The Mullnäset rapakivi granite, its chemical and mineralogical composition including a comparison with the Sörvik granite, west central Sweden.

Abstract

This contribution presents complimentary mineralogical and whole-rock chemical data for the Mullnäset rapakivi massif. Three rock types occur: gabbro, syenite with associated granite and “normal” rapakivi granite, here denoted rapakivi-textured granite. In the syenite-associated granite, extremely Mg-poor clinopyroxene and fayalite are the most important mafic minerals, whereas ferrohornblende and strongly chloritized biotite are most important in the rapakivi-textured granite. All mafic minerals are Fe-extreme. The two rapakivi granites are metaluminous straddling the boundary between calc-alkaline and alkaline granite, whereas the neighbouring, older Sörvik granite is peraluminous and typically calc-alkaline. The Mullnäset rapakivi granite is A-type. It is lower in incompatible trace elements than the Sörvik granite. The HFSE patterns differ between the two granites. In both the rapakivi and Sörvik granites, Archaean rocks form part of the source rocks. The Archaean input seems to be more important in the Mullnäset (and other rapakivi granites from the same general area) granite than in the Sörvik granite. No data contradict the suggested formation of rapakivi granite magma: upwelling mantle melts under a thickened continental crust triggering melting of the continental crust.

Keywords:

• rapakivi granite
• Sörvik granite
• source rock
• whole-rock chemistry
• mineral chemistry
• Mullnäset
• Jämtland
• west central Sweden

Acknowledgements

Elisabeth Thun performed the X-ray fluorescence whole-rock analyses, and Peter Stutz made the thin sections, both at the Mineralogisches-petrographisches Institut, Universität Hamburg, Germany. Prof Roland Vinx is thanked for making the Hamburg facilities available to me. Comments from Ulf Bertil Andersson and an anonymous reviewer are acknowledged.

Additional information

Funding

This study was partly financed with a grant from NFR/VR.