Isotopic character of Cambro‐Ordovician plutonism, southern Victoria Land, Antarctica

Abstract

Previous mapping of granitoid rocks in the Dry Valleys area of southern Victoria Land, Antarctica, identified the calc‐alkaline (DV1a), adakitic (DV1b), and monzonitic (DV2) suites. A fourth older suite comprising alkaline gabbro, syenite, and A‐type granite occurs in the Mt Dromedary area c. 80 km to the south. U‐Pb zircon dating of Bonney Pluton, the largest calc‐alkaline DV1a intrusion, indicates emplacement of this regional‐scale body at 505 ± 2 Ma. Pb‐loss and inherited zircon were common to Bonney Pluton analyses of this study. U‐Pb dating of monazite from Valhalla Pluton, a principal DV1b suite adakitic intrusion, indicates emplacement at 488 ± 2 Ma. The Bonney Pluton age constrains the peak of calc‐alkaline plutonism at 505 Ma and the Valhalla Pluton age records the major pulse of adakitic plutonism that is inferred to mark the final stages of subduction c. 490 Ma along this section of the East Antarctic margin. Nd and Sr isotope data for the calc‐alkaline DV1a suite and adakitic DV1b suite define distinct ranges for each suite, supporting their subdivision on the basis of field relationships, petrography, and whole‐rock geochemistry. Calc‐alkaline DV1a suite granite magmas have ϵNd(T) = ‐4–2 to ‐6.1 and Srj = 0.7071–0.7079, whereas the adakitic DV1b suite rocks have a wider range of ϵNd(T) = ‐1.9 to ‐7.2 and Srj = 0.7065–0.7097. The isotopic data suggest a significant mantle component and subordinate crustal component in the source region of both suites. Time‐dependent variations in the isotopic ratios of DV1a and DV1b suites imply a progressive increase in the proportion of more radiogenic material in the source region of the granitoid rocks, either mantle‐ or crust‐derived material. Larger adakitic DV1b plutons are more “evolved” than equivalent, smaller plutons of the same DV1b suite. Vanda Dikes and monzonitic DV2 suite intrusions are characterised by particularly low Srj = 0.7044–0.7067 and near‐constant ϵNd(T) = ‐4.8 to ‐5.3, which indicate a petrogenesis for these younger intrusions distinct from the older DV1a and DV1b suites. Gabbroic rocks from Mt Dromedary have ϵNd(T) values as low as ‐8.0 and Srj ratios as high as 0.7108, despite their mafic composition, confirming they are unrelated to granitoids in the Dry Valleys area. A granulite xenolith in the McMurdo Volcanics with calc‐alkaline DV1a‐type chemistry yielded a concordant U‐Pb zircon age of 490 ± 5 Ma. The age suggests that some of the lower crust in southern Victoria Land was emplaced during the Ross Orogeny rather than forming entirely during earlier Precambrian event(s). Isotopic ratios of metasediments and granitoids in the Dry Valleys correlate most closely with rocks that comprise the Beardmore “Microcontinent” in the Central Trans‐antarctic Mountains, rather than the Nimrod Group and crosscutting intrusions of the Miller Range. The DV1a suite granitoids in the Dry Valleys are petrographically and geo‐chemically similar to calc‐alkaline granitoids in northern Victoria Land, but have less‐evolved isotopic compositions that imply a lower proportion of crustal material in the source of the southern Victoria Land granitoid rocks. The isotopic data imply complex variations in the chemistry and genesis of granitoid rocks parallel to, as well as perpendicular to, the trend of the Ross Orogen.