Trevor R. Ireland

U/PB ZIRCON AGES CONSTRAIN THE ARCHITECTURE OF THE ULTRAHIGH-PRESSURE QINLING-DABIE OROGEN, CHINA

New SHRIMP and TIMS zircon ages, 40Ar/39Ar ages, and eclogite locations contribute significantly to our understanding of the ultrahigh-pressure Dabie Shan. (1) The geographic extent of the Yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the Dabie Shan. (2) The northern half of the Dabie Shan is a magmatic complex, intruded over a 10-Myr interval between 137 and 126 Ma, that accommodated ∼100% N–S stretching of the pre-existing collisional architecture. (3) Granitic orthogneisses and enclosing ultrahigh-pressure paragneisses have indistinguishable zircon populations. The population of Triassic zircon ages ranges from ∼219 to ∼245 Ma, leading us to question the prevailing assumption that 219 Ma zircons formed at ultrahigh pressure, and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation.

Tectonics of the Qinling (Central China): tectonostratigraphy, geochronology, and deformation history

Abstract The Qinling orogen preserves a record of late mid-Proterozoic to Cenozoic tectonism in central China. High-pressure metamorphism and ophiolite emplacement (Songshugou ophiolite) assembled the Yangtze craton, including the lower Qinling unit, into Rodinia during the ∼1.0 Ga Grenvillian orogeny. The lower Qinling unit then rifted from the Yangtze craton at ∼0.7 Ga. Subsequent intra-oceanic arc formation at ∼470–490 Ma was followed by accretion of the lower Qinling unit first to the intra-oceanic arc and then to the Sino-Korea craton. Subduction then imprinted a ∼400 Ma Andean-type magmatic arc onto all units north of the northern Liuling unit. Oblique subduction created Silurian–Devonian WNW-trending, sinistral transpressive wrench zones (e.g., Lo-Nan, Shang-Dan), and Late Permian–Early Triassic subduction reactivated them in dextral transpression (Lo-Nan, Shang-Xiang, Shang-Dan) and subducted the northern edge of the Yangtze craton. Exhumation of the cratonal edge formed the Wudang metamorphic core complex during dominantly pure shear crustal extension at ∼230–235 Ma. Post-collisional south-directed shortening continued through the Early Jurassic. Cretaceous reactivation of the Qinling orogen started with NW–SE sinistral transtension, coeval with large-scale Early Cretaceous crustal extension and sinistral transtension in the northern Dabie Shan; it presumably resulted from the combined effects of the Siberia–Mongolia—Sino-Korean and Lhasa–West Burma—Qiangtang–Indochina collisions and Pacific subduction. Regional dextral wrenching was active within a NE–SW extensional regime between ∼60 and 100 Ma. An Early Cretaceous Andean-type continental magmatic arc, with widespread Early Cretaceous magmatism and back-arc extension, was overprinted by shortening related to the collision of Yangtze–Indochina Block with the West Philippines Block. Strike–slip and normal faults associated with Eocene half-graben basins record Paleogene NNE–SSW contraction and WNW–ESE extension. The Neogene(?) is characterized by normal faults and NNE-trending sub-horizontal extension. Pleistocene(?)–Quaternary NW–SE extension and NE–SW contraction comprises sinistral strike–slip faults and is part of the NW–SE extension imposed across eastern Asia by the India–Asia collision.

Rare earth element chemistry of zircon and its use as a provenance indicator

Sedimentary mineral assemblages commonly contain detrital zircon crystals as part of the heavy-mineral fraction. Age spectra determined by U-Pb isotopic analysis of single zircon crystals within a sample may directly image the age composition—but not the chemical composition—of the source region. Rare earth element (REE) abundances have been measured for zircons from a range of common crustal igneous rock types from different tectonic environments, as well as kimberlite, carbonatite, and high-grade metamorphic rocks, to assess the potential of using zircon REE characteristics to infer the rock types present in sediment source regions. Except for zircon with probable mantle affinities, zircon REE abundances and normalized patterns show little intersample and intrasample variation. To evaluate the actual variation in detrital zircon REE composition in a true sediment of known mixed provenance, zircons from a sandstone sample from the Statfjord Formation (North Sea) were analyzed. Despite a provenance including high-grade metasediment and granitoids and a range in zircon age of 2.82 b.y., the zircon REEs exhibit a narrow abundance range with no systematic differences in pattern shape. These evidences show zircon REE patterns and abundances are generally not useful as indicators of provenance.

Considerations in zircon geochronology by SIMS

Secondary ion mass spectrometry (SIMS) is a versatile technique for measuring the chemical and isotopic composition of solid materials on a scale of a few microns. A beam of high-energy primary ions is focused onto the polished target surface, sputtering (ablating) atoms and molecules, and in the process ionizing a small fraction. These secondary ions, which reflect the target composition, are analyzed by mass spectrometry. The SIMS instrument most common in geoscience is the ion microprobe, which uses a focused primary ion beam in either static or scanning mode to sample target areas usually 10- to 50-μm diameter. Total sampling depth is typically less than 5 μm and the sampled mass only a few nanograms, making the analysis for most samples effectively non-destructive. Coupled with surface imaging techniques such as backscattered electron (BSE) and cathodoluminescence (CL), SIMS enables finely targeted chemical and isotopic analysis of specific domains exposed on a crystal surface. Zircon is particularly suitable for SIMS U–Th–Pb geochronology. It is a physically and chemically robust mineral that crystallizes under a range of geological conditions, incorporating trace U and Th, but little or no Pb. Zircon grains are commonly composite, having survived and grown during several geological events. This growth record is sometimes visible under an optical microscope, but is best revealed by CL and BSE imaging of polished sectioned grains (Fig. 1⇓). The U–Th–Pb closure temperature of unaltered zircon is very high (>900°C), and the growth domains commonly can preserve an isotopic record of thermal events spanning tens to thousands of millions of years. This record of provenance, and igneous and metamorphic history, is accessible only to microanalytical techniques such as SIMS. Figure 1. Both zircon crystals appear homogeneous in transmitted light but cathodoluminescence images show the true growth structures: (a) an igneous zircon with simple euhedral oscillatory zoning, …

Development of the early Paleozoic Pacific margin of Gondwana from detrital-zircon ages across the Delamerian orogen

Detrital-zircon age spectra have been determined for sedimentary rocks from the Delamerian orogen, southern Australia. In Neoproterozoic sedimentary rocks, patterns progressively change from Mesoproterozoic- to Neoproterozoic-dominated detritus and there are few zircons that are close to the depositional age. The base of the Cambrian Kanmantoo Group marks an abrupt change in provenance to detrital patterns dominated by Ross and Delamerian (600–500 Ma) and Grenvillean ages (1200–1000 Ma). These patterns are strikingly similar to those obtained from Lachlan fold belt sedimentary rocks, indicating that the sedimentation recorded in the Kanmantoo Group marks a change from deposition of sediments derived from the Australian cratons to those representative of the early Paleozoic Gondwana mudpile. If sedimentary rocks with zircon-provenance characteristics such as those of the Kanmantoo rocks extend under elements of the Lachlan fold belt, they would provide suitable protoliths for the S-type granites of southeastern Australia.

CARBON, NITROGEN, MAGNESIUM, SILICON, AND TITANIUM ISOTOPIC COMPOSITIONS OF SINGLE INTERSTELLAR SILICON CARBIDE GRAINS FROM THE MURCHISON CARBONACEOUS CHONDRITE

Seven hundred and twenty SiC grains from the Murchison CM2 chondrite, ranging in size from 1 to 10 micrometers, were analyzed by ion microprobe mass spectrometry for their C-isotopic compositions. Subsets of the grains were also analyzed for N (450 grains), Si (183 grains), Mg (179 grains), and Ti (28 grains) isotopes. These results are compared with previous measurements on 41 larger SiC grains (up to 15 x 26 micrometers) from a different sample of Murchison analyzed by Virag et al. (1992) and Ireland, Zinner, & Amari (1991a). All grains of the present study are isotopically anomalous with C-12/C-13 ratios ranging from 0.022 to 28.4 x solar, N-14/N-15 ratios from 0.046 to 30 x solar, Si-29/Si-28 from 0.54 to 1.20 x solar, Si-30/Si-28 from 0.42 to 1.14 x solar, Ti-49/Ti-48 from 0.96 to 1.95 x solar, and Ti-50/Ti-48 from 0.94 to 1.39 x solar. Many grains have large Mg-26 excesses from the decay of Al-26 with inferred Al-26/Al-27 ratios ranging up to 0.61, or 12,200 x the ratio of 5 x 10(exp -5) inferred for the early solar system. Several groups can be distinguished among the SiC grains. Most of the grains have C-13 and N-14 excesses, and their Si isotopic compositions (mostly excesses in Si-29 and Si-30) plot close to a slope 1.34 line on a Delta Si-29/Si-28 versus Delta Si-30/Si-28 three-isotope plot. Grains with small C-12/C-13 ratios (less than 10) tend to have smaller or no N-14 excesses and high Al-26/Al-27 ratios (up to 0.01). Grains with C-12/C-13 greater than 150 fall into two groups: grains X have N-15 excesses and Si-29 and Si-30 deficits and the highest (0.1 to 0.6) Al-26/Al-27 ratios; grains Y have N-14 excesses and plot on a slope 0.35 line on a Si three-isotope plot. In addition, large SiC grains of the Virag et al. (1992) study fall into three-distinct clusters according to their C-, Si-, and Ti-isotopic compositions. The isotopic diversity of the grains and the clustering of their isotopic compositions imply distinct and multiple stellar sources. The C- and N-isotopic compositions of most grains are consistent with H-burning in the CNO cycle. These and s-process Kr, Xe, Ba, and Nd suggest asymptotic giant branch (AGB) or Wolf-Rayet stars as likely sources for the grains, but existing models of nucleosynthesis in these stellar sites fail to account in detail for all the observed isotopic compositions. Special problems are posed by grains with C-12/C-13 less than 10 and almost normal and heavy N-isotopic compositions. Also the Si- and Ti-isotopic compositions, with excesses in Si-29 and Si-30 relative to Si-28 and excesses in all Ti isotopes relative to Ti-48, do not precisely conform with the compositions predicted for slow neutron capture. Additional theoretical efforts are needed to achieve an understanding of the isotopic composition of the SiC grains and their stellar sources.

Three-Dimensional Structure of Hayabusa Samples: Origin and Evolution of Itokawa Regolith

Laboratory analysis of samples returned from an asteroid establishes a direct link between asteroids and meteorites and provides clues to the complex history of the asteroid and its surface. Regolith particles on the asteroid Itokawa were recovered by the Hayabusa mission. Their three-dimensional (3D) structure and other properties, revealed by x-ray microtomography, provide information on regolith formation. Modal abundances of minerals, bulk density (3.4 grams per cubic centimeter), and the 3D textures indicate that the particles represent a mixture of equilibrated and less-equilibrated LL chondrite materials. Evidence for melting was not seen on any of the particles. Some particles have rounded edges. Overall, the particles’ size and shape are different from those seen in particles from the lunar regolith. These features suggest that meteoroid impacts on the asteroid surface primarily form much of the regolith particle, and that seismic-induced grain motion in the smooth terrain abrades them over time.

Geochronology and geochemistry of a Mesozoic magmatic arc system, Fiordland, New Zealand

The Median Tectonic Zone in Eastern Fiordland, SW New Zealand, comprises a tectonically disrupted belt of Mesozoic magmatic arc rocks related to subduction along the palaeo-Pacific margin of Gondwana. New ion microprobe (SHRIMP) U–Pb zircon ages confirm that the bulk of the plutonic rocks in eastern Fiordland range from Mid-Jurassic to Early Cretaceous (168–137 Ma) in age. Carboniferous age granitoids occur in SW Fiordland, along the western side of, and within the zone. Triassic plutonic rocks appear to be restricted to the eastern side of the zone. The Mid-Jurassic–Early Cretaceous igneous rocks (collectively referred to as the Darran Suite) are cut by several plutons of Na-rich granitoid (Separation Point Suite) that give ages of c. 124 Ma, slightly older than equivalent rocks in the NW part of the South Island. Early Cretaceous granulite facies orthogneisses (126–119 Ma) in western Fiordland (Western Fiordland Orthogneiss) are considered to be the lower crustal equivalent of the Separation Point plutons. The majority of the Darran Suite rocks are I-type, hornblende-bearing calc-alkaline igneous rocks, most likely derived from melting in the mantle wedge above a subducting slab of oceanic lithosphere. In contrast, the Separation Point-type plutons are Na-rich, alkali-calcic granitoids with high concentrations of Sr (typically >500 ppm and up to 1000 ppm) and low concentrations of Y (≤5 ppm) and heavy REE (<10 times chondritic). Isotopic compositions are primitive, with 87Sr/86Sr initial ratios of c. 0.7038, and åNd values of c. +3 at 120 Ma. Their geochemistry is consistent with melting of a mafic protolith of garnet amphibolite mineralogy. Mafic Darran Suite rocks have the appropriate chemical and isotopic compositions to generate the Western Fiordland Orthogneiss and the higher level Separation Point type plutons. We suggest that the sudden appearance of large volumes of Na-rich magma during the Early Cretaceous was triggered tectonically, perhaps by thrusting of the Median Tectonic Zone arc beneath western New Zealand. Melting of basal arc underplate at depths of >40 km would then have generated Na-rich granitoids, leaving residues of garnet + clinopyroxene + amphibole.

Low δ18O zircons, U-Pb dating, and the age of the Qinglongshan oxygen and hydrogen isotope anomaly near Donghai in Jiangsu Province, China

Zircons from metamorphosed granites exposed near Qinglongshan have δ18OVSMOW values of −7 to 0‰ in both grain rims and cores. The concordant 238U/206Pb ages of zircon cores are 684 to 754 Ma with rims at 221 Ma. Discordant 238U/206Pb ages range from 242 to 632 Ma. Results demonstrate a Neoproterozoic age for the origin of the Qinglongshan oxygen and hydrogen isotope anomaly. The low δ18O values were imprinted on the rocks by a hydrothermal system charged with meteoric water from a cold climate. Groundwater circulation was driven by heat from cooling granitic magma. The geologic age of the hydrothermal system correlates with that of the Nantuo tillite in the Sinian strata of the South China block, suggesting that Qinglongshan’s cold climate may be a manifestation of Neoproterozoic “snowball Earth.”

Timing of deposition, orogenesis and glaciation within the Dalradian rocks of Scotland: constraints from U–Pb zircon ages

Abstract: The stratigraphical and structural continuity of the Late Proterozoic Dalradian rocks of the Scottish Highlands is re-examined in the light of new U–Pb zircon ages on the tuffs belonging to the Tayvallich Volcanic Formation (601 ± 4 Ma), and on the late Grampian ‘Newer Gabbros’ (470 ± 9 Ma) of Insch and Morven–Cabrach in Aberdeenshire. These age data, together with the existing 590 ± 2 Ma age for the Ben Vuirich Granite, provide key radiometric constraints on the evolution of the Dalradian block, and the implications arising from these ages are critically assessed. Three main conclusions are drawn. (1) The entire Caledonian orogeny, although short-lived, is unlikely to have affected sediments of Arenig age and a break probably occurs between those Dalradian sediments of late Proterozoic (<600 Ma) age and the Ordovician rocks of the Highland Border Complex. (2) A period of crustal thickening probably affected some Dalradian rocks prior to 590 Ma. Such an event is indicated by both the polymetamorphic histories of the lower parts of the Dalradian pile and the contact metamorphic assemblages within the aureole of the Ben Vuirich Granite, which are incompatible with sedimentary thicknesses. (3) Age constraints on global Late Proterozoic glacial activity also suggest that the Dalradian stratigraphy is broken into discrete smaller units. Models involving continuous deposition of Dalradian sediments from pre-750 Ma to 470 Ma are rejected.