Contact partial melting of granitic country rock, melt segregation, and re-injection as dikes into Ferrar dolerite sills, McMurdo Dry Valleys, Antarctica

Abstract

Numerous, interconnected, granitic dikes (<30 cm in width and hundeds of meters in length) cut Ferrar dolerite sills of the McMurdo Dry Valleys, Antarctica. The source of the granitic dikes is partial melting of granitic country rock, which took place in the crust at a depth of about 2-3 km adjacent to contacts with dolerite sills. Sustained flow of doleritic magma through the sill generated a partial melting front that propagated into the granitic country rock. Granitic partial melts segregated and collected at the contact in a melt-rich, nearly crystal-free reservoir adjacent to the initial dolerite chilled margin. This dolerite chilled margin was subsequently fractured open in the fashion of a trapdoor by the granitic melt, evacuating the reservoir to form an extensive complex of granitic dikes within the dolerite sills. At the time of dike injection the dolerite was nearly solidified. Unusually complete exposures allow the full physical and chemical processes of partial melting, segregation, and dike formation to be examined in great detail. The compositions of the granitic dikes and the textures of partially melted granitic wall rock suggest that partial melting was characterized by disequilibrium mineral dissolution of dominantly quartz and alkali feldspar rather than by equilibrium melting. It is also unlikely that melting occurred under water-saturated conditions. The protolith granite contains only ∼7 vol.% biotite and estimated contact temperatures of 900-950°C suggest that melting was possible in a dry system. Granite partial melting, under closed conditions, extended tens of meters away from the dolerite sill, yet melt segregation occurred only over less than one-half a meter from the dolerite chilled margin where the degree of partial melting was of the order of 50 vol.%. This segregation distance is consistent with calculated length scales expected in a compaction-driven process. We suggest that the driving force for compaction was differential stress generated by a combination of volume expansion as a result of granite partial melting, contraction during dolerite solidification, and relaxation of the overpressure driving dolerite emplacement. On a purely chemical basis, the extent of melt segregation necessary under fractional and batch melting to match the Rb concentrations between melt and parent rock is a maximum of 48 and 83 vol.% melt, respectively. © The Author 2007. Published by Oxford University Press. All rights reserved.