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The common-Pb isotope composition (207Pb/204Pb vs. 206Pb/204Pb) of detrital K-feldspar was measured on the same clastic units from southeastern Laurentia that were previously characterized by detrital zircon and detrital monazite geochronology for provenance analysis. The purpose is to test a model that invokes late Paleozoic recycling of sediment initially sourced from erosion of exhuming Grenvillian basement in the Neoproterozoic. The approach takes advantage of the difference in Pb isotope compositions between Laurentian and Amazonian cratonic sources documented by previous workers. Neoproterozoic samples sourced from southern Amazonia and central Laurentian basement that serve as controls on methodology plot within Pb isotope space characteristic of their respective sources. K-feldspar in the Cryogenian Ocoee Supergroup in the southern Appalachian orogen falls within the field of Pb isotope compositions defined by south-central Appalachian basement (SCAB). The latter, in turn, exhibits Pb isotope compositions characteristic of Amazonia, because SCAB was transferred to Laurentia from Amazonia during Rodinian assembly. In contrast, K-feldspar in early Cambrian arenite falls within the Laurentian field, indicating a shift in the early Paleozoic to a sediment source from the Laurentian Craton. K-feldspar in Lower Pennsylvanian lithic arenites of the Central Appalachian Basin exhibit Pb isotope compositions that fall within the SCAB field but at higher Pb isotope ratios than in the inferred Ocoee sources. Incorporation of all provenance constraints requires an immediate source that is isotopically more radiogenic than the Ocoee but similar in all other petrologic and geochronologic characteristics, for example, other Cryogenian to Ediacaran units along strike in the Appalachian orogen. The results further demonstrate the importance of having multiple detrital mineral proxies for accurate provenance analysis rather than using detrital zircon geochronology alone. [ABSTRACT FROM AUTHOR]

This study addresses the timing and pressure-temperature (P-T) conditions of ophiolite obduction, one of the proposed causes of the ca. 470 Ma Grampian orogeny of Scotland and Ireland. This event gave rise to the main structural and metamorphic characteristics of the Grampian terrane--the type area for Barrovian metamorphism, the cause of which remains enigmatic despite a century of research. Zircons from the Highland Border ophiolite, Scotland, define a 499 [+ or -] 8 Ma U-Pb concordia age, which is interpreted as dating magmatism. Its metamorphism is dated by a 490 [+ or -] 4 Ma [sup.40][Ar.sup.-39]Ar hornblende age, and a 488 [+ or -] 1 Ma [sup.40][Ar.sup.-39]Ar muscovite age from a metasedimentary xenolith within it, from which P-T estimates of 5.3 kbar and 580[degrees]C relate to ophiolite obduction. Metamorphism of the Deerpark complex ophiolitic melange (Irish correlative of the Highland Border ophiolite) is constrained by a 514 [+ or -] 3 Ma [sup.40][Ar.sup.-39]Ar hornblende age, while mica schist slivers within it yield detrital zircon U-Pb ages consistent with Laurentian provenance and Rb-Sr and [sup.40][Ar.sup.-39]Ar muscovite ages of ca. 482 Ma. P-T values of 3.3 kbar and 580[degrees]C for the mica schist constrain the conditions of ophiolite obduction. Metamorphic mineral ages from the Grampian terrane (Dalradian Supergroup) are substantially younger (ca. 475-465 Ma) than those from the ophiolites. If conductive heating in overthickened crust was the cause of Barrovian metamorphism, then collisional thickening must have started soon after ophiolite obduction at ca. 490 Ma in order to generate the ca. 470 Ma metamorphic peak in the Grampian terrane. doi: 10.1130/B30139.1

Metasediments of the early Dalradian Grampian Group (Erris Group) and probable equivalents (Inishkea Division) structurally overlie Palaeoproterozoic to Neoproterozoic Annagh Gneiss Complex orthogneisses in NW Mayo, Ireland. Sm--Nd isotopic data suggest a Palaeoproterozoic source for the metasediments. K-feldspar and granitoid clasts from the Doonamo Formation, Erris Group, record U--Pb zircon ages of c. 1740 Ma and c. 980 Ma, respectively. These ages are within error of the c. 1730-1750 Ma Mullet gneisses and c. 990 Ma Grenvillian migmatitic leucosomes in the underlying Annagh Gneiss Complex. U--Pb detrital zircon data reveal that the Erris Group was deposited after c. 955 Ma, with predominant input from c. 1640, e. 1500 and c. 990 Ma interpreted Laurentian sources (Labradorian, Pinwarian and Grenvillian terranes, respectively). Limited detrital zircon data from the Inishkea Division yield similarly aged detritus and a tentative maximum depositional age of e. 1005 Ma. Correlation of both sequences with the Scottish Dalradian Grampian Group is considered valid based on detrital zircon U--Pb and whole-rock Sm--Nd data. The clast ages support the hypothesis that the Dalradian unconformably overlies the Annagh Gneiss Complex in north Mayo, whereas the detrital zircon data imply more distal Lanrentian sources. Dalradian deposition is thereby tied to the margins of Laurentia. Supplementary material: U--Pb zircon (secondary ion mass spectrometry) data and descriptions of dated zircon grains are available at http://www.geolsoc.org.uk/SUP18414. doi: 10.1144/0016-76492009-147.

Ion-microphone U-Pb zircon geochronology demonstrates that magmatic rocks that cut the Slishwood Division are early Ordovician in age and relate to the earliest stages of the Grampian Orogeny. These tonalite and granite intrusions yield ages of 474 [+ or -] 5 Ma, 472 [+ or -] 6 Ma, 471 [+ or -] 5 Ma and 467 [+ or -] 6 Ma. A Sm-Nd mineral isochron of 457 [+ or -] 36 Ma confirms an Ordovician age for one of the tonalite bodies and demonstrates that they were affected by Ordovician metamorphism whereas biotite Rb-Sr mineral ages from two of the tonalite bodies show that the Slishwood Division had cooled below c. 350[degrees]C at 449 [+ or -] 7 Ma. The early orogenic tonalite and granite instrusions are mylonitized in structures that overthrust the Dalradian Supergroup on the Slishwood Division during major nappe formation and, thus, date the onset of such deformation at or before 471 [+ or -] 5 Ma, the age of the youngest instrusion where such field relations can be demonstrated. Keywords: Dalradian, Caledonides, absolute age, zircon, granite.

The Palaeoproterozoic to early Neoproterozoic Annagh Gneiss Complex structurally underlies the Dalradian sequence in north Mayo, Ireland, and has been proposed as the depositional basement to Dalradian metasediments. The Annagh Gneiss Complex was deformed, metamorphosed and migmatized during the Grenville Orogeny and later reworked under amphibolite-facies conditions. This paper focuses on the timing of the post-Grenville events and particularly on the possible presence of post-Grenville, pre-Grampian deformation that could be attributed to the Knoydartian Orogeny. Seven U Pb tilanite analyses from Annagh Gneiss Complex gneisses have a weighted mean [sup.207]Pb/[sup.206]Pb age of 963 [+ or -] 8 Ma. which dates cooling after the main Grenville metamorphism. Locally, a later phase of titanite growth at 943 [+ or -] 8 Ma post-dates the last phase of Grenville deformation. The weak discordance of the titanite data suggests that post-Grenville events had little effect on the U-Pb system in titanite. If the discordance was caused by a tectonic event, this is likely to have occurred during the early Ordovician Grampian Orogeny rather than in the Neoproterozoic. Within the Annagh Gneiss Complex, cross-cutting metadolerites provide a structural marker allowing postGrenville deformation to be distinguished. In contrast, correlative metadolerites cutting the adjacent Dalradian metasediments share all Grampian deformation events affecting their host. Ar-Ar hornblende ages from the post-Grenville metadolerites indicate that reworking of the Annagh Gneiss Complex and the first episodes of Dalradian deformation occurred during the Grampian Orogeny in this pact of Ireland. One sample yields a 475 [+ or -] 4 Ma Ar-At plateau age, which is interpreted to date Grampian deformation. Younger Ar-Ar hornblende and Rb-Sr mica ages record post-Grampian cooling. Neither field nor isotopic evidence for the Knoydartian Orogeny has been found in this part of Ireland. Keywords: Grenvillian Orogeny, Caledonides, Grampian Orogeny, Dalradian, Ireland.