UC Santa Barbara

Due to limited exposure and the challenges of field access, the magmatic record of the Ross Orogeny along the Transantarctic Mountains, Antarctica remains ill-constrained. This results in a lack of clarity for Gondwanan tectonics and geodynamics along the paleo-Pacific margin. To better understand spatiotemporal trends in the timing, duration, and source(s) of magmatism, whole-rock geochemistry, U-Pb ages, and Lu-Hf isotopes were measured on zircon in 132 granitoids and 40 gneisses from northern Victoria Land (nVL), the Miller Range (MR), and the Ohio Range (OHR) mountains, spanning ~4,000 km of the Antarctic segment of the paleo-Pacific margin of Gondwana. U-Pb data reveal magmatism in the northern Transantarctic Mountains at nVL initiated by at least ca. 552 Ma, lasting until ca. 467 Ma, magmatism in the central Transantarctic Mountains (cTAM) at the MR spans ca. 573 Ma to ca. 487 Ma, and, the OHR in the southern Transantarctic Mountains (sTAM) records magmatism from ca. 540 Ma to ca. 423 Ma. These age ranges suggest that the duration of magmatism was near-synchronous along the entire margin, but that magmatism first initiated in the cTAM through sVL (ca. 595-590 Ma), before migrating northward and southward into nVL (ca. 552 Ma) and sTAM (i.e., OHR, ca. 540 Ma), respectively. Termination of magmatism followed a similar trend, ending in the cTAM through sVL ca. 485 Ma, then in nVL ca. 467 Ma, and finally in OHR ca. 423 Ma. Thus, our data do not support the long-held suggestion of arc magmatism decreasing in age northward along the margin.Zircon Hf isotope data indicate different magma sources contributed to magmatism as it migrated along the margin. Granitoids from nVL contain minimal mantle input, and were instead dominantly sourced from melting of (meta-)sedimentary source(s). Granitoids from the Ohio Range contain some juvenile input from a relatively young mantle source and contain almost no inherited zircon xenocrysts. Magmatic and metamorphic rocks from the Miller Range (MR) reveal abundant crustal contamination, indicating minimal mantle input over 2.5 billion years, and multiple episodes of crustal reworking. The combined geochemistry, age, and εHf data from nVL to OHR indicate that most of the magmatism along the paleo-Pacific margin of Antarctica was dominated by crustal contamination, with no continental growth, and can be dominantly attributed to reworking of ancient crust. We demonstrate no spatiotemporal patterns in the timing or duration of magmatism within nVL and find no support for previously defined distinctions between magmatic rocks associated with the high- to low-pressure metamorphic zones observed in the Wilson Terrane. Ross Orogen magmatism at nVL is dominated by peraluminous, S-type granitoids representing significant crustal melting and reworking of Proterozoic-aged continental crust, with possible inheritance of older, Neoarchean to early Paleoproterozoic crust. For the first time, a distinct step-like transition in magmatism is identified within sVL between the longer-duration Koettlitz Glacier (KG) and shorter-duration Dry Valleys (DV), likely due to lithosphere thickness constraining the onset of magmatism during subduction. Tectonic transitions also occur between the DV and nVL and between nVL and the Delamerian where subduction trench retreat is stalled along nVL due to accretion of additional continental lithosphere. Further, we produce the first record for OHR, demonstrating a magma source that increases in crustal contamination through time, and a nearly synchronous along-strike initiation of magmatism from the Queen Maud Mountains in the cTAM to the Pampean orogen in South America. Finally, we demonstrate that the East Antarctic craton, exposed in the MR, formed by at least 3.1 Ga and is likely the source material for the entire Ross Orogen granitic batholiths.