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

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 87Sr/86Sr = 0.7042) and dacite volcanics, peridotite–pyroxenite–gabbronorite (2.50–2.43 Ga, εNd = −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, 87Sr/86Sr = 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 (87Sr/86Sr = 0.7032; εNd = +1.6) and shallow tholeiitic basalt (87Sr/86Sr = 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 (1.98–1.90 Ga, 87Sr/86Sr = 0.7029; εNd = +1.5), and cogenetic, peridotite – olivine gabbro dyke swarms (1.96 Ga, εNd = +1.4), which are characterized by the elevated Fe, Ti, P, and light rare earth element contents. Intrusions of sulfide-bearing gabbronorite and websterite formed in the Lapland–Kolvitsa granulite belt, which experienced collision and high-grade (6–10 kbar (1 kbar = 100 MPa)) metamorphism. During the final stage (1.95–1.70 Ga), enclosure and orogenesis of the paleorift system took place; these events were accompanied by extensive development of mixed mantle–crustal and crustal sources, the formation of calc-alkaline volcanic and sedimentary orogenic associations, and the emplacement of P–Ti-bearing alkaline gabbro – nepheline syenite and U – Mo-bearing monzonite–granodiorite intrusions.

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Rift-wide correlation of 1.1 Ga Midcontinent rift system basalts: implications for multiple mantle sources during rift development

Canadian Journal of Earth Sciences ◽

10.1139/e17-041 ◽

1997 ◽

Vol 34 (4) ◽

pp. 504-520 ◽

Cited By ~ 53

Author(s):

Suzanne W. Nicholson ◽

Klaus J. Schulz ◽

Steven B. Shirey ◽

John C. Green

Keyword(s):

Mantle Source ◽

Lake Superior ◽

Flood Basalt ◽

Great Depth ◽

Rift System ◽

Midcontinent Rift ◽

Western Lake ◽

Isotopic Analyses ◽

Mantle Sources ◽

Midcontinent Rift System

Magmatism that accompanied the 1.1 Ga Midcontinent rift system (MRS) is attributed to the upwelling and decompression melting of a mantle plume beneath North America. Five distinctive flood-basalt compositions are recognized in the rift-related basalt succession along the south shore of western Lake Superior, based on stratigraphically correlated major element, trace element, and Nd isotopic analyses. These distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other MRS localities around western Lake Superior. Four of these compositions are also recognized at Mamainse Point more than 200 km away in eastern Lake Superior. These regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood-basalt magmatism during rift development, extending a model originally developed for eastern Lake Superior. In this refined model, the earliest basalts were derived from small degrees of partial melting at great depth of an enriched, ocean-island-type plume mantle source (εNd(1100) value of about 0), followed by magmas representing melts from this plume source and interaction with another mantle source, most likely continental lithospheric mantle (εNd(1100) < 0). The relative contribution of this second mantle source diminished with time as larger degree partial melts of the plume became the dominant source for the voluminous younger basalts (εNd(1100) value of about 0). Towards the end of magmatism, mixtures of melts from the plume and a depleted asthenospheric mantle source became dominant (εNd(1100) = 0 to +3).

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Re-Os isotopic compositions of Midcontinent rift system picrites: implications for plume – lithosphere interaction and enriched mantle sources

Canadian Journal of Earth Sciences ◽

10.1139/e17-040 ◽

1997 ◽

Vol 34 (4) ◽

pp. 489-503 ◽

Cited By ~ 58

Author(s):

Steven B. Shirey

Keyword(s):

Isotopic Composition ◽

Rift System ◽

Magmatic Activity ◽

Midcontinent Rift ◽

The North ◽

Enriched Mantle ◽

Mantle Sources ◽

Midcontinent Rift System ◽

Sedimentary Fill ◽

Nearly Continuous

Picrites and tholeiites from the Mamainse Point Formation, a 5.3 km thick section of Keweenawan (1100 Ma) volcanic and sedimentary fill on the eastern flank of the central portion of the Midcontinent rift system, contain a nearly continuous record of rift magmatic activity. Picrites occur primarily in the lowermost two units of the formation. In this study, they are compared to rarely exposed, slightly older Keweenawan basalts from the North Shore Volcanic Group and the Powder Mill Group to constrain mantle source compositions during early phases of rift magmatic activity. The most primitive picrites analyzed have low Re content (0.069–0.18 ppb), high Os content (0.8–2.1 ppb), and low 187Re/188Os (0.28–1.18). A Re–Os isochron with an age of 1128 ± 54 Ma and an initial 187Os/188Os of 0.1267 ± 0.0013 (γOs = +5.7) was obtained from a 24-point isochron on all but two analyzed samples. The Re–Os data, regressed separately for the older basalts, and the groups 1 and 2 samples from the Mamainse Point Formation, have barely resolvable initial 187Os/188Os that decrease up-stratigraphy from initial γOs(1100) of +12.2 to +6.2 and +4.2, respectively, and couple with changes in initial Nd isotopic composition. These data can be explained by mixing of melts of an enriched mantle plume and unradiogenic continental lithospheric mantle. A radiogenic initial Os isotopic composition (γOs of +8 or higher) for the Keweenawan plume marks the first known appearance of demonstrably radiogenic plume-derived magmas on Earth. Plume-derived magmas with radiogenic Os signatures are more common later. The radiogenic Os signatures of Keweenawan plume magmas may mark the appearance of melts derived from mantle containing recycled slab components from late Archean subduction.

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Origin of evolved magmas in the Midcontinent rift system, northeast Minnesota: Nd-isotope evidence for melting of Archean crust

Canadian Journal of Earth Sciences ◽

10.1139/e17-042 ◽

1997 ◽

Vol 34 (4) ◽

pp. 521-535 ◽

Cited By ~ 20

Author(s):

Jeffrey D. Vervoort ◽

John C. Green

Keyword(s):

Volcanic Rocks ◽

Bimodal Distribution ◽

Rift System ◽

Midcontinent Rift ◽

Nd Isotope ◽

The North ◽

Midcontinent Rift System ◽

Isotope Evidence ◽

Mantle Component ◽

Archean Crust

The North Shore Volcanic Group (NSVG) of northeast Minnesota is a thick (9 km) sequence of plateau volcanic rocks that constitutes an important part of the Midcontinent rift system. This volcanic sequence is unique among the Midcontinent rift lavas, because it is composed of up to 25% rhyolite flows. We have analyzed Sm- and Nd-isotope compositions of 20 of the largest rhyolite and icelandite flows from the NSVG and seven comparably sized granophyres in the subjacent Duluth and Beaver Bay complexes. The lavas vary in composition from primitive basalt and basaltic andesite to icelandite and rhyolite, with a bimodal distribution. The rhyolites have much lower initial εNd values (−2 to −15, most samples < −10) than either the icelandites (0 to −6) or granophyres (0 to −8). Most rhyolites cannot be related to either the icelandites or more mafic magmas by simple fractionation, but rather have been produced by melting and assimilation of older, evolved crust. We suggest that the bimodal magmatism in the NSVG, and probably throughout the Midcontinent rift, has been produced by two fundamentally different processes. The bulk of the magmatism is basaltic; magmas originate in the mantle and migrate through the lithosphère with minor compositional change. Assimilation and fractional crystallization occur to varying degrees in the crust and, in some cases, produce icelandites, some small-volume rhyolites, and the granophyres, with Nd compositions dominated by the mantle component. The melting that produced the large-volume rhyolites is the result of a multistage process induced by these mantle-derived magmas that pond within the crust. This process appears to occur during a period of slowed extension and causes widespread heating and eventually localized extensive melting of the crust.

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