The textures and genesis of metamorphic pyroxene in the Freetown Intrusion

Abstract

AbstractThe Freetown Intrusion, Sierra Leone, evolved by recurrent injection of olivine-bearing magma producing an aggregate thickness greater than 6000 m of rhythmically layered rocks. Sedimentation and constrained viscous flow of crystallizing magma concentrated olivine crystals during initial formation whilst concentration of volatiles locally lowered residual melt temperatures. This affected the crystallization of pyroxenes especially in pyroxene-troctolites where abundance of olivine caused deferred pyroxene growth. Three examples are described: (1) Flattened and elongated schlieren of augite enclosing plagioclase in ophitic intergrowths show preferred orientation cutting across primary layering defined by olivine, proving that pyroxene growth was secondary and influenced by stress directions in a relatively rigid rock. (2) In some pyroxene-troctolites, augite is segregated into coarse ophitic nodules which are widely and evenly spaced in a troctolitic host rock. Successful pyroxene nucleation and extensive diffusion caused development of domains of equilibration within a ‘cotectic’ pyroxene-plagioclase-volatile system. Crystallization within this was deferred compared with the surrounding olivine-plagioclase assemblage in which effects of granulitization are conspicuous. (3) Occasionally the pyroxenes are segregated in dispersed lenticular bodies of noritic pegmatite in troctolite. As in the previous cases, growth of pyroxene involved resorption and regrowth of olivine. This reflects the highest temperatures of crystallization in equilibrium with volatile-enriched pegmatite fluids. Temperatures decreased inwards giving a zonal structure and late growth of biotite, orthoclase, and apatite.The formation of late crystalline pyroxenes and pegmatites thus involved extensive diffusion of volatile and other components accompanying the establishment of domains of reduced pressure in hot and slowly cooled rocks subjected to gradual deformation due to progressive subsidence of the central parts of the intrusion. The final fabric is therefore metamorphic resulting from annealing rather than crystal accumulation.