scholarly journals Lithostratigraphy and lithogeochemistry of Ediacaran alkaline basaltic rocks of the Musgravetown Group, Bonavista Peninsula, northeastern Newfoundland, Canada: an extensional volcanogenic basin in the type-Avalon terrane

2021 ◽  
Vol 57 ◽  
pp. 207-234
Author(s):  
Andrea Mills ◽  
Hamish Sandeman
Keyword(s):  
Volcanic Rocks ◽  
Volcanic Belt ◽  
Geodynamic Setting ◽  
Alkaline Basalt ◽  
Fine Grained ◽  
Basaltic Rocks ◽  
Slab Rollback ◽  
Low Degree ◽  
Magmatic Underplating ◽  
Head Formation

Volcanic rocks of the Ediacaran Musgravetown Group on Bonavista Peninsula, Avalon terrane, Newfoundland, include basal ca. 600 Ma calc-alkaline basalt succeeded by continental tholeiite and alkaline rhyolite of the ca. 592 Ma Plate Cove volcanic belt (Bull Arm  Formation), indicating a change from subduction-related to extensionrelated tectonic regimes during that interval. Alkalic basalts on northeastern (Dam Pond area) and southwestern (British Harbour area) Bonavista Peninsula occur below and above, respectively, the  ca. 580 Ma glacial Trinity facies. Dam Pond basalt occurs in a structural dome intercalated with and flanked by fine-grained, siliciclastic deposits (Big Head Formation) overlain by Trinity facies. The British Harbour basalt occurs above the Trinity facies, in an upward- coarsening sandstone sequence (Rocky Harbour Formation) overlain by red beds of the Crown Hill Formation (uppermost Musgravetown Group). The Rocky Harbour and Big Head formations are likely stratigraphically interfingered proximal and distal  deposits, respectively, derived from erosion of the Bull Arm Formation and older Avalonian assemblages.The Big Head basalts have lower SiO2, Zr, FeOT, P2O5, TiO2 and higher Mg#, Cr, V, Co and Ni contents, and are therefore more primitive than the more FeOT-, TiO2-, and P2O5-rich British Harbour basalts. Large-ionlithophile and rare-earth-element concentrations and ratios indicate that both suites originated from low degree partial melts of deep, weakly garnet-bearing, undepleted asthenospheric peridotite sources, with magma conduits likely focused along regional extensional faults. The protracted and episodic extension-related volcanic activity is consistent with a geodynamic setting that evolved from a mature arc into extensional basins with slowly waning magmatism, possibly involving slab rollback and delamination followed by magmatic underplating. The duration and variation of both volcanism and sedimentation indicate that the Musgravetown Group should be elevated to a Supergroup in  order to facilitate  future correlation of its constituent parts with other Avalonian basins.

Density determinations of basic volcanic rocks of the Yellowknife supergroup by gravity measurements in mine shafts—Yellowknife, Northwest Territories

Geophysics ◽  
1980 ◽  
Vol 45 (1) ◽  
pp. 18-31 ◽  
Author(s):  
R. A. Gibb ◽  
M. D. Thomas
Keyword(s):  
Volcanic Rocks ◽  
Volcanic Belt ◽  
Mine Shaft ◽  
Average Interval ◽  
Basaltic Rocks ◽  
Surface Samples ◽  
Gravity Measurements ◽  
Vertical Density ◽  
Basic Volcanic

Gravity measurements were made in two gold mine shafts sunk in the Archean Yellowknife greenstone belt to determine the in‐situ densities of basic volcanic rocks of the Kam formation, Yellowknife supergroup. Thirteen stations were occupied between the surface and a depth of 608 m at an average interval of about 50 m in the C shaft of Giant Yellowknife Mines Limited, and 14 stations were occupied between the surface and a depth of 1598 m at an average interval of about 120 m in the Robertson shaft of Con mine, Cominco Limited. Densities were computed using the terminology of borehole gravimetry with appropriate corrections for surface terrain and underground voids such as shafts, drifts, and stopes. Weighted mean in‐situ densities of [Formula: see text] (36 to 608 m depth) and [Formula: see text] (surface to 1598 m depth) were obtained from the gravity measurements for the Giant and Robertson sections, respectively; these values compare with mean densities of 2.82 and [Formula: see text] obtained from rock samples collected at the underground gravity stations. Sheared specimens and massive specimens collected at both underground and surface gravity stations have mean densities of 2.80 and [Formula: see text], respectively. Unaltered surface samples collected at stratigraphic intervals of about 150 m throughout the entire volcanic sequence have a mean density of [Formula: see text]. Core samples obtained from holes drilled from the bottom of C shaft extend the vertical density profile for the Giant section from a depth of 608 to 1416 m; the mean density of these samples is [Formula: see text]. The lower bulk densities obtained from the mine shaft experiments reflect in part the high proportion of sheared rocks and in part the presence of lower‐density members of the Kam formation (andesite, dacite, tuff, breccia, and agglomerate) in the vicinity of the shafts, as opposed to purely massive basaltic rocks. A density of [Formula: see text] based on the proportion of low‐ and high‐density rocks in the volcanic belt is considered to be more representative of the Kam formation as a whole.

Geochemistry of Siluro-Devonian Tobique volcanic belt in northern and central New Brunswick (Canada): tectonic implications

1989 ◽  
Vol 26 (6) ◽  
pp. 1282-1296 ◽  
Author(s):  
J. Dostal ◽  
R. A. Wilson ◽  
J. D. Keppie
Keyword(s):  
New Brunswick ◽  
Mantle Source ◽  
Upper Mantle ◽  
Volcanic Rocks ◽  
Volcanic Belt ◽  
Continental Rift ◽  
Basaltic Rocks ◽  
Incompatible Elements ◽  
Tectonic Implications

Siluro-Devonian volcanic rocks of the northwestern mainland Appalachians are found mainly in the Tobique belt of New Brunswick where they consist predominantly of bimodal mafic–felsic suites erupted in a continental-rift environment. The axis of the Tobique rift trends north-northeast – south-southwest, obliquely to the regional northeast–southwest trend of the Appalachians. These geometric relationships are interpreted as being the result of rifting in a sinistral shear regime produced during emplacement of the Avalon terrene. The basaltic rocks are continental tholeiites and transitional basalts derived from a heterogeneous upper-mantle source that was enriched in incompatible elements relative to the primordial mantle. The mantle source was probably affected by the subduction processes.

Discerning asthenospheric, lithospheric, and crustal influences on the geochemistry of Quaternary basalts from the Iskut–Unuk rivers area, northwestern British Columbia

1995 ◽  
Vol 32 (9) ◽  
pp. 1451-1461 ◽  
Author(s):  
Brian L. Cousens ◽  
Mary Lou Bevier
Keyword(s):  
Volcanic Rocks ◽  
Volcanic Belt ◽  
Volcanic Field ◽  
Northwest Pacific ◽  
Small Range ◽  
Geochemical Composition ◽  
Basaltic Rocks ◽  
Depleted Mantle ◽  
Mid Ocean Ridge ◽  
Ocean Ridge

Pleistocene- to Holocene-age basaltic rocks of the Iskut–Unuk rivers volcanic field, at the southern terminus of the Stikine Volcanic Belt in the northern Canadian Cordillera, provide information on the geochemical composition of the underlying mantle and processes that have modified parental magmas. Basaltic rocks from four of the six eruptive centres are moderately evolved (MgO = 5.7–6.8%) alkaline basalts with chondrite-normalized La/Sm = 1.6–1.8, 87Sr/86Sr = 0.70336–0.70361, εNd = +4.4 to +5.9, and 206Pb/204Pb = 19.07–19.22. The small range of isotopic compositions and incompatible element ratios imply a common "depleted" mantle source for the basalts, similar to the sources of enriched mid-ocean ridge basalts from northwest Pacific spreading centres or alkali olivine basalts from the western Yukon. Positive Ba and negative Nb anomalies that increase in size with increasing SiO2 and 87Sr/86Sr indicate that the basalts are contaminated by Mesozoic-age, arc-related, Stikine Terrane crust or lithospheric mantle through which the magmas passed. Lavas from a fifth volcanic centre, Cinder Mountain, have undergone greater amounts of fractional crystallization and are relatively enriched in incompatible elements, but are isotopically identical to least-contaminated Iskut–Unuk rivers basalts. Iskut–Unuk rivers lavas share many of the geochemical characteristics of volcanic rocks from other Stikine Belt and Anahim Belt centres, as well as alkali olivine basalts from the Fort Selkirk volcanic centres of the western Yukon.

Stratigraphy, paleogeography, and tectonic setting of the Coldbrook Group in the Caledonia Highlands of southern New Brunswick

1977 ◽  
Vol 14 (6) ◽  
pp. 1263-1275 ◽  
Author(s):  
P. S. Giles ◽  
A. A. Ruitenberg
Keyword(s):  
New Brunswick ◽  
Tectonic Setting ◽  
Volcanic Rocks ◽  
Volcanic Belt ◽  
Fine Grained ◽  
Arc Model ◽  
Basement Rocks ◽  
Northwestern Margin ◽  
Intracratonic Basin ◽  
Volcanic Belts

The late Precambrian Coldbrook volcanic sequence and stratigraphic equivalents in southern New Brunswick can be divided into three distinct belts. These have been named the Eastern, Central and Western Volcanic Belts.The Eastern Volcanic Belt, along the Bay of Fundy coast, is characterized by intensely deformed mafic and felsic flows, tuffs, and abundant related volcanogenic sediments. Two thick arkosic sedimentary units in this belt reflect extensive intervals of volcanic quiescence. Fine-grained siliceous siltstone and conglomerate, locally intercalated with these rocks, have probably been derived from erosion of older Precambrian basement rocks to the northwest.The Central Volcanic Belt is composed of generally weakly deformed felsic and lesser mafic flows, and coarse lithic tuffs (including ignimbrites), and very minor intercalated sediments. The almost complete lack of water-lain sediments and presence of ignimbrites suggests subaerial deposition for most of these volcanic rocks. The relationship between rocks of the Central and Eastern Volcanic Belts is one of facies equivalence. The Western Volcanic Belt is also composed of felsic and minor mafic flows and tuffs that resemble those of the Central Volcanic Belt, but they are intensely deformed. Minor volcanogenic sedimentary rocks are intercalated with the volcanic rocks along the northwestern margin of this belt.The nature and distribution of major lithofacies belts in the Coldbrook Group and stratigraphic equivalents appear to be consistent with deposition along the margin of an intracratonic basin. It is possible, however, that further work may prove an ensialic island arc model to be a viable alternative.

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.

Quantitative Determination of some Carbonate Minerals in Greenschist Facies Meta-volcanic Rocks

1972 ◽  
Vol 9 (1) ◽  
pp. 36-42 ◽  
Author(s):  
Calvert C. Bristol
Keyword(s):  
Standard Deviation ◽  
Quantitative Determination ◽  
Volcanic Rocks ◽  
Internal Standard ◽  
Greenschist Facies ◽  
Carbonate Mineral ◽  
Mineral Contents ◽  
X Ray ◽  
Fine Grained

X-ray powder diffraction methods, successful in quantitative determination of silicate minerals in fine-grained rocks, have been applied to the determination of calcite, dolomite, and magnesite in greenschist facies meta-volcanic rocks. Internal standard graphs employing two standards (NaCl and Mo) have been determined.Carbonate mineral modes (calcite and dolomite) for 6 greenschist facies meta-volcanic rocks obtained by the X-ray powder method have been compared to normative carbonate mineral contents calculated for the same rocks. This comparison showed a maximum variation of 7.7 wt.% between the X-ray modes and the normative carbonate mineral contents of the rocks. Maximum standard deviation for the X-ray modes of these rocks was equivalent to 4.4 wt.%.

Three stages of arc migration in the Carboniferous-Triassic in northern Qiangtang, central Tibet, China: Ridge subduction and asynchronous slab rollback of the Jinsha Paleotethys

2021 ◽  
Author(s):  
Yin Liu ◽  
Wenjiao Xiao ◽  
Brian F. Windley ◽  
Kefa Zhou ◽  
Rongshe Li ◽  
...  
Keyword(s):  
Tibetan Plateau ◽  
Volcanic Rocks ◽  
Late Triassic ◽  
Geochemical Data ◽  
The Tibetan Plateau ◽  
Ridge Subduction ◽  
Temporal Relationships ◽  
Slab Rollback ◽  
Geodynamic Processes ◽  
Central Tibet

Carboniferous-Triassic magmatism in northern Qiangtang, central Tibet, China, played a key role in the evolution of the Tibetan Plateau yet remains a subject of intense debate. New geochronological and geochemical data from adakitic, Nb-enriched, and normal arc magmatic rocks, integrated with results from previous studies, enable us to determine the Carboniferous-Triassic (312−205 Ma), arc-related, plutonic-volcanic rocks in northern Qiangtang. Spatial-temporal relationships reveal three periods of younging including southward (312−252 Ma), rapid northward (249−237 Ma), and normal northward (234−205 Ma) migrations that correspond to distinct slab geodynamic processes including continentward slab shallowing, rapid trenchward slab rollback, and normal trenchward rollback of the Jinsha Paleotethys rather than the Longmuco-Shuanghu Paleotethys, respectively. Moreover, varying degrees of coexistence of adakites/High-Mg andesites (HMAs)/Nb-enriched basalt-andesites (NEBs) and intraplate basalts in the above-mentioned stages is consistent with the magmatic effects of slab window triggered by ridge subduction, which probably started since the Late Carboniferous and continued into the Late Triassic. The Carboniferous-Triassic multiple magmatic migrations and ridge-subduction scenarios provide new insight into the geodynamic processes of the Jinsha Paleotethys and the growth mechanism of the Tibetan Plateau.

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