GEOCHEMISTRY AND THERMOCHRONOLOGY OF GRANITOID MAGMATISM DURING THE ROSS OROGENY, CENTRAL TRANSANTARCTIC MOUNTAINS (CTAM)

Within the CTAM the Granite Harbour Intrusives (GHI) suite occurs as widespread, voluminous granitoids emplaced at several stages of the Ross Orogeny. Here we present new geochemical data and laser incremental heating ⁴⁰Ar/³⁹Ar ages for samples from the GHI, providing insights into the character and thermal evolution of arc magmatism at the convergent margin along the paleo-Pacific rim of Gondwana during the Ross Orogeny.

The bulk rock chemistry of GHI samples (n=50) indicates two distinctive groups of granite and granodiorite-to-diorite. For both groups, trace element systematics display enrichments in LILEs-Th-LREEs over HFSEs and HREEs coupled with negative Nb and Ti anomalies, typical of continental arc-related magmas. The mixing of pelitic and mafic melts is evident for both varieties through bulk rock signatures and initial ⁸⁷Sr/⁸⁶Sr ratios that vary between 0.70732-0.73439 for the dioritic samples (n=11), and 0.72641-0.81537 for the granitic samples (n=13). However, the granitic variety is peraluminous and has A-type characteristics, whereas the dioritic variety is peraluminous-to-metaluminous and has I- to S-type signatures. Consequently, the dioritic melts are interpreted to originate from lower crustal melts through crustal thickening and the granitic melts are associated with the roll-back of the subducting slab and subsequent creation of a short-lived back-arc extensional environment.

Multi-crystal incremental heating analysis yielded hornblende ⁴⁰Ar/³⁹Ar plateau ages (n=11) of ca. 499-485 Ma, interpreted to represent crystallization ages for the dioritic variety, conformable with available zircon U-Pb ages. From both groups, single-crystal incremental heating analysis of biotite yield ca. 495-460 Ma ⁴⁰Ar/³⁹Ar plateau/integrated ages (n=18) recording cooling through biotite closure temperatures. These ages support the overlap and shift in the chemical signature and represent cooling through mid-Ordovician time. In addition, several samples (n=4) yielded younger biotite ⁴⁰Ar/³⁹Ar integrated ages of ca. 435-337 Ma, interpreted to represent post-Ordovician brittle faulting, cooling, and potential fluid migration. The range of cooling rates is interpreted to represent variation in emplacement depths of these plutonic bodies and/or variation in exhumation along the CTAM.