Mantle magmas passing through an anatectic complex: textural and compositional evidence from lamproite micas, an example from the Neogene Volcanic Province of southeast Spain

Actualmente cobra cada vez más interés la evaluación de los componentes manto y corteza en la petrogénesis ígnea. En este contexto las rocas de la región neógena volcánica del sureste de España representan un excelente ejemplo de interacciones entre magmas mantélicos y derivados corticales. La caracterización de estos procesos en toda su magnitud se realiza mediante detallados estudios mineralógicos. En este trabajo mostramos cómo las micas de las rocas ultrapotásicas de Zeneta ponen de manifiesto una compleja interacción entre magmas lamproíticos y un complejo anatectico, evidenciado en las variaciones composicionales (Al2O3, FeOt y TiO2) de flogopitas y biotitas. Estudios a escala mineral cobran una gran relevancia para el desarrollo de modelos petrogenéticosA topic of particular current interest is the evaluation of mantle and crustal components in igneous petrogenesis. In this context, the rocks of the Neogene Volcanic Province of southeast Spain provide an excellent example of interaction between mantle and crustal components. The characterization of such processes, at all scales, is facilitated by detailed mineralogical studies. Here we conclude that micas from Zeneta ultrapotassic rocks reveal a complex interaction between lamproitic magmas and an anatectic complex. This is shown in compositional variations (Al2O3, FeOt y TiO2) in both phlogopites and biotites. Mineral-scale studies clearly have important implications for petrogenetic model

Zircon as a tracer of plumbing processes in an active magmatic system: insights from mingled magmas of the 2010 dome collapse, Montserrat, Lesser Antilles Arc, Caribbean

This project has received funding from the European Union's Hori-zon 2020 research and innovation programme under the Marie Skodowska-Curie grant agreement No. 749611 (JHS) . We also ac-knowledge funding from the Natural Environment Research Council Isotope Geoscience Facilities Steering Committee, grant IP-1746-1117 (JB) . The work has been financially supported by the Spanish grant CGL 2017-84469-P (JHS) . Funding for open access charge: Universidad de Granada/CBUA. Alexander Varychev at the University of Heidelberg, Germany, and Jeremy Rushton at the British Geological Survey, U.K., are thanked for help with analytical work. Jose Luis Macias is thanked for editorial handling. We are obliged to two anonymous reviewers for the time and effort they took to help us improve the clarity and rigour of our interpretations. 19Soufriere Hills Volcano, Montserrat, erupted from 1995 to 2010, with activity including dome growth, destructive pyroclastic density currents and Vulcanian explosions. Monitoring data, such as gas emissions, show the system is still in a state of unrest. The recent eruptions provide an opportunity to study, in real time, a complex subduction-related subvolcanic transaustal melt-mush reservoir, its magma fluxes, and the timing of crystal and melt storage prior to eruptive paroxysms. How and when mush destabilisation occurs prior to volcanic eruptions continues to be a question of intense debate. Evidence of mafic magma intrusion, a potential eruptive trigger, is preserved in enclaves with quenched and diffuse margins that are mingled with crystal-rich andesite. Here, in this first study of Soufriere Hills Volcano zircon, we report zircon ages and compositions for mafic-intermediate enclaves and host andesites from the most recent dome collapse in 2010 to place temporal constraints on magma reservoir processes. Zircon U-238-Th-230 ages disequilibrium crystallisation ages ranging between c. 2-250 ka constrain the longevity of the magmatic plumbing system. Uniform Hf isotopes, epsilon Hf 11.3 +/- 12 to 14.6 +/- 1.5, indicate invariant compositions that are typical for island arc magma sources. Zircon trace element concentrations and Ti-in-zircon crystallisation temperatures indicate crystallisation in isolated, small-volume, lenses with variable fractions of melt of heterogeneous compositions. We suggest amalgamation of assorted crystal cargoes from these lenses occurred prior to eruption during mush destabilisation triggered by mafic magma recharge. Zircon textures, on the other hand, shed light on recent centimetre-scale magma mingling immediately prior to eruption. Euhedral-subhedral zircon is preferentially preserved in or near quenched contacts of the least-evolved enclave and host andesite. By contrast, reheating of the andesite by the mafic magma recharge in the presence of zircon-undersaturated melts promoted zircon resorption. This led to the formation of subhedral-anhedral corroded zircon that is typical in the host andesite mush. Zircon thus reveals processes ranging from 100,000s of years of andesite storage to short-term partial destruction in response to transient heating and magma mixing events.European Commission 749611Natural Environment Research Council Isotope Geoscience Facilities Steering Committee IP-1746-1117Spanish Government CGL 2017-84469-PUniversidad de Granada/CBU

Zircon double-dating, trace element and O isotope analysis to decipher late Pleistocene explosive-effusive eruptions from a zoned ocean-island magma system, Ascension Island

In this first detailed study of zircon from Ascension Island, South Atlantic, we take a novel approach combining trace element and O isotope compositional data with double-dating (disequilibrium 238U–230Th and (U–Th)/He) to decipher timescales and dynamics of magmatic processes. The Echo Canyon (EC) sequence comprises small-volume explosive-effusive eruptions of trachyte that tapped a compositionally zoned magma system. Associated volcanic hazards may be constrained from the age of volcanism, duration of magma storage, and magma source and plumbing system character. Zircon U–Th–Pb dating of lithic lava clasts has revealed recurrent evolved volcanism at 1.34 and 0.6 Ma, and 95 ka. The (U–Th)/He zircon cooling ages indicate that most of the EC explosive-effusive sequence erupted in a brief episode at ca. 95 ka. Additionally, uniform 238U–230Th zircon crystallisation ages suggest moderately protracted magma storage with melt present at depth for at most 103–104 years before eruption. The enriched character of zircon trace element compositions, relative to MORB, in the absence of a continental crustal signature in the oxygen isotope values (δ18O range 2.67–5.63‰), suggests the presence of an enriched component in the EC magma source. Furthermore, low δ18O zircon compositions imply assimilation of high temperature hydrothermally altered country rock by the source magma. The mineral assemblage in crystal-poor pumices indicates equilibrium storage conditions: zircon saturation and Ti-in-zircon crystallisation temperatures are consistent with alkali feldspar-melt temperatures. Significantly, zircon crystals were preserved both as macrocryst inclusions and in the groundmass of EC explosive and effusive deposits. These rocks preserve evidence of magma evolution by fractional crystallisation. This process led to pre-eruptive compositional stratification, which is evidenced in the range of whole-rock major and trace element compositions and zircon Zr/Hf values. Notably, zircon crystallisation and cooling ages derived from pumice, lava, and accidental lithic lava clasts in highly explosive pyroclastic deposits, have revealed episodes of evolved magmatism that would otherwise have gone undetected. In addition, the zircon trace element and isotope compositions, in combination with the range of crystallisation ages, evidence progressively deeper tapping of less evolved magma stored in discrete lenses. Thus, a combined zircon geochronological-geochemical approach can place constraints on the ca. 0.6 Ma recurrence of past explosive-effusive pulses of millennial to decamillennial duration and their enriched magma sources. This information is relevant for assessing hazards and informing monitoring and forecasting efforts to assist in managing associated risks for small ocean island volcanoes with particularly vulnerable populations and infrastructure

K-rich mantle metasomatism control of localization and initiation of lithospheric strike-slip faulting

A conceptual model is proposed where bulk transtension, or local transtension during bulk simple shear (resulting from mantle anisotropy- or lithosphere rheology contrasts), of heterogeneously enriched lithospheric mantle, trigger localised K-rich magmatism, which focusses strain and causes nucleation of lithosphere-scale transtensional or strike-slip shear zones. Transtension-triggered magmatism is most likely to be located at sites of maximum metasomatism of the lithospheric mantle. Magma-generated fractures propagate upwards, nucleating zones of lithospheric weakness, which focus shear in narrow transcurrent faults or at basin margins. In this way, magmatism controls fault timing and location. Although volcanism will be coeval with fault development and volcanoes will appear fault-controlled, counterintuitively, our model suggests that faults are, in a sense, volcano-controlled. We suggest that this new transtension–K-rich magmatism–transcurrent faulting association represents a hitherto unrecognised genetic relationship as significant as, for example, the ocean island magma series

Petrogenesis of the tertiary lavas of the Isle of Skye, N. W. Scotland

The Tertiary lavas of Northern Skye, N.W. Scotland comprise a pile of flat-lying, predominantly basaltic, volcanic flows. The lavas are the earliest products of the igneous activity on Skye, later manifestations including the gabbroic Cuillin complex and the Red Hills granites. The activity occurred from approximately 65 Ma to 50 Ma (Palaeocene to Eocene) within a continental environment. The lava pile can be divided compositionally into three magma-types, the Skye Main Lava Series (SMLS), lavas of which constitute the majority of the pile, and the less abundant Preshal Mhor (PM) and Fairy Bridge (FB). This study concerns the petrogenesis of the three magma-types. The three magma-types have a normal basaltic mineralogy; phenocrysts include olivine, plagioclase and sparse clinopyroxene; groundmasses comprise varying proportions of olivine, plagioclase, clinopyroxene, and titanomagnetite. A typical flow comprises four zones: a basal amygdaloidal layer, a massive central portion possibly showing columnar jointing, a purple rotted amygdaloidal layer, and a capping red or brown bole. The fractional crystallisation of the SMLS w#s a two-stage process initially involving the precipitation of olivine (± minor Cr-spinel), and later fractionation of olivine and plagioclase ± clinopyroxene. The majority of the lavas assimilated some lower crustal Lewisian granulite en route to the surface. The most basic lavas are the most contaminated. Major and trace element modelling suggests that the SMLS magmas were generated by 15 % melting, at an above-average mantle potential temperature, within the spinel-garnet transition zone at a depth of - 100 km. The magmas subsequently last equilibrated with mantle host rocks at 15 kb (- 45 km).</p

A mineralogical quantitative X-Ray diffraction study of potassic volcanic rocks from the Neogene volcanic region of southeast Spain: ‘anomalous lamproites’

Dadas las características petrológicas de las rocas volcánicas, es de gran utilidad su estudio mineralógico mediante difracción de Rayos-X (DRX), presentando aquí un nuevo, rápido y económico método de cuantificación. De esta manera se caracterizaron las rocas potásicas de Zeneta y La Aljorra, localizadas en la región volcánica neógena del sureste de España. Las proporciones modales de ambos afloramientos dadas por DRX muestran una excelente correlación con las obtenidas por métodos petrográficos tradicionales. Los datos obtenidos indican que las rocas potásicas de La Aljorra muestran más afinidad lamproítica que las de Zeneta, considerando a estas últimas como ‘lamproitas anómalas’. Nuestros resultados sugieren una aplicabilidad general del nuevo método descrito a estudios petrológicosGiven the petrological characteristics of volcanic rocks X-Ray diffraction (XRD) study of their mineralogy is particularly useful. Here we present a new, fast and economical XRD quantification method that was used to characterize the potassic rocks from Zeneta and La Aljorra, Neogene volcanic region of southeast Spain. The XRD modal proportions for both outcrops show a clear correlation with those obtained using traditional petrographic methods. The data indicate that the potassic rocks from La Aljorra have a stronger lamproitic affinity than those from Zeneta, the latter may be considered ‘anomalous lamproites’. Our results suggest a general applicability of the new method described to petrological studie

Ophiolite obduction pulses as a proxy indicator of superplume events?

A major new synthesis of ophiolite geochronology and map of global Phanerozoic distribution indicates that previously noted pulses of ophiolite obduction can be linked to superplume-related tectonism. A marked cyclicity is evident in obduction ages obtained from minerals in ophiolite metamorphic soles and obduction-related minor intrusions. This episodicity is in phase with periods of predominantly uniform polarity of the geomagnetic field, formation of massive carbon-rich deposits, sea-level highstands, and formation of large flood basalt provinces; all generally considered to be superplume proxy indicators. A key to interpreting ophiolite obduction as a further proxy is the mid-Cretaceous, superplume-associated, ocean-margin compressional deformation. During this event, thermal rejuvenation and increased buoyancy of ocean lithosphere caused arc-terrane collision, marginal-basin shoaling, back-arc basin closure and ophiolite obduction. Convergent margins were placed in compression with increased coupling between subducting and overriding plates resulting in major ocean margin deformation. Being directly datable and tectonically and petrologically distinctive makes ophiolite assemblages of considerable use for identifying superplume events

Central volcanoes as sources for the Antarctic Peninsula Volcanic Group

From at least the Early Jurassic to the Miocene, eastward subduction of oceanic crust took place beneath the Antarctic Peninsula. Magmatism associated with the subduction generated a N-S linear belt of volcanic rocks known as the Antarctic Peninsula Volcanic Group (APVG), and which erosion has now exposed at about the plutonic/volcanic interface. Large central volcanoes from the APVG are described here for the first time. The structures are situated in north-west Palmer Land within the main Mesozoic magmatic arc. One centre, Zonda Towers, is recognized by the presence of a 160 m thick silicic ignimbrite, containing accidental lava blocks up to 25 m in diameter. This megabreccia is interpreted as a caldera-fill deposit which formed by land sliding of steep caldera walls during ignimbrite eruption and deposition. A larger centre, Mount Edgell-Wright Spires, is dominated by coarse-grained debris flow deposits and silicic ignimbrites which, with minor lavas and fine-grained tuffs, form a volcanic succession some 1.5 km thick. Basic intermediate and silicic sills c. 50 m thick intrude the succession. A central gabbro-granite intrusion is interpreted to be a high-level magma chamber of the Mount Edgell volcano