Crustal Architecture of the Paleo‐Pacific Rift Margin of East Antarctica: Evidence From U‐Pb Ages and O‐Hf Isotope Compositions of Ross Orogen Granitoids.

Granitic batholiths of the ∼500 Ma Ross Orogen in Antarctica are voluminous in scale, reflecting prolific magmatism along the active early Paleozoic convergent margin of Gondwana. New age and isotopic analysis of zircons from a large suite of Ross granitoids spanning >2,000 km along the orogen provide a wealth of geochronologic, tracer, and inheritance information, enabling us to investigate the pace of magmatism, along‐strike temporal and geochemical trends, magmatic sources, and tectonic modes of convergence. Because granitoids penetrate the crust of the earlier Neoproterozoic rift margin, they also provide insight into the age and composition of the largely ice‐covered East Antarctic craton. Zircon U‐Pb ages from these and other samples indicate that active Ross magmatism spanned 475–590 Ma, much longer than generally regarded. Most samples have heavy zircon δ18O values between 6.5 and 11.5‰ and initial εHf compositions between 0 and −15; their isotopic co‐variations are independent of age, as in other contemporary continental arcs, and reflect largely crustal melt sources. Samples near Shackleton Glacier have distinctly more mantle‐like isotope composition (i.e., radiogenic εHf and low δ18O) and separate two regions with distinctive isotopic properties and inheritance patterns—a more juvenile section of Mesoproterozoic crust underlying the southern TAM and an older, more evolved region of Paleoproterozoic and Archean crust in the central TAM. The isotopic discontinuity separating these regions indicates the presence of a cryptic crustal boundary of Grenvillian or younger age within the East Antarctic shield that may be traceable into the western Laurentian part of the Rodinia supercontinent. Plain Language Summary: Isotopic information obtained from granitic rocks formed during subduction‐zone convergence can help distinguish mantle and crustal sources of melting, evaluate the processes leading to chemical changes in the magmas, and reveal information about the age and composition of rocks in the upper plate. The Neoproterozoic to early Paleozoic Ross Orogen in Antarctica is a significant part of the active Gondwana‐margin convergent plate boundary stretching from present‐day eastern Australia to Argentina, but it is largely obscured by ice and younger sedimentary cover. The Ross Orogen hosts a prolific and widespread continental‐margin magmatic arc dominated by granitoid intrusions that serve as geochemical "samplers" of upper‐plate rocks with which they interact. We report U‐Pb ages and O‐Hf isotopic compositions in zircons from nearly 40 samples of Ross Orogen granitoids covering a large section of the modern Transantarctic Mountains that reveal patterns in magmatism best explained by variations in composition of the cratonic upper plate. They thus help to identify an otherwise hidden lithospheric boundary separating a region of juvenile Mesoproterozoic crust from evolved Paleoproterozoic and Archean crust that may represent a tectonic suture formed during Rodinia amalgamation. Key Points: Subduction‐related granites are effective geochemical samplers of the upper plate lithosphereU‐Pb, O and Hf isotope data from zircons in Ross Orogen granitoids reveal variations in age and magma sourcesContrasting magma compositions distinguish the East Antarctic upper‐plate lithosphere and reveal key boundaries related to Rodinia assembly [ABSTRACT FROM AUTHOR]