R.K. Shail

Geochemistry and provenance of Rhenohercynian synorogenic sandstones: implications for tectonic environment discrimination

Abstract The provenance of synorogenic greywackes from Devonian flysch successions occupying structurally similar positions at opposite ends of the Rhenohercynian zone in SW England and Germany is evaluated. Greywackes from both regions are petrographically and chemically similar, although minor differences are seen in the relative proportions of lithics; the Gramscatho group being richer in volcanic and metavolcanic clasts, whereas the Giessen group is richer in metasedimentary clasts. Absolute abundances of Ni-Cr-V and Zr-Hf-Y vary to a limited extent in each group and reflect variable mafic detritus and heavy mineral inputs respectively. Framework mode parameters and chemical data indicate the Rhenohercynian greywackes were derived mainly from a calc-alkali, acidic, dissected continental arc source, with minor MORB-like and argillaceous metasedimentary components. Upper continental crust-normalized multi-element patterns for the greywackes are characteristic of the continental arc/active margin tectonic environment. However, ‘Mid-Proterozoic’ model Nd ages for the Gramscatho greywackes suggest that an active Devonian arc source is unlikely. Instead the range of chemical and isotopic composition displayed mainly reflects mixing between acidic arc terranes of Proterozoic age and Devonian (Lizard-type) oceanic crust. Petrographic and geochemical discrimination diagrams alone cannot resolve the temporal decoupling between source and basin and may lead to an erroneous interpretation of tectonic setting.

Late Carboniferous to Triassic reactivation of Variscan basement in the western English Channel: evidence from onshore exposures in south Cornwall

South Cornwall represents the westernmost part of the Variscan basement massif in SW England. Field-based investigations into the distribution, geometry, kinematics and relative chronology of post-Variscan deformation have been accompanied by preliminary palaeostress analyses. These indicate three episodes of basement reactivation: (1) Stephanian to Early Permian NNW–SSE extension, (2) Early to Late Permian strike-slip deformation, and (3) Late Permian to Triassic ENE–WSW extension. These data have been used to test and refine previously published models, based upon seismic and well data, of late Carboniferous to Triassic basement reactivation and sedimentary basin development within the Western Approaches Trough. We confirm that NNW–SSE regional extension and reactivation of ENE-striking Variscan thrusts had initiated by the latest Carboniferous, and that ENE–WSW regional extension and dip-slip reactivation of NNW-striking high-angle faults occurred during the Triassic. Permian strike-slip deformation recognized onshore may have contributed to the development of widespread unconformities within the oVshore basins.

The role of post-Variscan extensional tectonics and mantle melting in the generation of the Lower Permian granites and the giant W-As-Sn-Cu-Zn-Pb orefield of SW England

Introduction: The Cornubian batholith of SW England is the largest in the British Isles and one of the most intensively investigated in the world. Production from the associated giant W-As-Sn-Cu-ZnPb orefield ceased with the closure of South Crofty Mine in 1998. Total historical output is estimated at ~1.75 Mt Sn, 1 Mt Cu and 0.13 Mt Zn, and the province played a major role in the development of early magmatic-hydrothermal mineralization models and the science of mining geology. Many of the petrological, geochemical and geochronological characteristics of the batholith, and the distribution, parageneses and broad fluid characteristics of the associated hydrothermal mineralisation have been elucidated. However, published models do not satisfactorily explain: (1) the generation and emplacement of a very substantial volume of granite within a relatively external part of the European Variscides, and (2) the complementary factors that favoured the development of a giant orefield. We present here the integrated results of tectonic/structural geological and helium/sulphur isotope studies. These data indicate that post-collisional lithospheric extension and mantle partial melting exerted a major control upon magmatism and associated mineralization. These processes are considered in the wider context of Permo-Carboniferous magmatism in Europe, and generic controls upon Sn-Cu mineralization.

40Ar/39Ar phlogopite geochronology of lamprophyre dykes in Cornwall, UK: new age constraints on Early Permian post-collisional magmatism in the Rhenohercynian Zone, SW England

The spatial and temporal association of post-collisional granites and lamprophyre dykes is a common but enigmatic relationship in many orogenic belts, including the Variscan orogenic belt of SW England. The geology of SW England has long been interpreted to reflect orogenic processes associated with the closure of the Rheic Ocean and the formation of Pangaea. The SW England peninsula is composed largely of Early Devonian to Carboniferous volcano-sedimentary successions deposited in synrift and subsequent syncollisional basins that underwent deformation and low-grade regional metamorphism during the Variscan orogeny. Voluminous Early Permian granitic magmatism (Cornubian Batholith) is considered to be broadly coeval with the emplacement of lamprophyric dykes and lamprophyric and basaltic lava flows, largely on the basis of geochronological data from lamprophyric lavas in Devon. Although published geochronological data for Cornish lamprophyre dykes are consistent with this interpretation, these data are limited largely to imprecise K–Ar whole-rock and biotite analyses, hindering the understanding of the processes responsible for their genesis and their relationship to granitic magmatism and regional Variscan tectonics. 40Ar/39Ar geochronological data for four previously undated lamprophyre dykes from Cornwall, combined with published data, suggest that lamprophyre magmatism occurred between c. 293.6 and c. 285.4 Ma, supporting previous inferences that their emplacement was coeval with the Cornubian Batholith. These data provide insights into (1) the relative timing between the lamprophyres and basalts, the Cornubian batholith and post-collisional magmatism elsewhere in the European Variscides, and (2) the post-collisional processes responsible for the generation and emplacement of lamprophyres, basalts and granitoids. Supplementary data: Complete datasets, photomicrographs and photographs of sample locations are available online at http://www.geolsoc.org.uk/SUP18838.

Shallow laccolithic emplacement of the Land's End and Tregonning granites, Cornwall, UK: Evidence from aureole field relations and P-T modeling of cordierite-anthophyllite hornfels

The Land’s End and Tregonning-Godolphin granites of the >250 km-long Permian Cornubian Batholith are heterogeneous medium- to coarse-grained peraluminous biotite-, tourmaline-, and lithium-mica granites traditionally thought to be emplaced as massive magmatic diapirs. Although S-type characteristics are dominant (quartz + biotite + muscovite + tourmaline ± topaz ± lithium-micas in the melt, numerous greisen and pegmatite veins, Sn-W mineralization), some characteristics of evolved I-type granites are also exhibited (hornblende-bearing enclaves, elevated ɛNd, Cu mineralization, batholithic dimensions). Here, we present an investigation focusing on the contact metamorphism and deformation of the aureole rocks adjacent to the Land’s End and Tregonning granites as an approach to better determine the method of granite emplacement and the depth at which it occurred. New 1:5000-scale geological maps are presented for ∼15 km of coastal exposure of the granites and their aureoles. We propose that the granites were emplaced non-diapirically by intrusion of sills that amalgamated to form a sheeted laccolith-type body. Granite contacts cleanly truncate all faults, folds, and cleavages generated during both Variscan convergence and subsequent latest Carboniferous–Early Permian (end-Variscan) extension, and it is likely that granite was emplaced during continuation of this extensional episode. There is evidence for stoping of the country rocks by an outward-migrated sill and dyke network, and uplift and doming of the host rocks can be partially attributed to laccolith inflation. Host meta-siltstones of the Devonian Mylor Slate Formation formed a contact aureole of cordierite + biotite + chlorite ± andalusite “spotted slates.” Several interspersed pillow basalts and dolerites, previously affected by hydrothermal alteration, underwent isochemical contact metamorphism to form cordierite- and orthoamphibole-bearing hornfels, including cordierite-anthophyllite rocks that are present in Kenidjack cliff, NW Land’s End aureole. THERMOCALC P-T modeling and pseudosection construction for these rocks in the large Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3 (NCKFMASHTO) chemical system indicates contact metamorphism occurred at 1.5 ± 1.0 kbar and 615 ± 50 °C. This ultra-low pressure metamorphism equates to a likely emplacement depth of 5–6 km for the adjacent granite sheets. The Cornubian Batholith is highly composite and likely comprises an amalgamation of discrete shallow-seated sheeted laccoliths that are dyke-fed from a common lower-crustal/upper-mantle melt region to result in the batholith’s mixed S-type/I-type character.

Composition of zircons from the Cornubian Batholith of SW England and comparison with zircons from other European Variscan rare-metal granites

Abstract Zircon from 14 representative granite samples of the late-Variscan Comubian Batholith in SW England was analysed for W, P, As, Nb, Ta, Si, Ti, Zr, Hf, Th, U, Y, La, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb, Al, Sc, Bi, Mn, Fe, Ca, Pb, Cu, S and F using electron probe microanalyses. Zircons from the biotite and tourmaline granites are poor in minor and trace elements, usually containing 1.0-1.5 wt.% HfO2, <0.5 wt.% UO2 and P2O5, <0.25 wt.% Y2O3, <0.2 wt.% Sc2O3 and Bi2O3 and <0.1 wt.% ThO2. Zircon from topaz granites from the St. Austell Pluton, Meldon Aplite and Megiliggar Rocks are slightly enriched in Hf (up to 4 wt.% HfO2), U (1-3.5 wt.% UO2) and Sc (0.5-1 wt.% Sc2O3). Scarce metamictized zircon grains are somewhat enriched in Al, Ca, Fe and Mn. The decrease of the zircon Zr/Hf ratio, a reliable magma fractionation index, from 110 - 60 in the biotite granites to 30-10 in the most evolved topaz granites (Meldon Aplite and Megiliggar Rocks), supports a comagmatic origin of the biotite and topaz granites via long-lasting fractionation of common peraluminous crustal magma. In comparison with other European rare-metal provinces, the overall contents of trace elements in Cornubian zircons are low and the Zr/Hf and U/Th ratios show lower degrees of fractionation of the parental melt.

Age and petrogenesis of the Lundy granite: Paleocene intraplate peraluminous magmatism in the Bristol Channel, UK

The Lundy granite forms part of the Lundy Igneous Complex, which is the southernmost substantive expression of magmatism within both the British Cenozoic Igneous Province and the wider North Atlantic Igneous Province. Its Qz + Pl + Kfs + Bt ± Grt ± Tpz mineralogy and peraluminous character contrast with other British Cenozoic Igneous Province granites farther north but are similar to the granites of the adjacent Early Permian Cornubian Batholith. We present the results of mapping, petrographical and mineral chemical analysis, and the first U–Pb zircon ages for the granite (59.8 ± 0.4 – 58.4 ± 0.4 Ma) and cross-cutting basic dykes (57.2 ± 0.5 Ma), which confirm a Paleocene age for magmatism. Zircon inheritance is limited but two cores imply the presence of Early Palaeozoic igneous rocks in the unexposed basement of SW England. The anomalous southerly location of the Lundy Igneous Complex is a consequence of mantle melting arising from the superimposition of localized lithospheric extension, related to intraplate strike-slip tectonics, with the distal ancestral Icelandic plume. Granite generation primarily reflects crustal partial melting during the emplacement of mantle-derived melts. The change in geochemical character between the Lundy granite (peraluminous) and other British Cenozoic Igneous Province granites (metaluminous or subalkaline) indicates a fundamental crustal source control between contrasting peri-Gondwanan and Laurentian basement provinces. Supplementary material: Sample locations, petrographic summaries, mineral chemistry data and secondary ionization mass spectrometry U–Pb analytical data are available at https://doi.org/10.6084/m9.figshare.c.3886783

Understanding the multi-episode formation of the world-class Hemerdon W–Sn deposit in the context of SW England granite evolution

The world-class granite-related, sheeted-vein, Hemerdon W–Sn deposit (Devon, SW England) has been intermittently mined since the seventeenth Century. In a major step forward for UK metal mining, or...

Using radioelement distributions to classify a composite granite batholith in the South West England Orefield

The South West England Orefield is well-known for its polymetallic magmatichydrothermal mineralisation associated with the composite Cornubian granite batholith. The mineralisation contains elevated concentrations of a range of metals (e.g. W, Sb, Bi, As, Be, Cd, Ga, Ge, In, Li, Nb, Ta) used in high-technology and clean energy applications and for which security of supply concerns exist [1,2,3]. Exploration for these elements requires improved understanding of the spatial distribution of granite types and their relationship to the different mineralisation styles and parageneses. Previous granite classifications defined granite types based on mineralogical and/or textural observations from field sampling [e.g. 4]. However, these divisions over simplify mineralogical variation (e.g. micas), and the sample density is inadequate to reflect the heterogeneity of the batholith. Classifications based on whole-rock geochemistry, provide a more objective classification method [e.g. 5] and can guide mineral exploration, particularly in poorly exposed areas.