THE ROLE OF STRAIN LOCALIZATION IN THE SEGREGATION AND ASCENT OF ANATECTIC MELTS, NAMAQUALAND, SOUTH-AFRICA

Granulite-facies gneisses of the late-Proterozoic Okiep Copper Distric t of the Namaqua Province in South Africa preserve evidence of a range of anatectic melt features that reflect the initial stages of segrega tion and ascent of crustally derived magmas during high grade metamorp hism. These melt bodies include both in situ and sharply transgressive , subvertical, pipe-like bodies, that show vertical dimensions of seve ral hundreds of metres and horizontal dimensions of tens to hundreds o f metres. Migmatite bodies are spatially closely linked with narrow, u pright zones characterized by intense high-strain fabrics, locally ref erred to as 'steep structures'. They display a progressive textural de velopment, from diktyonitic textures in in situ bodies, via schollen-a nd-raft textures to largely homogeneous intrusive granites that have m igrated vertically for distances of several hundreds of metres or more . The intimate association of the migmatite bodies with the steep stru ctures reflects strain-induced melt segregation, facilitated by increa sed permeabilities in these ductile deformation zones, and migration o f melts into sites of strain incompatibility at the intersections betw een the regional subhorizontal gneissosity and the superimposed high-s train zones. Subsequent melt migration was focused along the network o f subvertical structural anisotropies provided by the steep structures . Melt migration was controlled by a combination of buoyancy, shear-en hanced melt compaction during ongoing deformation, melt compaction due to the settling of wall-rock fragments from higher stratigraphic leve ls and subordinate brittle fracturing. The unusual geometry of the ste ep structures and the intensely heterogeneous nature of the strain, an d the absence of similar strain features and voluminous melt bodies el sewhere in the granulite-facies terrane, suggest a positive feed-back mechanism between melt generation and strain localization in steep str uctures. The structural development of the migmatite bodies illustrate s that the efficiency of melt segregation acid migration in a mid-crus tal segment is dependent not only on the fertility of its lithologies, but also on its deformational style. (C) 1998 Elsevier Science Ltd.