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New zircon U-Pb, trace element, and Lu-Hf laser ablation inductively coupled mass spectrometry (LA-ICP-MS) data are presented for five Early to Middle Devonian granitic and felsic volcanic rocks from the Cashes Ledge igneous suite, central Gulf of Maine, USA. These samples were previously analyzed by U-Pb LA-ICP-MS and whole-rock geochemical methods and the new data generally corroborate the earlier results. Coarse-grained alkali-feldspar granite from northwest of the Fundy magnetic anomaly, the interpreted fault boundary in the offshore between Gondwanan microcontinents Ganderia to the northwest and Avalonia to the southeast, yielded a crystallization age of 414 ± 2 Ma. Southeast of the inferred fault, crystallization ages are 385 ± 3 Ma and 386 ± 3 Ma for two crystal tuff samples near the fault, 403 ± 3 Ma for an alkali-feldspar granite ~50 km southeast of the fault, and 399 ± 5 Ma for syenogranite ~25 km southeast of the fault, which also yielded inherited grains at ~1.3 Ga and between 613 ± 15 Ma and 558 ± 9 Ma. Lu-Hf LA-ICP-MS data for zircon retaining igneous crystallization ages have εHf(t) between 2.9 and 13.1 and model ages based on felsic sources between 0.52 and 1.04 Ga, reflecting a mix of late Mesoproterozoic (Avalonian?) basement and primitive melt, possibly in an extensional setting. Zircon Nb/ Hf ratios generally greater than 0.001 indicate a predominately within-plate/anorogenic/rifr setting, consistent with their whole-rock chemistry. U/Yb-Nb/Yb and U/Yb-Hf tectonic setting discrimination diagrams show ocean island to continental arc signatures, with a stronger continental arc signature for the syenogranite. Most zircon grains have Eu/Eu* values less than 0.1, indicating a crustal thickness of ~30 km or less at the time of their crystallization. [ABSTRACT FROM AUTHOR]

Using a three-dimensional U-Th-Pb concordia diagram, electron microprobe (EMP) U-Th Pb analyses are shown to yield isotopic age constraints on isotopically concordant and discordant data that have not been recognized previously. The three-dimensional U-Th-Pb concordia diagram is discussed. It is demonstrated that a date obtained from an EMP analysis is as old as or younger than the [sup.207][Pb/.sup.235]U and [sup.207][Pb/.sup.206] Pb ages, and as old as or older than the [sup.208][Pb/.sup.232] Th age. EMP analyses always yield a minimum age for the oldest Pb component.

Newly compiled U-Pb detrital zircon data from eight geographic domains along the eastern Laurentian margin from Newfoundland (Canada) to Alabama (United States) show a highly consistent signature along strike, with only minor local variations. The Precambrian signature is characterized by a small ca. 2.7 Ga population and a major ca. 1.9-0.9 Ga population that peaks at ca. 1.2-1.0 Ga. Detrital zircon populations are from Laurentian Archean crust (ca. 2.7 Ga population), Paleoproterozoic orogens (ca. 1.9-1.6 Ga), the Granite-Rhyolite Province (ca. 1.5-1.4 Ga), and the Elzevir terrane and Grenville Province (ca. 1.3-0.9 Ga). The Mesoproterozoic populations vary in size depending on proximity to the ca. 1.5-1.4 Ga Granite-Rhyolite Province, the ca. 1245-1225 Ma Elzevir terrane, and the ca. 1.2-0.9 Ga Grenville Province. A middle Ordovician zircon population varies in size along strike depending on input from the Taconic orogenic belt, but it is strongest in the northern Appalachians. Because of the general along-strike consistency in detrital zircon age populations, the compilation of all 7534 concordant U-Pb detrital zircon data can be used in future U-Pb detrital zircon studies as an indicator for eastern Laurentian margin sources. [ABSTRACT FROM AUTHOR]

We present a U-Pb detrital zircon age and provenance study of a sequence of metasedimentary rocks in the northwestern Thor-Odin high-grade gneiss dome within the Omineca crystalline belt of the Canadian Cordillera. Despite strong overprint by defor mation and metamorphism, we successfully analyzed the age and provenance of six samples collected at various structural levels, using U-Pb detrital zircon laser-ablation- inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis. The Thor-Odin dome consists of Paleoproterozoic basement and a metasedimentary cover sequence of previously unknown age and tectonic signifi cance. Our results indicate that the oldest units of this sequence may be Paleoproterozoic, and some of the oldest known metasedimentary rocks in the Canadian Cordillera, originally deposited on top of Laurentian basement rocks. The youngest rocks are Devonian and deposited shortly before the onset of widespread Late Devonian to early Mississippian igneous activity in the Selkirk Domain or Kootenay arc. The cover sequence of the Thor-Odin dome thus preserves some of both the oldest and the youngest (meta)sedimentary rocks deposited between the formation of supercontinent Columbia and the onset of igneous activity and convergence that marked the beginning of Cordilleran deformation and metamorphism. Parts of as many as ∼1.4 b.y. of sedimentary history are preserved in the Thor-Odin dome, implying that much information on the sedimentary history of the Canadian Cordillera may be hidden in other Cordi lleran gneiss domes. [ABSTRACT FROM AUTHOR]

The concepts involved in the construction and interpretation of inverse isochron diagrams used in 40Ar/39Ar geochronology are reviewed. The diagrams can be useful as a means of recognising atmospheric argon and excess 40Ar, incorporated in the mineral lattice, which cannot be recognised from 40Ar/39Ar spectra. The age established using an inverse isochron plot, unlike that yielded by a spectrum, is not affected by trapped argon 40Ar/36Ar ratios that are different from the atmospheric argon ratio (e.g. due to excess 40Ar), and may contribute to a better age interpretation. However, a heterogeneous distribution of excess 40Ar or heterogeneous argon loss can cause ‘false’ isochrons, with axial intercepts indicating an incorrect age and an incorrect trapped argon composition. Inconsistency between the ages from a spectrum and from the associated inverse isochron plot may indicate the degree of inaccuracy of isochrons. However, it is possible that both the spectrum and inverse isochron yield the same incorrect age. The importance of considering all possible interpretations before assigning an age to a specimen is stressed. [Copyright &y& Elsevier]