The Pre‐Cenozoic magmatic history of the Thurston Island Crustal Block, west Antarctica

Abstract

New Rb‐Sr and K‐Ar geochronological data are presented for the majority of known pre‐Cenozoic outcrops in Thurston Island, the Jones Mountains, and the western Eights Coast, which collectively represent the basement geology of the Thurston Island crustal block of West Antarctica. Almost all are of calc‐alkaline igneous or metaigneous rocks, and indicate long‐standing proximity to a magmatic arc. The observable history began with Late Carboniferous (309±5 Ma) emplacement of mantle‐derived orthogneiss precursors in eastern Thurston Island. Nd model ages from these and later igneous rocks suggest that the underlying crust is no older than about 1200–1400 Ma throughout the area. A variety of cumulate gabbros was emplaced soon after gneiss formation, followed by crust‐contaminated diorites that have Triassic mineral cooling dates of 240–220 Ma. In the nearby Jones Mountains, the oldest exposed rock is a muscovite‐bearing granite with an Early Jurassic age of 198±2 Ma; its initial 87Sr/86Sr ratio of 0.710 and ϵNdt values of −5 to −7 indicate either anatexis or, at least, a high degree of crustal input during magma genesis. This belongs to a suite of such granites known throughout the Antarctic Peninsula and related to earliest rifting of the Gondwana supercontinent. The subsequent evolution of the Thurston Island area was dominated by I‐type magmatism, apparently in two major episodes at 152–142 Ma (Late Jurassic granites) and 125–110 Ma (Early Cretaceous bimodal suite). Most of these magmas had initial 87Sr/86Sr ratios of 0.705–0.706 and ϵNdt values of +2 to −4 and were derived from slightly enriched mantle or from juvenile lower crust. They are thought to signify subduction of Pacific Ocean floor as in the adjacent parts of West Antarctica, although the Late Jurassic episode was of greater intensity in Thurston Island than elsewhere. The Cretaceous magmatism was intense and of Andean‐type. Between 100 and 90 Ma, volcanism in the Jones Mountains became predominantly silicic, with increasing incorporation of crustal components (initial 87Sr/86Sr ratios of 0.706–0.709 and ϵNdt values of −3 to −6), as subduction‐related magmatism ceased in this part of the margin.

The Whitsunday Volcanic Province, Central Queensland, Australia: lithological and stratigraphic investigations of a silicic-dominated large igneous province

“…In eastern Gondwana, igneous rocks of Early Cretaceous age are widespread, occurring primarily in eastern Queensland Bryan et al, 1997), as well as New Zealand (e.g. Mortimer and Tulloch, 1996, and references therein; Muir et al, 1994;Waight et al, 1997), Marie Byrd Land (Pankhurst et al, 1993;Weaver et al, 1994), and Lord Howe Rise (McDougall and van der Lingen, 1974).…”

Jurassic silicic volcanism in the Transantarctic Mountains: Was it related to plate margin processes or to Ferrar magmatism?

“…Of the Lower Jurassic magmatic rocks, none has similar Nd isotope initial ratios and only the Jones Mountains granite has similar Sr isotope initial ratios (Pankhurst et al, 1993). None of these magmatic centers was the source for the Hanson tuffs.…”

“…Paleozoic and Mesozoic orogenic belts and magmatic arcs lie outboard of the TAM (Pankhurst et al, 1993(Pankhurst et al, , 1998Mukasa and Dalziel, 2000) but evidence for Early Jurassic magmatism along the Gondwana paleo-Pacific margin is scant (Fig. 5).…”

“…In the New Zealand region, parts of the Brook Street and Murihiku terranes indicate an Early Jurassic magmatic arc (Adams et al, 2002), and the Bounty Islands granite has an age of 183±9 Ma (Adams and Campbell, 2005). The Jones Mountains granite has an age of 198±2 Ma (Pankhurst et al, 1993), and a pluton in central Antarctic Peninsula an age of 181 Ma (Leat et al, 1995). In the southern Antarctic Peninsula, the Brennecke and Mt.…”

The Holocene history of George VI Ice Shelf, Antarctic Peninsula from clast-provenance analysis of epishelf lake sediments

“…Uncertainties are ±1-2% (Olive et al, 2001). Rb-Sr and Sm-Nd isotope analysis was undertaken at the NERC Isotope Geosciences Laboratory following the methods of Pankhurst et al (1993). Sr isotope compositions were determined using static multicollection on a Thermo-Finnegan Triton mass-spectrometer to an internal precision of better than 5 ppm (1 s.e.m.).…”