Daniel J. Dunkley

India-Antarctica-Australia-Laurentia connection in the Paleoproterozoic–Mesoproterozoic revisited: Evidence from new zircon U-Pb and monazite chemical age data from the Eastern Ghats Belt, India

We present zircon and monazite U-Pb data from ultrahigh-temperature (UHT) metamorphosed orthogneisses and paragneisses collected from key areas of the Eastern Ghats Belt, India. The results show contrasting tectonothermal histories in different isotopic domains of the Eastern Ghats Belt that were identified by previous workers. Of particular importance is the discovery of a ca. 1760 Ma event (concordia age) in the southern domain 1A, which is interpreted to be the age of an early UHT metamorphism event. This was followed by a second granulite-facies metamorphism event and partial melting at ca. 1600 Ma. This domain was presumably cratonized with India at around 1600 Ma. The record of the ca. 1760–1600 Ma events in domain 1A of the Eastern Ghats Belt allows us to speculate on modeling the Paleoproterozoic–Mesoproterozoic transcontinental correlation. The accretionary orogenic processes in the supercontinent Columbia encompassed Australia, Antarctica, Laurentia, and parts of India. The central part of Eastern Ghats Belt (isotopic domain 2), on the other hand, contains zircons showing inherited ages of ca. 1880–1700 Ma, with a concordant age group of ca. 1760 Ma. Moderately to strongly discordant ages in the time span of ca. 1600–1100 Ma in domain 2 are interpreted to be mixing ages as a result of strong overprint of a ca. 1030–900 Ma tectonothermal event(s) that affected this domain. An early UHT metamorphism event in this domain is inferred to have occurred at ca. 1030–990 Ma (chemical dating of included monazite grains). Zircon records the most pervasive tectonothermal event in this domain at ca. 980–900 Ma, which is correlative with the Rayner orogeny in East Antarctica as a part of the formation of Rodinia.

Geochronological constraints on the Late Proterozoic to Cambrian crustal evolution of eastern Dronning Maud Land, East Antarctica: a synthesis of SHRIMP U–Pb age and Nd model age data

Abstract In eastern Dronning Maud Land (DML), East Antarctica, there are several discrete, isolated magmatic and high-grade metamorphic regions. These are, from west (c. 20°E) to east (c. 50°E), the Sør Rondane Mountains (SRM), Yamato–Belgica Complex (YBC), Lützow-Holm Complex (LHC), Rayner Complex (RC) and Napier Complex (NC). To understand this region in a Gondwanan context, one must distinguish between Pan-African and Grenvillian aged magmatic and metamorphic events. Sensitive high-resolution ion microprobe U–Pb zircon ages and Nd model ages for metamorphic and plutonic rocks are examined in conjunction with published geological and petrological studies of the various terranes. In particular, the evolution of the SRM is examined in detail. Compilation of Nd model ages for new and published data suggests that the main part of eastern Dronning Maud Land, including the SRM, represents juvenile late Mesoproterozoic (c. 1000–1200 Ma) crust associated with minor fragments of an older continental component. Evidence for an Archaean component in the basement of the SRM is lacking. As for central DML, 1100–1200 Ma extensive felsic magmatism is recognized in the SRM. Deposition of sediments during or after magmatism and possible metamorphism at 800–700 Ma is recognized from populations of detrital zircon in metasedimentary rocks. The NE Terrane of the SRM, along with the YBC, was metamorphosed under granulite-facies conditions at c. 600–650 Ma. The SW and NE Terranes of the SRM were brought together during amphibolite-facies metamorphism at c. 570 Ma, and share a common metamorphic and magmatic history from that time. High-grade metamorphism was followed by extensive A-type granitoid activity and contact metamorphism between 560 and 500 Ma. In contrast, TDM and inherited zircon core ages suggest that the LHC is a collage of protoliths with a variety of Proterozoic and Archaean sources. Later peak metamorphism of the LHC at 520–550 Ma thus represents the final stage of Gondwanan amalgamation in this section of East Antarctica.

Syncollisional rapid granitic magma formation in an arc-arc collision zone: Evidence from the Tanzawa plutonic complex, Japan

The Tanzawa plutonic complex (TPC), central Japan, is a suite of tonalitic-gabbroic plutons exposed in a globally unique arc-arc collision zone, where an active intraoceanic Izu-Bonin-Mariana (IBM) arc is colliding against the Honshu arc. The TPC has been widely accepted as an exposed middle crust section of the IBM arc, chiefly because of geochemical similarities between the TPC and IBM rocks and previously reported precollisional Miocene K-Ar ages. However, new zircon U-Pb ages show that the main pulse of TPC magmatism was syncollisional and that plutons were emplaced rapidly and cooled soon after Pliocene collision. Trace element compositions of TPC zircon show distinctively elevated Th/Nb ratios compared to zircon from other noncollisional IBM silicic plutonic rocks, indicating the involvement of continental sediments from the Honshu arc in their magma genesis.

Sensitive high-resolution ion microprobe analysis of zircon reequilibrated by late magmatic fluids in a hybridized pluton

Zircon from a microgranular enclave in the ca. 315 Ma postcollisional Karkonosze pluton (Western Sudetes, northeastern Bohemian Massif) is characterized by unusual morphologies and reequilibration textures. Blocky, clustered, and skeletal Th-U–rich zircon grains are internally corroded along discrete boundary zones, and subsequently replaced by porous microgranular aggregates of zircon and various other minerals, including thorite. The boundary zones have textures and compositions characteristic of diffusion-controlled chemical reaction fronts, including enrichment in Ca, Ba, and light rare earth elements, whereas microgranular domains are typical of zircon replacement and regrowth by coupled dissolution and precipitation. Initial zircon crystallization occurred with the mingling of mafic magma into a cooler granitic melt, whereas zircon modification is attributed to the reaction of late magmatic fluids from the host granite with the enclave. Precise dating of reequilibrated zircon as 304 ± 2 Ma indicates that fluid activity, which is also responsible for scheelite mineralization, postdates the emplacement of the main part of the pluton by several millions of years.

Termination of backarc spreading: Zircon dating of a giant oceanic core complex

The Godzilla megamullion is the largest oceanic core complex (OCC) currently known, and is adjacent to the spreading center of the Parece Vela Basin (PVB), an extinct backarc basin in the Philippine Sea. The duration and termination of tectonomagmatic processes during OCC formation are poorly constrained, due to the weak geomagnetic anomalies in the region. Zircon U-Pb dating of gabbroic and leucocratic rocks from the Godzilla megamullion reveals that fault-induced spreading over the ∼125 km length of the OCC lasted for ∼4 m.y., with continuous magmatic accretion at the spreading axis. The latest magmatism constrains the cessation of PVB spreading to ca. 7.9 Ma or later, significantly younger than a previous estimate of ca. 12 Ma. The new ages show that backarc basin formation migrated to the present-day Mariana Trough soon after the cessation of spreading in the PVB.

Changes in zircon chemistry during archean UHT metamorphism in the Napier complex, Antarctica

Zircons from two paragneisses (from Mount Sones and Dallwitz Nunatak) and one orthogneiss (from Gage Ridge) in the Tula Mountains, Napier Complex (East Antarctica) were analyzed for U-Pb age, oxygen isotopes, REEs and by scanning ion imaging. A large number of zircons from all samples are reversely discordant. Mount Sones zircons show an age range from 3.0 Ga to 2.5 Ga and underwent high-grade metamorphism at both ∼2.8 Ga and 2.5 Ga. Zircons from Dallwitz Nunatak record detrital ages between 3.5 Ga and 2.5 Ga. Zircons from Gage Ridge record multiple age groups, with concordant data between 3.6 Ga and 3.3 Ga and reversely discordant data that form a discrete ∼3.8 Ga population. All of the grains show evidence of Pb mobility during metamorphism. Ion imaging of zircons reveals Y and U zonation, characteristic of magmatic zircon, together with a micro-scale patchy distribution of 206Pb and 207Pb that does not correspond to either growth zonation or crystal imperfections. Some of these patches yield 207Pb/206Pb ages >4 Ga, whereas others yield ages younger than the magmatic crystallization age. Reversely discordant data are the result of ancient Pb mobilization, which is independent of the degree of metamictisation, oxygen isotope and REE content of the zircons. This mobilization can result in spurious ages and was most likely caused by polymetamorphism under anhydrous conditions; that is two high-grade events; one poorly defined at ∼2.8 Ga and the other ultra-high temperature (UHT) metamorphism at 2.5 Ga.

Metallic lead nanospheres discovered in ancient zircons

Significance Metallic lead nanospheres have been discovered in ancient (>3.4 Ga) zircon grains from an Archean (2.5 Ga) high-grade metamorphic terrain in East Antarctica. Native Pb is present as 5–30 nm nanospheres, commonly in association with an amorphous silica-rich phase, along with titanium and aluminium-bearing phases. Together, these phases form nanoinclusions generated during the recovery of crystallinity in radiation-damaged zircon under high-grade metamorphic conditions. Once formed, the entrapment of nanospheres in annealed zircon effectively arrests Pb loss, explaining why zircon that has experienced such extreme conditions is not completely reset to its metamorphic age. The heterogeneous distribution of Pb can, however, affect isotopic measurement by microbeam techniques, leading to spurious age estimates. Metallic Pb is extremely rare in nature and has never previously been observed in high temperature rocks. Zircon (ZrSiO4) is the most commonly used geochronometer, preserving age and geochemical information through a wide range of geological processes. However, zircon U–Pb geochronology can be affected by redistribution of radiogenic Pb, which is incompatible in the crystal structure. This phenomenon is particularly common in zircon that has experienced ultra-high temperature metamorphism, where ion imaging has revealed submicrometer domains that are sufficiently heterogeneously distributed to severely perturb ages, in some cases yielding apparent Hadean (>4 Ga) ages from younger zircons. Documenting the composition and mineralogy of these Pb-enriched domains is essential for understanding the processes of Pb redistribution in zircon and its effects on geochronology. Using high-resolution scanning transmission electron microscopy, we show that Pb-rich domains previously identified in zircons from East Antarctic granulites are 5–30 nm nanospheres of metallic Pb. They are randomly distributed with respect to zircon crystallinity, and their association with a Ti- and Al-rich silica melt suggests that they represent melt inclusions generated during ultra-high temperature metamorphism. Metallic Pb is exceedingly rare in nature and previously has not been reported in association with high-grade metamorphism. Formation of these metallic nanospheres within annealed zircon effectively halts the loss of radiogenic Pb from zircon. Both the redistribution and phase separation of radiogenic Pb in this manner can compromise the precision and accuracy of U–Pb ages obtained by high spatial resolution methods.

SHRIMP Zircon U-Pb Dating of Sapphirine-Bearing Granulite and Biotite-Hornblende Gneiss in the Schirmacher Hills, East Antarctica: Implications for Neoproterozoic Ultrahigh-Temperature Metamorphism Predating the Assembly of Gondwana

We applied SHRIMP zircon U-Pb age dating to ultrahigh-temperature (UHT) sapphirine-bearing orthopyroxene garnet (SOG) granulite and biotite-hornblende (Bt-Hbl) gneiss in the Schirmacher Hills, East Antarctica. In the Bt-Hbl gneiss, concordant ages of and Ma were obtained from zircon overgrowth rims and zircon cores, with oscillatory and irregular zoning, respectively. The zircon overgrowth rims ( Ma) with low Th/U ratios from the Bt-Hbl gneiss are interpreted as having a metamorphic origin. Oscillatory-zoned and/or irregularly zoned zircon cores may have crystallized during an igneous event at Ma; 800-Ma igneous events have not previously been recognized in central Dronning Maud Land (DML) inland nunatak. Zircons in the SOG granulite yielded a concordant age of Ma, using analyses of sector-zoned and simple-zoned grains. These zircons have relatively high Th/U ratios with a narrow range, and they occur in association with garnet breaking down to form cordierite. The -Ma age obtained from these zircons is interpreted as the timing of crystallization from a high-Th/U partial melt soon after peak metamorphism. The combination of a ca. 800-Ma igneous age and 660–640-Ma metamorphic ages obtained from Schirmacher Hills is different from that of other neighboring parts of central DML. In addition, a metamorphic PT path involving ultrahigh temperatures at early and subsequent isobaric cooling (IBC) stages at around 650 Ma has not previously been known in the central DML nunatak region. The ca. 650-Ma UHT metamorphic event probably occurred in a back-arc tectonic setting and predates the main collisional event of central DML (ca. 550–500 Ma).

Geosciences research in East Antarctica (0°E–60°E): present status and future perspectives

Abstract In both palaeoenvironmental and palaeogeographical studies, Antarctica plays a unique role in our understanding of the history of the Earth. It has maintained a unique geographical position at the South Pole for long periods. As the only unpopulated continent, the absence of political barriers or short-term economic interests has allowed international collaborative science to flourish. Although 98% of its area is covered by ice, the coastal Antarctic region is one of the well-studied regions in the world. The integrity and success of geological studies lies in the fact that exposed outcrops are well preserved in the low-latitude climate. The continuing programme of the Japanese Antarctic Research Expedition focuses on the geology of East Antarctica, especially in the Dronning Maud Land and Enderby Land regions. Enderby Land preserves some of the oldest Archaean rocks on Earth, and the Mesoproterozoic to Palaeozoic history of Dronning Maud Land is extremely important in understanding the formation and dispersion of Rodinia and subsequent assembly of Gondwana. The geological features in this region have great significance in defining the temporal and spatial extension of orogenic belts formed by the collision of proto-continents. Present understanding of the evolution of East Antarctica in terms of global tectonics allows us to visualize how continents have evolved through time and space, and how far back in time the present-day plate-tectonic regime may have operated. Although several fundamental research problems still need to be resolved, the future direction of geoscience research in Antarctica will focus on how the formation and evolution of continents and supercontinents have affected the Earths environment, a question that has been addressed only in recent years.

Post-peak (<530 Ma) thermal history of Lützow-Holm Complex, East Antarctica, based on Rb–Sr and Sm–Nd mineral chronology

Abstract Rb–Sr and Sm–Nd mineral dating of metamorphic rocks from Skallen, Skallevikshalsen and Rundvågshetta, in the southwestern part of the Lützow-Holm Complex, Dronning Maud Land, assists in constructing a thermal history after peak metamorphism. The results fall into two groups: (1) a record of regional cooling after peak metamorphism (524–488 Ma); (2) local resetting 50–80 Ma after peak metamorphism (474–446 Ma). This grouping is consistently observed in published ages from various localities in the Lützow-Holm Complex. A Sm–Nd age of 524 Ma is indistinguishable from published zircon and monazite ages. Ages of 511 and 488 Ma are related to cooling after peak metamorphism. The younger age group overlaps with ages of post-metamorphic magmatism and related hydrothermal activity reported from localities throughout East Antarctica. This intracontinental, post-orogenic igneous activity continued after the tectonic assembly of Gondwana.