex extruded from six autonomous, contemporaneous volcanic centres. lcanic rocks belong to two geochemical cycles, each starting with tholeiitic and ending with calc-alkaline magmas. These changes reflect magmatic responses to major tectonic phases of crustal rifting and closure. Changes in the Sr and Nd isotope ratios indicate that the magmatic melts originated in various deep mantle and mantle-crustal source areas. A systematic, gradual geochemical change is observed between subsequent volcanic formations, notwithstanding the fact that the deposition of such volcanic formations is commonly interrupted by sedimentary events. Changes in the magmatic source were therefore gradual and the magmatic processes were continuous. la Greenstone Belt formed as an intracontinental rift (Kalevian rifting event) that has undergone a complex tectono-metamorphic evolution during the Karelian Orogenic Megacycle (2550–1700 Ma) of the northern Baltic Shield. The history of two complete tectono-magmatic cycles (the Lapponian Cycle and the Svecofennian Cycle) within this megacycle is recorded in the volcano-sedimentary successions of the Pechenga Fragment. mplete Lapponian Cycle is embodied within the volcanic sequence of the Imandra–Varzuga Fragment, where the composition of the volcanic and volcano-sedimentary rocks range from tholeiitic and boninitic basalts to basaltic-andesites, calc-alkaline rhyodacite, boninitic picrite-basalts and basaltic-andesites, subalkalic trachybasalts and trachyandesites. Here, the subsequent Svecofennian Cycle is represented by tholeiitic basalts and ferropicrites, followed Al-rich basalts and concluded by dacite, rhyolites, trachyandesites and trachytes. the final phases of the Kalevian rifting event, tholeiitic and ferropicritic magmas were generated from two distinct mantle sources. The tholeiitic magma was derived from a subabyssal, depleted mantle source. In contrast, the ferropicritic magma came from an abyssal, metasomatically enriched mantle source. The larger degree of mantle contamination in the latter was probably critical in the formation of the Ni–Cu ores of the Pechenga gabbro-wehrlite intrusions.