Chris Clark

Magmatic and metamorphic events during the early Paleozoic Wuyi-Yunkai orogeny, southeastern South China: New age constraints and pressure-temperature conditions

The early Paleozoic Wuyi-Yunkai orogen in South China is a major orogenic belt in East Asia that formed at a similar time as the classic Caledonian orogeny in Europe. Despite the possibility of its being one of the few examples of intraplate orogenesis in the world, details about the orogen remain poorly defined. In this study, we provide age constraints on metamorphic and magmatic events in the eastern segment of the orogen, and the protoliths of the amphibolite-facies metamorphic rocks found there. By combining previous work with our new metamorphic and petrogenetic analyses, we present the following findings: (1) the Wuyi-Yunkai orogeny occurred between mid-Ordovician (>460 Ma) and earliest Devonian (ca. 415 Ma) time; (2) amphibolite-facies metamorphism in the eastern Wuyi-Yunkai orogen occurred between ca. 460 and 445 Ma, whereas cooling below 500–300 °C occurred by ca. 420 Ma; (3) the orogen exhibits a clockwise pressure-temperature ( P - T ) path and a maximum pressure of >8 kbar, indicating crustal thickening during the orogeny; (4) protoliths of the high-grade metamorphic rocks in the eastern segment of the orogen were dominantly Neoproterozoic (840–720 Ma) volcanic and volcaniclastic rift successions and younger deposits formed in a failed rift, and Paleoproterozoic rocks account for only a small proportion of the outcrops; and (5) the analyzed granites indicate a mixed source of Paleoproterozoic basement and Neoproterozoic continental rift rocks, with elevated melt temperatures of >800 °C, which are interpreted as reflecting dehydration melting of basin sediments taken to below midcrustal levels.

The oac gene encoding a lipopolysaccharide O-antigen acetylase maps adjacent to the integrase-encoding gene on the genome of Shigella flexneri bacteriophage Sf6

Lysogens of Shigella flexneri harbouring the temperate bacteriophage, Sf6, have been previously shown to undergo a serotype conversion due to O-acetylation of the O-antigen of the lipopolysaccharide. A partial physical map of the phage genome has been constructed. Analysis of the phage DNA suggests that the phage packages by a headful mechanism and that the mature DNA molecules are terminally redundant. Cloning of the PstI fragments of Sf6 enabled the region encoding the serotype conversion to be localized, showing that this was clearly phage-encoded. The gene was further localized by mutagenesis with Tn5 and the nucleotide sequence of the entire 2693-bp PstI fragment was determined. Two major open reading frames (ORFs) were found capable of encoding proteins of 44.1 and 37.2 kDa. The latter corresponds to the O-antigen acetylase and its gene has been designated oac. The oac gene is capable of converting Sh. flexneri serotypes X, Y, 1a and 4a to 3a, 3b, 1b and 4b, respectively. The Oac protein bears a high degree of homology to the NodX protein of Rhizobium leguminosarum suggesting that it, too, may be a sugar acetylase. The second ORF immediately upstream from oac corresponds to the bacteriophage Sf6 integrase responsible for chromosomal integration and is highly homologous to the integrases of Escherichia coli bacteriophages P4 and phi 80, but less closely related to those of P1, P2, P22, 186 and lambda.

Mobilization of radiogenic Pb in zircon revealed by ion imaging: Implications for early Earth geochronology

Zircon is arguably the most commonly used geochronometer, but the reliability of ages obtained requires a full understanding of processes that might compromise the integrity of its U-Pb systematics. Here we present the results of a multifaceted ion microprobe study of zircon grains from the Napier Complex, East Antarctica, a region affected by pervasive high-temperature metamorphism at 2.5 Ga, and from which previous zircon geochronological interpretations have been problematic. Both U-Pb spot analysis (∼15 μm) and high spatial resolution (∼2 μm) scanning ion imaging of Pb isotopes have been applied in an attempt to quantify the effects of metamorphism. Spot analyses spread along concordia yielding 207 Pb/ 206 Pb ages from 2.5 Ga to 3.9 Ga, with the oldest grains reversely discordant. Ion images of uranogenic Pb reveal a surprising micrometer-scale patchy distribution that is unrelated to crystal morphology or damage. The 207 Pb/ 206 Pb ratios within these subdomains correspond to apparent zircon ages as old as 4.2 Ga. These are interpreted as artifacts of ancient redistribution of radiogenic Pb, a process that can generate meaningless ages, and are not relicts of ancient (including Hadean) zircon. Scanning ion imaging thus facilitates identification of unsupported radiogenic Pb and enables testing of the validity of old ages from zircon known to have a long and complicated history.

On the application of in situ monazite chemical geochronology to constraining P–T–t histories in high-temperature (>850 °C) polymetamorphic granulites from Prydz Bay, East Antarctica

We present electron microprobe-based in situ (Th + U)–Pb monazite chemical age data from granulite-facies metapelites in Prydz Bay, East Antarctica. The monazite age data define two distinct age populations, Late Neoproterozoic to Early Palaeozoic (c. 570–520 Ma) and Neoproterozoic (c. 950–820 Ma), that confirm the polytectonic nature of Prydz Bay. Our data suggest that similarity in lineation orientation along the Prydz Bay coast is not sufficient to necessarily indicate time-equivalance. The minimum duration of Early Palaeozoic tectonism, spanning at least 60 Ma, is constrained from an Mg–Al-rich metapelite: monazite hosted by coarse-grained orthopyroxene defines the oldest Early Palaeozoic population, whereas cordierite + orthopyroxene symplectites define the youngest Early Palaeozoic population. The spatial distribution of monazites and their ages is correlatable with the inferred mineral assemblage evolution. We are able to characterize the evolution of Early Palaeozoic tectonism in quantitative P–T–t–mineral assemblage space, demonstrating that ultrahigh-temperature tectono-metamorphism in Prydz Bay is of Early Palaeozoic age. The survival of Neoproterozoic inheritance in Fe–Al-rich metapelites has implications for high Pb retentivity in monazite when chemical and kinetic conditions are favourable. The approach and logic applied herein are entirely and directly transferable to the interrogation of any other metamorphic terrane.

Detrital zircons in basement metasedimentary protoliths unveil the origins of southern India

Coupled U-Pb and Hf isotopic analysis of detrital zircons from metasedimentary rocks of the Southern Granulite terrane (India) provides provenance information that helps unravel their paleotectonic position before Gondwana amalgamated. The metasedimentary packages of the Salem block (southernmost extension of Dharwar craton) record a restricted juvenile late Archean to early Paleoproterozoic (2.7–2.45 Ga) source provenance and epsilon Hf values between +0.3 and +8.8. Similar late Archean juvenile crust is found throughout the Dharwar craton and represents a likely source for the Salem block metasedimentary rocks. By contrast, the metasedimentary rocks of the Madurai block (south of the Salem block) show a predominantly Archean to Paleoproterozoic provenance (3.2–1.7 Ga) in the northern part of the Madurai block and a largely late Mesoproterozoic to Neoproterozoic provenance (1.1–0.65 Ga) in the southern part of the Madurai block. Collectively, the Madurai block metasedimentary rocks record a mixture of reworked Archean and Paleoproterozoic sources, as well as juvenile Paleoproterozoic, late Mesoproterozoic, and evolved Neoproterozoic sources. These detrital signatures best fit the combined basement ages of the Congo-Tanzania-Bangweulu block and central Madagascar (Azania), thus linking the tectonic evolution of the southernmost tip of India to these domains throughout much of the Proterozoic. The diachroneity of metamorphic ages obtained from the rims of Madurai block detrital zircons attests to their poly-metamorphic history that is different from that of the Salem block. The contrasting metamorphic and depositional histories between the Salem and Madurai blocks place them on opposite sides of the Mozambique Ocean until the latest Neoproterozoic when they came together to form Gondwana.

Cambrian reworking of the southern Australian Proterozoic Curnamona Province: constraints from regional shear-zone systems

Palaeoproterozoic to early Mesoproterozoic metamorphic rocks of the Curnamona Province, southern Australia, are crosscut by a system of regional-scale shear zones that locally dominate the terrain. Combined metamorphic and geochronological data from localities across the southern Curnamona Province indicate that the peak metamorphic shear-zone assemblages formed during the Cambrian (c. 500 Ma) Delamerian Orogeny, and not during the waning stages of the c. 1600 Ma Olarian Orogeny as has been previously asserted. A combination of monazite chemical U–Th–Pb and garnet Sm–Nd geochronology indicates that shear-zone fabrics formed between 497 and 517 Ma. Peak metamorphic conditions obtained from prograde garnet–staurolite–biotite–muscovite–chlorite–quartz assemblages are between 530 and 600 °C at pressures of around 5 kbar. The apparent absence of significant up-pressure prograde paths recorded by the mineral assemblages, together with modest (10–20%) Delamerian shortening, suggests that attainment of burial to depths of around 18 km was largely a function of pre-Delamerian sedimentation over the interval from c. 700 to 530 Ma. The spatial association between the pattern of basement metamorphism and reactivation during the Delamerian Orogeny is interpreted to reflect in part the distribution of pre-Delamerian sedimentation, and highlights the potential importance of pre-orogenic processes such as basin development in controlling the style and pattern of later terrain reactivation and reworking.

Hydrothermal brecciation due to fluid pressure fluctuations: examples from the Olary Domain, South Australia

Abstract Two breccias, the Doughboy and Cathedral Rock breccias, hosted by the Willyama Supergroup in the Olary Domain, northeastern South Australia, were mapped and described in this study. Mapping of the breccias has demonstrated that they are controlled by the regional deformational history of the area and they are intimately related to episodes of hydrothermal fluid flow and alteration. The clast morphology, particle size distribution, dilation ratio, structural position and contact relationships with the host rocks allow the degree of maturity and the nature of the fluid rock interactions for each breccia to be determined. Breccia clast size distribution and morphology were quantified using fractal analysis techniques. Textural analysis revealed significant differences in particle size distribution and clast morphology for the different breccias. The results of the fractal analysis indicate that the Doughboy breccia is the result of multiphase tectonic and fluid-assisted failure on a fault plane, whereas the Cathedral Rock breccia is the result of fluid-assisted failure during fold-related faulting. Due to the high fluid pressure constraints on reverse fault initiation, large fluid pressure differentials are generated during the opening of dilatant sites. It is the opening of these dilatant spaces which provides the room to accommodate and the mechanism for brecciation. Reverse faulting created dilatant spaces that, due to the large pressure differential between the wall rock and the dilational space, initiated failure of the wall rock. Whether this was a single event such as at Cathedral Rock or occurred during multiple events such as at Doughboy can be deduced via the fractal characteristics. Collectively, these observations shed light on the processes that occur during regional deformational events associated with multiple generations of hydrothermal activity. The relationship between fluid pressure and brecciation may explain why they are the sites where the most intense effects of hydrothermal activity in the Olary Domain occur.

Evidence for 930 Ma metamorphism in the Shetland Islands, Scottish Caledonides: Implications for Neoproterozoic tectonics in the Laurentia-Baltica sector of Rodinia

Abstract: Zircon and monazite laser-ablation inductively coupled plasma mass spectrometry U–Pb geochronological data for two metasediment samples from the Westing Group, northern Shetland Islands, Scottish Caledonides yield ages between 938 ± 8 and 925 ± 10 Ma (Tonian) for upper amphibolites-facies metamorphism. Texturally early metamorphism is recorded by a migmatitic garnet + sillimanite + plagioclase + muscovite + biotite assemblage, which formed at c. 650–700 °C and 7 kbar. Subsequent reworking resulted in the growth of a secondary garnet + kyanite + plagioclase + muscovite + biotite assemblage at c. 650 °C and 8–9 kbar. In situ electron probe microanalysis (EPMA) U–Th–Pb chemical dating of monazite hosted within garnet grains and the matrix of one sample also give Tonian ages, apparently indicating that all the metamorphism occurred during the Neoproterozoic. However, the dominant structural fabrics appear to have formed during the Ordovician–Silurian Caledonian orogeny, suggesting that the reworking was substantially younger despite the apparent absence of Caledonian monazite or zircon ages. Detrital zircons are consistent with Laurentia–Baltica provenance. Deposition of the Westing Group is constrained to between c. 1030 and 930 Ma. The timing of Tonian metamorphism suggests possible correlations with sequences elsewhere in the northern Caledonides, including the Krummedal Succession of East Greenland and Laurentian-derived successions in Svalbard and northern Norway. Supplementary material: U–Pb LA-ICPMS and EPMA data are available at http://www.geolsoc.org/SUP18379.

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.

Gene capture in Vibrio cholerae

We give our special thanks to Ruth Hall for many useful discussions, which have helped develop this work over the years. In addition, the financial support of the National Health and Medical Research Council of Australia is gratefully acknowledged.