J. S. Daly

Juvenile Middle Proterozoic crust in the Adirondack Highlands, Grenville province, northeastern North America

Nd isotope data indicate that minimal amounts of significantly older crust have contributed to the genesis of the oldest (ca. 1.3-13.5 Ga) plutons in the Adirondack Highlands. These are magmatic arc tonalites with positive initial {epsilon}{sub Nd} values and Sm-Nd depleted mantle model ages (t{sub DM}) that are within 70 m.y. of the time of their crystallization. Granitoids of the anorthosite-mangerite-charnockite-granite suite, dated at 1,156-1,134 Ma, as well as the 1,100-1,050 Ma plutons, associated with the Ottawan phase of the Grenvillian orogenic cycle, also have positive initial {epsilon}{sub Nd} values and t{sub DM} ages similar to the tonalites. Derivation of both groups of granitoids by crustal melting of the magmatic arc is consistent with the available isotopic and geochemical data. Juvenile late Middle Proterozoic crust that formed during or just prior to the Grenville cycle appears to dominate the southwestern Grenville province as well as the Grenville inliers to the south. In contrast, most of the contiguous Grenville province in Canada comprises largely reworked older crust.

Ion microprobe UPb zircon geochronology and isotopic evidence for a trans-crustal suture in the Lapland–Kola Orogen, northern Fennoscandian Shield

The Lapland–Kola Orogen (LKO; former Kola craton) in the northern Fennoscandian Shield comprises a collage of partially reworked late Archaean terranes with intervening belts of Palaeoproterozoic juvenile crust including the classic Lapland Granulite Terrane. Rifting of Archaean crust began at c 2.5–2.4 Ga as attested by layered mafic and anorthositic intrusions developed throughout the northernmost Fennoscandian Shield at this time. Oceanic separation was centred on the Lapland Granulite, Umba Granulite (UGT) and Tersk terranes within the core zone of the orogen. Importantly, SmNd data show that Palaeoproterozoic metasedimentary and metaigneous rocks within these terranes contain an important, generally dominant, juvenile component over a strike length of at least 600 km. Evidently, adjacent Archaean terranes, with negative eNd signatures, contributed relatively little detritus, suggesting a basin of considerable extent. Subduction of the resulting Lapland–Kola ocean led to arc magmatism dated by the NORDSIM ion probe at c 1.96 Ga in the Tersk Terrane in the southern Kola Peninsula. Accretion of the Tersk arc took place before c 1.91 Ga as shown by ion probe UPb zircon dating of post-D1, pre-D2 pegmatites cutting the Tersk arc rocks, juvenile metasediments as well as Archaean gneisses in the footwall of the orogen. Deep burial during collision under high-pressure granulite-facies conditions was followed by exhumation and cooling between 1.90 and 1.87 Ga based on SmNd, UPb and ArAr data. Lateral variations in deep crustal velocity and Vp/Vs ratio, together with reflections traversing the entire crust observed in reprocessed seismic data from the Polar Profile, may be interpreted to image a trans-crustal structure — possibly a fossilised subduction zone — supporting an arc origin for the protoliths of the Lapland Granulite, UGT and Tersk terranes and the location of a major lithospheric suture — the Lapland–Kola suture.

The tectonothermal evolution and provenance of the Tyrone Central Inlier, Ireland: Grampian imbrication of an outboard Laurentian microcontinent?

The Tyrone Central Inlier is a metamorphic terrane of uncertain affinity situated outboard of the main Dalradian outcrop (south of the Fair Head–Clew Bay Line) and could represent sub-arc basement to part of the enigmatic Midland Valley Terrane. Using a combination of isotopic, structural and petrographic evidence, the tectonothermal evolution of the Tyrone Central Inlier was investigated. Sillimanite-bearing metamorphic assemblages (c. 670 °C, 6.8 kbar) and leucosomes in paragneisses are cut by granite pegmatites, which post-date two deformation fabrics. The leucosomes yield a weighted average 207Pb/206Pb zircon age of 467 ± 12 Ma whereas the main fabric yields a 40Ar–39Ar biotite cooling age of 468 ± 1.4 Ma. The pegmatites yield 457 ± 7 Ma and 458 ± 7 Ma Rb–Sr muscovite–feldspar ages and 40Ar–39Ar step-heating plateaux of 466 ± 1 Ma and 468 ± 1 Ma, respectively. The metasedimentary rocks yield Palaeoproterozoic Sm–Nd model ages and laser ablation inductively coupled plasma mass spectrometry detrital zircon U–Pb analyses from a psammitic gneiss yield age populations at 1.05–1.2, 1.5, 1.8, 2.7 and 3.1 Ga. Combined, these data permit correlation of the Tyrone Central Inlier with either the Argyll or the Southern Highland Group of the Dalradian Supergroup. The inlier was thus part of Laurentia onto which the Tyrone ophiolite was obducted.

A precise U-Pb zircon age for the Inishtrahull syenitic gneiss, County Donegal, Ireland

Smd zircon fractions from the deformed alkaline igneous complex on Inishtrahd, County Donegal, Ireland yield near-concordant U-Pb ages of 1779 * 3 Ma interpreted as dating the igneous crystallization of the gneisss precursor. Sm-Nd depleted mantle model ages range from 1912 Ma to 1978 Ms. The similarity of the model ages and the crystallization age indicates that the protoliths were derived from a depleted mantle source essentially uncontaminated by older crust. The isotopic data confirm previous tentative correlations of the Inishtrahull rocks with those of Islay in southwestern Scotland. Archaean crust is apparently absent from this region, consistent with recent models placing a southern limit on the Archaean along the line of the Great Glen fault zone. Pre-Caledonian orthogneisses in Scotland and Ireland, south of the Great Glen fault zone, have generally been correlated with the late Archaean Lewisian Complex of northwestern Scotland (e.g. Wilkinson 1907; McCallien 1930; Bowes & Hopgood 1975; Dixon et al. 1990) although Aftalion & Max (1987) concluded that the Lewisian was absent from the Annagh Division gneisses of north Mayo, Ireland. There are now several lines of evidence to suggest that Archaean crust may be absent southeast of the Great Glen fault zone (e.g. Fitches et al. 1990; Daly 1990; Halliday et al. 1985). A large area of pre-Caledonian basement, here termed the Rhinns Complex, is thought to be present within the fault-bounded Colonsay-West Islay block (Fig. 1; Bentley et al. 1988; Fitches et al. 1990; Marcantonio et al. 1988)), which extends from Colonsay in

Grampian orogenesis and the development of blueschist-facies metamorphism in western Ireland

Rb–Sr and 40Ar/39Ar step-heating and in situ 40Ar/39Ar laserprobe dating of fabric-forming micas provide new constraints on the timing of Grampian orogenesis and the associated development of blueschist-facies metamorphism at the Laurentian margin in NW Ireland. Early (MP1) blueschist-facies assemblages were developed in metabasites of the Dalradian Supergroup deposited near the edge of the incipient Laurentian margin, contemporaneous with Barrovian metamorphism in the Dalradian closer to the Laurentian foreland. The regional D2 event is associated with the formation of orogen-scale fold nappes and is constrained in the Dalradian by S2 muscovite ages of c. 460 Ma, which are probably recording crystallization. Importantly, the Clew Bay Complex, previously considered as an exotic terrane (correlated with the Highland Border Complex of Scotland) and the Dalradian are in structural continuity. Muscovite from the S2 nappe fabric in the Clew Bay Complex also yields identical c. 460 Ma ages. During D3, dextral shearing tilted the recumbent D2 nappes into a vertical, downward-facing orientation adjacent to the Laurentian margin. D3 is constrained by S3 muscovite ages of c. 448 Ma. Synchronous deformation of the Dalradian Supergroup and the Clew Bay Complex in the mid-Ordovician Grampian orogeny casts doubt on both the validity of Silurian microfossil dates obtained from the Clew Bay Complex and the exotic status of both this ‘terrane’ and the correlative Highland Border Complex of Scotland.

Early Silurian magmatism and the Scandian evolution of the Kalak Nappe Complex, Finnmark, Arctic Norway

Three orogenic events, Porsanger (pre-800 Ma), Finnmarkian (490–540 Ma) and Scandian (400–425 Ma) have been proposed to explain the evolution of the Kalak Nappe Complex, a major component of the Caledonian orogen in Norway. The Kalak Nappe Complex has been considered as Baltoscandian passive margin metasediments telescoped eastwards onto the Baltic Shield. Two granitoid bodies with a weighted mean U–Pb zircon age of 438 ± 2 Ma cut the Hellefjord Schist, regarded as the highest stratigraphic unit in the Kalak Nappe Complex. A volcaniclastic unit within the Hellefjord Schist yields an identical U–Pb zircon age of 438 ± 4 Ma. Magmatism and sedimentation are synchronous within analytical error. The Hellefjord Schist cannot have been affected by Finnmarkian or Porsanger orogenesis and cannot be part of a continuous stratigraphic package. A Laurentian affinity for these sediments is suggested and radical revision of the tectonostratigraphy is required. The Hellefjord Schist is reassigned to the Magerøy Nappe of the Upper–Uppermost Allochthon. As the entire stratigraphy is infolded with the Hellefjord Schist a major deformation phase of Scandian age affected the Kalak Nappe Complex. The Hellefjord Schist represents a Laurentian flysch sequence that was juxtaposed with Baltica during the Scandian Orogeny.

Drainage reorganization during breakup of Pangea revealed by in-situ Pb isotopic analysis of detrital K-feldspar

Pb isotopes in detrital K-feldspar grains provide a powerful provenance tracer for feldspathic sandstones. Common Pb isotopic compositions show broad (hundred-kilometer scale) regional variation, and this signature can survive weathering, transport, and diagenesis. The feldspar Pb signature can be measured rapidly using laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-MC-ICP-MS), and careful targeting can avoid inclusions and altered regions within grains. Here, we combine a new Pb domain map for the circum–North Atlantic with detrital K-feldspar Pb isotopic data from Triassic and Jurassic sandstones from basins on the Irish Atlantic margin. The Pb isotopic compositions reveal otherwise cryptic feldspar populations that constrain the evolving drainage pattern. Triassic sandstones originated from distant Archean and Paleoproterozoic rocks, probably in Green-land, Labrador, and the Rockall Bank to the NW, implying long (>500 km) transport across a nascent rift system. Later, Jurassic sandstones had a composite Paleo- and Mesoproterozoic source in more proximal sources to the north (

Isotopic dating of overthrusting, collapse and related granitoid intrusion in the Grampian orogenic belt, northwestern Ireland

In the Grampian (Ordovician) orogenic belt in northwestern Ireland the Dalradian Supergroup abuts and interleaves with an enigmatic psammite-dominant unit, with relict latest Precambrian granulite-facies assemblages, known as the Slishwood Division. These two units were most likely tectonically juxtaposed during the D 3 phase of Dalradian deformation. Using Ar–Ar, Rb–Sr and Sm–Nd methods, 53 new mineral ages have been obtained from both rock units, and from pegmatites intruding them, in an attempt to constrain the timing of their juxtaposition and the history they subsequently shared. Prior to D 3 ( c. 480 Ma), retrograde hornblende poikiloblasts grew in granulite-facies metabasite pods within the Slishwood Division. Tectonic juxtaposition (D 3 ) with the Dalradian is loosely constrained between 479 Ma and 459 Ma but is likely to have occurred between 470 Ma and 459 Ma. Dalradian peak metamorphic conditions were attained shortly after D 3 , at around 460 Ma. Extensional collapse of the orogen, with rapid uplift and exhumation, is dated by an abundance of mineral cooling ages between 460 Ma and 450 Ma. Orogenic collapse is also dated, more precisely, by widespread pegmatite intrusion into both rock units at about 455 Ma. Initial 87 Sr/ 86 Sr ratios suggest that these pegmatites are partial melts from Dalradian metasediments. A second suite of pegmatites were intruded, along with the Ox Mountains Granodiorite, much later at or around 400 Ma during sinistral shearing.

Hidden Archaean and Palaeoproterozoic crust in NW Ireland? : evidence from zircon Hf isotopic data from granitoid intrusions

The presence of major crystalline basement provinces at depth in NW Ireland is inferred from in situ Hf isotope analysis of zircons from granitoid rocks that cut structurally overlying metasedimentary rocks. Granitoids in two of these units, the Slishwood Division and the Tyrone Central Inlier, contain complex zircons with core and rim structures. In both cases, cores have average ϵHf values that differ from the average ϵHf values of the rims at 470 Ma (the time of granitoid intrusion). The Hf data and similarity in U–Pb age between the inherited cores and detrital zircons from the host metasedimentary rocks suggests local contamination during intrusion rather than transport of the grains from the source region at depth. Rims from the Slishwood Division intrusions have average ϵHf 470 values of −7.7, consistent with a derivation from juvenile Palaeoproterozoic crust, such as the Annagh Gneiss Complex or Rhinns Complex of NW Ireland, implying that the deep crust underlying the Slishwood Division is made of similar material. Rims from the Tyrone Central Inlier have extremely negative ϵHf 470 values of approximately −39. This isotopic signature requires an Archaean source, suggesting rocks similar to the Lewisian Complex of Scotland, or sediment derived wholly from it, occurs at depth in NW Ireland.

RbSr isotopic equilibrium during Sveconorwegian (= Grenville) deformation and metamorphism of the Orust dykes, S.W. Sweden

Abstract The Orust dykes and their regional correlatives are an important time marker in the polymetamorphic Ostfold-Marstrand belt of S.E. Norway and S.W. Sweden. They were deformed and metamorphosed in the amphibolite facies during the Sveconorwegian (= Grenville) orogeny. Three narrow strongly deformed dykes yield RbSr whole-rock ages (1106 ± 52 m.y., 1062 ± 90 m.y., 1000 ± 153 m.y.). The weighted mean age (1087 m.y.) is interpreted as the age of the first post-dyke deformation and the accompanying amphibolite facies metamorphism. High initial 87Sr/86Sr ratios (0.7074 ± 9, 0.710 ± 2, 0.713 ± 3, respectively) and other geochemical evidence point to considerable exchange of mobile elements between the dykes and the country rocks. The emplacement age is not known precisely. However, the dykes may be correlated with widespread mafic magnatism which accompanied crustal instability in the N. Atlantic region early in the Sveconorwegian (= Grenville) orogeny about 1200 m.y. ago.