Robert J. Pankhurst

Antarctica-New Zealand rifting and Marie Byrd Land lithospheric magmatism linked to ridge subduction and mantle plume activity

Mid-Cretaceous igneous rocks of central Marie Byrd Land, Antarctica record a rapid change from subduction-related to rift-related magmatism. This correlates with the final stages of subduction of the Phoenix plate and the subsequent rifting of New Zealand from West Antarctica, prior to the opening of the Southern Ocean. Rift magmatism produced diverse A-type granitoids and mafic intrusive rocks of continental flood-basalt affinity that were derived ultimately from lithospheric mantle sources. Rifting was caused by changes in plate boundary forces; however, mantle plume activity may have begun in mid-Cretaceous time, triggering melting of the lithosphere and controlling the locus of rifting.

Pacific subduction coeval with the Karoo mantle plume: the Early Jurasssic Subcordilleran belt of northwestern Patagonia

Abstract The Early Mesozoic magmatism of southwestern Gondwana is reviewed in the light of new U-Pb SHRIMP zircon ages (181 ± 2 Ma, 181 ± 3 Ma, 185 ± 2 Ma, and 182 ± 2 Ma) that establish an Early Jurassic age for the granites of the Subcordilleran plutonic belt in northwestern Argentine Patagonia. New geochemical and isotopic data confirm that this belt represents an early subduction-related magmatic arc along the proto-Pacific margin of Gondwana. Thus, subduction was synchronous with the initial phase of Chon Aike rhyolite volcanism ascribed to the thermal effects of the Karoo mantle plume and heralding rifting of this part of the supercontinent. Overall, there is clear evidence that successive episodes of calc-alkaline arc magmatism from Late Triassic times until establishment of the Andean Patagonian batholith in the Late Jurassic involved westerly migration and clockwise rotation of the arc. This indicates a changing geodynamic regime during Gondwana break-up and suggests differential rollback of the subducted slab, with accretion of new crustal material and/or asymmetrical ‘scissor-like’ opening of back-arc basins. This almost certainly entailed dextral displacement of continental domains in Patagonia.

West Antarctica in Gondwanaland: Crustal blocks, reconstruction and breakup processes

Abstract A combined BAS-USARP West Antarctic Tectonic Project has provided new constraints on the crustal structure and geological evolution of West Antarctica and its relationship to the rest of Gondwanaland. Rb-Sr age dating has confirmed the presence of 1200-1000 m.y. old Precambrian gneisses within West Antarctica and aeromagnetic data have defined the extent of this Precambrian crustal block. Paleomagnetic data have constrained the position of four of the five geologically distinctive West Antarctic crustal blocks within Gondwanaland. They can be restored to their original position prior to breakup by 15°–25° counter clockwise rotation with little or no relative displacement between the blocks. A major period of within-plate middle Jurassic magmatism, associated with an important crustal melting event that is clearly related to the breakup of the supercontinent, has also been identified based on new isotopic and geochemical data.

Middle Jurassic within-plate granites in West Antarctica and their bearing on the break-up of Gondwanaland

Five post-tectonic granitic plutons isolated within the central Ellsworth-Whitmore mountains crustal block in West Antarctica form a distinctive geochemical suite. All have some characteristics of S-type granites and are atypical of active continental margins. They range in composition from a within-plate granite (WPG) end member, with the lowest 87Sr/86Sr initial ratio (0.707), to granites with a much more marked crustal signature and high initial ratios (0.722). The granitic suite was emplaced over a restricted Middle Jurassic time interval at the same time as the extensive Ferrar-Karoo-Tasman mafic suite and just prior to the disintegration of the supercontinent Gondwanaland. Petrogenetic modelling suggests that the WPG end member could have been derived entirely by differentiation of the enriched mantle-derived Ferrar magma, and the end member with the highest initial ratio by partial melting of a crustal source. Low initial 143Nd/144Nd ratios and Proterozoic model ages are compatible with a Precambrian crustal component but may alternatively, as in the case of Ferrar Supergroup magmas, reflect partial inheritance from enriched lithospheric mantle geochemically coupled to the lower crust since Precambrian differentiation. Data from these granites are consistent with large-scale underplating of mafic magma and crustal melting in response to a thermal disturbance in the Gondwanaland lithosphere related in some way to break-up of the supercontinent.

Cambrian-Ordovician magmatism in the Thiel Mountains, Transantarctic Mountains, and implications for the Beardmore orogeny

New field and laboratory studies result in a redefinition of the three main lithostratigraphic units of the Thiel Mountains. These are (1) the Thiel Mountains porphyry, a massive hypersthene-bearing monzonite, faulted against (2) the Mount Walcott Formation, a sequence of shallow-water volcaniclastic sedimentary rocks and dacitic tuffs or flows, and (3) the Reed Ridge granites, coarse-grained biotite granite/granodiorite stocks that cut the porphyry. Genetic relations between the porphyry and both the sedimentary rocks and the granite are proposed. Evidence for a Phanerozoic age indicated by the presence of fossils in the sediments is reinforced by Rb-Sr whole-rock dating that has conclusively established a Late Cambrian or Early Ordovician age (502 ±5 Ma) for the entire sequence. The stratigraphic and tectonic consequences refute all evidence for magmatism in the Transantarctic Mountains associated with the Precambrian Beardmore orogeny, the age and status of which are now in doubt.

Geochemistry of Palaeozoic–Mesozoic Pacific rim orogenic magmatism, Thurston Island area, West Antarctica

Thurston Island, and the adjacent Eights Coast and Jones Mountains, record Pacific margin magmatism from Carboniferous to Late Cretaceous times. The igneous rocks form a uniformly calc-alkaline, high-alumina, dominantly metaluminous suite; some relatively fractionated granitoids are mildly peraluminous. The magmas were hydrous, a result of subduction. Gabbros have compositions outside the range of mafic volcanic and hypabyssal rocks, as a result of cumulate processes. Trace element compositions of the mafic magmas range from a low La/Yb, Th/Ta end-member close to E-MORB in composition, perhaps contaminated by crust, to a high La/Yb, Th/Ta end-member, close to shoshonite, with strong magmatic arc trace element character. This variation may be a result of mixing of tholeiitic and shoshonitic end-members. Most silicic rocks could have been generated batch-wise from mafic magmas by fractional crystallization of a phenocryst assemblage dominated by plagioclase, pyroxene ± amphibole, as seen in the cumulates. Cessation of magmatism at about 90 Ma approximately coincided with collison of a spreading centre between the Phoenix and Pacific oceanic plates with the continent margin subduction zone. The rifting of New Zealand from West Antarctica and associated extension probably was responsible for emplacement of a coast-parallel Cretaceous dyke swarm.

The geology of Cape Dubouzet, northern Antarctic Peninsula: continental basement to the Trinity Peninsula Group?

Cape Dubouzet is mainly composed of a volcanic-subvolcanic complex of extrusive rhyolitic breccias, a banded rhyolite and a semi-annular body of dacite porphyry rich in xenoliths of metamorphic rocks. Major and REE geochemistry indicate that the volcanic rocks are calc-alkaline and that they are genetically related by fractional crystallization of a plagioclase-bearing assemblage from a common magma. Rb-Sr data suggest that the rhyolitic complex is of Middle-to-Late Jurassic age, and that it is intruded by Late Cretaceous stocks of banded diorite and gabbro. All these rocks are partially covered by moraines whose clasts are of local provenance. Xenoliths in the dacite porphyry suggest that the northern tip of the Antarctic Peninsula is underlain by a metamorphic complex composed of amphibolites, meta-tonalites and pelitic gneiss containing garnet, sillimanite, cordierite, hercynite, and andalucite. Such rocks are not known in the Scotia metamorphic complex, nor in the Trinity Peninsula Group and its low grade metamorphic derivatives, which also occur as rare xenoliths in the dacite. Previous dating of xenoliths collected from the moraines suggested a late Carboniferous age for this amphibolite-grade metamorphism. Both the Jurassic-Cenozoic magmatic arc of the Antarctic Peninsula and the accretionary complex rocks of the Trinity Peninsula Group were thus developed, at least in part, over pre-existing continental crust.