S.Y. O'Reilly

Two age populations of zircons from the Timber Creek kimberlites, Northern Territory, as determined by laser-ablation ICP-MS analysis

Two populations of kimberlitic zircon are present in the Timber Creek kimberlites, Northern Territory. Laser ablation ICP MS U—Pb dating yields an age of 1483 ± 15 (2σ) Ma for the main group and an age of 179 ± 2 Ma for the other group. This distinction of two age groups is strongly supported by Hf isotope data on the same zircons. Although the trace element patterns of both populations are typical of mantle derived zircons, the ‘young’ population has slightly higher concentrations of most trace elements, but has lower Hf, Nb, Ta and Pb contents. The distinct differences in trace element contents and Hf isotopic composition of the two zircon populations indicate that they were derived from different magma sources. The dating results indicate that the emplacement age of the Timber Creek kimberlites cannot be older than the age of the ‘young’ zircon population (i.e. 179 ± 2 Ma). This clarifies the inconsistency between the previously reported SHRIMP age of the Timber Creek zircons (1462 ± 53 Ma) and the much younger age (1200 Ma) of the sediments of the Victoria River Basin into which these kimberlites have intruded. The Timber Creek kimberlites are a newly recognised extension of the widespread Jurassic kimberlite activity known in Western Australia and South Australia (Wandagee, Orroroo, Cleve and Eurelia kimberlites).

A xenolith-derived geotherm and the crust-mantle boundary at Qilin, southeastern China

Pressure-temperature estimates on a group of well-equilibrated garnet pyroxenites from the Qilin locality, southeastern China have been used to construct the first detailed, locality-specific paleogeotherm for eastern China. This xenolith-defined geothermal gradient is comparable to that for northwestern Spitsbergen, Norway, and higher than that derived for southeastern Australia. The strong curvature of this geotherm between 20 and 50 km depth indicates that advection, rather than conduction was the dominant heat-transfer mechanism in the lower crust and upper mantle at that time. Underplating and overplating of basaltic magmas at the crust-mantle boundary is the likely cause of the advective heat transfer. Temperature estimates on two-pyroxene granulites suggest they were entrained from the lower crust. The crust-mantle boundary, defined from the lowest temperature estimates for spinel lherzolites, lies at ca 27 km depth in this locality. This is shallower than the Moho ( ~ 31 km) defined by seismic refraction. Calculation of Vp for the xenolith rock types, and comparison with observed Vp data, allow definition of a stratigraphie sequence. From 27 to 31 km depth, garnet granulites, spinel lherzolites and garnet pyroxenites may be interlayered in a transitional zone. Comparison with present-day heat flow measurements and seismic results suggest that this lithospheric section has undergone cooling since the Cenozoic volcanism.

Relict Proterozoic basement in the Nanling Mountains (SE China) and its tectonothermal overprinting

[1]xa0The basement beneath the Caledonian belt in SE China has been overprinted by Indosinian and Yanshanian events and is poorly known. In situ U-Pb and Hf-isotope analysis of zircons from granites and surrounding rocks in the Nanling Mountains area of the fold belt reveal that the Lanhe felsic gneiss is an outcrop of Mesoproterozoic basement, probably with a partly Late Archean protolith. This is consistent with Nd model ages of granites in this region, and exposures of Cathaysia basement in the coastal area of SE China. In the Nanling Mountains, this basement was reactivated to generate granitoid rocks during the Caledonian, Indosinian, and Yanshanian tectonothermal events. The Caledonian overprint is documented in extensive exposures of the Zhuguang migmatites (425–470 Ma) and by previous U-Pb zircon dating of adjacent granites. The Jiangnan granite may be of similar age, but strongly reworked. The Indosinian overprint resulted in the formation of granites such as the Luxi and Xiazhuang bodies (233–246 Ma) in the Guidong complex. The Yanshanian overprint produced the most widely distributed granites (circa 160 Ma) such as the Jiufeng and Fogang bodies, and parts of the Guidong complex. Hf-isotope data show that all of the Phanerozoic magmatic rocks were derived from the Mesoproterozoic basement (and minor Archean components), without obvious contributions from juvenile sources. The presence of zircons with Archean age (2.5–2.7 Ga) in both the Lanhe gneiss and Devonian sediments indicates that the Badu and Mayuan groups may not be the oldest basement of the Cathaysia block. Some Archean basement (circa 2.5–2.7 Ga) may still be exposed but unrecognized in this region, and Cathaysia may include Archean microcontinental fragments.

4-D LITHOSPHERE MAPPING : METHODOLOGY AND EXAMPLES

Abstract Heat is the primary driving energy for all geological processes: volcanism is the surface expression of thermal heterogeneities within the Earth. Mantle-derived volcanism provides a tool to unravel internal Earth processes: the nature of mantle convection, fluid transport, rheological properties and heat transfer mechanisms and the chemical and physical nature of the asthenosphere/lithosphere/crust system. The volcanic rocks yield geochemical averages of the melt/solid interfaces below the lithosphere but may be contaminated by overlying lithosphere during their ascent and may therefore reflect the geochemical signature of such lithosphere. Deep-seated xenoliths in volcanic rocks sample different levels and can be used to define empirical paleogeotherms characterizing the ambient thermal regime at the time of eruption. The characterization of the thermal structure of the Earth using xenoliths entrained in mantle-derived volcanics is a basic and very powerful tool for investigating the nature of the lithosphere in space and time. Xenoliths and high-pressure mineral debris in volcanic rocks yield the paleostratigraphies and thermal profiles of different types of lithosphere and measure the depths to the crust-mantle boundary and the lithosphere-asthenosphere boundary. Such xenoliths provide the raw materials to link geological, geochemical and petrophysical parameters with present-day remotely-sensed geophysical information. Repeated volcanic episodes that have occurred in the same crustal region can be used trace thermal, physical and chemical modifications of lithosphere through time. Ancient lithosphere rejuvenated by fluid infiltration (metasomatism) may source younger volcanism in response to a new thermal pulse as old cratonic keels erode and transform. Xenoliths in basaltic rocks, kimberlites and lamproites are the key to realistic interpretation of geophysical data in terms of lithosphere stratigraphy; geophysical data can then be used to extend the xenolith-derived stratigraphic profiles laterally. Xenoliths therefore provide the means to map the evolution of Earths lithosphere in space and time (4-D Lithosphere Mapping).

Archaean and Proterozoic crustal evolution in the Eastern Succession of the Mt Isa district, Australia: U – Pb and Hf-isotope studies of detrital zircons *

Over 500 zircon grains separated from modern sediments in 10 drainages covering the Eastern Succession of the Mt Isa Inlier have been analysed for U – Pb ages, Hf isotopes, and trace elements, using in situ LAM-ICPMS techniques, to evaluate the efficacy of this approach in characterising large-scale crustal evolution. U – Pb age spectra are used to estimate the timing of terrane-scale events, primarily magmatic episodes; Hf isotopes provide information on the relative contributions of juvenile material and reworked older crust at each stage of crustal evolution; trace-element patterns of zircons are used to characterise original magma types. The integration of these data for individual zircon grains produces an event signature that provides more information than that gained from U – Pb dating alone. The data define four major stages of crustal evolution in the area: 2550 – 2330 Ma, 1950 – 1825 Ma, 1800 – 1600 Ma, and 1590 – 1420 Ma. Each stage, except the last, involved crustal extension, and ended with a period of crustal homogenisation, which is reflected in the isotopic composition of magmatic rocks generated by crustal reworking in the succeeding stage. Reworking of Neoarchaean crust contributed significantly to crustal magmatism throughout the Proterozoic history. A major input of juvenile mafic material around 1625 Ma (interpreted as the magmatic age of the Toole Creek Volcanics) is poorly represented in the database of conventional geochronology but is prominent in the detrital zircon record. The major late-stage granitic magmatism of the Williams and Naraku Batholiths (1520 – 1490 Ma) was generated almost entirely from older crust, with little juvenile input. The study demonstrates that sampling of carefully selected modern drainages and the analysis of statistically large numbers of detrital zircons can provide insights additional to those gained from conventional analysis of U – Pb and Sm – Nd systematics in selected rock samples. The integrated use of U – Pb age, Hf-isotope composition and trace-element patterns in detrital zircons is a powerful and relatively inexpensive tool for the analysis of terrane-scale crustal evolution, and for the correlation of terranes.

Mesoarchean subduction processes: 2.87 Ga eclogites from the Kola Peninsula, Russia

The nature of tectonic processes on the early Earth is still controversial. The scarcity of high-pressure metamorphic rocks such as eclogite (the high-pressure equivalent of basalt) in Archean cratons has been used to argue that plate tectonics did not operate until Earth had cooled to a critical point, perhaps around the 2.5 Ga Archean-Proterozoic transition. However, eclogites occur as meter- to kilometer-sized lenses enclosed in Archean gneisses of the Belomorian Province of the Fennoscandian shield. Geochemistry and internal features suggest that the protoliths of the eclogites were interlayered olivine gabbros, troctolites, and Fe-Ti oxide gabbros. Greenschist facies mineral parageneses are enclosed in prograde-zoned eclogite garnets, and peak metamorphic conditions define an apparent thermal gradient (12–15 °C/km), consistent with metamorphism in a warm Archean subduction zone. We show here that these eclogites represent the oldest known high-pressure metamorphic rocks. U-Pb dating and Hf isotope analyses of zircons from the eclogites and a crosscutting felsic vein define a minimum age of 2.87 Ga for the Uzkaya Salma eclogite; a 2.70 Ga age for the Shirokaya Salma eclogite is interpreted as the age of a granulite facies overprint. Thermal overprinting and growth of new zircon also occurred during the Svecofennian (1.9–1.8 Ga) orogeny. These new data imply that plate tectonic processes operated at least locally in late Mesoarchean time. The adakitic nature of the felsic vein suggests that partial melting of hydrated eclogites could produce Archean tonalite-trondhjemite-granodiorite–type magmas.

Peridotite xenoliths in alkali basalts from the Sikhote-Alin, southeastern Siberia, Russia: trace-element signatures of mantle beneath a convergent continental margin

Abstract Abundant spinel peridotite xenoliths occur in late Cenozoic alkali basaltic rocks in the Sikhote-Alin region at the Pacific margin of the Asian continent. Major- and trace-element compositions of representative peridotite xenolith are documented for four occurrences located in different structural units of the continental margin. In each locality, the majority of xenoliths have distinctive microstructures, modal and chemical compositions that are typical for a given xenolith suite. Significant textural and compositional differences between the four xenolith suites suggest that the upper mantle beneath the Sikhote-Alin consists of distinct domains with contrasting composition. The inferred large-scale mantle heterogeneities may be due to juxtaposition of lithospheric blocks of different provenance during accretion of the Sikhote-Alin to the Asian continent. Trace-element patterns of the xenoliths and their minerals obtained ICP-MS technique provide evidence of depletion and enrichment events and indicate contrasting behaviour of REE, HFSE and other incompatible trace elements. The HFSE behave non-concordantly, in particular, some xenoliths have highly fractionated Zr/Hf, Ti/Zr, Nb/Ta, La/Nb and U/Th ratios relative to their values in the primitive mantle. The fractionated compositions may be related to the interaction of evolved subduction-related fluids and melts with lithospheric mantle at the Mesozoic-early Cenozoic active continental margin or to metasomatism during later continental rifting.

Thermal state and composition of the lithospheric mantle beneath the Daldyn kimberlite field, Yakutia

Abstract The proton microprobe has been used to study the distribution of trace elements in garnet and chromite concentrates from the Udachnaya kimberlite and three smaller, low-grade kimberlites from the Daldyn kimberlite field. Garnet thermobarometry and classical P-T estimates for megacrystalline peridotite xenoliths both suggest a Paleozoic geotherm beneath the Daldyn area that is close to a 35 mW/m 2 conductive model. Finer-grained xenoliths with T C scatter above this geotherm; high-temperature sheared xenoliths lie near or above a 40 mW/m 2 model geotherm at 1150–1400°C. The higher- T results are interpreted as the result of short-term heating, caused by magmatic intrusion and perturbation of a relatively cool conductive geotherm, especially near the base of the lithosphere. The stratigraphic distribution [inferred from nickel temperature ( T Ni )] of garnets with different major-element chemistry indicates that the lithosphere is strongly layered in terms of rock type; depleted lherzolites predominate to depths of ca. 150 km, harzburgites comprise up to 60% of the volume between 150 and 180 km, and these are underlain by a mixture of depleted and metasomatically enriched lherzolites. Zinc temperatures ( T Zn ) indicate that chromite-bearing peridotites are essentially absent at depths > 190 km. High- T lherzolite garnets carry a distinctive trace-element fingerprint showing enrichment in Zr, Ti, Y and Ga, interpreted as due to the infiltration of asthenosphere-derived melts. This melt-related metasomatic signature becomes the dominant one at ca. 220–230 km depth, and this is interpreted as the base of the lithosphere. This depth also corresponds approximately to the Lehman Discontinuity at the top of a pronounced low-velocity zone, observed in deep seismic sounding experiments across this part of the Siberian Platform. The techniques used here provide a means of mapping the lithosphere in terms of thermal structure, lithology and fluid-related processes; both lateral (3-D) and temporal (4-D) variations may be mapped using readily available garnet and chromite concentrates from the widespread kimberlite intrusions across the Siberian Platform.

Nature of the lithospheric mantle beneath the eastern part of the Central Asian fold belt: mantle xenolith evidence

The late Cenozoic potassic volcanic provinces of Wudalianchi, Erkeshan and Keluo are located at the boundary between the Great Xingan Mountains and the northwestern margin of the Songliao Basin, the eastern part of the Central Asian Fold Belt, in northeastern China. Mantle xenoliths found in the potassic rocks are mainly spinel peridotite (some phlogopite-bearing) associated with minor Fe-rich peridotite (some phlogopite-bearing) and pyroxenite. Clinopyroxene in the spinel peridotite xenoliths is generally low in Al2O3, Sc, Y and heavy rare earth elements and along with orthopyroxene and spinel has high Cr/(Cr+Al). The bulk rock has low basaltic components (Al2O3, CaO and TiO2), demonstrating their refractory nature. However, the relatively low forsterite contents (<0.915) of olivine and high bulk rock Ca/Al (1.3–4.1) and Mg/Si (1.2–1.6) ratios at given Mg/(Mg+Fe) distinguish them from either Archaean lithospheric mantle or the residue of melt extraction from Phanerozoic oceanic mantle. Clinopyroxenes from all the xenoliths are enriched in light rare earth elements, Th and Pb and depleted in high field strength elements, with similar incompatible element and rare earth element patterns. Clinopyroxenes from the spinel peridotite xenoliths have 87Sr/86Sr (0.70480–0.70516) higher than primitive mantle, but variable ϵNd values (+6.9 to −2.0). n nPetrological and geochemical evidence from the xenoliths supports a common two-stage evolution model for the upper part of the lithospheric mantle in the region. The mantle protolith was stabilised after moderate to high degrees of melt extraction (e.g. 8–13% for fractional melting) and then metasomatised extensively by SiO2-undersaturated potassic magma that was likely generated from low-degree partial melting of either deep asthenosphere or metasomatised lower part of lithospheric mantle. Both the melt extraction and metasomatic event(s) may have occurred during the Proterozoic. Geochemical signatures of the spinel peridotite xenoliths and the inferred garnet peridotites at the base of the subcontinental lithospheric mantle from chemistry of the host potassic rocks (Zhang et al., J. Petrol., 36 (1995) 1275; Zhang et al., AGU Geodynamic Series, 27 (1998) 197) indicates the preservation of a Proterozoic lithospheric mantle section within the Phanerozoic Central Asian Fold Belt.

Trace‐element signatures of apatites in granitoids from the Mt Isa Inlier, northwestern Queensland

The concentrations of trace elements in apatite from granitoid rocks of the Mt Isa Inlier have been investigated using the laser‐ablation inductively coupled plasma‐mass spectrometry (ICP‐MS) microprobe. The results indicate that the distribution of trace elements (especially rare‐earth elements (REE), Sr, Y, Mn and Th) in apatite strongly reflects the chemical characteristics of the parental rock. The variations in the trace‐element concentrations of apatite are correlated with parameters such as the SiO2 content, oxidation state of iron, total alkalis and the aluminium saturation index (ASI). The relative enrichment of Y, HREE and Mn and the relative depletion of Sr in the apatites studied reflect the degree of fractionation of the host granite. Apatites from strongly oxidised plutons tend to have higher concentrations of LREE relative to MREE. Manganese concentrations are higher in apatite from reduced granitoids because Mn2+substitutes directly for Ca2+. The La/Ce ratio of apatite is well‐correlated with the whole‐rock K2O and Na2O contents, as well as with the oxidation state and ASI. Because apatite trace‐element composition reflects the chemistry of the whole rock, it can be a useful indicator mineral for the recognition of mineralised granite suites, where particular mineralisation styles are associated with granitoids that have specific geochemical fingerprints.