New U-Pb dating of zircons separated from felsic volcanic rocks of the Patuxent and Gambacorta formations from the Pensacola Mountains in the Transantarctic Mountains, Antarctica, yields earliest Ordovician ages of 500 ± 8 and 501 ± 3 Ma respectively. The dated felsic volcanic rock of the Gorecki Felsite Member of the Patuxent Formation is important tectonically, as the felsite, together with mafic volcanic rocks, were previously considered to provide key evidence of a Neoproterozoic rifting event prior to separation of Laurentia from East Antarctica. The new data do not necessarily refute this event but indicate a previously unrecognized early Ordovician period of bimodal magmatism and extension along the Transantaractic Mountains.
Ar–Ar dating of high-strain ductile mylonites of the Eastern Palmer Land Shear Zone in the southern Antarctic Peninsula indicates that reverse movement on the shear zone occurred in late Early Cretaceous times (Albian), and not latest Jurassic times as previously supposed. The Eastern Palmer Land Shear Zone forms a major tectonic boundary, separating suspect arc terranes from rocks of Gondwana continental affinity. The dated mylonites are developed in Lower Jurassic plutonic rocks at Mount Sullivan, eastern Palmer Land, and form part of a zone of ductile reverse deformation up to 25 km wide. Biotite from a fine-grained mafic mylonite yields an Ar–Ar cooling age of 102.8±3.3 Ma. Movement of this age on the Eastern Palmer Land Shear Zone is coeval with circum-Pacific deformation, possibly related to a mantle superplume event, and provides support for allochthonous-terrane models for the Antarctic Peninsula with accretion in post-Early Cretaceous times.
Ellsworth-Whitmore Mountains crustal block (EWM) is a belt of small mountain ranges, hills, and nunataks which trend northward 500 km from the Thiel Mountains of the Transantarctic range to the Ellsworth Mountains. Granitic rocks compose most of the Pirrit Hills, Nash Hills, and Whitmore Mountains, and all of Pagano Nunatak. Mount Woollard is a migmatized complex of biotite schist, amphibolite, pegmatite, and massive biotite granite. A rhyodacite stock crops out in the Martin Hills. The Mount Seelig hornblende-bearing biotite granite of the Whitmore Mountains and the Mount Woollard biotite granite are metaluminous diopside-normative granitoids. All other granitic rocks of the EWM are mildly peraluminous, corundum-normative biotite (locally muscovite-bearing) leucogranites. Previously published radiometric ages range from 163 to 190 Ma. Petrography, geochemistry, and isotopic data suggest that these rocks are largely highly differentiated leucocratic S-type granites formed by anatexis of either metasedimentary rocks or more deeply seated, more mafic, plagioclase-rich granitoids. In these aspects they strongly resemble granites developed in intracontinental terranes such as the Hercynian belt of Europe and in continental collision-type settings such as the Bhutan and Nepalese Himalaya and the Seward Peninsula of western Alaska. We ascribe their origin to post-tectonic emplacement in a neutral or extensional (rifted) within-plate setting following deformation of the EWM. S-type Cambro-Ordovician granitic rocks and Precambrian quartz monzonite porphyries of the Thiel Mountains are geochemically similar to the EWM granites, but are correlative with the Granite Harbour Intrusive Series of the Transantarctic Mountains and the Wyatt Formation of the nearby La Gorce Mountains, respectively.
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