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.

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.

U–Pb zircon ages of volcanism and plutonism in the Mishibishu greenstone belt near Wawa, Ontario

1990 ◽  
Vol 27 (5) ◽  
pp. 649-656 ◽  
Author(s):  
A. Turek ◽  
R. Keller ◽  
W. R. Van Schmus
Keyword(s):  
Greenstone Belt ◽  
Volcanic Rocks ◽  
Superior Province ◽  
Calc Alkaline ◽  
Metasedimentary Rocks ◽  
Metavolcanic Rocks ◽  
Volcanic Pile ◽  
Archean Greenstone Belt

The Mishibishu greenstone belt, located 40 km west of Wawa, is a typical Archean greenstone belt and is probably an extension of the Michipicoten belt. This belt is composed of basic to felsic metavolcanic rocks of tholeiitic to calc-alkaline affinity and of metasedimentary rocks ranging from conglomerate to argillite. Granitoids, diorites, and gabbros intrude and embay supracrustal rocks as internal and external plutons.Six U–Pb zircon ages have been obtained on rocks in this area. The oldest is 2721 ± 4 Ma for the Jostle Lake tonalite. The bulk of the volcanic rocks formed by 2696 ± 17 Ma, which is the age of the Chimney Point porphyry at the top of the volcanic pile. The Pilot Harbour granite has a similar age of 2693 ± 7 Ma. The age of the Tee Lake tonalite is 2673 ± 12 Ma, and the age of the Iron. Lake gabbro is 2671 ± 4 Ma. The youngest age for volcanics in this part of the Superior Province is 2677 ± 7 Ma, obtained from, the David Lakes pyroclastic breccia. these ages agree with those reported for the adjacent Michipicoten and Gamitagama belts.

Chronology of Precambrian events in the Front Range, Colorado

1968 ◽  
Vol 5 (3) ◽  
pp. 749-756 ◽  
Author(s):  
Zell E. Peterman ◽  
Carl E. Hedge
Keyword(s):  
Regional Metamorphism ◽  
Volcanic Rocks ◽  
Time Span ◽  
Tectonic Activity ◽  
Front Range ◽  
Igneous Activity ◽  
Volcanic Pile ◽  
Metamorphic Terrane ◽  
Geologic Record ◽  
Intrusive Activity

The Precambrian rocks of the Front Range have undergone a long and complex history involving multiple periods of metamorphism, deformation, and intrusive activity. Before 1750 m.y. ago sedimentation and volcanism resulted in the accumulation of many thousands of feet of sandstone and shale, and lesser amounts of calcareous and volcanic rocks on a basement terrane that has not yet been recognized.Regional and dynamic metamorphism at 1700 to 1750 m.y. ago converted the sedimentary-volcanic pile to a metamorphic terrane of dominantly medium- to high-grade gneisses and schists. Plutonic rocks typified by Boulder Creek Granite were emplaced during this event. After this orogeny, there is a time span of 250 to 350 m.y. for which there is no obvious geologic record. The interval 1390 to 1450 m.y. was a period of major igneous activity during which the extensive Silver Plume Granite, Sherman Granite, granitic and mafic dikes, and pegmatites were emplaced. Regional heating attendant with this plutonism was effective in resetting most K-Ar and Rb-Sr mineral ages of the older rocks. Tectonic activity along the great shear zone that trends northeast through Idaho Springs probably began during this orogeny and may have continued intermittently until about 1200 m.y. ago.The Pikes Peak batholith and smaller related granites were emplaced at 1040 m.y. ago. This event was not accompanied by regional metamorphism. There is no recognized sedimentary record between the earlier orogeny and this igneous activity. The Pikes Peak igneous activity is the youngest recognized Precambrian event, but it appears to be limited to the Pikes Peak batholith and satellite plutons.

scholarly journals Report on field work along the north coast of Disko, 1971

1973 ◽  
Vol 53 ◽  
pp. 21-27
Author(s):  
A.K Pedersen
Keyword(s):  
Volcanic Rocks ◽  
Field Work ◽  
North Coast ◽  
Volcanic Zone ◽  
North West ◽  
The North ◽  
Volcanic Pile

During the summer field work was carried out along the north coast of Disko. This work was a continuation of investigations in 1968 (see Pedersen, 1969, 1970). The purpose of the work was to study the section across the Tertiary volcanic zone exposed along the north coast of Disko and to sample the volcanic rocks for petrological investigation. A zone about 40 km long, from the Kûgánguaq valley to just north-west of Qutdligssat, was mapped, representing a volcanic pile at least 2.3 km thick (see fig. 7).

TEM/AEM characterization of fine-grained clay minerals in very-low-grade rocks: Evaluation of contamination by empa involving celadonite family minerals

1996 ◽  
Vol 54 ◽  
pp. 694-695
Author(s):  
Gejing Li ◽  
D. R. Peacor ◽  
D. S. Coombs ◽  
Y. Kawachi
Keyword(s):  
Electron Microscopy ◽  
Volcanic Rocks ◽  
Analytical Electron Microscopy ◽  
Metamorphic Rocks ◽  
New Mineral ◽  
Chemical Compositions ◽  
Low Grade ◽  
Fine Grained ◽  
Depth Analysis ◽  
Mineral Aggregates

Recent advances in transmission electron microscopy (TEM) and analytical electron microscopy (AEM) have led to many new insights into the structural and chemical characteristics of very finegrained, optically homogeneous mineral aggregates in sedimentary and very low-grade metamorphic rocks. Chemical compositions obtained by electron microprobe analysis (EMPA) on such materials have been shown by TEM/AEM to result from beam overlap on contaminant phases on a scale below resolution of EMPA, which in turn can lead to errors in interpretation and determination of formation conditions. Here we present an in-depth analysis of the relation between AEM and EMPA data, which leads also to the definition of new mineral phases, and demonstrate the resolution power of AEM relative to EMPA in investigations of very fine-grained mineral aggregates in sedimentary and very low-grade metamorphic rocks.Celadonite, having end-member composition KMgFe3+Si4O10(OH)2, and with minor substitution of Fe2+ for Mg and Al for Fe3+ on octahedral sites, is a fine-grained mica widespread in volcanic rocks and volcaniclastic sediments which have undergone low-temperature alteration in the oceanic crust and in burial metamorphic sequences.

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