Abstract Preserved rocks in the Jurassic Ferrar Large Igneous Province consist mainly of intrusions, and extrusive rocks, the topic of this chapter, comprise the remaining small component. They crop out in a limited number of areas in the Transantarctic Mountains and southeastern Australia. They consist of thick sequences of lavas and sporadic occurrences of volcaniclastic rocks. The latter occur mainly beneath the lavas and represent the initial eruptive activity, but also are present within the lava sequence. The majority are basaltic phreatomagmatic deposits and in at least two locations form immense phreatocauldrons filled with structureless tuff breccias and lapilli tuffs with thicknesses of as much as 400 m. Stratified sequences of tuff breccias, lapilli tuffs and tuffs are up to 200 m thick. Thin tuff beds are sparsely distributed in the lava sequences. Lava successions are mainly 400–500 m thick, and comprise individual lavas ranging from 1 to 230 m thick, although most are in the range of 10–100 m. Well-defined colonnade and entablature are seldom displayed. Lava sequences were confined topographically and locally ponded. Water played a prominent role in eruptive activity, as exhibited by phreatomagmatism, hyaloclastites, pillow lava and quenching of lavas. Vents for lavas have yet to be identified.

Abstract The Lower Jurassic Ferrar Large Igneous Province consists predominantly of intrusive rocks, which crop out over a distance of 3500 km. In comparison, extrusive rocks are more restricted geographically. Geochemically, the province is divided into the Mount Fazio Chemical Type, forming more than 99% of the exposed province, and the Scarab Peak Chemical Type, which in the Ross Sea sector is restricted to the uppermost lava. The former exhibits a range of compositions (SiO 2 = 52–59%; MgO = 9.2–2.6%; Zr = 60–175 ppm; Sr i = 0.7081–0.7138; ε Nd = −6.0 to −3.8), whereas the latter has a restricted composition (SiO 2 = c. 58%; MgO = c. 2.3%; Zr = c. 230 ppm; Sr i = 0.7090–0.7097; ε Nd = −4.4 to −4.1). Both chemical types are characterized by enriched initial isotope compositions of neodymium and strontium, low abundances of high field strength elements, and crust-like trace element patterns. The most basic rocks, olivine-bearing dolerites, indicate that these geochemical characteristics were inherited from a mantle source modified by subduction processes, possibly the incorporation of sediment. In one model, magmas were derived from a linear source having multiple sites of generation each of which evolved to yield, in sum, the province-wide coherent geochemistry. The preferred interpretation is that the remarkably coherent geochemistry and short duration of emplacement demonstrate derivation from a single source inferred to have been located in the proto-Weddell Sea region. The spatial variation in geochemical characteristics of the lavas suggests distinct magma batches erupted at the surface, whereas no clear geographical pattern is evident for intrusive rocks.

Abstract: The Ferrar Large Igneous Province forms a linear outcrop belt for 3250 km across Antarctica, which then diverges into SE Australia and New Zealand. The province comprises numerous sills, a layered mafic intrusion, remnants of extensive lava fields and minor pyroclastic deposits. High-precision zircon geochronology demonstrates a restricted emplacement duration (<0.4 myr) at c. 182.7 Ma, and geochemistry demonstrates marked coherence for most of the Ferrar province. Dyke swarms forming magma feeders have not been recognized, but locally have been inferred geophysically. The emplacement order of the various components of the magmatic system at supra-crustal levels has been inferred to be from the top-down lavas first, followed by progressively deeper emplacement of sills. This order was primarily controlled by magma density, and the emptying of large differentiated magma bodies from depth. An alternative proposal is that the magma transport paths were through sills, with magmas moving upwards to eventually reach the surface to be erupted as extrusive rocks. These two hypotheses are evaluated in terms of field relationships and geochemistry in the five regional areas where both lavas and sills crop out. Either scenario is possible in one or more instances, but neither hypothesis applies on a province-wide basis. Supplementary material: The locations of samples, and trace element data and major element analyses of samples are available at: https://doi.org/10.6084/m9.figshare.c.3819454

Abstract The distribution and geochemical relationships of the Early Jurassic Ferrar large igneous province (LIP) are examined and it is concluded that they support the lateral flow model for the emplacement of the province, with a source along the strongly magmatic Early Jurassic Antarctica–Africa rifted margin. Published data and new analyses from the Pensacola Range are used to show that the dominant magma type in the Ferrar, the Mount Fazio chemical type (MFCT), occurs in the Theron Mountains, Shackleton Range, Whichaway Nunataks, Pensacola Mountains (all Antarctica) and South Africa, as well as well-known outcrops in Victoria Land, Antarctica, SE Australia and New Zealand. Chemical compositions are shown to be somewhat varied, but similar enough for them to be considered as representing closely related magmas. Examination of geochemical trends with distance from the interpreted magma source indicates that Mg# and MgO abundances decline with distance travelled, and it is argued that this is consistent with the lateral flow model. The Scarab Peak chemical type (SPCT), which occurs as sills in the Theron Mountains and Whichaway Nunataks, and as lavas in Victoria Land, is geochemically very homogeneous. Despite this, Mg#, MgO, Ti/Y and Ti/Zr all fall with distance from the interpreted source, consistent with fractional crystallization occurring during the lateral flow of the magmas. Flow took place in dykes or (more likely) sills. No feeder dyke swarm has been identified. The distances flowed, at least 4100 km for MFCT and 3700 km for SPCT, are the longest interpreted lateral magma flows on Earth.

Abstract Large-volume rhyolitic volcanism along the proto-Pacific margin of Gondwana consists of three major episodes of magmatism or ‘flare-ups’. The initial episode (V1) overlaps with the Karoo–Ferrar large igneous provinces at c. 183 Ma. A second (V2) episode was erupted in the interval 171–167 Ma, and a third episode (V3) was emplaced in the interval 157–153 Ma. The magmatic events of the V1 and V2 episodes of the Antarctic Peninsula are reviewed here describing major and trace elements, and isotopic (Sr, Nd, O) data from rhyolitic volcanic rocks and more minor basaltic magmatism. An isotopically uniform intermediate magma developed as a result of anatexis of hydrous mafic lower crust, which can be linked to earlier, arc-related underplating. The subsequent lower-crust partial melts mixed with fractionated mafic underplate, followed by mid-crust storage and homogenization. Early Jurassic (V1) volcanic rocks of the southern Antarctic Peninsula are derived from the isotopically uniform magma, but they have mixed with melts of upper-crustal paragneiss in high-level magma chambers. The V2 rhyolites from the northern Antarctic Peninsula are the result of assimilation and fractional crystallization of the isotopically uniform magma. This process took place in upper-crust magma reservoirs involving crustal assimilants with an isotopic composition akin to that of the magma. A continental margin-arc setting was critical in allowing the development of an hydrous, fusible lower crust. Lower-crustal anatexis was in response to mafic underplating associated with the mantle plume thought to be responsible for the contemporaneous Karoo magmatic province and rifting associated with the initial break-up of Gondwana.