Melt transport and magma accumulation in a migmatite-cored gneiss dome, Fosdick Mountains, West Antarctica.

Abstract

Expansive cliff-face exposures in the Fosdick Mountains (Antarctica) migmatite-cored gneiss dome enabled the documentation of suprasolidus deformation and melt migration structures in middle to lower crustal rocks at a scale significant for continental differentiation and crustal flow. Using field relations, U-Pb geochronology, and argon thermochronology, work in the Fosdick dome has examined the role of oblique deformation in the flow of partially molten crust, the internal segregation and emplacement of granite, and the relation between oblique detachment systems and exhumation of migmatite terrains. The Fosdick dome developed in the mid-Cretaceous during oblique plate convergence along the East Gondwana margin of Marie Byrd Land-New Zealand. Structures and fabrics in the dome record the middle to lower crustal response to intracontinental crustal extension and a transition from wrench to transtension associated with development of the West Antarctic Rift System. In the mid-Cretaceous, the dome was constructed and exhumed within a dilation zone that developed along an inferred, crustal-scale, dextral strike-slip fault owing to contrasting competency of granodiorite and metasedimentary gneiss. The pressure gradients created by the opening of the dilation zone induced melt migration into the low-pressure region, forming migmatites and leucogranite sheets. The opening of the dilation zone, influx of melt, and the accumulation of leucogranite sheets initiated movement on the dextral normal oblique South Fosdick Detachment zone, which led to cooling and exhumation of the dome. Mid-crustal migmatites and granites comprise the Fosdick dome. Residual metatexitic paragneiss, metatexitic orthogneiss units, and diatexite migmatites form an ~5 km thick section that partially preserves the lithologic heterogeneities developed in the Paleozoic. An ~2 km thick subhorizontal leucogranite sheeted complex was emplaced above the migmatites. The leucogranite sheets grade into, and syntectonically intrude, a km thick metatexitic migmatite unit that preserves solid-state fabrics related to the South Fosdick Detachment zone. Interconnected leucosome and leucogranite networks are the remnant pathways of a polyphase permeability network that allowed the transport of melt through, and the accumulation of magma in, the crustal layer of the Fosdick dome. In the Fosdick dome, lineations and fold axes record a stretching axis oriented 235 ± 5. Steep foliation domains that host granite and leucosome strike NE-SW. These foliations are crosscut and folded by subhorizontal foliation domains that strike ENE-WSW and also host granite and leucosome. The stretching axis of 055-235 is oblique to the strike of the bounding strike-slip fault, oriented 100-280, and the long axis of the dome, oriented 080-260. Solid-state structures in the detachment zone that trend NE-SW and record top-to-the-SW motion are overprinted by brittle structures that record N-S stretching, consistent with N-S stretching brittle structures in the hanging wall rocks. These fabrics and structures record a transition from wrench deformation to extension-dominated transtension within the Fosdick dome related to oblique plate convergence along the East Gondwana margin and development of the West Antarctic Rift System. U-Pb SHRIMP zircon ages of leucosome and granite indicate a 15 myr period of suprasolidus deformation between ca. 117-102 Ma. Steep foliation domains host an older group of leucosome and granite with ages between ca. 117-115 Ma, whereas subhorizontal foliation domains and the detachment zone host younger leucosome and granite with ages between ca. 109-102 Ma. These ages suggest the transition from wrench to extension-dominated transtension and detachment development occurred within 6 myr. Steep domains preserve steep former melt pathways that are relatively minor in abundance, and this may indicate melt transport through the crustal level of the Fosdick dome during wrench. Subhorizontal domains host thick and laterally extensive leucogranitic sheets, possibly suggesting magma accumulation during extension-dominated transtension. U-Pb SHRIMP zircon and titanite ages on discordant felsic and mafic dikes and 40Ar/39Ar biotite and amphibole cooling ages record cooling between ca. 101-97 Ma, suggesting rapid cooling of the Fosdick migmatites and granites. Rotation of the strain field that created a pressure gradient for the accumulation of magma and initiation of detachment-related exhumation is inferred to have caused rapid cooling of the rocks in the Fosdick dome.