Tregonning granite
The Tregonning Granite is a small composite Permian intrusion of the Cornu Bischen batholiths on the south coast of Cornwall .
etymology
The Tregonning Granite was named after its type locality , the 194 meter high Tregonning Hill .
geography
The Tregonning Granite is roughly the size of an oval turned in the south-south-east direction, which is 3.4 kilometers at its widest point and 5 kilometers along its longitudinal axis. The Godolphin Granite , which is a maximum of 1.2 kilometers wide and 4.4 kilometers long, has penetrated its center. Ashton is at the heart of the intrusion that reaches the south coast at Trewavas Head , along which the granite is exposed for over two miles.
geology
The Tregonning Granite had penetrated into the metapelite of the Mylor Slate Formation in the late phase of the Variscan Orogeny in Unterperm , which were transformed into Hornfels by contact metamorphism with the formation of andalusite and cordierite . In its roof area, the granite developed an approximately 25-meter-thick banded complex of leukogranites , aplites and pegmatites , which covers the entire intrusion. This lithium- enriched roof complex is likely to represent a leucocratic differentiate separated on the spot. On the eastern edge of the intrusion of the roof complex at sending Megiliggar Rocks sills along the foliation in the host rocks.
Inland, the Tregonning Granite is only very poorly exposed, as the rock here formed and kaolinized greisen - with the entire granite being replaced by kaolin in places .
The Godolphin Granite in the center is a biotite granite and is regarded as a precursor intrusion - confirmed by the inclusion of Godolphin Granite in the Tregonning Granite and by a coastal outcrop in which topaz granite veins cross a biotite granite dike. The actual contact between the two granites is nowhere to be seen because of the poor exposure conditions.
It is believed that the Tregonning Granite continues at not too great a depth over narrow ridges to the west towards Land's End Granite and to the north towards Carnmenellis Granite . This fact should be reflected in the current topographical depression between Godolphin Hill and Tregonning Hill.
Petrology
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Petrologically, the Tregonning Granite is a fine to medium-grain lithium mica or topaz granite of the G 5 type. It does not contain any megacrystals and has the same grain. Its minerals are orthoclase (Or 95 from 5 ), microcline , albite (Or 0.6 from 99.2 to 0.2 ), quartz , the lithium mica zinnwaldite and, as an accessory, topaz, amblygonite , tourmaline (Schörl), triplite , apatite ( Fluorapatite), fluorite and lollingite . Muscovite and lepidolite occur in the roof complex, which otherwise shows the same mineralogy, but can reach enormous grain sizes of up to 30 centimeters in pegmatitic layers .
Geochemical composition
| Oxide wt.% |
Cornubian batholith intersection |
Tregonning | Roof complex | Godolphin |
|---|---|---|---|---|
| SiO 2 | 72.35 | 71.13 | 72.52 | 72.28 |
| TiO 2 | 0.26 | 0.07 | 0.03 | 0.28 |
| Al 2 O 3 | 14.52 | 15.91 | 15.53 | 14.88 |
| Fe 2 O 3 | 0.30 | 1.39 | 0.73 | 2.18 |
| FeO | 1.56 | |||
| MnO | 0.06 | 0.06 | 0.08 | 0.02 |
| MgO | 0.41 | 0.10 | 0.07 | 0.38 |
| CaO | 0.79 | 0.60 | 0.31 | 0.57 |
| Na 2 O | 2.96 | 3.79 | 4.93 | 2.42 |
| K 2 O | 5.12 | 5.12 | 5.01 | 4.96 |
| P 2 O 5 | 0.25 | 0.50 | 0.45 | 0.27 |
| H 2 O - | ||||
| H 2 O + |
The Tregonning Granite is a peraluminous topaz granite of the S-type with A / CNK ≥ 1.1. Its geochemical composition clearly differs from the average values in the Cornubian batholith and also from the neighboring Godolphin Granite. Its contents of Al 2 O 3 , Na 2 O and P 2 O 5 are increased, whereas TiO 2 and MgO are decreased. The trace elements show a depletion of zirconium , generally low levels of rare earths , a clear europium anomaly and an enormous accumulation of niobium (79 ppm) and especially rubidium (1530 ppm). These facts can in no way be explained by a continued differentiation of the G 1 - G 2 granites or the G 3 - G 4 granites in the Cornubian batholith. The G 5 granite thus represents a completely unique type of magma. The partial melting of a biotite- rich restite that was left in the magmas of G 1 and / or G3 is now used as an explanation . Its melting was facilitated by granulite facial fluids of the lower crust. The topaz granite also differs significantly from intraplate granites of the A-type.
Age
Darbyshire and Shepherd (1987) dated the Tregonning Granite 280 million years ago in the Artinskian .
literature
- B. Simons, Robin K. Shail and Jens C.Ø. Andersen: The Petrogenesis of the Early Permian Variscan granites of the Cornubian Batholith - lower plate post-collisional peraluminous magmatism in the Rhenohercynian Zone of SW England . In: Lithos . 2016, doi : 10.1016 / j.lithos.2016.05.010 .
- M. Stone: Structure and petrology of the Tregonning-Godolphin granite, Cornwall. In: Proc. Geol. Assoc. tape 86 , 1975, pp. 155-70 .
Individual evidence
- ^ M. Stone: Structure and petrology of the Tregonning-Godolphin granite, Cornwall. In: Proc. Geol. Assoc. tape 86 , 1975, pp. 155-70 .
- ^ AV Bromley: Field guide to the Cornubian Orefield . In: Sixth international congress on waterrock interaction . International Association of Geochemistry and Cosmochemistry, Malvern 1989.
- ^ JR Hawkes and J. Dangerfield: Petrological accounts of the igneous rocks in the Land's End District . In: Mineralogy and petrology report No. 86/31 . British Geological Survey, 1986.
- ^ P. Golley and R. Williams: Cornish mineral reference manual . Endsleigh Publications, Truro 1995, pp. 71 .
- ↑ B. Simons, Robin K. Shail and Jens C.Ø. Andersen: The Petrogenesis of the Early Permian Variscan granites of the Cornubian Batholith - lower plate post-collisional peraluminous magmatism in the Rhenohercynian Zone of SW England . In: Lithos . 2016, doi : 10.1016 / j.lithos.2016.05.010 .
- ^ DPF Darbyshire and TJ Shepherd: Chronology of magmatism in south-west England: the minor intrusions . In: Proceedings of the Ussher Society . tape 6 , 1987, pp. 431-438 .