The Crummock Water aureole, an ENE-trending elongate zone of bleached and recrystallized Skiddaw Group rocks, 24 km in length and up to 3 km wide, is a zone in which pervasive metasomatism has modified the composition of the dominantly siltstone and mudstone lithologies. The bleached rocks show a substantial net gain of As, B, K and Rb and loss of Cl, Ni, S, Zn, H 2 O and C. Carbon loss is responsible for the bleaching. There are smaller and more localized net losses of Cu, Fe, Li and Mn, and gains of Ca, F and Si, whilst Co, Pb and REE are at least locally redistributed. Many chalcophile elements show evidence of initial widespread depletion and subsequent local enrichment. The mineralogy of the rocks is little affected by the geochemical changes. Like their counterparts outside of the bleached zone, the metasomatized rocks consist essentially of quartz, chlorite, muscovite, paragonite and rutile. Small aggregates and porphyroblasts of white mica and chlorite are developed. The metasomatism, which was accompanied by tourmaline veining, is superimposed on a contact metamorphic event. It post-dates the main Caledonian cleavage but pre-dates late Caledonian minor folds. Rb-Sr whole rock isochrons suggest that the metasomatic event occurred at c. 400 Ma and was thus associated with the Lower Devonian Shap-Skiddaw granite magmatism and not the earlier Eskdale Granite or Ennerdale Granophyre magmatic events. Modelling of Bouguer anomalies indicates that geological and geochemical constraints are most simply satisfied if the metasomatism is attributed to a buried, elongate, highly evolved granitic body intruded along the northern margin of a major granitic-granodioritic component of the Lake District batholith. The bleached zone is associated with a major lineament, which may reflect basement control on the location and form of the buried intrusion. Loss of metals from the bleached rocks is related to penecontemporaneous and subsequent hydrothermal vein mineralization and demonstrates that Skiddaw Group sedimentary rocks were a source of ore metals in the Lake District.
Abstract The outcrop of Lower Palaeozoic rocks on the Isle of Man is dominated by thin- to medium-bedded sand-rich turbidites over most of the southeast side of the island (Lamplugh’s Lonan and Agneash Grits, comprising greywacke and quartz arenite, respectively, interbedded with mudstone) and on the northwest coast south of Peel (the Niarbyl Flags). Recent work has shown that the latter, the Niarbyl Formation, is Silurian in age and thus distinct from the otherwise early Ordovician sequences. The composition of the Niarbyl sandstone is also distinct, with comparatively low silica (SiO 2 60–66%) but elevated CaO, MgO and Cr, relative to the Ordovician sandstones. The Ordovician sandstones fall into two compositional groups: a very mature, silica-rich (SiO 2 78–95%) quartz arenite (Agneash type) and a greywacke (Lonan type) with lower silica (SiO 2 65–78%). Most element contents vary with silica but there is a compositional hiatus. It is here inferred that the two sandstone groups represent material from separate source areas. The three tectonostratigraphical sequences distinguished in the southeast of the island all include sandstone of both compositional types in different proportions. These usually occur as units of one or other composition, but in one sequence the two are locally closely interbedded while remaining compositionally distinct. Sandstone in two possibly equivalent units may, however, vary gradationally between the two types, implying more intimate mixing. In the absence of biostratigraphical control, the geochemical data are used to constrain the various ways in which the tectonostratigraphical sequences might correlate. The chemical signature of the Isle of Man sandstones also provides constraints on possible correlatives in adjacent areas. The Lonan type is very similar to mid-Arenig sandstone in the upper part of the Skiddaw Group of the English Lake District. The more siliceous Agneash type has no compositional comparative in the main Skiddaw Group outcrop, although sandstone comprising the enigmatic Redmain Formation is similar in composition. The Wenlock Niarbyl Formation is lithologically and chemically comparable with Wenlock turbidite sequences in the Southern Uplands terrane and in the Windermere Supergroup in the Lake District, and they may all be closely related. None provides a precise match but compositionally the Niarbyl sandstone closely resembles the sandstone of the same age in the Birk Riggs Formation of the Windermere Supergroup. Subject to a number of constraints, the chemical composition of sandstone can also provide information on the probable tectonic environment of the source rocks. On this basis, the Lonan and Niarbyl sandstones include substantial components of first- or second-cycle volcanic debris, whereas the Agneash type is dominated by mature debris reworked from older sedimentary rocks or derived from a granite/gneiss basement source.
Two intrusion complexes, the Bay and the Glen,
probably representing an early phase of the
Devonian magmatic episode, and a number of
dykes, are emplaced within a folded succession of
Silurian sedimentary rocks at Caimgarroch Bay.
Only the roof of the Bay Complex, which consists
of microtonalite and granodiorite, is exposed.
The Glen Complex, of uncertain form, comprises
quartz porphyry, porphyritic quartzmicrodiorite
and quartz-microdiorite. Exposure is good along
the shore line, but much of the area is covered in
thick, dmmlinised drift.
Local high chargeability zones were identified
along three geophysical traverse lines. Soil samples
were collected on a 50 m grid over the area of IP
anomalies. In addition, water, base of slope talus,
and rock samples were chemically analysed
Both the intrusion complexes and some of the
sedimentary rocks show locally intense hydrothermal
alteration. In the Bay Complex narrow
zones of bleached rock are rich in calcite, chlorite
and pyrite and contain minor chalcopyrite and
pyzrhotite. The Glen Complex displays network
fracturing, brecciation and locally intense alteration
to sericite or calcite. There is locally abundant
pyrite in veins and disseminated and rare chalcopyrite.
Arsenopyrite is present in wall rock adjacent
to the Bay Complex.
Rock geochemistry indicates a pervasive but
patchy Cu-Fe-As-MO mineralisation in all rock
types, with copper enrichment greatest in the Bay
Complex where the highest level recorded is over
600 ppm Cu. The mineralisation is accompanied
by irregular barium, potassium and strontium
enrichment. The K/Rb ratios suggest that the
hydrothermal liquors were not entirely late
magmatic. The mineralisation and alteration have
some characteristics of a porphyry system and it
is conceivable that copper enrichment might
increase with depth.
Abstract Granitic rocks from all the exposed components of the Lake District batholith have been analysed for ammonium to look for evidence of an ammonium-rich sedimentaryprotolith. The results indicate that the ammonium content of Lake District granites is related to the type and degree of alteration experienced by the rocks. Fresh, little-altered granites from all parts of the batholith have low ammonium contents (< 30 ppm), whereas highly altered rocks contain up to 250 ppm ammonium. The most ammonium-rich rocks are highly altered specimens of Skiddaw Granite containing appreciable secondary muscovite, sericite and clay. The degree of ammonium enrichment is related to the style of alteration and host-rock lithology. Evidence for the source of ammonium is provided by strong depletion (< × 0.3) of ammonium in silty mudstones within the cordierite andgarnet hornfels zones of the Skiddaw Granite aureole. In view of the low ammonium content of some sediments and the losses known to accompany high-grade metamorphism and associated dehydration, the low primary ammonium contents of the granites are not considered to be evidence either for or against a sedimentary protolith.
Abstract The Latterbarrow Formation, 400 m thick, has been mapped and subdivided informally into three members. These consist of quartz wacke sandstone and, in the upper member only, intercalated mudstones. The formation unconformably overlies the fossiliferous Skiddaw Group of late Tremadoc to middle Arenig age and is overlain disconformably by volcanic rocks that have been attributed to the Borrowdale Volcanic Group. Chemically, the sandstones are characterized by very low concentrations of CaO, Na 2 O, and K 2 O and unusually high total iron. MnO and MgO, such that iron, as Fe 2 O 3 , exceeds Al 2 O 3 . Mudstone in the upper member is illite rich, has a high K 2 O content and is compositionally similar to K‐bentonites derived from volcanic ash. Sedimentary structures and petrography suggest that the sediments were deposited in a sandy estuary and were derived from a similar source area to that of the Skiddaw Group. Throughout the succession there is evidence of distal volcanism, probably representing the earliest eruptions of the Borrowdale volcanic episode. The Redmain Formation, 100 m thick, is unconformably overlain by Carboniferous rocks but its relationship to the underlying Skiddaw Group is unknown. Though this lithic arenite shows some petrographic and geochemical similarities with the Latterbarrow sandstone, differences are such that it is possible they are not equivalent in age. The Redmain sandstone may be derived from the erosion of Skiddaw Group rocks.
Three composite andesitic lava flows, within the Ordovician Borrowdale Volcanic Group, are described. They are 35-250 m thick and in their simplest form consist of a massive, flow-jointed andesite lower part and a flow-laminated to blocky basaltic andesite upper part with a zone of interlayering between them. One flow has a very thin, discontinuous, basal unit of xenolithic, flow-laminated basaltic andesite. The interlayered zone comprises layers and lenses of andesite, basaltic andesite and mixed rocks consisting of closely packed inclusions of pale coloured andesite in a chlorite-rich matrix. The rocks have undergone several stages of alteration: (a) deuteric alteration, including silicification associated with autobrecciation of the upper component and inter-magma reactions in the interlayered zone; (b) soda-metasomatism, preferentially of basaltic andesite; (c) metamorphism, attributed to the Lake District granite batholith, giving an actinolite-sericite assemblage; (d) regional metamorphism to give a chlorite-epidote-actinolite-sphene assemblage with rare prehnite and pumpellyite; (e) calcite veining. These alterations have modified the bulk chemistry of the rocks, but the concentrations of Al, P, Ti, V, Cr, Co, Ni, Ga, Y, Zr, Nb, La, Ce and Th are largely unaffected on the scale of a 2 kg sample. Using these elements the composition of the lavas compares with high-K orogenic andesites emplaced through continental crust. Each of the flows shows distinct chemical characteristics which may be explained by variations of phenocryst content or the assimilation of cognate xenoliths. Variations within flows can be explained by magma mixing and fractional crystallization of plagioclase, minor pyroxene, ilmenite and zircon. A mechanism for eruption is proposed in which a volatile-rich basic magma intruded de-gassed cooler, less dense andesite magma in a high level sub-volcanic chamber. The basic magma formed the lower layer in the chamber. On cooling and crystallization of plagioclase near the interface the volatile content of the residual basic liquid rose and the density decreased allowing the liquids to form an emulsion-like layer at the interface. This separated and rose into the andesite magma and the process was repeated to form an interlayered zone at the junction. Before mixing was complete the magma was erupted as a series of composite flows.
Abstract An unusual nodular form of monazite has been found to account for abnormally high levels of cerium in panned heavy mineral concentrates from stream sediments in several areas in Wales and pan of Exmoor. Textural features indicate a pre‐metamorphic, diagenetic origin within Lower Palaeozoic shale or siltstone host‐rocks but REE patterns show only minor variation, including relative enrichment in europium, from ‘average’ granitic monazite. The similarities between these occurrences, described for the first time from the British Isles, and those previously described from Brittany and elsewhere are discussed.
The Skiddaw Granite and its contact metamorphic aureole in the English Lake District provide an excellent opportunity to test the capability of the N-isotope system to trace devolatilization and large- scale transfer of crustal fluids. In the aureole, Skiddaw Group metasedimentary rocks with relatively uniform lithology and major-element compositions show a dramatic decrease in N content toward the granite contact (from ≥ 800 ppm at distances >2.5 km from the contact, to <410 ppm ≤0.55 km from the contact). Far from the intrusive body (>1.5 km), these rocks have extremely uniform δ 15 N air near +3.7‰, whereas closer to the contact (≤1 km) δ 15 N is shifted to higher values (up to +8.7‰). The coupled decreases in N content and increases in δ 15 N are compatible with the removal of N having low δ 15 N in fluids during continuous, prograde devolatilization reactions involving the breakdown of white mica and the stabilization of biotite-, cordierite-, and andalusite-bearing assemblages. In the same metasedimentary rocks, the lack of obvious trends in major-element concentrations (including SiO 2 /TiO 2 , SiO 2 /Al 2 O 3 , and the ratios of other major oxides to TiO 2 and Al 2 O 3 ) with distance from the granitic contact is consistent with minimal change in major element composition during the contact metamorphism. Ratios of whole-rock N, B, Rb, and Ba concentrations to whole-rock K 2 O content are believed to reflect the differing fluid-mica partitioning (and involving varying relative proportions of white mica and biotite) of these trace elements during devolatilization reactions. Greisenized Skiddaw Granite from a borehole is enriched in N (range of 17-225 ppm for whole rocks and white mica separates) relative to the unaltered granite (whole-rock <30 ppm), and has δ 15 N of+1.0 to +4.8‰. The N concentrations and δ 15 N of the wall-rocks and greisenized granites, combined with C isotopic data (carbonate and carbonaceous matter) for the same rocks, are consistent with the mobilization of fluids having low δ 15 N and δ 13 C PDB values from the devolatilized aureole into the cooling intrusive body. Such transport is consistent with the predictions of recent theoretical models of late-stage hydrothermal evolution in cooling intrusive systems.
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