Abstract

High-grade metamorphic tectonites of the Nimrod Group in the central Transantarctic Mountains compose a major ductile shear zone that formed within the paleo-Pacific margin of Gondwana. Despite demonstrated Precambrian protoliths, the timing of metamorphism and tectonite development has been poorly constrained. Igneous rocks of diverse compositions intrude the Nimrod tectonites. Four intrusive units with incipient to well-developed ductile fabrics yield U-Pb zircon ages of 541-521 Ma, and a nondeformed pegmatite has a U-Pb zircon age of ∼515 Ma. These data show that early Paleozoic Ross magmatism was compositionally, texturally, and temporally more heterogeneous than previously recognized. Fabrics in the igneous rocks are concordant with those in their host tectonites, indicating that Nimrod tectonism was in part synchronous with plutonism. U-Pb ages of 525-522 Ma for metamorphic monazite from two pelitic tectonites support this interpretation. Thus, ductile deformation was in its peak to waning stages between about 540 and 520 Ma. This timing provides compelling evidence for transcurrent basement involvement in oblique plate convergence along the Neoproterozoic to Early Cambrian Antarctic margin of Gondwana.

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