Metamorphic Segregation Banding in the Loch Kerry Basite Sheet from the Lewisian of Gairloch, Ross-shire, Scotland

The Loch Kerry basite varies from amphibolite through stripedvarieties to strongly banded amphibolite containing many lensesof hornblendite and bands of plagioclasite. The striping andbanding is interpreted as the result of metamorphic segregationwith the complementary development of mafic and felsic types.The geochemical changes are set out and discussed on the basisof thirty-one new rock analyses. Foliation planes acted as pressure minima and differentiationdue to heterogeneous pressure led to the development of finelystriped amphibolite and subsequently to banded amphibolite.With continued deformation, the mafic parts of the banded assemblageacted passively while the felsic material moved along the foliationplanes. By preferential residual enrichment, hornblendite bandsup to 380 metres long and 30 metres across were formed. Theextent to which segregation progressed appears to have beencontrolled by the intensity of deformation in and adjacent toa zone of intense movement between a crystalline basement blockand cover rocks of basite and metasediment.