The Lewisian complex of NW Scotland is dominantly composed of Archaean tonalitic
to granodioritic gneisses, ultramafic bodies and minor metasedimentary components.
Although the area is internationally well known and has been much studied for over a
century, the recognition and precise timing of some high-grade metamorphic events
has proven difficult to ascertain. This is partly due to repeated deformational and
metamorphic episodes in the Palaeoproterozoic which overprint and obscure earlier
events. We present data from both laser ablation (LA) ICP-MS and an adaptation of a
U-Pb chemical abrasion ID-TIMS technique applied to multi-age component zircons
from the Assynt (“Central”) block of this region. The new data reveal a previously
unrecognised complexity and provide the first unequivocal proof of an Archean to
Paleoproterozoic granulite metamorphic event in the Assynt area.
LA-ICP-MS U-Pb dating has indicated a ca 2.8 Ga protolith age for a tonalitic gneiss
with evidence for a ca. 3.5 Ga xenocrystic component (the oldest discovered in the
UK). Non-conventional U-Pb ID-TIMS utilising a combination of high-temperature
annealing followed by multi-step incremental dissolution on single grains allows
identification of Pb-loss and multi-generational age trajectories on 206Pb/238U-
207Pb/235U plots. A combination of LA-ICP-MS and this non-conventional TIMS work
has dated zircon growth at ca 2.7 Ga (“Badcallian”) and 2.5 Ga (“Inverian”) with
later Pb-loss occurring at ca 1.9 Ga and ca 1.7 Ga (early and late “Laxfordian”
respectively). This TIMS method is unique in that it combines a pseudo-spatial
resolution normally associated with an in-situ technique but benefits from the highprecision
analysis required to differentiate between these metamorphic events at ca
2.7 and 2.5 Ga.
Zircon Hf isotopes indicate that some gneisses from the Assynt area are typical of
Archaean continental crust (epsilon Hf ca -1). The tonalite gneisses however have
strongly negative epsilon Hf values of -7 to -10 indicating a more complex history of
derivation through partial melting of ancient crust with residual garnet as a long-lived
control on Hf. Moreover, consistent zircon epsilon Hf values from inherited cores,
igneous overgrowths and two separate metamorphic events indicate that the tonalitic
gneisses were formed by crustal recycling, rather than new additions to the crust.
These events may be summarised as: zircon crystallisation from a magma at ca 3.5
Ga, partial melting and crustal recycling producing the tonalite gneiss protoliths at ca
2.8 Ga, a prolonged lower crustal residence in granulite P-T conditions by ca 2.7 Ga,
further metamorphism in amphibolite conditions at ca 2.5 Ga and later deformation
associated with punctuated terrane amalgamation events between ca 1.9 Ga and ca 1.7
Ga. The occurrence of a 2.7 Ga metamorphic event preserved in gneisses from Assynt
contradicts the assertion of some previous studies that it does not exist in this region
and suggests at least some local terrane amalgamation occurred in the Archean.
Abstract: U–Pb ages determined by laser-ablation multi-collector inductively coupled plasma mass spectrometry on 199 zircons from a lithic pumice tuff are the first isotopic ages from the Grangegeeth volcanic terrane of Ireland. The youngest zircon population ( n = 24) yields an age of 465.4 +2.4/−3.1 Ma. Inherited zircons yield ages of c . 1120–1520, c . 1650–1914 and c . 2540–2890 Ma. The absence of zircons between c . 550 and 700 Ma argues against involvement of Avalonia as a source of inherited crystals. Overall, the inherited age spectra are characteristic of Laurentian crust or metasedimentary successions of peri-Laurentian provenance. These findings suggest that the Grangegeeth volcanic terrane does not represent a continuation of the intra-Iapetus Exploits arc of Newfoundland. Supplementary material: Full analytical protocols, applied data corrections, processing and plotting rationales and data table for zircon LA-MC-ICP-MS analysis are available at http://www.geolsoc.org.uk/SUP18399 .
An intrusion of trachy-andesite, representative of a newly discovered suite of high-K–Ba–Sr, calc-alkaline minor intrusions (termed herein the Sperrin Mountains suite), hosted within the Grampian terrane in the north of Ireland, has been dated by U–Pb zircon at 426.69 ± 0.85 Ma (mid-Silurian; Wenlock–Ludlow boundary). Geochemistry reveals a close association with the Fanad, Ardara and Thorr plutons of the Donegal Batholith and the Argyll and Northern Highlands Suite of Scotland. The deep-seated Omagh Lineament appears to have limited eastward propagation of the Sperrin Mountains suite from beneath the main centre of granitic magmatism in Donegal. A Hf depleted mantle model age (TDMHf) of c. 800 Ma for trachy-andesite zircons indicates partial melting from a source previously separated from the mantle. Whole-rock geochemistry of the suite is consistent with a model of partial melting, triggered by slab break-off, following thrusting of Ganderia–Avalonia under the Southern Uplands–Down–Longford accretionary prism (i.e. Laurentian margin). The new age constrains the timing of this event in the north of Ireland and is consistent with the petrogenesis of Late Caledonian high-K granites, appinites and minor intrusions across the Caledonides of northern Britain and Ireland.
In this study the formation of the polygenetic High Tatra granitoid magma is discussed. Felsic and mafic magma mixing and mingling processes occurred in all magma batches composing the pluton and are documented by the typical textural assemblages, which include: mafic microgranular enclaves (MME), mafic clots, felsic clots, quartz-plagioclase-titanite ocelli, biotite-quartz ocelli, poikilitic plagioclase crystals, chemically zoned K-feldspar phenocrysts with inclusion zones and calcic spikes in zoned plagioclase. Geochemical modelling indicates the predominance of the felsic component in subsequent magma batches, however, the mantle origin of the admixed magma input is suggested on the basis of geochemical and Rb-Sr, Sm-Nd and Pb isotopic data. Magma mixing is considered to be a first-order magmatic process, causing the magma diversification. The cumulate formation and the squeezing of remnant melt by filter pressing points to fractional crystallization acting as a second-order magmatic process.
A model based on new structural and geochemical data is presented. It unifies the structural history of the Izera, Rudawy Janowickie and Kaczawa complexes with the Fore Sudetic block, despite their current separation by the Intra-Sudetic and Marginal Sudetic faults. Above the granitoid Izera, Kowary and Wa¸droże gneisses, at the base of the structural sequence, the ductile Kowary shear zone marks the basal decollement of the Świerzawa thrust sheet, comprising often highly strained metasediments associated with enriched tholeiitic and alkaline metabasites. Above it, the ductile Kaczorów shear zone, corresponding to the main mylonitic zone within the Leszczyniec shear zone in the Rudawy Janowickie Complex, marks the base of the Dobromierz thrust sheet, characterized by voluminous MORB-like meta-tholeiites and minor metasediments in the higher parts of the Rudawy Janowickie and Kaczawa complexes and the Pyszczyńska Hill area of the Fore-Sudetic Block. In the east the Sle¸za ophiolite and the Góry Sowie Block override the entire nappe stack. Kinematic fabrics in the major and related shear zones indicate D 1 compressional transport towards the northwest, followed by minor D 2 extensional movements. The thrust stack was deformed during D 3 by southwest verging folds, was subsequently intruded by post-orogenic granites, and later disrupted by the Intra-Sudetic, Marginal Sudetic and associated faults.
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