HR: 08:45h
AN: V11A-04    [PDF]
TI: Layering by Self-Made Kinetic Sieves Through Selective Phase Coarsening: Dais Intrusion, McMurdo Dry Valleys, Antarctica
AU: * Marsh, B D
EM: bmarsh@jhu.edu
AF: Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218
AB: The Dais Intrusion is a small (400m thick), sheet-like, ultramafic layered body (anorthosite \& orthopyroxenite) that is a lobe of the Basement Sill in Wright Valley. Although its overall form, layering and composition are similar to much larger bodies, like Dufek and Bushveld, because of its size it solidified unusually quickly (1000 yrs.), quenching in original, seldom seen, textures prior to serious annealing. As is abundantly clear from its association with the pervasive regional Opx Tongue of the Basement Sill, the Dais body formed by progressive infilling and sorting during flow and avalanching of massive amounts of entrained cumulus opx and finer plagioclase. The large opx (up to 5 x 20 mm) acted during shear as a sieve to the small sugary plagioclase (0.1 mm), forming abundant stringers and layers of anorthosite up to 50 cm thick and leaving overlying massive brows of concentrates of pyroxenite. Overall, there is organization on many scales. The body as a whole gets increasingly felsic upward from massive orthopyroxenite at the base to felsic dolerite at the top, showing both modal and cryptic layering. Within this there are three major felsic bands, each 4-5 m thick, containing distinct small scale, 2-3 cm, layers. Scattered throughout the sequence are numerous well-sorted anorthositic layers (white, buff \& tan sandstone-like rock) that laterally pinch in and out in continuing at precise horizons across the body. There are also innumerable delicately sorted pockets and one large trough. Regardless of position within the body, the anorthositic layers are fine grained and the cumulus pyroxene is coarse grained, reflecting more a mechanical sorting process than progressive inward crystallization. Yet there are very curious textures within the fine-grained rocks that reveal the incipient transition to a fully coarse grained body. First, within the purest anorthosite (white) are patches of massive grey rock that in thin section turnout to be large grains (2-3 mm) of interlocking plagioclase. Close inspection shows these large crystals to be formed of many small sub-grains quenched in the process of annealing, evidently through grain boundary diffusion, into single large crystals. Second, the buff and tan anorthosites consist of as much as 50-50 mixtures of plagioclase and opx, all of a similar size. In some of these close packed mixtures, the opx has begun to anneal into large (1-2 mm), optically continuous patches, of opx. These patches are very reminiscent of incipient alumino-silicates forming under hornfels grade metamorphism in pelites. All the inter-granular plagioclase is included in the opx patch and the pattern of included plagioclase grains shows an evolution of the shear environment. Careful inspection of the included grains, show them in the early process of rounding, possibly through dissolution, which is also much as in alumino-silicates. Near the opx patches are satellite opx grains, tenuously connected and optically continuous to the patch, quenched in the process of annexation. Clusters of these opx patches are all of the same optical orientation and collectively they, evidently, form an interconnected 3-D sieve through which the remaining plagioclase are draining to form an enriched (white) anorthosite layer. Remarkable enough, the fine-grained rocks are not only coarsening into massive monomineralic rocks, but in the process they are building the mechanical means to further sort and refine the textures. The important lesson is: Were the Dais Intrusion much thicker, it would have had ample solidification time, like all larger layered bodies, to go to textural completion and erase all vestiges of these fundamental processes.
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting