The Farsund Shear Zone: geochemical evidence for lithological diversity in the wall rock of the Rogaland anorthosite province, South Norway

The Rogaland anorthosite province (RAP) – a typical Proterozoic Anorthosite-Mangerite-Charnockite (AMC) plutonic complex exposed in the Sveconorwegian orogen in South Norway – was emplaced diapirically through the crust, along a shear zone, at c. 933–916 Ma. The shear zone, recently defined as the Farsund Shear Zone, is outcropping along the eastern flank of the anorthosite province, and it is made up of banded gneisses, strongly foliated and verticalized. The banded gneisses comprise a diversity of lithologies: metabasites, granitoid gneisses, augen gneisses and kinzigitic gneisses. Major and trace elements composition of samples mostly from the banded gneisses and the neighbouring granite gneiss permit to unravel the nature of the various protoliths. The kinzigitic gneiss result from metamorphism of pelitic sediments, the augen gneisses belong to the high-K calc-alkaline series similar to the Feda suite (c. 1050 Ma), a major component of the Sirdal magmatic belt (SMB, 1070–1020 Ma). The metabasites comprise jotunites comparable to the intermediate rocks of the AMC suite (933–916 Ma) as well as amphibolites and norites with trace element signatures consistent with an oceanic origin. Charnockitic gneisses could result from anatexis in CO2-rich conditions or from fractionation of felsic magma similar to those of the charnockite-granite Farsund intrusion (931–926 Ma). Some leucogranitic layers have typical REE distribution of migmatitic leucosome. Other granite layers can be distinguished from the massive granite gneiss that has higher Th, Pb, Rb concentrations and [La/Yb]N ratios, but all granites display high-K calc alkaline affinities compatible with island arc origins. The granite gneiss is comparable to the SMB rocks and granite layers to the Suldal arc lithologies (c. 1520–1480 Ma). The Farsund shear zone hosts thin interleaved layers of rocks corresponding to the major lithologies exposed at regional scale. This includes jotunites and charnockites probably genetically related to intrusion of the c. 930 Ma anorthosite province, and a variety of granitic gneiss, metasediments, augen gneisses, and mafic rocks with protolith age ranging from c. 1020 Ma to c. 1500 Ma.