Devonian Igneous Rocks Research Articles

Evolution of Silurian to Devonian magmatism associated with the Acadian orogenic cycle in eastern and southern Newfoundland Appalachians: Evidence for a three-stage evolution characterized by episodic hinterland- and foreland-directed migration of granitoid magmatism

The migration and character of magmatism over time can provide important insights into the tectonic evolution of an orogen. We present evidence for three separate stages of compositionally distinct granitoid magmatism associated with the Acadian orogenic cycle in the eastern and southern Newfoundland Appalachians. The interpretations are based on new zircon U-Pb ages, geochemical data, and Sr-Nd-Hf-O isotopic data for 18 samples from 15 Silurian and Devonian granitoid plutons, combined with previously published data. The three stages outline hinterland- and foreland-directed migration trends and represent subduction (435−420 Ma), syncollision (415−405 Ma), and postcollision (395−370 Ma) settings in the Acadian orogenic cycle. The Silurian plutons (435−420 Ma) of the first stage consist mainly of quartz diorite, tonalite, granodiorite, monzogranite, and syenogranite, with high-K calc-alkaline and enriched Sr-Nd-Hf-O isotopic compositions (e.g., εNd[t] = −5 to −2; εHf[t] = −3 to −1; δ18O = +6‰ to +8‰). They are interpreted to record the subduction of oceanic lithosphere of the Acadian seaway that separated the leading edge of composite Laurentia, represented by the Gander margin, and Avalonia. Early Devonian plutons (415−405 Ma) of the second stage contain more voluminous monzogranite and syenogranite; they have calc-alkaline to high-K calc-alkaline features, adakite-like compositions, and more depleted Sr-Nd-Hf-O isotopic compositions (e.g., εNd[t] = −6 to 0; εHf[t] = +1 to +3; δ18O = +5‰ to +6‰). Plutons of this stage occur mostly to the northwest of the Silurian granitoids, indicating a regional-scale northwestward (hinterland-directed) migration of magmatism with a rate of >9 km/m.y. The migration is interpreted to have been related to the progressive shallow underthrusting of Avalonia beneath the Gander margin (composite Laurentia) at least as far as 90 km inboard. The Middle to Late Devonian plutons of the third stage (395−370 Ma) consist mainly of monzogranite, syenogranite, and alkali-feldspar granite, which are silica- and alkali-rich granites with large negative Eu anomalies. These rocks are concentrated along both sides of the Dover−Hermitage Bay fault zone, which represents the boundary between Avalonia and composite Laurentia, to the southeast of the Silurian and Early Devonian igneous rocks. This stage of magmatism represents a foreland-directed (retreating) migration. The Early Devonian and Middle to Late Devonian episodes of magmatism were separated by a gap between 405 Ma and 395 Ma and recorded an evolution from (high-K) calc-alkaline to alkaline compositions, ascribed to partial delamination of Avalonian lithospheric mantle in a postcollisional setting.

Long-lived subduction retreating led to continental rifting along the northern Gondwana: Insights from Devonian igneous rocks and ophiolite in the Beishan orogenic collage

The Beishan orogenic collage is an ideal setting for investigating the temporal and spatial relationships between extensional tectonics in the Central Asian Orogenic Belt and the rifting of Gondwana. This study presents new geochemical and geochronological data for Devonian igneous rocks and the Zhangfangshan ophiolitic mélange in the southern Beishan region. The Zhangfangshan ophiolitic mélange contains a Late Devonian gabbro block (ca. 369 Ma) with extremely depleted zircon Hf isotopic compositions (εHf[t] = +16.5 to +22.4). The Lucaogou diorites (ca. 367 Ma) have low MgO and compatible element contents, low Sr/Y and La/Yb ratios, and positive zircon εHf(t) values, indicative of derivation by dehydration melting of Mesoproterozoic mafic igneous rocks in a high-temperature and medium- to low-pressure environment. The Lucaogou K-feldspar granites (ca. 374 Ma) and Dundunshan silicic volcanic rocks (ca. 375 Ma) have A-type granite affinities, and were probably derived from magmas formed by mixing between Neoproterozoic orthogneiss-derived melts and depleted mantle-derived mafic magmas in a high-temperature shallow-crustal environment. Compiled geochronological data for subduction-related magmatism and supra-subduction zone-type ophiolites are consistent with protracted subduction–accretion until the early Carboniferous along the northern margin of the Dunhuang Block. Slab retreat in this long-lived subduction zone led to the generation of oceanic lithosphere in the southern Beishan region. Furthermore, Devonian extensional tectonics in the southern Beishan area coincided with the opening of the Paleo-Tethys Ocean, indicating that protracted subduction retreat caused the separation of the Dunhuang Block from East Gondwana.

Zircon inheritance, sources of Devonian granitic magmas and crustal structure in central Victoria

In central Victoria, inherited zircon in Devonian igneous rocks and detrital zircon in metasedimentary country rocks and an amphibolite-facies xenolith show that Mesoproterozoic parts of the underlying Selwyn Block cannot be the source for all the silicic magmas. Zircon inheritance in S-type samples reveals significant thermal events at 525–425 Ma and 1200–1100 Ma. Both S- and I-type samples have prominent zircon age peaks at 420–410 Ma, which record high-grade metamorphism of the deep crust during the terminal phases of the Benambran and Bindian orogenies. All I-type rocks have 650–500 Ma peaks, suggesting derivation from an arc-related metavolcanic source in the upper Selwyn Block. Protoliths of the greenschist-facies Ordovician metasediments and the amphibolite-facies Cambrian metasedimentary xenolith were deposited in distal backarc settings. Most inherited zircon cores are metamorphic, and the strongest zircon inheritance occurs in hornblende-bearing I-type rocks, highlighting their largely crustal origin. Zircon populations at ca 1400 Ma, thought to signal sediment derivation from East Antarctica and Rodinia-Nuna, are mostly absent in I-type samples and some S-types. The ca 1400 Ma signal probably applies to the upper, metasedimentary Selwyn Block, so Devonian S-type magmas were sourced mainly in the deeper sections. Zircon inheritance in the Devonian igneous rocks was not influenced by the exposed metasedimentary country rocks. Two samples from one of the smaller plutons have contrasting patterns of zircon inheritance, suggesting relatively small-scale source heterogeneity. Many rounded and corroded cores in zircon crystals yield the same ages as the crystallisation dates for the rocks, and thus are antecrysts. Higher whole-rock Zr contents generally correlate with higher proportions of inherited zircon, and differentiation does not affect this relationship. The degree of partial melting of a magma source and the efficiency of crystal entrainment are critical in governing zircon inheritance.

The Middle Paleozoic Magmatism of the Central Tuvinian Trough (Eastern Altai–Sayan Fold Area): Petrogenesis, Tectonics, and Geodynamics

Abstract —The Tuvinian trough is one of the large grabens of the rift system formed in the Devonian–Carboniferous in the eastern part of the Altai–Sayan fold area. Based on the results of comprehensive studies, the age was refined, and the geochemical features of igneous rocks formed during two stages of tectonic and magmatic activity within the Tuvinian trough were studied. In the Early Devonian (397 Ma, Emsian), at the stage of the initiation of the Tuvinian trough in the stretching setting, the volcanic and subvolcanic rocks of the Kendei Formation formed, which make a bimodal series. The Early Devonian igneous rocks of mafic composition have geochemical features of both intraplate (low values of Mg#, high contents of K2O (up to 2.9 wt.%) and TiO2 (up to 2.2 wt.%), and enrichment in LREE relative to HREE) and suprasubductional (enrichment in Pb and Sr and depletion in Ta and Nb) formations and are characterized by high values of εNd(T) (+5.9 to +8.0). They are assumed to have formed from a mixed source including the depleted mantle and components modified by subduction. The Early Devonian felsic volcanic rocks, which are the extreme member of the bimodal sequence, also combine the geochemical features of rocks of intraplate (high Fe, low Sr, P, and Ti contents, Zr and Hf enrichment) and island arc (Ta and Nb depletion) origin. These rocks with εNd(T) values from +4.0 to +7.0 resulted from the melting of a heterogeneous source corresponding in composition to the lower continental crust. In the Middle Devonian–early Carboniferous (390–350 Ma), the Tuvinian rift trough evolved into a mature stage, at which the mafic rocks of the Torgalyk complex were intruded. The Middle Devonian–early Carboniferous mafic rocks are similar in isotope and geochemical characteristics, including the Nd isotopic composition (εNd(T) = +6.7), to the Early Devonian formations. In contrast to the Early Devonian rockes, the magmas for the Middle Devonian–early Carboniferous mafic rocks were generated a relatively homogeneous mantle source without significant metasomatic transformations, the features of which are better manifested in the Kendei rocks.

Amphibole-rich xenoliths from Devonian igneous rocks of the Pripyat rift, Southeastern Belarus: a window into cratonic lower-crust–upper-mantle boundary

Amphibole-rich xenoliths from Devonian igneous rocks of the Pripyat rift, Southeastern Belarus: a window into cratonic lower-crust–upper-mantle boundary

The age and tectonic significance of the Warraweena Volcanics and related rocks, southern Thomson Orogen

Magmatic-textured zircon from medium- to high-K calc-alkaline Warraweena Volcanics (WV) in two drill holes have yielded concordant U–Pb dates of 417 ± 3.5 and 414 ± 4.0 Ma and are interpreted as maximum emplacement ages. The Warraweena volcanics were previously considered to be either Neoproterozoic or Macquarie arc equivalents. Whole-rock εNdt values of these volcanics are +4.5 and +4.8. Along strike of the drill holes, Devonian zircon U–Pb ages (411 ± 5.5 Ma) were obtained from coherent S-type rhyolite flows that have highly negative εNdt values (–7.9 and –7.8). These are a component of the Oxley volcanics. The ages of the Warraweena and Oxley volcanics are identical within uncertainty.The Oxley volcanics (OV) are interbedded with predominantly fine- to medium-grained metasedimentary and so imply a Lower Devonian deposition age for these host rocks. Based on their geophysical characteristics, the metasediments are widely distributed. These metasedimentary rocks yield a wide range of maximum depositional ages, from Early Devonian to earliest Ordovician–latest Cambrian, similar to the Cobar Basin. The absence of complex fabric development typical of Ordovician supracrustal rocks in the region, and conformity with the OV where observable suggest the widespread sedimentation was synchronous with rift-related volcanism in the Early Devonian.Regionally, the WV is temporally, geochemically and isotopically (εNd values) similar to the calc-alkaline Louth Volcanics located over 100 km to the southwest of the WV. Louth Volcanics define a complexly folded belt in geophysical data. Other potentially correlative Early Devonian igneous rocks occur in the nearby Cobar Superbasin and elsewhere in the eastern Lachlan Orogen and are considered to represent the products of a post-orogenic, nascent continental back-arc rift system.

Evolution of the Palaeozoic basement of the North Patagonian Andes in the San Martín de los Andes area (Neuquén, Argentina): petrology, age and correlations

In San Martín de los Andes area (Argentinian Patagonia) the Palaeozoic basement consists of metamorphic and plutonic rocks. The metamorphic rocks include strongly deformed schists, gneisses and migmatites. Their geochemical and petrographic characteristics suggest that the protholith could have been a sequence of pelites and greywackes. Detrital zircon analysis (U-Pb Q-ICP-LA-MS) yielded a maximum depositional age of 501±14 Ma (Series 3 Cambrian) for this sedimentary protolith. Metasedimentary rocks are affected by a regional foliation defined by the minerals of the metamorphic peak. This is a S2 foliation, since relics of a former foliation are present in some samples. This regional foliation is locally affected by open folds that develop an incipient crenulation cleavage (S3). The high-grade metamorphism includes partial melting processes, where the incipient segregates intrude parallel to the regional foliation and also cut it in presence of abundant melt. Zircons from anatectic granites formed during this partial melting process yielded a U-Pb Concordia age of 434.1±4.5 Ma (Llandovery-Wenlock, Silurian). The age of maximum sedimentation and the anatectic age constrain the metamorphic evolution of the basement into the lower Palaeozoic (between upper Cambrian and lower Silurian). The igneous rocks of the basement are granodiorites, tonalities, and some gabbros that cut the metamorphic basement and contain xenoliths and roof pendants from the country rocks. These plutonic rocks are affected by low-grade metamorphism, with the development of discrete, centimetric to hectometric, brittle-ductile shear zones. The age of these igneous rocks has been constrained through U-Pb zircons analysis, carried out by several authors between ca. 370 and 400 Ma (Devonian). The maximum sedimentation age for the protolith and its metamorphic evolution seems to be related to an early Palaeozoic orogenic event, probably the Patagonian Famatinian orogeny. In contrast, the Devonian igneous rocks of San Martín de los Andes could represent a Devonian magmatic arc, related to subduction processes developed at the beginning of the Gondwanan orogenic cycle, which culminates with the Gondwanan orogeny.

Pennsylvanian glacimarine sedimentation in the Cushamen Formation, western North Patagonian Massif

The metasedimentary sequence of the Cushamen Formation in the western North Patagonian Massif is 540 m thick and comprises six sedimentary lithofacies associations related to a glacimarine environment. Four of these lithofacies represent distal glacimarine environments, whereas another one was deposited in proximal glacimarine environments, and the last includes subglacial environments. The organization and configuration of these lithofacies associations represent the advance and retreat of the glacier masses. The maximum glacial advance is correlatable with the G2 glacial interval of the Pennsylvanian Pampa de Tepuel, Las Salinas and Valley Chico, formations of the Extraandean Chubut, and the southern part of Neuquén Cordillera. Contemporaneously, in southern Chile there are marine and glacimarine sediments. The chronostratigraphic relationships between the Silurian to Permian units allow five paleogeographic stages to be distinguished. The middle Silurian–late Devonian igneous rocks represent the first magmatic stage. The second stage, which is transitional to the first, is represented by a marine basin that includes the late Devonian–early Carboniferous Esquel and Río Pescado formations and the Llanquihue Complex. The third stage (early–late Carboniferous) includes granitoids of the second magmatic event that partially overlapped the first magmatic igneous belt. The fourth stage belongs to the late Carboniferous sedimentation of the Cushamen and equivalent formations. The extended early Permian magmatism was the last Paleozoic event in the studied area.

Was the Devonian geomagnetic field dipolar or multipolar? Palaeointensity studies of Devonian igneous rocks from the Minusa Basin (Siberia) and the Kola Peninsula dykes, Russia

Abstract Defining variations in the behaviour of the geomagnetic field through geological time is critical to understanding the dynamics of Earth's core and its response to mantle convection and planetary evolution. Furthermore, the question of whether the axial dipole dominance of the recent palaeomagnetic field persists through the whole of Earth's history is fundamental to determining the reliability of palaeogeographic reconstructions and the efficacy of the magnetosphere in shielding Earth from solar wind radiation. Previous palaeomagnetic directional studies have suggested that the palaeofield had a complex configuration in the Devonian period (419–359 Ma). Here we present new high-quality palaeointensity determinations from rocks aged between 408 and 375 Ma from the Minusa Basin (southern Siberia), and the Kola Peninsula that enable the first reliable investigation of the strength of the field during this enigmatic period. Palaeointensity experiments were performed using the thermal Thellier, microwave Thellier and Wilson methods on 165 specimens from 25 sites. Six out of eight successful sites from the Minusa Basin and all four successful sites from the Kola Peninsula produced extremely low palaeointensities (<10 μT). These findings challenge the uniformitarian view of the palaeomagnetic field: field intensities of nearly an order of magnitude lower than Neogene values (except during relatively rare geomagnetic excursions and reversals) together with the widespread appearance of strange directions found in the Devonian suggest that the Earth's field during this time may have had a dominantly multipolar geometry. A persistent, low intensity multipolar magnetic field and associated diminished magnetosphere would increase the impact of solar particles on the Earth's magnetosphere, ionosphere and atmosphere with potential major implications for Earth's climate and biosphere.

Geochemistry and Sr–Nd–Hf isotopes of Middle Devonian igneous rocks of the Sarsuk polymetallic Au deposit: implications for understanding the tectonic evolution of the south Altay Orogenic Belt, Northwest China

ABSTRACTThe medium-tonnage Sarsuk polymetallic Au deposit is located in the Devonian volcanic–sedimentary Ashele Basin of the south Altay Orogenic Belt (AOB), Northwest China. Within the deposit, the rhyolite porphyries and diabases are widespread, emplaced into strata. The orebodies are hosted by the rhyolite porphyries. We studied the petrography, geochemistry, and Sr–Nd–Hf isotopes of the rhyolite porphyries and diabases, in order to understand the petrogenesis of these rocks and their tectonic significance. They display typical bimodality in geochemistry compositions. The diabases are characterized by SiO2 contents of 44.84–59.77 wt.%, high Mg# values (43–69), enrichment in large ion lithophile elements (LILE) and light rare earth elements (LREE), depletion in Nb and Ta, low (87Sr/86Sr)i (0.706687–0.707613) values, positive εNd(t) (4.8–6.8) values, and positive and high εHf(t) (7.15–15.19) values, suggesting a depleted lithosphere mantle source that might have been metasomatized by subduction-related components. The rhyolite porphyries show affinity to sanukitoid magmas contents [high SiO2 (78.6–81.82 wt.%) and MgO (3.38–5.94 wt.%, one sample at 0.61 wt.%), and enrichments in LILE and LREE], they were derived from the equilibrium reactions between a mantle source and subducted oceanic crust materials. Those characteristics together with the positive εNd(t) (4.1–8.4) and εHf(t) (2.88–15.17) values indicate that the diabases and rhyolite porphyries were generated from the same mantle peridotite source. But the rhyolite porphyries underwent fractional crystallization of Fe–Ti oxides, plagioclase, and apatite due to their negative Eu (δEu = 0.21–0.28) and P anomalies. According to the geochemical and isotopic data, the Sarsuk Middle Devonian igneous rocks are considered to be the products of the juvenile crustal growth in an island arc setting. The Sarsuk polymetallic Au deposit formed slightly later than the Ashele Cu–Zn deposit in the Ashele Basin, but they have the same tectonic setting, belonging to the trench–arc–basin system during extensional process in the south AOB.

Devonian magmatism in the accretionary complex of southern Chile

Supposed or potential Devonian igneous rocks in the accretionary complex of southern Chile were investigated using sensitive high-resolution ion microprobe U–Pb dating of zircon, with Hf- and O-isotope analyses of selected grains. Ages of 384 ± 3 and 382 ± 2 Ma are confirmed for two igneous bodies (another having been previously dated at 397 ± 1 Ma). Detrital zircon ages in the host rocks, some associated with Devonian marine fossils, indicate maximum possible sedimentation ages of c . 330 – 385 Ma. Devonian ages of 391 ± 10 and 374 ± 3 Ma for plutonic rocks at the western edge of the North Patagonian Massif are somewhat older than those of orthogneisses in the western flank of the Andes near Chaitén (361 ± 7 and 364 ± 2 Ma). O and Hf isotopes indicate that the Devonian intrusions in the accretionary complex crystallized from mantle-derived magmas, whereas those in the North Patagonian Massif show a strong crustal influence, corresponding to oceanic and continental margin subduction environments of magma genesis, respectively. Devonian zircon provenance in the accretionary complex was from the North Patagonian Massif and not from the mantle-derived intrusions, suggesting that the accretionary complex formed an integral part of the Gondwana margin during Devonian–Carboniferous times. Supplementary material: Description of analytical methods and tables of isotope analytical data are available at http://doi.org/10.6084/m9.figshare.c.2336728 .

Source characteristics and tectonic setting of the Early and Middle Devonian volcanic rocks in the North Junggar, Northwest China: Insights from Nd–Sr isotopes and geochemistry

Andesite and pyroxene diorite porphyry in the North Junggar, NW China yield zircon U–Pb age of 412.9±1.7Ma and 380.7±3.8Ma, suggesting that they were formed in the Early and Middle Devonian. The Early Devonian lavas consist of basalt, basaltic andesite and andesite, dacitic porphyry and rhyolite. The enrichment of LILE and LREE, remarkably negative Nb, Ta and Ti anomalies, and low (87Sr/86Sr)i ratios and high εNd(t) values in these volcanic rocks, indicate that the Early and Middle Devonian igneous rocks were formed in an intra-oceanic arc, and no continental crust was involved.The wide range of lava types, abundant phenocrysts and overall chemical and isotopic coherence of the Early Devonian lavas clearly suggest that fractional crystallization was the dominant process controlling the evolution of Early Devonian lavas, and is accompanied by the reinjection of parental melt into the magma chamber which can be clearly verified by reverse zoning of plagioclase phenocrysts and their abrupt increase of An content from sieved-textured core to rim. Andesite and basaltic andesite displaying adakitic characteristics have negligible relation to melting of subducted basaltic crust; instead, they are products of basaltic replenishment and fractional crystallization of plagioclase+clinopyroxene+hornblende from mafic magmas. The mantle source of Early Devonian lavas was enriched by much more slab-derived component than that of pyroxene diorite porphyry. With the evolvement of an island arc, involvement of slab-derived components in arc magmatism could decrease; moreover, the high-level crustal contamination may be a more important mechanism than source enrichment.

Geochronological and geochemical constraints for a middle Paleozoic continental arc on the northern margin of the Tarim block: Implications for the Paleozoic tectonic evolution of the South Chinese Tianshan

In order to reveal the tectonic evolution of the South Chinese Tianshan orogenic belt, we conducted structural, geochemical, and geochronological studies and identified granitic and volcanic rocks along the northern margin of the Tarim block. Zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating of two samples from granitic plutons yielded crystallization ages from 404.8 ± 2.0 Ma to 388.1 ± 2.2 Ma, and two samples of dacite and rhyolite yielded ages of volcanism at ca. 404 Ma, highlighting a middle Paleozoic igneous event. Geochemical data suggest that these Devonian igneous rocks are metaluminous, high-K calc-alkaline felsic volcanic and plutonic rocks. All the samples display relatively enriched light rare earth element (LREE) contents, weak or no negative Eu anomalies, and relative depletion in Nb, Ta, Ti, and P, but they show enrichment in K, Rb, and Ba. In situ zircon Hf isotopic results show a positive range of e Hf (t). These geochemical and isotopic features suggest that this magmatic event probably occurred in an Andean-type arc. Combined with zircon U-Pb and Hf isotopic studies on the basement rocks, the results indicate multiphase Precambrian (ca. 2600–2300 Ma, 1900–1800 Ma, 1140–830 Ma) continental growth and minor involvement of basement in Devonian arc magmatism. Finally, we integrated the structural, geochemical, and geochronological data into a geodynamic model of the South Chinese Tianshan that emphasizes south-directed subduction of the South Tianshan Ocean along the northern margin of the Tarim block during the early to middle Paleozoic.

Geochemistry of the Karamea Batholith, New Zealand and comparisons with the Lachlan Fold Belt granites of SE Australia

The Karamea Batholith in the Buller terrane of the South Island New Zealand forms part of an extensive Middle-Late Devonian belt of magmatic activity along, or close to, the Paleo-Pacific margin of Gondwana. The belt includes the I- and S-type granites of the Lachlan Fold Belt in SE Australia and coeval rocks in Antarctica. The northern half of the Karamea Batholith comprises five main intrusive phases: Zetland Diorite, Whale Creek Granite, Karamea Granite, O'Sullivans Granite and Dunphy Granite. To the east of the Karamea Batholith in the Takaka terrane, ultramafic-mafic Devonian igneous rocks are represented by the Riwaka Complex. The rocks forming the Karamea Batholith are a high-K calc-alkaline suite ranging in composition from metaluminous (ASI for Zetland Diorite = 0.8) to strongly peraluminous (ASI for Dunphy Granite = 1.2–1.3). Initial 87 Sr 86 Sr ratios exhibit a large range from 0.705 in the Zetland Diorite to 0.719 in the Dunphy Granite. The corresponding values for ϵ Nd are −0.3 and −9.2. There is a strong inverse correlation between ϵ Nd and initial 87 Sr 86 Sr , which suggests that the Karamea rocks were generated by a simple mixing process. The mafic end-member (with ϵ Nd = 0), which is itself probably derived from a mixed lithospheric source, is taken to be the Zetland Diorite/Riwaka Complex, and the crustal end-member is represented by Ordovician Greenland Group greywackes that form the country rocks to the batholith. Mixing is also supported by recent U-Pb zircon studies. The inherited zircon population in the granites matches the detrital zircon population in the Greenland Group greywackes. The Whale Creek Granite, Karamea Granite and O'Sullivans Granite can be modelled by 20–30% crustal material, whereas the Dunphy Granite appears to represent 65–85% crustal material. In terms of the I-S classification scheme developed for the Lachlan Fold Belt granites in SE Australia, both types are present in the Karamea Batholith. However, in New Zealand there appears to be a continuum from one extreme to the other, which is consistent with the mixing model presented here.

Short Paper: Movement on proposed terrane boundaries in northern Britain: constraints from Ordovician–Devonian igneous rocks

Regional chemical and isotopic differences seen in Ordovician–Devonian igneous rocks of northern Britain are thought to be due to subduction-related and collision magmatism tapping a variety of mantle and crustal sources. Matching of spatial chemical variation across major fault zones restricts post-magmatic transcurrent movement north of the Southern Upland Fault (SUF) to a few tens of kilometres on each fault. A unique mantle isotopic signature present in the same area may restrict post-1.5 Ga movement to less than a few hundred kilometres. No such restrictions exist on displacement across the SUF, to the south of which all magmas show a Pb isotopic component derived from southern sources.

Cliff Profiles of Devon and Cornwall

The coast scenery of North Devon and of North and West Cornwall con? sists mainly of rugged cliffs, ranging in profile from the hog's-back slopes around Lynmouth to the sheer precipices of Morwenstow. The loftiest hog'sback cliffs (Plate i) are those in north-east Devon between Combe Martin and Foreland Point, extending still farther east across the Somerset border. The Great Hangman near Combe Martin has a crest 1044 feet above sealevel about 300 yards inland, whence a straight vegetated slope of gradient approaching 1 in 1 descends to the very base, which alone is now subject to wave attack. The clifTs between Hartland Point and Widemouth afford a complete contrast. An inland plateau sloping gently towards the sea is cut clean by an almost vertical cliff face, 400 feet high near Morwenstow and nearly 300 feet high at Hartland Quay (Plate 2). Where the profile, as at the Lizard and the Land's End, does not conform to either of these two extremes, a vegetated slope curves in a bevel from the level surface above to the vertical cliff face below. The curve may be concave (Plate 3) or convex (Plate 4). The greater part of the area considered in this paper (see map) consists of Devonian and Carboniferous rocks. Along the coast the commonest of these are hard grits, shales, sandstones and slates; there are also small outcrops of Devonian igneous rocks. At the end of the Carboniferous period the region was evidently subjected to severe pressure along Armorican lines, resulting in a series of folds with east and west strike. The north of the area is occupied by a great synclinorium, so that in North Devon the regional dip is to the south while along the coast, roughly from Boscastle to Morwenstow, it is to the north. During the same orogeny the great granite domes of Devon and Cornwall were intruded, which outcrop on the mainland coast at the Land's End.

West Highland Tectonics: Loch Leven to Glen Roy

I. Introduction The present paper is a sequel to my paper on the “Recumbent Folds in the Schists of the Scottish Highlands”, published in this journal in 1910, and expanded with certain corrections and discussions in a Geological Survey Memoir on the “Geology of Ben Nevis and Glen Coe”, 1916. The existence of these two publications allows of concentration on new features of interpretation. The Loch Leven referred to in the title is the salt-water loch of that name, between Argyll and Inverness-shire. The coastal parts of the district are well served by roads, railways, and steamers. There are also inland roads through Glen Etive, Glen Coe, Glen Spean, and Glen Roy. The main villages or small towns are Ballachulish, Kinlochleven, Onich, Fort William, and Spean Bridge. Accommodation is easy to get. The geological facilities include good exposures, high relief, and wonderfully definite lithology. Some of the mountains are carved out of Devonian igneous rocks, as at Ben Nevis and around Glen Coe, but much the greater number consist of the schistose rocks which concern us here. These peaks include a large number rising well above 3000 feet in height, and one, Aonach Beag, surpassing the 4000-foot level. When I described the Ballachulish stratigraphical succession in 1910, and again in 1916,1 was careful not to suggest an original order of superposition . I described recumbent folds, but left it to the future to decide which set of limbs was normal and which inverted. In 1922, however, I attempted a comprehensive treatment

II.—The Igneous Rocks of the Ashprington Area

In South Devon the outcrops of Middle Devonian igneous rocks form a series of roughly parallel bands, which broaden out and occupy a continuous area about twelve square miles in extent in the neighbourhood of Ashprington village. No detailed account of the petrological characters of these rocks has been given, although similar types have been described fully in the Plymouth Survey Memoir. For this reason, and also in the hope that further study might explain more completely the cause of the broadened outcrop, I investigated many exposures in the Ashprington area.