Field and biostratigraphic evidence for an unconformity at the base of the Eycott Volcanic Group in the English Lake District

1992 ◽  
Vol 129 (1) ◽  
pp. 77-92 ◽  
Author(s):  
David Millward ◽  
Stewart G. Molyneux
Keyword(s):  
Volcanic Rocks ◽  
Tectonic Deformation ◽  
Lake District ◽  
Critical Data ◽  
Volcanic Group ◽  
Late Cambrian ◽  
Water Formation ◽  
English Lake District ◽  
Volcaniclastic Rocks ◽  
Lower Palaeozoic

AbstractThe Eycott Volcanic Group of the Lake District Lower Palaeozoic inlier consists of basaltic and andesite and and andesite sheets and associated, mainly coarse, volcaniclastic rocks. The volcanic rocks have been regarded previously as interdigitated with, and equivalent in age to, mudrocks of the upper part of the Skiddaw Group (Tremadoc–Llanvirn). Microfloral evidence has been quoted in support of this interpretation, but has not been substantiated by re-assessment of the critical data. Furthermore, a recent examination of the base of the Eycott Volcanic Group has shown that it rests with angular unconformity on the Skiddaw Group. Skiddaw Group rocks beneath the unconformity range in age from possible late Cambrian to early Llanvirn. The lowest part of the Eycott Volcanic Group, the Over Water Formation, consists of siltstones and tuffaceous sandstones yielding a diverse microflora, and is intercalated with two andesite sheets interpreted herein as sills. Since the currently accepted Llanvirn age for the Eycott Volcanic Group cannot be confirmed, the volcanism may have been penecontemporaneous with the Llandeilo–Caradoc Borrowdale Volcanic Group episode. There are implications for a pre-volcanic tectonic deformation episode.

An Acadian mountain front in the English Lake District: the Westmorland Monocline

1993 ◽  
Vol 130 (2) ◽  
pp. 203-213 ◽  
Author(s):  
B. C. Kneller ◽  
A. M. Bell
Keyword(s):  
Shear Zone ◽  
Foreland Basin ◽  
Lake District ◽  
Volcanic Group ◽  
The North ◽  
English Lake District ◽  
Mountain Front ◽  
Group A ◽  
Lower Palaeozoic ◽  
Wide Zone

AbstractThe structure of the southern and central English Lake District is that of a southeast-facing monocline, named here the Westmorland Monocline. This 10 km wide zone of highly cleaved, southeast-dipping rocks separates gently dipping, poorly cleaved Borrowdale Volcanic Group to the north from extensively folded but regionally subhorizontal Windermere Group (foreland basin) rocks to the south. The monocline formed early in the local Acadian deformation sequence, and accommodates at least 8 km of uplift. It coincides with the steep concealed margin of the Lake District batholith. A major northwest-dipping shear zone is revealed in the deepest levels now exposed within the monocline, in the Skiddaw Group rocks of the Black Combe inlier.The monocline has the characteristics of a mountain front, providing significant tectonic elevation across a foreland-dipping panel of rocks, with no hinterland-dipping thrust visible at the surface. We interpret the uplift as the consequence of a southeast-vergent thrust with a gently northwest-dipping ramp beneath the central Lake District, continuing southeastwards as a flat detachment beneath the Windermere Group. A displacement up the ramp of about 20 km is accommodated by backthrusting within the monocline and by shortening within the Windermere Group of the hangingwall southeast of the monocline. The tip lies beyond the limit of the Lower Palaeozoic inlier, beneath Carboniferous cover.

Age of the Eycott Volcanic Group and its conformable relationship to the Skiddaw Slates in the English Lake District

1972 ◽  
Vol 109 (3) ◽  
pp. 259-268 ◽  
Author(s):  
C. Downie ◽  
N. J. Soper
Keyword(s):  
Volcanic Rocks ◽  
Island Arc ◽  
Lake District ◽  
Volcanic Group ◽  
Volcanic Sequence ◽  
Volcanic Formation ◽  
English Lake District

SummaryAn earliest Llanvirn age is established on micropalaeontological grounds for the interbedded lava–pelite sequence at the base of the Binsey Volcanic Formation in the northern Lake District. The northern volcanic sequence, here termed the Eycott Volcanic Group, is earlier than, and chemically distinct from, the main Borrowdale Volcanic Group. The evidence precludes a regional unconformity between the Skiddaw Slates and the overlying volcanic rocks, but is compatible with the hypothesis that the vulcanicity occurred in an island arc environment.

The timing of Ordovician magmatism in the English Lake District and Cross Fell inliers

1993 ◽  
Vol 130 (3) ◽  
pp. 369-377 ◽  
Author(s):  
Richard A. Hughes ◽  
Peter Kokelaar
Keyword(s):  
Volcanic Rocks ◽  
Arc Magmatism ◽  
Lake District ◽  
Early Ordovician ◽  
Extensional Tectonic ◽  
English Lake District ◽  
Lower Palaeozoic ◽  
Ordovician Magmatism ◽  
Tectonic Style ◽  
Subduction System

AbstractVolcanic and hypabyssal intrusive rocks of the Lower Palaeozoic English Lake District and Cross Fell inliers are elements of the Ordovician destructive plate margin system of microcontinental Avalonia. Two igneous sheets within the marine sedimentary Skiddaw Group of these inliers, previously described as lavas, are reinterpreted as sills. Sedimentary rocks enclosing these sills are of late Tremadoc-early Arenig (c. 493 Ma) and early Llanvirn (> 476 Ma) age, and breccias along the upper contacts of both were produced by steam explosivity and fluidization ahead of theadvancing tips of the intrusions. Previous interpretation of the breccias on the older sheet, as sediment deposited on the eroded top of a lava flow, implied an early Ordovician onset of arc magmatism. Such early magmatism would have been virtually coincident with the latest Tremadoc initiation of arc magmatism in Wales, but evidence for such a near synchronous response tothe putative onset of subduction is lacking. Respective onsets of magmatismwere probably separated by at least 17 m.y., and possibly by as much as 29 m.y. The apparent contemporaneity of mid and late Ordovician volcanic episodes in England and Wales, and similarities in extensional tectonic style, suggest that the two areas then were part of the same subduction system responding similarly to plate-scale magma-generating and tectonic processes. The early Ordovician situation is uncertain, but the absence of arc volcanic rocks of this age in the English Lake District suggests that this area and Wales are not tectonically juxtaposed elements of a former simple linear arc.

Palaeomagnetism of the Borrowdale and Eycott volcanic groups, English Lake District: primary and secondary magnetization during a single late Ordovician polarity chron

1997 ◽  
Vol 134 (4) ◽  
pp. 481-506 ◽  
Author(s):  
J. D. A. PIPER ◽  
J. C. STEPHEN ◽  
M. J. BRANNEY
Keyword(s):  
Volcanic Rocks ◽  
Field Tests ◽  
Late Ordovician ◽  
Caldera Collapse ◽  
Lake District ◽  
Volcanic Group ◽  
Group I ◽  
Iapetus Ocean ◽  
English Lake District ◽  
Normal Polarity

Late Ordovician volcanic rocks of the English Lake District typically have a magnetic remanence dominated by a single characteristic component. Previous investigations have interpreted this remanence as both of primary (pre-folding) and secondary origin. Palaeomagnetic field tests have been conducted on (a) andesite blocks from an autobrecciated lava top, (b) andesite blocks in mass-flow breccias, and (c) fault-blocks tilted during Ordovician caldera collapse to establish the time of remanence acquisition. All three tests show that the lavas retain a magnetization acquired during initial cooling: magnetizations of the breccias are coherent within clasts and random between clasts, whilst magnetizations of the tilted fault blocks converge with better that 95% confidence when corrected for the effects of caldera collapse. In contrast, the volcaniclastic sedimentary and pyroclastic rocks possess an Ordovician secondary remanence acquired after strata had been tilted by volcano-tectonic subsidence. A distributed sample of 65 andesite and basalt sheets through the Borrowdale Volcanic Group has a mean remanence direction D/I=341.9/−48.9° (α95=4.0°) yielding a positive fold test and a palaeomagnetic pole at 12.7°E, 4.3°S (dp/dm=3.5/5.3°). A progressive steepening of the palaeofield direction is recorded during emplacement of the Borrowdale Volcanic Group (∼I=−39° to I=−51°) which continued into the interval of volcanotectonic overprinting (I=−62°); the equivalent motion of Eastern Avalonia is ∼20° into higher southerly latitudes.Both the Eycott and Borrowdale volcanic groups exhibit uniform normal polarity throughout. Correlation with the geomagnetic time scale for the Ordovician restores the broad correlation between the two groups by constraining their emplacement and partial overprinting to a single long normal polarity chron occupying the Nemagraptus gracilis and earlier part of the Diplograptus multidens biozones (late Llandeilo and early Caradoc). All the volcanism, therefore, occurred within a period of no more than ∼5 Ma. The palaeomagnetic evidence confirms that the Borrowdale Volcanic Group was affected by both syn-volcanic deformation (caldera collapse) and regional compressive deformation prior to deposition of the (late Ordovician–Silurian) Windermere Supergroup. The succession of primary and secondary Ordovician palaeomagnetic poles from the Lake District inlier defines an anticlockwise apparent polar wander (APW) loop with the apex correlating with ‘soft’ closure of the Iapetus Ocean and late Ordovician deformation. The APW paths from Avalonia and Baltica converge at this point as subduction ceased and the arc subsided beneath the sea after mid-Caradoc times.

A pre-caldera plateau-andesite field in the Borrowdale Volcanic Group of the English Lake District

1992 ◽  
Vol 149 (6) ◽  
pp. 889-906 ◽  
Author(s):  
M. G. PETTERSON ◽  
B. BEDDOE-STEPHENS ◽  
D. MILLWARD ◽  
E. W. JOHNSON
Keyword(s):  
Lake District ◽  
Volcanic Group ◽  
English Lake District

Pre-Acadian copper mineralization in the English Lake District

1999 ◽  
Vol 136 (2) ◽  
pp. 159-176 ◽  
Author(s):  
D. MILLWARD ◽  
B. BEDDOE-STEPHENS ◽  
B. YOUNG
Keyword(s):  
Quartz Crystal ◽  
Volcanic Rocks ◽  
Wall Rock ◽  
Lake District ◽  
Early Devonian ◽  
Rock Fragments ◽  
Copper Mineralization ◽  
English Lake District ◽  
Fibre Strain ◽  
Devonian Age

The Ordovician sedimentary and igneous rocks of the English Lake District host a widespread suite of epigenetic metalliferous veins dominated by copper sulphides with abundant arsenopyrite, pyrite and accessory galena and sphalerite. New field and microstructural evidence from examples of this suite at Coniston, Wasdale, Honister, Newlands and Borrowdale shows that the veins were strongly cleaved during the Early Devonian (Emsian) Acadian orogenic event. The principal evidence includes the continuity of wall-rock cleavage fabrics with pressure solution seams in the veins and consistently orientated cleavage through enclosed, rotated wall-rock fragments and chloritic mats. There is also widespread complex intracrystalline deformation in quartz, cataclasis of arsenopyrite and pyrite, fracturing and/or buckling of bladed hematite, and growth of quartz or mica-fibre strain fringes. Chalcopyrite was partially or totally remobilized, enabling it to migrate along quartz crystal boundaries, and invade brecciated pyrite. Previous K–Ar Early Devonian age determinations for the mineralization are considered to have been reset. The pre-Acadian age of this mineralization, its style and relationship to the volcanic rocks permits a genetic link with the final phases of Caradoc magmatism.

Stable isotope compositions of alteration fluids in low-grade Lower Palaeozoic rocks, English Lake District

1985 ◽  
Vol 49 (352) ◽  
pp. 425-434 ◽  
Author(s):  
L. J. Thomas ◽  
R. S. Harmon ◽  
G. J. H. Oliver
Keyword(s):  
Stable Isotope ◽  
Fluid Inclusion ◽  
Isotopic Composition ◽  
Low Grade ◽  
Secondary Phases ◽  
Lake District ◽  
English Lake District ◽  
Lower Palaeozoic ◽  
Hydrogen And Oxygen Isotope ◽  
Isotope Analyses

AbstractA combination of hydrogen and oxygen isotope analyses and fluid inclusion studies has defined the composition of fluids involved in the metamorphism of Lower Palaeozoic rocks in the English Lake District. Three fluid fields have been defined from secondary phases: 1, syn-burial metamorphic D-enriched fluids from epidote and chlorite at a temperature between 250 and 350°C; D-depleted fluid measured from groundmass and quartz inclusions; 3, a mixed magmatic-meteoric fluid with an intermediate H-isotopic composition estimated from W/R granite data and calculated from illite.

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