The Paleoproterozoic (2.5–1.7 Ga) Midcontinent rift system of the northeastern Fennoscandian Shield

• Published in: Canadian Journal of Earth Sciences
• Main Author: Smolkin, V. F.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Canadian Science Publishing 1997
• Subjects: Fennoscandian; karelian; Lapland
• Online Access: http://dx.doi.org/10.1139/e17-036; http://www.nrcresearchpress.com/doi/pdf/10.1139/e17-036

The Karelian epoch of tectono-magmatic activity resulted in an intensive structural–tectonic rearrangement of Archean crustal blocks, origination, development, and orogenesis of the Pechenga – Varzuga belt. Being emplaced on the Archean crust of the continental type, the Pechenga – Varzuga belt is an intracontinental paleorift system formed during four stages: prerifting (2.55–2.30 Ga), early rifting (2.30–2.20 Ga), late rifting (2.20–1.95 Ga), and orogenic (1.95–1.70 Ga). During the stage of 2.55–2.30 Ga, as a result of formation of an extensive asthenolens whose projection to the surface covered most of the Kola – Lapland – Karelian province, there appeared paleoaulacogen depressions and mantle and crustal magma associations with normal alkalinity: gabbronorite dykes (2.55–2.40 Ga), low-Ti picrite–basalt (2.45–2.41 Ga), basalt, andesite–basalt (initial 87 Sr/ 86 Sr = 0.7042) and dacite volcanics, peridotite–pyroxenite–gabbronorite (2.50–2.43 Ga, ε Nd = −1, −2), lherzolite–gabbronorite (or drusite) (2.45 Ga), and gabbro–anorthosite (2.45 Ga) layered intrusions characterized by chromite, platinum, and titanomagnetite mineralization. As the rocks of Archean blocks were generally warmed up, intracrustal chambers of granitoid magmas were common. During the stage of 2.30–2.20 Ga, the asthenolens broke up and differentiation of its fragments significantly increased. Over the most heated fragments, the paleorift system (Pechenga–Varzuga belt) appeared, accompanied by generation of mantle melts with higher alkalinity (volcanic series of picrite – trachybasalt – trachyandesitic basalts, 87 Sr/ 86 Sr = 0.7035). During the third stage (2.20–1.95 Ga), rifting reached its maximum owing to intense sinistral fault-rifting, and mantle sources of deep ferropicritic ( 87 Sr/ 86 Sr = 0.7032; ε Nd = +1.6) and shallow tholeiitic basalt ( 87 Sr/ 86 Sr = 0.7021) melts formed at different depths; eruption of these magmas gave rise to thick volcanic sequences (1.98 Ga), Ni – Cu-bearing differentiated gabbro–wehrlite intrusions ...