| Summary: | The Jokkmokk granitoid is exposed in a plutonic massif northwest of Jokkmokk in northern Norrbotten. It is light grey to white, fine-grained, with megacrysts of feldspar and glomeroporphyritic hornblende and biotite. Small enclaves of mafic rocks and synplutonic mafic dykes are products of mingling with a coeval and possibly cogenetic mafic magma. A couple of smaller occurrences of Jokkmokk-type granitoid are known south of Jokkmokk. All are spatially associated with metasedimentary rocks of the Norvijaur formation. The Jokkmokk granitoid was previously considered to belong to the c.1.8 Ga Lina S-type intrusive suite, but the Jokkmokk granitoid has a unique calc-alkaline to alkali-calcic, metaluminous to weakly peraluminous character with a steep REE-profile, positive Eu-anomaly, and a low Zr content. U-Pb TIMS zircon dating of the Jokkmokk granitoid gives an age of 1882 +-19 Ma which is within limits the same age as that obtained for the Haparanda suite, but contrary to the Haparanda suite it has a positive epsilon Nd value of 2.8, indicating a more juvenile Palaeoproterozoic character similar to the Jörn suite in the Skellefte district. Compared to other granitoids in the Jokkmokk area it is geochemically most similar to the older 1.93 Ga Norvijaur granitoid. This type of magma seems to be restricted to the palaeoboundary between the Archaean craton in the north and Palaeoproterozoic juvenile crust in the south. Spatial correlation with low angle, south dipping, WNW-trending shear zones and NNE-trending subvertical shear zones, highlight the possibility that this unique magma type is related to transtension in the overriding plate and partial melting in sub-arc mantle wedge during NE directed subduction processes related to the early stages of the Svecokarelian orogen. Possibly magma was ponded in shallow chambers below pull-apart basins in a transtensional regime. This type of setting has been advocated as the potentially most favourable tectonic setting for porphyry copper formation. The Cu-Au-(Mo) ...
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