| Summary: | Sensitive high-resolution ion-microprobe (SHRIMP) U-Pb zircon dating of four gneissic granitoid intrusions, whose field relationships indicate that they are among the oldest plutonic rocks exposed in the Dry Valleys region of southern Victoria Land, Antarctica, gives ages between 531 ± 10 and 502 ± 9 Ma. Two samples from 1- to 3-km-scale gneissic plutons characterized by the distinctive enrichment in Sr and depletion in Y (DV1b suite) typical of "adakitic" igneous rocks yielded dates of 531 ± 10 Ma and 516 ± 10 Ma, respectively. Two other samples of gneissic, K-feldspar– megacrystic granodiorite intercalated with the Skelton Group country rocks that are characterized by more typical Cordilleran I-type chemistries (DV1a suite) yielded dates of 505 ± 9 Ma and 502 ± 9 Ma, respectively. A fifth sample from the large Bonney pluton (DV1a suite) emplaced at the peak of magmatic activity during the Ross orogeny, gave a date of 499 ± 6 Ma. The new dates indicate that subduction and magmatism associated with the Ross orogeny had begun by ca. 530 Ma along this segment of the East Antarctic margin. They also require cessation by this time of the Neoproterozoic–Cambrian rifting that predated the Ross orogeny. None of the gneissic granitoid plutons was emplaced during a Neoproterozoic Beardmore orogeny, as has been suggested by some previous workers. Emplacement of small plutons characterized by an "adakitic" chemistry, punctuated by folding events, continued for 10–30 m.y. after ca. 530 Ma before major, more typical Cordilleran-style magmatism occurred between 505 and 499 Ma. Large volumes of calc-alkalic plutonic rocks in the Ross orogen south of the Dry Valleys region were emplaced 10–40 m.y. earlier than those in the Dry Valleys region, indicating marked variations in the timing of calc- alkalic granitoid magmatism parallel to the Ross orogen. The full extent and significance of these variations remain to be defined.
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