COVERING THEIR TRACKS - COLLAPSED MAGMA-DRAINAGE NETWORKS IN THE MIDDLE CRUST

The formation of large granitic plutons in the mid- to upper-crust requires the concentration of large magma volumes from diffuse source regions and the rapid transfer thereof, through largely subsolidus crust, to shallower emplacement sites. Yet, magma pathways linking anatectic lower-crustal levels with the shallower emplacement sites of granitic plutons remain elusive.

Exceptional exposure of mid-crustal (5 kbar, 700 °C) gneisses in the Pan-African (550-500 Ma) Damara Belt of Namibia preserve a km-scale network of intrusive granite sheets that record former magma transfer pathways. The interconnected networks are comprised of foliation parallel and foliation oblique granite sheets and culminate in large (up to > 30 m wide and > 100 m long), subvertical, lensoid granite pipes. Granites in the subvertical pipes show multiple intrusive relationships and contain variable amounts of granite, but are most obvious where they are developed as wide blows that commonly suspend large, dismembered and rotated wall rock blocks. In contrast, the former existence of pipes is recorded by foliation collapse structures and the presence of dismembered and rotated fragments of wall rocks separated by only small amounts of granite. The resulting breccia-like textures and foliation collapse structures suggest deflation during the evacuation of magma from the subvertical pipes, emphasizing the transiency of magma transfer.

We suggest that the fracture networks preserve the inflation and subsequent magma escape and closure of magma-filled fractures that have accommodated rapid magma extraction from the mid-crust. Such fracture networks have the potential to rapidly transfer large quantities of granitic magma, yet they may remain largely undetected, leaving behind only subtle evidence that is most easily detectable in cross section.