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The Coldbrook Group and related plutons in the Caledonian Highlands of southern New Brunswick contain voluminous late Ediacaran silicic rocks formed in a magmatic event not recognized in other parts of Avalonia in the northern Appalachian orogen. To better constrain the age and origin of these rocks, we used U-Pb zircon dating by laser ablation inductively coupled plasma mass spectrometry to check for older inherited zircon and obtain trace element data, followed by chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-TIMS) to obtain precise dates. Four silicic samples were dated from the Coldbrook Group, one from the Bonnell Brook pluton, and, for comparison, a felsic lithic-crystal tuff sample from the older arc-related Broad River Group. Overlapping CA-TIMS dates of 551.57 [+ or -] 0.23, 551.38 [+ or -] 0.24, and 551.70 [+ or -] 0.20 Ma for samples from the lower, middle, and upper Coldbrook Group, respectively, and 551.71 [+ or -] 0.19 Ma for granite from the Bonnell Brook pluton show that these units crystallized in 760 000 years or less, consistent with a super-eruption event. Rhyolite from the uppermost unit of the Coldbrook Group yielded a younger date of 549.18 [+ or -] 0.09 Ma, but the large extent of that unit is consistent with the possibility of a second younger super-eruption. The felsic lithic-crystal tuff sample from the Broad River Group yielded a date of 615.48 [+ or -] 0.16 Ma, consistent with previously published dates from that group and associated plutons. Differences in zircon chemistry between the Broad River Group sample and the late Ediacaran samples are consistent with the contrasting subduction-related vs within-plate extensional tectonic settings as suggested by previous studies of whole-rock petrological characteristics of the two age groups. Key words: U-Pb zircon dating, super-eruptions, late Ediacaran, Avalonia, zircon chemistry, Introduction The Caledonia terrane of southern New Brunswick, Canada, is a component of Avalonia in the northern Appalachian orogen (Fig. 1, inset). The original relations among the now-geographically separated parts [...]

The oldest rocks in the Avalonian Antigonish Highlands of northern mainland Nova Scotia, Canada, are late Neoproterozoic (>618 Ma) volcanic and sedimentary rocks of the Georgeville Group intruded by gabbroic/dioritic to granitic plutons. New U-Pb zircon ages presented here for five samples from plutons intruded into the James River and (or) Keppoch formations of the Georgeville Group have ages ranging from ca. 615 to 604 Ma. They have petrological characteristics of expanded calc-alkalic 'Andean-type' suites but are compositionally biased toward evolved high-silica compositions and are interpreted as evolved I-type granites. They were emplaced at shallow depths and some were likely comagmatic with felsic volcanic components of their host rocks. These plutons are younger and show less varied Sm-Nd isotopic compositions than most plutonic rocks formed in the early Ediacaran 'main arc phase' elsewhere in Avalonian terranes in the northern Appalachian orogen, although they are similar in age to plutons in southeastern New England and in the Bass River and Jeffers blocks of the Cobequid Highlands, Nova Scotia. The Jeffers block of the Cobequid Highlands appears to be most similar to the Antigonish Highlands but both areas record a Neoproterozoic history less protracted than in other parts of Avalonia. Key words: Avalonia, Appalachian orogen, Ediacaran, granitoid, U-Pb zircon age, chemistry. Les roches les plus anciennes des hautes terres avaloniennes d'Antigonish du nord de la Nouvelle-Ecosse continentale (Canada) sont des roches volcaniques et sedimentaires d'age neoproterozoique tardif (>618 Ma) du Groupe de Georgeville recoupees par des plutons gabbro'iques/dioritiques a granitiques. De nouveaux ages U-Pb sur zircons pour cinq echantillons provenant de plutons qui recoupent les Formations de James River et (ou) de Keppoch du Groupe de Georgeville vont d'environ 615 Ma a 604 Ma. Ces roches presentent des caracteristiques petrologiques de suites calcoalcalines de type andin elargies, mais leurs compositions sont biaisees vers des compositions evoluees riches en silice, et elles sont interpretees comme etant des granites de type I evolues. Ces plutons ont ete mis en place a faible profondeur et certains d'entre eux etaient probablement comagmatiques de composants volcaniques felsiques de leurs encaissantes. Les plutons sont plus jeunes et presentent des concentrations d'isotopes de Sm-Nd moins variables que la plupart des roches plutoniques formees durant la[much less than] phase d'arc principale [much greater than] d'age ediacarien precoce ailleurs dans des terranes avaloniens de l'orogene du nord des Appalaches, bien qu'ils soient d'age semblable a ceux de plutons dans le sud-est de la Nouvelle-Angleterre et dans les blocs de Bass River et de Jeffers des hautes terres de Cobequid (Nouvelle-Ecosse). Le bloc de Jeffers semble presenter la plus grande similitude avec les hautes terres d'Antigonish, mais les deux regions temoignent d'une evolution neoproterozoique moins longue que dans d'autres parties de l'Avalonie. [Traduit par la Redaction] Mots-cles: Avalonie, orogene des Appalaches, Ediacarien, granito'ide, age U-Pb sur zircons, chimie., Introduction Since the pioneering work of Williams (1978, 1979), the Antigonish Highlands of northern mainland Nova Scotia have been included in the Avalon zone in the northern Appalachian orogen, now [...]

Three conodont fossil occurrences stratigraphically above and below a rhyolitic tuff in the Lower Ordovician Crow Formation, southeast Yukon, constrain the tuff to the early Tremadocian Rossodus manitouensis Zone and possibly the Rossodus tenuis Zone. U-Pb dates were obtained from zircon from the tuff that was imaged with cathodoluminescence and chemically abraded to reduce the likelihood of dating grains that contain older components or suffered Pb loss, respectively. Six dates from grains with oscillatory parallel zoning are equivalent, with a weighted mean [sup.206]Pb/.sup.238]U date of 491.04 ± 0.13 Ma. These grains are interpreted as being primary volcanic crystals, and the date is therefore taken as the depositional age of the tuff. Four other dates from grains with sector zoning are slightly older, up to 492.0 Ma, and are interpreted as inherited or recycled from earlier volcanics. The currently defined Cambrian-Ordovician boundary is inferred to be below the tuff and separated from it by at least two conodont biostratigraphic zones that are estimated to span at least 2.25 Ma based on the top of the Rhabdinopora flabelliformis parabola graptolite zone being older than the base of the Rossodus tenuis conodont zone. We interpret the evidence in southeast Yukon to suggest that the global Cambrian-Ordovician boundary is older (>493.3 Ma) than previous estimates of ~488 and 491 Ma based upon legacy [sup.207]Pb/[sup.206]Pb zircon ages. Trois occurrences de fossiles de conodontes situees stratigraphiquement au-dessus et en dessous d'un tuf rhyolitique dans la Formation de Crow de l'Ordovicien inferieur du sud-est du Yukon placent le tuf dans la zone a Rossodus manitouensis et possiblement dans la zone a Rossodus tenuis du Tremadocien precoce. Des ges par datation U-Pb ont ete obtenus de zircons provenant du tuf qui ont ete caracterises par cathodoluminescence et abrases chimiquement afin de minimiser la possibilite de dater des grains contenant des composantes plus anciennes ou ayant perdu du Pb respectivement. Six ges U-Pb obtenus de grains presentant un zonage oscillatoire parallele sont equivalents pour un ge [sup.206]Pb/[sup.238]U moyen pondere de 491,04 ±0,13 Ma. Ces grains sont interpretes comme etant des cristaux volcaniques primaires cet ge U-Pb refletant donc l' ge de depot du tuf. Quatre autres ges obtenus de grains a zonage sectoriel sont legerement plus vieux allant jusqu' a 492,0 Ma. Il s'agirait de cristaux herites ou recycles de roches volcaniques plus anciennes. La limite Cambrien-Ordovicien telle qu' elle est actuellement definie se situerait sous le tuf dont elle serait separee par au moins deux biozones a conodonte qui representeraient au moins 2,25 Ma etant donne que le sommet de la zone a graptolite a Rhabdinopora flabelliformis parabola est plus ge que la base de la zone a conodonte a Rossodus tenuis. Les donnees pour le sud-est du Yukon suggerent que a l'echelle planetaire la limite Cambrien-Ordovicien est plus reculee (>493,3 Ma) que ce qu'indiquaient les estimations anterieures qui la situaient a ~488 et 491 Ma a la lumiere d'ges [sup.207]Pb/[sup.206]Pb existants obtenus de zircons., Introduction The Ordovician period was a time of major evolutionary turnover and diversification of marine faunas that resulted in establishment of communities that would typify the succeeding 200 million years [...]

Chinese loess is regarded as one of the most detailed and long-term archives of climate on land. However, there is still significant controversy over the deposit's origin, limiting interpretation of the sedimentological and paleoclimatic mechanisms responsible for its emplacement. Here this is addressed through morphology and the first laser ablation--inductively coupled plasma--mass spectrometer (LA-ICP-MS) U-Pb ages of detrital zircons from loess (last glacial age; northern Loess Plateau). These are compared to zircon U-Pb age spectra from desert and sandy lands surrounding the Loess Plateau. Surface samples were taken from the Tengger and Mu Us deserts, as well as the Horqin and Otindag sandy lands. The results demonstrate that zircon U-Pb ages can discriminate between potential source areas and highlight both similarities and differences in age spectra for the desert and sandy land samples. Most significantly, the loess age spectrum shows no single affinity to any of these regions and exhibits zircon ages associated with granitoid rocks representing tectonic events in both west and east northern China. Furthermore, and in contrast to proximal deserts, a significant proportion of zircons from the loess show affinities with rocks cropping out in the Qilian Mountains. The euhedral form of many of these grains further suggests direct transport from these crystalline source rocks, in contrast to previously hypothesized production or storage in deserts. Thus, dust-transporting storms tracked from the west during the last glacial maximum, although this does not explain all the zircon variability and implies multiple sources and storm-track variation over the depositional period. doi: 10.1130/B30102.1

We suggest that the Salzachtal-Ennstal-Mariazell-Puchberg (SEMP) fault, a major strikeslip system in the European Alps, is active. It has accommodated lateral extrusion of the central part of the Eastern Alps toward the Pannonian Basin. The main tectonic activity of this fault dates back to Oligocene and Miocene time, but until now it was largely unknown whether the fault was still active. We present here the first field evidence of neotectonic activity from a cave in the Hochschwab karst massif (Styria, Austria) that intersects a segment of the SEMP fault zone. Damaged speleothems in this cave include scratched flowstone (a hitherto undescribed feature), massive flowstone disrupted by a fault, and ruptured flowstone. The superposition of younger flowstone layers allows constraining the time frame of the tectonic events using U/Th dating. The youngest flowstone of the pre-damage generation is ca. 118 ka (end of the Last Interglacial) and the oldest post-event layer is ca. 9 ka (early Holocene). The tectonic event bracketed by these layers coincided with a growth hiatus during the last glacial period, consistent with the high alpine setting of the cave. Geologic evidence precludes deformation mechanisms other than tectonic. These new data are consistent with vectors of continuous global positioning system measurements as well as instrumental seismicity data, and collectively suggest that the SEMP is an active fault and that lateral extrusion of the Eastern Alps is ongoing. doi: 10.1130/G30854.1

The shortening history of the Andes is important for understanding retroarc deformation along convergent margins and forcing mechanisms of Cenozoic climate. However, the timing of uplift in the northern Andes is poorly constrained, with estimates ranging from Cretaceous to Pliocene. Detrital zircon U-Pb ages from the Middle Magdalena Valley Basin in Colombia reveal two provenance shifts during Cenozoic time. The first shift occurs between early and late Paleocene strata, where U-Pb results show a switch from Proterozoic-dominated to Phanerozoic-dominated age spectra. We attribute this change to uplift-related exhumation of the Central Cordillera. The second shift occurs between middle-late Eocene and late Oligocene strata, where increased Grenville ages and diminished Mesozoic ages can be linked to uplift of the Eastern Cordillera. Our results show that significant pre-Neogene deformation affected the northern Andes, underscoring the potential importance of Andean uplift on the dynamics of Paleogene climate. doi: 10.1130/G30775.1

New high-precision U/Pb geochronology from volcanic ashes shows that the Triassic-Jurassic boundary and end-Triassic biological crisis from two independent marine stratigraphic sections correlate with the onset of terrestrial flood volcanism in the Central Atlantic Magmatic Province to doi: 10.1130/G30683.1

Paleozoic rocks of the Maya (Yucatan) block of Central America include granitoids, rhyolitic-dacitic volcanic rocks, clastic sedimentary strata ranging from conglomerate to shale/phyllite, and minor limestone. To date, published isotopic ages, paleontologic data, and interpretations of field relations have been contradictory. For instance, U/Pb ages of zircon and monazite from granitoids yield Silurian-Devonian ages (ca. 420-405 Ma); the granitoids are bordered by metasedimentary contact aureoles, yet the bulk of their protolith has been assigned to the Pennsylvanian-Permian (Macal Formation or Santa Rosa Group). In this paper, we resolve the paradox by showing that pre--Lower Devonian beds are present in the Maya Mountains and constitute the wall rock for the intrusions. We dated 23 igneous zircons from a rhyolite interbedded with conglomerates obtaining a 406 [sup.+7][/.sub.-6] Ma (2s) median U/Pb age, which represents the time of eruption of the previously recognized volcaniclastic Bladen Formation. Furthermore, U/Pb geochronology of detrital zircon grains from a sandstone cobble, and outcrops of sandstone and quartzite yielded equivalent age spectra, indicating that they represent a separate geologic unit, which we herein define as the Baldy unit. Recognition of a distinct, pre-Devonian unit is .justified because the sandstone cobble was collected from a conglomerate associated with the dated rhyolite, implying that the Baldy unit must have been deposited, lithified, exposed, and eroded before deposition of the Bladen Formation. The Baldy unit predates Silurian--Devonian magmatism because it does not contain interbedded volcanic rocks and is intruded by the dated granites. The five youngest detrital zircons of Baldy sedimentary rocks yielded ages in the 600-520 Ma range. These features imply that Baldy deposition took place some time between the Cambrian and Silurian. Baldy zircon age spectra have peaks at ca. 1.52 Ga, ca. 1.21 Ga, and ca. 1.02 Ga, consistent with provenance from Baltica or Amazonia, not Laurentia. Based on our new data and previously known geologic attributes of the Maya block, we propose that, during the Cambrian--Silurian, the block's position was along the West Amazonia side of Gondwana's periphery. doi: 10.1130/B26405.1

Absolute ages for sedimentary rocks are required to construct a temporal framework in which to decipher Earth's history. The most reliable method for dating Precambrian sedimentary rocks is U-Pb geochronology of zircon in intercalated volcanic rocks. However, extracting sufficient zircons involves destructive mineral separation procedures, often requiring several kilograms of sample. We have dated felsic tufts doi: 10.1130/G30567.1

The Tanzawa plutonic complex (TPC), central Japan, is a suite of tonalitic-gabbroic plutons exposed in a globally unique are-are collision zone, where an active intraoceanic Izu-Bonin-Mariana (IBM) arc is colliding against the Honshu arc. The TPC has been widely accepted as an exposed middle crust section of the IBM arc, chiefly because of geochemical similarities between the TPC and IBM rocks and previously reported precoilisional Miocene K-Ar ages. However, new zircon U-Pb ages show that the main pulse of TPC magmatism was syncoilisional and that plutons were emplaced rapidly and cooled soon after Pliocene collision. Trace element compositions of TPC zircon show distinctively elevated Th/Nb ratios compared to zircon from other noncollisional IBM silicic plutonic rocks, indicating the involvement of continental sediments from the Honshu arc in their magma genesis. doi: 10. l 130/G30526.1

The Variscan Central Bohemian Plutonic Complex crops out between the upper-crustal Tephi Barrandian and the high-grade Moldanubian units (Bohemian Massif). Much of the complex is made up of calcalkaline plutonic rocks: (1) the geochemically more primitive, Na-rich 354 [+ or -] Ma Sazava suite, which was emplaced syntectonically during regional shortening; (2) the younger, more evolved, potassic Blatna suite, which records both the shortening along its NW contact and the onset of normal shearing related to exhumation of the Moldanubian Unit to the SE. New ion microprobe U-Pb zircon ages for the high-K calc-alkaline Blatna suite pinpoint this major switch in the tectonic regime. Both Blatna and Kozarovice granodiorites (346 [+ or -] 2 Ma and 347 [+ or -] 2 Ma (2[sigma])) were generated by melting of heterogeneous crust and were emplaced contemporaneously to form the Blatna composite pluton. From age spectra preserved in inherited zircons and whole-rock Sr-Nd isotope signatures, the likely crustal sources for the magmas were immature greywackes rich in Neoproterozoic (615 [+ or -] 10 Ma, Kozarovice) or Late Cambrian Early Ordovician (492 [+ or -] Ma, Blatna) volcanogenic detritus. An additional important petrogenetic process was variable mixing with enriched mantle-derived monzonitic magmas, which may also have supplied the extra heat for crustal anatexis. Supplementary material: Analytical techniques, selected whole-rock major- and trace-element geochemical analyses and U-Th-Pb data are available at http://www.geolsoc.org.uk/SUP18391. doi: 10.1144/0016-76492009-008.

Using laser ablation ICP-MS techniques, U-Pb ages are reported for 338 detrital zircons separated from four Franciscan metagraywacke samples from imbricate thrust sheets cropping out in the Pacheco Pass area, east central California Coast Ranges. Structurally higher slabs are clastic quartz [+ or -] albite-bearing rocks typified by neoblastic lawsonite + jadeitic pyroxene; the lowest exposed slab lacks jadeitic pyroxene but contains traces of newly grown pumpellyite and lawsonite in addition to abundant quartz + albite. Studied specimens contain moderate amounts of phengite + titanite [+ or -] chlorite, stilpnomelane, carbonate, iron oxides, rock fragments, and carbonaceous matter. These Diablo Range clastic sediments were deposited in mid-Cretaceous and Late Cretaceous time. The highest allochthon, unit I, was deposited after ~ 102 Ma, unit IV was deposited after ~90-102 Ma, and unit V was deposited after ~86 Ma. Metagraywacke depositional-accretionary ages within slab IV young upward, but ages of the slabs decrease progressively downward. In the four-sample aggregate, most zircons have igneous ages falling in the 135-170 Ma range, with a smaller population at 85-120 Ma. The rocks contain rare zircons of Middle Proterozoic--Late Archean ages. A minor source in northern California and NW Nevada seems likely for the Pacheco Pass metagraywackes, but the Sierran-Klamath calcalkaline arc provided most of the clastic debris. Judging by the volcanic nature of lithic clasts in these Eastern Belt metagraywackes, and because massive Sierran plutonism occurred at ~85-120 Ma, the abundant 135-170 Ma zircons probably were derived chiefly from the eroding comagmatic volcanic arc rather than from the less voluminous Jurassic arc plutons. Citation: Ernst, W. G., U. Martens, and V. Valencia (2009), U-Pb ages of detrital zircons in Pacheco Pass metagraywackes: Sierran-Klamath source of mid-Cretaceous and Late Cretaceous Franciscan deposition and underplating, Tectonics, 28, TC6011, doi: 10.1029/2008TC002352.

U-Pb ages determined using sensitive high-resolution ion microprobe-reverse geometry on 205 single-grain zircons from 16 ash beds within submarine fan deposits of the Ecca Group provide the first evidence of a marine Permian-Triassic (P-T) boundary in the Karoo Basin of South Africa. These U-Pb ages provide an objective basis for correlating the deep-marine sediments of the southwest Karoo Basin with fluvial-deltaic deposits in the central and eastern parts of the basin where the P-T boundary is recorded in a diverse macrofauna. Furthermore, these new zircon ages and their correlation imply asymmetric subsidence and variable sedimentation rates across the basin.

Avalonia is the largest accreted crustal block in the Appalachian orogen and comprises a collection of late Neoproterozoic volcano-sedimentary sequences that are overlain by a Palaeozoic platformal sedimentary succession. Detrital zircons from the Conception Group are dominated by 570-620 Ma ages and contain a significant component generated by erosion of coeval igneous arc-volcanic rocks. Overlying samples from the Cuckold and Crown Hill formations are dominated by Neoproterozoic populations with ages between 600 and 650 Ma and are interpreted to be derived from the underlying calc-alkaline arc-plutonic rocks. Early Palaeozoic platform units are dominated by c. 620 Ma zircons with lesser Mesoproterozoic and Palaeoproterozoic zircons. The range of detrital zircon ages is inconsistent with a West African provenance and suggests that Avalonia originated along the Gondwanan margin of the Amazon craton. The influx of Mesoproterozoic and Palaeoproterozoic detritus in the Avalonian platform suggests a major change in tectonic regime. The prominent change in provenance is interpreted to be related to separation of Avalonia from Gondwana during the Early Ordovician opening of the Rheic Ocean. The Redmans Formation is interpreted to represent the rift--drift transition of the Rheic Ocean, which imposes important constraints on the palaeotectonic evolution of Avalonia. Supplementary material: U--Pb isotopic data of LA-ICP-MS analysis of detrital zircons are available at http://www.geolsoc.org.uk/SUP18346.

Ages of Permian volcanic rocks from the Ligurian Brianconnais domain (Western Italian Alps) have been determined by laser ablation inductively coupled plasma mass spectrometry U-Pb dating of zircon. Three major volcanic units yielded zircons that were dated: calc-alkaline rhyolites (285.6 [+ or -] 2.6 Ma), andesites (with inherited cores yielded ages c. 476 Ma and older) and voluminous rhyodacites-rhyolites (272.7 [+ or -] 2.2 Ma). Following an amagmatic, sediment-starved time gap of c. 14 Ma, alkaline volcanic activity is recorded, at the top of the sequence, by K-alkaline rhyolites dated at 258.5 [+ or -] 2.8 Ma. The Ligurian segment of the Southern Variscan belt records transtensional and then extensional tectonics associated with the volcanic activity. The switch from calc-alkaline to alkaline activity corresponds to the transition from a post-orogenic to an anorogenic setting in the Southern Variscides; it may represent progressive and increasing delamination of the continental lithosphere, accompanied by partial melting of the lithospheric mantle. Supplementary material: Technical and data acquisition parameters, U-Th-Pb isotope analyses and calculated ages of zircons from samples and standard, and trace element compositions of selected zircons are available at: http://www.geolsoc.org.uk/SUP18322.

Ion-probe U-Pb dating of plutonic rocks from the northern Arabian-Nubian Shield in Sinai and southern Israel constrains the timing of late East African batholithic post-collisional calc-alkaline (CA2) magmatism and within-plate alkaline to peralkaline (AL) magmatism to c. 635-590 Ma and c. 608-580 Ma, respectively. The earliest dated CA2 rocks are slightly deformed to undeformed, indicating that penetrative deformation ceased by c. 630 Ma. Within the CA2 suite a change from mafic to felsic magmatism is manifested in most of the region, peaking in a voluminous pulse of granodiorite to granite intrusion at 610-600 Ma. The AL magmatism started contemporaneously with the peak in CA2 felsic activity at c. 608 Ma and lasted until 580 Ma. It includes mostly alkaline and peralkaline granites, probably representing variable degrees of differentiation of similar parental magmas. Thus CA2 and AL granites do not represent different tectonic settings, but coeval derivation from variable sources during crustal extension. The majority of rocks dated in this study show minor to non-existent zircon inheritance and thus indicate very minor interaction with previously formed felsic crust. The rare zircon xenocrysts span a typical East African age range (900-607 Ma) and confirm the absence of older crustal components in the juvenile Arabian-Nubian Shield. Supplementary material: Geochemical and geochronological methods, sample descriptions and data are available at http://www.geolsoc.org.uk/SUP18327.

The Rheic Ocean was a major oceanic domain between Avalonia and Gondwana in Ordovician-Silurian times. Most of the Paleozoic plate reconstructions assume that the Rheic Ocean suture lies within southern Iberia, coinciding with the contact between the South Portuguese Zone and the Ossa-Morena Zone. This paper reports four Sensitive High Resolution Ion Micro-Probe (SHRIMP) U-Pb zircon ages from midocean ridge basalt (MORB)-featured rocks of the Beja-Acebuches Amphibolite unit, which crops out along the boundary between the Ossa-Morena and the South Portuguese Zone, and is considered its most conspicuous suture unit. The obtained ages range from 332 [+ or -] 3 to 340 [+ or -] 4 Ma, corresponding to the crystallization of the mafic protoliths. These Early Carboniferous ages for the Beja-Acebuches amphibolites imply that this unit can no longer be viewed as an ophiolite belonging to the Rheic Ocean suture, since this oceanic domain was presumably closed in Devonian times. Tectonic reconstructions joining in a single suture line the Beja-Acebuches Amphibolite unit in southern Iberia to either the Devonian Lizard ophiolite in southern England or the root zone of the Devonian/ Ordovician ophiolitic units in northwest Iberia must be therefore reconsidered because of the age difference. We interpret the Beja-Acebuches Amphibolite unit to represent a narrow and very ephemeral realm of oceanic-like crust that opened in Early Carboniferous times, after total consumption of the Rheic Ocean. We suggest that a mantle plume underneath southern Iberia in Early Carboniferous times is the most plausible large-scale geodynamic scenario for the formation of these MORB-featured rocks.

Zircons and monazites from 6 samples of the North Ayilari dextral shear zone (NAsz), part of the Karakorum fault zone (KFZ), have been dated with the U-Th-Pb method, using both ID-TIMS and SIMS techniques. The ages reveal (1) inheritance from several events spanning a long period between the late Archean and the Jurassic; (2) an Eocene-Oligocene magmatic event (~35-32 Ma); (3) an Oligo-Miocene magmatic event (~25-22 Ma), at least partly synkinematic to the right-lateral deformation; and (4) a period of metamorphism metasomatism (~22-14 Ma) interpreted as thermal and fluid advection in the shear zone. The Labhar Kangri granite located ~375 km farther Southeast along the KFZ is dated at 21.1 [+ or -] 0.3 Ma. Such occurrence of several Oligo-Miocene granites along the KFZ, some of which show evidence for synkinematic emplacement, suggests that the fault zone played an important role in the genesis and /or collection of crustal melts. We discuss several scenarios for the onset and propagation of the KFZ, and offset estimates based on the main sutures zones. Our preferred scenario is an Oligo-Miocene initiation of the fault close to the NA range, and propagation along most of its length prior to ~19 Ma. In its southern half, the averaged long-term fault-rate of the KFZ is greater than 8 to 10 mm/a, in good agreement with some shorter-term estimates based on the Indus river course, or Quaternary moraines and geodesy. Our results show the KFZ cannot be considered as a small transient fault but played a major role in the collision history.

Secondary ionization mass spectrometry (SIMS) U/Pb dating of zircon has been applied to rocks of the Lewisian Complex of South Harris (Outer Hebrides, NW Scotland), yielding insights into the complex geological evolution of this region and enhancing our understanding of terranes in the Lewisian Gneiss Complex. Results show that tonalite--trondhjemite--granodiorite magmatism occurred in the Tarbert Terrane between c. 2850 and 2830 Ma, equivalent to similar rocks in other parts of the Outer Hebrides. Magmatism was accompanied or closely followed by high-grade metamorphism at c. 2830 Ma, the first reported occurrence of such an event at this time. These rocks were affected by a subsequent high-grade metamorphic event at c. 2730 Ma, which has correlatives both in the Outer Hebrides and in the mainland Lewisian Gruinard Terrane. These data suggest that the 'terranes' forming the Outer Hebrides and parts of the mainland Lewisian were once contiguous and were fragmented by post-2730 Ma events. The c. 1700-1660 Ma ages from felsic sheets and pegmatites within the Langavat Shear Zone may record the reassembly of these crustal fragments.

Detrital zircons from Devonian and Carboniferous synorogenic flysch deposits occurring in an imbricate stack have been dated by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) to: (1) obtain a maximum depositional age to constrain the maximum age limit for thrusting of exotic terranes in the NW Iberian Massif; (2) correlate the zircon age populations with published ages in nearby units to establish their possible source areas. The maximum depositional ages are Late Devonian for rocks high in the structural nappe pile (Gimonde Formation), in accordance with palynomorph dating, and around the Devonian--Carboniferous boundary for structurally lower samples (San Vitero Formation). Used in conjunction with previously published ages, the new ages are interpreted in terms of the advance of the thrust system responsible for the emplacement of exotic terranes upon the Iberian autochthon during the Variscan collision. Early Variscan zircon population ages indicate the exotic terranes as the source of synorogenic sediments, whereas their scarcity suggests derivation from the Iberian autochthon. One of the samples analysed lacks Variscan detrital zircons; this feature, together with the absence of an Early Palaeozoic zircon age population, puts into question its synorogenic character and suggests that the sample may be representative of the preorogenic parautochthon.

High-grade gneisses of the north Qaidam high-pressure/ultrahigh-pressure metamorphic terrane enclose minor eclogites and ultramafic rocks. In combination with petrological data, sensitive high-resolution ion microprobe U-Pb geochronology on the granulitized gneisses in the Luliangshan and Xitieshan, western north Qaidam Mountains, reveals a polyphase tectonothermal history including Early Neoproterozoic and Early Paleozoic events. The rocks investigated are two granulite-facies paragneisses and one orthogneiss that both surround garnet peridotite in the Luliangshan, and two paragneisses that enclose eclogite in the Xitieshan. The inherited zircon cores from paragneiss and orthogneiss yield ages between ca. 1000 Ma and ca. 2500 Ma, representing Mesoproterozoic to Archaean source material for these gneisses. A ca. 900 Ma age obtained from one orthogneiss and one paragneiss is interpreted as the age of simultaneous Early Neoproterozoic magmatism and metamorphism. This Early Neoproterozoic tectonothermai event is similar in age to the Jinning orogeny, which is commonly recognized in the metamorphic basement of the south China block, and suggests that the north Qaidam Mountains have an affinity to the south China block. Early Paleozoic metamorphism is recorded in all gneiss samples. In conjunction with cathodoluminescence imagery and mineral inclusions, U-Pb dating of zircons reveals two Early Paleozoic age groups: ca. 450 Ma represents the age of high-pressure granulite metamorphism (Grt + Rt inclusions in zircon; kyanite porphyroblasts), whereas ca. 425 Ma reflects the time of medium-pressure, granulite-facies metamorphism (Pl + Sil inclusions in zircon) and associated anatexis during decompression. The ages obtained suggest that the granulitefacies metamorphism lasted for ~25 m.y. and was related to the Early Paleozoic continental collision between the Qilian and Qaidam blocks, and to subsequent thermal relaxation and exhumation. Keywords: polyphase tectonothermal event, gneiss, north Qaidam Mountains, U-Pb geochronology.

The source of buoyancy for the uplift of cratonic plateaus is a fundamental question in continental dynamics. The ~1.9 km uplift of the Colorado Plateau since the Late Cretaceous is a prime example of this problem. We used apatite (U-Th)/He thermochronometry (230 analyses; 36 samples) to provide the first single-system, regional-scale proxy for the unroofing history of the southwestern quadrant of the plateau. The results confirm overall southwest to northeast unroofing, from plateau margin to plateau interior. A single phase of unroofing along the plateau margin in Late Cretaceous to Early Tertiary (SevierLaramide) time contrasts with multiphase unroofing of the southwestern plateau interior in Early and mid- to Late Tertiary time. The Early Cretaceous was characterized by northeastward tilting and regional erosion, followed by aggradation of [greater than or equal to] 1500 m of Upper Cretaceous sediments along the eroded plateau margin. Sevier-Laramide denudation affected the entire southwestern plateau, was concentrated along the plateau margin, and migrated from northwest to southeast. Following a period of relative stability of the landscape from ca. 50-30 Ma, significant unroofing of the southwestern plateau interior occurred between ca. 28 and 16 Ma. Additional denudation north of the Grand Canyon took place in latest Tertiary time. Mid-Tertiary dates from the Grand Canyon basement at the bottom of the Upper Granite Gorge limit significant incision of the modern Grand Canyon below the Kaibab surface to 1500 m difference in vertical structural position. If these models are correct, they indicate that a 'proto-Grand Canyon' of kilometer-scale depth had incised post-Paleozoic strata by the Early Eocene. Evidence for kilometerscale mid-Tertiary relief in northeast-flowing drainages along the plateau margin, as well as the mid-Tertiary episode of plateau interior unroofing, imply that the southwestern plateau interior had attained substantial elevation by at least 25-20 Ma, if not much earlier. These observations are inconsistent with any model calling for exclusively Late Tertiary uplift of the southwestern plateau. Sevier-Laramide plateau surface uplift and incision thus result from one or more processes that enhanced the buoyancy of the plateau lithosphere, expanding the Cordillera's orogenic highlands into its low standing cratonic foreland. The onset of the Laramide slab's demise at ca. 40 Ma and the major pulse of extension in the Basin and Range from ca. 16-10 Ma appear to have had little influence on the denudation history of the southwestern plateau. In contrast, the post-Laramide unroofing episodes may be explained by drainage adjustments induced by rift-related lowering of regions adjacent to the plateau, without the need to otherwise modify the plateau lithosphere. Our data do not preclude a large component of post-Early Eocene elevation gain (or the geodynamic mechanisms it may imply), but they do point toward Laramide-age buoyancy sources as the initial cause of significant surface uplift, ending more than 500 m.y. of residence near sea level. Keywords: Colorado Plateau, (U-Th)/He, Grand Canyon, unroofing, incision, uplift, thermochronometry.

Abstract: New Rb-Sr phlogopite and U-Pb perovskite age determinations for 12 of 38 known ultramafic bodies in the Buffalo Head Hills area of north-central Alberta show that kimberlitic magmatism occurred [...]

[1] The NNW-trending White-Inyo Range includes intrusive and volcanic rocks on the eastern flank of the Sierran volcano-plutonic arc. The NE-striking, steeply SE-dipping Barcroft reverse fault separates folded, metamorphosed Mesozoic White Mountain Peak mafic and felsic volcanic flows, volcanogenic sedimentary rocks, and minor hypabyssal plugs on the north from folded, well-bedded Neoproterozoic-Cambrian marble and siliciclastic strata on the south. The 163 [+ or -] 2 Ma Barcroft Granodiorite rose along this fault, and thermally recrystallized its wall rocks. However, new SHRIMP-RG ages of magmatic zircons from three White Mountain Peak volcanogenic metasedimentary rocks and a metafelsite document stages of effusion at ~115-120 Ma as well as at ~155-170 Ma. The U-Pb data confirm the interpretation by Hanson et al. (1987) that part of the metasedimentary-metavolcanic pile was laid down after Late Jurassic intrusion of the Barcroft pluton. The Lower Cretaceous, largely volcanogenic metasedimentary section lies beneath a low-angle thrust fault, the upper plate of which includes interlayered Late Jurassic mafic and felsic metavolcanic rocks and the roughly coeval Barcroft pluton. Late Jurassic and Early Cretaceous volcanism in this sector of the Californian continental margin, combined with earlier petrologic, structural, and geochronologic studies, indicates that there was no gap in igneous activity at this latitude of the North American continental margin. Citation: Scherer, H. H., W. G. Ernst, and R. Brooks Hanson (2008), Geologic implications of new zircon U-Pb ages from the White Mountain Peak Metavolcanic Complex, eastern California, Tectonics, 27, TC2002, doi:10.1029/2007TC002141.

The Tanneron massif belongs to the southernmost segment of the Variscan belt in France and is composed of migmatitic orthogneisses and paragneisses cross-cut by a main tonalite-granite complex. Late Carboniferous detrital basins are bounded by north-south-trending ductile to brittle normal faults and delineate three distinct zones. Our new isotope dilution U-Pb monazite dating indicates a pre-Variscan history in the central part of the massif recorded by monazites from an orthogneiss yielding ages from 440 to 410 Ma, whereas monazites from a migmatitic paragneiss record a late Carboniferous high-Tevent at 317 [+ or -] 1 Ma. In the eastern part, a migmatization event is recorded by monazites from a synkinematic leucogranitic layer and a mylonitic orthogneiss yielding ages of 309 [+ or -] 5 and 310 [+ or -] 2 Ma, respectively. Later post-collisional magmatism related to lithospheric thinning is recorded by the intrusion of undeformed granitic bodies at 302 [+ or -] 4 and 297 k 5 Ma, and marks the final stage of the Variscan evolution. The data indicate the presence of two crustal blocks, displaying different levels of exposure of the Variscan crust. The overall tectonomagmatic evolution of this massif suggests a strong affinity with the internal part of the Variscan belt.

We have discovered a robust microcrystalline record of the early genesis of North American lithosphere preserved in the U-Pb age and oxygen isotope zoning of zircons from a lower crustal paragneiss in the Neoarchean Superior province. Detrital igneous zircon cores with [[delta].sup.18]O values of 5.1 [per thousand]-7.1 [per thousand] record creation of primitive to increasingly evolved crust from 2.85 [+ or -] 0.02 Ga to 2.67 [+ or -] 0.02 Ga. Sharp chemical unconformity between cores and higher [[delta].sup.18]O (8.4 [per thousand]-10.4 [per thousand]) metamorphic overgrowths as old as 2.66 [+ or -] 0.01 Ga dictates a rapid sequence of arc unroofing, burial of detrital zircons in hydrosphere-altered sediment, and transport to lower crust late in upper plate assembly. The period to 2.58 [+ or -] 0.01 Ga included ~80 m.y. of high-temperature (~700-650 [degrees]C), nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root. Huronian continental rifting is recorded by the youngest zircon tip growth at 2512 [+ or -] 8 Ma (~ 600 [degrees]C) signaling magma intraplating and the onset of rigid plate behavior. This >150 m.y. microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of U, Pb, and O in zircon throughout protracted regional metamorphism, and the consequent advances now possible in reconstructing planetary dynamics with zircon zoning. Keywords: zircon, U-Pb, oxygen isotopes, ion probe, lower crust, Kapuskasing, Archean.

The Shiqiao-Pingshang low-grade metasedimentary rocks sporadically crop out in the Sulu ultrahigh-pressure (UHP) belt in east-central China. Major and trace element data indicate that they were deposited in a passive margin basin, probably located at the southern margin of the North China Craton. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating of detrital zircons from a quartzite (SD53) from Pingshang and a quartz schist (SD54) from Shiqiao records ages ranging from 2800 to 1900 Ma. Three age populations are defined: at 2660-2500, 2350-2150 and 2100-1900 Ma, with peak ages at 2522, 2212 and 2020 Ma, respectively. Muscovite Ar-Ar dating of the quartz schist (SD54) yields a plateau age of 265.9 [+ or -] 1.2 Ma. These data support the view that the Shiqiao-Pingshang low-grade metasedimentary rocks originated from the North China Craton, and underwent deformation in the Permian during subduction of the South China Craton (Yangtze block) beneath the North China Craton. This requires that the low-grade metasedimentary rocks were overthrust for several tens of kilometres onto the South China Craton, explaining why source materials from the North China Craton are common in Triassic to Jurassic sedimentary basins adjacent to the Dabie-Sulu orogenic belt.

Combined geochemistry and geochronology of the Myanmar jadeitite were determined. Bulk-rock trace element compositions display U-shaped REE patterns with pronounced positive Eu anomalies. The total REE abundances are very low, less than half chondritic, and the high field strength elements and some large ion lithophile elements are moderately enriched. These features indicate a metasomatic origin. There are three groups of zircons with different interior characteristics, cathodoluminescence, mineral inclusions, chemical compositions and sensitive high-resolution ion microprobe U-Pb ages. Group-I zircons, with a mean age of 163.2 [+ or -] 3.3 Ma, mostly have distinct oscillatory zoning, highest U and Th contents, and Na-free, Mg-rich mineral inclusions, and thus indicate an igneous (formation of oceanic crust) or hydrothermal (serpentinization and/or rodingitization) event in the Middle Jurassic. Group-II zircons, with a mean age of 146.5 [+ or -] 3.4 Ma, have bright luminescence without oscillatory zoning and include jadeite and jadeitic pyroxene inclusions, suggesting that formation of the Myanmar jadeitites, as well as subduction of the eastern Indian oceanic plate, occurred in the Late Jurassic. Group-III zircons have an age of 122.2 [+ or -] 4.8 Ma, which represents a later unknown thermal event. Discovery of the Middle Jurassic zircons provides geochronological constraint on the tectonic evolution of the eastern Indo-Burman Range.

In the Agly Massif (Pyrenees), two Variscan plutons, the Saint-Arnac pluton and the Ansignan chamockite, intrude different levels of a c. 10 km thick crustal section. The Saint-Arnac pluton intrudes through upper crustal rocks and the Ansignan charnockite cuts mid-crustal country-rocks. A structural study of these plutons based on the anisotropy of magnetic susceptibility technique, combined with a kinematic study of the country-rocks, shows that the structures and emplacement modes of the plutons are compatible with those of the other plutons of the Pyrenees emplaced during the [D.sub.2] transpressive phase. U-Pb dating on zircons from the Saint-Arnac pluton yields a 308.3 [+ or -] 1.2 Ma age for a diorite and a 303.6 [+ or -] 4.7 Ma age for a granodiorite. The charnockite was previously dated at 315 Ma. The emplacement ages of these two intrusions are thus separated by at least 5 Ma. First, numerous sills and laccoliths, such as the Ansignan laccolith, were injected in the middle crust, and this induced heating and thickening. Subsequently, the large Saint-Arnac pluton was injected in the upper crust at the beginning of formation of a gneissic dome.

The extent and nature of the Archaean-Palaeoproterozoic craton of Rio de la Plata of southern South America, a major but poorly understood crustal component in Neoproterozoic plate reconstructions, as well as the depositional, metamorphic and magmatic history of the surrounding orogenic belts, are reviewed and reassessed, in part through the analysis of material recovered from deep boreholes in western Argentina that penetrated Palaeozoic cover into basement. U--Pb SHRIMP zircon ages for these samples of 2162[+ or -]6 Ma (diorite), 2189[+ or -]14 Ma (amphibolitic schist) and 2088[+ or -]6 Ma (granite) encompass the range of ages determined for the major Palaeoproterozoic orogenic events in the exposed parts of the craton close to the Atlantic coasts of Uruguay and Argentina. Taken together with the geochemical and Nd-isotope characteristics of these samples and an olivine gabhro from a further borehole that failed to yield zircon, these results strongly suggest that the Rio de la Plata craton is extremely uniform in its dominant ehrono-tectonic and lithological make-up, and that it extends westwards as far as the 535-520 Ma Pampean orogenic belt, against which it probably has a fault contact. U--Pb SHRIMP zircon detrital age patterns are presented for representative metasedimentary samples from the craton cover in the Tandilia belt of eastern Argentina, and from the Pampean (Cambrian) and Famatinian (Ordovician) belts to the west of the craton. Whereas the oldest cover rocks in Tandilia clearly show material derived from the underlying craton, such detritus only appears in the younger (Ordovician) units to the west. Sedimentary protoliths in the Pampean belt were dominated by Neoproterozoic (broadly ~ 600 Ma) and late Mesoproterozoic (broadly ~ 1100 Ma) provenance, and derivation from the Rio de la Plata craton is highly unlikely. Regional considerations, including previously published zircon data, palaeocurrent and structural data, suggest that these rocks must have had an origin within Gondwana-forming blocks, for which the closest identifiable sources are 'Brazilian' and 'African' (Namaqua-Natal). Consequently, the preferred model for the Pampean orogeny is that the Rio de la Plata craton reached its present position by large-scale dextral strike-slip movement against fore-arc sedimentary sequences that had developed on the southern and western margins of the Kalahari craton during the Early Cambrian. In the final stage the displaced sedimentary sequences outboard of the RPC collided with the Mesoproterozoic Western Sierras Pampeanas terrane, which was at the time attached to the large Amazonia craton and other smaller continental blocks, such as Arequipa--Antofalla and Rio Apa. Protracted relative displacement of the RPC after the Pampean Orogeny led to its final position. Keywords: Rio de la Plata craton; Gondwana assembly; Palaeoproterozoic; Neoproterozoic; Cambrian; Brasiliano orogeny; Pampean orogeny; U--Pb zircon dating

The Vindhyan Supergroup of central India, the focus of many paleontological studies, has been reported to contain Cambrian small shelly fossils, Ediacaran fossils, trace fossils, and Proterozoic microfossils and carbonaceous megafossils. New U-Pb zircon and [sup.87]Sr/ [sup.86]Sr isotopic data from the Lower Vindhyan Supergroup require that the rocks are latest Paleoproterozoic to earliest Mesoproterozoic in age. Two rhyolitic volcanic horizons from the Deonar Formation, between the Kajrahat and Rohtasgarh Limestones and below the unit containing trace fossils, yield U-Pb zircon ages of 1631 [+ or -] 5 Ma and 1631 [+ or -] 1 Ma. The Kajrahat and Rohtasgarh Limestones of the Semri Group that are below and above the reported Mesoproterozoic trace fossils have [sup.87]Sr/[sup.86]Sr ratios of 0.70460 and 0.70479, respectively. The Bhander Limestone from the Upper Vindhyan Supergroup has an [sup.87]Sr/ [sup.86]Sr ratio of 0.70599, consistent with a Neoproterozoic age for this formation. These results indicate that the Kajrahat Limestone is of latest Paleoproterozoic age and the Rohtasgarh Limestone is of probable Mesoproterozoic age. These findings are in conflict with the report of Cambrian small shelly fossils and fossils of articulate brachiopods in the Rohtasgarh Limestone and argue for a Mesoproterozoic age for the formation that contains the alleged trace fossils. Reports of an Ediacaran fossil Spriggina(?) from the Lower Vindhyan Supergroup from the northern margin of the Vindhyan Basin suggest either incorrect stratigraphic correlation of units or misidentification of this fossil. Keywords: Vindhyan Supergroup, metazoan evolution, U-Pb zircon dating, Sr isotope stratigraphy.

U-Pb and Pb isotopic measurements of cassiterite from the 200 Ma tin deposits of Belitung, Indonesia, and 2100 Ma Zaaiplaats deposit in South Africa demonstrate that cassiterites can be used to directly date mineral deposits, rather than associated minerals such as zircon and rutile. A precise U-Pb age of 2099 [+ or -]3 Ma has been determined for cassiterites hosted in granites from the Bushveld Complex. On conventional [Pb.sup.207/Pb.sup.204-Pb.sup.206/Pb.sup.204] plots, the data for the Indonesian deposits give an age apparently older than that obtained from U-Pb plots, perhaps indicative of inheritance of Pb from an older source.

The Little Rincon thrust fault is a mylonitic shear zone that juxtaposes Middle Proterozoic Continental Granodiorite over metasedimentary rocks of Proterozoic and early Paleozoic age. This fault is structurally beneath the San Pedro detachment fault and associated ductile deformational fabrics, which formed during early Oligocene to early Miocene time. A syntectonic leucogranite sill within the Little Rincon shear zone yields a U-Pb concordia-intercept age of 66 [+ or -]10 Ma for zircon and a concordant age of 51 [+ or -]2 Ma for fractions composed of monazite and xenotime. This demonstrates that compressional deformation in the Catalina and Rincon mountains is generally coeval with Laramide thrust faults that extend at least from southeastern California to southeastern Arizona. A peraluminous granite pluton that truncates the shear zone but displays extension-related fabrics yields a lower-intercept age of 24 [+ or -]12 Ma for zircon and an age of 30 [+ or -]6 Ma for monazite. This indicates that some peraluminous plutons in the region were emplaced during regional crustal extension.

The techniques developed by scientists at the Jack Satterly Geochronology Laboratory, Royal Ontario Museum, Toronto, reduce the laboratory lead (Pb) background allowing the identification and isolation of low levels of zircon. The technique developed for the first thousand-fold reduction expanded uranium-Pb dating all over the world. The second thousand-fold reduction helped resolve the lead-loss mechanism debate. The recent million-fold reduction allows the establishment of the primary ages of single shocked zircons present in Cretaceous/Tertiary boundary ejecta.