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A ?lower carboniferous dyke from Castleton, Gwent, South Wales
SHORT COMMUNICATION
A ?Lower Carboniferous dyke from Castleton, Gwent, South Wales D. J. D. Lawrence, R. W. Sanderson and R. A. Waters D. J. D. Lawrence and R. A. Waters, Institute of Geological Sciences, 5, Princes Gate, London, SW 7 IQ N. R. W. Sanderson, Institute of Geological Sciences, Exhibition Road, Lon don, S W7 2DE
1. INTRODUCTION
The dyke occurs about one kilometre northeast of Castleton , Gwent at the junction of the M4 motorway and the A4 8(M) (Figs. 1 & 2). It was expo sed during the con struction of the A48(M) in the summer of 1976 and discovered by two ofthe authors (D.l.D.L. and R.A.W.) during the exa mination of the temporary excavations there . Dolerite was exposed in three places: in two excavations for drains situated on the north and south edges of the eastbound carriageway of the A4 8(M) (ST 25828422 and ST 25888420 respecti vely) and in the excavat ion for the northern pier of the road bridge over the M4/A4 8(M) junction (ST 26218421 ), 350 m to the east.
5
10 Miles
!
I
5
0
10
I
Kilometres
t 90
2. FIELD RELATIONSHIPS
The dolerites are intruded into the St. Maughan's Group of the Lower De vonian Lower Old Red Sand stone, which here con sists of red and purple mudstones and siltstones with scattered caliche nodul es and subordinate sand stones. All three expo sure s show the dolerite to be part of a dyke or dykes, the cont acts striking N 105° and dipp ing 55° to 78°S in the northern dr ain excavation, N 105° and dipping ste eply south in the southern drain and N 080°, dipping 85° to 90 in the bridg e pier excavat ion. The two western exposures are obviously part of the same dyke but whether the eastern one represents an eastw ard continuation is uncertain from the field evidence , though it seems likely. The dyke ranges in width from 1,2-2·6 m. The country rock is massive purple siltstone of variable strike, baked for up to 0·3 m from the dyke . The dyke was not traced beyond the temporary exp osure s owing to a cover ing of glacial dr ift. 0S
3. PETROLOGY
The dyke cons ists of a grey, carbonated, amygdaloidal porphyritic olivine -dolerite, weathering buff to purple and locally deeply weathered. Two thin sections (E 49335 and E 49336: numb er s preceded by E refer to thin sections in the England and Wale s collections of sliced rock s of the Institute of Geological Sciences .) were exam ined , respectivel y from the northern drain
60 10
20
40
50
60
Fig. 1. Location map showing position of igneous and volcanigenic rocks referred to in the text . Numbers refer to 10-km. squares within the National grid 100-km. square ST. A, Great House; B, Castleton, rectangle encloses area of Fig. 2; C, Cwrt-yr- ala ; D, Tickenham; E, Middle Hope ; F, Goblin Combe.
excavation and from the bridge pier excavation and proved to be identical; that from the northern drain exca vation is illustrated in Plate 1. Und er the microscope, remarkably fre sh laths , < 0·75 mm long, and uncommon interstit ial plates of labradorite are seen to be set in a completely altered base. Microprobe anal yses indicate that the feldspar compositions vary little from a mean of Or, Ab a7 AI1fi2 .
125
126
D . J. D . LAWR ENCE, R. W . SA ND ERSON & R. A . WATERS
N
t 84
OUTCROP OF OYKE ..-- INCLI N EO STRATA 5 OIP IN DEGREES
, 9
50
100
METRES
Fig. 2. Map showing location of the dyke near Cas tleto n.
Ori ginal groundm ass ferromagne sians and a spa rse mesostasis are repl aced by turbid iron- stained carbon at e together with so me chlorite and chalcedonic silica. Th e accesso ry phases are restricted to inte rstitia l finegrained aggregates of o paque Ti-m agnetite and minor ilmenit e showing hem atitic, but not leucoxenoid, alte ratio n. A maximum of6 % ZnO det ected in the magnetite app ears to be incorpor ated in the latt ice rather th an in a distinct Zn -spin el phase . Pseudomorphs after olivine and pyroxene phen ocrysts are recogni zable and attain a maximum len gth of 0·9 mm. Where good crystal outline is present the typical acutely terminat ed shapes after olivine are formed of turbid gra nular Mg-bearing carbon ate and minor silica; more prismatic for ms commonly possess a cor e of weIl crystallised ' bastite' serpe ntine and presumably re present an original pyroxene . PLATE 1 Alt ered olivine-
127
SHORT COMMUNICATION
The numerous small round amygdales are filled mainly with granular carbonate in which isolated rhombs of ankerite occur together with chlorite and silica. Against the amygdales the dolerite presents a narrow tachylitic rim which, like the mesostasis, is now largely replaced by carbonate but also contains some felsic devitrification products. Alternation of the dolerite may be attributed to hydrothermal action by alkaline fluids, resulting in the loss of silica, with hydrolysis and replacement by carbonate of the ferromagnesian phases. There is no evidence for late stage alkali enrichment of the rock . 4. AGE The dolerite of the Castleton dyke is too altered for reliable isotopic dating, but a consideration of it in relation to the other igneous and volcanigenic rocks in the region enables us to propose a Lower Carboniferous age . A. In South Wales the only stratigraphically welldefined volcanigenic rock known to be younger than the St. Maughan's Group is a thin bentonite very low in the Tournaisian sequence of the Cwrt-yr-ala borehole near Cardiff (Waters, 1978 , p. 9), although Eyles & Blundell (1957) also suggest a Lower Carboniferous age for a volcanic vent at Great House , 6·5 km southeast of Usk (Fig. I). B. South of the Mouth of the Severn, basalt lavas and tuffs are known from two levels within the Lower
Carboniferous (George. Johnson. Mitchell. Prentice. Ramsbottom, Sevastopulo & Wilson , 1976 , p. 14) namely: (i) in the upper part of the Tournaisian at Middle Hope , north of Weston-super-Mare (see Fig. 1), (ii) in the Arundian. within a fairly restricted geographical area , at Weston-super-Mare, Goblin Combe and near Tickenham (Fig. 1) . C. The mineralogy and texture of the Castleton dolerite, especially the nature of the fresh feldspars and the altered ferromagnesium minerals, is reminiscent of that of the Arundian basalt lavas at Weston-super-Mare. We therefore suggest that the Castleton dyke belongs to fairly localised igneous activity that took place on both sides of the eastern end of the mouth of the Severn during the Lower Carboniferous. ACKNOWLEDGEMENTS Mr. P. H. A. Nancarrow undertook the electron microprobe analyses of selected minerals and Mr. B. R. Young the X-ray identification of the opaque phases. The authors would like to thank Sir Owen Williams and Partners, agents for the Welsh Office, and M. J. Gleeson Ltd., the contractors, for permitting them to examine the temporary sections provided by the con struction of the A48(M). The paper is published by permission of the Director. Institute of Geological Sciences (N.E.R.C.).
References EYLES, V. A. , & C. R. K. BLUNDELL, 1957 . On a volcanic vent and associated monchiquite intrusions in Monmouthshire . Geol. Mag., 94, 54-7. GEORGE, T. N., G. A. L. JOHNSON , M. MITCHELL, J. E. PRENTICE. W. H. C. RAMSBOTTOM. G . D. SEVAS-
TOPULO & R. B. WILSON, 1976. A correlation of Dinantian rocks in the British Isles. Geol. Soc. London Spec. Rep., 7,87 pp. WATERS, R. A., 1978 . Cwrt-yr-ala Borehole. In, IGS Boreholes 1977 . Rep. Inst. Geol. Sci., 78/21 , p. 9.
A ?Lower Carboniferous dyke from Castleton, Gwent, South Wales D. J. D. Lawrence, R. W. Sanderson and R. A. Waters D. J. D. Lawrence and R. A. Waters, Institute of Geological Sciences, 5, Princes Gate, London, SW 7 IQ N. R. W. Sanderson, Institute of Geological Sciences, Exhibition Road, Lon don, S W7 2DE
1. INTRODUCTION
The dyke occurs about one kilometre northeast of Castleton , Gwent at the junction of the M4 motorway and the A4 8(M) (Figs. 1 & 2). It was expo sed during the con struction of the A48(M) in the summer of 1976 and discovered by two ofthe authors (D.l.D.L. and R.A.W.) during the exa mination of the temporary excavations there . Dolerite was exposed in three places: in two excavations for drains situated on the north and south edges of the eastbound carriageway of the A4 8(M) (ST 25828422 and ST 25888420 respecti vely) and in the excavat ion for the northern pier of the road bridge over the M4/A4 8(M) junction (ST 26218421 ), 350 m to the east.
5
10 Miles
!
I
5
0
10
I
Kilometres
t 90
2. FIELD RELATIONSHIPS
The dolerites are intruded into the St. Maughan's Group of the Lower De vonian Lower Old Red Sand stone, which here con sists of red and purple mudstones and siltstones with scattered caliche nodul es and subordinate sand stones. All three expo sure s show the dolerite to be part of a dyke or dykes, the cont acts striking N 105° and dipp ing 55° to 78°S in the northern dr ain excavation, N 105° and dipping ste eply south in the southern drain and N 080°, dipping 85° to 90 in the bridg e pier excavat ion. The two western exposures are obviously part of the same dyke but whether the eastern one represents an eastw ard continuation is uncertain from the field evidence , though it seems likely. The dyke ranges in width from 1,2-2·6 m. The country rock is massive purple siltstone of variable strike, baked for up to 0·3 m from the dyke . The dyke was not traced beyond the temporary exp osure s owing to a cover ing of glacial dr ift. 0S
3. PETROLOGY
The dyke cons ists of a grey, carbonated, amygdaloidal porphyritic olivine -dolerite, weathering buff to purple and locally deeply weathered. Two thin sections (E 49335 and E 49336: numb er s preceded by E refer to thin sections in the England and Wale s collections of sliced rock s of the Institute of Geological Sciences .) were exam ined , respectivel y from the northern drain
60 10
20
40
50
60
Fig. 1. Location map showing position of igneous and volcanigenic rocks referred to in the text . Numbers refer to 10-km. squares within the National grid 100-km. square ST. A, Great House; B, Castleton, rectangle encloses area of Fig. 2; C, Cwrt-yr- ala ; D, Tickenham; E, Middle Hope ; F, Goblin Combe.
excavation and from the bridge pier excavation and proved to be identical; that from the northern drain exca vation is illustrated in Plate 1. Und er the microscope, remarkably fre sh laths , < 0·75 mm long, and uncommon interstit ial plates of labradorite are seen to be set in a completely altered base. Microprobe anal yses indicate that the feldspar compositions vary little from a mean of Or, Ab a7 AI1fi2 .
125
126
D . J. D . LAWR ENCE, R. W . SA ND ERSON & R. A . WATERS
N
t 84
OUTCROP OF OYKE ..-- INCLI N EO STRATA 5 OIP IN DEGREES
, 9
50
100
METRES
Fig. 2. Map showing location of the dyke near Cas tleto n.
Ori ginal groundm ass ferromagne sians and a spa rse mesostasis are repl aced by turbid iron- stained carbon at e together with so me chlorite and chalcedonic silica. Th e accesso ry phases are restricted to inte rstitia l finegrained aggregates of o paque Ti-m agnetite and minor ilmenit e showing hem atitic, but not leucoxenoid, alte ratio n. A maximum of6 % ZnO det ected in the magnetite app ears to be incorpor ated in the latt ice rather th an in a distinct Zn -spin el phase . Pseudomorphs after olivine and pyroxene phen ocrysts are recogni zable and attain a maximum len gth of 0·9 mm. Where good crystal outline is present the typical acutely terminat ed shapes after olivine are formed of turbid gra nular Mg-bearing carbon ate and minor silica; more prismatic for ms commonly possess a cor e of weIl crystallised ' bastite' serpe ntine and presumably re present an original pyroxene . PLATE 1 Alt ered olivine-
127
SHORT COMMUNICATION
The numerous small round amygdales are filled mainly with granular carbonate in which isolated rhombs of ankerite occur together with chlorite and silica. Against the amygdales the dolerite presents a narrow tachylitic rim which, like the mesostasis, is now largely replaced by carbonate but also contains some felsic devitrification products. Alternation of the dolerite may be attributed to hydrothermal action by alkaline fluids, resulting in the loss of silica, with hydrolysis and replacement by carbonate of the ferromagnesian phases. There is no evidence for late stage alkali enrichment of the rock . 4. AGE The dolerite of the Castleton dyke is too altered for reliable isotopic dating, but a consideration of it in relation to the other igneous and volcanigenic rocks in the region enables us to propose a Lower Carboniferous age . A. In South Wales the only stratigraphically welldefined volcanigenic rock known to be younger than the St. Maughan's Group is a thin bentonite very low in the Tournaisian sequence of the Cwrt-yr-ala borehole near Cardiff (Waters, 1978 , p. 9), although Eyles & Blundell (1957) also suggest a Lower Carboniferous age for a volcanic vent at Great House , 6·5 km southeast of Usk (Fig. I). B. South of the Mouth of the Severn, basalt lavas and tuffs are known from two levels within the Lower
Carboniferous (George. Johnson. Mitchell. Prentice. Ramsbottom, Sevastopulo & Wilson , 1976 , p. 14) namely: (i) in the upper part of the Tournaisian at Middle Hope , north of Weston-super-Mare (see Fig. 1), (ii) in the Arundian. within a fairly restricted geographical area , at Weston-super-Mare, Goblin Combe and near Tickenham (Fig. 1) . C. The mineralogy and texture of the Castleton dolerite, especially the nature of the fresh feldspars and the altered ferromagnesium minerals, is reminiscent of that of the Arundian basalt lavas at Weston-super-Mare. We therefore suggest that the Castleton dyke belongs to fairly localised igneous activity that took place on both sides of the eastern end of the mouth of the Severn during the Lower Carboniferous. ACKNOWLEDGEMENTS Mr. P. H. A. Nancarrow undertook the electron microprobe analyses of selected minerals and Mr. B. R. Young the X-ray identification of the opaque phases. The authors would like to thank Sir Owen Williams and Partners, agents for the Welsh Office, and M. J. Gleeson Ltd., the contractors, for permitting them to examine the temporary sections provided by the con struction of the A48(M). The paper is published by permission of the Director. Institute of Geological Sciences (N.E.R.C.).
References EYLES, V. A. , & C. R. K. BLUNDELL, 1957 . On a volcanic vent and associated monchiquite intrusions in Monmouthshire . Geol. Mag., 94, 54-7. GEORGE, T. N., G. A. L. JOHNSON , M. MITCHELL, J. E. PRENTICE. W. H. C. RAMSBOTTOM. G . D. SEVAS-
TOPULO & R. B. WILSON, 1976. A correlation of Dinantian rocks in the British Isles. Geol. Soc. London Spec. Rep., 7,87 pp. WATERS, R. A., 1978 . Cwrt-yr-ala Borehole. In, IGS Boreholes 1977 . Rep. Inst. Geol. Sci., 78/21 , p. 9.