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We present a new method for Lu-Hf geochronology using lawsonite, an index mineral of high-pressure, low-temperature metamorphism. The method uses common cation exchange chromatography to purify Lu and Hf from the sample and analysis by muiticollector inductively coupled plasma--mass spectrometry. Lawsonite [sup.176]Lu/[sup.177]Hf ratios are elevated and are similar to other minerals commonly used in Lu-Hf geochronology such as garnet or apatite. We obtain an age of 145.5 [+ or -] 2.4 Ma for lawsonite-blueschist facies metamorphism at Ring Mountain within the Franciscan Complex, California, the lawsonite type locality. The age agrees well with published geochronology and thermochronology for the Franciscan Complex. The new method provides a powerful tool to investigate processes and time scales of blueschist to eclogite facies metamorphism, crust-mantle recycling via subduction, and the geodynamics of convergent margins. doi: 10.1130/G30292A.1

We present a combined garnet Lu–Hf and Sm–Nd, zircon and monazite U–Pb, hornblende Ar–Ar, and zircon U–Th/He geochronology study on a garnet-bearing amphibolite from the Cathaysia Block in Southern China. Pseudosection modeling and garnet–clinopyroxene thermometry indicate that the amphibolite attained peak metamorphic conditions of approximately 0.9 GPa and 780 °C. The Lu–Hf garnet date of 402.7 ± 1.0 Ma and Sm–Nd date of 331.6 ± 3.6 Ma define a ~ 70 Myr difference for the same garnet fractions. Reconciling the large discrepancies between these three geochronological systems with the observed major element diffusion profiles in garnet requires that REEs and Hf were fully re-equilibrated at peak temperatures and that subsequent rapid cooling prevented any disruption of the Lu–Hf or Sm–Nd systems throughout cooling from high temperatures. The Sm–Nd garnet age was partially reset during a thermal pulse associated with intrusion of leucocratic veins, which is inferred to have occurred at ca. 226 Ma based on the coinciding hornblende Ar–Ar (222.3 ± 0.9 Ma) and monazite U–Pb (226.2 ± 2.3 Ma) dates. The T–t path of a reheating event at 226 Ma necessary to produce both the major divalent cation diffusion profiles in garnet and partial resetting of the Sm–Nd garnet age, such that it records an apparent age of ~ 332 Ma, requires an effective diffusion radius of 45–60 μm in garnet during reheating, which is in agreement with the observation of pervasive chlorite-filled fractures within single garnet grains. Diffusion kinetics of Lu and Hf in garnet are sufficiently slow that this reheating episode would not impact the Lu–Hf system, and the 402.7 ± 1.0 Ma garnet age likely reflects the onset of rapid cooling from peak P–T conditions. The zircon U–Pb date of 434.8 ± 2.7 Ma predates the Lu–Hf garnet date by approximately 30 Myr and is interpreted to reflect zircon crystallization during prograde metamorphism. The ~ 226 Ma reheating event is contemporaneous with the intrusion and crystallization of the widespread Triassic A-type granites and alkaline syenites across the Cathaysia Block, indicating an extensional environment. We interpret the zircon U–Th/He date of 143.1 ± 4.2 Ma as reflecting a third thermal pulse based on the concurrence with the early Cretaceous volcanic rocks and granitoids, which overlies the amphibolite. [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]

We report Hf isotope data for 3467 Ma igneous zircons from the Owens Gully Diorite of the Pilbara Craton. These zircons, designated OG1 or OGC, have simple, well-defined U-Pb age systematics. Measurements obtained by single crystal dissolution yield a moderate range in 176Hf/177Hf (mean 0.280633 ± 34) and 176Lu/177Hf (mean 0.00119) that translate into initial (3467 Ma) 176Hf/177Hf values that are equivalent within measurement uncertainty (mean 0.280554 ± 7, εHf = +0.6 ± 0.2, 2 s). Laser ablation analysis yielded clustered initial 176Hf/177Hf ratios (mean 0.280560 ± 20, 2 s) indistinguishable from those obtained by solution analysis. The antiquity and moderately high 176Lu/177Hf of these zircons result in significant corrections for radiogenic ingrowth of 176Hf (to 6 εHf units). Depth profiling by concurrent Pb-Hf isotope determination reveals trends to lower 207Pb/206Pb in several crystals, although these are not accompanied by shifts in 176Hf/177Hf. We conclude that zircons from the Owens Gully Diorite are homogeneous for Hf isotopes at the scale of sampling and within the uncertainty limits of the measurement techniques employed. Zircon OG1/ OGC would appear to be an ideal quality monitor for laser ablation Hf isotope determination in ancient zircons, and for laser ablation techniques where Hf isotope and age information are determined concurrently or simultaneously. [ABSTRACT FROM AUTHOR]

Zircon U-Pb analyses of 18 orthogneisses from the Clearwater and Priest River complexes of northern Idaho have identified important exposures of Neoarchean and Paleoproterozoic basement rocks in northwest Laurentia. All samples have ages that fall into two tightly defined age ranges: in the Neoarchean from 2.67 to 2.65 Ga and in the Paleoproterozoic from 1.88 to 1.84 Ga. The Neoarchean orthogneisses show no evidence of older components, whereas some of the Paleoproterozoic orthogneisses have xenocrystic zircon cores with ages overlapping with those of the Neoarchean gneisses. Collectively, these two packages of rocks represent the two main periods of crust formation in northwest Laurentia, one wholly juvenile in the Neoarchean, and the other a mixture of juvenile Paleoproterozoic and inherited Neoarchean components. Based on existing data, the ages of orthogneisses in the Clearwater and Priest River complexes are identical and probably represent one continuous crustal block; we refer to this basement domain as the Clearwater block. The Paleoproterozoic 1.86 Ga magmatism described here is distributed throughout the northwest Laurentian margin and does not coincide with the proposed trend of the Great Falls tectonic zone. Therefore, a model of a single linear arc preserved within the Great Falls tectonic zone is inadequate in describing the majority of the known 1.86 Ga crust in the region. [ABSTRACT FROM AUTHOR]

The Lhasa terrane, the main tectonic component of the Himalayan–Tibetan orogen, has received much attention as it records the entire history of the orogeny. The occurrence of Permian to Triassic high-pressure eclogites has a significant bearing on the understanding of the Paleo-Tethys subduction and plate suturing processes in this area. An eclogite from the Bailang, eastern Lhasa terrane, was investigated with a combined metamorphic P – T and U–Pb, Lu–Hf, Sm–Nd and Ar–Ar multichronometric approach. Pseudosection modeling combined with thermobarometric calculations indicate that the Bailang eclogite equilibrated at peak P – T conditions of ~ 2.6 GPa and 465–503 °C, which is much lower than those of Sumdo and Jilang eclogites in this area. Garnet–whole rock–omphacite Lu–Hf and Sm–Nd ages of 238.1 ± 3.6 Ma and 230.0 ± 4.7 Ma were obtained on the same sample, which are largely consistent with the corresponding U–Pb age of 227.4 ± 6.4 Ma for the metamorphic zircons within uncertainty. The peak metamorphic temperature of the sample is lower than the Lu–Hf and Sm–Nd closure temperatures in garnet. This, combined with the core-to-rim decrease in Mn and HREE concentrations, the slightly U-shaped Sm zonation across garnet and the exclusive occurrence of omphacite inclusion in garnet rim, are consistent with the Lu–Hf system skewing to the age of the garnet core and the Sm–Nd system favoring the rim age. The Sm–Nd age was thus interpreted as the age of eclogite-facies metamorphism and the Lu–Hf age likely pre-dated the eclogite-facies metamorphism. 40 Ar/ 39 Ar dating of hornblende from the eclogite yielded ages about 200 Ma, which is interpreted as a cooling age and is probably indicative of the time of exhumation to the middle crust. The difference of peak eclogite-facies metamorphic conditions and the distinct metamorphic ages for the Bailang eclogite (~ 2.6 GPa and ~ 480 °C; ca . 230 Ma), the Sumdo eclogite (~ 3.4 GPa and ~ 650 °C; ca . 262 Ma) and Jiang eclogite (~ 3.6 GPa and ~ 750 °C; ca . 261 Ma) in the same (ultra)-high-pressure belt indicate that this region likely comprises different slices that had distinct P – T histories and underwent (U)HP metamorphism at different times. The initiation of the opening the Paleo-Tethys Ocean in the Lhasa terrane could trace back to the early Permian. The ultimate closure of the Paleo-Tethys Ocean in the Lhasa terrane was no earlier than ca . 230 Ma. [ABSTRACT FROM AUTHOR]