An occurrence of apatite-rich rocks of carbonatitic affinity near the Jotnian graben structure in Gävle, central Sweden

Abstract

An occurrence of apatite-rich rocks of carbonatitic affinity at Främlingshem, SW of Gävle, central Sweden, is reported. Boulders and cobbles of two lithologically related types (dominated by apatite-calcite and chlorite-apatite, respectively) have been found. Major constituents are fluorapatite with a ⁸⁷Sr/⁸⁶Sr ratio of 0.7027, Sr-rich calcite (2600 ppm) with (δ¹³C = -3.9 ± 0.2% vs PDB and δ¹⁸O = 14.9 ± 0.5% vs SMOW, chlorite completely replacing biotite or phlogopite, and fragments of granitoids and other rocks. The apatite-rich rocks were apparently emplaced as a ‘crystal-mush’, at a low temperature. They are post-Svecokarelian in age, probably formed between 1750 Ma and 1100 Ma ago, a timespan comprising two or more periods of tensional movements in central Sweden. The location of the Främlingshem rocks near a graben structure is consistent with their carbonatitic affinity.

Key Words:

• Apalite
• carbonatite
• carbonatitic affinity
• graben structure
• strontium isotope data
• sub-Jotnian
• Jotnian
• Främlingshem
• Gävle
• Alnö
• central Sweden
• N6030
• N6031
• E1655
• E1656