Geo-Amble 4th September 2017

Overview of the Site and its Geology (thanks to i.a. EHT, Prof. Ian Fairchild and Prof. Donny Hutton)

The Carboniferous marks the time in which there was a sharp draw down of atmospheric CO2, which produced cycles of glaciations and also led to the deposition of the massive coal measures that were subsequently exploited during the industrial revolution. It is from the study of these coal measures that the idea of the Carboniferous system was developed. The rocks of this period outcrop in the north and north-western part of Worcestershire, centred on the Wyre Forest Coalfield. They consist of deposits laid down in a flat, swampy, deltaic environment, ideal for coal formation.

The prevalent Coal Measures have not been used for aggregate, however igneous intrusions of the same age have been quarried. Igneous intrusions are discordant bodies, a few cm to over 100 m thick, and can be of any length. They are produced when magma is injected along fractures in the country rock. The tectonic settings responsible for the emplacement of Carboniferous intrusions have not been established, although it may be related to crustal extension following the Variscan Orogeny.

Lithology

A Dolerite dike intrudes in to the Raglan Mudstone Formation at Brockhill in the Teme Valley. It is visible on the east bank of the river in a quarry and again on the opposite side but in smaller exposures. The dike runs in a westerly direction for approximately 1200 metres and has affected the course of the Teme, which runs along it for some way before breaching the hard rock barrier to produce a noticeable meander. At its maximum the dike is around 7.5 m thick at its centre. Its mineralogy is of an alkali gabbro (teschenite) with pyroxene, serpentized olivine, intermediate plagioclase and interstitial calcite. Along its margins it consists of a 2.5 to 5 cm band of quartz dolerite with pyroxene, lathy feldspar and quartz. There is no contact between these two igneous types suggesting that the quartz-dolerite has resulted from acidification of alkali gabbro magma prior to emplacement. The country rock is altered to a distance of around 9 m on each side of the intrusion converting the marls to a purple rock with light spots of calcite, analcite, chlorite and garnet. Tridymite needles have also developed around the quartz grains in the sandstone indicating a temperature range of 870 to 1470˚C. The cornstone has become a quartz-calcite-garnet-hornfels. The age of the dike is likely to be Carboniferous.

Exposed Units: Brockhill Dike, Raglan Mudstone

Conservation Status: Local Geological Site

The Brockhill Dike is a type of intrusive igneous rock with a dolerite/teschenite chemistry, which has been extensively quarried for aggregate at this site. It was intruded into the surrounding sandstones and marls at a time when these country rocks were cold. This created a marked temperature gradient between the sedimentary ‘country rocks’ and the hot, molten igneous intrusion and resulted in the formation of a baked margin.

From TVGS Web Site following a Field Trip led by Prof. Donny Hutton (http://www.geo-village.eu/?cat=14):

Brockhill Dike, Shelsley Beauchamp, where a teschenite dike is exposed in an old pit. Little remains of the dike except high up in the eastern end of the pit. However, good fresh specimens of the rock were obtained after a stiff scramble halfway up the face. Sodium rich, it belongs to the syeno-gabbro suite of rocks. It’s mineral composition is very similar to gabbro but the inclusion of an alkaline mineral, (either nepheline or analcite – in this case analcite) distinguishes it from gabbro. Plagioclase feldspar, clinopyroxene, analcite (easily distinguishable), minor amphibolites and biotite make up this medium- grained basic rock. The dike extends east-west for about 1200 metres and is exposed in small pits on the western side of the Teme. The river itself runs along the line of the dike until it finds a way through, just below Brockhill Court.  Emplaced in the Downton Series of red marls and sandstones it is about ten metres wide and dips almost vertically.  No in situ examination of the margins was possible but the Droitwich Memoire has it that narrow doleritic edges to the dike can be seen. Loose specimens were found of what may have been a fine grained rock from the chilled margin of the dike. Excellent examples of spheroidal (onion skin) weathering can be found in the debris of the pit and on the exposed face.

The country rock, marls, silts and sandstones were ‘baked’ by the hot (1600 degrees C?) magma. The sandstones are now hornfels, a very hard, metamorphic rock. During the baking some layers of the purple marls were sufficiently plastic to allow the escape of volatile gases and the development of vesicles and tubes which were later lined with calcite, chlorite and analcite.  Extreme baking produced vitrified black specimens with conchoidal fracturing. Good examples of all of these rocks can be found in the garden walls of the nearby Brockhill Court.

An explanation of the cause of the Brockhill dike was given by our very knowledgeable guide, Prof Donny Hutton. The dike is one of a suite emplaced in late Carboniferous times (300 Ma) during the Variscan Orogeny. Similar dikes with similar E-W orientation can be found in Northern England and the Midland Valley of Scotland. Variscan subduction with consequent loading and downbending of the lithosphere induced ‘flexural bulging’ with uplift and tensional fracturing of the crust. Low degrees of adiabatic melting produced buoyant syeno-gabbros which rose and pushed into the fractures.

NOTE ON SOME OF THE WORDS USED IN THE ABOVE

And also a useful note about the commonly used words-acid, basic et al:

Because most igneous rocks are composed of silicate minerals, the earliest chemical classification used was one based on weight percentage of SiO2 (silicon dioxide) in the rock. This led to the subdivision of igneous rocks into four categories—acid(ic), intermediate, basic, and ultrabasic. Over the years, different authors have varied slightly in the limits of SiO2 percentage for the four groups, but many petrologists designate igneous rocks with ≫66% SiO2 as acidic, 52–66% SiO2 as intermediate, 45–52% SiO2 as basic, and ≪45+ SiO2 as ultrabasic (≫65%, 65–55%, 55–45% and ≪45% are also used). Some authors use the term sub-silicic and others use the term mafic synonymously with basic although mafic is mainly used in relation to dark-colored Mg–Fe minerals or rocks rich in these minerals.

Word or Expression Meaning Derivation
Adiabatic Melting Hot magma rises through weak points in the crust but critically retains most of its heat and temperature.  As it rises it is subject to lower and lower pressures.  This allows melting to occur.  Adiabatic refers to constancy of temperature Anglicisation of the Greek for impassable and by Rankine then Maxwell to describe a process whereby heat cannot or does not escape from the process, so the temperature remains constant (in a perfect system of course). See HERE
Amphibole (amphibolite) mineral supergroup, is the name of an important group of generally dark-coloured, inosilicate minerals, forming prism or needlelike crystals Greek amphi–both and ballein–to throw
Analcite White, colourless silicon based mineral, a tectosilicate (cubic crystalline as opposed to sheet crystalline). Found in basalt and other igneous rocks.  Greek analkimos– “weak
Baked Margin That part of the country rock that is immediately adjacent to an igneous intrusion. High temperatures experienced by the country rock during the intrusion of an igneous body can cause clay-rich rocks to become baked in the immediate vicinity of the intrusion. The effect of this baking decreases with distance from the intrusion.
Biotite A common phyllosilicate (i.e. a material where the crystals, based around Silicon, form sheets), part of the mica group Named from French physicist Jean-Baptiste Biot who worked on the optical properties of mica
Calcite Carbonate mineral CaCO3, the most common form of this compound German Calcit from Latin calx for chalk
Chlorite Also a sheet silicate (phyllosilicate). See HERE for silicates and HERE for chlorites. Greek chloros—green, the main tint of chlorite
Conchoidal Fracturing describes the way that brittle materials break or fracture when they do not follow any natural planes of separation. Materials that break in this way include quartz, flint, quartzite, jasper, and other fine-grained or amorphous materials with a composition of pure silica, such as obsidian and window glass. Greek Konchos—mussel (shellfish)
Country Rock The host rock into which an igneous rock has been intruded. It is also termed ‘surrounding rocks’ in this entry.
Dike A body of igneous rock that has been intruded into the surrounding rocks and has a ‘sheeted’ geometry. This ‘sheet’ cuts across the sedimentary layering in the surrounding rocks.
Dolerite is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt (i.e. basalt that has erupted on to the surface) or plutonic gabbro (i.e. the same material as basalt but that remained underground). Greek doleros-‘deceptive’ (as it is difficult to distinguish from diorite!) via French dolerite
Gabbro a dark, medium- to coarse-grained intrusive igneous rock composed of calcium plagioclase, pyroxene, and minor olivine, but no quartz. It is the intrusive equivalent (i.e. formed underground, not on the surface) of a basalt.
Hornfels Hornfels derives from mudstone or shale (clay rich rocks) being affected by heat from contact with hot magma –contact metamorphism German—literally horn rock as it reminded them of the toughness of animal horns
Intrusive Igneous Rock Rock derived from magma that cooled underground
Mafic an adjective describing a silicate mineral or igneous rock that is rich in magnesium and iron and relatively low in Silicon (old term was base or basic) Ma (magnesium) fic (iron—short for Ferric)
Marl A type of mudstone that consists of clay and carbonate, i.e. a lime-rich mudstone.
Plagioclase Feldspar Feldspar-a group of rock-forming tectosilicate minerals that make up about 40% of the Earth’s continental crust. Feldspar from German for field and spar for non-ore containing mineral
Pyroxene The pyroxenes (commonly abbreviated to Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes are silicon-aluminum oxides with Ca, Na, Fe, Mg, Zn, Mn, Li substituting for Si and Al early 19th century: from pyro-‘fire’ + Greek xenos ‘stranger’ (because the mineral group was supposed alien to igneous rocks).
Teschenite Teschenite, coarse- to fine-grained, rather dark-coloured, intrusive igneous rock that occurs in sills (tabular bodies inserted while molten between other rocks), dikes (tabular bodies injected in fissures), and irregular masses and is always altered to some extent. It consists primarily of plagioclase feldspar, analcime, and titaniferous augite, with barkevikite, nepheline, and olivine usually in lesser amounts. The plagioclase crystals often are encased in the augite to give teschenite an ophitic texture. In central Scotland it is abundant in thick sills. Teschenite grades into picrite when the olivine content increases. From Teschen a site in Poland where presumably first noted
Variscan Orogeny A mountain building episode that occurred in the development of super continent Pangaea some 380-280 million years ago Medieval Latin name for the district Variscia, the home of a Germanic tribe, the Varisci; Eduard Suess, professor of geology at the University of Vienna, coined the term in 1880
Vesicles a small, usually spherical cavity in a rock or mineral, formed by expansion of a gas or vapor before the enclosing body solidified. from Latin ‘vesicula’ small bladder