Marlina A. Elburg

One Hundred Fifty Million Years of Intrusive Activity in the Sør Rondane Mountains (East Antarctica): Implications for Gondwana Assembly

New U-Pb zircon ages for the younger phase of magmatism in the Sør Rondane Mountains (East Antarctica) are combined with published igneous and metamorphic zircon ages and show evidence for at least four thermal pulses: at 650–600 Ma, 580–550 Ma, ca. 530 Ma, and a magmatic tail between 510 and 500 Ma. No igneous U-Pb ages younger than 500 Ma have been found, in contrast to the situation in central and western Dronning Maud Land. Zircon Lu-Hf isotopic data are best explained as reflecting both crustal reworking and juvenile input, with the latter more obvious during the 580–550 Ma period. The Hf isotopic data, together with the presence of mafic and silica-undersaturated intrusives, argue against purely intracrustal melting as a petrogenetic process. Apart from the observed temporal trend, there is also a geographic trend in Hf isotopic compositions, with lower initial ε Hf values toward the northeast. However, the Hf isotopic shifts are gradual and do not show evidence for a dramatic change between the two previously defined metamorphic terranes. This observation, together with the long duration of magmatism, suggests that the Sør Rondane Mountains may be a collage of several different (sub-)terranes that were amalgamated over a longer period of time.

Detrital zircon in a supercontinental setting: locally derived and far-transported components in the Ordovician Natal Group, South Africa

U–Pb and Lu–Hf signatures of detrital zircon from conglomerates and sandstones of the Ordovician Natal Group, South Africa were determined using laser ablation inductively coupled plasma mass spectrometry. The basal conglomerates are dominated by Palaeo- to Mesoarchaean detrital zircon with εHf values from +3 to −4 with minor Mesoproterozoic input, indicating a proximal source in the Kaapvaal Craton and minor input from rocks of the Natal Sector of the Mesoproterozoic Namaqua–Natal Province. The sandstones are all dominated by a combination of juvenile Mesoproterozoic zircon and Neoproterozoic zircon derived from Mesoproterozoic rocks that were reworked during the Pan-African Orogeny. Several sedimentary sequences from former Gondwana with Neoproterozoic to Permian depositional ages show similar detrital zircon signatures. Sedimentary sequences of such vast temporal and geographical distribution are unlikely to have been fed by a single source, making it more likely that these sequences were fed by several different (Pan-Gondwana) source terranes with closely similar U–Pb and Lu–Hf zircon signatures. The results show that source terrane non-uniqueness can make ascertaining sedimentary provenance from detrital zircon impossible, and should be taken as a reminder when using detrital zircon as evidence for far-reaching conclusions in basin evolution studies and palaeogeography. Supplementary materials: U–Pb and Lu–Hf data are available at http://www.geolsoc.org.uk/SUP18880.

U–Pb and Lu–Hf zircon data in young sediments reflect sedimentary recycling in eastern South Africa

Detrital zircon from unconsolidated, Cenozoic sediments from eastern South Africa has been analysed for U–Pb and Lu–Hf isotopes by laser ablation inductively coupled plasma mass spectrometry. Identifiable bedrock sources have made local contributions to the detrital zircon populations, but the dominant zircon components are of regional distribution: late Mesoproterozoic (εHf = –5 to +10), Neoproterozoic to early Palaeozoic (εHf = –10 to +10), and minor late Palaeozoic (εHf ≈ 0). Archaean zircons are scarce even in sediments deposited on exposed Archaean basement or by rivers eroding it. The dominant components cannot be tied to specific first-generation sources in southern Africa or its former Gondwana neighbours. Instead, we see the effect of mixing and remobilization of debris from large parts of the supercontinent in the early Phanerozoic, which was stored in the Karoo basin and other continental cover sequences and shed from there to the present site of deposition. Therefore, data from detrital zircon in these deposits tell us less about the path of detritus from source to sink in a recent sedimentary system than about processes in much earlier erosion–transport–deposition cycles. To facilitate comparison of detrital zircon age distribution patterns, a simple and intuitive method that takes sampling uncertainty explicitly into account is proposed. Supplementary materials: U–Pb and Lu–Hf data, and concordia diagrams and discussion of effects of discordance are available at http://www.geolsoc.org.uk/SUP18884.

Layered intrusions and traffic jams

A wide range of explanations has been proposed for the origin of repetitive layering in mafic-ultramafic and in (per)alkaline intrusions. Here we propose that the interaction of mineral grains that sink and float in the crystallizing magma is an alternative mechanism that can explain many of the features of layered intrusions, without the need to invoke extrinsic factors. Similar to traffic jams on a motorway, small perturbations in crystal density develop that impede further ascent or descent of buoyant or heavy minerals, respectively. These “traffic jams” separate layers of magma from the rest of the magma chamber. The magma in the individual layers further evolves as a largely independent subsystem, with gravitational sorting organizing the mineral distribution within each layer. Layering can develop in the intermediate range between full mineral separation in low-viscosity or slowly cooling magma chambers and homogeneous crystallization in high-viscosity or fast-cooling chambers. This self-organization mechanism provides a novel explanation for the formation of rhythmic layering in low-viscosity magmas, for example in the Ilimaussaq igneous complex in southwest Greenland.

The Main Shear Zone in Sør Rondane, East Antarctica: Implications for the late-Pan-African tectonic evolution of Dronning Maud Land

Structural investigations in western Sor Rondane, eastern Dronning Maud Land (DML), provide new insights into the tectonic evolution of East Antarctica. One of the main structural features is the approximately 120 km long and several hundred meters wide WSW-ENE trending Main Shear Zone (MSZ). It is characterized by dextral high-strain ductile deformation under peak amphibolite-facies conditions. Crosscutting relationships with dated magmatic rocks bracket the activity of the MSZ between late Ediacaran to Cambrian times (circa 560 to 530 Ma). The MSZ separates Pan-African greenschist- to granulite-facies metamorphic rocks with “East African” affinities in the north from a Rayner-age early Neoproterozoic gabbro-tonalite-trondhjemite-granodiorite complex with “Indo-Antarctic” affinities in the south. It is interpreted to represent an important lithotectonic strike-slip boundary at a position close to the eastern margin of the East African-Antarctic Orogen (EAAO), which is assumed to be located farther south in the ice-covered region. Together with the possibly coeval left-lateral South Orvin Shear Zone in central DML, the MSZ may be related to NE directed lateral escape of the EAAO, whereas the Heimefront Shear Zone and South Kirwanveggen Shear Zone of western DML are part of the south directed branch of this bilateral system.

Petrogenetic processes in granitic magmas and their igneous microgranular enclaves from Central Victoria, Australia: match or mismatch?

Using chemical evidence, particularly the variations between titanium and iron + magnesium, we demonstrate that the petrogenetic processes that operated within Central Victorian granitic host magmas and their igneous microgranular enclave suites are dissimilar. Chemical variations within the granitic series result from a variety of what might be called ‘orderly’ processes, resulting in linear or curvilinear trends in chemical variation diagrams. Those that affected the enclave suites commonly resulted in scattered, chaotic variations. Even in cases where an enclave suite displays a more orderly chemical variation, it is demonstrable that the hypothesis of simple mixing between an enclave magma and a host granitic magma, to bring about the overall chemical variations, cannot be supported. The enclave magmas had vastly smaller volumes compared with their host granitic magmas. Thus, they have commonly undergone hybridisation through mixing with deep crustal melts and both chemical and mechanical interactions with wall rocks and their enclosing granitic magmas. As a result of this complex and chaotic set of processes, it remains extremely difficult to unravel the precise mechanisms that produced a given suite of enclave magmas. Due to the similarities between the studied granites and their enclaves with occurrences worldwide, we suggest that our findings are likely to have general applicability.

Possible spatial variability in the Selwyn Block of Central Victoria: evidence from Late Devonian felsic igneous rocks

ABSTRACT The Neoproterozoic to Cambrian Selwyn Block in Central Victoria forms the mainly unexposed basement to the Paleozoic metasediments, granitic rocks and felsic volcanic complexes of the Melbourne Zone of the Lachlan Orogen. The Late Devonian felsic rocks are largely products of partial melting of the Selwyn Block, and their chemistry implies that their sources were most probably arc-related andesite, dacite, volcaniclastic greywackes and some pelites. When plotted against the median longitudes of the plutons and volcanic complexes, the average values for 87Sr/86Srt and ϵNdt (at 370 Ma) reveal broad trends interpreted to reflect possible compositional and/or age structure in the Selwyn Block. Assuming that the trends are real, from W to E, I-type sources are progressively less crustally evolved, probably younging eastward. The S-type sources show no trend in ϵNdt, suggesting that there was efficient sediment mixing. The 87Sr/86Srt values, however, become more evolved eastward (opposite in sense to the apparent variation in the I-type sources). This is interpreted as the original Selwyn Block sediments having been more pelitic eastward, perhaps suggesting a deepening of the basin in this direction, as well as structurally upward in the succession. The opposite senses of variation highlights the spatial separation of the S- and I-type sources and suggest that the granitic magmas here are unlikely to represent any sort of mixing continuum.

Alps to Apennines zircon roller coaster along the Adria microplate margin

We have traced the particle path of high-pressure metasedimentary rocks on Elba Island, Northern Apennines, with the help of a U-Pb-Hf detrital zircon study. One quarter of the analysed zircons are surprisingly young, 41-30 Ma, with a main age peak at ca. 32 Ma, indicating an unexpected early Oligocene maximum deposition age. These Oligocene ages with negative εHf indicate a volcanic source region in the central-southern Alps. Though young by geological means, these zircons record an extraordinary geodynamic history. They originated in a volcanic arc, during the convergence/collision of the the Adria microplate with Europe from ca. 65 to 30 Ma. Thereafter, the Oligocene zircons travelled ca. 400 km southward along the Adria margin and the accretionary prism to present-day Tuscany, where they were subducted to depths of at least 40 km. Shortly thereafter, they were brought to the surface again in the wake of hinge roll back of the Apennine subduction zone and the resulting rapid extensional exhumation. Such a zircon roller coaster requires a microplate that has back-to-back subduction zones with opposing polarities on two sides.

P–T Conditions, Mechanism and Timing of the Localized Melting of Metapelites from the Petronella Shear Zone and Relationships with Granite Intrusions in the Southern Marginal Zone of the Limpopo Belt, South Africa

Evidence is presented for the localized melting of cordierite–orthopyroxene–biotite metapelites within the Petronella shear zone in the Southern Marginal Zone of the Limpopo Belt of Southern Africa. This process is expressed by the formation of centimetre-scale, K-feldspar-rich, garnet– orthopyroxene-bearing leucosome patches. Structural data indicate that the leucosomes formed after regional peak metamorphic and deformational events, but were synto late tectonic with respect to the shear fabric of the host metapelites. Phase equilibria modelling using the PERPLE_X software shows that the leucosomes were produced via fluid-deficient partial melting of the metapelites at temperatures above 900 C and pressures of 6 0–6 5 kbar. Melting reactions involving mainly biotite, plagioclase, quartz and accessory phases (including sulfides) produced peritectic Cr-enriched orthopyroxene, as well as Sc, Y, Cr, P-enriched garnet along with the potassium-rich melt. The melt was segregated around the peritectic minerals into the leucosomes. During nearisobaric cooling, the segregated melts crystallized to produce abundant K-feldspar and Sc, Y, Cr and P-poorer garnet inside the leucosome patches. Partial melt loss from the patches on cooling assisted with the preservation of anhydrous assemblages inside the patches and with re-hydration of the surrounding melanosome, with extensive formation of biotiteþquartz6sillimanite assemblages after orthopyroxene and cordierite, as well as late Sc, Y, Cr, and P-free garnet (þsillimaniteþquartz) at temperatures down to<600 C. Field and P–T data suggest a link between the localized melting of metapelites and trondhjemite intrusions that were synto late-tectonically injected into the Petronella shear-zone at about 2 67 Ga after the peak metamorphic and deformational event in the Southern Marginal Zone. This link is proven by U–Pb ages for monazites (a weighted average Pb/U age of 2666 6 4 Ma) from the leucosome patches. The trondhjemites provided additional heat and, probably, also a small amount of fluids, for the leucosome formation.