Plume related MORB and high-Ti basalts in the Kalix greenstones, northern Sweden

• Main Authors: Martinsson, Olof, Billström, Kjell
• Format: Conference Object
• Language: English
• Published: Luleå tekniska universitet, Geovetenskap och miljöteknik 2006
• Subjects: Geology; Geologi; Kalix; Kautokeino; Kiruna; Lofoten; Fennoscandian; Northern Sweden
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-31988

Paleoproterozoic greenstones are widely distributed in the northeastern part of the Fennoscandian Shield. Due to obvious similarities of the local stratigraphy and the mainly tholeiitic character of the mafic volcanic rocks, Pharaoh (1985) suggested these greenstone areas to be coeval and representing a major tholeiitic province. Based on petrological and chemical studies of the mafic volcanic rocks and associated sediments, a continental rift setting is favoured for these greenstones. According to the present distribution of Archean rocks the paleocontinent was rifted in a northwest-southeast direction from Lofoten to Ladoga and a passive margin with deposition of greywacke developed at 2.1-2.0 Ga.In the Kalix area in northern Sweden well preserved Paleoproterozoic greenstones occur 100 km inland from the rift margin. Basaltic lava interlayered with volcanoclastic rocks and dolomite is exposed in a more than two km thick sequence that is overlain by graywackes. The chemostratigraphy of the Kalix greenstones including Sm-Nd isotopes suggest variable magma sources and progressive changes during magma evolution. Most of the basaltic lava and the volcanoclastic rocks have a depleted mantle source, a low to moderate content of Ti, and a chemical composition resembling MORB. However, extremely TiO2 rich (5.0-6.0 %) basaltic lava and tuff occur in a 150 m thick unit in the upper part of the greenstone pile. These are succeeded by basalt with a transitional character interlayered with stromatolitic dolomite. A major change in depositional environment is then recorded by the overlying graywackes. The lithostratigraphy and the chemostratigraphy of the greenstones are suggested to monitor the magmatic evolution during the final stage of continental break up and the interaction with a mantle plume that caused extrusion of thick units of volcanic rocks in the Kalix-Kiruna-Kautokeino-Kittili area. Godkänd; 2006; 20111007 (andbra)