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Discussion papers
https://doi.org/10.5194/se-2017-3
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
https://doi.org/10.5194/se-2017-3
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 23 Jan 2017

Research article | 23 Jan 2017

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This discussion paper is a preprint. It has been under review for the journal Solid Earth (SE). The revised manuscript was not accepted.

Unravelling the internal architecture of the Alnö alkaline and carbonatite complex (central Sweden) using 3D models of gravity and magnetic data

Magnus Andersson and Alireza Malehmir Magnus Andersson and Alireza Malehmir
  • Department of Earth Sciences, Uppsala University, Villavägen 16, SE-75236, Uppsala, Sweden

Abstract. The Alnö complex in central Sweden is one of the largest alkaline and carbonatite ring-shaped intrusions in the world. Presented here is the 3D inversion of ground gravity and aeromagnetic data that confirms some of the previous ideas about the 3D geometry of the complex but also suggests that the complex may continue laterally further to north than earlier expected. The gravity and aeromagnetic data show the complex as (i) a strong positiver Bouguer anomaly, around 20 mGal, one of the strongest gravity gradients observed in Sweden, and (ii) a strong positive magnetic anomaly, exceeding 2000 nT. Magnetic structures are clearly discernible within the complex and surrounding area. Petrophysical measurements (density, bulk magnetic susceptibility, and magnetic remanence) were used to constrain the 3D inversion. Both gravity and magnetic inversion models suggest that dense (> 2850 kg/m3) and magnetic (> 0.05 SI) rocks extend down to about 3.5–4 km depth. Previous studies have suggested a solidified magma reservoir at this approximate depth. The inversion models further suggest that two apparently separate regions within the intrusion with gravity and magnetic highs are likely connected at depth, starting from 800–1000 m, implying a common source for the rocks observed in these two regions. The modelling of the aeromagnetic data indicates that a more than 3 km wide ring-shaped magnetic high in the bay that can be a hidden part of the complex, linking a satellite intrusion in Söråker on the northern side of the bay to the main intrusion on the Alnö Island. While the rim of the ring must consist of highly susceptible rocks to support the magnetic anomaly, the centre has a relatively low magnetisation and is probably made up of low-susceptible wall-rocks or metasomatised wall-rocks down to about 2 km. Below this depth the 3D susceptibility model shows higher magnetic susceptibility values. From these observations the solidified magma chamber is interpreted to extend further to north than has previously been suggested.

Magnus Andersson and Alireza Malehmir
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Interactive discussion
Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement
Magnus Andersson and Alireza Malehmir
Magnus Andersson and Alireza Malehmir
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Short summary
Magmatic rocks in the Alnö alkaline and carbonatite complex of central Sweden are unique and likely to be found only in a few places in the world. In this study, plausible 3D geometry of the intrusion based on 3D modelling of gravity and magnetic data constrained by petrophysical measurements is given. We support that the intrusion may extend down to about 3–4 km depth but also suggest that it may have some lateral continuation, as a major volcanic centre, in the bay off from the Alnö Island.
Magmatic rocks in the Alnö alkaline and carbonatite complex of central Sweden are unique and...
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