Abstract
Alkali metasomatism is a common overprint in Archean medium- to high-grade metamorphic terranes. Since mafic-intermediate rocks are able to host ore mineralization, it is important to understand the effect of metasomatism on its distribution. This study presents field, petrographic, mineralogical, geochemical, and U-Pb/40Ar-39Ar geochronologic characteristics of metasomatized amphibolite, amphibole gneiss and gabbro along a regional E-W section of the southeastern Motloutse Complex in eastern Botswana. The progressive metasomatism accompanies shear deformation and is visually manifested by varying degrees of pinkish discoloration of the host rocks. The metasomatic reactions in amphibolite, amphibole gneiss and gabbro are manifested in replacement of amphibole, clinopyroxene, ilmenite and plagioclase by the assemblages of chlorite, epidote, titanite, magnetite, sericite, albite and K-feldspar, up to near complete albitization and K-feldspathization with few relics of earlier minerals. Mineral compositions reflect the effects of a progressive metasomatic overprint. The metasomatic overprint resulted in the break down and dispersal of primary Cu-Ni sulphides. The pseudosection modelling (PERPLE_X) in terms of lg(aK2O) and lg(aH2O) parameters for representative compositions of leucocratic and melanocratic amphibolites, amphibole gneiss and gabbro indicates that the metasomatic process involved an increase of potassium activity followed by hydration. The modelling explains variable extent of metasomatism in different rock types. Whole-rock compositional variations indicate a general enrichment in Na2O, K2O and SiO2, and depletion in FeO and MgO in rocks during the metasomatism. 40Ar/39Ar amphibole and U-Pb titanite geochronology from metasomatized amphibolite and gabbro gave comparable ages of c.2.01–1.96 Ga. Alkaline fluids producing metasomatism were likely of crustal origin and moved along the reactivated regional c.2.01–1.95 Ga shear zones at the boundaries of the Motloutse-Limpopo complexes with the adjacent Zimbabwe and Kaapvaal cratons. Implications for a Paleoproterozoic Large Alkali Metasomatic Province (LAMP) in southern Africa, and its relation to the Bushveld large igneous province (LIP) is explored.
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