Segment E2 is situated in the back-arc East Scotia Ridge. The segment is unusual in that it has an axial topographic high underlain by a seismically imaged melt lens. The axis of the segment, which ...is 70 km long, was sampled at ~2 km spacing. There is strong correlation between compositions and bathymetry, and there is no evidence that lateral flow of magmas along dykes within the segment was more than 25 km. Magmas are more evolved towards the summit, indicating that magma fractionation took place within the imaged melt lens. Na<8middot0< is roughly constant at ~2middot6, implying uniform degree of partial melting, except for some samples at the summit that have Na<8middot0< = 2middot2. Compositions closest to N-MORB occur at the segment tips, and LREE/HREE ratios increase toward the summit. None of the magmas were derived from depleted sub-arc mantle. Nevertheless, most compositions in the segment were modified by slab-derived components. The low-Na<8middot0< samples have high Ba/Nb compared with the rest of the segment. Dredged lavas from the lateral flanks of the summit have the most extreme compositions, including ones derived from plume mantle, and are end-members for magma mixing processes that controlled compositions.
The Jutulstraumen ice stream in western Dronning Maud Land may conceal a Jurassic continental rift. Delineating the geometry and the magmatic patterns of this inferred glaciated rift in East ...Antarctica is important to improve our understanding of the regional tectonic and magmatic processes associated with Gondwana break-up. A high-resolution aeromagnetic survey provides new insights over the largely buried tectonic and magmatic patterns of the Jutulstraumen area. Prominent NE-SW oriented aeromagnetic trends are detected over the Jutulstraumen. These trends delineate major inherited structural boundaries, active in Grenvillian (about 1.1 Ga) and Pan-African times (about 500 Ma), which appear to strongly control the location of the later Jurassic rift. The postulated eastern flank of the rift is marked by a broad positive anomaly over H.U. Sverdrupfjella. Buried Grenvillian age rocks may be the source of the long-wavelength anomaly. However, the higher frequency components correlate with granitoids of late Pan-African age. The inferred western flank of the rift features short-wavelength anomalies over the Borgmassivet and Ahlmannryggen areas, indicating a considerably greater extent of mid-Proterozoic tholeiitic sills than apparent in outcrop. In contrast, aeromagnetic signatures suggest that alkaline plutons, which relate to Jurassic rifting, are restricted to outcrop areas along the eastern rift flank. The prominent magnetic low over the Jutulstraumen indicates either a largely amagmatic rift, or perhaps subglacial sediments within the rift basin.
The Grunehogna Craton (GC, East Antarctica) is interpreted as part of the Archaean Kaapvaal Craton of southern Africa prior to Gondwana breakup. The basement of the GC is exposed only within a small ...area comprising the dominantly leucocratic Annandagstoppane (ADT) S-type granite. The granite (and hence the craton) has been dated previously only by Rb–Sr and Pb–Pb mica and whole-rock methods. Here, the crystallization age of the granite is determined to be 3067 ± 8 Ma by U–Pb dating of zircon. This age is coeval with that of granitoids and volcanic rocks in the Swaziland and Witwatersrand blocks of the Kaapvaal Craton. Inherited grains in the ADT granite have ages of up to 3433 ± 7 Ma, and are the first evidence of Palaeoarchaean basement in Dronning Maud Land. The age spectrum of the inherited grains reflects well-known tectono-magmatic events in the Kaapvaal Craton and forms important evidence for the connection of the GC to the Kaapvaal Craton for at least 2·5 billion years and probably longer. Whole-rock chemistry and zircon O isotopes demonstrate a supracrustal sedimentary source for the granite, and Hf model ages show that at least two or three crustal sources contributed to the magma with model ages of ∼3·50, ∼3·75 and possibly ∼3·90 Ga. The 3·1 Ga granites covering ∼60% of the outcrop area of the Kaapvaal–Grunehogna Craton played a major role in the mechanical stabilization of the continental crust during the establishment of the craton in the Mesoarchaean. Combined zircon Hf–O isotope data and the lack of juvenile additions to the crust in the Mesoarchaean strongly suggest that crustal melting and granite formation was caused by the deep burial of clastic sediments and subsequent incubational heating of the crust. Intracrustal recycling of this type may be an important process during cratonization and the long-term stabilization of continental crust.
Primitive magmas representing mantle partial melts minimally affected by fractionation and assimilation are rare in the magmatic arc environment. Most examples are either associated with high rates ...of arc-parallel extension, or occur along faults and dykes perpendicular to the trend of the arc and related to arc compression. In two cases, the Vanuatu and Solomon Islands arcs, such arc compression is being caused by collision of seamounts. In the Antarctic Peninsula, primitive mafic dykes were emplaced perpendicular to the continental arc. Ar-Ar and K-Ar data suggest intrusion of the dykes atc. 126-106 Ma, possibly during mid-Cretaceous regional compression of the arc. The dykes form two compositional groups. One group has low LaN/YbN ratios (0.31-0.49), lower Nb/Yb and higher Th/Nb than N-MORB, age-corrected εNd values of +7.3 to +7.9, and are interpreted as melts of subduction modified sub-arc asthenosphere. The other has high LaN/YbN ratios (3.86-8.92), higher Nb/Yb and Th/Nb than N-MORB, age-corrected εNd values of -2.8 to +3.4, and are interpreted as melts of sub-arc lithosphere. The absence of dykes compositionally between these groups suggests that the primitive magmas avoided storage and mixing in magma chambers.
A suite of dolerite dykes from the Ahlmannryggen region of western Dronning Maud Land (Antarctica) forms part of the much more extensive Karoo igneous province of southern Africa. The dyke ...compositions include both low- and high-Ti magma types, including picrites and ferropicrites. New 40Ar/39Ar age determinations for the Ahlmannryggen intrusions indicate two ages of emplacement at ∼178 and ∼190 Ma. Four geochemical groups of dykes have been identified in the Ahlmannryggen region based on analyses of ∼60 dykes. The groups are defined on the basis of whole-rock TiO2 and Zr contents, and reinforced by rare earth element (REE), 87Sr/86Sr and 143Nd/144Nd isotope data. Group 1 were intruded at ∼190 Ma and have low TiO2 and Zr contents and a significant Archaean crustal component, but also evidence of hydrothermal alteration. Group 2 dykes were intruded at ∼178 Ma; they have low to moderate TiO2 and Zr contents and are interpreted to be the result of mixing of melts derived from an isotopically depleted source with small melt fractions of an enriched lithospheric mantle source. Group 3 dyke were intruded at ∼190 Ma and form the most distinct magma group; these are largely picritic with superficially mid-ocean ridge basalt (MORB)-like chemistry (flat REE patterns, 87Sr/86Sri ∼0·7035, εNdi ∼9). However, they have very high TiO2 (∼4 wt %) and Zr (∼500 ppm) contents, which is not consistent with melting of MORB-source mantle. The Group 3 magmas are inferred to be derived by partial melting of a strongly depleted mantle source in the garnet stability field. This group includes several high Mg–Fe dykes (ferropicrites), which are interpreted as high-temperature melts. Some Group 3 dykes also show evidence of contamination by continental crust. Group 4 dykes are low-K picrites intruded at ∼178 Ma; they have very high TiO2–Zr contents and are the most enriched magma group of the Karoo–Antarctic province, with ocean-island basalt (OIB)-like chemistry. Dykes of Group 1 and Group 3 are sub-parallel (ENE–WSW) and both groups were emplaced at ∼190 Ma in response to the same regional stress field, which had changed by ∼178 Ma, when Group 2 and Group 4 dykes were intruded along a dominantly NNE–SSW strike.
The Antarctic Peninsula (AP) consists of a long lived and uniquely well preserved magmatic arc system. The broad tectonic structure of the AP arc is well understood. However, magmatic processes ...occurring along the arc are only constrained by regional geophysical and relatively sparse geological data. Key questions remain about the timing, volume, and structural controls on magma emplacement. We present new high resolution aeromagnetic data across Adelaide Island, on the western margin of the AP revealing the complex structure of the AP arc/forearc boundary. Using digital enhancement, 2-D modelling and 3-D inversion we constrain the form of the magnetic sources at the arc/forearc boundary. Our interpretation of these magnetic data, guided by geological evidence and new zircon U-Pb dating, suggests significant Palaeogene to Neogene magmatism formed ∼25 per cent of the upper crust in this region (∼7500 km3). Significant structural control on Neogene magma emplacement along the arc/forearc boundary is also revealed. We hypothesize that this Neogene magmatism reflects mantle return flow through a slab window generated by Late Palaeogene cessation of subduction south of Adelaide Island. This mantle process may have affected the final stages of arc magmatism along the AP margin.
Ocean floor magnetic anomalies show that New Zealand was the last continental fragment to separate from Antarctica during Gondwana break-up, drifting from Marie Byrd Land, West Antarctica, about 84 ...Ma ago. Prior to continental drift, a voluminous suite of mafic dykes (dated by Ar-Ar laser stepped heating at 107±5 Ma) and anorogenic silicic rocks, including syenites and peralkaline granitoids (95-102 Ma), were emplaced in Marie Byrd Land during a rifting event. The mafic dyke suite includes both high-and low-Ti basalts. Trace element and Sr and Nd isotope compositions of the mafic dykes may be modelled by mixing between tholeiitic OIB (asthenosphere-derived) and alkaline high- to low-Ti alkaline magmas (lithospheric mantle derived). Pb isotopes indicate that the OIB component had a HIMU composition. We suggest that the rift-related magmatism was generated in the vicinity of a mantle plume. The plume helped to control the position of continental separation within the very wide region of continental extension that developed when the Pacific-Phoenix spreading ridge approached the subduction zone. Separation of New Zealand from Antarctica occurred when the Pacific-Phoenix spreading centre propagated into the Antarctic continent. Sea floor spreading in the region of the mantle plume may have caused an outburst of volcanism along the spreading ridge generating an oceanic plateau, now represented by the 10-15 km thick Hikurangi Plateau situated alongside the Chatham Rise, New Zealand. The plateau consists of tholeiitic OIB-MORB basalt, regarded as Cretaceous in age, and similar in composition to the putative tholeiitic end-member in the Marie Byrd Land dykes. The mantle plume is proposed to now underlie the western Ross Sea, centred beneath Mount Erebus, where it was largely responsible for the very voluminous, intraplate, alkaline McMurdo Volcanic Group. A second mantle plume beneath Marie Byrd Land formed the Cenozoic alkaline volcanic province.
The Ellsworth Mountains of West Antarctica represent part of a displaced terrane once situated along the palaeo-Pacific margin of Gondwana, prior to supercontinent break-up, adjacent to South Africa ...and the Weddell Sea coast of East Antarctica. Middle Cambrian sedimentary rocks of the southern Ellsworth Mountains host locally thick volcanic and subvolcanic rocks forming five igneous centres. Geochemically, most of the igneous samples are mafic, with a subordinate suite of evolved types. The mafic suite is geochemically varied, ranging from MORB (mid-ocean ridge basalt)-like compositions to shoshonitic and lamprophyric (e.g. La
N/Yb
N = 0.95 to 15.2), with εNd
i values ranging from +5.2 to −2.0, correlating with Ti/Y. They are interpreted as representing melts derived from more than one mantle source, with the MORB-like rocks being derived from a depleted mantle source, and the more enriched compositions representing partial melting of lithospheric mantle. Silicic rocks contain melt contributions from Late Proterozoic crust, which is inferred to form the basement of the Ellsworth Mountains. We interpret these igneous rocks as having been formed in a continental rift environment, with MORB-like basalts erupted near the rift axis, and melts from lithospheric mantle emplaced on the rift shoulder. Such an interpretation is consistent with the sedimentary host-rock palaeogeography and contemporaneous structures. This Middle Cambrian rift event is correlated spatially and temporally with rift-related sedimentary rocks in South Africa. It is currently unclear what rifted off the southern African–Weddell Sea sector of the Gondwana palaeo-Pacific margin at that time.
PDF
