The Highwood Mountains potassic igneous province, Montana: mineral fractionation trends and magmatic processes revisited

2012 ◽  
Vol 76 (4) ◽  
pp. 1005-1051 ◽  
Author(s):  
C. M. B. Henderson ◽  
F. R. Richardson ◽  
J. M. Charnock
Keyword(s):  
Major Element ◽  
Magma Mixing ◽  
Alkali Feldspar ◽  
Bulk Rock ◽  
Magma Chambers ◽  
Igneous Province ◽  
Rock Analysis ◽  
Volatile Loss ◽  
Primary Magmas ◽  
Alkali Feldspars

AbstractPotassium-rich mafic dykes and lavas from the Highwood Mountains Igneous Province, USA were studied by electron-microprobe and bulk-rock analysis. For the mafic phonolites, compositional trends for olivine and augite phenocrysts and groundmass biotite, alkali feldspar and titanomagnetites are presented and substitution mechanisms discussed. Phenocrysts of biotite and augite in the minettes are also characterized, together with groundmass alkali feldspar and titanomagnetite. The alkali feldspars and biotites are commonly enriched in Ba. Olivine, clinopyroxene and biotite phenocrysts are generally quite magnesium-rich, which is consistent with the primitive natures of the least evolved rocks.Bulk-rock major-element compositions are combined with modal and microprobe data for the principal phenocrysts to calculate model residual liquid compositions for mafic phonolites, minettes and a syenitic rock. On the basis of phase-equilibria, it is suggested that the main controls of differentiation are polybaric involving crystallization during transport of primary magmas from the mantle for the minettes, and low-pressure differentiation for the mafic phonolites. Whereas magma mixing might have contributed to petrogenesis, many of the disequilibrium features exhibited by clinopyroxene and biotite phenocrysts can also be attributed to pre-existing phenocrysts undergoing decompression melting during magma uprise from its mantle source, followed by rapid crystal growth and episodic volatile loss in sub-volcanic magma chambers.

scholarly journals Radiometric age determinations in the Kærven area, Kangerdlugssuaq, Bast Greenland Tertiary igneous Province: 40Ar/39 Ar, Kl Ar and Rb/Sr isotopic results

1991 ◽  
Vol 38 ◽  
pp. 183-201
Author(s):  
Paul Martin Holm
Keyword(s):  
Isotope Analysis ◽  
Alkali Feldspar ◽  
Magmatic Activity ◽  
Sr Isotopes ◽  
Sr Isotope ◽  
Stepwise Heating ◽  
Alkaline Intrusion ◽  
Igneous Province ◽  
Age Determinations ◽  
Alkali Feldspars

The Kærven syenite complex was emplaced as part of the magmatic activity related to continental rifling in the Paleogene. Radiometric age determinations have been carried out on samples from selected parts of the complex, which consists of more !han 10 significant units. Five amphiboles and two alkali feldspar have been analysed by the 40Arl'9Ar method with stepwise heating, five amphiboles and one biotite K/Ar analyses are presented together with Rb/Sr isotope analysis of 6 amphiboles, 2 biotites, 3 alkali feldspars and 32 whole rocks. The results reveal that a Iate, probably 36 Ma, thermal event caused Ar-loss in the alkali feldspars and excess 40Ar-gain in variable amounts in the analysed minerals. Also the Sr isotopes were disturbed by the secondary heating. Crystallisation ages for parts of the Kærven complex can be established as 58 ± 1 Ma, while other units have younger ages of 56.1 ± 0.8 Ma and 50.4 ± 0.8 Ma. The age for the early Kærven rocks is significantly older than other recorded syenites in the Kangerdlugssuaq area and is comparable only to the estimated age of the initiation of basaltic volcanism along the East Greenland coast at 57 Ma. An Rb/Sr isochron for four nordmarkite whole rocks and a 40Ar/39 Ar age plateau of an amphibole from one nordmarkite date the part of the Kangerdlugssuaq alkaline intrusion adjacent to the Kærven complex as 54.6 ± 2.4 Ma. This is older than other parts of the Kangerdlugssuaq intrusion, and indicate that this intrusion was multiple and emplaced over several million years

An experimental study of Sr, Ba and Rb partitioning between alkali feldspar and silicate liquid in the system nepheline–kalsilite–quartz at 0.1 GPa P(H2O): a revisitation and reassessment

2012 ◽  
Vol 76 (1) ◽  
pp. 157-190 ◽  
Author(s):  
C. M. B. Henderson ◽  
W. J. Pierozynski
Keyword(s):  
Partition Coefficients ◽  
Microprobe Analysis ◽  
Electron Microprobe Analysis ◽  
Major Element ◽  
Alkali Feldspar ◽  
Silicate Melts ◽  
Water Vapour Pressure ◽  
Regression Equations ◽  
Physical Variables ◽  
Alkali Feldspars

AbstractThe partitioning of Sr, Ba and Rb between alkali feldspar and melt has been determined at 0.1 GPa water vapour pressure, mainly on one Na-rich series and one K-rich series within the system nepheline-kalsilite-quartz. Experiments were also carried out with small amounts of the anorthite molecule or peralkaline components (Na, K metasilicates). The compositions of the alkali feldspars and coexisting quenched glasses were determined by electron microprobe analysis. Except for some peralkaline compositions, the crystal/liquid partition coefficients for Sr and Ba are always >1; the crystal/liquid partition coefficient for Rb is always <1. For sodic alkali feldspars DSr > DBa and for potassic feldspars DSr < DBa. Partition coefficients for Sr and Ba increase: (1) with decreasing temperature; (2) with increasing Or content of feldspar; (3) with increasing silica-undersaturation of the melt; (4) with decreasing peralkalinity. The variation in the value of DRb is less clear, but it is higher for K-rich feldspars. Multiple linear regression equations are fitted to correlate ln(D) with independent compositional and physical variables. Where rock/groundmass major-element data are available for felsic natural rocks, equations are recommended for obtaining reliable alkali feldspar partition coefficients for modelling fractional crystallization processes. The structural properties of silicate melts and crystal chemical relations are used to rationalize trends in partition coefficients.

Metabentonite geochemistry: magmatic cycles and graptolite extinctions at Dob's Linn, southern Scotland

1988 ◽  
Vol 79 (1) ◽  
pp. 19-41 ◽  
Author(s):  
R. A. Batchelor ◽  
J. A. Weir
Keyword(s):  
Trace Elements ◽  
Deleterious Effect ◽  
Major Element ◽  
Magma Mixing ◽  
Volcanic Origin ◽  
Intermediate Composition ◽  
Ash Composition ◽  
Mineralogical Study ◽  
Local Extinctions ◽  
Phosphorus Levels

ABSTRACTThe Moffat Shale Group is a condensed, variable and partly pelagic sequence of mudrocks of Llandeilo—Llandovery age. The sequence has a five-fold lithological subdivision based mainly on the occurrence of grey mudstones within a succession otherwise dominated by fully euxinic black graptolitic mudrocks. Associated with the black mudrocks, especially in the Llandovery, are metabentonite beds which achieve a climax, both in thickness and in number, within the top quarter of the mudrock sequence. A geochemical and mineralogical study has confirmed a volcanic origin for the metabentonites. Major element data highlight a carbonate-dominated environment above the gregarius—convolutus Zones boundary. Phosphorus levels reach a peak at the same boundary, as well as at the Caradoc—Ashgill boundary where phosphorite horizons are known from Wales and Norway. Immobile trace elements have highlighted regular changes in source magma composition. Prolonged periods of crystal fractionation in magmas of intermediate composition gave rise, on eruption, to large volumes of silicic ash which had a deleterious effect on graptolite species and led to local extinctions. Regular fluctuations in ash composition from silicic to intermediate are ascribed to alternating fractionation and magma mixing cycles.

Geochronology and geochemistry of igneous rocks in the southeastern Lesser Xing’an Range, northeastern China: petrogenesis and implications for the early Mesozoic tectonic evolution

2020 ◽  
Vol 57 (4) ◽  
pp. 506-523
Author(s):  
Jin-hua Qin ◽  
Cui Liu ◽  
Jin-fu Deng
Keyword(s):  
Field Strength ◽  
Inductively Coupled Plasma ◽  
High Field ◽  
Magma Mixing ◽  
Tectonic Setting ◽  
Alkali Feldspar ◽  
Igneous Rocks ◽  
Geochemical Data ◽  
High Field Strength ◽  
Alkaline Granite

We present systematic U–Pb age data collected by laser ablation multi-collector inductively coupled plasma mass spectrometry, precise geochemical data, and Nd isotope data for igneous rocks from the southeastern Lesser Xing’an Range (SE LXR). The results indicate that the formation ages as follows: Maojiatun alkaline granite, 207.2 ± 0.84 Ma and 204.6 ± 0.93 Ma; Diorite porphyrite, 164.5 ± 0.97 Ma; and Tieli syenogranite, 186.7 ± 1.50 Ma. The alkaline granite has high silicon, potassium, alkali, and FeOT contents; it is enriched in high field strength elements, Zr, Hf, Th, Rb, and U; is depleted in Ba, Sr, Nb, Ta, P, Ti, etc.; and has high ratios of 10000Ga/Al. It shows an A2-type granite affinity. The Tieli alkali-feldspar granite has high total alkali contents and is enriched in high field strength elements and rare earth elements and depleted in Sr, Ba, Ti, and P, and shows varying degrees of alkalinity. Rocks from SE LXR display similar εNd (t) values with corresponding to Nd model ages of 1095 to 813 Ma. The igneous rocks from the SE LXR are proposed to be derived from melting of the Neoproterozoic lower crust and potential magma mixing with ancient crystalline basement. The formation of the Maojiatun alkaline granite occurred in response to a postorogenic event following the closure of the Paleo-Asian Ocean. However, the SE LXR exhibited an extensional back-arc tectonic setting in the Early Jurassic. The Middle Jurassic diorite porphyrite could be related to the temporary stagnation of the westward subduction of the Paleo-Pacific plate.

Distribution of gallium between phenocrysts and melt in peralkaline salic volcanic rocks, Kenya Rift Valley

2010 ◽  
Vol 74 (2) ◽  
pp. 351-363 ◽  
Author(s):  
R. Macdonald ◽  
N. W. Rogers ◽  
B. Bagiński ◽  
P. Dzierżanowski
Keyword(s):  
Rift Valley ◽  
Inductively Coupled Plasma ◽  
Magma Mixing ◽  
Volcanic Rocks ◽  
Alkali Feldspar ◽  
Volcanic Complex ◽  
Kenya Rift ◽  
Inductively Coupled ◽  
Plasma Mass Spectrometry ◽  
Kenya Rift Valley

AbstractGallium abundances, determined by laser ablation-inductively coupled plasma-mass spectrometry, are presented for phenocrysts and glassy matrices from a metaluminous trachyte and five peralkaline rhyolites from the Greater Olkaria Volcanic Complex, Kenya Rift Valley. Abundances in the glasses range from 28.9 to 33.3 ppm, comparable with peralkaline rhyolites elsewhere. Phenocryst Ga abundances (in ppm) are: sanidine 31.5–45.3; fayalite 0.02–0.22; hedenbergite 3.3–6.3; amphibole 12; biotite 72; ilmenite 0.56–0.72; titanomagnetite 32; chevkinite-(Ce) 364. The mafic phases and chevkinite-(Ce) are enriched in Ga relative to Al, whereas Ga/Al ratios in sanidine are smaller than in coexisting glass. Apparent partition coefficients range from <0.01 in fayalite to 12 in chevkinite-(Ce). Coefficients for hedenbergite, ilmenite and titanomagnetite decrease as melts become peralkaline. The sharp increase in Ga/Al in the more fractionated members of alkaline magmatic suites probably results from alkali feldspar-dominated fractionation. Case studies are presented to show that the Ga/Al ratio may be a sensitive indicator of such petrogenetic processes as magma mixing, interaction of melts with F-rich volatile phases, mineral accumulation and volatile-induced crustal anatexis.

scholarly journals Mid-infrared spectroscopy of alkali feldspar samples for space application

2020 ◽  
Vol 114 (5) ◽  
pp. 453-463 ◽  
Author(s):  
Maximilian P. Reitze ◽  
Iris Weber ◽  
Herbert Kroll ◽  
Andreas Morlok ◽  
Harald Hiesinger ◽  
...  
Keyword(s):  
Alkali Feldspar ◽  
Reference Database ◽  
Space Application ◽  
X Ray Diffraction ◽  
Exchange Method ◽  
Infrared Measurements ◽  
Planetary Bodies ◽  
Terrestrial Planetary Bodies ◽  
End Members ◽  
Alkali Feldspars

Abstract Feldspars are major components of terrestrial planetary surfaces. For future space application and the setup of a comprehensive reference database, Na- and K-rich alkali feldspars, NaAlSi3O8 – KAlSi3O8, have been investigated by infrared reflectance spectroscopy. We related the feldspar spectra to the chemical composition and state of Al,Si order/disorder. The infrared measurements were analyzed with respect to band shifts and peak shapes using the autocorrelation function. Natural samples served as starting materials. Some samples were treated by the alkali exchange method to produce pure end-members, which were then heated to generate various states of Al,Si disorder. X-ray diffraction (XRD) methods served to determine the Al,Si distribution. Our autocorrelation allowed to differentiate between the compositional and the order/disorder influences seen in the spectra in the wavelength range between 7 μm up to 14 μm (1429 cm− 1 to 714 cm− 1). Space missions often analyze the surfaces of planetary bodies using remote sensing. Therefore, our results are essential to characterize and distinguish alkali feldspars on the surfaces of terrestrial planetary bodies like Mercury.

scholarly journals Eruptive Activity on the Western Flank of Piton de la Fournaise (La Réunion Island, Indian Ocean): Insights on Magma Transfer, Storage and Evolution at an Oceanic Volcanic Island

2019 ◽  
Vol 60 (9) ◽  
pp. 1717-1752 ◽  
Author(s):  
G Boudoire ◽  
Y -A Brugier ◽  
A Di Muro ◽  
G Wörner ◽  
I Arienzo ◽  
...  
Keyword(s):  
Major Element ◽  
Magma Mixing ◽  
Central Area ◽  
Magma Ascent ◽  
Plumbing System ◽  
Large Variability ◽  
Piton De La Fournaise ◽  
Western Flank ◽  
Apparent Homogeneity ◽  
Conduit System

Abstract Petrological and geochemical (major element, trace element, Sr–Nd isotope) data for recent (&lt;5 kyr old) basalts that sporadically erupt on the western flank of Piton de la Fournaise (PdF), one of the most active volcanoes on Earth, allow the tracking of magma transfer and evolution from mantle to crustal depths. In the western peripheral area of PdF we document the broadly synchronous eruptions of (1) primitive olivine and olivine–clinopyroxene transitional basalts with tholeiitic affinity that are closely associated in space with (2) transitional olivine basalts with alkaline affinity, and (3) hybrid lavas, intermediate between the ‘alkaline’ and the ‘tholeiitic’ end-members. The composition of the latter overlaps with that of the lavas frequently erupted from the conduit system feeding the main summit cone. AlphaMELTS modelling, and fluid inclusion and clinopyroxene barometry, constrain the conditions of magma storage at 10–30 km, and the ascent of magma from the upper mantle to the shallow crustal plumbing system. Variable degrees of mantle melting, together with minor source heterogeneity and contamination with cumulate-derived partial melts, contribute to the diversity of PdF magmas. However, all these processes do not represent the dominant factors that produce the large variability we found in major element composition. Indeed, the composition of basalts erupted from PdF peripheral centers is strongly controlled by polybaric olivine–clinopyroxene fractionation at pressures higher than 3 kbar. Crystal textures and geochemical modelling suggest that fast magma ascent is critical to prevent clinopyroxene dissolution. Conversely, long-lasting magma stagnation promotes pyroxene resorption and magma differentiation. ‘Central’ eruptions occurring close to the PdF summit cone emit variably more evolved melts, which result from olivine–clinopyroxene–plagioclase differentiation at intermediate–shallow pressure (&lt;3 kbar and in most cases &lt;1 kbar). Deep and extensive magma mixing before injection into the crustal magma conduit system, located below the summit region, results in the apparent homogeneity of basalts erupted from the central area. As regards ‘peripheral’ eruptions, deep-seated stagnation of basaltic melts and differentiation at the mantle–crust transition zone (c. 4 kbar) produces a range of magma compositions. We demonstrate that rapid magma ascent from deep-seated reservoirs can bypass the central plumbing system. The eruptions of these magmas both in the central area and on the densely populated flanks have major consequences in terms of volcanic hazard at PdF.

The Tertiary Kærven Syenite Complex, Kangerdlugssuaq, East Greenland: Mineral Chemistry and Geochemistry

1988 ◽  
Vol 52 (367) ◽  
pp. 435-450 ◽  
Author(s):  
Paul Martin Holm ◽  
Niels-Ole Prægel
Keyword(s):  
Magma Mixing ◽  
Crustal Contamination ◽  
Outer Part ◽  
Alkali Feldspar ◽  
Mineral Chemistry ◽  
Crystal Fractionation ◽  
Maximum Pressure ◽  
General Sequence ◽  
East Greenland ◽  
Rock Suite

AbstractThe Kærven syenite complex, which reflects the hitherto earliest recorded stages in the Tertiary of East Greenland, outcrops in the middle reaches of the Kangerdlugssuaq Fjord as a peripheral intrusion to the Kangerdlugssuaq intrusion. The rocks of the Kærven complex range from syenite through alkali feldspar quartz-syenite to alkali feldspar granite. The general sequence of crystallization of the Kærven magmas was: alkali feldspar ± olivine(Fa96−99) ± plagioclase(An41−11), clinopyroxene (augite, ferrosalite, ferrohedenbergite), quartz and amphibole. Whole-rock major and trace-element data show coherent geochemical trends which suggest comagmatism. The data reveal that the Kærven rocks are distinct from the rocks from the adjacent Kangerdlugssuaq intrusion (e.g. higher TiO2, FeOT in low-SiO2 samples, lower Na2O, approx. constant Zr/Nb). The mineral chemistry supports this conclusion, as the Kærven samples typically have calcic amphiboles and clinopyroxenes with a very limited Na-enrichment in contrast to the sodic trends of the Kangerdlugssuaq intrusion. Normative feldspar compositions plot near to the Ab-Or cotectic in the Q-Ab-Or system and a maximum pressure of crystallization of 3–5 kbar with moderate to low PH2O is indicated.Trace elements preferently incorporated in plagioclase and alkali feldspar, i.e. Sr, Ba and Rb, show systematics which are not compatible with an evolution of the rock suite by crystal fractionation of these phases, though possibly alkali feldspar may be partially accumulated in a few very evolved rocks. Numerical calculations do not suggest a magmatic evolution by fractional crystallization of the observed phases. The variation of Sr, Ba and Rb as well as of the incompatible elements Nb, Zr and Th support a derivation of the rock suite mainly by mixing two components, a syenitic and a granitic end-member. It is concluded that magma mixing was the most significant process in the formation of the Kærven rock suite accompanied by some crystal fractionation. Evidence for crustal contamination is detected in a few samples from the outer part of the intrusion but has not affected the main suite of rocks.

Field, petrographical and geochemical characterization of a layered anorthosite sequence in the Upper Main Zone of the Bushveld Complex

2020 ◽  
Vol 123 (3) ◽  
pp. 277-296
Author(s):  
J.E. Bourdeau ◽  
S.E. Zhang ◽  
B. Hayes ◽  
A. Logue
Keyword(s):  
Bushveld Complex ◽  
Major Element ◽  
Parental Magma ◽  
Incompatible Trace Element ◽  
Limpopo Province ◽  
Main Zone ◽  
Bulk Rock ◽  
Magmatic Differentiation ◽  
Slurry Injection

Abstract A sequence of eight poikilitic anorthosite layers (labeled 1 to 8), within the Upper Main Zone in the eastern lobe of the Bushveld Complex, are exposed along a road-cut, 5.3 km northeast of the town of Apel, Limpopo Province. The anorthosite layers are meter-scale in thickness (0.4 to 10 m), have sharp contacts and are defined on the size and shape of pyroxene oikocrysts they contain. The anorthosite sequence is bounded by typical Main Zone gabbronorites. Euhedral, zoned primocrystic laths of plagioclase (An62.5-80.6; 0.2 to 4 mm long) are morphologically identical throughout the anorthosite sequence and define a moderate to strong foliation that is typically aligned parallel to the plane of layering. Interstitial clinopyroxene and orthopyroxene typically occur as large (0.8 to 80 cm) oikocrysts enclosing numerous partly rounded plagioclase chadacrysts. Rarely, orthopyroxene appears as subophitic crystals enclosing few and significantly smaller (0.08 to 0.4 mm), equant plagioclase inclusions. Detailed plagioclase and pyroxene mineral compositions for layers 2 to 5 show minimal variations within layers (0.1 to 2.3 mol% An and 0.7 mol% Mg#), whereas compositional breaks occur between layers (0.5 to 3.8 mol% An and 1.3 mol% Mg#). In layers 2 to 5, the An-content of plagioclase cores and the Mg# of both clinopyroxene and orthopyroxene crystals decrease by 2.5 mol%, 8.6 mol% and 13.0 mol% upwards, respectively. Bulk-rock incompatible trace element concentrations and patterns are similar for all analyzed anorthosite layers indicating that they are related to the same parental magma. However, bulk-rock major element oxides (e.g. Al2O3, TiO2, K2O) and atomic Mg# become more evolved upwards, consistent with magmatic differentiation. Based on the consistent plagioclase crystal morphologies and relatively constant chemistries within each anorthosite layer, we propose that each layer was formed by the intrusion of a plagioclase slurry. The upwards-evolving mineral chemistries, bulk-rock major element oxides and atomic Mg# suggests that each plagioclase slurry injection, that yielded an anorthosite layer, was derived from a slightly more fractionated parental magma prior to emplacement.

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