Search

Secondary ion mass spectrometry (SIMS) zircon U-Pb dating has been widely used to confine the absolute ages of the magmatic or metamorphic events and to distinguish multiple events. Here, a data set consisting of the zircon standards dating data (∼15 000 Plešovice and ∼8 000 Qinghu items) accumulated for more than 8 years using the CAMECA IMS 1280HR of the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) is compiled to evaluate the long-term external reproducibility. With Plešovice zircon as the calibration standard and Qinghu zircon as an unknown sample, the analytical uncertainties for single-spot (n = 7 723) and session analyses (n = 691, with more than 7 single spot analyses in one session) are 2.6% and 0.9% (2RSD, relative standard deviation), respectively. It means that single-spot U-Pb dating for a standard-level zircon sample could vary 5.2% at 95% confidence level, while the average value (n > 7) for each session may vary 1.8%. Thus, these values should be considered as the minimum uncertainty when comparing single spot and individual session analysis results for multiple dating works on a certain geological event. [ABSTRACT FROM AUTHOR]

We present multitechnique U‐Pb geochronology and Hf isotopic data from zircon separated from rapakivi biotite granite within the Eocene Golden Horn batholith in Washington, USA. A weighted mean of twenty‐five Th‐corrected 206Pb/238U zircon dates produced at two independent laboratories using chemical abrasion‐isotope dilution‐thermal ionisation mass spectrometry (CA‐ID‐TIMS) is 48.106 ± 0.023 Ma (2s analytical including tracer uncertainties, MSWD = 1.53) and is our recommended date for GHR1 zircon. Microbeam 206Pb/238U dates from laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) and secondary ion mass spectrometry (SIMS) laboratories are reproducible and in agreement with the CA‐ID‐TIMS date to within < 1.5%. Solution multi‐collector ICP‐MS (MC‐ICP‐MS) measurements of Hf isotopes from chemically purified aliquots of GHR1 yield a mean 176Hf/177Hf of 0.283050 ± 17 (2s, n = 10), corresponding to a εHf0 of +9.3. Hafnium isotopic measurements from two LA‐ICP‐MS laboratories are in agreement with the solution MC‐ICP‐MS value. The reproducibility of 206Pb/238U and 176Hf/177Hf ratios from GHR1 zircon across a variety of measurement techniques demonstrates their homogeneity in most grains. Additionally, the effectively limitless reserves of GHR1 material from an accessible exposure suggest that GHR1 can provide a useful reference material for U‐Pb geochronology of Cenozoic zircon and Hf isotopic measurements of zircon with radiogenic 176Hf/177Hf. Key Points: We suggest a new Eocene zircon reference material, GHR1.U‐Pb and Hf isotopic systematics in GHR1 zircon are homogeneous with a 206Pb/238U date of 48.106 +/‐ 0.023 Ma (2s) and a 176Hf/177Hf of 0.283050 +/‐ 17 (2s).GHR1 fulfills a need for abundant and easily available Cenozoic zircon reference materials. [ABSTRACT FROM AUTHOR]

Zircon is the most useful mineral for studies in U-Pb geochronology and Hf and O isotope geochemistry. Matrix effect is a major problem of the microbeam techniques such as SIMS and LA-(MC)-ICPMS. Therefore, external standardization using well-characterized natural zircon standards is fundamental for accurate microbeam measurements. While the isotopic geochronology and geochemistry laboratories equipped with microbeam analytical facilities have been increasingly established in China during the past decade, applications of the isotopic microanalysis are still limited due to shortage of available standards. We report here the Qinghu zircon as a potential new working reference for microbeam analysis of zircon U-Pb age and O-Hf isotopes. This zircon was separated from the Qinghu quartz monzonite from the western Nanling Range, Southeast China. It is fairly homogeneous in U-Pb age and Hf and O isotopes in terms of large amounts of mircobeam measurements by LA-MC-ICPMS and SIMS at the scales of 20-60 μm. SIMS measurements yield consistent Pb/U age within analytical uncertainties with that obtained by ID-TIMS. Precise determinations of O isotopes by IRMS and Hf isotopes by solution MC-ICPMS are in good agreement with the statistical mean of microbeam measurements. We recommend U-Pb age of = 159.5 ± 0.2 Ma (2SE), δO = 5.4‰± 0.2‰ (2SD) and Hf/Hf = 0.283002 ± 0.000004 (2SD) as the best reference values for the Qinghu zircon. [ABSTRACT FROM AUTHOR]

The Baizhangyan skarn-porphyry type W- Mo deposit is located in a newly defined Mo- W- Pb- Zn metallogenic belt, which is in the south of Middle- Lower Yangtze Valley Cu- Fe- Au polymetallic metallogenic belt in SE China. The W- Mo orebodies occur mainly within the contact zone between fine-grained granite and Sinian limestone strata. There are two types of W- Mo mineralization: major skarn W- Mo mineralization and minor granite-hosted disseminated Mo mineralization which was traced by drilling at depth. Eight molybdenite samples from Mo-bearing ores yield Re- Os dates that overlap within analytical error, with a weighted average age of 134.1 ± 2.2 Ma. These dates are in close agreement with SIMS U- Pb concordant zircon age for fine-grained granite at 133.3 ± 1.3 Ma, indicating that crystallization of the granite and hydrothermal molybdenite formation were coeval and likely cogenetic. The Baizhangyan W- Mo deposit formed in the Early Cretaceous extensional tectonic setting at the Middle- Lower Yangtze Valley metallogenic belt and the Jaingnan Ancient Continent. Based on mineral compositions and crosscutting relationships of veinlets, hydrothermal alteration and mineralization, the ore mineral paragenesis of the Baizhangyan deposit is divided into four stages: skarn stage ( I), oxide stage ( II), sulfide stage ( III), and carbonate stage ( IV). Fluid inclusions in garnet, scheelite, quartz and calcite from W- Mo ores are mainly aqueous-rich ( L + V) type inclusions. Following garnet deposition at stage I, the high-temperature fluids gave way to progressively cooler, more dilute fluids associated with tungsten-molybdenite-base metal sulfide deposition (stage II and stage III) (162-360°C, 2.7-13.2 wt % NaCl equivalent) and carbonate deposition (stage IV) (137-190°C, 0.9-5 wt % NaCl equiv.). Hydrogen-oxygen isotope data from minerals of different stages suggest that the ore-forming fluids consisted of magmatic water, mixed in various proportions with meteoric water. From stage I to stage IV, there is a systematic decrease in the homogenization temperature of the fluid-inclusion fluids and calculated δ18O values of the fluids. These suggest that increasing involvement of formation water or meteoric water during the fluid ascent resulted in successive deposition of scheelite and molybdenite at Baizhangyan. [ABSTRACT FROM AUTHOR]

The Jitoushan W–Mo ore body is a typical skarn-type deposit with the potential for porphyry Mo mineralization at depth. As it is newly discovered, only a few studies have been conducted on the geochronology and ore genesis of this deposit. The ore district consists of Cambrian to Silurian sedimentary and low-grade metasedimentary strata, intruded by granodiorite, diorite porphyry, granite porphyry, and quartz porphyry. Skarn W–Mo ore bodies are hosted in the contact zone between the granodiorite and Cambrian limestone strata. Within the granodiorite near the contact zone, quartz vein type and disseminated sulphide mineralization are well developed. The Mo-bearing granite porphyry has been traced at depth by drilling. Our results reveal two discrete magmatic events at ca. 138 and ca. 127 Ma in the study area. The molybdenite Re–Os isochronal age of 136.6 ± 1.5 million years is consistent with the first magmatic event. The zircon Hf isotope (ϵHf(t) = −12.55−3.91), sulphide isotopes (δ34S = 3.32–5.59‰), and Re content of molybdenite (Recontent = 6.424–19.07 μg) indicate that the ore-forming materials were mainly derived from the deep crust. The regional tectonic system switched from a Late Jurassic transpressive regime to an earliest Cretaceous extensional regime at ca. 145 Ma, and at ca. 138 Ma, the Jitoushan W–Mo deposit formed in an extensional setting. [ABSTRACT FROM AUTHOR]

Abstract: We report the petrography, mineralogy, trace element abundance geochemistry, and Pb–Pb geochronology of the lunar meteorite Northwest Africa (NWA) 4734 and make a comparison with the LaPaz Icefield (LAP) 02205/02224 low-Ti lunar basaltic meteorites. NWA 4734 is an unbrecciated low-Ti mare basalt composed mainly of subophitic-textured pyroxene (60vol%) and plagioclase (30%). Pyroxene, plagioclase, and olivine exhibit large compositional variations and intra-grain chemical zoning. Pyroxene and plagioclase in NWA 4734 have rare earth element (REE) concentrations and patterns similar to those of the LAPs. The crystallization age of NWA 4734, determined in situ in baddeleyite, is 3073±15Ma (2σ), nearly identical to that of the LAPs (3039±12Ma). NWA 4734 and the LAPs have similar textures, modal abundances, mineral chemistry, and crystallization ages, and are most likely source-crater paired on the Moon. One baddeleyite grain in LAP 02224 displays distinctively older and spatially variable ages, from 3349±62 to 3611±62Ma (2σ), similar to another baddeleyite grain (3109±29 to 3547±21Ma) reported by for the same meteorite. Raman spectra, cathodoluminescence, and stoichiometric studies of the baddeleyite suggest that the two older grains were not endogenic but were trapped by the parental magma. Equilibrium partition calculation shows that the parental melt from which the NWA 4734 plagioclase crystallized has much lower REE contents than its whole rock, indicating an open system during magma evolution. NWA 4734 could have originated from a parental melt with REE concentrations similar to that of the Apollo 12 olivine basalt. The magma likely assimilated a small amount (∼4wt%) of KREEP-rich material during its ascent through the lunar crust. [Copyright &y& Elsevier]

Abstract: Although petrologic data from the North Qinling orogen in central China have provided essential information on the geodynamic history of the orogenic cycle, the timing of the continental collision between the North China block and the South China block in the North Qinling orogen remains essentially unconstrained. To refine existing geodynamic models, two eclogites from the North Qinling were dated by U–Pb and Lu–Hf geochronology. The upper limit of the pressure during the eclogite facies metamorphism was restricted within the quartz stability field at 2.6–2.8GPa within a temperature range of 660–710°C by combining conventional thermobarometry and pseudosection estimates. Mineral–whole rock dating for multi-garnet-(omphacite) fractions from two localities yielded ages of 516.4±5.8Ma (2σ) and 494.3±2.7Ma (2σ) with corresponding weighted mean zircon 206Pb/238U ages of 507.0±8.7Ma (2σ) and 490.0±4.1 (2σ) Ma, respectively. Garnet porphyroblasts from the two samples bore evidence of prograde elemental zonation, but the major portion of the garnet grains grew exclusively under eclogite facies conditions as indicated by the occurrence of omphacite inclusions from core to rim. Therefore, these Lu–Hf ages were interpreted to reflect high-pressure eclogite facies metamorphism. The difference between the two Lu–Hf ages likely represents the minimum duration of garnet growth and thus brackets the onset and the closure of the subduction. These results allow for more detailed insight into the subduction history of the North Qinling orogen. These new U–Pb and Lu–Hf ages mark the timing of the onset of the continental collision between the North China block and the South China block in the Qinling belt, suggesting that the entry of the leading edge of the South China block continent into the North China block trench occurred from ca. 516Ma to 490Ma in the North Qinling orogen. [Copyright &y& Elsevier]

Abstract: Quartz-rich veins, together with their host high-pressure (HP) to ultra-high-pressure (UHP) rocks, are ideal for deciphering the fluid–rock interaction history in subduction zones. Previous studies of samples from Dabie–Sulu have shown that most quartz-rich veins within UHP/HP eclogite were formed during the prograde or early exhumation stage of host rocks. In contrast, a combined study of petrology and U–Pb, Sm–Nd and Lu–Hf geochronologies for a quartz-rich vein and its host retrogressed eclogite from North Qinling, which is associated with a granite intrusion and obliterated with amphibolite-facies overprint, reveals that the amphibolite-facies overprint is likely due to the late regional magmatism and the quartz-rich vein might derive from the nearby granite. A pool of U–Pb analyses of zircon grains from the retrogressed eclogite reveals zircons grew over an extended period of time but with two caveats at ca. 490Ma and ca. 473Ma. Analyses of zircon grains from the quartz vein yield a weighted mean 207Pb/206Pb ages of 428±5Ma (1σ), which is indistinguishable from that of formation of the adjacent granite. Combined elemental zoning patterns in the garnet with the spherical geometry effect, the Lu–Hf age of 416±5Ma (2σ) of the retrogressed eclogite is likely a mix age of eclogite-facies and amphibolite-facies metamorphism while the garnet–amphibole Sm–Nd age of 400±8Ma (2σ) approximates amphibolite-facies retrogression metamorphism. The latter is consistent with the garnet–biotite Sm–Nd age of 393±4Ma (2σ) of the adjacent granite, suggesting contact metamorphism. The >70Myr gap between the eclogite-facies metamorphism and the amphibole-facies overprint and the consistency of the timing the amphibolite-facies overprint for the eclogite and the cooling of the magma intrusion in this area indicate that the eclogite-facies metamorphism and amphibolite-facies metamorphism might be distinct metamorphic events, and the latter might be trigged by the granite intrusion. [Copyright &y& Elsevier]