Land's End Granite

The Land's End Granite is a single intrusion of the Cornubian batholith located on the south-western tip of Great Britain . It took place during the late phase of Variscan orogenesis in the lower Permian in weakly metamorphic sediments of the Upper Devonian .

geography

Coarse-grained CGG granite from Porthcurno

The granite, named after Land's End , underlies almost the entire southwestern tip of Cornwall (Penwith Peninsula) - with the exception of the immediate vicinity of Penzance and an almost 1 kilometer wide coastal strip from Cape Cornwall to St Ives . The northern part, the so-called Zennor lobe , is a rectangle about 20 kilometers long and 7 kilometers wide, rotated in the northeast direction. The likewise rectangular and parallel offset southern part - the St Buryan lobe - measures 10 kilometers in length and 5 kilometers in width. The Land's End Granite thus covers an area of around 190 square kilometers.

geology

Fine-grained FGG granite rocks at Land's End

The Land's End Granite is the youngest of five mainland plutons within the sub-Permian Cornubian batholith. It is a prime example of a peraluminous , anatectic biotite granite of the sedimentary S-type , which nevertheless contains relictual signs of I-magmatism. The granite is mainly of crustal origin.

Towards the end of the Carboniferous , the convergence movements of the Variscan orogen in northwestern Europe had transformed into an east-west trending, dextral, transtensional stretching regime controlled by major northwest-southeast trending faults . In the south-west of England the suture between the Rhenohercynic and the Rheischer Ocean was reactivated while stretching and old Variscan thrusts and north-west-south-east trending transfer faults were moved again. The northern reclining plate of the suture slowly rose to the surface and was successively exhumed. The resulting pressure relief caused the Cornubian batholith and thus also the Land's End Granite to melt and settle at a depth of around 5 to 6 kilometers.

Host rocks

The Land's End Granite intruded metasediments and metavulcanites of the Upper Devonian Mylor Slate Formation , with all structures such as B. breaks, faults, folds and foliations were clearly cut off. Like other Devonian successions in England, the host rocks experienced three deformation phases D 1 (isoclinal folding), D 2 (NNW-verging upright folding) and D 3 (SSO-verging back-folding) during the Carboniferous, which were caused by the north-north-west-south-south-east-oriented Variscan convergence movements had been caused. The deformations were coupled with a low-grade green slate metamorphosis , which had already reached its maximum intensity during D 1 - with 0.3 GPa of pressure and temperatures of 300 ° C. Towards the end of the Carboniferous, the convergence movements then changed into a north-northwest-south-southeast stretch regime, which ultimately enabled the Land's End Granite to penetrate.

The Mylor Slate Formation is a clayey to silty sediment sequence from the Famenne , deposited on the seabed of the Gramscatho Basin , containing pillow lavas and dolerite . Even before it was metamorphically changed by Land's End Granite, it had already experienced the effects of hot hydrothermal solutions on the sea floor. The contact metamorphism produced the mineral associations cordierite - anthophyllite and hornblende - plagioclase in some of the basalts . Lime silicate horn rock was created by the metasomatosis caused by granite intrusion . The main mass of the sediments turned to blotch slate with the mineral paragenesis cordierite + biotite + chlorite ± andalusite.

The host rocks were raised and bulged, which was most likely caused by a distension of the laccolith intrusion. Their scouring (English stoping ) through an outwardly penetrating network of corridors and storage corridors is also documented.

The contact aureole surrounds the granite intrusion at a distance of 2 to a maximum of 3 kilometers. For example, it runs roughly parallel to the intrusion from Penzance to Carbis Bay .

Petrology

Porphyry Coarse Grain Facies CGG at Lamorna Cove

The intrusion is mainly composed of different petrological facies of biotite granites ( G 3 granites). The largest proportion of the area is taken up by a porphyry , coarse-grained G 3a granite ( English Coarse Grained Granite or CGG for short ), which makes up practically the entire southern part and a good 50 percent of the northern part. The southwest half of the northern part is formed by a coarse-grained G 3b granite, which, however, in contrast to the first facies, does not contain any phenocrystals (and is therefore aphyric). This is followed by a medium-grain G 3c granite, also known as MGG . Finally, a fine-grain G 3d granite or FGG appears , divided into a porphyry and an aphyric facies. The main deposits of fine-grained granite are north and west of Penzance, with smaller deposits appearing at Land's End, St Levan and Cape Cornwall.

In addition to these main facies of the early Monzonitic biotite granites, there are also smaller, isolated G 4 granites (tourmaline granites G 4 a and G 4b , lithium mica granite G 4c and a solid, fine-grained quartz tourmaline rock MQT ) as late facies . In addition, an albite microgranite is associated with the early biotite granites.

These different facies are probably to be assigned to different magmas that came into contact with each other and sometimes also mixed (English magma mingling and mixing ). Overall, Land's End Granite is structurally and chemically quite inhomogeneous.

mineralogy

The protrusion on Bosigran Cliff is made of medium-grain granite MGG , which merges into the coarse-grained facies CGG in the hanging wall .

The coarse and medium-grain biotite granites contain large hypidomorphic feldspar phenocrystals ( orthoclase ), which are embedded in a matrix of plagioclase (An _10-33 ), biotite (Mg-siderophyllite), interstitial xenomorphic quartz , alkali feldspar and Fe-polylithionite. Accumulations of cordierite up to 10 millimeters in size can also be present . Tourmaline is the most common accessory mineral, and apatite , ilmenite , monazite , muscovite , ruti , xenotime and zircon are also represented . The alkali feldspars are usually microperthitic and show a Rapakiwi texture. They can also contain plagioclase, quartz and biotite. The concentric inclusions usually follow irregular internal structure boundaries. Tabular phenocrystals made from plagioclase are hypidomorphic to xenomorphic and can reach 4 centimeters. They show typical magmatic growth patterns and often also screen textures . Chemically caused zoning within the plagioclase suggests imbalances during the crystallization process.

The fine-grained granites have the same mineral structure and differ only in their smaller grain size .

Geochemical composition

Main elements

The main elements have the following compositions:

Oxide wt.%	Cross section of the Cornubian batholith	Coarse-grained CGG Lamorna	Fine-grain FGG Castle at Dinas	Grossly aphyrical pendeen	Tourmaline granite Pellitras Point	Lithium mica granite Porth Nanven	MQT Porth Ledden	Albite micro-granite Porthmeor Cove
SiO ₂	72.35	70.46	72.21	73.23	75.52	75.46	76.63	75.29
TiO ₂	0.26	0.38	0.16	0.15	0.10	0.15	0.08	0.04
Al ₂ O ₃	14.52	14.81	14.67	14.13	13.42	13.45	13.43	14.84
Fe ₂ O ₃	0.30	0.32	0.32	0.52	0.97	0.94	6.26	0.49
FeO	1.56	2.19	1.15	0.77	0.36	0.65	0.65	0.43
MnO	0.06	0.07	0.07	0.02	0.01	0.02	0.05	0.02
MgO	0.41	0.65	0.32	0.22	0.05	0.18	0.17	0.10
CaO	0.79	0.85	0.64	0.65	0.29	0.42	0.43	1.30
Na ₂ O	2.96	2.52	3.05	2.69	2.50	2.62	0.70	5.45
K ₂ O	5.12	5.57	4.99	5.24	5.84	4.96	0.14	0.95
P ₂ O ₅	0.25	0.29	0.27	0.31	0.16	0.23	0.25	0.23
H ₂ O ^-		0.22	0.30	0.30
H ₂ O ⁺		1.00	0.90	0.80	0.80	0.81	0.81	0.61

All granites are generally very peraluminous, as their A / CNK ratio is between 1.1 and 1.4. They are also calcareous and belong to the high-K series. Their phosphorus content is low and is less than 0.4 percent by weight P ₂ O ₅ .

The CGG show quite homogeneous SiO ₂ values between 70 and 73 percent by weight, whereas the FGG are scattered and in some cases have very low values between 66 and 73 percent by weight. The CGG's FeO, MgO and CaO contents are lower than those of the FGG .

The G 4 granites differ significantly from the G 3 granites, in particular due to an increased SiO ₂ and Fe ₂ O ₃ content, with simultaneously decreased values of TiO ₂ , FeO, MnO, MgO and CaO. The MQT and the albite micro-granite show extreme differences, the latter being the end stage of an independent magmatic secretion. This geochemical development of the G 3 to the G 4 granites is very nicely expressed in a Zr / Hf diagram and reflects the general trend of crustal zirconium fractionation. Here, too, MQT and albite micro-granite clearly leave this trend due to late magmatic hafnium enrichment.

Even if the characteristics for an S-type granite predominate, which is underlined by numerous old men , pegmatite veins and the tin - tungsten mineralization, in addition to the pereraluminosity, signs of developed granites of the I-type, manifested in hornblende, can still be seen -rich inclusions, in an increased εNd and in an enrichment of copper .

Trace elements

A selection of trace elements is listed here :

Trace element ppm	Cross section of the Cornubian batholith	Coarse-grained CGG Lamorna	Fine-grain FGG Castle at Dinas	Grossly aphyrical pendeen	Tourmaline granite Pellitras Point	Lithium mica granite Porth Nanven	MQT Porth Ledden	Albite micro-granite Porthmeor Cove
Li	280	200	410	450	60	80	60	40
B.	242	220	550	910	600	1000	1000	200
Rb	491	415	484	675	520	592	2	94
Ba	181	280	135	48	17th	26th	2	48
Zr	113	169	68	66	36	66	38	40
Sr	73	97	53	29	15th	18th	7th	139
Ce	63	99	38	33.5	16.9	29.6	5.9	10.8
Cs	50	31	52	48	23.7	29.9	1.3	10.3
Zn	45	67	42	23	40	30th	60	30th
La	30th	42	15.6	13.6	7.1	12.3	2.3	5.8
Pb	28	30th	22nd	16	13.2	10.6	2.7	14.8
Nd	28	47.5	18.8	7th	7.6	13.7	3.1	4.9
Ga	24	22.6	23	25.8	30th	30th	40	20th
Sn	13	9	28	9	100	15th	5	20th
Nb	18th	19th	20th	26th	21st	23	24	27
Th	15.3	25.9	9.3	10.9	6.9	11.2	3.3	4.4
U	12.8	17.9	22.3	6.7	5.5	8.1	4.1	6.4

With the trace elements, the clear differentiation of the granites is reflected in the high contents of rubidium (with the exception of the MQT and the albite micro-granite ), cesium and niobium . The difference between the G 3 granites on the one hand and the G 4 granites (including MQT and albite micro-granite ) on the other is very clearly expressed in them . In general, a clear decrease in the concentrations can be observed; exceptions to this behavior are, for example, the concentration increases of boron (tourmalinization), gallium and niobium. The zinc and tin values remain roughly the same, but the abnormally high tin value of the tourmaline granite from Pellitras Point is remarkable. The MQT and the albite micro-granite behave very differently to these fractionations . The latter is also the only rock to show a positive europium anomaly.

Isotope ratios

Isotope ratio	CGG	FGG	Albite micro-granite
⁸⁷ Sr / ⁸⁶ Sr	0.70944	0.70877	0.71307
¹⁴³ Nd / ¹⁴⁴ Nd	0.512144	0.512209	0.512090

The isotope ratios ⁸⁷ Sr / ⁸⁶ Sr and ¹⁴³ Nd / ¹⁴⁴ Nd indicate a sequence starting from FGG via CGG to albite microgranite.

structure

Porthmeor Cove with tourmaline granite in the lying wall and Hornfels in the hanging wall, pierced by white albite micro-granite dikes

The coarse and medium-grain facies are characterized by feldspar phenocrystals that show strong variations in size and mode. The igneous structure can be recognized with the naked eye due to the 2 to 4 centimeters large idiomorphic alkali feldspar crystals. The latter are usually more or less the same size, but can also assume a bimodal distribution - with giant crystals up to 20 centimeters in length and a remaining population of 2 centimeters. The crystals usually assume a preferred adjustment. More recent studies by Müller and colleagues suggest that the giant feldspars crystallized very early as xenocrystals at higher temperatures from a poorly differentiated magma.

Age

Previous radiometric surveys of the Land's End Granite have indicated ages from the Kungurian . They range from 277.1 ± 0.4 to 274.4 ± 0.4 million years. A xenotime sample even delivered 279.3 ± 0.4 million years as the oldest known date. The intrusions thus extend over around 5 million years. In relation to the Cornubian batholith, the age of which covers 298.3 to 272.3 million years and which was put together over a very long period of 26 million years, the Land's End Granite represents one of its last magma pulses.

Emergence

Contact between Mylor Slate Formation and CGG at Tater-du

It can no longer be doubted that the Land's End Granite is a very complex intrusion. A large number of newly discovered internal contacts confirm that a sequence of different granitic magmas was involved in its formation. The radiometric ages, for their part, show several stages in the taking place and the subsequent cooling of the pluton. All of this contradicts earlier model ideas , which assumed a diapiric penetration from one or more separate magma chambers. The variability of the contacts indicates the extent of the physical and chemical interactions between the magmas involved. Although it provides ideas about the mechanisms involved in taking a seat, it cannot provide any information about the ascent paths of magmas through the earth's crust.

The general consensus is now moving towards other seating mechanisms than just diapirism. Many new interpretations contemplate gait ascent and layered seating. In the work of Pownall and colleagues (2012), a laccolithic layer body 3 kilometers thick is assumed that penetrated at a depth of about 5.5 kilometers. The physical conditions of the intrusion were 0.15 ± 0.1 GPa pressure load coupled with temperatures of (615 ± 50) ° C.

Contact relationships

View from Porth Ledden over Cape Cornwall, which is made up of Hornfels spotted schist. Contact with the CGG is made in the bottom right corner of the picture .

Due to its sometimes complex internal storage and contact conditions, the Land's End Granite provides important insights into this problem. In comparison to the structure of the host rock (e.g. igneous foliation), contacts of the intrusive rock can be concordant or discordant and can be sharp or blurred or gradual, flat or irregular (bulging, wavy and nebulous). Fault contacts can also be observed. Flat contacts that sever previous foliations or grain boundaries suggest complete solidification of the host. Flat, but more indistinct contacts indicate incomplete solidification of the host below the critical melt fraction (English Critical Melt Fraction or CMF ). Diffuse or gradual contacts suggest physical and / or chemical interactions and at the same time require a higher content of residual melt and thus a lower crystallinity in the precursor. Wavy or irregular contacts are explained by the coexistence of two magmas above their CMF . They are usually sharp, but can also appear diffuse in part and then indicate a certain interaction between the two magmas. All these forms of contact (except for the razor-sharp contacts) are an indication of the presence of Kristallbreien (English crystal mushes ) - whereby the respective type of contact information on the rheological state provides (stiffness) of these pulps.

Land's End Granite manifests all forms of contact, above all corridors and layer intrusions. From them it can be seen that the precursors were already solidified or that their crystallization was in an advanced stage. However, irregular shapes such as B. in the FGG at Treen Cliff, which suggest that a fluid magma pulse penetrated a still fluid magma. In this case, the intrusive magma pulse, which is only influenced by the viscosity of the host magma, can definitely assume a diapir-like shape.

Conclusion

The Land's End Granite is built up from a complex sequence of intrusive magma pulses. In general, three stages can be distinguished:

Megacrystalline fine-grain biotite granites FGG
Megacrystalline coarse-grained biotite granites CGG
Fine to medium-grain lithium mica granite.

These three main units form layers in the tens to hundreds of meters, which are fed by corridors. The early FGG are completely absorbed by the later units and form inclusions in them in the centimeter to hundred meter range. Small inserts below 300 meters made of tourmaline granite and MQT are viewed as differentiates of the lithium mica granite . Tourmaline granite layers also occur at the contacts to the biotite granites - with the formation of crest textures . The granites are covered in places by horn rocks of the Mylor Slate Formation and develop ridge textures here too. The younger intrusive rocks are intense fractionation in place and by pressing be filtered (English filter pressing , leading) characterizes an accumulation of boron in the dome area. The tourmaline granites developed a significant vapor phase which was most likely responsible for the transport and deposition of the metals from St Just. The MQT are the deaf end product of the separation and immiscibility of igneous phases during the transition into the hydrothermal area. This can be seen in the compositional zoning of quartz and tourmaline as well as in melt inclusions. The albite micro-granite represents an independent, less differentiated magma development.

It can be assumed that the laccolithic layers were fed by tunnels, which in turn had access to a magma reservoir located on the border between the lower crust and the upper mantle. This explains the simultaneous S and I characteristics of Land's End Granite.

Mineralization

The Botallack Mine at St Just

The Land's End Granite was mineralized in its late stage. The tin - copper - arsenic deposit near St Just in its western section is of importance . Lithium mica granite, related tourmaline granites and massive quartz tourmaline rock are considered the source of mineralization. The ore elements had accumulated in the hot gas or vapor phase and were then transported further hydrothermally and finally deposited in a so-called emanative center west of St Just .

literature

DB Clarke: Granitoid Rocks . Chapman and Hall, 1992.
PA Floyd, CS Exley and MT Styles: Igneous Rocks of South-West England . In: Geological Conservation Review Series, Joint Nature Conservation Committee London . Chapman & Hall, 1993.
SP Hughes, RJ Stickland, RK Shail, NG LeBoutillier, AC Alexander and M. Thomas: The chronology and kinematics of late Paleozoic deformation in the NW contact metamorphic aureole of the Land's End Granite . In: Geoscience in South-West England . v. 12, 2009, p. 140-152 .
AR Müller et al: The magmatic evolution of the Land's End pluton, Cornwall, and associated pre ‐ enrichment of metals . In: Ore Geol. Rev. Band 28 (3) , 2006, pp. 329-367 , doi : 10.1016 / j.oregeorev.2005.05.002 .
Jonathan M. Pownall, David J. Waters, Michael P. Searle, Robin K. Shail and Laurence J. Robb: Shallow laccolithic emplacement of the Land's End and Tregonning granites, Cornwall, UK: Evidence from aureole field relations and PT modeling of cordierite -anthophyllite hornfels. In: Geosphere . v. 8th; no. 6, 2012, p. 1467-1504 , doi : 10.1130 / GES00802.1 .
EB Selwood, EM Durrance and CM Bristow: The Geology of Cornwall . University of Exeter Press, 1998, ISBN 978-0-85989-432-6 .
GK Taylor: Pluton shapes in the Cornubian Batholith: New perspectives from gravity modeling . In: Geological Society (London) Journal . v. 164, no.3, 2007, p. 525-528 , doi : 10.1144 / 0016-76492006-104 .

Individual evidence

^ ^A ^b A. JJ Goode and RT Taylor: Geology of the area around Penzance . In: Memoire of the British Geological Survey, sheets 351 and 358 (England and Wales) . 1988.
^ BJ Chappell and R. Hine: The Cornubian Batholith: An example of magmatic fractionation on a crustal scale . In: Resour. Geol. Band 56 , 2006, p. 203-244 .
↑ ^a ^b ^c A. R. Müller et al .: The magmatic evolution of the Land's End pluton, Cornwall, and associated pre ‐ enrichment of metals . In: Ore Geol. Rev. Band 28 (3) , 2006, pp. 329-367 , doi : 10.1016 / j.oregeorev.2005.05.002 .
↑ PA Ziegler and P. Dezes: Crustal evolution of western and central Europe . In: DG Gee and AA Stephenson, European lithosphere dynamics (Eds.): Geological Society of London Memoirs . tape 32 , 2006, pp. 43-56 .
↑ RK Shail and BE Leveridge: The Rhenohercynian passive margin of SW England: Development, inversion and extensional reactivation . In: CR ACAD SCI IIA . tape 341 , 2009, p. 140-155 .
^ PR Rattey and DJ Sanderson: The structure of SW Cornwall and its bearing on the emplacement of the Lizard Complex . In: J. Geol. Soc. London . tape 141 , 1984, pp. 87-95 .
↑ RK Shail and JJ Wilkinson: late- to post-Variscan extensional tectonics in south Cornwall . In: Proceedings of the Ussher Society . tape 66 , p. 263-272 .
↑ S. Salmon: Mingling between coexisting granite magma within the Land's End granite-preliminary observations . In: Proceedings of the Ussher Society . tape 8 , 1994, pp. 219-223 .
↑ AH Clark et al: Refinement of the time / space relationships of intrusion and hydrothermal activity in the Cornubian batholith . In: Abstracts Volume, Ussher Society Annual Meeting . Minehead 1994.
^ T. Powell, S. Salmon, AH Clark and RK Shail: Emplacement styles within the Land's End Granite, west Cornwall . In: Geoscience in south-west England . tape 9 , 1999, p. 333-339 .
↑ DR Baker: Granitic melt viscosity and dike formation . In: Journal of Structural Geology . tape 20 , 1998, pp. 1395-1404 .
↑ Jonathan M. Pownall, David J. Waters, Michael P. Searle, Robin K. Shail and Laurence J. Robb: Shallow laccolithic emplacement of the Land's End and Tregonning granites, Cornwall, UK: Evidence from aureole field relations and PT modeling of cordierite-anthophyllite hornfels. In: Geosphere . v. 8th; no. 6, 2012, p. 1467-1504 , doi : 10.1130 / GES00802.1 .
^ DB Clarke: Granitoid Rocks . Chapman and Hall, 1992.

[Goode-1] A ^b A. JJ Goode and RT Taylor: Geology of the area around Penzance . In: Memoire of the British Geological Survey, sheets 351 and 358 (England and Wales) . 1988.

[2] BJ Chappell and R. Hine: The Cornubian Batholith: An example of magmatic fractionation on a crustal scale . In: Resour. Geol. Band 56 , 2006, p. 203-244 .

[Müller-3] A. R. Müller et al .: The magmatic evolution of the Land's End pluton, Cornwall, and associated pre ‐ enrichment of metals . In: Ore Geol. Rev. Band 28 (3) , 2006, pp. 329-367 , doi : 10.1016 / j.oregeorev.2005.05.002 .

[4] PA Ziegler and P. Dezes: Crustal evolution of western and central Europe . In: DG Gee and AA Stephenson, European lithosphere dynamics (Eds.): Geological Society of London Memoirs . tape 32 , 2006, pp. 43-56 .

[5] RK Shail and BE Leveridge: The Rhenohercynian passive margin of SW England: Development, inversion and extensional reactivation . In: CR ACAD SCI IIA . tape 341 , 2009, p. 140-155 .

[6] PR Rattey and DJ Sanderson: The structure of SW Cornwall and its bearing on the emplacement of the Lizard Complex . In: J. Geol. Soc. London . tape 141 , 1984, pp. 87-95 .

[7] RK Shail and JJ Wilkinson: late- to post-Variscan extensional tectonics in south Cornwall . In: Proceedings of the Ussher Society . tape 66 , p. 263-272 .

[8] S. Salmon: Mingling between coexisting granite magma within the Land's End granite-preliminary observations . In: Proceedings of the Ussher Society . tape 8 , 1994, pp. 219-223 .

[9] AH Clark et al: Refinement of the time / space relationships of intrusion and hydrothermal activity in the Cornubian batholith . In: Abstracts Volume, Ussher Society Annual Meeting . Minehead 1994.

[10] T. Powell, S. Salmon, AH Clark and RK Shail: Emplacement styles within the Land's End Granite, west Cornwall . In: Geoscience in south-west England . tape 9 , 1999, p. 333-339 .

[11] DR Baker: Granitic melt viscosity and dike formation . In: Journal of Structural Geology . tape 20 , 1998, pp. 1395-1404 .

[12] Jonathan M. Pownall, David J. Waters, Michael P. Searle, Robin K. Shail and Laurence J. Robb: Shallow laccolithic emplacement of the Land's End and Tregonning granites, Cornwall, UK: Evidence from aureole field relations and PT modeling of cordierite-anthophyllite hornfels. In: Geosphere . v. 8th; no. 6, 2012, p. 1467-1504 , doi : 10.1130 / GES00802.1 .

[13] DB Clarke: Granitoid Rocks . Chapman and Hall, 1992.