Zoning in highly alkaline magma bodies

1984 ◽  
Vol 121 (6) ◽  
pp. 563-575 ◽  
Author(s):  
John A. Wolff ◽  
Michael Storey
Keyword(s):  
Trace Elements ◽  
Late Quaternary ◽  
Calc Alkaline ◽  
Pyroclastic Deposits ◽  
Phenocryst Assemblage ◽  
Alkaline Magma ◽  
Element Abundances ◽  
High Silica ◽  
Plausible Mechanism ◽  
Reverse Zoning

AbstractWe present chemical data on magmatically heterogeneous pyroclastic deposits of late Quaternary age erupted from zoned magma systems underlying Tenerife (Canary Islands), Sao Miguel and Faial (Azores), and Vesuvius. The most fractionated magmas present at each centre are respectively Na-rich phonolite, trachyte, and K-rich phonolite. Within any one deposit, chemical variation is either accompanied by changes in the phenocryst assemblage (petrographic zonation) or is largely manifested in trace element abundances, unaccompanied by any petrographic change (occult zonation). Zoning is analogous to that in calc-alkaline systems where the most fractionated products are high-silica rhyolites. When a range of magma types are considered, a correlation emerges between roofward depletion of trace elements (especially REE) in the zoned system and compatability of those same trace elements in the accessory phenocryst phases present. Thus, allanite- or chevkinite-bearing rhyolitic systems are light-REE depleted roofwards, the sphene-bearing Tenerife system is middle-REE depleted roofwards, the melanite-bearing Vesuvius system is heavy-REE depleted roofwards, while the Azores systems, which lack these phases, display roofward REE enrichment. Therefore, the behaviour of trace elements may in each case be explained by fractionation of observed phenocryst assemblages. The resemblance between features of zoned magma systems and published work on the dynamic consequences of cooling saturated aqueous solutions prompts us to suggest that sidewall crystallization and consequent boundary-layer uprise to form a capping layer at top of the system may be a plausible mechanism for the generation of both petrographic and occult zonation. Reverse zoning occurs among the first-erupted tephra of some deposits, demonstrating that the most highly differentiated magma available is not always the first to be tapped during an eruption from a zoned system.

Identification of Dacitic Tephra by Activation Analysis of their Primary Mineral Phenocrysts

1973 ◽  
Vol 3 (2) ◽  
pp. 307-315 ◽  
Author(s):  
M.J. Dudas ◽  
M.E. Harward ◽  
R.A. Schmitt
Keyword(s):  
Rare Earth ◽  
Activation Analysis ◽  
Pacific Northwest ◽  
Late Quaternary ◽  
Minor Element ◽  
Transition Elements ◽  
Pyroclastic Deposits ◽  
Element Abundances ◽  
The Pacific ◽  
Primary Mineral

AbstractPrimary mineral phenocrysts from eight different late Quaternary pyroclastic deposits were fractionated for neutron-activation analysis with the purpose of characterizing each of the deposits on the basis of trace and minor element compositions. In hornblende separates, contents of several rare earth and transition elements were found to be distinctive for the Mazama, Glacier Peak, and several St. Helens deposits. In magnetites, abundances of transition elements are characteristic and serve as good discriminants for the pyroclastic deposits examined in this investigation. Contents of transition and rare earth elements in hyperthenes also appear useful in distinguishing volcanic ash deposits. Trace and minor element abundances in plagioclase phenocrysts did not appear adequate for identification of pyroclastics due to elemental depletion and similarity of contents for feldspar separates. It was found that contents of Sm and Yb in hornblende phenocrysts would serve to distinguish between several pyroclastic deposits from the Pacific Northwest.

Felsic plutonic rocks from the southern portion of the New Caledonian Ultramafic Belt

1973 ◽  
Vol 110 (5) ◽  
pp. 431-446 ◽  
Author(s):  
K. A. Rodgers
Keyword(s):  
New Caledonia ◽  
Ultramafic Rocks ◽  
Calc Alkaline ◽  
Mafic Rocks ◽  
Alkaline Magma ◽  
Southwest Pacific ◽  
Plutonic Rocks

SummaryGranodiorite stocks were intruded into the alpine peridotites of southern New Caledonia in the Eocene following overthrusting of the ultramafics onto the sialic core of the island. Strong zoning, from mela-diorite to granodiorite, is developed in one pluton and is believed to be the result of assimilation of ultramafic and mafic rocks by the calc-alkaline magma. Evidence in favour of a consanguineous relationship between the felsic and ultramafic rocks is largely circumstantial. In their petrography, mineralogy and chemistry, the rocks show few differences from other felsic plutonics of Tertiary age in the southwest Pacific.

scholarly journals Petrogenesis of the synplutonic high-magnesian porphyritic dykes from the mafic-granitoid plutons of the East-European Craton, Voronezh crystalline massif

2019 ◽  
Vol 64 (4) ◽  
pp. 356-371
Author(s):  
R. A. Terentiev
Keyword(s):  
Trace Elements ◽  
Magma Chamber ◽  
Earth Element ◽  
Parent Magma ◽  
Chrome Spinel ◽  
Calc Alkaline ◽  
East European ◽  
Fine Grained ◽  
Mineral Inclusions ◽  
Metasomatized Mantle

This paper documents the data on high-Mg porphyrite dykes (PDs) from the mafic to felsic (~2.09 Ma) plutons of Elan complex (EC). The low-thickness (first centimeters) synplutonic dykes are characterized by sharp straight contacts without visible chilling zones, in contrast to the larger (up to 119 m) dykes that have gradual transitions. The dykes are fresh, porphyritic (bronzite, Al-enstatite, labradorite) and has fine-grained mainly quartzo-feldspathic (+biotite, sulfides, accessories, ±hypersthene) matrix. Based on geochemistry data the PDs are intermediate rocks (SiO2 = 58.9–60.3 wt. %) and plot into calc-alkaline series with high magnesian of whole rock (Mg# ~0.7) and felsic (68.9–70.2 wt. %) matrix (Mg# ~0.5). The PDs show differentiated rare-earth element patterns with negligible Eu anomalies. The bronzite phenocrysts varying sizes are characterized by block zoning and contain irregular inclusions of olivine (Mg# ~0.85), clinopyroxene (Mg# ~0.88), phlogopite (Mg# up to 0.94), labradorite, chrome spinel, graphite and sulfides. The Al-enstatite phenocrysts are practically sterile with respect to trace elements and mineral inclusions. The geochemical features as well as diffusion zones, reaction rims, and resorbed faces of the phenocrysts such as orthopyroxene and plagioclase indicate processes of recrystallization and/or partial dissolution of nonequilibrium crystals in the melt and indicate intratelluric nature of the dyke phenocrysts that cores are inherited from the EC derivatives/cumulate. The mineral thermometry estimates are: (1) the parent magma starting temperatures of 1200–1400 °С and (2) the EC crystallization temperatures 1080–1155 °С, (3) the PD emplacement temperatures 910–1070 °С. The petrogenetic model supposes the generation of EC high-temperature magmas similar to boninites from an upper metasomatized mantle. The melt is contaminated with continental crust lithologies. It implies the half-way evolved magma chamber in the crust. The PD melt, as a result of ending of the half-way magma chamber evolution, was emplaced into the still unheated EC plutons.

Emmons Lake Volcanic Center, Alaska Peninsula: source of the Late Wisconsin Dawson tephra, Yukon Territory, Canada

2003 ◽  
Vol 40 (7) ◽  
pp. 925-936 ◽  
Author(s):  
Margaret T Mangan ◽  
Christopher F Waythomas ◽  
Thomas P Miller ◽  
Frank A Trusdell
Keyword(s):  
Compositional Data ◽  
Late Quaternary ◽  
Yukon Territory ◽  
Volcanic Center ◽  
Pyroclastic Deposits ◽  
Alaska Peninsula ◽  
Aleutian Arc ◽  
Pumice Flow ◽  
North And South ◽  
The Bering Sea

The Emmons Lake Volcanic Center on the Alaska Peninsula of southwestern Alaska is the site of at least two rhyolitic caldera-forming eruptions (C1 and C2) of late Quaternary age that are possibly the largest of the numerous caldera-forming eruptions known in the Aleutian arc. The deposits produced by these eruptions are widespread (eruptive volumes of >50 km3 each), and their association with Quaternary glacial and eolian deposits on the Alaska Peninsula and elsewhere in Alaska and northwestern Canada enhances the likelihood of establishing geochronological control on Quaternary stratigraphic records in this region. The pyroclastic deposits associated with the second caldera-forming eruption (C2) consist of loose, granular, airfall and pumice-flow deposits that extend for tens of kilometres beyond Emmons Lake caldera, reaching both the Bering Sea and Pacific Ocean coastlines north and south of the caldera. Geochronological and compositional data on C2 deposits indicate a correlation with the Dawson tephra, a 24 000 14C BP (27 000 calibrated years BP), widespread bed of silicic ash found in loess deposits in west-central Yukon Territory, Canada. The correlation clearly establishes the Dawson tephra as the time-stratigraphic marker of the last glacial maximum.

Effects of volatiles on mixing in calc-alkaline magma systems

Nature ◽  
1982 ◽  
Vol 297 (5867) ◽  
pp. 554-557 ◽  
Author(s):  
Herbert E. Huppert ◽  
R. Stephen J. Sparks ◽  
J. Stewart Turner
Keyword(s):  
Calc Alkaline ◽  
Alkaline Magma

Amphibole fractionation in calc-alkaline magma genesis

1976 ◽  
Vol 276 (3) ◽  
pp. 309-329 ◽  
Author(s):  
R. G. Cawthorn ◽  
M. J. O'Hara
Keyword(s):  
Calc Alkaline ◽  
Alkaline Magma ◽  
Magma Genesis ◽  
Amphibole Fractionation

Compositional zonation and cumulus processes in the Mount Mazama magma chamber, Crater Lake, Oregon

1988 ◽  
Vol 79 (2-3) ◽  
pp. 289-297 ◽  
Author(s):  
Timothy H. Druitt ◽  
Charles R. Bacon
Keyword(s):  
Magma Chamber ◽  
Crater Lake ◽  
Calc Alkaline ◽  
Alkaline Magma ◽  
Complete Section ◽  
State Growth ◽  
Derivatives Of ◽  
Insight Into ◽  
Compositional Zonation ◽  
Water Contents

ABSTRACTThe 6845 ± 50 BP climactic eruption of Mount Mazama discharged 47 ± 9 km3 of vertically zoned calc-alkaline magma, affording a virtually complete section through the chamber. Evidence for two andesitic parents with different trace-element (particularly Sr) and water contents is preserved in the ejecta. Prior to eruption, a dominant volume of rhyodacite was underlain successively by high-Sr andesite, high-Sr crystal mushes, and low-Sr crystal mushes. Intergranular liquids in the high-Sr magmas were probably richer in water than those in the low-Sr magmas. Thermal continuity throughout the ejecta favours eruption from a single, zoned reservoir. Insight into chamber development is given by preclimactic rhyodacitic lavas and tephra erupted between about 30,000 BP and the climactic eruption. The oldest of these lavas, contaminated derivatives of low-Sr magma, contain crystal-poor magmatic inclusions of low-Sr andesite; the youngest has inclusions of high-Sr andesite and, like rhyodacitic pumice in the climactic ejecta, is hybrid magma containing an admixed high-Sr component. A model for steady-state growth of the chamber is inferred whereby repeated recharge, first by low-Sr then high-Sr andesite (± basalt), builds up a cumulate succession, while derivative liquid fractionates convectively, segregates, and mixes with an incrementally growing silicic volume. The magma chamber at Mount Mazama may provide insight into the evolution of some granitoid plutons.

Late quaternary explosive silicic volcanism on St Lucia, West Indies

1984 ◽  
Vol 121 (1) ◽  
pp. 1-15 ◽  
Author(s):  
J. V. Wright ◽  
M. J. Roobol ◽  
A. L. Smith ◽  
R. S. J. Sparks ◽  
S. A. Brazier ◽  
...  
Keyword(s):  
Late Quaternary ◽  
Large Area ◽  
Lava Domes ◽  
Flow Deposit ◽  
Pumice Flow ◽  
Valley Fills ◽  
High Silica ◽  
Chemical Character ◽  
Central Highlands ◽  
High Level

AbstractMany explosive eruptions of dacitic magmas have occurred on St Lucia during the late Quaternary. These have produced widespread aprons and fans of pumice flow and ash flow deposits radiating around the central highlands, with co-eruptive air-fall and surge layers interbedded with palaeosols and epiclastic deposits. Vents in the highlands have not been located because of the dense tropical jungle but we suspect they are now plugged by lava domes surrounded by aprons of block and ash flow deposits. Young magmatically related dacitic lava domes have been extruded in the Qualibou depression. The pumice succession can be divided into older quartz-poor deposits forming the Choiseul Pumice and younger crystal-rich deposits with abundant large quartz which are called the Belfond Pumice. The Choiseul Pumice groups together scattered remnants of the products of many eruptions of different low-silica dacitic magma types. The Belfond Pumice is the product of several eruptions of a high-silica magma type and 14C ages have dated these between 20900 to 34200 years B.P.The pumice flow deposits occur as small-volume valley fills. A granulometric study of Belfond pumice flow deposits shows them to be strongly depleted in finer ash and vitric components. It is suggested that the narrow, winding and vegetated valleys on the island locally induced turbulence and the flows moved with large, highly fluidized and inflated heads, resulting in substantial loss of fine vitric ash. One ash flow deposit which is extremely rich in crystals and carbonized vegetation is highly depleted in fines and shows enhanced vitric losses. This flow may have been a much more violent ash hurricane or blast which surmounted topography ingesting large amounts of lush vegetation. Ignition of this released the large quantities of gas needed to elutriate most of the fines.A model is suggested for the recent volcanic activity on St Lucia in which separate batches of silicic magma, each having a distinctive petrological and chemical character, rose into high level chambers over a large area. Eruptions of volatile-rich magma led to highly explosive pumice-forming activity from vents in the central highlands. Degassed and more crystal-rich magma was extruded later from the same vents or in the attenuated flank of the Qualibou depression to from lava dome complexes.

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