Allen K. Kennedy

SHRIMP U-Pb zircon geochronological and geochemical evidence for Neoproterozoic arc-magmatism along the western margin of the Yangtze Block, South China

Abstract The magmatic and tectonic history of the Yangtze Block and its possible affinity with other Neoproterozoic arc terranes are important in the reconstruction of Neoproterozoic plate tectonics. In the Panxi Belt, adjacent to the eastern margin of the Tibetan Plateau, there are many metamorphic complexes associated with Neoproterozoic granites. These are granitic gneisses of upper greenschist to amphibolite metamorphic facies, which have traditionally been considered the Archean basement of the Yangtze Block, although their origin and age of formation were poorly understood. This study provides the first reliable, SHRIMP U–Pb zircon dating results for the gneissic complexes and the Neoproterozoic granites. Three samples of the Kangding gneissic complex yielded identical ages of 797±10, 795±13 and 796±14 Ma. The Gongcai gneissic complex has zircons dated to be 824±14 Ma with metamorphic rims of 177±3 Ma, whereas the Gezong granite has an older age of 864±8 Ma. Other gneissic complexes include the Miyi complex that has a younger age of 764±9 Ma. Geochemical data show that the Kangding gneissic complex has arc signatures, representing metamorphic products of Neoproterozoic, arc-related acidic plutons. This scenario suggests subduction of oceanic lithosphere eastward (present-day orientation) underneath the Yangtze Block. There is a well-defined arc assemblage with an identical Neoproterozoic age along the eastern margin of the Yangtze Block. Thus, during Neoproterozoic time, both the western and eastern margins of the block were active arcs separated by the Trans-Yangtze basin. The Yangtze Block must, therefore, have been an isolated continent, although it was presumably located near the Rodinian supercontinent.

A temporal link between the Emeishan large igneous province (SW China) and the end-Guadalupian mass extinction

Abstract Previous studies have suggested that there were two mass extinction events in the Late Permian: one that occurred at the Permo-Triassic (P/T) boundary (251 Ma) and a second, smaller mass extinction that occurred 5–8 Myr earlier at the end of the Guadalupian. Many workers have argued that there is a causal relationship between large-scale volcanic activity and mass extinctions. The major mass extinction event at the P/T boundary coincides with the outpouring of huge quantities of lava that formed the Siberian flood basalt province in Russia. Courtillot et al. [Earth Planet. Sci. Lett. 166 (1999) 177–195] and Wignall [Earth Sci. Rev. 53 (2001) 1–33] suggested that the earlier Late Permian mass extinction coincided with the eruption of the lavas that formed the Emeishan flood basalt (EFB) province in SW China. However, the age of eruption of the EFB lavas is poorly constrained. Using the Sensitive High-Resolution Ion Microprobe to analyze zircons, we have established the age of the Xinjie intrusion, believed to be a feeder to the main phase of EFB volcanism, to be 259±3 Ma. Hence, the formation of the EFB is coincident with a proposed extinction event at 256–259 Ma. This result supports a temporal link between the Emeishan large igneous province and the end-Guadalupian mass extinction.

Neoproterozoic Arc‐Related Mafic Intrusions along the Northern Margin of South China: Implications for the Accretion of Rodinia

South China has been considered as part of the Rodinian supercontinent during Neoproterozoic time, although its paleogeographic position within this supercontinent is still a matter of debate. The Wangjiangshan and Bijigou complexes along the northern margin of South China are among the largest mafic intrusions in China. New SHRIMP zircon U‐Pb results indicate that these two intrusions have crystallization ages of 820 Ma and 780 Ma, respectively. Enrichment of large ion lithophile and light rare earth elements and depletion of high field‐strength elements in these intrusions suggest derivation from an active arc along a continental margin. This interpretation of these intrusions as part of a continental arc assemblage is in contrast with the previous view that they were products of a Neoproterozoic mantle plume that initiated the breakup of Rodinia. The presence of a Neoproterozoic magmatic arc suggests that Rodinian oceanic lithosphere was subducted beneath the (present) northern margin of South China and therefore that South China flanked the Rodinian ocean.

Linking growth episodes of zircon and metamorphic textures to zircon chemistry; an example from the ultrahigh-temperature granulites of Rogaland, SW Norway

Abstract In-situ U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland anorthosite-norite intrusive complex; and (b) to constrain the age of M2 ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M3 event. All samples yield magmatic inherited zircon of c. 1035 Ma, some an additional group at c. 1050 Ma. This suggests that loss of Pb by volume diffusion in non-metamict zircon is not an important factor even under extreme crustal conditions. Furthermore, the identical inheritance patterns in aluminous (garnet, cordierite ± osumilite-bearing) migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite. Results for the M1 metamorphic event at c. 1000 Ma BP are consistent in all samples, including those from outside the orthopyroxene-in isograd. The latter do not show evidence for zircon growth during the M2 metamorphic episode. Zircon intergrown with or included within M2 metamorphic minerals (magnetite, spinel, orthopyroxene) give an age of 927 ± 7 Ma (2σ, n = 20). The youngest observed results are found in zircon outside M2 minerals, some overgrown by M3 mineral assemblages (late garnet coronas, garnet + quartz and orthopyroxene + garnet symplectites) and yield a slightly younger pooled age of 908 ± 9 Ma (2σ, n = 6). These textures are relative time markers for the crystallization of zircon overgrowths during discrete stages of the UHT event. These youngest age groups are consistent with the emplacement age of the Rogaland intrusive complex and the last magmatic activity (Tellnes dyke intrusion), respectively. This is direct and conclusive evidence for UHT metamorphism in the regional aureole being caused by the intrusions, and corrects earlier notions that the events are not linked. Trace element behaviour of zircon (Tb/U and Y content) has been tracked through time in the samples and shows variations both within and between samples. This heterogeneous behaviour at all scales appears to be common in metamorphic rocks and precludes the use of ‘rules of thumb’ in the interpretation of zircon chemistry, but chemical tracers are useful for recognition of zircon growth or recrystallization during metamorphism.

Incomplete retention of radiation damage in zircon from Sri Lanka

Abstract A suite of 18 zircon gemstones from placers in the Highland/Southwestern Complex, Sri Lanka, were subjected to a comprehensive study of their radiation damages and ages. The investigation included X-ray diffraction, Raman and PL spectroscopy, electron microprobe, PIXE and HRTEM analysis, as well as (U-Th)/He and SHRIMP U-Th-Pb age determinations. Zircon samples described in this study are virtually homogeneous. They cover the range from slightly metamict to nearly amorphous. Generally concordant U-Th-Pb ages averaging 555 ± 11 Ma were obtained. Late Ordovician zircon (U-Th)/He ages scattering around 443 ± 9 Ma correspond reasonably well with previously determined biotite Rb-Sr ages for rocks from the HSWC. Slightly to moderately metamict zircon has retained the radiogenic He whereas only strongly radiation-damaged zircon (calculated total fluences exceeding ~3.5 × 1018 α-events/g) has experienced significant He loss. When compared to unannealed zircon from other localities, Sri Lanka zircon is about half as metamict as would correspond to complete damage accumulation over a ~555 m.y. lasting self-irradiation period, suggesting significant annealing of the structural radiation damage. Insufficient consideration of this has often resulted in significant underestimation of radiation effects in zircon. We suggest to estimate “effective α-doses” for Sri Lanka zircon by multiplying total α-fluences, which were calculated using the zircon U-Th-Pb age, by a correction factor of 0.55. This conversion may be applied to literature data as well, because all gem-zircon samples from Sri Lanka (this work and previous studies) seem to reveal the same general trends of property changes depending on the radiation damage. The use of “effective α-doses” for Sri Lanka zircon contributes to more reliable quantitative estimates of radiation effects and makes possible direct comparison between natural and synthetic radiation-damaged zircon.

Origin and age of the earliest Martian crust from meteorite NWA 7533

The ancient cratered terrain of the southern highlands of Mars is thought to hold clues to the planet’s early differentiation, but until now no meteoritic regolith breccias have been recovered from Mars. Here we show that the meteorite Northwest Africa (NWA) 7533 (paired with meteorite NWA 7034) is a polymict breccia consisting of a fine-grained interclast matrix containing clasts of igneous-textured rocks and fine-grained clast-laden impact melt rocks. High abundances of meteoritic siderophiles (for example nickel and iridium) found throughout the rock reach a level in the fine-grained portions equivalent to 5 per cent CI chondritic input, which is comparable to the highest levels found in lunar breccias. Furthermore, analyses of three leucocratic monzonite clasts show a correlation between nickel, iridium and magnesium consistent with differentiation from impact melts. Compositionally, all the fine-grained material is alkalic basalt, chemically identical (except for sulphur, chlorine and zinc) to soils from Gusev crater. Thus, we propose that NWA 7533 is a Martian regolith breccia. It contains zircons for which we measured an age of 4,428 ± 25 million years, which were later disturbed 1,712 ± 85 million years ago. This evidence for early crustal differentiation implies that the Martian crust, and its volatile inventory, formed in about the first 100 million years of Martian history, coeval with earliest crust formation on the Moon and the Earth. In addition, incompatible element abundances in clast-laden impact melt rocks and interclast matrix provide a geochemical estimate of the average thickness of the Martian crust (50 kilometres) comparable to that estimated geophysically.

A double focusing mass spectrometer for geochronology

Abstract The performance characteristics of a double focusing mass spectrometer designed specifically for geochronology are described. The sensitive high resolution ion micro probe (SHRIMP II) mass spectrometer is ideally suited for in-situ analysis of uranium and/or thorium-bearing minerals to provide geochronological information on micron-sized domains of the minerals. SHRIMP II simultaneously meets the requirements of high mass resolution and high ion transmission efficiency together with good abundance sensitivity. SHRIMP II’s excellent spatial and mass resolution, coupled with its high sensitivity, has enabled zircon geochronology to be revolutionized.

Annealing radiation damage and the recovery of cathodoluminescence

The structural recovery upon heat treatment of a highly metamict, actinide-rich zircon (U~6000 ppm) has been studied in detail using a range of techniques including X-ray powder diffraction, Raman spectroscopy, SHRIMP ion probe, electron microprobe, transmission electron microscopy and cathodoluminescence analysis. The structural regeneration of the amorphous starting material depends on random nucleation. It starts between 800 and 950°C when amorphous ZrSiO4 decomposes to form crystalline ZrO2 and amorphous SiO2. At around 1100°C, well-crystallised ZrSiO4 grows at the expense of the oxides. U has been retained in the newly grown zircon whereas Pb was evaporated during the heat treatment. This process is in marked opposition to the reconstitution of moderately metamict minerals, which experience a gradual recovery controlled by the epitaxial growth at the crystalline–amorphous boundaries. Both of these recovery processes are not the direct inverse of metamictisation. The structural regeneration was found to be connected with a significant increase in the emission of CL. In all cases (annealing heavily damaged zircon and moderately damaged zircon and monazite), we observe that the final, wellcrystallised annealing products emit more intense CL than their radiation-damaged starting minerals, although having almost identical elemental composition. Our observations are taken as evidence that the CL is not only determined by the chemical composition of the sample but is also strongly controlled by structural parameters such as crystallinity or the presence of defect centres.

Magmatic flare-up at the Carboniferous/Permian boundary in the NE German Basin revealed by SHRIMP zircon ages

SHRIMP ages of 12 volcanic samples indicate intense magmatic activity to have occurred at the Carboniferous/Permian boundary throughout much of the NE German Basin. Rhyolitic crystal-rich samples have been taken from quarries in the Halle Volcanic Complex (HVC) and in the Flechtingen Ignimbrite, and from drill cores of the Kotzen, Mirow, Friedland and Penkun areas. Ten samples yielded 206Pb/238U ages between 302 and 297 Ma (±3 Ma, respectively) which indicate that the magmatic activity took place concentrated in a relatively short time span throughout much of the NE German Basin. Two HVC samples have ages of 307 and 294 Ma. This remarkably synchronous magmatic activity occurred during the initial phase of the basin development. Considering published volume estimates (48,000 km3), extrusion rates during this time period probably were in the order of 0.01 km3 a−1, implying magma production rates of 0.1 km3 a−1. The calc-alkaline SiO2-rich volcanic rocks originated from magmas that presumably formed during anatexis and subordinate magma mixing in an intra-continental transtensional setting. The basaltic magma which must have provided the thermal input into the lower crust probably formed during decompressional melting of lithospheric mantle fertilised by previous magmatic processes. The dated volcanic rocks occur in three geotectonic provinces, namely the (a) Mid German Crystalline Rise, which forms the northern part of the Internal Variscides, the (b) External Variscides and the Variscan foreland which is considered as (c) Eastern Avalonia. Many of the old zircons found in the HVC samples reflect the magmatic activity of the Mid German Crystalline Rise (325–400 Ma). However, Cadomian (500–650 Ma) and older Gondwanian elements (1674–2373 Ma) are also present. Although the Flechtingen and Kotzen areas are located within the External Variscides, it seems more likely that the middle to lower crust, that experienced anatexis, formed part of the Eastern Avalonia Plate overridden by the Variscan Front. Thus the old zircons found in Flechtingen (350 and 538 Ma) and Kotzen (345 Ma), but certainly those present in the Penkun drill cores (1483 Ma) represent Eastern Avalonia tectonic history. The anatectic magmas in the Friedland area probably formed partly from Baltic crust thrusted below Eastern Avalonia during the Caledonian Orogeny. Among the old zircons from the Friedland samples, in addition to Proterozoic ages (1456 Ma), we found testimony of Caledonian (443 Ma) and of clearly post-Caledonian (387 Ma) magmatism.

Dating of zircon and monazite from diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge – hints at burial and exhumation velocities

Abstract In order to better understand the formation and evolution processes of ultrahigh pressure (UHP) felsic rocks, we determined the ages of various domains of zircon and monazite crystals from the diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge. According to cathodoluminescence imagery and Th/U ratios, three zircon zones were distinguished. Each was dated using several spot analyses from a sensitive high-resolution ion microprobe analysing Pb, U and Th isotopes. The results were: (1) core zone - 21 analyses: Th/U ≤0.023 and 337.0±2.7 Ma (2σ, combined 206Pb/238U-207Pb/235U age); (2) diamond-bearing intermediate zone - 23 analyses: Th/U ≥0.037 and 336.8±2.8 Ma; and (3) rim zone -12 analyses: Th/U = 0.015-0.038 (plus one analysis of 0.164) and 330.2±5.8 Ma. The U-Pb obtained ages are virtually concordant. Furthermore, two oscillatory zoned zircon cores (Th/U ≥0.8) yielded (~concordant) ages of ~400 Ma. Six SHRIMP analyses of monazites gave an age of 332.4±2.1 Ma. In addition, Pb, Th and U contents in monazite were analysed with an electron microprobe (EMP). A mean age of 324.7±8.0 (2σ) Ma was acquired from 113 analyses. By combining the defined ages with previously published P-T conditions, minimum velocities for burial and exhumation were estimated. In addition, we present a likely geodynamic scenario involving age data from the literature as well as this study: beginning 340 million years ago, gneisses at the base of a thickened continentalcrust (~1.8 GPa, 650ºC) were transported to depths of at least 130 km, possibly as deep as 250 km. Here they were heated (>1050ºC) and partially melted and as a result began to rise rapidly. The burial and subsequent ascent back to a depth of 50 km, where zircon rims and monazite formed, took only a few million years and perhaps significantly less.