Wholesale, massive partial melting and melt separation in granite host rock, McMurdo Dry Valleys, Antarctica

During Gondwana breakup, over 104 km3 of basaltic magma was erupted and emplaced through a deep-seated magmatic mush column forming the Ferrar Igneous Complex (FIC) of the McMurdo Dry Valleys. The FIC consists of a 3.5 km interconnected stack of four thick (~350 m) and extensive (~100 km) sills capped by a series of flood basalts. The lowermost sill (Basement Sill) is floored and hosted by a complex of granitic plutons, which generally has undergone no melting at sill contacts. In the vicinity of the newly discovered central feeder vent in central Bull Pass, however, the granitc wall rock is extensively and thoroughly melted for over 40 m above the dolerite contact. The transition from essentially fully melted to un-melted rock is 100% exposed over many kms along the contact, delineating a huge block of crust, perhaps the foot of a massive caldera block, that evidently floated in the sill. Moreover, mafic dikes in the country rock vanish in the melt zone, indicating that the sliding block produced enough shear to stretch, thin, and erase the dikes. The degree and extent of melting clearly reflects the feeding locus for the establishment of the FIC. Besides the loss of dikes, it is the full loss of the coarse granitic texture and the bulk chemical profile that are most distinctive. The partial melt zone thus basically consists of two essential features: post melting residuum granite and granophyric segregations, reflecting locally separated melt. The residuum granite consists of varying amounts of unmelted feldspars and large (5-10 mm) distinctive globular clusters of quartz grains, thermally rounded by partial melting and tightly rimmed with biotite and iron oxides that fade away into unmelted granite. With distance from the contact the style and abundance of the granophyre segregations change in response to the degree of melting. Near the contact, they form irregular, bulbous masses up to several meters across, sometimes containing xenoliths of residuum granite. Beyond about 15 m they form large aspect ratio sills, parallel to the contact, 10s of meters long and a few centimeters thick. This variation seems to reflect melting relative to the point of critical crystallinity at about 55 vol.%. At low crystallinities, melt and crystals have freely moved relative to one another, strongly segregating the melt. And at high crystallinities, the melt segregates in response to shear, becoming almost migmatitic. This overall process of melt production and segregation is closely reflected by the bulk chemical variations recorded by the residuum granite spatially throughout the melt zone. These compositions compare in most cases favorably with melt components predicted by MELTS as a function of degree of melting, which implies a close relation between melt production and distance and spatial temperature variations. In addition to the intimate record of melts and melt segregation, which allows standard models of melting to be evaluated, this allows the duration of magma flow to be gauged and related to the establishment of the Ferrar Igneous Complex itself.