Pseudoleucite syenites at Loch Borralan, Scotland: Petrology and a genetic model

2020 ◽  
Vol 58 (5) ◽  
pp. 597-623
Author(s):  
Rebekka Reich ◽  
Michael A. W. Marks ◽  
Thomas Wenzel ◽  
Gregor Markl
Keyword(s):  
Genetic Model ◽  
Alkali Feldspar ◽  
Host Magma ◽  
Rock Types ◽  
Enriched Mantle ◽  
Primitive Magma ◽  
Thrust Zone ◽  
Magma Compositions ◽  
Low Pressures ◽  
Subsequent Decomposition

ABSTRACT The alkaline Loch Borralan intrusion (Assynt Region, NW Highlands of Scotland) consists of a composite arrangement of several ultramafic to felsic plutonic rock bodies which were emplaced around 430 Ma into the Moine Thrust Zone during the Caledonian Orogeny. Some of the Loch Borralan rocks are ultrapotassic and contain pseudoleucite, i.e., a pseudomorph of alkali feldspar and nepheline after leucite. In total, 25 samples have been investigated, representing garnet-bearing pseudoleucite syenites and accompanying rock types such as nepheline-garnet-bearing syenites, alkali feldspar syenites, an amphibole syenite, a biotite-clinopyroxene syenite, and calcite-bearing glimmerites. Pseudoleucite is always associated with garnet, biotite, orthoclase, and minor clinopyroxene and titanite. Mineral chemical data indicate rather primitive magma compositions with no major differences between the various investigated main rock units. The abundant occurrence of up to 2 cm large, mostly euhedral pseudoleucite crystals and petrological phase considerations suggest that magmatic leucite physically separated from its host magma as a flotation cumulate. Based on our data and a comparison with previous field-based and experimental work, K-rich basanitic to tephriphonolitic melts that originated from a K-enriched mantle source may be parental to these rocks. The high liquidus temperatures at low pressures (e.g., ∼1100 °C at 1 bar PH2O) required to crystallize leucite could have resulted from the ascent of successive melt batches in a composite intrusion. Later melt batches would increase the temperature in earlier, already partially cooled batches, causing an increase in temperature and a decrease in pressure during ascent. The subsequent decomposition of leucite to pseudoleucite is interpreted to result from either dry breakdown or autometasomatism, i.e., involvement of late-magmatic fluids.

scholarly journals Fertility of crustal rocks during anatexis

1996 ◽  
Vol 87 (1-2) ◽  
pp. 1-10 ◽  
Author(s):  
Alan Bruce Thompson
Keyword(s):  
Experimental Investigations ◽  
Crustal Rock ◽  
Dehydration Melting ◽  
Specific Pressure ◽  
Crustal Rocks ◽  
Rock Types ◽  
Increasing Temperature ◽  
Low Pressures ◽  
Kbar Pressure ◽  
Melting Relations

ABSTRACT:After many years of systematic experimental investigations, it is now possible to quantify the conditions for optimum fertility to melt production of most common crustal rock types as functions of temperature and to about 30 kbar pressure. Quartzo-feldspathic melting produces steady increases in melt proportion with increasing temperature. The exact melt fraction depends on the mineral mode relative to quartz-feldspar eutectics and the temperatures of mica dehydration melting reactions. Mica melting consumes SiO2 from residual quartz during the formation of refractory Al2SiO5, orthopyroxene, garnet or cordierite.A simple graphical interpretation of experimental results allows a deduction of the proportions of mica and feldspar leading to optimum fertility. In effect, the mica dehydration melting reactions, at specific pressure and are superimposed on quartz-feldspar melting relations projected onto Ab-An-Or. Fertility to melt production varies with the mica to feldspar ratio and pressure. Pelites are more fertile than psammites at low pressures (e.g. 5 kbar), especially if they contain An40 to An50 plagioclase. At higher pressure (e.g. 10-20 kbar) and for rocks containing albitic plagioclase, psammites are more fertile than pelites. For a typical pelite (e.g. with An25 at 20 kbar), the cotectic with muscovite lies at higher (≍·) and XAb (≍0·42) than with biotite :≍0·35; XAb(≍·), thus dehydration melting of muscovite requires 10% more plagioclase for fertility than does biotite.The first melts from dehydration melting of muscovite (with Plg + Qtz) are more sodic and form at lower temperatures than the first melts from Bio + Plg + Qtz. With increasing pressure, to at least 30 kbar, granite minimum and mica dehydration melts become more sodic. This indicates that of such melts is greater than 0·3.

scholarly journals An Overview on the Classification and Tectonic Setting of Neoproterozoic Granites of the Nubian Shield, Eastern Desert, Egypt

Geochemistry ◽  
2021 ◽  
Author(s):  
Gaafar A. El Bahariya
Keyword(s):  
Continental Crust ◽  
Tectonic Setting ◽  
Alkali Feldspar ◽  
Eastern Desert ◽  
Granitic Rocks ◽  
Rock Types ◽  
Nubian Shield ◽  
Magma Sources ◽  
Tectonic Settings ◽  
Ocean Ridge

Granites constitute the main rock components of the Earth’s continental crust, which suggested to be formed in variable geodynamics environments. The different types of granitic rocks, their compositional characteristics, tectonic settings and magma sources are outlined. Mineralogical classification of granites includes four rock types: tonalites, granodiorites, granite (monzogranite and syenogranites) and alkali-feldspar granites. Alphabetical classification subdivided granites into: I-type, S-type, A-type and M-type granites. Moreover, formation of granitic magmas requires distinctive geodynamic settings such as: volcanic arc granite (Cordilleran); collision-related granites (leucogranites); intra-plate and ocean ridge granites. The Eastern Desert of Egypt (ED) forms the northern part of Nubian Shield. Both older and younger granites are widely exposed in the ED. Old granites (OG) comprise tonalites and granodiorites of syn- to late-orogenic granitoid assemblages. They are calcalkaline, I-type, metaluminous and display island arc tectonic setting. Younger granites (YG) on the other hand, include granites, alkali-feldspar granites and minor granodiorites. They are of I- and A-type granites and of post-orogenic to anorogenic tectonic settings. The majority of the YG are alkaline, A-type granite and of within-plate tectonic setting (WPG). The A-type granites are subdivided into: A2-type postorogenic granites and A1-type anorogenic granites. Granite magma genesis involves: (a) fractional crystallization of mafic mantle-derived magmas; (b) anatexis or assimilation of old, upper crustal rocks (c) re - melting of juvenile mafic mantle – derived rocks underplating the continental crust. Generally, older I-type granitoids were interpreted to result from melting of mafic crust and dated at approximately 760–650 Ma, whereas younger granites suggested to be formed as a result of partial melting of a juvenile Neoproterozoic mantle source. Moreover, they formed from anatectic melts of various crustal sources that emplaced between 600 and 475 Ma.

Phase relations and pre-eruptive conditions at low f(O2) in Pantelleria peralkaline rhyolites

2021 ◽  
Author(s):  
Ernestina Appiah ◽  
Paola Stabile ◽  
Fabio Arzilli ◽  
Alessandro Fabbrizio ◽  
Michael Robert Carroll
Keyword(s):  
Water Pressure ◽  
Alkali Feldspar ◽  
Single Step ◽  
Volcanic System ◽  
Rhyolitic Magma ◽  
Transport Dynamics ◽  
Reducing Conditions ◽  
Magma Compositions ◽  
Magma System ◽  
Water Saturated

<p>The volcanic system of Pantelleria is an example of volcanism in a continental rift basin which over the years has attracted much researcher due to the different eruptive styles it exhibits, ranging from effusive to explosive. Investigating the cooling history as well as the magma transport dynamics of peralkaline rhyolitic magma is useful to understand the eruptive behaviour of the pantelleritic magma system.</p><p>The present work seeks to obtain information on the liquidus temperature of alkali feldspar in pantellerite from the Fastuca pumice fall unit (PAN13) under water-saturated conditions. Alkali feldspar is one of the most abundant crystalline phases in peralkaline rhyolitic melts as well as in evolved, alkali-rich magma compositions (e.g., trachyte, phonolite).</p><p>A series of water-saturated isobaric single-step cooling experiments were performed at reducing conditions (graphite filler rod, water P-medium, ~NNO-2) with final temperature range of 670 °C-880 °C and water pressure of 20-150 MPa. Phase equilibria show that clinopyroxene is the first liquidus phase always appearing by 750 °C, followed by alkali feldspar over the entire pressure and temperature (P-T) range investigated, with also the presence of aenigmatite crystallizing near the liquidus at P of 50 MPa. Providing experimental constraints on pre- and syn-eruptive magma crystallization is fundamental to better understand the eruptive dynamics of peralkaline rhyolitic magmas. This is important for volcanic hazard assessments of peralkaline rhyolitic magmatic systems.</p>

A Discussion on volcanism and the structure of the Earth - Aspects of magmatic evolution of Réunion Island

1972 ◽  
Vol 271 (1213) ◽  
pp. 105-130 ◽  
Keyword(s):  
Partial Melting ◽  
Olivine Basalt ◽  
Low Velocity ◽  
Rock Types ◽  
Degree Of Coherence ◽  
Primitive Magma ◽  
Rapid Ascent ◽  
Basalt Composition ◽  
High Level ◽  
Velocity Layer

As with the Hawaiian islands, the volcanic construction of Reunion can be related to two main phases of activity—a shield-form ing stage of predominantly olivine-basalt composition, and a declining stage comprising more varied products (basalt-trachyte). The older of the two Reunion volcanoes (Piton des Neiges) appears to have completed both these stages, but the younger volcano (Piton de la Fournaise) is still in the shield-forming stage, and is lagging approximately 1.5 Ma* behind Piton des Neiges in its evolution. The chemical data indicate a considerable degree of coherence between the various rock types produced during the different stages of development, and it is concluded that they all stem from essentially the same hypersthene-normative, picritic, primitive magma, generated by partial melting of peridotite in the low-velocity layer of the upper mantle. The shield-forming lavas are believed to represent relatively rapid ascent of this magma, only modified by olivine fractionation, but the declining stage seems to require intermediate-depth fractionation (olivine + pyroxene) to account for its initially nepheline-normative character, followed by high level fractionation (olivine+plagioclase + pyroxene + magnetite, etc.) to produce the hawaiite—mugearite—benmoreite—trachyte sequence. Volume considerations appear to favour an open system of basalt extraction, involving a relatively modest (7 to 15 %) degree of partial melting and continuous replenishment of the mantle source beneath Reunion, rather than a closed system with its restricted basalt potential even if as much as 50 % partial melting is postulated.

The Petrology of the Tarosero Volcanic Complex: Constraints on the formation of extrusive agpaitic rocks

2021 ◽  
Author(s):  
S Braunger ◽  
M A W Marks ◽  
T Wenzel ◽  
A N Zaitsev ◽  
G Markl
Keyword(s):  
Trace Element ◽  
Water Activity ◽  
Fractional Crystallization ◽  
Genetic Relationships ◽  
Alkali Feldspar ◽  
High Water ◽  
Low Pressure ◽  
Water Soluble ◽  
Volcanic Complex ◽  
Rock Types

Abstract The Quaternary Tarosero volcano is situated in the East African Rift of northern Tanzania and mainly consists of trachyte lavas and some trachytic tuffs. In addition, there are minor occurrences of extrusive basalts, andesites, latites, as well as peralkaline trachytes, olivine trachytes and phonolites. Some of the peralkaline phonolites contain interstitial eudialyte, making Tarosero one of the few known occurrences for extrusive agpaitic rocks. This study investigates the genetic relationships between the various rock types and focuses on the peculiar formation conditions of the extrusive agpaitic rocks using a combination of whole-rock geochemistry, mineral chemistry, petrography, thermodynamic calculations, as well as major and trace element modelling. The Tarosero rocks formed at redox conditions around or below the fayalite-magnetite-quartz buffer (FMQ). During multi-level magmatic fractionation at depths between ∼40 km and the shallow crust, temperature decreased from > 1100 °C at near-liquidus conditions in the basalts to ∼ 700 °C in the peralkaline residue. Fractional crystallization models and trace element characteristics do not indicate a simple genetic relationship between the trachytes and the other rock types at Tarosero. However, the genetic relationships between the primitive basalts and the intermediate latites can be explained by high pressure fractional crystallization of olivine + clinopyroxene + magnetite + plagioclase + apatite. Further fractionation of these mineral phases in addition to amphibole and minor ilmenite led to the evolution towards the peralkaline trachytes and phonolites. The eudialyte-bearing varieties of the peralkaline phonolites required additional low-pressure fractionation of alkali feldspar and minor magnetite, amphibole and apatite. In contrast to the peralkaline trachytes and phonolites, the peralkaline olivine trachytes contain olivine instead of amphibole, thus indicating a magma evolution at even lower pressure conditions. They can be modelled as a derivation from the latites by fractional crystallization of plagioclase, clinopyroxene, magnetite and olivine. In general, agpaitic magmas evolve under closed system conditions which impedes the escape of volatile phases. In case of the extrusive agpaitic rocks at Tarosero, the early exsolution of fluids and halogens was prevented by a low water activity. This resulted in high concentrations of Rare Earth Elements (REE) and other High Field Strength Elements (HFSE) and the formation of eudialyte in the most evolved peralkaline phonolites. Within the peralkaline rock suite, the peralkaline olivine trachytes contain the lowest HFSE and REE concentrations, consistent with mineralogical evidence for a formation at a relatively high water activity. The lack of amphibole fractionation, which can act as a water buffer of the melt, as well as the evolution at relatively low pressure conditions caused the early exsolution of fluids and loss of water-soluble elements. This prevented a strong enrichment of HFSE and REE before the magma finally extruded.

scholarly journals The petrography and origin of gneisses, amphibolites and migmatites in the Qasigialik area, South-West Greenland

1970 ◽  
Vol 83 ◽  
pp. 1-70
Author(s):  
F Kalsbeek
Keyword(s):  
Alkali Feldspar ◽  
Mineralogical Composition ◽  
Gradual Change ◽  
General Introduction ◽  
Quartz Content ◽  
Modal Composition ◽  
Rock Types ◽  
South West ◽  
Basic Volcanic

The migmatites from a small area (approx. 200 km2) in SW Greenland are described with special attention to the modal composition of the different rock types. After a general introduction a description of the field relationships of the rocks is given. The leucosome of the migmatites consists of leucocratic veins which mostly have a quartz-dioritic to granodioritic composition; a typical melanosome has rarely been found. The paleosome consists of banded biotite gneisses, hornblende-biotite gneisses and amphibolites. In subordinate quantities occur anorthositic gneisses and garnet-rich gneisses and amphibolites. Spread throughout the area, homogeneous biotite gneisses occur, which locally form large masses, but generally occur as minor outcrops full of inclusions of banded gneisses and amphibolites. These homogeneous gneisses are generally hardly migmatized. The different rock types are petrographically described. Histograms of measured An contents of plagioclase in the different rocks are given. It is shown from a large number of modal analyses that there is a gradual change in plagioclase and quartz content from the banded biotite gneisses via the hornblende-biotite gneisses to the amphibolites (in part). Some of the amphibolites do not fit into this pattern, these amphibolites are also in other respects difierent from the others, among others through the presence of garnet and/or diopside. The homogeneous biotite gneisses may have a granodioritic composition, but the majority of the samples contain hardly any alkali feldspar and agree in modal composition with the banded biotite gneisses, but for a slightly higher amount of biotite in the latter. The leucocratic veins also have often a quartz-dioritic composition which agrees with most of the gneisses, but with a much lower content of dark minerals. The paper is concluded with a discussion of the origin of the difierent rock types, based on the data collected. Most of the rocks are thought to be formed by isochemical metamorphism of geosynclinal sediments. The banded biotite gneisses probably represent original sediments of greywacke to arkose type. Part of the amphibolites probably represent original basic volcanic and tufiaceous rocks, which may have undergone erosion, transport and sedimentation. The hornblende-biotite gneisses are thought to derive from mixtures of normal sediment with varying amounts of tufiaceous material. Alternative interpretations of the gneisses and amphibolites are discussed. The homogeneous gneisses may have formed from the metasediments by a process of homogenization and mobilization. The origin of the leucocratic veins is discussed. The mineralogical composition of the veins and the An content of the plagioclase seem to exclude an origin by anatexis in situ of the gneisses, but the evidence is not conclusive.

scholarly journals The importance of in situ crystallisation and loss of interstitial melt during formation of the Kærven Syenite Complex, Kangerlussuaq, East Greenland

2020 ◽  
Vol 67 ◽  
pp. 107-146
Author(s):  
Paul Martin Holm ◽  
Niels-Ole Prægel
Keyword(s):  
Alkali Feldspar ◽  
Fractional Crystallisation ◽  
Geochemical Data ◽  
Magma Chambers ◽  
Quartz Syenite ◽  
East Greenland ◽  
The North ◽  
Rock Types ◽  
Host Rocks ◽  
Residual Melt

The Kærven Syenite Complex (KSC) is one of the oldest felsic intrusions in the Tertiary East Greenland province. Here we update our previous description of the KSC and supply a greatly expanded and comprehensive geochemical dataset. New data allow us to present a more detailed petrogenetic model for the evolution of the KSC and to investigate the geochemical characteristics of igneous cumulates subjected to loss and, occasionally, replacement of residual liquid. The KSC comprises eleven mappable units that generally young westwards. Rock types range from quartz syenite to quartz alkali feldspar syenite and alkali feldspar granite. Individual intrusive units are relatively narrow and steep-sided and are collectively suggested to represent a ring dyke complex. Basement gneiss and gabbro host rocks have locally contaminated the oldest quartz syenite KSC unit, but most of the main part of the complex escaped significant influence from host rocks. A late suite of E–W to NE–SW striking peralkaline dykes of trachytic to phonolitic compositions intrude the KSC. Compositions of the KSC rocks span a considerable range in SiO2, 59–73 wt%. Concentrations of several elements vary widely for a given SiO2 (especially at SiO2 < 66 wt%), and variation diagrams do not suggest a single model for the evolution of the units of the complex. A cumulative origin is envisaged for several KSC units. Geochemical modelling suggests that KSC magmas were derived from more than one primary magma, and that the complex evolved through a four-stage process: fractional crystallisation in precursory magma chambers was followed by final emplacement of each unit, establishment of a crystal/melt mush, expulsion of part of the residual melt and, finally, crystallisation of the remaining melt. Trace element disequilibria between alkali feldspar and host rocks in two closely associated quartz alkali feldspar syenite units indicate that highly evolved residual melt was replaced by a less evolved melt phase. Modelling of potential parent melt compositions to the Kærven magmas suggests an origin not in the Iceland plume asthenosphere, but rather in a moderately enriched source, possibly in the continental lithosphere. The course of melt evolution by fractional crystallisation is indicated to have taken place in magma chambers at depth, and repeated rise of magma into the upper crustal magma chambers and crystallisation there formed the KSC. Based on our survey of published geochemical data, the inferred parental magmas seem to have few equivalents in the North Atlantic Igneous Province and may have been generated mainly from melting of enriched dry lithospheric mantle of possibly Archaean age.

scholarly journals Structural, petrological, and tectonic constraints on the Loch Borralan and Loch Ailsh alkaline intrusions, Moine thrust zone, northwestern Scotland

Geosphere ◽  
2021 ◽  
Author(s):  
Robert Fox ◽  
Michael P. Searle
Keyword(s):  
High Temperature ◽  
Alkali Feldspar ◽  
Passive Margin ◽  
Ductile Deformation ◽  
Large White ◽  
Ductile Shearing ◽  
Geological Evolution ◽  
Thrust Zone ◽  
Moine Thrust Zone ◽  
Laurentian Margin

During the Caledonian orogeny, the Moine thrust zone in northwestern Scotland (UK) emplaced Neoproterozoic Moine Supergroup rocks, metamorphosed during the Ordovician (Grampian) and Silurian (Scandian) orogenic periods, westward over the Laurentian passive margin in the northern highlands of Scotland. The Laurentian margin comprises Archean–Paleoproterozoic granulite and amphibolite facies basement (Scourian and Laxfordian complexes, Lewisian gneiss), Proterozoic sedimentary rocks (Stoer and Torridon Groups), and Cambrian–Ordovician passive-margin sediments. Four major thrusts, the Moine, Ben More, Glencoul, and Sole thrusts, are well exposed in the Assynt window. Two highly alkaline syenite intrusions crop out within the Moine thrust zone in the southern Assynt window. The Loch Ailsh and Loch Borralan intrusions range from ultramafic melanite-biotite pyroxenite and pseudoleucite-bearing biotite nepheline syenite (borolanite) to alkali-feldspar–bearing and quartz-bearing syenites. Within the thrust zone, syenites intrude up to the Ordovician Durness Group limestones and dolomites, forming a high-temperature contact metamorphic aureole with diopside-forsterite-phlogopite-brucite marbles exposed at Ledbeg quarry. Controversy remains as to whether the Loch Ailsh and Loch Borralan syenites were intruded prior to thrusting or intruded syn- or post-thrusting. Borolanites contain large white leucite crystals pseudomorphed by alkali feldspar, muscovite, and nepheline (pseudoleucite) that have been flattened and elongated during ductile shearing. The minerals pseudomorphing leucites show signs of ductile deformation indicating that high-temperature (~500 °C) deformation acted upon pseudomorphed leucite crystals that had previously undergone subsolidus breakdown. New detailed field mapping and structural and petrological observations are used to constrain the geological evolution of both the Loch Ailsh and the Loch Borralan intrusions and the chronology of the Moine thrust zone. The data supports the interpretation that both syenite bodies were intruded immediately prior to thrusting along the Moine, Ben More, and Borralan thrusts.

Alkali-feldspar and Fe-Ti oxide exsolution textures as indicators of the distribution and subsolidus effects of magmatic ‘water’ in the Klokken layered syenite intrusion, South Greenland

1980 ◽  
Vol 71 (1) ◽  
pp. 1-12 ◽  
Author(s):  
Ian Parsons
Keyword(s):  
Gas Phase ◽  
Water Distribution ◽  
Alkali Feldspar ◽  
Sharp Change ◽  
Magmatic Evolution ◽  
Fine Grained ◽  
Rock Types ◽  
Ti Oxides ◽  
Normal Zoning ◽  
Granular Layers

ABSTRACTThe layered syenites in the Klokken intrusion consist of horizons of fine-grained, granular-textured ferroaugite syenite showing inverted cryptic layering, interleaved with coarser, laminated, more fractionated hedenbergite syenite. Distribution of hydrous mafic phases indicates build-up of water in parallel with magmatic evolution, and druses and pegmatitic segregations in the laminated syenites are evidence for late development of a gas phase. Feldspar bulk compositions are close to the minimum on the Ab-Or binary, with An decreasing from An7 to An1 with fractionation, and normal zoning in cryptoperthite crystals. Feldspars in granular syenites are transparent coherent cryptoperthites or braid microperthites; An-content is probably the main control of fine-perthite coarseness. Laminated syenite feldspars are turbid, coarse patch microperthites with rare relics of braid textures. This non-coherent coarsening was caused by interactions between feldspars and water entrapped at magmatic temperatures which was retained within the original lithologies to low subsolidus temperatures. Fe-Ti oxides reflect this water distribution, with regular trellis ilmenite-titanomagnetite intergrowths in less fractionated rocks and ragged granule exsolution in more advanced syenites. The sharp change in exsolution textures at granular-laminated syenite boundaries implies steep water-gradients within these interleaved rock types. Water was unable to penetrate the granular layers and did not circulate freely in the cooling intrusion.

scholarly journals Melt Inclusions in Plagioclase Macrocrysts at Mount Jourdanne, Southwest Indian Ridge (~64° E): Implications for an Enriched Mantle Source and Shallow Magmatic Processes

Minerals ◽  
2019 ◽  
Vol 9 (8) ◽  
pp. 493 ◽  
Author(s):  
Wei Li ◽  
Chunhui Tao ◽  
Wen Zhang ◽  
Jia Liu ◽  
Jin Liang ◽  
...  
Keyword(s):  
Mantle Source ◽  
Melt Inclusions ◽  
Parental Magma ◽  
Southwest Indian Ridge ◽  
Host Magma ◽  
Enriched Mantle ◽  
Magma Supply ◽  
General Chemical ◽  
Indian Ridge ◽  
Ocean Ridge

Plagioclase ultraphyric basalts (PUBs) with up to 40% millimeter-sized plagioclase crystals, were sampled from the Mount Jourdanne volcanic massif (~64° E) in the Southwest Indian Ridge. The geochemistry of the host glass, the glassy melt inclusions and their host plagioclase macrocrysts (An60-69) are used to reveal the mantle heterogeneity and to discuss the origin of Mount Jourdanne PUBs. The melt inclusions trapped in plagioclase display low MgO and high SiO2 contents and show rare earth element (REE) patterns resembling enriched mid-ocean ridge basalts (E-MORB). Together with their positive Sr and Eu anomalies, these features indicate that they were derived from an enriched mantle source, likely a refertilized peridotite or a pyroxenite. In contrast to some 61–67° E basalts, there is a lack of negative Eu anomalies in the PUB host glasses, precluding large amounts of plagioclase crystallization from their parental magma. Petrographic observations and the general chemical similarity between melt inclusions and melts equilibrated with the clinopyroxene cores in regional gabbros and/or troctolites suggest that these plagioclase macrocrysts originate from gabbroic mush within the lower crust. The density contrasts allow the effective segregation of plagioclase prior to their incorporation into the host magma. We propose that these plagioclase macrocrysts were entrained when a new batch of magma passed through the crustal mush zone, and resulted in the formation of the PUB. Eruption of Mount Jourdanne PUBs requires a minimum ascending velocity of 5 m d−1 for the host magma, which is not as high as the eruption rate for typical MORB samples. It is likely that the PUB host magma erupts during a period with reduced magma supply, whereas eruption of aphyric lavas correspond to the fast volcanic formation of the Mount Jourdanne massif.

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