Exposed Units: Malverns Complex
Alternative Site names: Lower Tolgate Quarry, Wyche Quarry
Conservation Status: Local Geological Site
The Malverns Complex at Earnslaw Quarry is mostly represented by coarse grained pink granite that is tinged green in places because of the presence of the mineral chlorite. Diorite is present in the north of the quarry but is separated from the granite by a fault zone. Exposed in the quarry are the contacts between the Malverns Complex and two younger intrusive rocks. The smaller of these intrusions is a ~20cm thick microdiorite dyke that intrudes the granite at a high angle. Minor copper mineralisation has taken place at this site.
The second intrusion is a potassium-rich trachyte dyke that forms an east-west step across the quarry floor in the south part of the quarry. This is the only intrusion of its kind in the Malverns Complex. It is a fine-grained, dull grey rock with scattered spots and veins of carbonate. It is composed mostly of the mineral potassium feldspar with minor chlorite, iron-titanium oxides, a colourless fibrous mineral and quartz. Pseudomorphs of the mineral nepheline may also be present.
The intrusion exhibits a marked chilled margin, indicating its intrusion into the granite after it had cooled and solidified. Coarse-grained carbonate patches occur in the centre of the intrusion and in the highly deformed margin of the granite, which also contains common carbonate veins. Carbonate mineralisation occurred either during or after the intrusion of the trachyte, as it cross cuts both the granite and the trachyte.
The trachyte dyke crops out on an E-W step across the southern part of the quarry, and appears to be confined to the western part of the step. It is perhaps truncated by a NNE fault running parallel to the trend of the quarry. It runs NW-SE and is oriented near-vertical, and may be Caledonian in age. A fault zone immediately to the north of the body separates diorite to the north from coarse-grained, pink granite to the south.
Terminology
Trachyte – Fine grained igneous rock dominated by the mineral potassium feldspar, with smaller amounts plagioclase feldspar and either quartz or a feldspathoid mineral, such as nepheline.
Microdiorite – Fine-medium grain igneous rock containing the minerals plagioclase feldspar, iron- or magnesium-bearing minerals, such as amphibole or pyroxene, and up to 10% quartz.
Calendonian – Descriptive term for rocks/structures formed during the Caledonian orogeny, a period of mountain building in Britain, North America and Scandinavia that took place ~400 million years ago.
Photos
General view of the lake and land slip at the Earnslaw Quarry.
The scar left by the landslip exposes Malverns Complex diorite that is cut by pale pink pegmatite. The top of the scree slope left by the land slip is visible towards the bottom of the image.
The quarry face behind the upper car park at Earnslaw Quarry.
Granite pegmatite vein intruding altered ultramafic rocks between the upper and lower car park.
References
Barclay, W.J., Ambrose, K., Chadwick, R.A., and Pharaoh, T.C., (1997), ‘Geology of the country around Worcester’, Memoir of the British Geological Survey, British Geological Survey, London.
Beckinsale, R.D., Thorpe, R.S., Pankhurst, R.J. and Evans, J.A., 1981, ‘Rb-Sr whole rock isochron evidence for the age of the Malvern Hills Complex’, Journal of the Geological Society, London, vol. 138, 1, pp. 69-73.
Carney, J.N., Horak, J.M., Pharaoh, T.C., Gibbons, W., Wilson, D., Barclay, W.J., Bevins, R.E., Cope, J.C.W. & Ford, T.D., 2000, ‘Precambrian Rocks of England and Wales’, Geological Conservation Review, 20, Joint Nature Conservation Committee, Peterborough.
Thorpe, R.S., 1987, ‘Pseudotachylite from a Precambrian shear zone in the Malvern Hills’ Proceedings of the Geologists’ Association, vol. 98, 3, pp. 205-210.
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