Julian F. Menuge

Petrogenesis of metaluminous A-type rhyolites from the St Francois Mountains, Missouri and the Mesoproterozoic evolution of the southern Laurentian margin

Abstract Predominantly acid volcanic and intrusive rocks of the approximately 1.48 Ga Eastern Granite–Rhyolite Province are exposed only in the St Francois Mountains inlier, Missouri. Combined Nd isotopic, major and trace element analyses are presented for the predominantly acid volcanic rocks of the St Francois Mountains. They are metaluminous to mildly peraluminous, A-type compositions with positive initial e Nd values of 3.1–5.0. They are interpreted to be the result of crystal fractionation of partially melted calc-alkaline igneous rocks formed at a recently active subduction zone. The likelihood of a juvenile, calc-alkaline, subduction-generated protolith for the St Francois Mountains rhyolites leads to a hypothesis in which dacitic parental magmas resulted from partial melting in a continental ‘back-arc’ setting. Basaltic magmatism due to decompression melting of the mantle resulted from lithospheric extension and thinning and provided the heat source for melting in the middle or lower crust. Penecontemporaneous magmatism elsewhere in Laurentia included subduction related calc-alkaline rocks in the Pinware Terrane of Labrador; some felsic igneous rocks of uncertain plate tectonic setting within the Grenville Province also have 1.5–1.6 Ga T DM ages. These are interpreted as magmatic products of approximately 1.5 Ga northward dipping subduction beneath much of the length of the southern margin of Laurentia. The Eastern Granite Rhyolite Province is consequently seen as an integral inboard expression of this subduction related igneous activity. Petrogenetic models involving a mantle plume or the development of a continental rift are not required to explain this granite–rhyolite terrane and evidence against their involvement is discussed.

The petrogenesis of massif anorthosites: a Nd and Sr isotopic investigation of the Proterozoic of Rogaland/Vest-Agder, SW Norway

Sm-Nd and Rb-Sr isotopic analyses of charnockitic migmatite, augen gneiss, anorthosite-leuconorite and two acid plutons from the Rogaland and Vest-Agder districts of southwest Norway constrain their crustal residence ages, origin and evolution. The charnockitic migmatites, which are a major component of the metamorphic basement complex, represent the oldest and largest episode of accretion, in which new crust was derived 1.5–1.9 Ga ago from a mantle source of depleted Nd isotopic composition. The basement complex was intruded by a number of large anorthositic to granitic plutons during and after the Sveconorwegian orogenic period. Samples from the ca. 1050 Ma old, synorogenic Håland anorthosite-leuconorite massif exhibit substantial variation of initial ɛNd of +2.1 to +4.4 at an anorthosite locality and −0.5 to +2.3 at a leuconorite locality, but display significant variation of initial 87Sr/86Sr ratio only between the localities (anorthosite mean=0.70369, leuconorite mean=0.70560). A model is proposed whereby the anorthosite and leuconorite were derived by major crustal contamination of, and fractional crystallization from, a picritic magma derived from isotopically-depleted mantle. Two younger acid intrusions, the 950 Ma old Lyngdal granodiorite and the 930 Ma old Farsund charnockite, both have initial Sr and Nd isotope ratios consistent with massive contamination of depleted-mantle-derived magma by old continental crustal material.

Geochemistry of late Mesoproterozoic volcanism in southwestern Scandinavia: implications for Sveconorwegian/Grenvillian plate tectonic models

Abstract: The last magmatic stage before the Sveconorwegian orogeny in the Baltic Shield is represented by bimodal, c.1.16 Ga volcanics from the Bandak Group in southern Norway and largely coeval basalts from the Dal Group in SW Sweden. In both areas, the volcanic rocks are intercalated with sedimentary units and the onset of basin development is marked by deposition of clastic sediments. In the lower part of the Bandak Group, the Morgedal basalts are relatively evolved and geochemical signatures suggest assimilation of lower crustal components. In contrast, the subsequent Gjuve basalts are more primitive and record assimilation of upper crustal rocks similar to exposed basement rocks. In the Bandak Group, the chemistry of the basalts changes within the stratigraphy, such that the older Morgedal basalts (MgO = 5.2–8.8 wt%, initial ϵNd 2.81–3.96, Zr/Y = 4.0–5.1, Zr/Nb = 34–50, La/Nb = 2.4–3.7) are more fractionated and have more enriched Nd isotope ratios compared to the Gjuve basalts (MgO = 6.4–12.3 wt%, initial ϵNd 4.05–4.97, Zr/Y = 3.4–4.7, Zr/Nb = 20–30, La/Nb = 1.5–2.4). This suggests that there earliest basalts are more contaminated, and it appears that the composition of the crustal component changed with time. A thin felsic volcanic unit, the Dalen Formation, separates the two basalt sequences. It represents melting of upper crustal rocks, triggered by injection of mafic magmas into crustal magma chambers. In the Dal Group, basalts form a relatively thin unit (< 500 m) where flows with high MgO values (> 9%) were least affected by assimilation of crustal components. Whole-rock geochemistry and Nd isotope ratios for the Bandak and Dal basalts indicate shallow melting of sub-continental lithospheric mantle in response to extension in a continental back-arc setting, related to subduction along the western margin of Baltica. These constraints envision a similar tectonic evolution with that of eastern Laurentia, which support models of a pre-Grenvillian supercontinent with a long-lived, active margin that reached western Baltica.

Metamorphic overprinting of SmNd isotopic systems in volcanic rocks: the Telemark supergroup, southern Norway

Abstract The Telemark Supergroup of southern Norway consists of a series of metamorphosed Mesoproterozoic volcanics and sediments. The volcanic pile records three metamorphic events, a pervasive regional greenschist facies event (ca. 1050 Ma, [Starmer, 1993]), a more localised early (pre-regional metamorphism) burial-type event and a late thermal metamorphism related to granite intrusion (ca. 900 Ma [Priem et al., 1973]). In the present study, major and trace element data are employed to illustrate the influence of these metamorphic events on apparent initial Nd isotopic compositions in metamorphosed volcanics. In the case of the basaltic volcanics, burial and thermal metamorphism caused substantial fractionation of the REE and alteration of Nd isotopic compositions by interaction with open system fluids, such that spurious ages might result from age calculations using the Nd isotopic data. During the regional metamorphism, element mobility was limited to the LILE (Rb, Sr, K and Ba); the REE were not fractionated and the original Nd isotopic signatures were preserved. The regional metamorphic effects on the acid volcanics are related to the degree of matrix recrystallisation, since this matrix contains a large fraction of the REE. Where groundmass recrystallisation was limited, the original Nd isotopic signatures were retained, but where the groundmass has been extensively recrystallised the REE have been fractionated resulting in a SmNd isochron age [Menuge, 1985] ca. 300 Ma younger than the UPb zircon age [Dahlgren et al., 1990]. The results of this study emphasise the importance of distinguishing the nature of metamorphic fluid/rock reactions, the potential for REE mobility during low-grade metamorphism and the need for careful selection and geochemical screening of samples for Nd isotopic studies in metamorphosed volcanic terrains.

Mesoproterozoic anorogenic magmatism in southern Norway

Abstract The 1500 Ma Rjukan Group of the Telemark Supracrustal Suite, south Norway, consists of a metamorphosed sequence of acid volcanic rocks (Tuddal Formation) overlain by a comparable thickness of metabasalts (Vemork Formation). Both acid and basic volcanic rocks have within-plate chemical compositions. The acid rocks have initial εNd values of 1.0 to 4.5, similar to the isotopic signature of some components of the Telemark gneiss complex, which is the probable basement to the Telemark Supracrustal Series. The metabasalts have initial εNd values of 3.0 to 4.3, indicating a depleted mantle source region. A mantle plume origin for the Rjukan Group is unlikely given the absence of high temperature igneous rocks such as picrites. Magma genesis by lithospheric extension following orogeny is therefore proposed, with the Tuddal Formation acid magmas derived by anatexis of arc-generated crustal rocks 1500–2000 Ma old, whilst the Vemork Formation basaltic magmas were derived by decompression mantle melting. The Rjukan Group is correlated with the granite-rhyolite province of the US midcontinent, implying large-scale continental rifting. The substantial volume of basic volcanic rocks suggests greater or more prolonged extension in south Norway than in North America.

Silurian turbidite provenance and the closure of Iapetus

Subduction of Iapetus crust during the Caledonian orogeny has been seen by some authors as terminating at the end of Ordovician times and by others as continuing through the Silurian. We present petrological and chemical data which support a coeval arc origin for detritus in the Lettergesh Formation (Silurian), western Ireland. The arc, based on continental crust, lay to the north of the present outcrops of this formation, along the Laurentian margin of Iapetus, implying active oceanic subduction in the Silurian. This subduction may have been followed by some continental underplating around the middle of the Wenlock giving rise to a successor basin in Ireland and Scotland.

Neodymium Isotope Evidence for the Age and Origin of the Proterozoic of Telemark, South Norway

Nd isotope data indicate derivation of 0.9 Ga old post — tectonic granites from acid gneisses. Both granites and gneisses consist of material accreted ∼1.5 Ga ago from a depleted mantle source (∈Nd(T) ∼5.5). A Sm-Nd whole — rock isochron age of 1190 +/− 37 Ma from the Rjukan metavolcanics is believed to date their extrusion; the initial ratio (∈Nd(T) = 2.1 +/− 0.7) suggests depleted — mantle — derived magma contaminated by 1.5 Ga old crust. Basic Bandak volcanics also comprise depleted — mantle — derived magma (∈Nd(T) ∼7) variably contaminated by older,acid rocks. The acid Bandak volcanics were derived essentially from 1.5 Ga old crust. Combined with published results, the data suggest gradual mantle depletion and complementary crustal growth for at least 3.8 Ga.

The Southern Part of the Fongen-Hyllingen Layered Mafic Complex, Norway: Emplacement and Crystallization of Compositionally Stratified Magma

The 160km2 Fongen-Hyllingen complex is a syn-Caledonian, layered mafic intrusion, located south-east of Trondheim, Norway. It crystallized at 5–6kb from a basaltic parent magma under elevated PH2O. In the 1500–4000m thick Hyllingen Series modal layering strikes directly towards the wall where layered diorites interfinger with country rock metabasalts and metapelites. The lower part of the layered sequence consists of dioritic layered rocks with restricted cryptic variation, followed by a gradual, major regression to more primitive compositions. In the upper part the rocks become increasingly evolved, reaching quartzbearing syenite at the roof. There are abundant metabasaltic rafts in the layered sequence. Cryptic and modal layering are discordant: at the at the top of the regressive sequence olivine and plagioclase become increasingly evolved along the strike of modal layering towards the wall, varying over 7km from Fo75:An63 to Fo13:An42. The apparent angle between cryptic and modal layering is usually less than 20°. The start of the major regression, reflecting the influx of fresh magma at the floor of the chamber, is concordant with modal layering which implies that modal layering represents the crystallization front. Crystallization of compositionally zoned magma along an inclined floor is envisaged, with modal layering representing the crystallization front and cryptic layering reflecting the composition of the stratified magma. This is supported by laboratory experiments involving the crystallization of stratified aqueous solutions along inclined surfaces.

A local source for the Ordovician Derryveeny Formation, western Ireland: implications for the Connemara Dalradian

Abstract Facies and palaeocurrent data indicate that the conglomeratic Ordovician Derryveeny Formation is the deposit of an alluvial fan whose apex was located a few kilometres east of the present outcrop. Clasts in the conglomerates include migmatite, schist, gneiss, granite, acid porphyry, spilite and vein quartz. Sillimanite bearing migmatite clasts closely resemble rocks of the neighbouring Lough Kilbride Formation, a unit of the Connemara Dalradian (Lower Proterozoic) basement from which they are presumed to be derived. This local source for the conglomerates is also suggested by the similarity of the Nd-model ages (c. 2.15 Ga) from the metamorphic clasts and from the Lough Kilbride Formation. Moreover peraluminous granite clasts give the same model ages, suggesting the granites are intracrustal melts from the same source. Interpretation of the Connemara Dalradian as a suspect terrane requires that it docked before Upper Ordovician times. Differences between the clast assemblage in the Derryveeny Formation and the presently exposed Lough Kilbride Formation are related to stripping of the original cover, emphasizing the value of clastic detritus in the study of the uplift history of metamorphic basement.

Age and crustal evolution of crystalline basement in western Ireland and Rockall

Great diversity exists in the age, composition and tectonic history of the crystalline basement in Ireland, northwestern Britain and adjacent Atlantic plateaux. Because the crystalline basement provides the fundamental framework for basin development, an understanding of the temporal and spatial variation in basement geology is an important goal of current research efforts. Economic basement in western Ireland and southwestern Scotland chiefly comprises Proterozoic and Lower Palaeozoic supracrustals. However, small areas of Proterozoic orthogneiss have recently been identified, including exposures of the Rhinns Complex on Inishtrahull and Islay (Muir et al. 1994), and the Annagh Gneiss Complex in north Mayo (Fig. 1; Menuge & Daly 1994). Offshore, Proterozoic gneisses have also been recognized on the Rockall Bank (Morton & Taylor 1991). Until the Proterozoic age of these rocks was recognized, it was generally assumed that the crystalline basement was similar to the Archaean Lewisian Complex, which outcrops in northwestern Scotland and in the Outer Hebrides and forms the basement to the West Shetland Basin. Until recently, our understanding of these basement occurrences has, of necessity, often been based on poor quality geochronology, Nd model ages or geological inferences alone. Recent developments in U-Pb geochronology (air abrasion and reduction in laboratory blanks) allow very small sample sizes to be used and can achieve unprecedented accuracy and precision. The reduction in sample size not only allows very small-scale sampling, such as may be necessary with drillcore, but is intrinsically important in deconvoluting complex zircon populations where failure to do so is likely to do so