GRANITIC DIKES GENERATED BY AND INJECTED INTO DOLERITE SILLS; MCMURDO DRY VALLEYS, ANTARCTICA: FUNDAMENTALS OF CRUSTAL CONTAMINATION OF BASALTIC MAGMATIC SYSTEMS

Well-formed granitic dikes up to 30 cm thick and often over 100 m long cut deeply into the Basement Sill, a massive 350 m thick, 180 Ma Ferrar dolerite sill. The dike magma was formed by partial melting of the granitic country rock over a wide region at the sill margins with the largest and most numerous dikes being in the area of Bull Pass. This is an uncommon and curious process to have occurred in the upper crust where the wall rock is cool and generally cannot be melted by sills. Although exposures are excellent, the exact roots of the dikes have been difficult to discern, but during the past (03’) field season a dike was traced and unearthed at the sill contact where it ends abruptly in a 10 cm wide continuous zone of granitic melt along the contact. The silicic melt has ruptured the well-formed dolerite chilled margin (~15 cm thick), opening it much like a trap door, and propagated deeply into the dolerite. That the dolerite chilled margin was well formed before the dike was formed is clear, and the fact that the granitic melt existed side by side with the obviously solid chilled dolerite sets valuable conditions on the relative temperatures and on the temperature of formation of the granitic melt. This also places an important constraint on the timing between dike injection and final solidification of the dolerite itself. This sequence poses the fundamental question of the driving force of injection. Melting of the granitic wall rock creates a local overpressure due to the volume excess attending melting. At the same time progressive solidification of the dolerite magma causes contraction, fostering inward propagation of cooling joints, which the granitic melt may follow. A local dipole pressure field is established, which in effect coordinates and drives the melt from the wall rock into the dolerite. This sequence is fundamentally important to establish. With 100% exposure, the processes of melt production with distance (i.e., temperature) normal to the contact and melt extraction and transport both normal to and along the contact can be determined. Deeper in the crust where the sill/conduit may become reactivated these dikes represent a pervasive, thorough, and efficient means of contaminating basaltic magma with no later discernable lithologic effect due to fluid deformation and diffusive mixing.