Petrogenesis and thermal evolution of the Granite Harbour Intrusives Suite (Central Transantarctic Mountains, Antarctica): Insights on the Ross orogeny

The Granite Harbour Intrusives (GHI) Suite occurs within the Transantarctic Mountains as widespread voluminous granitoids emplaced during the Ross-Delamerian orogeny. Here, we present new bulk-rock geochemistry, bulk-rock Sr-Nd-Pb isotope geochemistry, and laser incremental heating hornblende and biotite 40Ar-39Ar thermochronology data for samples of the GHI Suite from the Central Transantarctic Mountains, providing insights into the tectono-magmatic and tectono-thermal evolution of this syn- to post-collisional magmatism at the Paleo-Pacific margin of Gondwana during the early Paleozoic.

The spatially extensive sample set revealed two distinctive groups of (1) granite and (2) diorite/granodiorite. For both groups, trace-element systematics display enrichments in large ion lithophile elements−Th−light rare earth elements over high field strength elements and heavy rare earth elements, typical of continental arc-related magmas. However, where Nb-Ta is coupled for the dioritic variety, it is decoupled for the granitic variety. The mixing of pelitic and mafic melts, and crustal contamination are evident for both varieties through major and trace elements and Sr-Nd-Pb isotopic compositions. Even though Sr-Nd isotopes do not clearly discriminate the two varieties, the first-ever Pb isotopic data from the GHI Suite indicates a higher amount of crustal material involved in the source of the granitic magmas. The tectono-magmatic signatures of the GHI Suite indicate magmatism occurring in two modes, as dioritic continental arc magmatism at an Andean-type margin followed by granite generation with remelting of the crustal material (including the diorites) due to later-stage extension of the thickened crust caused by slab rollback of the subducting oceanic lithosphere.

Multi-crystal incremental heating analysis of hornblendes from both granitic and dioritic varieties yielded 40Ar-39Ar plateau ages of ca. 499−485 Ma, whereas single-crystal incremental heating analysis of biotite yielded ca. 495−460 Ma 40Ar-39Ar plateau and/or integrated ages, recording cooling through hornblende and biotite closure temperatures, respectively. The hornblende-biotite pairing for both varieties indicates the complex thermal evolution of the GHI Suite, raising different scenarios for their emplacement chronology and post-emplacement thermotectonic evolution.