A revision of the Ordovician stratigraphy for the western Ireland Caledonides and implications for tectonic models

The western Ireland Ordovician stratigraphy has been previously used to constrain the timing of docking of an island arc and its fore‐arc basin with the margin of Laurentia for the British and Irish Caledonides. New field relationships and age data indicate that one of the key formations, the Rosroe Formation (459.2 ± 0.8 and 465.1 ± 2.1 Ma), and its supposed lateral equivalent, the Maumtrasna Formation are younger than previously interpreted. New age data for a tuff band in the Maumtrasna Formation (468.9 ± 1.3 Ma) also support previous studies showing it can be correlated to the adjacent Mweelrea Formation. The new field evidence, age data and geochemistry contradict some previous studies and show that the Maumtrasna, Rosroe and Derrylea formations can no longer be considered lateral equivalents. Based on the new stratigraphy a revised tectonic model is required with sedimentation in this part of the Caledonides taking place in a fore‐arc basin outboard of a continental arc and the oceanic arc was an along‐strike equivalent of this arc situated in an embayment of the Laurentian margin. Copyright © 2010 John Wiley & Sons, Ltd.     

Extensional deformation along the late Precambrian-Cambrian Baltoscandian passive margin: the Sarektjåkkå Nappe,Swedish Caledonides

The structural evolution of a part of the late Precambrian Baltoscandian passive margin just before the inception of seafloor spreading is described, recording the change from deformation by faulting to dominantly magmatic extension of the crust. The allochthon of the Scandinavian Caledonides contains the imbricated passive margin of continental Baltica overlain by various exotic terranes. The Sarektjåkkå Nappe in the Seve Nappe Complex, which contains the outer parts of Baltica's passive margin, consists of sedimentary rocks, occurring as screens between Vendian (573±74 Ma) diabase dykes. These dykes constitute 70–80% of the nappe and locally form sheeted dyke complexes. The Sarektjåkkå Nappe largely escaped penetrative Caledonian deformation and preserves igneous, metamorphic and structural elements that are linked to the evolution of a pre-Caledonian rift to a passive continental margin. Extensional deformation before dyke emplacement is recorded by normal faults, pull-apart structures and folds. Unconformities, dykes affected by brittle deformation, and fluidization of sediments during dyke emplacement indicate close relations between the deposition of sediments, extensional deformation and dyke emplacement. The Sarektjåkkå Nappe is compared with other parts of the Baltica's passive margin and its tectonic evolution is discussed.     

Contrasting textures in metamorphic and anatectic migmatites: an example from the Scottish Caledonides

Abstract. A method for the quantitative analysis of the spatial relations of minerals is described. Dispersed distributions are formed by annealing and destroyed in post-tectonic migmatization. Aggregate distributions characterize solid-state differentiation, whereas leucosomes formed in systems of high fluid:rock ratio (in the examples studied, anatectic melts) show random distributions.
Quantitative textural analysis can be used to indicate whether migmatization was post-tectonic or earlier, though caution is necessary if post-migmatite cooling is slow or if there is some minor deformation. More importantly, it can be used to discriminate melt-present from melt-absent leucosomes; this is exemplified by a suite of metamorphic and anatectic migmatites from the Scottish Caledonides.
The textural evolution of anatexites with increasing melt percentage is traced. Initial feldspar porphyroblastesis occurs by Ostwald ripening via grain boundary melts; subsequently ophthalmites develop with fabrics and chemistry inherited from the palaeosome. At greater than 30% melt these inherited fabrics are wholly destroyed. Deformation prompts segregation into melanosome and leucosome; resultant leucosomes contain no inherited crystals. The scale of anatectic systems is fixed at the point at which segregation begins; ophthalmites provide evidence for melt and crystal transfer beyond original palaeosome boundaries. In contrast, metamorphic migmatites are necessarily small-scale systems because of diffusive constraints, and melanosomes are invariably produced.     

Multi-textured garnets from a single growth event: an example from northern Norway

Abstract Textural relationships between porphyroblasts of biotite and garnet in metasediments in the Nordkinn Peninsula area of the Finnmarkian Caledonides of North Norway are apparently complex. There is evidence for two textural zones in both mineral phases and superficially the development of these appears to have overlapped, at least in part, in time and space. This apparently complex porphyroblast growth history can be considerably simplified if only one period of garnet growth occurred and if different inclusion fabrics developed where garnet replaced biotite porphyroblasts and where it overgrew the matrix foliation. The possibility that porphyroblasts with textural evidence for multiphase growth histories actually grew during a single crystallization event is of importance in the interpretation and elucidation of tectonometamorphic relationships.     

Incipient metamorphism in the Lower Palaeozoic marginal basin of Wales

Abstract A diagenctic through anchizone to epizone transition is demonstrated in pelitic rocks of the Lower Palaeozoic marginal basin of Wales by examination of variations in phyllo-silicate mineralogy, illite crystallinity and b_o parameter of white micas. This transition represents a temperature range from ∼ 150°C to ∼ 400°C and the metamorphism is of a low-pressure facies series type, with a geothermal gradient of ∼ 40°Ckm^-1. Variations in grade can be correlated largely with the original basin and shelf form, suggesting a depth-related metamorphism. However, in areas closer to the site of Caledonian plate collision an increasingly syn-tectonic metamorphic event is apparent.
Correlation of pelite data with metabasite assemblages is variable, the most consistent relationship being between epizone crystallinity values andepidote-actinolite (greenschist facies) assemblages. Diagenetic clay mineral assemblages are found associated with prehnite-pumpellyite assemblages in metabasites and it is suggested that the latter represent non-buffered, and therefore non-diagnostic, assemblages.     

An Archean(?) to Paleozoic Evolution for a Garnet Peridotite Lens with Sub-Baltic Shield Affinity within the Seve Nappe Complex of Jamtland, Sweden, Central Scandinavian Caledonides

Mineralogical, isotopic, geochemical and geochronological evidencedemonstrates that the Friningen body, a garnet peridotite bodycontaining garnet pyroxenite layers in the Seve Nappe Complex(SNC) of Northern Jämtland, Sweden, represents old, certainlyProterozoic and possibly Archean, lithosphere that became incorporatedinto the Caledonian tectonic edifice during crustal subductioninto the mantle at c. 450 Ma. Both garnet peridotite and pyroxenitecontain two (M₁ and M₂) generations of garnet-bearing assemblagesseparated by the formation of two-pyroxene, spinel symplectitearound the M₁ garnet and the crystallization of low-Cr spinel_1Cin the matrix. These textures suggest initial high-pressure(HP) crystallization of garnet peridotite and pyroxenite succeededby decompression into the spinel stability field, followed byrecompression into the garnet peridotite facies. Some pyroxenitelayers appear to be characterized solely by M₂ assemblages withstretched garnet as large as several centimeters. Laser ablationmicroprobe–inductively coupled plasma mass spectrometryRe–Os analyses of single sulfide grains generally definemeaningless model ages suggesting more than one episode of Reand/or Os addition and/or loss to the body. Pentlandite grainsfrom a single polished slab of one garnet peridotite, however,define a linear array on an Re–Os isochron diagram that,if interpreted as an errorchron, suggests an Archean melt extractionevent that left behind the depleted dunite and harzburgite bodiesthat characterize the SNC. Refertilization of this mantle bymelts associated with the development of the pyroxenite layersis indicated by enriched clinopyroxene Sr–Nd isotope ratios,and by parallel large ion lithophile-enriched trace elementpatterns in clinopyroxene from pyroxenite and the immediatelyadjacent peridotite. Clinopyroxene and whole-rock model Sm–Ndages (T_DM = 1·1–2·2 Ga) indicate that fertilizationtook place in Proterozoic times. Sm–Nd garnet₂–clinopyroxene₂–wholerock ± orthopyroxene₂ mineral isochrons from three pyroxenitelayers define overlapping ages of 452·1 ± 7·5and 448 ± 13 Ma and 451 ± 43 Ma (2     

The geometry and kinematics of flow perturbation folds

Minor folds formed synchronous with ductile deformation in high strain zones can preserve a record of the scale and kinematics of heterogeneous flow. Using structures associated with WNW-directed Caledonian thrusting in N Scotland, we show that localised perturbations in flow resulted in the generation of predominantly cylindrical minor folds with hinges lying at low angles to the transport direction. These define a series of larger-scale fold culminations (reflecting ‘surging flow’) or depressions (reflecting ‘slackening flow’) that are bisected by transport-parallel culmination and depression surfaces. The fold patterns suggest a dominance of layer-normal differential shearing due to gradients in shear strain normal to transport. Culmination surfaces are marked by along-strike reversals in the polarity of structural facing and vergence of minor folds which, contrary to classic fold patterns, define reverse asymmetric relationships. Culmination surfaces separate folding in to clockwise (Z folds) and anticlockwise (S folds) domains relative to the transport lineation. The dip of fold axial planes systematically increases as their strike becomes sub-parallel to transport resulting in a 3D statistical fanning arrangement centred about the transport direction. Thus, mean S- and Z-fold axial planes intersect precisely parallel to the transport lineation and potentially provide a means of determining transport directions in cases where lineations are poorly preserved. Culminations display convergent fold patterns with fold hinges becoming sub-parallel to transport towards the culmination surface and underlying detachment, whilst axial planes define overall concave up listric geometries which are bisected by the culmination surface. Thus, around culminations and depressions there are ordered, scale-independent relationships between transport direction, shear sense, fold facing, vergence and hinge/axial plane orientations. The techniques described here can be applied and used predictively within any kinematically coherent system of ductile flow.     

Magmatic evolution of the Løkken SSZ Ophiolite,Norwegian Caledonides: Relationships between anomalous lavas and high-level intrusions

The Lower Ordovician Løkken ophiolite fragment shows a tripartite subdivision of its thick volcanic sequence into: (1) an Upper Volcanic Member (UVM) of transitional MORB/IAT character, comprising basalts or, in a related, thrust-bounded sequence, a bimodal basalt-rhyolite assemblage; (2) a Middle Volcanic Member (MVM) derived from widely variable, generally MORB-related magmas; and (3) a Lower Volcanic Member (LVM) of N-type MORB basalts which locally passes into an underlying sheeted dyke complex. The UVM and LVM were fed from relatively deep-seated magma chambers, and lavas were erupted at moderate flow rates to form predominantly pillow lavas. The whole sequence is interpreted as having formed in a marginal basin setting. The MVM is characterized by abundant voluminous sheet basalts thought to have been erupted at high flow rates. The volcanites had their source, through extensive dyke swarms, in high-level magmas which intruded the sheeted dyke complex and parts of the LVM, possibly in a near-axial seamount setting. The magmas differentiated to form plagiogranitic melts, with a wide range of conjunctive, mafic to intermediate cumulates. This plutonic assemblage, possibly in crystal mush form, was subsequently intruded by new, mafic magma, leading partly to large-scale remelting or assimilation of cumulus phases. These processes are reflected in the MVM by common hybrid flows, as well as local intermediate lavas having the geochemical characteristics of cumulates, erupted during a period of intense faulting. Major tapping of the primary, mafic magma occurred toward the end of the period of MVM volcanism. A predominance of voluminous, apparently fluid, massive flows in the MVM is mainly ascribed to the shallow depth of magma reservoirs.