Fractionation in the feeder system at a Proterozoic rifted margin

1984 ◽  
Vol 21 (4) ◽  
pp. 489-499 ◽  
Author(s):  
Jean H. Bedard ◽  
Donald M. Francis ◽  
Andrew J. Hynes ◽  
Serge Nadeau
Keyword(s):  
Fractional Crystallization ◽  
Volcanic Rocks ◽  
Crystallization Mechanism ◽  
Evolutionary Trend ◽  
Titanium Oxides ◽  
Magma Chambers ◽  
Volcanic Stratigraphy ◽  
Volcanic Pile ◽  
Dominant Process ◽  
Rifted Margin

In the Proterozoic Cape Smith Foldbelt of Ungava, Quebec, basal basalts of continental affinity are succeeded upward and basinward by cyclic sequences of MgO-rich (≤ 19 wt.% MgO) to MgO-poor submarine basalts of oceanic affinity belonging to the Chukotat Group. The more primitive komatiitic basalts of the Chukotat Group evolved via fractional crystallization of olivine within a crustal feeder system that is represented by large, layered sills and discordant dyke – sill complexes. These intrusions occupy horizons of mechanical weakness such as sedimentary or hyaloclastite-rich horizons within the volcanic stratigraphy. Peridotite and peridotite – gabbro sills predominate at the base of the Chukotat volcanic pile, whereas gabbroic sills are more common higher in the stratigraphy, reflecting the progressive fractionation within the feeder system. Gravitationally controlled settling of crystals or crystal clots is thought to be the dominant process responsible for fractionation in these crustal sills. Fractional crystallization of olivine within the feeder system produced the olivine-phyric to pyroxene-phyric evolutionary trend observed in the coexisting volcanic rocks. Continuing extraction of clinopyroxene, plagioclase, and iron – titanium oxides in subcrustal sills or magma chambers is thought to have generated the MORB-like upper plagioclase-phyric Chukotat basalts. The compositional gap between the pyroxene-phyric and plagioclase-phyric basalts is a by-product of the fractional crystallization mechanism: liquids with compositions typical of the gap are so highly charged with suspended plagioclase crystals that they resist extrusion.

The petrogenetic significance of chemically related plutonic and volcanic rock units

1986 ◽  
Vol 123 (6) ◽  
pp. 619-628 ◽  
Author(s):  
D. Wyborn ◽  
B. W. Chappell
Keyword(s):  
Fractional Crystallization ◽  
Source Region ◽  
Volcanic Rocks ◽  
Granitic Rocks ◽  
Magma Chambers ◽  
Southeastern Australia ◽  
Lachlan Fold Belt ◽  
Magma Compositions ◽  
High Level ◽  
Heterogeneous Crystallization

AbstractComagmatic granitic and volcanic rocks are divided into two types depending on whether or not the primary magma contains restite crystals. Examples of both of these types are discussed from the Lachlan Fold Belt of southeastern Australia.Volcanic rocks containing restite phenocrysts are chemically identical to the associated plutonic rocks containing the same amount of restite. The more mafic granitic rocks correspond in composition to the most phenocryst-rich volcanics (up to 60% phenocrysts), and thus cannot be cumulate rocks produced by fractional crystallization, but must represent true magma compositions. These restite-bearing magmas result from partial melting in a source region up to the rheological critical melt percentage, which we estimate to be about 40% in the S-type Hawkins Suite of volcanics.Melts which escape their restite at the source, before the critical melt percentage is reached, are able to undergo fractional crystallization in high level magma chambers by heterogeneous crystallization on chamber walls. In this case volcanic products from the top of the chamber are more felsic than the plutonic products, the plutonics are crystal cumulates and the volcanics are composed of the complementary fractionated liquid. Those phenocrysts present in the volcanics were probably eroded from the chamber walls and are less abundant (< 20%) than in the restite-retentive volcanic products.

Relative roles of source composition, fractional crystallization and crustal contamination in the petrogenesis of Andean volcanic rocks

1984 ◽  
Vol 310 (1514) ◽  
pp. 675-692 ◽  
Keyword(s):  
Subduction Zone ◽  
Continental Crust ◽  
Fractional Crystallization ◽  
Magma Mixing ◽  
Volcanic Rocks ◽  
Crustal Contamination ◽  
Oceanic Lithosphere ◽  
Alkaline Basalt ◽  
Complex Process ◽  
Heterogeneous Mantle

There are well established differences in the chemical and isotopic characteristics of the calc-alkaline basalt—andesite-dacite-rhyolite association of the northern (n.v.z.), central (c.v.z.) and southern volcanic zones (s.v.z.) of the South American Andes. Volcanic rocks of the alkaline basalt-trachyte association occur within and to the east of these active volcanic zones. The chemical and isotopic characteristics of the n.v.z. basaltic andesites and andesites and the s.v.z. basalts, basaltic andesites and andesites are consistent with derivation by fractional crystallization of basaltic parent magmas formed by partial melting of the asthenospheric mantle wedge containing components from subducted oceanic lithosphere. Conversely, the alkaline lavas are derived from basaltic parent magmas formed from mantle of ‘within-plate’ character. Recent basaltic andesites from the Cerro Galan volcanic centre to the SE of the c.v.z. are derived from mantle containing both subduction zone and within-plate components, and have experienced assimilation and fractional crystallization (a.f.c.) during uprise through the continental crust. The c.v.z. basaltic andesites are derived from mantle containing subduction-zone components, probably accompanied by a.f.c. within the continental crust. Some c.v.z. lavas and pyroclastic rocks show petrological and geochemical evidence for magma mixing. The petrogenesis of the c.v.z. lavas is therefore a complex process in which magmas derived from heterogeneous mantle experience assimilation, fractional crystallization, and magma mixing during uprise through the continental crust.

Origin of the Tulameen ultramafic-gabbro complex, southern British Columbia

1969 ◽  
Vol 6 (3) ◽  
pp. 399-425 ◽  
Author(s):  
D. C. Findlay
Keyword(s):  
British Columbia ◽  
Internal Structure ◽  
Fractional Crystallization ◽  
Volcanic Rocks ◽  
Age Determination ◽  
Upper Triassic ◽  
Ultramafic Rocks ◽  
Metasedimentary Rocks ◽  
Gabbroic Intrusion

The Tulameen Complex is a composite ultramafic-gabbroic intrusion that outcrops over 22 sq. mi. (57 km2) in the Southern Cordillera of British Columbia. The complex intruded Upper Triassic metavolcanic and metasedimentary rocks of the Nicola Group, and on the basis of geologic relations and a K–Ar age determination (186 m.y.) is tentatively dated as Late Triassic.The principal ultramafic units — dunite, olivine clinopyroxenite, and hornblende clinopyroxenite — form an elongate, non-stratiform body whose irregular internal structure is best explained by deformation contemporaneous with crystallization of the rocks. The derivation of the ultramafic rocks is attributed to fractional crystallization of an ultrabasic magma. The gabbroic mass, which consists of syenogabbro and syenodiorite, partly borders and partly overlies the ultramafic body and was apparently intruded by it.The ultramafic and gabbroic parts of the complex probably formed from separate intrusions of different magmas, but the two suites have sufficient mineralogical and chemical features in common to indicate an ultimate petrogenic affinity of the magmas. Comparison of the Tulameen rocks with nearby intrusions of the same general age, in particular the Copper Mountain stock, suggests that they are members of a regional suite of alkalic intrusions. The possibility is also raised that these intrusions may be comagmatic with the Nicola volcanic rocks.

scholarly journals Multi-stage arc magma evolution recorded by apatite in volcanic rocks

Geology ◽  
2020 ◽  
Vol 48 (4) ◽  
pp. 323-327 ◽  
Author(s):  
Chetan L. Nathwani ◽  
Matthew A. Loader ◽  
Jamie J. Wilkinson ◽  
Yannick Buret ◽  
Robert H. Sievwright ◽  
...  
Keyword(s):  
Fractional Crystallization ◽  
Volcanic Rocks ◽  
Explosive Volcanism ◽  
Ore Deposits ◽  
Magma Evolution ◽  
Arc Magmas ◽  
Deep Crust ◽  
Multi Stage ◽  
Novel Approach ◽  
Arc Magma

Abstract Protracted magma storage in the deep crust is a key stage in the formation of evolved, hydrous arc magmas that can result in explosive volcanism and the formation of economically valuable magmatic-hydrothermal ore deposits. High magmatic water content in the deep crust results in extensive amphibole ± garnet fractionation and the suppression of plagioclase crystallization as recorded by elevated Sr/Y ratios and high Eu (high Eu/Eu*) in the melt. Here, we use a novel approach to track the petrogenesis of arc magmas using apatite trace element chemistry in volcanic formations from the Cenozoic arc of central Chile. These rocks formed in a magmatic cycle that culminated in high-Sr/Y magmatism and porphyry ore deposit formation in the Miocene. We use Sr/Y, Eu/Eu*, and Mg in apatite to track discrete stages of arc magma evolution. We apply fractional crystallization modeling to show that early-crystallizing apatite can inherit a high-Sr/Y and high-Eu/Eu* melt chemistry signature that is predetermined by amphibole-dominated fractional crystallization in the lower crust. Our modeling shows that crystallization of the in situ host-rock mineral assemblage in the shallow crust causes competition for trace elements in the melt that leads to apatite compositions diverging from bulk-magma chemistry. Understanding this decoupling behavior is important for the use of apatite as an indicator of metallogenic fertility in arcs and for interpretation of provenance in detrital studies.

Iron-titanium oxides and oxygen fugacities in volcanic rocks

1967 ◽  
Vol 72 (18) ◽  
pp. 4665-4687 ◽  
Author(s):  
I. S. E. Carmichael ◽  
J. Nicholls
Keyword(s):  
Volcanic Rocks ◽  
Titanium Oxides

Evolution of Mt. St. Joseph—an Archaean Volcano

1971 ◽  
Vol 8 (1) ◽  
pp. 150-161 ◽  
Author(s):  
Paul M. Clifford ◽  
Robert H. McNutt
Keyword(s):  
Volcanic Rocks ◽  
Gas Pressure ◽  
Eruptive History ◽  
Iron Enrichment ◽  
Severe Deformation ◽  
Continuous Activity ◽  
Volcanic Pile ◽  
Mixed Flows

A 6650-m thickness of volcanic rocks at Lake St. Joseph is here interpreted as an Archaean composite strato-volcano. Despite severe deformation, primary mesoscopic fabrics are well-preserved. They permit the inference of a physical eruptive history, to which the chemistry can be related.The lowermost 2700-m of flows, pillow breccias, and autobreccias are exclusively basaltic, and indicate quiet, probably semi-continuous activity. Interruption of this activity is shown by an intraformational conglomerate developed on a metadiorite, which has thermally metamorphosed argillite lenses intercalated with the flows. Subsequently, activity became more episodic and violent, yielding, first, 750-m of mixed flows and fragmental rocks, substantially andesitic; and then 3200-m of fragmental rocks, substantially salic. Basaltic dikes ramify through the volcanic pile. Several rhyolitic or dacitic flows are regarded as flank eruptions.Generally, these volcanic rocks have trends somewhat similar to trends for other Archaean volcanics. However, there is an iron enrichment trend which we suggest indicates an initially low pO2 increasing as the volcano ages. This increase may indicate an increase in total gas pressure, compatible with the change from quiet to violent eruptive mode.

Eocene Challis-Kamloops volcanism in central British Columbia: an example from the Buck Creek basin

1998 ◽  
Vol 35 (8) ◽  
pp. 951-963 ◽  
Author(s):  
J Dostal ◽  
D A Robichaud ◽  
B N Church ◽  
P H Reynolds
Keyword(s):  
British Columbia ◽  
Rare Earth ◽  
Rare Earth Element ◽  
Fractional Crystallization ◽  
Earth Element ◽  
Volcanic Rocks ◽  
Volcanic Belt ◽  
The United States ◽  
Calc Alkaline ◽  
Heavy Rare Earth Element

Eocene volcanic rocks of the Buck Creek basin in central British Columbia are part of the Challis-Kamloops volcanic belt extending from the United States across British Columbia to central Yukon. The volcanic rocks include two units, the Buck Creek Formation, composed of high-K calc-alkaline rocks with predominant andesitic composition, and the overlying Swans Lake unit made up of intraplate tholeiitic basalts. Whole rock 40Ar/39Ar data for both units show that they were emplaced at 50 Ma. They have similar mantle-normalized trace element patterns characterized by a large-ion lithophile element enrichment and Nb-Ta depletion, similar chondrite-normalized rare earth element patterns with (La/Yb)n ~4-14 and heavy rare earth element fractionation, and overlapping epsilonNd values (2.4-3.1) and initial Sr-isotope ratios ( ~ 0.704). These features suggest derivation of these two units from a similar mantle source, probably garnet-bearing subcontinental lithosphere. The differences between tholeiitic and calc-alkaline suites can be due, in part, to differences in the depth of fractional crystallization and the crystallizing mineral assemblage. Fractional crystallization of the calc-alkaline magmas began at a greater (mid-crustal) depth and included fractionation of Fe-Ti oxides. The volcanic rocks are probably related to subduction of the Farallon plate under the North American continent in a regime characterized by transcurrent movements and strike-slip faulting.

GEOCHEMISTRY OF THE YELLOWKNIFE VOLCANIC ROCKS

1966 ◽  
Vol 3 (1) ◽  
pp. 9-30 ◽  
Author(s):  
W. R. A. Baragar
Keyword(s):  
Bulk Composition ◽  
Volcanic Rocks ◽  
River Section ◽  
Magma Chambers ◽  
Tholeiitic Magma ◽  
Composite Form ◽  
High Level ◽  
Mafic Lavas ◽  
Stratigraphic Height ◽  
Volcanic Cycles

Results of rapid-method chemical analyses of samples taken at about 500-ft stratigraphic intervals through two sections of Yellowknife Group volcanic rocks are presented in graphical and composite form. The Yellowknife section is about 40 000 ft thick with the base undefined; the Cameron River section, about 45 mi northeast, is about 7 000 ft thick and may be complete.Two aspects of the volcanic chemistry considered are (1) variation in composition with stratigraphic height; (2) bulk composition of the volcanic rocks.Chemical data of the Yellowknife section define two volcanic cycles in each of which mafic lavas show a small but significant increase in sialic components with stratigraphic height culminating abruptly in acidic layers. The Cameron River section shows a similar but less well-defined trend. Iron–magnesium ratios stage a succession of systematic increases, each persisting for a few thousand stratigraphic feet, but no overall systematic variation. The two types of chemical variation correspond to calc-alkali and tholeiitic differentiation trends respectively. The tholeiitic trend is attributed to fractionation in high-level magma chambers, demonstrated for Yellowknife magma by the Kam Point sill, and the calc-alkali trend to contamination of tholeiitic magma by sialic crust.Frequency distribution diagrams show Yellowknife volcanic rocks to be similar to Chayes' circumoceanic basalts in TiO2, CaO, and MgO and to his oceanic basalts in Al2O3. The characteristic rock type is basalt.

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