Chalk and landscape of the South Downs, England

The Upper Cretaceous Chalk hills of the South Downs form the southern flank of the Wealden anticline in south-east England, with older Wealden and Purbeck sediments exposed at its core. With prominent chalk escarpments on each side of it, this major structure is up to 70 km wide, and extends eastwards for over 200 km from eastern Hampshire to the area around Boulogne-sur-Mer in Northern France, dissected by the English Channel.     

The evolution from Miocene potassic to Quaternary sodic magmatism in western Turkey: implications for enrichment processes in the lithospheric mantle

New K-Ar dating and major- and trace-element analyses from the U ak-Selendi-Emet (USE) area constrain the timing of changes in the nature of volcanism in the Miocene in western Turkey. The data reveal a change from dominantly calc-alkaline and silicic in the Early Miocene to largely alkaline and more mafic volcanism in the Middle Miocene. This probably reflects a decreasing amount of crustal contamination with time, a result of extensional tectonics. High levels of various incompatible elements (including K) in the more mafic members, suggest an enriched subcontinental lithospheric source region for the Middle Miocene USE lavas. Highly variable Nb/Y, Ti/Y and Th/Nb ratios suggest a lithospheric mantle heterogeneously enriched by two processes: (1) enrichment by subduction-related processes producing high Th/Nb but low Nb/Y and Ti/Y; and (2) enrichment by small degree melts of depleted upper mantle producing low Th/Nb but high Nb/Y and Ti/Y. Both of these enrichment processes have variably contributed to Middle Miocene K-rich lavas in the USE area. The mechanism which initiated the melting of the enriched lithosphere is considered to be extension which produced decompression melting. Comparisons with the nearby Kula lavas reveals that by the Pliocene to Quaternary, volcanism, although still enriched in incompatible elements, had become sodic. It seems likely that continued extension up to this time thinned the lithosphere to such an extent that asthenospheric melts were produced which ascended and mixed with previously enriched lithosphere.     

Combining CSD and isotopic microanalysis: Magma supply and mixing processes at Stromboli Volcano, Aeolian Islands, Italy

Integrating isotopic microanalysis with other analytical techniques creates powerful new methodologies for understanding the evolution of rock samples at the sub-grain scale. Here we present Crystal Size Distribution (CSD) data for a 26,000 year old sample from Stromboli Volcano and accompanying isotopic microanalysis of the phenocrysts. A technique, called the ICSD plot, is introduced which given stated assumptions allows the integration of both sets of data to generate timelines of isotopic evolution through the volcanic system. The combined approach is powerful, allowing investigation of the magma supply, mixing, crystallisation and contamination processes prior to eruption of a volcanic sample. For Stromboli Volcano, the combined analysis suggests that the change in magma type following a cone collapse took roughly five years to complete, similar to the timescale of changes seen in recent decades.     

Palaeomagnetism of the Tan y Grisiau granite,North Wales: Evidence for a subvolcanic origin in Late Ordovician times

Progressive thermal demagnetization of samples from the Tan y Grisiau granite defines a coherent easterly positive characteristic remanence (D/I = 124.9/60.3°;, 42 samples, R = 40–51, a₉₅ = 4.8°;) residing in magnetite. An ancient reversal of magnetization is recovered in the highest blocking temperature spectrum of a few samples and suggests that a cooling-related dipolar axis is recorded by this pluton. Only facies of the granite which have been reddened, probably by submagmatic streaming, have recorded a stable remanence. Adjustment for tilt yields a very steep remanence (D/I = 193/88°;) incompatible with any known Early Palaeozoic and younger field direction from Britain. The in situ remanence has a similar declination to the primary magnetization in Late Ordovician dolerites from the Welsh Borderlands and yields a comparable palaeolatitude (41.5°;S). It is concluded that the Tan y Grisiau pluton was magnetized in Late Ordovician times after deformation. Folding in this region is therefore interpreted to be substantially of Taconic (Late Ordovician) origin and not Acadian in age. As both in situ and tilt-adjusted remanence directions are incompatible with Silurian and younger palaeofield directions from Britain, the pluton is interpreted as a subvolcanic component of the North Wales igneous province. Large anticlockwise rotation of Avalonia is identified between Late Ordovician and Late Silurian times.     

Ben Nevis and geologically driven changes in elevation

With a great deal of fanfare in March 2016 an Ordnance Survey press release announced 1345 m as the newly rounded height for Ben Nevis, the highest mountain in Scotland. After a series of complex calculations and surveying, new measurements underpinned by geophysical data revealed that the highest point in the United Kingdom had gone up to by only a few cm since 1949, when it was last measured. This time they were able to use GPS readings from satellites maintained by the US military to pin this down to millimetre precision. The height rose to 1344.527 metres (or 4411 feet and 1½ inches), but Britain's national mapping agency does not reveal exactly what the old height was. Oddly the seventh series 1: 63 360 map (Sheet 47‐Glencoe) published in 1960 was still using 4406 feet, 1343 m. This antiquated height was transferred from the 4406.3 ft inscribed on the six‐inch 1 : 10 560 (Inverness‐shire—Mainland sheet CLI) map published in 1902, relative to the Liverpool datum originally used by the Ordnance Survey. This was probably the base of the cairn upon which the triangulation pillar is built, and not even the nearby summit of Ben Nevis itself. Indeed the OS explains in a video on its website why modern 1 : 50 000 editions show two heights, with brackets around that for the natural high point, to make this distinction clear.     

Geology of Marseilles to Italy high‐speed TGV railway line compared to HS2 in Britain

In a radical departure, SNCF, the French state railway company, launched a public consultation in 2016 about the route for the first of a series of piecemeal extensions to the proposed French high‐speed railway (ligne à grande vitesse) LGV network in Provence‐Alpes‐Côte d'Azur. A cheaper proposal to build an inland line, bypassing Marseilles and Toulon, met strong opposition as it would have traversed landscapes made famous by Cézanne's nineteenth century paintings. This alternative offered options for tunnelling under Marseilles, including an underground TGV station due for completion in 2033. Eventually heading eastwards, a line about 35 km long with a lengthy tunnel into the port city of Toulon will be built by around 2040 from Aubagne. Unlike Britain and problems building HS2 bypassing the West Coast railway mainline into London, France has well‐established relatively generous compensation procedures for anybody losing property due to new infrastructure schemes. Also, this SNCF budget is split into many portions, while HS2 has been hampered by poor geological knowledge of likely ground conditions along parts of the route resulting in massive cost overruns. Even after the Oakervee review into how to bring these cost overruns under control gave the green light for construction of HS2 to continue in February 2020, these underlying factors were hardly considered in this technical and economic report.     

Bathymetric and geological maps from the British Antarctic Survey

Since my 2012 feature about British Antarctic Survey (BAS) geological maps, the survey has continued to produce a series of ad hoc maps highlighting the results of their ongoing research, often in collaboration with their international colleagues. Not only has BAS added to their geological mapping series with James Ross Island off the Antarctic Peninsula, they have also produced a map of the bedrock topography of the entire continent, together with two bathymetric and geological settings sheets of the seas to the north of Antarctica. The first oceanographic map focuses on the South Sandwich Islands, some of the most remote United Kingdom overseas territories, roughly 700 km south‐east of South Georgia. Finally, the Drake Passage sheet shows the sea bed morphology from the Falklands and Tierra del Fuego to the South Shetland Islands just off the Antarctic Peninsula, spanning 70–50°W in longitude and 52–63°S in latitude. Below 60° south, all international territorial claims have been frozen by the Antarctic Treaty, which has demilitarized the entire continent since 1961 and ensured it remains untainted by mineral exploration and which has now expanded to a membership of over 50 nations.     

Origin of CFB Magmatism: Multi-tiered Intracrustal Picrite-Rhyolite Magmatic Plumbing at Spitzkoppe, Western Namibia, during Early Cretaceous Etendeka Magmatism

Early Cretaceous tholeiitic picrite-to-rhyolite dykes aroundSpitzkoppe, western Namibia, are part of the extensive HentiesBay–Outjo swarm, penecontemporaneous with 132 Ma Etendekalavas 100 km to the NW. Although only intermediate to rhyoliticdykes contain clinopyroxene phenocrysts, the behaviour of Ca,Al and Sc in the dyke suite shows that liquidus clinopyroxene—togetherwith olivine—was a fractionating phase when MgO fell to9 wt %. Both a plot of CIPW normative di–hy–ol–ne–Qand modelling using (p)MELTS show that a mid-crustal pressureof 0·6 GPa is consistent with this early clinopyroxenesaturation. Sr, Nd, Hf and Pb isotope variations all show trendsconsistent with AFC contamination (assimilation linked to fractionalcrystallization), involving Pan-African Damara belt continentalcrust. The geochemical variation, including isenthalpic AFCmodelling using (p)MELTS, suggests that the picrites (olivine-richcumulate suspensions) were interacting with granulite-faciesmetamorphic lower crust, the intermediate compositions withamphibolite-facies middle crust, and the rhyolitic dykes (anda few of the basalts) with the Pan-African granites of the uppercrust. The calculated densities of the magmas fall systematicallyfrom picrite to rhyolite and suggest a magmatic system resemblinga stack of sills throughout the crust beneath Spitzkoppe, withthe storage and fractionation depth of each magma fraction controlledby its density. Elemental and isotopic features of the 20 wt% MgO picrites (including Os isotopes) suggest that their parentalmelts probably originated by fusion of mid-ocean ridge basalt(MORB) source convecting mantle, followed by limited reactionwith sub-continental lithospheric mantle metasomatized justprior to the formation of the parental magmas. Many of the distinctivefeatures of large-volume picritic–basaltic magmas maynot be derived from their ultimate mantle sources, but may insteadbe the results of complex polybaric fractional crystallizationand multi-component crustal contamination. KEY WORDS: flood basalts; Spitzkoppe; picrite; trace elements; hafnium isotopes; Etendeka     

Evidence of diverse depletion and metasomatic events in harzburgite–lherzolite mantle xenoliths from the Iberian plate (Olot, NE Spain): Implications for lithosphere accretionary processes

Mantle xenoliths from the Olot volcanic district (NE Spain) comprise a bi-modal suite consisting of protogranular spinel lherzolites (cpx 12–14%) sometimes with pargasitic amphibole, and highly refractory spinel harzburgites (cpx ≤ 1%) with coarse-grained granular textures. The lherzolites range from slightly depleted to moderately LREE-enriched with flat HREE patterns between 1.5 and 2.7 × chondrite (Ch). In contrast, the harzburgites are extremely depleted in HREE (down to 0.2 × Ch) and strongly LREE-enriched (La_N/Yb_N = 12.3–17.2). LA-ICP-MS analyses of clinopyroxene and amphibole of the lherzolites highlight variable degrees of LREE depletion (HREE up to 13 × Ch, La_N/Yb_N down to 0.01), with the exception of a single sample in which both clinopyroxene and amphibole are LREE-enriched (La_N/Yb_N up to 19). In the harzburgites, clinopyroxenes display totally different REE distributions, characterized by extreme HREE depletion (down to 0.4 × Ch) and upward convex positively fractionated middle-light REE patterns (Nd_N/Yb_N up to 20.7 × Ch; La_N/Yb_N up to 12 × Ch). Sr–Nd–Hf isotopic data for both whole-rocks and cpx separates, coherently indicate depleted mantle (DM) compositions for the lherzolites (εSr = − 15 to − 26, εNd = + 9 to + 17, εHf = + 18 to + 68) and enriched mantle (EM) compositions for the harzburgites (εSr = − 10 to + 36, εNd = − 1 to − 6, εHf = + 3 to + 8). Modelling of the clinopyroxene REE data and isotopic systematics suggest that some lherzolites were affected by pre-Paleozoic (0.6–1 By) low-degree partial melting processes, while others probably reflect some extent of refertilization of the mantle protolith by metasomatizing melts similar to the Triassic rift-related tholeiites reported from several Pyrenean localities. The harzburgites represent extreme refractory residua, resulting from a complex depletion history due to multistage melt extraction as often observed in the cratonic mantle. The distinctive REE patterns and isotopic systematics of their clinopyroxenes suggest that the harzburgites were formed by the interaction of an ultra-depleted peridotite matrix with highly alkaline basic melts similar in composition to the Permo-Triassic alkaline lamprophyres which are widespread within the Iberian plate. Lherzolites possibly represent younger lithosphere (accreted asthenosphere?) up-lifted and juxtaposed to the older subcontinental lithospheric mantle (harzburgites) during the post-Variscan rifting of the Iberian margin. These two genetically different, but adjoining, mantle domains intimately mingled along the northern Iberian margin during the subsequent plate convergence processes, leading to the close association of harzburgites and lherzolites observed in the Olot mantle xenoliths and in some Pyrenean peridotite massifs.