Silicic Melt Generation, Segregation, and Injection by Dolerite Partial Melting of Granitic Wall Rock, McMurdo Dry Valleys, Antarctica

Numerous, long (100's m), thin (< 30 cm), interconnected fine-grained granitic dikes cut Ferrar dolerite sills in the McMurdo Dry Valleys. The source of at least one dike is completely exposed at the upper contact of the Basement Sill and granite country rock. The dike emanates from a thin (5 cm) melt sheet separating chilled dolerite from partially melted granite. Residual interstitial granophyric melt decreases away from the contact from 55% to zero within a distance of < 20 m. Higher than expected dolerite contact temperatures of 900-950°C calculated using two-pyroxene thermometry suggest that the dolerite feeder acted as an open conduit for sustained flux of magma. As a consequence of this flow, the contact temperature was pinned above the `dry' granite minimum, the most restrictive condition necessary to generate granitic melt. Although closed-system partial melting of granite clearly occurred beyond 50 cm from the dolerite chilled margin, compositional moment balances on the feldspar ternary between the orthoclase-enriched melt sheet and granite dike whole-rock compositions are reconciled by melts segregated from increasingly orthoclase-depleted partially melted granite at 12.3 cm and closer to the dolerite chilled margin. Melting models and mass balance calculations predict a range of between 48 to 83% maximum volumes of segregated granitic melt, but these are only estimates as the samples are not exclusively residuum. If granitic melt segregation occurs by viscous compaction of the restitic crystal matrix, then, employing commonly used properties, the compaction length scale is ~3 m. This is an upper bound as the compaction model assumes constant melt fraction, but the result is nevertheless only an order of magnitude larger than the distance over which the partially melted granite has a composition that differs from unmelted granite. Contraction attending cessation of doleritic magma flow and dolerite solidification likely generated deviatoric stresses within the partially melted zone that initiated crack formation at and parallel to the contact, into which, interstitial melts flowed in response to a pore pressure gradient. Excess pore pressure within this granitic melt reservoir along the contact subsequently tore open the brittle dolerite chilled margin like a trap door and emplaced, essentially by evacuation, granitic dikes into the (nearly?) solidified dolerite. Granite partial melting, segregation, and dike emplacement likely occurred within a period of several years as suggested by corroborative evidence from thermal modeling and the time estimated to produce, by interdiffusion between the granitic melt and dolerite, a thin (2 mm) distinctive planar orthopyroxenite zone within the dolerite chilled margin. Reactivation of similarly injected basaltic feeders deeper in the crust, with dikelet stretching and absorption by simultaneous diffusion, presents a viable and efficient means of extensive and subtle crustal contamination of basaltic magma.