Geochemistry and petrogenesis of high-Mg tholeiites and lamprophyres in the Vestfold Hills, Antarctica
Creators: name: Seitz, H-M
Publisher: name: University Of Tasmania
Year: 2023

description: The Vestfold Hills, Antarctica, an ice-free region of ea. 400 km\\(^2\\), is a well exposed terrane of deep Archaean and Proterozoic crust which has been strongly interfolded and transposed. Between 2350 and 1100 Ma a dense network of mafic dykes intruded at mid- to upper crustal depths. Crosscutting relationships distinguish 9 different dyke sets intruded in three magmatic episodes. These three episodes exhibit distinct directional trends suggesting rotation of the regional crustal stress field over the period 2350 to 1150 Ma. During the first magmatic episode high-Mg tholeiites were emplaced at about 2350 Ma together with Fe-rich tholeiites. A second magmatic period has been inferred at ~1800 Ma in which a further set of Fe-rich tholeiites was emplaced. Radiogenic dating of the 1800 Ma dykes is not well constrained and field observations from this study did not clarify the relationship of these dykes to the older (2350 Ma) Fe-rich tholeiites. A third magmatic episode, which occurred between 1350 and 1150 Ma, emplaced two sets of lamprophyric dykes and Fe-rich tholeiites, in at least four separate events. Previous studies have concluded that the 2350 Ma old high-Mg tholeiites were emplaced at deep crustal levels (7 - 8 kbar). New pressure estimates based on aluminium exchange between clinopyroxene and plagioclase and on pressure dependent trends of magmatic evolution reveal that these dykes were more likely emplaced at considerably lower levels ( &lt; 5 kb), possibly as low as 1-2 kbar. This requires much faster uplift of the Vestfold Hills block, during the late Archaean and early Proterozoic, than previously thought. The 2350 Ma high-Mg tholeiites can be divided into 3 geochemically distinct groups which are not related by simple crystal fractionation processes. The high SiO\\(_2\\) and MgO, and low TiO\\(_2\\) magmas and are enriched in platinum group elements and gold: all characteristic of siliceous high magnesian basalts (SHMB). A fundamental question arises as to whether these geochemical characteristics are primary features inherited from the mantle or whether the magmas have been modified through assimilation of continental crust. This has been assessed by evaluating the extent to which differentiation processes are able to account for geochemical variations within one of the largest dykes in the Vestfold Hills has been studied in detail. This dyke is found to be of virtual homogeneous composition, apart from some minor variations in major- and trace elements which are consistent with limited in situ differentiation. Major and minor element geochemistry shows no evidence exists for crustal contamination in this dyke. A large noritic ring complex (ca. 7 - 8 km in diameter) is associated with the intrusion of the 2350 Ma high-Mg tholeiite suite. It consists of several partly isolated bodies which vary in width from 70 to 400 m. This complex comprises three lithological units: (1) Homogeneous Norite, the most uniform and voluminous unit, containing cumulus orthopyroxene with clinopyroxene and plagioclase as intercumulus phases, (2) the Mottled Norite, a unit characterized by large 1 - 2 cm plagioclase - alkali-feldspar aggregates, and (3) the Rubbly Norite, a less common variant, distinguished by patches of bronzitic orthopyroxene, cognate and inherited xenoliths, and globules of sulphide. The xenoliths are mostly orthopyroxenites but also include websterites, sapphirine-bearing fine-grained quartzite, and feldspathic gneiss. The sulphide-rich zones range in size from ~ 4 to 30 m in width, with extensions up to 400 m in length in the southern part of the complex. Evidence for multiple intrusion suggests the Rubbly Norite was subsequently emptaced into the the Homogeneous and Mottled Norite units.