Root zone of a continental rift: the Neoproterozoic Kebnekaise Intrusive Complex, northern Swedish Caledonides

Abstract

Mafic magmatic rocks formed between ca. 615 and 560 Ma along the Neoproterozoic margins of Baltica and Laurentia are classically attributed to continental rifting heralding the opening of the Iapetus Ocean. We report new data for the Kebnekaise Intrusive Complex (KIC) exposed in the Seve Nappes in northern Sweden. The KIC consists of sheeted dolerite dykes and gabbroic bodies with mutually intrusive relations. Major and trace element data exhibit a transitional-to normal-mid-ocean-ridge basalt-type geochemical signature. Differentiation processes and late-stage liquid immiscibility of a tholeiitic melt are considered to account for the compositional range exhibited by the KIC igneous rocks. U/Pb SIMS geochronological data from zircon rims yield an emplacement age of 578^9 Ma. The KIC is thus younger and more depleted than coeval mafic rocks found in the Seve Nappe, and is interpreted to represent a highlevel magma plumbing system in a late-stage continental rift. The composition and volume of rift-related igneous rocks in the Seve Nappes are inconsistent with a mantle plume origin, but are thought to record progressive lithospheric thinning and increasing involvement of an asthenospheric mantle component.

Timing of Breakup and Thermal Evolution of a Pre‐Caledonian Neoproterozoic Exhumed Magma‐Rich Rifted Margin

“…Ages of mafic magmatism obtained by CA‐ID‐TIMS generally fall within the time interval 608 to 596 Ma (Figure ; Svenningsen, ; Baird et al, ; Kumpulainen et al, ), suggesting that most of the SDC was emplaced during a short time period similar to the duration of the breakup of the NALIP. A younger SIMS age of 578 Ma has, however, also been found at Kebnekaise (Kirsch & Svenningsen, ).…”

“…Opening of the Iapetus Ocean occurred in stages between about 620 and 550 Ma, beginning with rifting of rocks now found mainly in Caledonian nappes on Baltica and ending with breakup of southern Iapetus between Laurentia and Gondwana (Cawood et al, ). Rifting and opening of the portions of the Iapetus between Laurentia and Baltica is constrained by dating of mafic as well as felsic rift‐related magmatic rocks in sedimentary basins and basement from both the Laurentian side (e.g., Dempster et al, ; Halliday et al, ; Kamo et al, ) and the Baltican side (Baird et al, ; Bingen et al, ; Kirsch & Svenningsen, ; Kumpulainen et al, ; Root & Corfu, ; Svenningsen, ; this study; Figure a). The oldest rift‐related age on the Baltican side is that of the Egersund dikes, 616 ± 3 Ma (Bingen et al, ), whereas the youngest dikes at around 578 ± 9 Ma has been found in the Kebnekaise area (Kirsch & Svenningsen, ).…”

“…Rifting and opening of the portions of the Iapetus between Laurentia and Baltica is constrained by dating of mafic as well as felsic rift‐related magmatic rocks in sedimentary basins and basement from both the Laurentian side (e.g., Dempster et al, ; Halliday et al, ; Kamo et al, ) and the Baltican side (Baird et al, ; Bingen et al, ; Kirsch & Svenningsen, ; Kumpulainen et al, ; Root & Corfu, ; Svenningsen, ; this study; Figure a). The oldest rift‐related age on the Baltican side is that of the Egersund dikes, 616 ± 3 Ma (Bingen et al, ), whereas the youngest dikes at around 578 ± 9 Ma has been found in the Kebnekaise area (Kirsch & Svenningsen, ). However, many ages of both mafic and felsic magmatism fall between 608 and 596 Ma (Figure b), suggesting that although the magmatic record shows prolonged activity, the final breakup of Baltica and Laurentia was punctuated by a major dike emplacement event between 608 and 596 Ma.…”

“…The southern segment includes the areas of Sarek, Kebnekaise, Abisko, and Indre Troms and covers an area of approximately 1,660 km 2 (Figure ). The dike complex in Sarek and Kebnekaise has yielded U‐Pb zircon ages ranging from 608 to 578 Ma (Baird et al, ; Kirsch & Svenningsen, ; Svenningsen, ) and intrudes presumed late Neoproterozoic syn‐rift sediments and basement (Kumpulainen & Nystuen, ; Nystuen et al, ). In the lower part of the stratigraphy, the sedimentary rocks also comprise a carbonate unit with local evaporite deposits (Spika Fm) overlain by thick‐bedded sandstones, locally with more micaceous domains (cf.…”

A Mantle Plume Origin for the Scandinavian Dyke Complex: A “Piercing Point” for 615 Ma Plate Reconstruction of Baltica?

“…570 Ma; Shumlyanskyy et al, ), and several carbonatite and kimberlite complexes (590–520 Ma; Ernst & Bell, , Torsvik et al, ). The parental magmas to the Seiland Igneous Province include picrites similar to OIB and are often explained as deep, hot mantle plume melts (e.g., Larsen et al, ), whereas the origin of the dominantly tholeiitic basalts of the dyke complexes is debated (Andréasson et al, ; Baird et al, ; Hollocher et al, ; Kirsch & Svenningsen, ).…”