FIRST GARNET LU-HF AGES FOR THE ROSS OROGEN, ANTARCTICA: GARNET GROWTH PRECEDING ARC MAGMATISM

The Ross orogen is an extensive (~3000 km long) belt of deformed and metamorphosed sedimentary rocks and batholith-scale intrusions, interpreted as the result of subduction of paleo-Pacific lithosphere beneath East Gondwana in the Neoproterozoic–early Paleozoic. Though broadly synchronous with major collisional events of the pan-African Orogeny, details of the metamorphic history (especially the timing) of the Ross orogeny are limited. Garnet Lu-Hf geochronology was employed for the first time in this major orogenic belt to constrain the timing of metamorphism.

In the Walcott Glacier area of the southern Victoria Land segment of the orogen, multiply deformed marbles, calc-silicates, and metapelites host numerous small igneous intrusions. The metapelites are sillimanite-grade and commonly contain garnet. The phase is texturally related to the assemblage grt+qtz+bt+pl±ms±sil. Garnet zoning and mineral compositions indicate this assemblage formed on the prograde path at ~ 600 °C and 0.5 GPa. Four samples were subjected to Lu-Hf analyses, yielding dates between 572–616 Ma (0.7–3%, 2 s.d.; garnet + whole rock or garnet-only isochrons). These results are interpreted to represent the age of garnet-grade metamorphism in this area. The oldest igneous zircon age from this area is ca. 565 Ma, suggesting that garnet-grade metamorphism shortly preceded, or was broadly synchronous with, the earliest Ross-stage magmatism in this area. Our tectonic interpretation is that this segment of the margin underwent crustal thickening (and garnet grade-metamorphism) during the earliest phase of subduction. Continued subduction resulted in the introduction of hot asthenosphere into the mantle wedge and associated thermal maturation and magmatism in the overlying arc.

Ongoing work is focused on measuring the trace element composition and U-(Th-)Pb ages of monazite and zircon from the samples subjected to Lu-Hf geochronology. Together, these data will provide a detailed P-T-t path for this segment of the Ross Orogen, which will be of crucial importance to reconstructing the subduction-exhumation history of this extensive orogenic belt.