Dartmoor Granite, Tor Formation and the Haytor Granite Tramway 12Th Sept, 2010
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Dartmoor Granite, Tor Formation and the Haytor Granite Tramway 12th Sept, 2010 Key Features. Haytor (or Heytor) is a good example of an 'avenue' or 'valley' tor where 2 bosses of granite have been left after weathering and erosion of the core. The tor is composed of megacrystic granite, arguably with evidence of flow alignment, and xenoliths of country rock are incorporated in places. The 'tor' granite is underlain by a fine grain sheet intrusion and the junction can be clearly seen in the north-west face of the western boss of the tor, where the effects of periglacial weathering are also in evidence. Further weathering effects on both vertical and horizontal joints can be seen along with rock basins and other periglacial features such as clitters. The wider area is also underlain by poorly megacrystic 'blue' or 'quarry' granite, which has been extensively quarried. In this is exposed a third set of joints with evidence of late stage mineralisation, and aplite intrusions, evidence of late stage magma evolution. Stone from the quarries was transported to the lowlands and ultimately on to Teignmouth Port on the unique Granite Tramway. Emplacement of the Dartmoor Granite. The History of the Dartmoor granite began in the Devonian, 400Ma, when the area was a vast flood plain. In time as the coastline moved northward the area was inundated by a shallow tropical sea whilst the area to the north remained dry and arid. The ocean contained coral reefs around which mud sediments and volcanic deposits accumulated. Shallow sea conditions persisted well into the Carboniferous when the swampy coastline moved steadily northward leaving coal deposits; only poor quality in North Devon, and interbeded with mudstones. Subsequently as offshore conditions became prevalent the sediments became increasingly sandy. Towards the end of the Carboniferous these sediments were driven up into a mountain chain by continental collision and thrusting from the south and magma was intruded into the basement of the mountain chain approximately 290 – 285Ma. The manner of the actual intrusion of the magma is still debated. It rose through the Devonian rocks but was forced to spread out when it encountered the more competent Carboniferous sediments. There is some suggestion that the magma migrated northwards along the thrust weaknesses and it was long maintained that this occurred as a series of sheets, based largely on the evidence of feldspar phenocryst size layering and apparent alignment parallel to the direction of supposed flow. Both these features can be seen in the granite of the cliffs surrounding Land's End and were supposed to correspond to features in the Dartmoor granite. Seismic studies have indicated that the Dartmoor pluton is deep rooted and steep sided with the upper surface tilted, rising slightly towards the north. As the magma rose it incorporated a large amount of the surrounding rocks but on the northern margin it OUGS 12 Sept, 2010 The Granite and Quarries at Haytor Page 1 appears that the granite is only 6 miles or so thick. Some maintain that this is evidence of a sheet intrusion but another string of evidence points to a massive shelf of country rock that broke from the chamber roof and sank into the solidifying magma at a late stage. A further line of evidence points to the tilting of the granite as a consequence of the much more recent alpine orogeny at the same time as the sticklepath fault was active. Variation of texture and composition within the granite. Figure 1 shows a view of the western face of Haytor Rocks. OUGS 12 Sept, 2010 The Granite and Quarries at Haytor Page 2 There are 2 textural varieties of granite commonly found on Dartmoor. 'Tor Granite' as the name suggests occurs principally around the high points of the moor and is regarded as remnants of the batholith roof. The ground mass typically has grains of quartz and feldspar in excess of 3mm with phenocrysts principally of orthoclase often exhibiting carlsbad twinning and typically 5-7cm in length. It is consequently classed as coarse megacrystic. The tor granite incorporated a high proportion of country rock as it was intruded but though the present day upper surface of the granite may represent a surface that has been eroded with the loss of as much as 50m of granite, xenoliths occur frequently at Haytor. They are usually highly altered having indistinct contact surfaces with the granite. The assimilation of the xenoliths altered the magma composition from sodium rich increasing the potassium content so that it is sometimes viewed as a sub-type and called contact granite. [Locality1 The flat granite surface leading up from the avenue between the 2 bosses of Haytor Rocks to just below the steps cut into the sharp rise of the western boss. This is a clean exposure of tor granite and shows clear megacryst alignment. A large lichen coated xenolith can be seen almost at the foot of the steps another about fist sized is at the top of the steps where it has been polished by passing feet.] [Locality1a On top the eastern boss are the best rock basins and a view to the ball clay pits at Teigngrace and on to Teignmouth. There is also plenty of growan in the joints. Optional depending on weather and ability as it is a bit of a scramble.] Beneath the tor granite lies the 'blue' or 'quarry' granite. This has a moderately coarse ground mass with grains in the 2-3mm size range with feldspar phenocrysts typically 2- 3cm in length but they are very few and is consequently termed coarse, poorly megacrystic. There is a further minor compositional variation between these 2 types as the mica component of tor granite is almost exclusively biotite whereas the blue granite has a higher muscovite content. A third type of granite; fine, poorly megacrystic is seen at Haytor. This appears as a strikingly sharp boundary at the base of the north-western face of the tor, which has been seen as evidence supporting the theory for granite intrusion as several distinct sheets. [Locality2 The grassy area below the north-west face of the western boss. This shows clear vertical and horizontal joints and the very sharp boundary between the tor and the fine, poorly megacrystic granites. The fine granite has proved more susceptible to weathering and in places erosion has formed a substantial overhang or 'rock shelter'. There is extensive clitter of both tor and fine granite and some blocks show feather and tare grooves and there is a shallow quarry.] Finally in places there are intrusions of aplite with an enriched quartz content and sugary OUGS 12 Sept, 2010 The Granite and Quarries at Haytor Page 3 fine grain texture. Aplite intrusions are common throughout Dartmoor usually appearing as narrow, high angle veins or dykes, though at Meldon the thickness exceeds 20m. The aplite veins often show chilled margins, evidence that they were intruded at a late stage after granite mass had already solidified. Exhumation. The Dartmoor granite was intruded at a relatively shallow depth beneath as little as 8 – 10km of crust when the area was south of the equator. During the Permian and Triassic, as the land mass moved northward, there was rapid exhumation from beneath the semi-arid mountain range which was periodically eroded by flash floods. This led to extensive deposits of red sandstones and breccia on the lower ground. By Jurassic times the roof granite was exposed at surface in places though this was a short-lived situation. Rising sea level and crustal subsidence saw the area gradually inundated so that by the late Jurassic the high points of Dartmoor were a series of islands. Crustal subsidence continued during the Cretaceous until Dartmoor was completely submerged and became covered in an extensive thickness of chalk. During the past 50 million years the ocean retreated, warm sub-tropical conditions prevailed and Dartmoor was rapidly exhumed from beneath the chalk. However, 2 million years ago the ocean briefly returned leaving a shore line which is still visible around Dartmoor at about 200m above present O.D. Weathering and Tor Formation. Weathering of the Dartmoor granite began almost as soon as it had solidified. Greissen formation occurred in places such as at Hemerdon where hydrothermal fluids decomposed the feldspars replacing them with muscovite whilst also depositing metalliferous mineralised veins and stockworks. Mineralised or not these hydrothermal veins can be readily identified on Dartmoor and across the south-west granite as a whole as a result of an unusually high boron content, which manifests as black deposits of tourmaline. The high boron content, relative to granites globally, indicates a mantle component in the magma. [Locality3 Middle Quarry to the north of Haytor Rocks. The main quarry face shows good clean vertical and horizontal joints in an aplite vein. Pieces of aplite are lose on the quarry floor as are clean pieces of blue granite. In the western wall of the quarry is a joint at an inclination of about 35degrees with a black tourmalinised centre and alteration of the granite for up to 3cm on each side.] Vein mineralisation is associated with tin, tungsten, lead, copper and zinc ores at various locations. Later stages of hydrothermal activity were responsible for the kaolinisation of large volumes of feldspar leaving workable china clay deposits in some areas. OUGS 12 Sept, 2010 The Granite and Quarries at Haytor Page 4 Some geologists argue that the intruded magma was at a low temperature whereby it was already highly viscous. This coupled with intrusion at relatively shallow depth and relatively rapid removal of the over-lying crust lead to the formation of 2 joint sets through rapid cooling and decompression.