Structural, petro-geochemical and modelling constraints on melt migration by porosity waves in sub-arc mantle

Petrographic and structural observations in the mantle lithospheres provided evidence for two end-member processes of melt migration in natural rocks: (1) impregnation features indicate pervasive flow and (2) dykes denote fully segregated flow. Replacive dunite represents an intermediate mode in which porous flow has been channelized. Because the retrograde thermal regime in the upper portion of the mantle wedge is hostile to melt transport, the dominant mechanism by which melts ascend from their source through the mantle remains uncertain. We studied the petro-structural features of melt percolation in a exhumed sub-arc mantle section of the Sapat area in the Kohistan Paleo-Island Arc (NE Pakistan). Our observations indicate a continuum of transport mechanisms ranging from pervasive to fully segregated melt flow: The dominantly harzburgitic mantle section of Sapat exposes tens to hundred of meters size dunitic domains, which comprises clinopyroxene-rich cores associated with gabbroic lenses. The clinopyroxene zones show isolated Cpx blasts, aligned Cpx “trails” and Cpx bands. Gabbro lenses are 3-dimensional lenses terminated horizontally and vertically by clinopyroxene proto-lenses. Proto-lenses refer to Cpx bands prior to plagioclase appearance. Based on bulk and mineral composition, dunite zones formed by orthopyroxene dissolution and olivine crystallization via the peritectic reaction opx + melt = ol; and clinopyroxene-rich parts and gabbros were recognized to have formed from the same melt as dunite. The melt forming gabbros and cpx-rich parts were in near-equilibrium with dunite, but not with the surrounding harzburgite. The structural relationships (i.e parallelism of gabbroic lenses and cpx-trails, and 3D evolution of trails into gabbroic lenses) strengthen the co-genetic origin of the lithologies. Isolated clinopyroxene porphyroblasts evolve into trails that turn into bands in which plagioclase appears. From these observations we infer that the percolation of melt into dunite formed a succession of structures, with increasing melt flux: Isolated clinopyroxene porphyroblasts -> clinopyroxene trails -> clinopyroxene bands -> gabbroic lenses. From the sub-vertically oriented lens-like geometry of the gabbroic lenses and magmatic foliation and lineation we concluded that isolated lenses represent a dynamic mode of melt transport, similar to a self-propagating dyke or porosity wave that has been frozen in place. Indeed, the petrographic-structural features are consistent with numerical two-phase thermo-mechanical model of melt flow, which shows that pervasively distributed melt is channelized by melt-filled waves of porosity. Also, high precision U-Pb ages and Hf isotopic datas on zircons, coupled with bulk rock isotopic analyses, show decoupled isotopic systems, which may be due to this mode of melt transport.